emission and laser-induced fluorescence imaging methods in experimental combustion

ELSEVIER

Emission and Laser-Induced Fluorescence Imaging Methods in Experimental Combustion

K. McManus*

B. Yip*

S. Candel Laboratoire EM2C, CNRS, Ecole Centrale Paris, Chatenay-Malabry, France

• Imaging methods provide new insights into many fundamental combustion processes. Many imaging techniques have been devised in recent years and applied to a range of experiments. One particularly useful method is to seed the flow with oil particles and illuminate the domain of interest with a planar sheet of laser light. The droplets evaporate and vanish when they pass through the flame. The light scattered by the particles may be imaged for example with a CCD camera or with high-speed cinematography to show the structure and dynamics of the flame front. This technique, sometimes called laser tomography, is based on Mie scattering. It provides essentially qualitative information on the geometry and motion of the flame front. Another valuable method relies on spontaneous emission imaging. In this method the light emitted by certain radicals produced by the chemical reaction is detected by a camera and delivered to a computer for further processing. In some circumstances it is possible to deduce from this measurement the spatial distribution of heat release in the reactive flow. More quantitative data may be gathered with planar laser-induced fluorescence (PLIF) imaging. The reactive flow is illuminated with a planar laser sheet delivered by a tunable laser. The laser light excites the fluorescence of a species that is present in the flow, which is then detected with an intensified CCD camera. The data obtained in this way can be processed to obtain spatial measurements of the species concentration. The basic principles, equipment requirements, and experimental aspects of these three imaging techniques are reviewed. Practical applications to turbulent flames are emphasized. It is shown that emission imaging applied to turbulent ducted flames yields interesting information for modeling. A second example of application is the ignition sequence of a multiple-injector combustor, of importance to modern cryogenic rocket engines. Emission and PLIF imaging have been used to obtain data on the development of the initial flame kernel and on its propagation from the first injector to the next. The images gathered in this experiment yield a unique view on the flame patterns that lead to the final stabilization of the reactive fronts. While current imaging methods are essentially qualitative, it is possible to deduce quantitative results from the data, and some of the present limitations may be overcome with more refined measurement procedures. These issues are analyzed, and future developments in this area are evaluated.

Keywords: emission imaging, laser-induced fluorescence, combustion

*Current address: Physical Sciences Inc., Andover, MA 01810. *Current address: University of Heidelberg, Heidelberg D6900,

Germany.

Address correspondence to Professor S6bastien Candel, Laboratoire EM2C, UPR 288 CNRS, Ecole Central Paris, 92295 Chatenay-Malabry Cedex, France.

Experimental Thermal and Fluid Science 1995; 10:486-502 © Elsevier Science Inc., 1995 655 Avenue of the Americas, New York, NY 10010

0894-1777/95/$9.50 SSDI 0894-1777(94)00078-M

Emission and LIF Imaging Methods 487

I N T R O D U C T I O N

Imaging of reactive flow fields has provided new views into the complex chemical and fluid-mechanical phenomena typically found in combustion devices. These phenomena include the chemical processes of flame initiation, spread- ing, propagation, and heat release along with the flow processes leading to vortex shedding, turbulent combustion, and flame interactions with the flow and the boundaries. Combustion imaging has specifically supplied information on two- and three-dimensional flame structures, allowing detailed identification of the basic processes governing reactive flow.

For many years the most common imaging techniques used in combustion have been shadowgraph, schlieren, and interferometric techniques. These methods yield two- dimensional renderings of refraction index distributions, and in flows of constant or nearly constant pressure the visualization yields the density gradients in the flow field and effectively locates interfaces separating gases of different density. In most subsonic combustion flow fields, the pressure may be taken as nearly constant, and one may interpret the image in terms of temperature gradients. The boundary separating the fresh and burned gases may be located on this basis, and the image can be used to identify the approximate position of the flame front. How- ever, the interpretation remains ambiguous because it is not always possible to distinguish a simple interface between gases of different density from an active flame sheet.

Another major limitation of classical visualization methods concerns their spatial resolution in flows that are not two-dimensional with respect to the optic axis. The information is collected over the optical path along the line of sight, and the image is not easily deconvolved. Because most combustion flows are at least partly three- dimensional, standard techniques provide spatial resolution that is only adequate for qualitative interpretation. It is worth noting, however, that classical techniques remain valuable, especially in combination with modern imaging methods.

In recent years digital imaging techniques based on solid-state array detectors in combination with planar light sheets formed with the radiation emitted by lasers have been extensively developed and have become a powerful experimental tool. These new methods allow time- resolved measurements and yield spatially resolved maps of the reactive flow field, and in this they are far superior to the aforementioned line-of-sight techniques. Other advantages become apparent when one considers that many species-specific spectroscopic measurement techniques, typically performed at a point, may be extended to two dimensions using more recently available high-power nar- row band lasers along with sensitive detector arrays. Typi- cally, these techniques deliver a two-dimensional image of specific flow-field variables through the collection of ei- ther scattered or induced radiation from the laser light plane using a detector array such as photographic film or an electronic camera array [i.e., a charge-coupled device (CCD) matrix].

The most popular two-dimensional imaging techniques used in combustion studies derive their signal from Lorenz-Mie scattering [1], Rayleigh scattering [2], natural

light emission from free radicals (e.g., [3, 4], or laser induced fluorescence (LIF) (see, e.g., [5, 6]).

In this article we focus on combustion investigations in which two-dimensional imaging is used as a diagnostic tool. Without trying to be exhaustive, we focus on a reduced number of topics and dwell on practical aspects. Subjects not covered here are examined in other publica- tions cited as references. We specifically describe laser light scattering, line-of-sight direct-emission time-average and time-resolved imaging, and planar laser-induced fluorescence (PLIF). Initially developed in laminar flame experiments, these methods have recently found important applications in studies of turbulent combustion.

Laser light scattering (often called laser tomography because the flow is visualized as a slice) provides basic information on the shape and dynamics of the flame front and, in combination with digital processing, may be used to determine the density of flame surface (the flame surface per unit volume). Time-average emission imaging gives, under some circumstances, access to the volumetric rate of heat release. In other applications, time-resolved light emission from free radicals may be used to visualize a dynamical flame phenomenon like ignition and extinction or the flame response to external perturbations or simply to determine the flame stand-off distance. Planar laser-induced fluorescence yields time-resolved spatial distributions of species of interest such as OH. We describe these four methods along with examples of results obtained. Special attention is given to providing sufficient information so that experiments similar to those described herein may be performed by others with a minimum of difficulty.

