The theory of advanced multi-layer thin film heat transfer gauges

This paper describes the theory of a transient method of measuring heat transfer rate to metal substrates coated with an electrical insulator, using thin film resistance thermometers. This builds on the already well-established system which uses semi-infinite insulating substrates. It is intended that the new technique will have application in rotating turbine test rigs, since there is at present a lack of suitable instrumentation which can be easily manufactured, and which does not interfere with the flow. The new system described here shows that multi-layer substrate gauges can be used. This paper presents analyses of layered gauges and gives sample predictions and calibrations.     

Transient heat fluxes measurement analysis from platinum based thin film gauges in open and closed cavities

Platinum thin film gauges (PTFGs) measure heat fluxes in the applications involving very short duration of the heating environment. Heat transfer measurement is the frequently used technique for measuring the surface heat flux using thin film gauges. The present investigation has been focused on the design and manufacturing methods for heat transfer gauge, their stability, and dynamic calibrations in certain situations where the heat load suddenly build up. PTFGs measure heat fluxes in heating environments applications during the very duration. The measurement for heat transfer is a technique used often with thin film gauges to measure the surface heat flux. The convection devices are regarded as the best measuring units in short-term transient temperature measurement applications and are usually used when the heat transfer mode is dominant means gas turbine engines, high speed aircraft, etc. In addition to that, there are many difficulties incurred for convection based measurement practically and few interdisciplinary research fields. A convective heat load is provided with a hot air gun to get the temperature signal. By using thin film gauge through present investigations, it is very ambitious to explore the possibility of short term conduction based transient measurements with pure conduction heat transfer mode. A simple experimental set up is used to supply the thin film gauges with heat flux, which is manually manufactured with platinum as a sensing material and quartz as a substrate material. The body's nose tip to high speed flow is expected to be the maximum heat transfer at the stagnation point. The stagnation point probes are fabricated for PTFGs, and baking material is quartz. The recorded temperature histories are compared with the experimentally recorded temperature signals from the gauges through the finite element method. The heat flux forecast was configured by using the one dimension thermal conduction equation convolution integral and by comparison with the heat input loads. This study reveals the ability of PTFGs to be used for a short period.     

Design and fabrication of coaxial surface junction thermocouples for transient heat transfer measurements

Low cost coaxial surface junction thermocouples (CSJTs') have been fabricated in-house and calibrated to measure the transient surface temperature rise within a UNITEN's shock tube wall facility, consisting of K-type coaxial thermocouple elements. The aim of this paper is to explain the design technique of the CSJTs' and the difficulties that have occurred during the fabrication process. The microstructural analysis and the chemical characterization for these types of thermocouples have also been carried out to verify the surface morphology and to qualitatively evaluate the CSJT materials composition. The preliminary testing was performed to demonstrate the performance of these thermocouples to be used for measuring the surface temperatures and heat transfer rates under transient conditions. The preliminary results from shock tube tests have shown that these thermocouples have a time response on the order of microseconds and were suitable for making heat transfer measurements in highly transient conditions. It was concluded that the current construction technique produced gauges that were reliable, reproducible, rugged and inexpensive.     

Heat transfer mechanisms in thin film with laser heat source

The present study deals with the effect of laser radiation on the propagation phenomenon of a thermal wave in a very thin film subjected to a symmetrical heating on both sides. Pulsating laser heating is modelled as an internal heat source with various time characteristics. The Cattaneo heat flux law together with the energy conservation equation is solved by a numerical technique based on explicit scheme, i.e., MacCormack’s predictor–corrector scheme. Results are obtained for the time history of heat transfer behaviour before and after symmetrical collision of wave fronts from two sides of a film. The study concludes (1) if the absorption coefficient of the continuously-operated- and pulsating-laser heat source increases, temperature overshoot causes in a very thin film within a very short period of time, and (2) the overshoot and oscillation of thermal wave depend on the frequency of the heat source time characteristics. This trend becomes minor in a thick film.     

