development of a picosecond lidar system for large-scale combustion diagnostics

Development of a picosecond lidar system forlarge-scale combustion diagnostics

Billy Kaldvee,* Andreas Ehn, Joakim Bood, and Marcus AldénLund University, Division of Combustion Physics, Box 118, 221 00 Lund, Sweden

*Corresponding author: [email protected]

Received 15 July 2008; revised 28 October 2008; accepted 9 November 2008;posted 10 November 2008 (Doc. ID 98840); published 5 December 2008

In the present work, a picosecond lidar system aiming at single-ended combustion diagnostics in full-scale combustion devices with limited optical access, such as power plants, is described. The highest over-all range resolution of the systemwas found to be<0:5 cm. A demonstration has been made in a nonsootyand sooty Bunsen burner flame. A well-characterized ethylene flame on a McKenna burner was evalu-ated for different equivalence ratios using Rayleigh thermometry. The results indicate both that pico-second lidar might be applicable for single-shot Rayleigh thermometry, even two-dimensional, andthat there is a possibility to qualitatively map soot occurrence. Furthermore, differential absorption lidarhas been investigated in acetone vapor jets for fuel visualization purposes. © 2008 Optical Society ofAmerica

OCIS codes: 120.1740, 280.3640, 280.1910, 280.6780.

1. Introduction

In combustion research, most laser diagnostic mea-surements have been limited to small and moder-ately sized combustion devices, such as laboratoryflames, internal combustion engines, and gas tur-bines [1]. Characterization of the burning conditionsinside full-scale industrial combustors such as boi-lers in power plants is important in order to maxi-mize the efficiency and minimize emissions ofpollutants. These large boilers, however, representreally challenging measurement conditions, primar-ily because of their large size and limited optical ac-cess. Nevertheless, laser diagnostics using tunablediode lasers (TDLs) have been demonstrated in full-scale power plants for temperature and speciesdetection [2,3]. Since TDL sensors usually are line-of-sight integration devices, they normally do notprovide spatially resolved information.Lidar is a well-established method for remote de-

tection and ranging of objects and gaseous plumes.The fundamental principles of remote sensing werethoroughly described by, e.g., Measures [4]. Lidar has

been used extensively in atmospheric research for avariety of measurements, such as species detection,wind speed measurements, and temperature mea-surements. Theory, practice, and analysis methodsof elastic lidar are described in detail by Kovalevand Eichinger [5], while a comprehensive survey oflidar research, including numerous examples of ap-plications, is given in a review article by Argall andSica [6]. Geiser et al. [7] demonstrated a subnanose-cond laser source for lidar measurement of methane.Lidar has so far, to the best of our knowledge, notbeen used for diagnostics in combustion devices.Cavalcabo et al. [8] described a lidar instrument con-structed for species concentration measurements,with a range resolution of 40 cm, inside power plants.The sensitivity, specified at 1500 K, is 10ppm for NOand SO2 and 1000 ppm for CO detection. However, nomeasurement results have been reported. Picose-cond lidar has also been used by Gai et al. [9] forsimulation of lidar signals from clouds in laboratoryexperiments.

The current work focuses on development of anear-field lidar system, aiming at applications inlarge scale combustion devices, such as powerplants and industrial burners, and fire research.

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1 February 2009 / Vol. 48, No. 4 / APPLIED OPTICS B65

Traditionally, laser pulses with duration of the orderof 10ns have been used in atmospheric lidar mea-surements. Such pulse durations result in a range re-solution of the order of 1m. Given that the typicalsize of power plants is of the order of 10m, obviously10ns laser pulses do not provide satisfactory resolu-tion. In order to improve the range resolution we uselaser pulses of 30ps duration. Ideally, such pulseswould lead to a range resolution of 4:5mm. The lidartechnique uses backscattered light, which makes itpossible to use only one optical access for both trans-mitted and received light. This is desirable for situa-tions with limited optical and physical access, whichis typical in full-scale power plants or gas turbines.In order to utilize the high temporal resolution

provided by picosecond laser pulses, fast detection isrequired. In the present work a streak camera wasused. The overall time resolution of the system,which determines the range resolution along the op-tical axes, has been characterized for all availablestreak rates of the streak camera.Since one potential application of the method is to

