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.     

Conduction based calibration of handmade platinum thin film heat transfer gauges for transient measurements

Heat transfer measurement using thin film gauges (TFG) is the most prevalently used technique for determination of surface heat flux. They are best suited for short duration transient surface temperature measurements and typically used in the applications where convection is a dominant mode of heat transfer such as gas turbine engines, high speed flights etc. However, in few interdisciplinary research areas, there are practical issues and difficulties in exposing the gauges for convection based measurements. These present investigations are aimed at exploring the possibility of using thin film gauges for short duration conduction based transient measurements with pure conduction mode of heat transfer. A simple calibration set-up has been used to supply known heat flux of different magnitudes to the thin film gauges that are fabricated in-house with platinum as sensing element and pyrex as an insulating substrate. Experimentally recorded temperature signals from the gauges are compared with simulated temperature histories obtained through finite element analysis. Convoluted integral of one-dimensional heat conduction equation is used to predict the surface heat flux and compared with input heat loads. The presently developed calibration setup is seen to be very useful for conduction based measurements of thin film gauges.     

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.     

Analytical solution of the parabolic and hyperbolic heat transfer equations with constant and transient heat flux conditions on skin tissue

In this article, the parabolic (Pennes bioheat equation) and hyperbolic (thermal wave) bioheat transfer models for constant, periodic and pulse train heat flux boundary conditions are solved analytically by applying the Laplace transform method for skin as a semi-infinite and finite domain. The bioheat transfer analysis with transient heat flux on skin tissue has only been studied by Pennes equation for a semi-infinite domain. For modeling heat transfer in short duration of an initial transient, or when the propagation speed of the thermal wave is finite, there are major differences between the results of parabolic and hyperbolic heat transfer equations. The non-Fourier bioheat transfer equation describes the thermal behavior in the biological tissues better than Fourier equation. The outcome of transient heat flux condition shows that by penetrating into the depths beneath the skin subjected to heat, the amplitude of temperature response decreases significantly. The blood perfusion rate can be predicted using the phase shift between the surface temperature and transient surface heat flux. The thermal damage of the skin is studied by applying both the parabolic and hyperbolic bioheat transfer equations.     

Measurements and analysis of load and speed effects on the instantaneous wall heat fluxes in a direct injection air‐cooled diesel engine

This work presents an experimental analysis which is carried out to study the instantaneous heat fluxes, during the engine cycle, in the cylinder head and exhaust manifold of a direct injection, air‐cooled, four‐stroke diesel engine. For temperature measurements, a new pre‐amplification unit for fast response thermocouples, appropriate heat flux sensors and an innovative, object‐oriented, control code for fast data acquisition have been designed and developed at the authors' laboratory. The experimental installation separates the engine transient temperature signals into two parts; namely the ‘long’‐ and the ‘short’‐term response ones; followed by their discrete processing in two independent data acquisition systems. One‐dimensional heat conduction with Fourier analysis of the raw temperature data are implemented in order to calculate the instantaneous engine combustion chamber and exhaust pipe heat fluxes. This study concentrates on the correct interpretation of the measured temporal variations of heat fluxes and the examination of the effect of engine load and speed on the cylinder head and exhaust manifold heat flux losses. Many interesting aspects of transient engine heat transfer are revealed. The simultaneous presentation of heat fluxes on the cylinder head and exhaust manifold, together with the engine indicator diagram, sheds light into the mechanisms governing transient heat transfer during an engine cycle. Copyright © 2000 John Wiley & Sons, Ltd.     

Experimental evaluation of local instantaneous heat transfer characteristics in the combustion chamber of air-cooled direct injection diesel engine

An experimental analysis is conducted investigating the differences between the variations of overall and local instantaneous heat transfer coefficients, during the engine cycle, in the combustion chamber walls of a direct injection (DI), air-cooled diesel engine located at the authors’ laboratory. For this purpose, a novel experimental installation is developed, which separates the engine transient temperature signals into two parts, namely the long- and the short-term response ones, processed in two independent data acquisition systems. Moreover, a new pre-amplification unit for fast response thermocouples, appropriate heat flux sensors and an object-oriented control code for fast data acquisition have been designed and applied. Experimentally obtained cylinder pressure diagrams are used as a basis for the calculation of the overall heat transfer coefficients, whereas one-dimensional heat conduction theory with Fourier analysis techniques, combined with an iterative procedure between calculated and measured temperature data, are implemented in order to calculate the instantaneous local heat transfer coefficients in the engine cylinder. Analysis of the experimental results reveals interesting aspects of transient engine heat transfer. Significant differences are disclosed between the overall and local heat transfer coefficient variations, with the importance of the latter one on engine design being emphasized. The local heat transfer coefficient on the cylinder head is quantified based on the experimental data. The effect of engine speed and load as well as of the air swirling motion on the heat transfer variations are presented. From the analysis results it is concluded that the instantaneous heat transfer variation is non-uniform, unlike its values calculated from standard correlations that assume spatial uniformity, noting that such information, especially for air-cooled diesel engines, seems to be very scarce in the open literature.     

