Heat Transfer Gauges Research Articles

1D analytic and numerical analysis of thin film heat transfer gauges and infra-red cameras in non-planar applications

The impulse response method is widely used for heat transfer analysis in turbomachinery applications. Traditionally, the 1D method assumes a: linear time invariant, isotropic, semi-infinite block with planar surfaces and does not accurately model the true geometric behaviour. This paper evaluates the error introduced by the planar assumption and outlines the required modifications for accurate freeform surface analysis. Adapted cylindrical basis functions are defined for the impulse response method and used to evaluate the impact of the 1D planar assumption. The analytic solutions for both convex and concave surfaces are presented. A penta-diagonal algorithm, for a modified numerical Crank-Nicolson scheme, is also evaluated for fast stable implementation of curved geometry simulation. The scheme shows comparable performance to the impulse response, whilst removing the requirement for linear time invariance. The new methods are demonstrated in the case of aerothermal analysis for heat transfer in a turbine nozzle guide vane. A 3D ANSYS simulation is used as a benchmark and further highlights the importance of the curvature effects. The methods are extended using curvature mapping for infra-red camera data, enabling direct 3D thermal simulation or the fast calculation of freeform curvature. This paper defines the methodology to analyse heat transfer measurements on non-planar geometry.

Heat-Transfer Gauge Arrays for Identifying Spot Initiation in Hypersonic Transitional Boundary Layers

The initiation and growth of turbulent spots in transitional hypersonic boundary layers can influence the transition length and the distribution of heat transfer and skin friction through the transitional region. There is debate in the literature about whether spots initiate in a small band at the start of transition or throughout the transitional region. This paper presents a new design of arrays of thin-film heat transfer specifically designed to detect whether spots initiate in a small band. The gauges were arranged in two spanwise rows at different streamwise locations and were tested on a 7° blunt cone in the T4 Stalker Tube. It is shown that such instrumentation is capable of identifying that some spots initiate between the rows when other spots have already formed upstream of the first row. This demonstrates that the breakdown of laminar boundary layers is not concentrated in a region of small streamwise extent in the current hypersonic flows.

1D analytic and numerical analysis of multilayer laminates and thin film heat transfer gauges

The impulse response method is widely used for heat transfer analysis in turbomachinery applications. Traditionally, the 1D method assumes a linear time invariant, isotropic, semi-infinite block and does not accurately model the behaviour of laminated materials. This paper evaluates the error introduced by the single layer assumption and outlines the required modifications for multilayer analysis. The analytic solution for an N layer, semi-infinite laminate is presented. Adapted multilayer basis functions are derived for the impulse response method and used to evaluate the impact of uniform, isotropic assumptions. A numerical solution to the laminate problem is also presented. A penta-diagonal inversion algorithm, for a modified Crank-Nicolson scheme, is evaluated for fast stable implementation of multilayer simulation. The scheme shows comparable performance to the impulse response, whilst removing the requirement for linear time invariance. The methods are demonstrated in the case of analysing a thin film gauge, used in laboratory analysis of heat transfer in a turbine nozzle guide vane. Thin film gauge manufacturing techniques have advanced significantly in recent years. Advanced multilayer constructions are now used however, post-processing commonly relies on outdated single layer methods. This paper provides a universal methodology, required to analyse modern-day multilayer heat transfer measurements.

Magnetohydrodynamic Experiments of Total Heat Flux Mitigation for Superorbital Earth Reentry

A set of experiments was conducted with the aim of evaluating total heat flux mitigation using magnetohydrodynamic flow control over ionized shock layers surrounding planetary entry vehicles. The University of Queensland’s X2 expansion tube was used to generate a flow condition representative of a Mars return trajectory. Spherical permanent magnets were fitted into a scaled test model based on NASA’s Stardust capsule. Thermocouples located at the stagnation point and shoulder of the test model, as well as thin-film heat transfer gauges located in the aft section, measured total heat flux for both magnetic and nonmagnetic experiments. High-speed imaging of the shock layer showed a significant increase in shock standoff when using the magnet, and polynomial fits were derived, describing standoff distance along the model’s forebody. Despite significant noise affecting the heat flux gauges signals for the magnetic experiments, a simple methodology allowed to compare heat flux during the useful test time. It was found that the applied magnetic field significantly reduces total heat flux at the stagnation point and shoulder of the test model, although heat flux to the aft section was found to significantly increase. Reasonable agreement with the numerical predictions was found for the aft section of the test model, but discrepancies were observed at the other locations.