locate occurrences of soot inside a full-scale powerplant, picosecond lidar (ps-lidar) measurements wereperformed with a Bunsen burner producing a non-sooty and a sooty flame, respectively, positioned 3maway from the receiver. Other applications usingthe directly scattered Rayleigh signals are, e.g., tolocate the presence of soot in fire applications or tolocate aerosols used for reducing pollutants in largescale power plants, e.g., ammonia to reduce nitricoxides (NOx).To evaluate the potential for temperature mea-

surements, Rayleigh scattering thermometry wasconducted in a well-controlled ethylene/air flameon a McKenna burner. The temperature was evalu-ated for different equivalence ratios and comparedwith coherent anti-Stokes Raman spectroscopy(CARS) measurements by Vestin et al. [10].As acetone might be used as a fuel marker in forth-

coming fuel visualization applications, experimentshave also been carried out in gaseous acetone jets.Since acetone is strongly absorbing light at 266nm,it was also selected in order to facilitate an investi-gation of the differential absorption lidar (DIAL)technique for quantitative species-specific detection

in fuel visualization applications. The DIAL methodhas been described by, e.g., Kovalev and Eichin-ger [5].

2. Experimental Arrangement

A schematic experimental setup for the lidar systemis shown in Fig. 1. A mode-locked Nd:YAG lasersystem (Ekspla, PL 2143C) with the possibility touse the fundamental, 1064nm, second harmonic,532nm, and fourth harmonic, 266nm, radiationwas used. The laser pulse duration was 30ps, the re-petition rate was 10Hz, and the laser beam diameterwas 12mm with a divergence angle of <0:5mrad. Astreak camera (Optronis, Optoscope) with a 19:5mmCCD chip, providing streak rates from 10ps=mm to1ns=mm, was used to give time-resolved imaging of aline, or a spectrally resolved point if a spectrograph ispositioned in front of the streak camera. The streakcamera was triggered by a pulse from the laser pas-sing through a delay box (Optronis, TRRC1) withvariable delay ranging from 0 to 63:75ns plus a min-or inherent system delay.

Backscattered laser light was collected with a10 cm diameter quartz lens with f ¼ 15 cm (lens 1). Atelescope, f -number matched to the streak camera,collimated the collected light and focused it onto theentrance slit of the streak camera. The telescopeconsisted of two quartz lenses with a 3 cm diameterand focal lengths of 5 cm (lens 2) and 15 cm (lens 3).

Hard target returns can be a significant problemmeasuring in enclosed environments. However, withthe streak camera used in this setup it is possible tochoose both when and how long the microchannelplate will actually register photoelectrons. This op-tion means that the streak camera may be triggeredand configured so that electrons at the cathode, cre-ated from photons diffusely scattered of hard targets,do not hit the microchannel plate. Hence, there willneither be any problem with the dynamic range ofthe detector, nor will any part of the detector exceptthe photocathode be exposed to the signal from thehard target return. On the other hand, the photo-cathode of the streak camera might be damaged ifit is exposed to a direct laser reflex. With direct re-flexes we mean coherent laserlike beams. To avoidharmful direct reflexes it is imperative to make lidarmeasurements with a carefully designed experimen-

Fig. 1. Schematic setup for the ps-lidar system. M1–M4 designate mirrors and the x − y coordinate system aims to describe how themeasurement objects are placed.

B66 APPLIED OPTICS / Vol. 48, No. 4 / 1 February 2009

tal geometry, where all direct reflexes fall outside thereceiver. A proper procedure to ensure this experi-mentally is to make sure that the probe laser beamdirection is not normal to any scattering surface andcontrol that no direct reflexes impinge on the detect-ing optics before the detector is put in place.For the range resolution measurements, two thin

metallic wires were placed next to each other alongthe y axis (see Fig. 1). One of the wires was positionedon a translator to be movable along the x axis. Allthe mirrors were coated for high reflectivity at532nm.For the Bunsen flame measurements, the laser

wavelength was 266nm, and mirrors M1–M3 werecoated for high reflectivity at 266nm. A Bunsen bur-ner was positioned in the transmitted laser beam.The inner diameter of the burner is 10mm. Two pro-pane/air flames were investigated, a fuel-rich non-premixed flame and a partially premixed flame. Theformer flame, generated by closing the throat hole onthe burner, produced strong Planck radiation indi-cating considerable soot production. In the lattercase, fuel and air were premixed by leaving thethroat hole fully open, resulting in a bluish flamethat was nonsooty.For the Rayleigh temperature measurements in