IMAGING BASED ON LASER L I G H T SCATTERING

A simple method of flame visualization was devised by Boyer [1]. It consists of injecting an oil-aerosol into the unburned mixture (Fig. la). The droplets evaporate and burn as they pass through the flame front. This process may then be visualized by illuminating the combustion zone with a thin laser sheet. It is thus possible to measure the flame displacement as a function of time (as described by Boyer [1]) or to take a direct image of the flow. The method yields the shape of the flame front. When com- bined with a high-speed camera and modulated laser light, it also reveals the dynamical properties of the flame front and may be used to study flame interactions with orga- nized structures [7], pressure waves [8], and flow turbulence [9-11].

Experimental Application

A typical experimental configuration is shown in Fig. lb. A strained diffusion flame is formed in the counterflow of a jet of fuel and a jet of air. A laser sheet is formed with two cylindrical lenses and illuminates the combustion zone. In a first experiment [12], oil droplets are injected in both streams and the fuel is propane. The counterflow pattern is visualized in this case in Fig. 2a The light emitted by the flame is also superposed on the scattered laser light. Consider now the same counterflow geometry, but this time the gaseous fuel is replaced by a cloud of liquid

488 McManus et al.

Photomultiplier

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ra

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Figure 1. Typical arrangement for laser "tomography" of flames. (a) Tomography of a premixed laminar flame in a duct; (b) tomography of a counterflow diffusion flame.

droplets of heptane convected by a stream of nitrogen [13]. The flow is not seeded in this second experiment. The laser sheet illuminates the heptane droplets. It is observed that the droplets vaporize at a small distance from the flame front, and this indicates that a regime of "group combustion" prevails in which the flame forms at a distance from the droplet cloud vaporization.

Figure 2. Laser tomography of counterflow diffusion flames. (a) The flame is formed by opposed jets of propane and air. (Courtesy of C. Rolon.) (b) The flame is formed by a jet of nitrogen convecting droplets of heptane encountering a stream of air. (Courtesy of F. Lacas.)

recent models of turbulent combustion, and it may be determined from successive visualizations of the flame front. The domain of interest is first subdivided into elementary cells. The flame length in each cell is determined, and the instantaneous density is calculated for all the ceils. This operation is repeated for an ensemble of flame images. An averaged density is then determined for each cell and constitutes the mean flame surface density.

High-speed tomography based on modulated laser sheets provides information on dynamical processes like flame propagation, instabilities, and pattern formation. Such applications are well illustrated in [8].

Perspect ive

Laser tomography is now used extensively in qualitative investigations of flames. It is particularly valuable in studies of dynamical phenomena and in turbulent combustion analysis. Using digital processing of images obtained with laser tomography, one may analyze flame shapes and determine flame speeds, wrinkle geometry, scales, and fractal characteristics and dimensions (see, e.g., [14, 15]). It is also possible to compute the flame surface density with algorithms like those described in Delhaye et al. [16] or Deschamps et al. [17].

The density of the flame surface is of interest in some

TIME-AVERAGE IMAGING BASED ON LINE-OF-SIGHT EMISSION FROM

FREE RADICALS

Objective

Imaging of the light emission from free radicals such as C 2, CH, and OH is specifically valuable in turbulent combustion modeling, in combustion instability studies, and in the analysis of dynamical flame phenomena such as ignition and extinction. In general, the chemiluminescent emission may be interpreted as a signature of chemical reaction and can be used to delimit regions of reaction and heat release [18-22].


Under certain conditions the light emission from these radicals can be interpreted as indicative of the rate of heat release in the reactive flow. The spatial distribution of the light emission can be used to analyze the interactions between turbulence and chemical reaction, to understand the coupling between pressure waves and the nonsteady heat release, to visualize the dynamics of a flame submit- ted to external perturbations, and to examine the ignition/extinction sequence of a combustor. We wish to emphasize the utility of such measurements for combustion modeling. An application to the analysis of ignition is also given in the next section.

Although most of the recent experiments on turbulent flames have focused on measurements of temperature, velocity, or concentration profiles, it is worth gathering information on the mean source terms and especially on the mean heat release distribution in the flow. There are good reasons that justify this statement. First, it is found that the spatial distribution of the mean heat release is influenced by the equivalence ratio and the flow velocity, thus providing a unique view of the interaction between the chemical kinetics and the flow turbulence. Second, the heat release data allow a direct evaluation of the theoretical expressions used to model the mean consumption rates appearing in the averaged balance equations. Of course, standard measurements of the flow variables and of their fluctuations remain valuable, but much can be learned from the less standard evaluation of the rate of heat release per unit volume.

It is worth emphasizing that the detailed examination of the mean source term differs from the more common tests performed in the literature on velocity, temperature, and mass fraction profiles, all of which are only indirectly related to the modeled reaction rates. Because the flow variables are obtained by integrating the dynamic equations, they are less sensitive to the modeling assumptions and do not allow a direct assessment of the combustion models. It is a fact that reasonable mean flow profiles can be obtained with the simplest assumptions.

It is, then, difficult to see if the model is adequate. If one wishes to describe the effects of finite rate chemistry and turbulence on the structure of the flame, it appears clear that the mean source terms should be examined and precisely represented. With this goal in mind it is useful to measure the distribution of light emission from free radicals such as C 2 and CH. As already indicated, the spatial distributions of the radiated light may be interpreted as giving a qualitative mapping of the local mean heat release in the turbulent flame. These distributions may be used as guidelines for combustion modeling. Now, an examination of the heat release pattern indicates that finite-rate chemistry effects are quite pronounced.

On these grounds it appears that the modeling of the flow constitutes a challenging problem. Indeed it is now well known that standard "fast" chemistry models are unable to reproduce the trends observed. Improved de- scriptions are needed that account explicitly for the interaction between the chemical kinetics and the flow turbulence.

Light Emission Imaging Principles

Although exact measurements of the local heat release are not available, many observations indicate that the light

emitted by the combustion zone is related to the reaction intensity and hence to the heat-release process. Certain radicals, for example, C 2 and CH, appear almost exclu- sively in reactive zones, and their concentrations are always small and their lifetimes quite short [19]. Hence the self-absorption of the light emitted by these radicals is not important, and the radiated light is directly related to the reaction rate or equivalently to the heat release rate. Early studies of John and coworkers [20, 21] and Barr~re and Barr~re [18] indicate that radiation from free radicals may be related to the heat release rate.

In a more recent study of sound generation by flames, Hurle et al. [23] establish that C 2 and CH radical emission varies linearly with the volumetric heat release. By chang- ing the airflow rate and the equivalence ratio, they show that the emission intensity is proportional to the volume consumption of the mixture. Another proof of this relation is provided by experiments on the combustion of soap bubbles filled with a mixture of ethylene and air. When a bubble is ignited, its contents burn and a sound wave is radiated. The sound pressure signal at a distance from this source is proportional to the rate of heat release dQ/dt. It is shown by Hurle et al. [23] that this rate may be deduced by simply differentiating the light intensity emitted by free radicals such as CH or C 2. Taking dQ/dt = kdI/dt , it is possible to predict the sound pressure signal, which is remarkably close to the experimental waveform detected by a microphone. Further indications on the relationship between light radiation and heat release rate can be found in many studies of combustion instabilities (see Candel [24] for a review and Poinsot et al. [25], Yu et al. [26], and Yip and Samaniego [27] for conditional imaging of nonsteady combustion).

