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TRANSIENT 'THERMAL BEHAVIOR OF LAYERED STRUCTURES

L. VozAr, W. Hohenauer, I. Stubna

TRANSIENT THERMAL BEHAVIOR OF LAYERED STRUCTURES

1 Department of Physics, Faculty of Natural Sciences, Constantine the Philosopher University, Tr. A. Hlinku 1, SK-94974 Nitra, Slovakia 2 Department of Materials Technology, Austrian Research Centers, A-2444 Seibersdorf, Austria 3 Department of Phy sics, The University of TrenCin, Studentska 2, SK-9 1 1 0 1 TrenEin, Slovakia Email: vozar@uvt.uniag. sk, wolfgang. hohenauer@arcs.ac.at, stubna@uniag. sk

Abstract

The use of layered materials has rapidly increased in a great number of applications - in case of building industry especially as thermal barriers, partially as electric insulation and wear, erosion and corrosion resistance protection. Because such structures are utilized under different transient thermal conditions it is of a great interest to study their transient thermal behavior.

The knowledge of thermophysical properties of these structures is the primary information needed for performing analytical as well as numerical analyses. One may study contact thermal resistance between layers that may indicate the quality of the layered composite. In situ measurement on layered systems allows testing whether and how thermophysical properties of layers differ from those values received for bulk materials. It has been shown that transient photothermal methods like the laser flash technique can be successfully utilized for a reliable estimation the thermal difisivity andlor the thermal contact resistance.

The paper deals with an investigation of transient thermal behavior of layered systems. In case of analytical simulations the initial and boundary conditions corresponding to the laser flash experimental method are considered. The paper presents a draft of analytical theory and gives some results of an estimation of the thermal difisivity of a composite consisted of a metallic and non-metallic two-layer.

Key words: layered structures, thermal diffusivity, thermal contact resistance

1 Introduction

Building constructions are frequently exposed on one side to variable weather conditions and on the other face enclosure an indoor space at a constant and uniform (room) temperature. The determination of the heat transfer through a wall is ofien treated as an example of the linear heat flow in a slab of finite thickness bounded by a pair of parallel planes of finite extent. They assume that each of the layers that compose the wall consists of a material of different thermal properties. The equation for the conduction in each layer of the wall is

where k is the thermal conductivity - the basic thermophysical property used to relate heat fluxes to steady state temperature gradients, C , is the specific heat at constant pressure and p the density.

For homogeneous isotropic materials whose thermal conductivity is independent on temperature and position the equation (1) can be rewritten as

Here the thermal difhsivity a represents the transport property that describes how quickly heat propagates through a material during a transient state.

If there occurs a temperature drop AT across the interface between two materials when analyzing a heat transfer through a layered composite the thermal contact resistance R should be considered. Then the heat flow q through the interface conforms to the equation [I]

2 Theory and the laser flash method

In the flash method, the front face of a wall shaped sample is subjected to a pulse of radiant energy coming from a laser [2]. If material boundaries are flat and parallel to the sample front and rear surfaces and if there are no heat losses from the radial surface an one-dimensional heat transfer occurs across the sample. Analyzing the resulting temperature rise on the opposite (rear) face of the sample any thermophysical property value (the thermal difisivity of one layer, or the thermal contact resistance) can be computed [3].

The analytical theory assumes one-dimensional heat flow across a wall composed fi-om n layers. It considers the sample initials at constant equilibrium (zero) temperature. The front face of the wall is in the initial (zero) time uniformly subjected to the instantaneous heat pulse with the heat Q supplied to the unit area. In case of non-ideal experimental conditions - when heat losses from the front and rear face is not negligible, appropriate boundary conditions include heat flux term with the heat transfer coefficients at the front hl and rear face h2. The expression for the transient temperature rise T=T(x,t) in the sample can be received solving the heat conduction equation for each layer

with the initial

and the boundary condition at the front and rear face as

where 6(1) is the Dirac's delta function. The boundary conditions at the interface between the ith and jUl layer describe the relations

The problem (equations 4-9) is negotiable using the Green's function approach as well as utilizing the Laplace transform technique [4]. Very eficient is to take the quadmpole formalism of the Laplace transform technique and the approach described in [5] that yields to simple expressions even though analyzing general complex cases.