the flame of the McKenna burner the setup was ba-sically the same as for the Bunsen flame measure-ments. The McKenna burner produced a stabilizedflame positioned in the transmitted laser beam withthe top of the burner 9mm below the center of thelaser beam profile. The distance between the collect-ing lens, lens 1 in Fig. 1, and the center of theMcKen-na burner was 207 cm. The ethylene/air mixture wasgenerated using two mass flow controllers (Bronk-horst) calibrated for ethylene (F-201C-FAC-33-Z)and air (F-201C-FAC-33-V), respectively.The setup used for DIAL measurements was simi-

lar to the setup shown in Fig. 1. Now the measure-ment objects were two acetone gas jets locatedalong the x axis. As both 532 and 266nm laser wave-lengths were used, dual pathways were aligned withmirrors coated for 266 and 532nm, respectively. Themirrors, M3 and M4, on which the laser beams wereoverlapping, were replaced with quartz prisms to getsimilar reflectivity at both wavelengths. The lenssystem was replaced with a single achromatic quartzlens (B. Halle) of focal length 250mm and diameter50mm to enable efficient light collection at bothwavelengths. In order to suppress the fluorescenceinduced when using 266nm laser light, a spectro-graph (Spectral Products, DKSP-240) with a150 grooves=mm grating (blazed at 500nm) wasput in front of the streak camera so that the spec-trally dispersed signal was focused onto its entranceslit. The acetone vapor jets were generated by bub-bling air, controlled by a mass flow regulator (Bron-khorst, F-201-FA-33-Z), through acetone bathscontained in glass flasks. The outlet of each bubblerflask has an inner diameter of 20mm, which also de-fined the size of the acetone jets.

3. Measurements and Results

A. Determination of Range Resolution

The range resolution of the lidar system was deter-mined for different streak rates by imaging two thinmetallic wires located at different positions along theoptical axis. To get a variable distance between thewires, one of themwas fixed and the other was placedon a translator movable along the optical axis, i.e.,the x axis in Fig. 1. The distance between the nearestwire and lens 1 was 450 cm. The laser wavelengthused was 532nm and the laser power was kept low(1–2mJ=pulse). All seven available streak rates,i.e., 10, 25, 50, 100, 250, 500, and 1000 ps=mm, wereinvestigated. For each streak rate, measurementswere performed for typically ten different distancesbetween the wires. Three streak camera images wererecorded for each distance.

Figure 2(a) shows a streak-camera image takenwith the streak rate of 25ps=mm and with a distanceof 7:5mm between the wires. For all images, pixelvalues along each column were summed up in thesoftware. The resulting curve corresponding to theimage in Fig. 2(a) is shown in Fig. 2(b). For each curvethe average value of the two peak values were di-vided with the minimum value between them.We de-fine this ratio as the resolution parameter of thesystem. The resolution parameter for each distancebetween the wires was averaged from the threevalues corresponding to the three recorded images.Figure 2(c) shows the resolution parameter versusdistances between the wires for the streak rate of25ps=mm. We consider two peaks resolved when theresolution parameter ≥2. As can be seen in Fig. 2(c)this condition is fulfilled for distances between thewires of at least 5:0mm, hence the range resolutionis in this case 5:0mm. In the same way, the rangeresolution has been determined for each streak rateas shown in Fig. 3. The highest range resolution is0:46 cm and is obtained with a streak rate of10ps=mm.

There is a trade-off between range resolution andmeasurement range due to the design of the streakcamera; high resolution, requiring high streak rates,results in low measurement range and vice versa. Itis worth noting that changing the streak rate from 25to 10ps=mm changes the measurement range from7.5 to 3:0 cm but only leads to a change in resolutionfrom 0.50 to 0:46 cm. The reason for this is that theresolution is limited by the duration of the laserpulse at the fastest streak rate. A laser pulse dura-tion of 30ps converts into a range resolution of0:45 cm, using that ΔR ¼ c ×Δt=2, where ΔR isthe range resolution, c is the speed of light, and Δtis the laser pulse duration. Hence, the range resolu-tion measured with the fastest streak rate is in ex-cellent agreement with the resolution determinedby the laser pulse duration.