While a linear relation between the heat release source term and the light emission from free radicals exists in some situations, one can assume that in general a mono- tonic quasi-linear relation prevails. The measurement of the heat release source term relies on this assumption.

Calibration

Distributions of light emission provide qualitative information on the regions where the heat release takes place. To obtain quantitative maps one has to apply a calibration procedure as explained in detail in [28]. It is assumed that the volumetric rate of heat release is proportional to the light intensity:

W = a I . To determine the constant a one has to measure the global power P released in the combustor (the region that is being imaged). This information can be deduced from an enthalpy balance between the inlet and exhaust of the combustor,

The enthalpy and density in the exhaust section can be obtained from gas analysis. The constant a that relates the volumetric heat release to the light intensity is then given by

490 McManus et al.


The general arrangement used in emission imaging is shown in Fig. 3. The emission image may be formed by displacing a photomultiplier on a predetermined grid or by collecting the light emission with a fixed CCD camera. The natural radiation from the flame is integrated over a line of sight of the detector as shown in Fig. 4. As a consequence this type of method is well suited to configurations that are two-dimensional. It is also possible, at least in principle, to use emission measurements in axi- symmetric geometries and deconvolve the data with an Abel transformation.

The spectral bands for emission imaging are the (0, 0) C 2 and (0, 0) CH bands at wavelengths of 516 and 430 nm, respectively. Each wavelength is isolated in the emission spectrum with a narrowband interferential filter (AA = 5 nm). The light emitted by the flame is collected by an f = 100 mm convex lens located at 200 mm from the combustor center plane. The light beam is detected by a photomultiplier through a 2-ram-diameter pinhole to provide a local measurement; the emitting area actually seen by the detector has a diameter of about 2 ram. The detector output is amplified and transmitted to a computer, which also controls the optical displacement in the vertical and horizontal directions (see Fig. 3). The photomultiplier scans a grid comprising 540 points (30 points horizontally and 18 vertically). At each point of the grid, 30,000 data samples are acquired at a sampling frequency of 30 kHz (after a low-pass filtering with a cutoff at 6 kHz to prevent aliasing) and averaged.

This process yields the spatial distribution of the mean light emission from C 2 and CH radicals. Images of free radical emission may also be acquired with a video camera (Pulnix TM 440 including a CCD matrix of 768 x 596

Photomultiplier

Computer I Y

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Figure 3. Typical setup for time-average light emission measurement. The photomultiplier displaced on a predetermined grid and a C C D camera collect the light emitted by radicals such as CH and C 2. A turbulent premixed flame is stabilized in a rectangular duct.

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pixels) equipped with a C-mount lens (20 mm focal length, f /1 .5) and an interference filter at 516 nm (peak transmis- sion 0.55 at 516 nm, bandwidth 8 nm). The camera is located about 2 m from the combustion chamber and oriented perpendicularly to the quartz windows, thus integrating the C 2 emission over the transverse dimension of the combustor. The measurement window comprises 600 x 100 pixels. This very high definition of the image provides a spatial resolution of 0.5 mm. The output video signal is digitized, and the digital images are stored on a Macintosh II. The time exposure of each image is of 1/50 s due to the video standard. By averaging 10 images, effects of turbulent fluctuations are minimized.

Experiments on Turbulent Flames Stabilized in a Channel

Among the many imaging experiments carried out in various geometries, we consider the case of premixed flames stabilized in a channel. Figure 5a provides a direct view of this flame.

The experimental setup used in this study is shown in Fig. 3. A mixture of air and propane is injected through a long duct into a rectangular combustor. The inflow stream has a pressure of 1 bar and a temperature of 300 K. The maximum mass flow rate is 120 g/s . The height, depth, and length of the chamber are respectively 50, 80, and 300 ram. The upper and lower combustor walls are made of ceramic material, and the lateral walls are transparent artificial quartz windows allowing maximum optical access into the combustion zone. The inlet plane comprises a V-gutter flame holder placed at the duct center that produces 50% blockage. Combustion is stabilized by hot gases recirculating behind the V gutter. The outlet is of the same cross-sectional area as the combustion chamber.

Experimental Results: Effects of the Equivalence Ratio

The purpose of this experiment is to obtain information on the local heat release source term distribution. Imag- ing is performed with the photomultiplier and the CCD camera. The light radiated by C 2 and CH is detected.

Experiments are carried out in the two-dimensional combustion chamber shown in Fig. 3 fed with a mixture of air and propane. The mass flow rate of air and the


Figure 5. Turbulent premixed flames stabilized in a duct. (a) Direct view of the flame. (b) CH Radical emission imaging. The maps were obtained by displacing a photomultiplier on a grid. The five images correspond respectively to equivalence ratios of 0.60, 0.65, 0.70, 0.75, 0.80, 0.90. rh a = 75 g/s. (c) Calculated flame structure: ~b = 0.75, rh a = 75 g/s. The maps give the temperature, fuel mass fraction, flame surface density, and fuel consumption rate.

equivalence ratio determine the operational conditions of the combustor. This last quantity, defined as

/'h f / / ' h a

( rh f / /Fha)st '

compares the mass flow rates of fuel and air to stoichio- metric values. The present results correspond to a velocity of 32 m / s at the stabilizer lips (an upstream velocity of 16 m / s ) and to equivalence ratios ranging from 0.60 to 0.90.

The average Reynolds number based on the inlet stabilizer size is about 53,000. Spatial distributions of the light emitted by CH radicals are displayed in Fig. 5b. These maps correspond to a fixed airflow rate (75 g / s ) but to different values of the equivalence ratio (0.60 < ~b < 0.90). Measurements of the light emission of C 2 radicals yield similar patterns, confirming that C 2 and CH may be used as tracers of the local heat release in the flame. If one now examines the evolution of the mean flame structure with the equivalence ratio, one observes that at low values

of ~b concentrated regions of reaction are in the vicinity of the flame holder. For ~b = 0.60 these regions form a merged reaction zone. When the equivalence ratio increases, two flames develop from the lips of the flame holder. These flames become more and more separated as f increases. For the highest equivalence ratios, the light emission reaches a local maximum in the vicinity of the two side walls.