Development of coated heat flux gauges for fast responding measurements

Thermal systems often exhibit transient behaviors that have important implications for the operation of the system and can be difficult to predict. For these reasons, experimental testing is often required to ensure system durability requirements are achieved. One important parameter governing the survivability of components in hot, high-stress environments is the heat flux into the part that dictates the temperature distribution for the component. However, sensors required to experimentally characterize heat fluxes in extreme environments must also be resilient. This study presents the development of coated heat transfer gauges capable of robust, high-frequency measurements in turbine research facilities. The addition of a protective coating increases the durability of the gauge, but inherent of that coating is the attenuation of high-frequency temperature penetrations. As a result, this study first outlines the use of analytical solutions to define a gauge design for a specific frequency range and heat transfer, ensuring that subsurface signals can be rectified to surface conditions through inverse methods. Then, the fabrication of polyimide substrate sensors with a parylene-F coating is described. Micro surface heaters added to the custom sensors were used to determine important geometric and thermal properties necessary to calculate accurate surface heat flux. Ultimately, this work shows increased sensor robustness in a turbine test bed and experimentally validates that the frequency response of the fabricated sensors meet the design intent.

Fast-Response transient heat flux measurements in a plasma wind tunnel

A fast response heat transfer gauge has been developed to measure the radial distribution and the fluctuations of heat flux in a supersonic arcjet plasma wind tunnel. The heat flux gauge consists of 3.2 mm diameter coaxial surface junction thermocouple which was positioned behind a thin layer of PTFE and mounted in a ESA standard 50 mm diameter flat faced copper probe head. A non-dimensional impulse response for the heat flux gauge was identified using pulsed optical power deposition from a laser. The impulse response was used in combination with reference measurements from a calorimeter at a single location to determine the distribution and fluctuations in heat flux. Eight traverses of the PWK4 jet at the University of Stuttgart confirmed a symmetric Gaussian-like heat flux distribution with a centreline heat flux consistent to within ±6%. These distributions were measured in less than one second, representing a significant gain in productivity when compared to calorimeter-based heat flux measurements where probes must be held stationary at a number of locations in order to resolve spatial distributions. At this particular PWK4 operating condition, which had a flow stagnation enthalpy of approximately 15 MJkg−1, heat flux fluctuations of up to ± 140 kWm-2 (corresponding to a relative variation of 15% of the centreline heat flux) were identified near the vicinity of the nozzle centreline for frequencies from 4 Hz to 1 kHz.

A review on thin film fast response heat transfer gauges

In this paper, an effort has been made to compile various research investigations done so far on fast response gauges for the short duration measurement in the transient state. The review paper is not only about summarization of all-important research work; rather, a detailed review on the development of the fast response gauges, its application, and various methods of heat estimation from the measured transient temperature history is also presented. Before discussing the development of fast response gauges, various heat flux measurement techniques and newer trends in heat transfer gauges are presented. The focus of this Review is mainly on the thin-film gauge, a robust fast response temperature detector best suited for the short duration measurement. The literature on the application of thin-film gauges for various engineering systems such as turbomachinery and aerodynamic heating systems is broadly categorized as single-layered and multi-layered gauges. The applications for the internal combustion engine and low heat flux measurement are also discussed later. The summary tables with studies arranged in chronological order are also provided. Finally, the estimation of the heat transfer rate from the measured transient temperature data is presented.

Analysis of nitrogen recombination activity on silicon dioxide with stagnation heat-transfer

Investigation of surface recombination activity of nitrogen on certain materials is crucial in the design of hypersonic re-entry vehicles. In the present work, an experimental study using a conventional shock tube facility has been conducted to investigate the phenomena of nitrogen recombination. Stagnation heat-transfer measurements were conducted behind reflected shock waves in a dissociated shock-tube flow for a mixture of 20% nitrogen and 80% argon. A platinum thin-film gauge was used to measure the stagnation heat-transfer in a shock heated gas mixture under a well-defined test condition. The surface of the heat-transfer gauge was coated with silicon dioxide (SiO2), the principal material used in thermal protection systems (TPS) for the hypersonic vehicles. Spectroscopic analyses were performed for in-depth characterization of the initial surface condition of the test specimen. Recombination activity behavior of nitrogen was evaluated by combining the heat-transfer data with catalytic heat transfer theories. The nitrogen recombination efficiency of silicon dioxide is found to be from 0.0026 to 0.035 at surface roughness factors of 1.01 to 1.10. Compared to the oxygen recombination activity, the nitrogen recombination activity showed a more than five times higher sensitivity to the roughness factor of the SiO2-coated surface.