It should be pointed out that the results of theresolution study are based on single-pulse measure-ments, in which temporal jitter is not an issue. In


measurements where a number of recordings are ac-cumulated time jitter will generally degrade therange resolution. The time jitter of the experimentalsystem used here was 50ps. This contribution alonewould correspond to a range resolution of ∼5mm.Although it is possible to compensate for time jitterwith the present streak camera system, no jitter cor-rection was carried out on the accumulated data pre-sented in the following sections.

B. Bunsen Flame Measurements

In order to investigate the potential of the ps-lidarmethod for mapping soot occurrence and Rayleigh-scattering thermometry, applications that would beof great interest in full-scale power plants, measure-ments were performed in Bunsen burner flames.Figure 4 shows the results from measurements ina nonsooty (gray curve) and a sooty flame (blackcurve), respectively. These curves are the result of100 accumulated recordings. Typically, the laserpulse energy was 35mJ. Both curves clearly indicatethe presence of a flame at a distance of 3:0m. As ex-pected, the nonsooty flame gives rise to a dip in thecurve, due to the lower number density in the hotflame compared to the ambient air, while thesooty flame results in a strong peak due to Mie scat-tering from soot particles. The intensity at the centerof the peak in the black curve, shown in Fig. 4, is afactor of 4.6 higher than the intensity at the sameposition without the flame. Although quantitativesoot concentration measurement would be difficult

Fig. 3. Minimum resolvable distance versus streak rate.

Fig. 2. Results from range-resolution measurements using astreak rate of 25ps=mm. (a) Streak camera image of elastic back-scattering from two metallic wires located 7:5mm apart alongthe x axis. (b) Summed up pixel values from each column ofthe image in (a) versus the x position. (c) Resolution parameterversus distance between the metallic wires. It is indicated wherethe two peaks are considered to be resolved, i.e., the resolutionparameter ≥2.

Fig. 4. Lidar curves recorded in ambient air with a Bunsen flamelocated 3:0m from the receiver lens. The black curve was recordedwith a sooty flame on the burner, while the gray curve was ob-tained with a nonsooty flame.


with the present technique, the result shows thatqualitative data can be obtained. Such informationcould, however, be of large value in characterizingcombustion inside power plants and fires.For the nonsooty flame (gray curve), the signal in-

tensity at the center of the dip is a factor of 3.6 lowerthan the intensity at the same distance withoutthe flame. Assuming pure Rayleigh scattering andthe same Rayleigh cross section in the flame as inthe air, this intensity ratio implies that the averagenumber density in the flame is 3.6 lower than in theambient air. With an ambient temperature of 295Kand using the ideal gas law, this ratio between thenumber densities results in an average flame tem-perature of 1050K. It should be emphasized that thisis an average temperature in a probe volume about6 cm long (see Fig. 3 for a streak rate of 500ps=mm),covering both the flame and a significant region inthe ambient air adjacent to the flame. Thus, giventhat the flame diameter was ∼1:5 cm, the averageflame temperature is certainly underpredicted. Ob-viously, it is possible to improve the temperature ac-curacy by using a higher range resolution, i.e., usinga higher streak rate (see Fig. 3). However, that wouldimply a shorter measurement range.The result of a single-shot measurement in the

nonsooty flame located 2:52m from the receiver lensis shown in Fig. 5. The streak rate was increased to100ps=mm in this measurement, providing a rangeresolution of ∼1:2 cm (see Fig. 3). Figure 5(a) showsthe two-dimensional lidar image of the elastic back-scattering of the laser beam. The temporal axis, i.e.the horizontal axis, of the streak-camera image cor-responds to the spatial axis along the laser beam(x axis in Fig. 1), while the axis orthogonal to thetemporal axis corresponds to the spatial axis perpen-dicular to the laser beam direction, (y axis in Fig. 1).Since the laser beam was not focused, the horizontalentrance slit of the streak camera set the spatial

resolution along the vertical axis (z axis in Fig. 1),which was 500 μm for a slit width of 100 μm.