This peculiar phenomenon is observed because the flame touches the boundaries, which are in the present case nearly adiabatic. In the interaction region, the flow in the boundary layer is decelerated and its velocity is less than that existing in the main flow, and the flame angle increases (this angle is measured with respect to the axial direction). The wall temperature takes large values, and the wall region acts like a secondary stabilization zone for the incoming stream of fresh mixture. Combustion is activated, and the light emission is enhanced. For ~b = 0.90 the two flames are separated by a region of reduced reaction. In each flame region the radical light emission

492 McManus et al.

increases progressively and reaches its maximum in the vicinity of the combustor walls. These data clearly show that the equivalence ratio significantly affects the flame structure. The modifications associated with this parame- ter are not extensively documented in the previous literature (see Libby et al. [29] for a review of available experimental data on premixed turbulent flames).

The previous data may be compared with results of calculations. The mean source terms describing the consumption of reactants and the heat release are obtained from a flamelet model based on a balance equation for the flame surface density (the coherent flame model). Detailed comparisons have been carried out in several configurations and for a range of operating conditions [3, 4, 28, 30]. A typical result of numerical modeling is shown in Fig. 5c. The computed fields of temperature, fuel mass fraction, flame surface density, and mean reaction rate are displayed in this figure. Comparison of the computed heat release rate field (the last map in Fig. 5c) with the experimental distribution of light emission (the fourth image in Fig. 5b) shows that the model is able to reproduce the general pattern of combustion inside the test section.

The presence of two main combustion zones is fairly well predicted, especially regarding the locations of the heat release rate maxima. A more quantitative comparison can be carried out with the calibrated maps of heat release (Figs 6a and b). One has to remember that the mean flow is nearly two-dimensional. This allows a direct comparison of the mean distributions of heat release determined experimentally and calculated numerically. These distributions are quite similar. It is also found that the model predicts a maximum heat release rate of 2.2 × 108 W / m 3 while the experimental value is 2.5 × 10 s W / m 3. This may be considered an excellent result. One has to recall that the comparison is carried out directly on the modeled mean source terms appearing in the time- averaged equations governing the flow.

Figure 7 gives a second example of emission imaging. The data obtained with the CCD camera corresponds to flames stabilized on a small cylinder (diameter = 3 mm). In this case two oblique flames spread in the flow. Here again, the heat release pattern is strongly influenced by the airflow rate and the equivalence ratio. Effects associ-

ated with a change in the flow rate are illustrated in this figure (the equivalence ratio is fixed and equal to 0.90).

TIME-RESOLVED IMAGING OF FREE RADICAL LIGHT EMISSION

We have shown in the previous section that time-average images of the light emission from radicals existing in flames could be interpreted in terms of the mean heat release in the flow. Instantaneous images of the light emitted by the same radicals are also valuable in studies of dynamical phenomena such as ignition, combustion instability, and combustion under forced oscillations. Syn- thetic visualization can be obtained by performing a large ensemble of point measurements as shown, for example, by Poinsot et al. [25], but the method is time-consuming and can be used only when the phenomenon is periodic and stable.

Advances in imaging technology now allow direct acquisition of the light emitted from a large number of points in the flame zone [27, 31, 32]. Each element of the detector gathers the light emitted from a small flame volume through a set of lenses (Fig. 8). The sensitive surface of the CCD camera is cooled to diminish the noise. An intensifier amplifies the signal level and provides means for controlling the time duration of exposure. In light emission experiments, the energy radiated by the free radicals is quite small but the intensifier allows time exposures of the order of 50 #zs.

Time-resolved emission imaging is now illustrated with an experiment on the ignition of a multiple-injector nonpremixed combustor. The objective of this study was to gather basic information on the dynamics of ignition in a geometry having some similarity with a rocket engine backplane. An interpretation of the results is given else- where [33]. The same combustor was also used for PLIF imaging, and some results are provided in the next section for comparison.

Experimental Configuration

The test combustor was designed to loosely represent the geometry of a cryogenic rocket backplane. In comparison with real motors our model features only three injectors

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Figure 6. (a) Calibrated map of mean volumetric heat release rate. The map is deduced from the CH emission image. (b) Calculated volumetric heat release rate. & = 0.75, rh~ = 75 g/s.

Figure 7. CH radical emission imaging of oblique flames stabilized on a small cylindrical obstacle. The equivalence ratio is fixed 4, = 0.90, and the mass flow rate of air takes values of 35, 50, 75, and 100 g/s. Images were obtained with the CCD camera.

and is fed with air and hydrogen at atmospheric pressure. In addition, the geometry is two-dimensional to allow complete optical access (Figs. 9a and b). The chamber is rectangular (30 × 10 × 5 cm) and features quartz side walls for optical access. The upper and lower walls are made of ceramic material and equipped with rectangular quartz windows (10 x 2 cm) that transmit the laser sheet into the chamber. Hydrogen is fed to the device on the upstream end through three injection blocks (Fig. 9b). The blockage produced is 40% of the total inlet section. Each block has two (0.3 mm x 5 cm) side slits through which the hydrogen is injected. The slits are slightly re- cessed with respect to the backward-facing side of each block. Air supplied by a blower is fed into the chamber between the injectors after passing through a long settling duct.

Igniter Characteristics

The igniter source is a commercially available spark ignition unit designed for use in gas turbine applications. It is mounted on the top wall of the combustor at a distance of 2 cm downstream of the injection plane. The initial discharge takes place in a region adjacent to one of the inlet airstreams (in the nonpremixed case) and not in a region

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where premixed gases exist. The nominal discharge energy was 0.5 J, and the spark duration was on the order of 275 p,s. The discharge sequence may be described qualitatively as consisting of an initial breakdown phase of very short duration (order of 10 ns), a transitional period called the arc phase, and then a glow discharge phase lasting much longer.

494 McManus et al.

Imaging Method and Instrumentation

The imaging setup is shown in Fig. 9c. The detector is an intensified CCD camera placed on one side of the combustor at right angles to the axis of the device. The CCD camera is equipped with a gateable light intensifier that controls the time of acquisition and exposure duration of the CCD matrix and amplifies the incident light intensity. A photomultiplier located on one side of the combustor monitors the total light radiated by the free OH radicals. The signal of this detector will be shown together with the emission images.

Intensified CCD Camera System The imaging detector used in this work is an intensified liquid-cooled CCD array camera system (Photometrics Model CC200). The system consists of several elements including the image intensifier (and controller), CH200 liquid-cooled camera head (LC200 cooling unit), CE200 camera electronics unit, CC200 camera controller, and a Mac II microcomputer acting as the host computer (Fig. 9c). All of the system elements are coupled via standard interface connections, and the system performs in a turnkey fashion with the exception of the image intensifier.