Development of a Fast-Response Diamond Calorimeter Heat Transfer Gauge

A robust fast-response calorimeter heat transfer gauge called the Diamond Heat Transfer Gauge has been developed for use in transient hypersonic ground test facilities. Gauges have been produced using discs of synthetic diamond thick as calorimeters with platinum thin-film resistance temperature detectors on the rear surface to measure temperature rise. Depending on calorimeter thickness and the grade of diamond used, the 99% rise time of the gauges is between 10.6 and . Test-time heat fluxes have been measured with the new gauges on a sharp wedge model and a blunt-nosed wedge model in flows with total enthalpies of over multiple shots in an expansion tunnel. Stagnation point measurements exceeded . Experimentally measured heat fluxes are shown to agree with surface-junction coaxial thermocouples and an empirical correlation on the stagnation point.

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.

Dynamic calibration of temperature sensors from light rays for transient measurement

Measurement of transient heat fluxes in the applications involving very short duration of a heating environment which has been promised for the candidate by measurement of surface heating rates with thin film gauges (TFG). They are basically resistance temperature detectors having to measure in a very short duration of time. In the present study, a platinum based TFG has been fabricated and dynamically calibrated (radiation based) in the laboratory with a view to assessing the performance of platinum thin film gauges (PTFG) in the dynamic environment. These examinations are focusing to explore the probability of using TFG for small duration transient measurements with pure radiation mode of heat transfer. Radiation based heat flux is applied on the gauge by using the halogen bulb in a square box and its response is obtained through measured transient temperature. Subsequently, the surface heat fluxes are estimated by using radiation-based heat transfer. The purpose of this work is to statically calibrate each handmade heat transfer gauges by using quartz as substrate material deposited on platinum paste. This experiment has been carried out by oil bath based experimental technique. The similar experimental environment is also studied to observe the transient temperature response by using numerical simulation. The experiments are carried out by exposing the platinum TFG to various known step heat load of known input wattage, for the duration of 10 seconds. Then, the voltage signals are recorded due to change in temperature of air-flow past the TFG. The numerical simulation (ANSYS-Fluent v. 14.5) is performed in the similar experimental environmental conditions, for the same heating loads. Experimentally recorded temperature signals from the gauges are compared with simulated temperature histories obtained through finite element analysis. Cubic spline methods of the 1-D heat conduction equation are used to predict surface heat flux and compared with input heat loads. The presently developed calibration set-up is seen to very useful for radiation-based measurements of TFG.

Aerothermodynamic effects of controlled heat release within the hypersonic shock layer around a large angle blunt cone

Effects of controlled heat addition into the high temperature, chemically reacting shock layer of a large angle (60°) blunt cone with a spherical nose have been experimentally investigated. The exothermic oxidation of ablated chromium from the surface of the cone at hypersonic Mach numbers triggers the heat release process. A conical skirt with a base diameter of 70 mm and a semi-apex angle of 30°, culminating into a nose of radius 30 mm, has been used as the test model. An in-house hypersonic free-piston driven shock tunnel facility, HST3, was used for the experiments at a stagnation enthalpy of 6.31 MJ/kg and a freestream Mach number of 9.84. The temperature distribution in the shock layer was experimentally measured by the two-color ratio pyrometry technique, using a Digital Single Lens Reflex (DSLR) camera as a pyrometer. The temperature field was corrected for discrete radiative line emissions obtained through emission spectroscopy. Surface heat flux measurements were taken using carefully calibrated thin film platinum heat transfer gauges mounted on an insulating Macor substrate. Shock stand-off distances were measured through Schlieren imaging of the flow using a high-speed camera at 20 000 frames per second, and also by a new intensity-scan based method using the processed color image from the DSLR camera. Calculations showed a 173 K rise in the temperature of the gas layer in the stagnation region due to chromium oxidation. The net surface heat flux on the blunt cone was also found to increase by about 31 W/cm2. The shock stand-off distance, as ascertained from Schlieren images, increased from about 3.82 mm (±1.4%) to 4.45 mm (±1.5%), a 17% rise. Analytical calculations, taking chromium oxidation reaction kinetics into consideration, to relate the total exothermic heat release to its distribution into various processes demonstrated that chromium oxidation releases about 78 W/cm2 energy in the stagnation region of the shock layer. 1.9% of this energy increases the temperature of the gas layer, 40% is convected back into the airframe, 0.4% is lost from the rear of the Macor substrate by conduction, and about 8% is radiated into the model. The remaining 50% of the heat was used up in pushing the shock layer away from the body by raising its density, thereby increasing shock stand-off distance.