Thus, the present setup leads to images of the Ray-leigh scattered intensity appearing as if they wereviewed from above, which is clearly illustrated inthe image recorded in the Bunsen flame [Fig. 5(a)].The circular flame front between the premixed innercone and the outer diffusion flame is faintly indicatedby the darker (lower intensity) regions surrounding alighter (higher intensity) circular area located at252 cm. The flame structure is, however, more clearlyshown by taking a horizontal cross section at the cen-ter of the image. Figure 5(b) shows a signal intensityprofile in which 20 pixel rows have been summed up.

Again, assuming pure Rayleigh scattering, idealgases, and neglecting differences in Rayleigh crosssections, the intensity profile can be used to deter-mine the temperature profile in the same way as de-scribed above. The temperature profile is displayedin Fig. 5(c). As expected, the temperature profileshows two peaks, indicating the flame front betweenthe inner premixed region and the outer diffusionflame. Despite having five times higher rangeresolution than in the previously described measure-ment, the flame temperatures are still underpre-dicted because of too low range resolution (∼1:2 cm),but in this case the two reaction zones of the flamecone are resolved. In the previous measurement,shown in Fig. 4, the average temperature of the flameand the surrounding air was calculated. Neverthe-less, the result indicates that ps-lidar might be usefulfor single-shot thermometry, even two dimensionally,in large-scale combustion environments with insig-nificant particle concentrations.

C. Rayleigh Scattering Thermometry in a Flat Flame

To further investigate the potential of lidar Rayleighthermometry, an ethylene/air flame on a McKennaburner was studied. This burner ideally produces aflat flame, i.e., temperature and species concentra-tions in the flame vary with height above the burnerwhile they are constant at a fixed height. The equiva-lence ratio was varied from 0.5 to 2.0. The laser pulseenergy was set to 13mJ. As in the previous Bunsenburner experiments a two-dimensional lidar image ofthe elastic backscattering of the laser beam was col-lected. The horizontal axis of the streak-camera im-age corresponds to the spatial axis along the laserbeam (x axis in Fig. 1), and the vertical axis on thestreak-camera image corresponds to the spatial axisperpendicular to the laser beam direction, (y axis inFig. 1). A streak rate of 100ps=mm was used, whichgave the same resolution, 1:2 cm, as in the single shotBunsen burner experiments. As theMcKenna burnerhas a flame diameter of 6 cm the resolution was highenough, even for accumulated measurements, add-ing the contribution due to time jitter, to be able tomake temperature evaluation that is not influencedby the cooler ambient air.

Figure 6(a) shows an accumulation of 100 two-dimensional lidar images of an ethylene flame with

Fig. 5. Lidar measurements in a Bunsen flame located 2:52mfrom the receiver lens. (a) Two-dimensional single shot lidar imageof the Rayleigh scattering intensity. (b) Corresponding horizontalintensity profile (sum of 20 pixels along the vertical axis) aroundthe vertical center position (0 cm). (c) Temperature profile ex-tracted from the curve shown in b.


an equivalence ratio of 1.0. The elastic backscatter-ing of the laser beam is seen as the horizontal linein the middle of the image. The backscattering de-creases when the laser beam passes above theMcKenna burner, due to the lower number densityin the flame.Before temperature evaluation, superpixels

consisting of 30 × 90pixels, corresponding to thephysical dimension of 0:6 cm × 0:4 cm, were formedin all images. The overall resolution in the setupin this experiment is lower than the physical dimen-sion corresponding to the superpixels, which meansthat no information is lost due to the formation ofsuperpixels. Figure 6(b) shows the evaluated tem-perature in the same area as shown in Fig. 6(a).The temperature has been evaluated by comparingthe image with a reference image recorded in airat room temperature. The ratios between the inten-sities in the super pixels in the burner measure-ments and the reference measurement areproportional to the ratios of the corresponding num-ber densities if the gas mixture is constant. As theconstituents are not the same in air and the productgases of the flame, the temperatures have been cor-

rected for the contribution to the total Rayleigh crosssection from CO, CO2, H2, H2O, N2, and O2 using thesame mole fractions as Vestin et al. [10] and usingRayleigh cross sections from Sutton and Driscoll [11].

The high temperature pixels in the peripheralparts of the laser beam in Fig. 6(b) are due to numer-ical noise induced when calculating the Rayleigh ra-tio with a denominator close to zero. This is not aproblem in the main region of the laser beam.