The CH220 camera head comprises a liquid-cooled Thomson 7882 CCD array (576 × 384 pixels). The liquid circulation unit (LC200) maintains the chip surface at a temperature of - 25°C.

The image intensifier (PCO Model XX1410/SP41021- 160) is mounted to the front of the CCD array via a fiber-optic bundle. The gain of the intensifier can be adjusted by varying the voltage across the microchannel plate (MCP). Voltage levels of 600-1000 V correspond to luminous gain factors between 7500 and 15,000. Fast gating of the intensifier is performed by means of the voltage between the photocathode and the MCP and is also controlled by the power supply.

Imaging Lenses The camera head is equipped with a Nikon lens mount and can accommodate any camera lens with this type of bayonet mount. For emission and PLIF experiments of OH, it is necessary to use lenses that give a high transmissivity in the UV (near 305 nm), and a UV Nikkor lens (105 mm, f /4 .5) is available for this purpose.

Triggering Electronics Three timing circuits provide several possibilities for camera-intensifier synchronization. The three circuits allow single-exposure synchronization with manual triggering or external triggering and multiple-exposure synchronization with external triggering. With these circuits it is easy to make single-shot as well as multiple-shot phase-locked images when a periodic synchronization signal is available.

CE200 Camera Electronics Unit The CE200 camera electronics unit attached directly to the camera head supplies all the required signals and performs all signal acquisition necessary to operate the Thomson CCD array. The charge-to-digital signal conversion is also performed by this unit. The digitization is over 14 bits, and there is a software-configurable gain control to adjust the dynamic range for the particular light level of the application.

CC200 Camera Controller The CC200 camera controller comprises a 68000 microprocessor. The controller

card provides the interface between the CC200 system and the CE200 camera electronics unit. Timing and control sequences are generated by the CC200 controller and are sent to the CE200 to control camera functions. The camera controller card also provides for DMA transfer of image data from the CE200 into CC200 image memory.

Results

We now describe results obtained for one set of operating conditions from among the many imaging experiments. The air and hydrogen flow velocities are respectively Uai r = 18 m / s and UH2 = 25 m/s . Corresponding to these conditions the global equivalence ratio is 4~ = 0.100. For this operating point, a large number of images were acquired during a time interval of approximately 20 ms following the igniter discharge. The acquisition time was varied in order to gather images representing the different phases of the ignition sequence leading to the steady-state combustion.

Three images were chosen from this ensemble to give an idea of the type of data that can be obtained with the method (Fig. 10). In each part of this figure the flow is from right to left and the injection blocks appear as thick red lines on the left. The global signal of OH emission detected by the side-looking photomultiplier is given under each image. The instant corresponding to the image acquisition time is marked as a red dot. In the first image corresponding to t = 3.8 ms after the discharge (Fig. 10a), the first injector has been ignited and a large region of intense radiation is visible in the immediate neighborhood of the second block. The second image (t = 6.6 ms) indicates that two injectors are ignited and that a ball of premixed combustion has formed downstream from the third injector (Fig. 10b). At the third instant in time (t = 8.1 ms), the flame has propagated to the third injector, which is in turn ignited (Fig. 10c). An examination of the complete set of images indicates that flame kernels initially appear in the wakes of the injection blocks where the reactants have formed a partially mixed stream. A flame front then propagates upstream and initiates the reaction in the vicinity of the injection blocks. The premixed gases burn out, and the hot products are convected downstream. A regime of nonpremixed combustion is then established in the chamber.

Perspectives

Time-resolved emission imaging has the advantage of sim- plicity and can be applied to many practical flames. The method provides a signal integrated over the line of sight and is therefore mainly useful in two-dimensional situations. The fraction of OH radicals that are in an electroni- cally excited state is small, and consequently the photon flux is relatively weak. This induces a limitation on the time resolution of the method. The exposure could not be diminished below 50 p~s in the experiment just described. One drawback of the method comes from the self-absorption of radiation. The intensity of the signal is modified by this effect, and its amplitude is not easily interpreted in quantitative terms. The method remains essentially qualitative but provides valuable information


Figure 10. Time-resolved OH emission images corresponding to three instants in the ignition sequence of the nonpremixed combustor. Uai r = 18 m/s, UH2 = 25 m/s, ~b = 0.100. (a) t = 3.8 ms: (b) t = 6.6 ms; (c) t = 8.1 ms.

on combustion dynamics and is specifically useful in studies of unstable and transient reactive flows.

LASER-INDUCED FLUORESCENCE IMAGING

The use of species-specific imaging in combustion provides important advantages over standard line-of-sight techniques. One of these advantages resides in the ability to identify the spatial structure of reaction zones by imaging chemical species that are known to exist in appreciable quantities in these regions only. Laser-induced fluorescence imaging has been extensively exploited in combustion research as an imaging technique that provides this feature. Although Rayleigh and Raman techniques allow species-specific measurements as well, they are less practical due to the characteristically weak signals produced.

Laser-induced fluorescence is a well-established technique for detecting the population densities of molecular or atomic species in specific quantum states. In combustion applications this information can be used to determine relevant quantities such as mole fractions, density, temperature, and velocity [6, 34-40]. The technique was initially developed for point measurements; however, the principles can be applied directly to multipoint measurements for fluorescence imaging [5, 41-46].

The basic arrangement for PLIF imaging is shown in Fig. 11. A laser sheet traverses the flame zone. The light is absorbed in part by certain species and reemitted as fluorescence. This radiation is collected by an intensified camera. The image can then be interpreted to yield a distribution of flow properties of interest. In what follows we consider only the measurement of species concentrations.

496 McManus et al.

z - ~ CCD Ll~se [ ~ Camera ~ ]

~ [I ~ Interferential filter

Flame J ~ "

~ Cylindrical lens Figure 11. Principle of laser-induced fluorescence imaging.

Theory of Laser-Induced Fluorescence

It is worth giving some theoretical elements in order to understand the type of signal provided by LIF. Initial theoretical work on fluorescence was carried out by Piepmeier [47] to describe the molecular dynamics of fluorescence experiments. The objective was to make quantitative measurements of atomic species seeded into analyzer flames. This was achieved with a rate equation analysis of an ideal two-level system by assuming that the populations of these levels reach a steady state.

Some fluorescence models are founded on a completely classical theory [48]. The quantum-mechanically correct theory for ensembles of two-level systems (the density matrix approach) is contained in standard texts (see, e.g., [49]). The validity of the rate equation devised by Piep- meier was examined by Daily [50] with a direct derivation from the density matrix formalism. For pulsed multimode lasers, the rate equations are valid as long as the pulse rise time is slow compared to the collisional dephasing time of the coupled system. In flames, this time is on the order of 10 10 s, whereas the pulse rise time is approximately 10 9 s.

In the following section, we give a brief account of the rate equation analysis.