Base heat flux measurement techniques in impulse hypersonic test facilities

Safe and efficient hypersonic missions require exhaustive testing to capture the surface heat flux used in optimizing thermal protection systems. Though low, base heat transfer rates are still relevant and their estimation is of the utmost importance. Impulse hypersonic test facilities like the shock tunnel are commonly used for such estimation tests. The present paper compares contemporary heat flux measurement techniques, namely, E‐type coaxial thermocouples, Pt‐thin films, and atomic layer thermopiles in a hypersonic shock tunnel at the base of a scaled‐down re‐entry capsule. Base flow establishment is ensured using pressure measurements. The measurements indicated that the base‐stagnation point heat flux was approximately 2% to 3% of the forebody stagnation heat flux and it stabilized at an approximate nondimensional establishment time of 43.2 ± 2.4.     

Underhood thermal management: Temperature and heat flux measurements and physical analysis

Aerodynamic cooling drag, caused by car underhood cooling, can be reduced by better underhood aerothermal management. This study addresses the aerothermal phenomena encountered in the vehicle underhood compartment by physical analysis of the heat transfer modes in complex internal flows. We report here underhood heat flux and temperature measurements on a vehicle in wind tunnel S4 of Saint-Cyr-France using a new experimental method. The underhood is instrumented by 40 surface and air thermocouples and 20 fluxmeters. Measurements are carried out for three thermal functioning points: the engine in operation and the front wheels positioned on the test facility with power-absorption-controlled rollers. The ultimate aim is to reengineer the underhood architecture so as to reduce the cooling air flow rate in the underhood component and hence the aerothermal cooling drag.     

Nonequilibrium heat conduction in a nanofluid layer with periodic heat flux

Nonequilibrium heat conduction in a nanofluid layer with periodic heat flux on one side and specified temperature on the other side is studied numerically. The energy equations for the nanoparticles and base fluid are nondimensionalized and the problem is described by four dimensionless parameters: heat capacity ratio, volume fraction of nanoparticles, period of surface heat flux, and the Sparrow number. The Sparrow number is to describe the coupling between the energy equations for nanoparticles and base fluid. Nonequilibrium between nanoparticles and base fluid, as well as heat transfer enhancement in nanofluid, of three nanofluids (diamond–water, diamond–ethylene glycol, and copper–ethylene glycol) is investigated. The results showed that the nonequilibrium between the nanoparticles and base fluid exist for all three nanofluids at low Sparrow number and short period of surface heat flux. The results also showed that heat transfer in a liquid layer can be enhanced by adding nanoparticles to the base fluid, but the level of enhancement is not as high as those reported by using transient hot wire (THW) method.     

Experimental validation of CFD modelling for heat transfer coefficient predictions in axial flux permanent magnet generators

This paper describes the experimental validation of CFD modelling for heat transfer coefficients in an axial flux permanent magnet (AFPM) generator. A large scale low speed test rig was designed and constructed. The geometric parameters and the rotational speed of the test rig were determined by dimensional analysis, to ensure the flow characteristic remains unchanged as compared with commercial AFPM generators. The heat transfer coefficients in the test rig were measured at rotational Reynolds number, Re_ω from 0 to 2 × 10⁶, non-dimensional flow rate, C_w up to 11,000 and gap ratio, G = 0.016, by using the combination of heat flux sensors and thermocouples. Due to the large size of the scaled-up rig, natural convection played a significant part in the heat transfer and this had to be compensated for in the forced convection heat transfer coefficient calculations. Extra experiments were designed and conducted to identify the effect of natural convection on the machine’s cooling. The experimentally determined results were compared to heat transfer coefficients predicted by CFD models and good agreement was obtained.     