3 Simulations

For a better understanding the phenomena a simple two-layered geometry is studied here. We consider to have sample that consists fi-om 1.97 mm thick molybdenum and 0.21 mm ceramic layer with appropriate thermal properties as measured at 500 "C (molybdenum - k = 1 11.4 W l d , a = 39.3.10" m2/s, ceramic k = 6.09 W/&, a = 2.83.10-~ m2/s). From Fig 1 one can see, that thermal contact resistance below

0.00 0.02 0.04 0.06 0.08 0.10

Time [s]

Fig. 1 Simulated rear face temperature rise vs. time curves calculated for various thermal contact resistance

1om6 W - ' ~ K does not observably influence the temperature response investigated at the wall (sample) rear face. From the other point of view the result indicate the natural border of the thermal contact resistance estimation for the presented composite.

Fig 2 presents normalized sensitivity to thermal difisivity of molybdenum SaM to thermal difisivity of ceramic Saceram, to Biot number (H = helk, where e is the sample (layer) thickness) SH and sensitivity to thermal contact resistance SR VS. time curves. We see that sensitivities to thermal diffusivity and sensitivity to thermal contact resistance have similar shape, what indicate that all these three parameters influence the temperature rise vs. time evolution in a similar and therefore they can not be estimated simultaneously .

-4.0 h h 0 0.02 0.04 0.06 0.08 0.1

Time [s]

Fig. 2 Normalized sensitivities vs. time curves

Fig 3 presents a typical experimental temperature rise vs. time curve (No 1) and its least-squares-fit using the analytical model that considers the sample acting as a homogeneous medium (No 2) (here the unknown property is the 'apparent' thermal difisivity). The curve No 3 considers a two layered Molceramic sample with above mentioned thermal properties with non-zero thermal contact resistance - that is estimated here. The curve No 4 is based on an 'ideal two layered Molceramic sample' model with Zero thermal contact resistance - here the thermal diffusivity of ceramics is determined. We see that the case of an ideal two-layered composite suits the best to the experimental temperature rise vs. time evolution. Therefore a data reduction process based on the used analytical model can give a reliable data of thermal diffusivity of the ceramics material.

4 Measurement of thermal diffusivity

Fig 4 presents results of the thermal diffusivity estimation of the ceramic material on three samples of two-layered Molceramic composite. The sample No 1 has 2.1 8 mm

0.00 0.02 0.04 0.06 0.08 0.10

Time [SI

Fig 3 Experimental temperature rise vs. time data and its fit

thick ceramic layer and the thickness of molybdenum layer is 0.29 mm, the sample No 2 has layers thickness 2.31 mm and 0.35 mm; and No 3 - 2.295 mm and 0.329 mm. Acquired values are compared with those thermal diffusivities measured on a homogeneous ceramic samples. What can be Seen is also for a thermophysical property measurement unusually high dispersion of result.

2.0 0, 0 200 400 600 800 1000

Temperature ['C]

Fig 4 Thermal diffusivity of ceramic material

In fact an estimation of the thermal diffusivity from experimental data measured on a layered composite is a dependent measurement. The measurement of the thermal difisivity of one layer requires besides the knowledge of other relevant properties (the density, the heat capacity and the thickness of both components) to know the thermal difisivity of the remained layer. Errors in measurement of these additional parameters are propagated through the data reduction and result as inaccuracy of the thermal difisivity determination. On the other hand physical borders limit the accuracy and the reproducibility of the method [ 3 ] Therefore the flash method approach is not very suitable for a study of poor thermal conductive coatings on much better conductive sub strate.

5 Conclusion

The paper shows that an experimental investigation of transient thermal behavior of a layered composite may be used as a suitable basis for measurement of a thermophysical property of a component. Because such measurement is a dependent measurement the accuracy of the required property estimation depends besides the accuracy of other relevant properties on an information that experimental data contains about the estimated property. This depends on the geometry and materials properties.

Acknowledgement

Authors wish to thank the Her

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