Rate Equation Analysis of Laser-Induced Fluorescence

Theories of laser-induced fluorescence describe the intensity of spontaneous light emission from an ensemble of molecules that have been excited to an upper electronic energy level through the absorption of monochromatic light radiation. Most theoretical developments begin with the consideration of two or more quantum states. Balance equations for the molecular populations in each state yield general expressions for the laser-induced fluorescence signal. The classical model for a two-level system may be found in [51]. More elaborate multilevel models have been developed for various limiting cases, and exam-

pies of these are given in [52]. In this section we discuss the two-level model in which only the laser-coupled states are included. Much of this discussion is based on the work of Allen [46], and the reader is directed to this reference for a more detailed explanation. In addition, Eckbreth [35] has provided an excellent text describing various aspects of this model as well as general considerations in laser- based combustion diagnostics.

In the two-level model of fluorescence, one considers only two molecular quantum states that are directly populated or depopulated through interaction with the laser light. Transfer of energy resulting in the population of neighboring quantum states is neglected. The energy transitions and the transfer mechanisms that are considered in this model are summarized in Fig. 12. Each mechanism is represented by a rate (s -1) and a direction. The rates of stimulated emission and absorption of photons resulting from laser interaction are designated by IvB2t and IvB12, respectively, where I v is the laser spectral intensity [J/(cm 2 s Hz)] and B21 and BI2 are the Einstein B coefficients for the transition (cm 2 Hz/J) . Spontaneous light emission from the upper energy level is described by the Einstein A coefficient A21 (s-t), and the collisional quenching rate from the upper level to the lower level is denoted by the term Q21 ( s - l ) • The laser spectral bandwidth is assumed to be larger than the molecular absorption linewidth so that there is a complete overlap, rendering the details of the absorption lineshape irrelevant. In our application of laser-induced fluorescence, the temporal dynamics of the excitation process are not resolved, and we use average intensities,

IvT = £~Iv( t ) dt, (1)

where ~- is the laser pulse duration. In typical flame environments, the upper-state lifetime is on the order of 10 -9 S whereas the laser pulse duration is about 10 -s s, and in this case one can also use average population densities,

N2T = £~N2(t) dt. (2)

Stimulated emission from molecules transitioning from the upper level to the lower level possesses the same momentum and phase as the incident laser radiation. Spontaneous emission, however, has random momentum and phase and is emitted into 47r steradians. It is a

Upper Level

/vB12 lvB 21 Q21

\

Lower Level

A 21

Figure 12. Schematic diagram of two-level model of induced fluorescence.


portion of this radiation that is collected and constitutes the fluorescence signal. The fluorescence signal can be described by the equation

f~ Sf = "rT I ~ N2A21V , (3)

where ~ is the efficiency of the collection optics, which collect photons through a solid angle 1); N 2 represents the number density of molecules in the upper state due to laser excitation; and V is the collection volume imaged onto one detector pixel. The collection volume is defined by the thickness of the laser sheet multiplied by the area of the sheet imaged onto a single pixel.

The rate equation describing the population in the upper state may be written as

d N 2 dt = N l l vBI2 - N2(IvB21 + Q21 + A21)" (4)

The first term on the right represents the rate of population transfer from the lower state to the upper state due to stimulated absorption, and the remaining terms represent depopulating mechanisms: stimulated emission, collisional quenching, and spontaneous emission, respectively. In situations where the duration of the laser pulse is long compared to the collisional quenching time, it may be assumed that the system reaches a steady state; thus Eq. (4) becomes

NllvB12 = NZ(IvB21 + Q21 +A21). (5)

In flames where the temperature seldom exceeds 3000 K, the upper energy level state is initially empty. The steady- state populations then satisfy the constraint

N 1 + N 2 = N °, (6)

where N ° is the initial population in the lower state. Substituting Eq. (6) into Eq. (5) and introducing the result into Eq. (3) leads to

~ IvBIzA21

Sf = TT/-~ VN?Iv(B12 + B21) + Q21 + A21 (7)

The initial population of the lower level is related to the total number density N t of the species being probed by the Boltzmann fraction fB, so that Eq. (7) becomes

~Z IvBl2A21

Sf = "rr I - ~ V f B N t iv(Bi2 + B21) + Q21 + A21 (8)

This is the basic fluorescence equation, which may be used to relate the measured fluorescence signal to the total number density N t. A particularly simple form is obtained for weak excitation [i.e., when Iv(B~2 + B21) << Q21 + A21]-In this limit,

l'~ IvB12A21 Sf = rrl ~ VfBNt Q2I + A21 " (9)

Since the quenching is much larger than the spontaneous emission probability (Q21 >> A21),

A21 Sf = ~7 - ~ Vf B - - B 1 2 E v N t , (10)

Q21

where r I v is replaced by E v, the laser spectral fluence [J(cm 2 Hz)].

Using published data for OH (see, e.g., [53, 54]), one finds that the saturation intensity for the excitation// detection scheme described here is on the order of 10 -3 (J/cm2). Typical spectral intensities for the experiments reported herein were on the order of 10 -5 ( J / cm 2) (i.e., 10-mJ laser pulse with At; = 20 GHz and r = 10 -s s focused into a 10 cm × 500 /~m sheet), thus justifying the use of the weak excitation limit.

Multilevel Models

The two-level model as presented above is quite appealing because it is simple; however, it does not account for many physical processes that are potentially important in laser-induced fluorescence measurements. In particular, the model neglects the presence of other molecular energy levels that may play a role in the energy transfer processes. This aspect is described in refined theories that take into account energy transfer between the level being directly populated by the laser excitation and nearby rotational and vibrational energy levels. Under certain experimental conditions these additional levels must be included, especially when the laser intensity is great and the weak excitation limit is no longer valid.

Nonresonant Fluorescence

In typical combustion experiments, sources of elastic scattering may deflect the laser radiation toward the collection optics, resulting in signal contamination. This parasitic phenomenon may be caused by scattering from fuel droplets in two-phase combustion experiments as well as from scattering from solid surfaces as the beam traverses the measurement region. The distinguishing feature between the light elastically scattered and that resulting from the induced fluorescence is their spectral nature. Elastic scattering occurs only at .the laser wavelength, whereas fluorescence occurs at the wavelengths of all allowed transitions from the upper laser populated level as well as from nearby levels subsequently populated through collisional energy transfer. A spectral separation may exist between the laser source and the resulting fluorescence signal, allowing the possibility of spectrally filtering the detected signal in order to separate parasitic signals resulting from elastic scattering of the laser light. This procedure is known as nonresonant fluorescence. This mode of operation is implemented in the experiment described below.