Transient heat transfer in a heat‐generating fin with radiation and convection with temperature‐dependent heat transfer coefficient

The transient heat transfer in a heat‐generating fin with simultaneous surface convection and radiation is studied numerically for a step change in base temperature. The convection heat transfer coefficient is assumed to be a power law function of the local temperature difference between the fin and its surrounding fluid. The values of the power exponent n are chosen to include simulation of natural convection (laminar and turbulent) and nucleate boiling among other convective heat transfer modes. The fin is assumed to have uniform internal heat generation. The transient response of the fin depends on the convection‐conduction parameter, radiation‐conduction parameter, heat generation parameter, power exponent, and the dimensionless sink temperature. The instantaneous heat transfer characteristics such as the base heat transfer, surface heat loss, and energy stored are reported for a range of values of these parameters. When the internal heat generation exceeds a threshold the fin acts as a heat sink instead of a heat source. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21012     

Transient effects in natural convection cooling of vertical parallel plates

This work presents results from a numerical study of transient natural convection between vertical parallel plates. Two boundary conditions – uniform wall temperature and uniform heat flux – are considered. Results presented include the rate of heat transfer for uniform wall temperature and the maximum wall temperature for uniform heat flux. Also presented are simple correlations to calculate the minimum heat transfer and the maximum wall temperature during the transient period. It is found that for uniform wall temperature the ratio of the minimum heat transfer to the steady state heat transfer decreases with length of the channel, and for uniform heat flux the maximum transient temperature has a maximum of about 9% over the steady state temperature.     

Determination of skin temperature distribution and heat flux during simulated fires using Green's functions over finite-length scales

One of the current practices for measuring heat flux during flash fire testing, forest fires, and other industrial cases focuses on the use of semi-infinite models to predict the heat flux during exposure through surface temperature measurements on simulated skin sensors. For short time frames, these models can be shown to have acceptable accuracy. However, when considering longer time exposures at reduced heat fluxes, such as with firefighters in a forest fire, the accuracy of these models could be brought into question. A one-dimensional, finite length scale, transient heat conduction model was developed using a Green's function approach on a rectangular sensor. The model was developed using transient temperature boundary conditions to avoid the use of complicated radiation and convection conditions at each boundary. For comparison, a semi-infinite model utilizing the same boundary condition on the exposed face was solved using both the Laplace transform method and Green's function method. Experimental data was obtained during exposure to a cone calorimeter. All measurements were taken for a minimum duration of 2 min. This temperature data was used to develop appropriate curves for the boundary conditions and validate the analytical models. It was found that the temperature obtained from the one-dimensional transient heat conduction model based on Green's functions agreed well with the experimental results over longer exposure times, and with reduced error when compared with the semi-infinite model. This suggests that modeling the problem on a finite-length scale will produce more accurate or more conservative temperature and heat flux results over extended periods of exposure in high heat load applications.     

Inner cylinder diameter effects on free convection heat transfer in a stable stratified fluid

An experimental study was performed to identify the effects of the inner cylinder diameter on free convection heat transfer in a stable stratified fluid between coaxial cylinders. The stratified layer was formed using a sucrose aqueous solution, and was heated from the outer cylinder (70 mm I.D.) at a constant heat flux and cooled from the inner cylinder at a constant temperature. The experiment was conducted for an inner cylinder of 50.8 mm O.D. and the results were compared with those for an inner cylinder of 30 mm O.D., which were previously reported by the authors. The results show that the inner cylinder size affects the heat transfer in the early stage of heating: increasing the diameter of the inner cylinder decreased the distance between the heating and cooling surfaces. It consequently reduced both the duration of heat conduction prevailing and the resulting temperature increase of the heating surface during that period. Increasing the diameter of the inner cylinder also made the heat transfer characteristics similar to those between vertical plates. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(3): 172– 186, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20148     

Laminar free convection from a vertical plate with uniform surface heat flux in chemically reacting systems

Two-dimensional steady laminar free convection from a vertical plate with uniform surface heat flux rate is studied in a gas where a reversible very fast reaction of dissociation A↔2B takes place at atmospheric pressure. The effective thermophysical properties of the gas in the interval of dissociation are evaluated and the governing boundary-layer equations are solved numerically by a finite-difference method with control volume formulation for a wide range of values of the independent variables which have a significant influence on the phenomenon. In the case of undisturbed fluid temperature T_∞ smaller than T_0.5, corresponding to a rate of dissociation α=0.5, three different heat transfer regimes, marked by two critical heat fluxes, may be distinguished as the surface heat flux rate increases. The theoretical results obtained for the critical heat fluxes as well as the coefficient of convection are expressed in terms of correlations among dimensionless parameters defined through the mixture effective properties.     