Design fabrication and satic calibration of thermocouple and thin film gauges

Thin film gauges are basically resistance temperature detector (RTD) sensors. It is a combination of high conducting (Sensor) and low conducting (Substrate) material. It operates on the simple principle that penetration of heat pulse from the high conducting material to the low conduction one is very small during the time span of the measurement. The present work is focused at design and fabrication of fast response thermocouples and thin film gauges (TFG) in the laboratory. Three types of thermocouples have been fabricated (E-type, E-type (Welding) and K-type) whereas platinum paste is deposited on the insulating substrate (Quartz) for thin film heat transfer gauge. The purpose of this work is to statically calibrate to each handmade heat transfer gauges by using oil bath technique. Both thermocouples and thin film gauges are calibrated under the same experimental conditions. The sensitivity of thin film gauge is found 268.919 % higher than the other sensing materials.

A New Transient High Heat Flux Convection Calibration Facility for Heat Transfer Gauges in High Enthalpy Flows

This paper presents a novel transient method for calibrating heat transfer gauges for convective wall heat flux measurements in high enthalpy flows. The new method relies on the transient heating of sensor substrates under rapid exposure to a hot flow in order to obtain the necessary reference heat flux. Compared to previous calibration facilities, the new facility is simple, inexpensive, easy to adapt for different flow configurations and sensor geometries, and quick to run across a wide range of conditions. In this paper, the design of the new calibration facility is described and the transient calibration method explained. The method is demonstrated by calibrating a Gardon gauge at convective heat transfer coefficients between 300 and 600 W/m2 K. Typical facility data and calibration results are presented in terms of voltage-heat flux sensitivity and calibration correction ratio. These results are shown to agree with theoretical estimates within the estimated calibration uncertainty.

Estimating Turbulent Spot Initiation Rates from Transition Lengths in Hypersonic Boundary Layers

Experimental evidence indicates that boundary layers in hypersonic flows transition from being laminar to turbulent via the mechanism of formation and propagation of turbulent spots. Turbulent spots have been detected using either surface heat transfer gauge signals or pressure traces, or they have been visualized using high-speed schlieren techniques. The extent of the transition zone is strongly dependent on the rate of initiation of turbulent spots and spot propagation parameters. However, it is not possible at present to predict the turbulent spot initiation rates in hypersonic transitional flows. As a first step toward developing an effective predictive model for the purpose, a methodology is developed to enable estimation of the spot initiation rate and intermittency distribution from experimental heat transfer data. The methodology is validated by comparison with experimental data where both the heat transfer and intermittency were measured. The methodology is then used in a predictive mode on three experimental datasets, and it is found to give good agreement. This enables prediction of other important variables as well, including skin friction and a wide variety of other boundary-layer properties in the transition zone at hypersonic Mach numbers.

On the response of coaxial surface thermocouples for transient aerodynamic heating measurements

Surface thermocouples are widely used in transient aerodynamic heating measurements, but their response often exhibits uncertainty and unpredictability, resulting in poor accuracy of measurement. To address this issue and provide reference information on their fabrication, the response of coaxial surface thermocouples was investigated numerically and experimentally. From the numerical simulations, it was observed that the heat blocking effect of the insulation layer can change the response of a thermocouple which strongly depends on the structure of the junction at short test times. Nevertheless, with increasing time, the response tends to be independent of the junction and be consistent with the prediction of the commonly used one-dimensional heat conduction model. Owing to the difficulty in controlling the junction, these observations not only account for the uncertainty and unpredictability of the response, but also suggest that for ensuring accurate measurements, a sufficiently long test time is necessary. The simulation also shows that the response of a thermocouple is insensitive to the properties of the insulation layer and that the duration of an uncertain response decreases dramatically with the thickness of the layer. To improve the performance of a surface thermocouple, additional effort should be directed at reducing the thickness of the insulation layer rather than enhancing its thermal properties. The shock tube experiments confirmed the achieved numerical results, and demonstrated a practical calibration technique for heat transfer gauges.