Exci ta t ion/Detec t ion Strategy for OH

The A-X system of OH possesses many strong transitions in the wavelength region near 300 rim, easily accessed by frequency-doubled tunable dye lasers. Excitation or detection from off-diagonal transitions of this system (i.e., where Av =g 0) allow wide wavelength separation between the elastically scattered laser radiation and the fluorescence signal. Figure 13 shows a schematic diagram of the energy levels for the ground and first excited states of OH with an off-resonant fluorescence scheme indicated with ar-

498 McManus et al.

v '=2 A ~ R~

v ' = O

IB 01

= 283

( 2/7

V " = 0

R(j) = Rotational energy transfer V(j) = Vibrational energy transfer

A,B = Einstein coefficients for spontaneous emission and absorption

Q = Quenching

Figure 13. Multilevel energy diagram for off-resonant OH fluorescence. R(j) = Rotational energy transfer; V~s ) = vibrational energy transfer; A, B = Einstein coefficients for spontaneous emission and absorption; Q = quenching.

rows. In evaluating potential PLIF modes of operation, it is useful to define a figure of merit from Eq. (11),

F = "rlA21B12 E, (12)

where r / is the quantum efficiency of the intensifier photocathode at the fluorescence wavelength, E is the laser pulse energy in joules, and the remaining coefficients are as defined earlier.

Excitat ion/detect ion strategies that maximize this figure of merit will result in the highest fluorescence signals on the photodetector. This aspect is investigated by Allen [46]. It is found that an excitation/detection scheme utiliz- ing the (1,0)/(1, 1) vibrational bands has a figure of merit that is 50 times greater than that of a scheme based on (0, 0)/(0, 1) bands. The analysis is based on the theoretical response curve of a Type S-20 extended UV photocathode and on data from published sources (see [53, 54]). The ( 0 , 0 ) / ( 0 , 0 ) scheme is not considered because the fluorescence signal might be confused with scattered laser light.

The calculations indicate that the (1, 0)/(1, 1) scheme is more desirable from a signal level standpoint. This scheme is adopted here. The excitation wavelength is near 280 rim, and the broadband detection is near 315 rim. [How- ever, with the filtering scheme used, some fluorescence resulting from the (0, 0) is detected as well.]

It is worth noting that the quoted figure of merit is not the only criterion and that other aspects such as saturation should also be taken into account.

Choice of Pumping Wavelength for Species Measurements

The fluorescence signal as given by expression (11) de- pends on the temperature and on the number density of OH radicals. One may try to minimize the dependence of the signal with respect to the temperature by selecting a

particular pumping wavelength. To understand this point, consider again expression (11). The fluorescence signal is proportional to the Boltzmann function describing the population distribution fB among the available energy levels, and it is inversely proportional to the quenching factor Q. One may write

A Sf o~ ~Felec(O)Fvib(C'" , T ) F m t ( J " , T)Nt,

where the F factors are given by Boltzmann distributions describing the fraction of OH radicals that exist in the state being pumped. The rate of nonradiative transitions (the quenching factor) Q can be approximated with a kinetic model in terms of a relative velocity v of the molecules in the gas with respect to the OH species and a collision cross section Crq. According to this model the quenching rate can be expressed as

p o : ( 3 k T l 1/2 1 O ~ n t ° t ° ' q U = U q ~ - m ] ~ T ]/2"

The quenching is proportional to T i/2 Using this result and substituting in the fluorescence

signal expression one finds that

S f (3{ Xtfvib(/'" , T)Frot(J" , T ) T I/?.

It is then possible to choose the rotational level J" to be pumped [43] to minimize the dependence with respect to temperature. This can be achieved by minimizing the rate of change of the function

d [ F r o t ( J " , T ) / T 1/2 ]

d T

The function appearing in the brackets is plotted in Fig. 14. It is observed that if one excites a transition from the J" = 6 level, in a range of temperatures 1500 < T < 3500 K, the fluorescence signal will only weakly depend on the temperature, and as a consequence it will be essentially proportional to the concentration of OH radicals.

The c " = 0 vibrational level is chosen because this state is the most populated according to the Boltzmann distribution and the fluorescence signal will be maximum. It is then adequate to excite the Q](6) transition of the (1,0) vibrational band (at 282.9 nm) and to detect the fluorescence in the (1, 1) band around ~ 315 nm, for the reasons given above. In principle, this will give a signal

LIF signal S(J,T) 0 . 2 . . . . , . . . . , . . . . , -

0.15

0.1

0.05

, - • • , • • i •

J - - 4 J = 6

5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0 3 5 0 0

Figure 14. Variation of the fluorescence signal with temperature.

that will be essentially proportional to the concentration of OH radicals (with a variation of about 8% caused by the weak temperature dependence of the signal). To obtain absolute values of the concentration it is necessary to calibrate the method with a known source of OH radicals, but this has not been done in the present experiment. For many purposes, the relative concentration of OH is in fact sufficient.

Experimental Calibration of the Wavelength Tuning of the Laser Pumping System

To excite the chosen Q1(6) transition of the (1, 0) vibrational band of OH, it is important to calibrate the dye laser tuning system. For this calibration we computed excitation spectra for the OH radical by assuming that the temperature has a fixed value of 2000 K and by making use of published spectral data for this species. Experi- ments were then conducted on a small propane-ai r flame that provided a convenient source of OH radicals. The laser beam traverses the flame, and the response to excitation is measured by a photomultiplier that detects the fluorescence in the (0, 0) vibrational band and by a microphone that monitors the photoacoustic response of the flame. The comparison between the computed and measured spectra is quite good, allowing (Fig. 15) a fine tuning of the laser on the selected Q1(6) transition of the (1,0) vibrational band.


Many applications of PLIF to combustion problems are reported in the recent literature. The method has been used to study the structure of reacting shear layers (e.g., [55]), to analyze turbulent diffusion flames [38, 39], or to examine the interaction between a laminar flame and a vortex pair [56]. It is illustrated in this section with the problem of ignition in the nonpremixed combustor already described in the section on time-resolved imaging of free radical light emission. This will allow a direct comparison between emission and PLIF imaging. The detection setup is the same as in the time-resolved emission imaging. The general arrangement for PLIF imaging is shown in Fig. 16.

The imaging system components were described previ- ously. A N d : YAG laser amplified and frequency-doubled ( = 340 mJ at 532 nm) pumps a tunable dye laser system (--- 105 mJ at 566 nm). The beam passes through a crystal doubler ( = 8 mJ at 283 nm) and through a system of cylindrical lenses to form a thin sheet of light (10 cm × 0.3 mm), which is then used to excite specific energy transitions of the OH radical.