For each equivalence ratio the average tempera-ture is calculated using the superpixels in the centralarea of the recorded image [see Fig. 6(b)], rangingfrom 206 to 209 cm horizontally and from −0:6 to0:2 cm vertically. To validate our results the flametemperatures were compared with flame tempera-tures from CARS measurements made by Vestinet al. [10], as shown in Fig. 7. The average tempera-tures from our measurements are indicated by cir-cles, while the CARS data are indicated by stars.The error bars correspond to standard deviationsof the temperatures evaluated from all superpixelswithin the region of interest as specified previously.

The Rayleigh temperatures are in good agreementwith the CARS temperatures up to an equivalenceratio of 1.3. At higher equivalence ratios the tem-peratures are underpredicted. This behavior is dueto increased soot and particle content in the productgases of rich flames. The average standard deviationof our data is 27K, i.e., the average temperature pre-cision (due to differences between temperatures eval-uated for different superpixels) is 27K. The meanabsolute deviation from the CARS temperatures is33K. The measurements indicate that two-dimen-sional thermometry can be conducted with an uncer-tainty of less than �60K in each superpixel.

D. DIAL Measurements of Acetone Concentrations

As mentioned in Section 1, lidar may deliver species-specific information by tuning the laser frequency toa molecular absorption line. In order to enhance thesignal contrast and facilitate absolute concentra-tion measurement, usually DIAL is applied. In this

Fig. 7. Measured Rayleigh temperatures (circles) and tempera-tures measured with CARS (stars) [10].

Fig. 6. (a) Two-dimensional lidar image (100 accumulations) of astoichiometric ethylene/air flame. (b) Temperature map extractedfrom the image shown in (a) and a corresponding Rayleigh scatter-ing image recorded in air at room temperature.


method, two laser pulses of different wavelengths areused, one tuned to the peak of a molecular absorptionline and the other one positioned off the line. Byforming the ratio between the two lidar signals,many unknown factors are eliminated, leaving thefollowing simple expression [12]:

Pλ1ðRÞPλ2ðRÞ

¼ e−2½σðλ1Þ−σðλ2Þ�R

R

0NðrÞdr; ð1Þ

where Pλ1ðRÞ is the signal intensity at wavelength λ1,Pλ2ðRÞ is the signal intensity at wavelength λ2, R isthe range coordinate along the laser beam, σðλ1Þ isthe absorption cross section at wavelength λ1, σðλ2Þis the absorption cross section at wavelength λ2,and NðrÞ is the range-dependent number densityof the absorbing species. Equation (1) is commonlyreferred to as the DIAL equation. Solving the equa-tion for NðrÞ results in the following expression:

NðRÞ ¼ 12ðσðλ1Þ − σðλ2ÞÞ

ddR

�ln

Pλ1ðRÞPλ2ðRÞ

�: ð2Þ

This equation is only strictly valid as long asthe following conditions are fulfilled: the total (mole-cular and particulate) scattering coefficients for thetwo wavelengths are equal, the total backscatteringcoefficients are the same for both wavelengths, andthere is no background absorption at any of thetwo wavelengths.In order to investigate the potential for DIAL mea-

surements in fuel visualization applications usingpicosecond laser pulses, we performed measure-ments along a path intersected by two jets of acetonevapor. Although no tunable picosecond laser systemwas used, the concept could still be tested by makingone measurement using 266nm light, which is ab-sorbed by acetone, and another measurement usinglight at 532nm, which is not absorbed by acetone.The laser pulse energies were 30mJ ð532nmÞ and15mJ ð266nmÞ, respectively. The lidar curves (200accumulated recordings) corresponding to the twolaser wavelengths are displayed in Fig. 8(a). Usingthe ratio of the two signals, shown in Fig. 8(b),and the absorption cross sections, σð532nmÞ ¼ 0and σð266nmÞ ¼ 4:36 × 10−20cm2 [13], acetone num-ber densities were extracted using Eq. (2). The re-sulting range-resolved number density plot is shownin Fig. 8(c).The acetone number density at the center of the

two peaks shown in Fig. 8(c) is 1:7 × 1018 molecules=cm3 at the distance of 340 cm and 2:5 ×1018 molecules=cm3 at 384 cm, respectively. The30% difference in number density might well bedue to differences in the two acetone vaporizationchannels. The total flow of bubbling air going tothe two acetone baths was measured with a massflow controller. However, the separate flows fed toeach bath were not controlled. In addition, there isuncertainty due to the low signal-to-noise ratio on

the curve shown in Fig. 8(c), especially for the peakat 384 cm.