纳米流体颗粒沉积对薄液膜蒸发换热的影响

通过建立基于动量守恒和能量守恒的数学模型,比较纳米流体不同程度沉积造成表面粗糙度变化下,薄液膜的蒸发换热特性变化,分析得出表面粗糙度变化对薄液膜蒸发换热的影响。     

Analysis of transient heat transfer in multilayer thin films with nonlinear thermal boundary resistance

Transient energy transport in thin-layer films with a nonlinear thermal boundary resistance is analyzed theoretically within the framework of the dual-phase-lag heat conduction model. An iterative finite difference numerical method is used and is verified using a derived semi-analytical solution of the problem. Effects of the thermo-physical properties on energy transport when a two-layer film is exposed to a thermal pulse of certain duration and strength are presented. The thermal boundary resistance, the heat flux and temperature gradient phase lags and the thermal conductivities and heat capacities all are important factors that characterize energy transport through the interface and the temperature distribution in the two layers. The maximum interfacial temperature difference that takes place in the transient process of thermal pulse propagation is found to be the proper choice to measure the perfect-ness of the interface with a finite thermal boundary resistance. The results show that even with high values of the thermal boundary resistance the maximum interfacial temperature difference can be very small when the thermal pulse propagates from a high-thermal conductivity and heat capacity layer to a low-thermal conductivity and heat capacity layer. For a certain range of the thermal conductivities and heat capacities, the maximum interfacial temperature difference approaches zero even with high values of the thermal boundary resistance. Thermal conductivities and heat capacities are much more important in characterizing transient heat transfer through the imperfect interface than the phase lags of the heat flux and temperature gradient.     

Influence of lateral conduction due to flow temperature variations in transient heat transfer measurements

An analytical model is presented addressing the effect of lateral conduction due to varying adiabatic wall temperature distributions in transient film cooling experiments using thermochromic liquid crystals (TLC). Applying the analysis for a typical experimental situation shows, that results evaluated without taking the lateral conduction effect into account can lead to erroneous results especially in the regions of high film cooling effectiveness. An alternative data evaluation procedure is suggested considering the lateral conduction effects based on the given analysis.     

A hybrid optimization technique for developing heat transfer correlations based on transient experiments

A new approach for developing a Nusselt number correlation, in terms of relevant non-dimensional parameters, for turbulent forced convection flows in vertical channels using a judicious combination of transient cooling experiments with a hybrid optimization technique is reported. The temperature–time history, during the cooling of a heated plate, idealized as a lumped capacity heat transfer model, is recorded using a PC based data acquisition system. A numerically computed temperature–time history of the plate is then compared with the experimentally known temperature–time history to estimate the residual. The minimization of sum of the squares of the residual is done using a hybrid numerical optimization technique, i.e. a combination of Genetic Algorithm and the Levenberg–Marquardt method, in order to obtain the coefficient and the exponents of the pertinent non-dimensional parameters in the Nusselt number correlation. The parameters in the correlation are also retrieved using another global optimization technique, the Simulated Annealing (SA), for evaluating the consistency in parameter estimation. As a validation exercise, Nusselt number values estimated using the proposed correlation are compared with steady state experimental results and a good agreement of results endorses the efficacy of this approach.     