Local convective boiling heat transfer and pressure drop of nanofluid in narrow rectangular channels

This paper reports an experimental study on convective boiling heat transfer of nanofluids and de-ionized water flowing in a multichannel. The test copper plate contains 50 parallel rectangular minichannels of hydraulic diameter 800 μm. Experiments were performed to characterize the local heat transfer coefficients and surface temperature using copper–water nanofluids with very small nanoparticles concentration. Axial distribution of local heat transfer is estimated using a non-intrusive method. Only responses of thermocouples located inside the wall are used to solve inverse heat conduction problem. It is shown that the distribution of the local heat flux, surface temperature, and local heat transfer coefficient is dependent on the axial location and nanoparticles concentration. The local heat transfer coefficients estimated inversely are close to those determined from the correlation of Kandlikar and Balasubramanian [An extension of the flow boiling correlation to transition, laminar and deep laminar flows in minichannels and microchannels, Heat Transfer Eng. 25 (3) (2004) 86–93.] for boiling water. It is shown that the local heat flux, local vapor quality, and local heat transfer coefficient increase with copper nanoparticles concentration. The surface temperature is high for de-ionized water and it decreases with copper nanoparticles concentration.     

Forced convection heat transfer from a heated circular cylinder with arbitrary surface temperature distributions

A Fredholm-type boundary integral expression for evaluation of the forced convection heat transfer from an object with arbitrary surface temperature distributions is proposed. The Fredholm kernel function for a heated circular cylinder was calculated by numerical simulation of the forced convection fields, and then generalized heat transfer coefficients for arbitrary surface temperature distributions were defined. By use of the generalized heat transfer coefficients, it is shown that the difference in local heat transfer characteristics between the case of an isothermal cylinder and that of a uniform heat flux one can be interpreted only as the difference of the surface temperature distributions. Moreover, the mechanism of the effect of the surface temperature distribution on the characteristics of forced convection heat transfer from a cylinder is clarified in detail through the generalized heat transfer coefficients. © 1999 Scripta Technica, Heat Trans Asian Res, 28(6): 484–499, 1999     

Recommendations and guidelines for the performance of accurate heat transfer measurements in rapid forming processes

The published requirements for accurate measurement of heat transfer at the interface between two bodies have been reviewed. A strategy for reliable measurement has been established, based on the depth of the temperature sensors in the medium, on the inverse method parameters and on the time response of the sensors. Sources of both deterministic and stochastic errors have been investigated and a method to evaluate them has been proposed, with the help of a normalisation technique. The key normalisation variables are the duration of the heat input and the maximum heat flux density. An example of application of this technique in the field of high pressure die casting is demonstrated. The normalisation study, coupled with previous determination of the heat input duration, makes it possible to determine the optimum location for the sensors, along with an acceptable sampling rate and the thermocouples critical response-time (as well as eventual filter characteristics). Results from the gauge are used to assess the suitability of the initial design choices. In particular the unavoidable response time of the thermocouples is estimated by comparison with the normalised simulation.     

CFD analysis of heat transfer in hexagonal subchannels of super fast reactor in upward flow

A super fast reactor is a fast spectrum, supercritical, water‐cooled reactor. This paper represents CFD analysis of heat transfer in hexagonal subchannels of super fast reactor using FLUENT in ANSYS. The numerical simulation of grid stability was done by considering different mesh sizes and the turbulence model for heat transfer of supercritical water was also carried out and compared with the experimental data. RNG k‐? turbulence model with enhanced wall treatment was considered for simulations. Heat transfer and heat generation rate analysis of the outer surface rod wall is carried out with different subchannels by changing various parameters like boundary conditions and pitch‐to‐diameter ratio. The analyses reveal that the outer surface of the rod wall temperature decreases with increase in pitch‐to‐diameter ratio. Maximum coolant temperature rises in edge subchannels more than corner subchannels. Further analysis is carried out with different mass fluxes. Increases in mass flux has minimal effect on the maximum rod wall surface temperature. Maximum cladding surface temperature for the corner subchannel is less compared to the edge subchannel. Heat generation rate also decreases with increase in pitch‐to‐diameter ratio. This paper also investigates the buoyancy effect on subchannels with varying heat flux as boundary conditions considering constant mass flux.     

Experimental and theoretical investigation of surface temperature non-uniformity of spray cooling

In this study, a test system for spray cooling, in which the heating surface temperatures were simultaneously measured by thermocouples and an infrared imager, was set up. A mathematical model of spray cooling heat transfer characteristics was presented based on the fundamentals of dynamics and heat transfer. The temperature distribution on the heating surface was investigated by the experimental and theoretical methods, the surface temperature non-uniformity and its influencing factors were analyzed. The predictions by the model coincided with the experimental results well, and a comparison was demonstrated with a deviation below 10%. It can be concluded that the surface temperature non-uniformity is influenced by the spray characteristics, nozzle-to-surface distance, inlet pressure, heat flux, spray angle and the system pressure. In the case of the same heat flux, the surface temperature non-uniformity can be reduced by the small spray angle, low system pressure, low nozzle-to-surface distance, and the high inlet pressure.