The vapor pressure of acetone at 295K is201:6Torrð26:9kPaÞ [14]. Assuming that the air/acetone mixtures in the bubbler flasks were satu-rated and assuming ideal gases, the acetone numberdensity would then be 6:6 × 1018 molecules=cm3.This number density is about 3 times higherthan the average of the two measured values(2:1 × 1018molecules=cm3). The major reason forthe deviation is the same as for the measurementin the Bunsen flame as described in a previoussection, namely, the fact that the probe volume is sig-nificantly longer than the width of the acetone jet.Thus, the measured values are number densitiesaveraged along a probe volume covering not onlythe acetone jet, but also a significant fraction ofthe surrounding air. Our calculation shows thataround three times lower temperature is expecteddue to the limited resolution.

Another reason for discrepancy is the large differ-ence between the two laser wavelengths, i.e.,Δλ ¼ 266nm. With such a large Δλ, there is a signif-icant difference in the scattering coefficients at the

Fig. 8. DIALmeasurements of two acetone jets placed at 340 and380 cm, respectively. (a) LIDAR curves for laser wavelengths of 266and 532nm, respectively. (b) Ratio between the two lidar curves in(a). (c) Corresponding acetone number density extracted fromEq. (2).


two wavelengths, and the prerequisites of Eqs. (1)and (2) to be strictly valid are not fulfilled. It is pos-sible to take the difference in scattering coefficientsinto account by adding two extra terms to Eq. (2), oneterm for backscatter correction and another for ex-tinction correction [5]. However, in our laboratorymeasurements, assuming only Rayleigh scattering,the correction terms have been calculated to bearound 0.02%, which is insignificant in this context.The correction may also be experimentally deter-

mined by recording a 266nm lidar signal, generatedwith the acetone flow switched off, and use this curveas the off-resonance case. Such a measurement wasnot made in direct connection with the measure-ments presented here. The present results show thepotential of picosecond DIAL. The applicability of thetechnique could be enhanced by using toluene as thefuel tracer and using the wavelengths 266.9 and266:1nm as on and off resonance wavelengths, re-spectively [15]. One of the great advantages of thetechnique is clearly shown in Fig. 8. The large peakat around 210 cm, visible on both curves in Fig. 8(a),most likely due to some spurious reflection in the sys-tem, does not influence the final result. The reasonfor this is that experimental disturbances that arecommon for both measurements are canceled whenforming the intensity ratio between the two signals,as can be seen in Fig. 8(b). Furthermore, Eq. (2)clearly shows that the number density is, exceptfor the absorption cross sections, only influencedby range-dependent parameters, which means thatsystem parameters, such as efficiency of transmit-ting and receiving optics, detector efficiency, andlaser intensities, are in principle unimportant.

4. Conclusions

We have described the development of a ps-lidar sys-tem, aiming at combustion diagnostics in large-scalecombustion environments. The range resolution ofthe system has been determined for each of sevenavailable streak rates of the streak camera. Thehighest overall resolution was found to be 0:46 cmat a streak rate of 10ps=mm, in excellent agreementwith 0:45 cm set by the 30ps laser pulse duration.The results from measurements in a nonsooty Bun-sen flame indicate that ps-lidar might be applicablefor single-shot Rayleigh thermometry, even two-dimensional, in combustion devices with insignifi-cant particle concentrations. The corresponding re-sults from a sooty flame show that the techniquealso might be used for qualitative soot distributionmeasurements in sooty environments. Rayleigh ther-mometry with ps-lidar in a well-haracterized ethy-lene flame shows good agreement with CARSmeasurements up to an equivalence ratio of 1.3.Two-dimensional thermometry has been demon-strated with an uncertainty of less than �60K.

Furthermore, results from DIAL measurements inacetone jets indicate a good potential for using DIALfor fuel visualization. Overall, the results are promis-ing with respect to future applications in large-scalecombustion devices, where often only one opticalaccess is available.

The authors would like to thank the Centre ofCombustion Science and Technology (CECOST), theSwedish Energy Agency, E.ON Sweden, and theWallenberg Foundation for financial support.

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development of a picosecond lidar system for large-scale combustion diagnostics

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