Performance assessment of thermal sensors during short-duration convective surface heating measurements

The determination of convective surface heating is a very crucial parameter in high speed flow environment. Most of the ground based facilities in this domain have short duration experimental time scale (~milliseconds) of measurements. In these facilities, the calorimetric heat transfer sensors such as thin film gauges (TFGs) and coaxial surface junction thermocouple (CSJT) are quite effective temperature detectors. They have thickness in the range of few microns and have capability of responding in microsecond time scale. The temperature coefficient of resistance (TCR) and the sensitivity are calibration parameter indicators that show the linear change in the resistance of the gauge as a function of temperature. In the present investigation, three of types of heat transfer gauges are fabricated in the laboratory namely, TFG made out of platinum, TFG made out of platinum mixed with CNT and chromel–alumel surface junction coaxial thermocouple (K-type). The calibration parameters of the gauges are determined though oil-bath experiments. The average value TCR and sensitivity of platinum TFG is found to be 0.0024 K−1 and 465 μV/K, while similar values of CSJT are obtained as, 0.064 K−1 and 40.5 μV/K, respectively. The TFG made out of platinum mixed with CNT (5 % by mass) shows the enhancement of TCR as well as sensitivity and the corresponding values are 0.0034 K−1 and 735 μV/K, respectively. The relative performances of heat transfer gauges are compared in a simple laboratory scale experiment in which the gauges are exposed to a sudden step heat load in convection mode for the time duration of 200 ms. The surface heat fluxes are predicted from the temperature history through one dimensional heat conduction modeling. While comparing the experimental results, it is seen that prediction of surface heat flux from all the heat transfer gauges are within the range of ±4 %.

Roughness-Induced Transition of Hypervelocity Boundary Layers

Experiments were performed in the T4 free-piston reflected shock tunnel to explore the transition process of hypervelocity boundary layers. The experiments took place on a 1000-m-long by 300-m-wide flat plate with a sharp leading edge at representative conditions pertaining to those of an unpublished test of a Mach 12 scramjet. A streamwise row of thin-film heat transfer gauges along the centerline of the plate was used to determine the state of the boundary layer. It was found that, without any boundary-layer trips, the boundary layer remained laminar for the entire length of the plate. Multiple combinations of trip geometry and size were then used in an attempt to force the boundary layer into turbulence. The most successful geometry was the diamond trip configuration, although transition did not occur directly behind the trip. Transition occurred after a period of relaminarization, with the length of the delay region agreeing well with the literature.

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.

Radiative and Total Heat Transfer Measurements to a Titan Explorer Model

This paper reports on the design and testing of a heat transfer gauge suitable for the spectrum of radiation encountered during entry to the atmosphere of Titan. Experiments were performed on a 70° sphere-cone in the X3 expansion tube at average freestream velocity, pitot pressure, static pressure, and density of 5:83 km=s, 76 kPa, 1.15 kPa, and 2:43 × 10 -3 kg=m 3, respectively. The radiation gauges used thin-film-sensing elements mounted behind a borosilicate window on the windward surface and were therefore shielded from the convective heat transfer. The assembled gauges are uniformly sensitive for radiation in the wavelength range from 0.347 to 2 mand give an integrated measurement of the total radiant flux in this band. Externally mounted thermocouple gauges were used to measure the total heat transfer and the ratio of radiant to the total heat flux found. The Titan atmosphere was simulated by a mixture of 5% methane (by volume) in nitrogen. Comparative tests using pure nitrogen confirmed that, without the methane content, negligible radiation was produced, which is consistent with cyanogen being the primary radiator. Results from this study demonstrate that the gauges and the superorbital expansion tube facility are useful resources for the study of nonequilibrium radiating flows.