The laser setup shown in Fig. 17 excites the A2E -~ X2H system of OH. The dye-laser system can be tuned to access atomic or molecular transitions in spectral regions other than that used in the present work; thus fluorescence of other chemical species is possible following relatively simple configuration changes. The unique feature of the present setup is the triggering system, which allows synchronization of the image acquisition with the igniter discharge. The timing electronics are piloted by a signal from the Nd : YAG laser that indicates the precise time of each laser pulse. A special delay circuit was used to synchronize the igniter discharge and subsequent image

0.030

Emission and LIF Imaging Methods

Computed OH absorption at 2000K

499

0.025

0.020

0.015

0.010

0.005

0.0 281

a

l, Ill t,, 281.5 282 282.5

Wavelength (nm)

il 283 283.5

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

b

Fluorescence signal in a pilot f lame experiment

281 281.5 282 I J, 1

282.5 283

Wavelength (nm)

83.5

Photoacoustic response of a pilot f lame excited by laser radiation 25

2O

15

I0

5.0

0.0 281

C

Maximum Pressure (Pa)

[Ihth lit 281.5 282

.,,,.t l, J 282.5 283 283.5

Wavelength (rim)

Figure 15. Computed and measured spectra. (a) Computed OH absorption at a temperature of 2000 K. (b) Measured fluorescence signal in a pilot flame experiment. (c) Photo- acoustic response of a pilot flame excited by laser radiation.

acquisition so the user could control the delay time between these two events. Due to experimental jitter in the ignition system discharge circuitry, there was an uncer- tainty in the delay time adjustment of approximately _+ 1 ms. Only one image could be acquired during a single ignition event. This limitation made it necessary to repeat the experiment several times to build up a time history representation of the entire transient ignition phase for a given operating condition. Further difficulties are encoun- tered due to test-to-test experimental jitter that results from spatial variations in the mixing properties of the flow and from shot-to-shot variations in the discharge characteristics.

Typical Results

We show only three images selected from a large collection of images. The operating conditions differ from those used in the emission studies reported in the previous section: Uai r ~ 14.5 m/s , U H = 29.0 m/s , ~b = 0.141. Ad-

• . 2

ditional results and discussions can be found in [33]. In Fig. 18 the flow is from left to right and the geometry of the three injectors is displayed. After a short delay, the spark initiates a flame kernel in the premixed gas region

CCD Camera Image Intensifier

CCD camera 307 nm ~

Hydrogen injection [ I

Air ~ ~

~Nd:YAG ~ D y e laser ~ paser V[

532 nm ~566 nm

UV lens (105mm, f/4.5)

Camera Controller (CC200) ~ ' ~ Igniter

C2

C

~Xq]

Nd-YAG (10 pps) Dye Laser f×2

b

Figure 16. Experimental configuration. (a) Combustion system, laser and imaging arrangement. (A detailed view of the combustor is shown in Fig. 9.) (b) General arrangement for PLIF imaging of the ignition sequence of the nonpremixed combustor.

that forms as a result of fuel and air mixing in the recirculation zone established behind the injector block. Rapid combustion of this region takes place and reaches the initial shear layer formed by the streams of H 2 and air. Diffusion flames are initiated. The volumetric expansion associated with the combusting mixture in the wake of the first injector transports hot products into the wake

I PDL-3 Dye Laser

500 McManus et al.

Laser output beam Prism

I

I I Figure 17. Schematic diagram of Nd : YAG pumped dye laser system.

Figure 18. OH PLIF images acquired during ignition. Oper- ating conditions: Udi r = 14.5 m/s, UH2 = 29.0 m/s, ~b = 0.141. (a) t = 4.3 ms; (b) t 13.5 ms; (c) t = 18 ms.

region of the second injector, and this starts a subsequent ignition phase. Ignition of a pocket takes place in the vicinity of the injector exhaust plane (Fig. 18a). Premixed reaction fronts progress in this region and establish a pair of diffusion flames near the second injector in the Hz/a i r


confluence layers. The products of premixed combustion are transported downstream and simultaneously ignite the premixed pocket formed behind the third injector (Fig. 18b). Because the third injector is close to the lower wall, the volumetric expansion produces an upward motion of the hot products, which perturbs the flow. After a certain while a steady state is reached (Fig. 18c). In fact, the final regime of combustion is periodic due to a low-frequency instability. Images like Fig. 18c show alternating patterns of intense combustion and local extinction.

Comparison and Perspectives

A comparison with the time-resolved emission images indicates that the same general processes are observed with the two imaging techniques. However, PLIF images have better temporal resolution because the time exposure is much shorter (of the order of the laser pulse duration, 10 ns, instead of 50 /~s). The signal-to-noise ratio is also enhanced in the case of PLIF as it is controlled by the laser intensity. The spatial resolution is improved in PLIF images because the fluorescence signal originates from a slice whereas emission images integrate the natural radiation along a line of sight.

Clearly, PLIF imaging constitutes a remarkable qualitative tool for combustion analysis. The method has excellent time resolution, and its spatial resolution is good and will continue to improve in the future with the application of new detectors comprising more than a million pixels.

CONCLUSIONS

Considerable progress has been accomplished in recent years in the application of imaging to combustion problems. These advances have resulted from the development of laser sources and from remarkable improvements in solid-state detector technology. It is now possible to image flame front motion in real turbulent flames and to obtain time-resolved maps of species of interest to combustion modeling. The images can be used to understand fundamental phenomena and to evaluate quantities of basic interest such as the flame surface density or the mean reaction rate. Further work will be necessary to refine the current methods and make them more quantitative, to apply them to problems of increased complexity, and to devise advanced image processing schemes.

We thank all those who contributed to the work presented in this article. Particular thanks are given to Denis Veynante, Franqois Lacas, Carlos Rolon, Eric Maistret, Can Huynh, and FrEderic Aguerre. Also, great thanks are due to Jean Chambon and Gilbert Lucas for their technical support. Various parts of this work have been supported by SNECMA, DRET, SEP, and CNES.

NOMENCLATURE

,4 Einstein transition probability for stimulated emission, s t

B Einstein coefficient for stimulated emission, cm 2 H z / J

E v laser spectral fluence, J / ( c m 2 Hz) f focal length, m

fB Boltzmann distribution function, dimensionless

F Boltzmann distribution factor, figure of merit I v laser spectral intensity, J / ( c m 2 Hz) J rotational quantum number, dimensionless

rh mass flow rate, kg / s N population number density, m -3

1 Q quenching rate, s S number of photons collected, dimensionless T temperature, K U flow velocity, m / s v vibrational quantum number, dimensionless V collection volume per detector pixel, m 3

Greek letters • / collection optics efficiency, dimensionless A wavelength, m tr collisional cross section, m 2 ~- pulse duration, s

4' equivalence ratio, dimensionless collection solid angle, rad

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Received January 4, 1994; revised August 22, 1994

emission and laser-induced fluorescence imaging methods in experimental combustion

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