Evaporation heat transfer of steady thin film in micro capillary tubes of micron scale

This paper reports that the heat transfer mechanism of phase change in a capillary tube belongs to liquid film conduction and surface evaporation. The surface evaporation is influenced by vapor temperature, vapor‐liquid interfacial temperature, and vapor‐liquid pressure difference. In the vapor‐liquid flow mechanism, flow is effected by both the gradient of disjoining pressure, and the gradient of capillary pressure. The mechanism of vapor‐liquid interaction consists of the shear stress caused by momentum transfer owing to evaporation, and frictional shear stress due to the velocity difference between vapor and liquid. In the model presented for a capillary tube, the heat transfer, vapor‐liquid flow, and their interaction are more comprehensively considered. The thin film profile and heat transfer characteristics have close relations with a capillary radius and heat transfer power. The results of calculation indicate that the length of the evaporating interfacial region decreases to some extent with decreasing capillary radius and increasing heat transfer power. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 513–523, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ).DOI 10.1002/htj.10050     

Frequency response and spatial resolution of a thin foil for heat transfer measurements using infrared thermography

A technique for measuring the spatio-temporal distribution of convective heat transfer has been developed using a test surface fabricated from a thin foil heated electrically. If the heat capacity of the test surface is sufficiently low, the fluctuating temperature on the foil can be measured using high-frame-rate infrared thermography. This method, however, has an inherent problem in that the temperature on the test surface attenuates both in time and space due to thermal inertia and conduction. In the present study, the frequency response and the spatial resolution of a thin foil were examined analytically considering heat losses. In order to derive general relationships, non-dimensional variables of fluctuating frequency and spatial wavenumber were introduced to formulate the temporal and spatial amplitudes of the temperature on the test surface. Based on these relationships, the upper limits on the detectable fluctuating frequency and spatial wavenumber were successfully formulated using governing parameters of the measurement system. This enables us to evaluate quantitatively the reliability of the heat transfer measurement by infrared thermography. The values, evaluated here for the practical conditions, indicated that this measurement technique is promising for investigating the spatio-temporal behavior of heat transfer caused by flow turbulence.     

Analysis of Evaporation Heat Transfer of Thin Liquid Film in a Capillary of Equilateral Triangular Cross—Section

IntroductionThe study on evaPoration heat transfer in caPillmpbodies is imPOrtant for design and development of higIilyefficient heat transfer equipment, such as heat pipe. P.C.W and Y.K. Kao[i] stUdied the interline heattransfer coefficients of an evaPorating wetting film.L.SoloVyev and S.A. KovalevI2l discussed the mechAnsmabout evaPoration of a liquld film from a porous sdse.A. Schonberg and P.C. Wayner['l developed Shae's(l953)['l evaPoration heat transfer model that based ontem…     

Interferometry based whole-field heat transfer measurements of an impinging turbulent synthetic jet

The present work is concerned with exploring the potential of refractive index-based imaging techniques for investigating the heat transfer characteristics of impinging turbulent synthetic jets. The line-of-sight images of the convective field have been recorded using a Mach Zehnder interferometer. Heat transfer experiments have been conducted in infinite fringe setting mode of the interferometer with air as the working fluid. The effect of the excitation frequency of the synthetic jet on the resultant temperature distribution and local heat transfer characteristics has been studied. The fringe patterns recorded in the form of interferograms have first been qualitatively discussed and thereafter, quantitatively analyzed to determine the two-dimensional temperature field. Local heat transfer coefficients along the width of the heated copper block have been determined from the temperature field distribution thus obtained from the interferograms. The results have been presented in the form of interferometric images recorded as a function of frequency of the synthetic jet, corresponding two-dimensional temperature distributions and local variation of heat transfer coefficients. Interferometric measurements predicted maxima of the heat transfer coefficient at the resonance frequency of the synthetic jet and at a jet-to-plate surface spacing (z/d) of 3. These observations correlate well with the thermocouple-based measurements of temperature and heat transfer coefficient performed simultaneously during the experiments. The interferometry-based study, as reported in the present work for the first time in the context of synthetic jets, highlights the importance of refractive index-based imaging techniques as a potential tool for understanding the local heat transfer characteristics of synthetic jets.     

An alternative sensitivity method for a two-dimensional inverse heat conduction-radiation problem based on transient hot-wire measurements

An inverse approach based on hot-wire measurements has been proposed to simultaneously estimate radiative and conductive properties of metallic/ceramic foam samples, from a unique experiment. A theoretical model of two-dimensional transient conduction-radiation heat transfer in a cylindrical medium allowed to accurately simulate real temperature measurements in a hot-wire cell. Based on this model, the Levenberg–Marquardt method (LMM) has been adopted as the inverse technique. According to a sensitivity and correlation analysis, measuring points were suitably chosen and a proposed two-stage estimation technique was successfully performed. Due to the dimension of the model and to the number of identified parameters, a large computational time was required to calculate the sensitivity matrix. The Broyden’s combined update method has been adopted as an alternative sensitivity method, whose combination with the LMM reduced considerably the amount of computation. This reduction reached 69% for each of the studied samples.     

Characteristics of transient heat transfer during quenching of a vertical hot surface with a falling liquid film

An experimental study has been conducted to elucidate characteristics of transient heat transfer during quenching of a vertical hot surface with a falling liquid film. The experiment was done at atmospheric pressure for the following conditions: an initial surface temperature from 200 to 400^°C, a subcooling of 20– 80 K, average velocity of 0.52– 1.24 m/s, and the block material is copper and carbon steel. The surface temperature and heat flux are estimated from the measured temperatures in the block during the quench by a two‐dimensional inverse solution. It follows that as the position of wetting advances downward, the position at which the heat flux becomes a maximum also advances downward. The time at which the position of maximum heat flux begins to move is one of the most important parameters and can be predicted by a proposed correlation. In addition, it is revealed that the maximum heat flux for copper depends on the length to which it occurs from the leading edge. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 345– 360, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20167     

An engineering foundation for controlling heat transfer in one-dimensional transient heat conduction problems

This paper develops an engineering foundation for controlling heat transfer in one dimensional transient heat conduction problems based upon concepts borrowed from vibration control problems. The foundation distinguishes between modal control, distributed control, discrete control and direct feedback control and then singles out direct feedback control because its simplicity. An example demonstrates modal control and direct feedback control of the variation of the transient temperature field in a one dimensional slab geometry.     

Experimental results and a user-friendly model of heat transfer from a thin film resistance temperature detector

The research reported in this paper has focused on the different modes of heat transfer – conductive (to the substrate), conductive and convective (to the environment) and radiative (to the environment) – from an on-chip resistance temperature detector (RTD). The study has been carried out at various input voltages, various pressures ranging from atmospheric to vacuum, and for two classes of platforms for the device – thermal insulators (glass wool and ceramic), and a thermal conductor (aluminum block). The transient temperature–time response of the RTD under the various conditions stated above was recorded. A heat transfer model approximately accounting for all the modes of heat transfer was introduced. The calibration parameters of the model allowed us to quantify the different modes of heat transfer. The model uncovers the fact that the heat losses to the environment via conduction and convection are almost as much as the heat lost by radiation (radiative effects were unequivocally confirmed experimentally). Compared to these losses, conductive heat losses from the RTD to its underlying substructure are far more dominant (almost five times). We also give an analysis originating from the exact form of conservation of energy and demonstrate that the use of the simplified model has led to the most dominant heat transfer mode of conduction to the substrate being underestimated by no more than 7.89% (at the highest input power tested).     

X-ray fluorescence measurements of thin film chalcopyrite solar cells

X-ray fluorescence has turned out to be a very suitable and reliable tool for the characterization of thin film chalcopyrite solar cells. Besides the composition determination in atomic percent the total mass per unit square (mg/cm²) of the analyzed elements and the film thickness can be measured accurately. Furthermore, a real multi-layer analysis allows in addition to determine the CdS, ZnO and Mo thickness simultaneously with the absorber measurement. By the use of etching techniques, information about a vertical composition gradient can also be obtained. This work shows the possibilities and limitations of the X-ray fluorescence technique for the chalcopyrite solar cell characterization and emphasizes the advantages over the widespread electron probe microanalysis.