Thin Film 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.

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.

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.

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.

Impulse Response Processing of Transient Heat Transfer Gauge Signals

A new, computationally efficient method is presented for processing transient thin-film heat transfer gauge signals. These gauges are widely used in gas turbine heat transfer research, where, historically, the desired experimental heat transfer flux signals, q, are derived from transient measured surface-temperature signals, T, using numerical approximations to the solutions of the linear differential equations relating the two. The new method uses known pairs of exact solutions, such as the T response due to a step in q, to derive a sampled approximation of the impulse response of the gauge system. This impulse response is then used as a finite impulse response digital filter to process the sampled T signal to derive the required sampled q signal. This is computationally efficient because the impulse response need only be derived once for each gauge for a given sample rate, but can be reused repeatedly, using optimized MATLAB filter routines and is highly accurate. The impulse response method can be used for most types of heat flux gauge. In fact, the method is universal for any linear measurement systems which can be described by linear differential equations where theoretical solution pairs exist between input and output. Examples using the new method to process turbomachinery heat flux signals are given.

Preliminary study of CVD diamond film to improve the performance of heat flux sensors for hypersonic experiments

The present work investigated the use of conventional hot filament chemical vapor deposition technique (HFCVD) diamond film as a coating on Platinum Thin Film Heat Transfer Gauges to improve their operation performance in the range of Mach 4 to Mach 15 airflow, in order to acquire accurate heat transfer data in hypersonic experiments. The gauge under development has a substrate made of sintered silicon nitride (Si 3N 4). It has cylindrical shape, with 5 mm diameter and 15 mm length. On it's top surface there is a platinum thin film, which is the heat flux-sensing element. Diamond films were deposited at 870 K and 6.5 kPa by HFCVD using a gas mixture of methane 1.0% or 1.5% vol. in hydrogen. For improving the adherence, films were grown onto Pt/Si 3N 4. A SiO 2 interlayer was also used to minimize the dissolution of carbon into the platdinum substrate during film growth. Scanning Electron Microcopy (SEM) has evidenced a faceted polycrystalline grains with orientation (111) and (100) covering the entire surface. Raman spectroscopy has demonstrated a film with high quality and low stress, by considering the small peak shift presents in the spectrum compared to natural diamond.

Boundary-Layer Transition Measurements in Hypervelocity Flows in a Shock Tunnel

Experiments to investigate the transition process in hypervelocity boundary layers were performed in the T4 free-piston shock tunnel. An array of thin-film heat-transfer gauges was used to detect the location and extent of the transitional region on a 1500 mm long x 120 turn wide flat plate, which formed one of the walls of a duct. The experiments were performed in a Mach 6 flow of air with 6- and 12-MJ/kg nozzle-supply enthalpies at unit Reynolds numbers ranging from 1.6 x 10(6) to 4.9 x 10(6) m(-1). The results show that the characteristics typical of transition taking place through the initiation, growth, and merger of turbulent spots are evident in the heat-transfer signals. A 2-mm-high excrescence located 440 turn from the leading edge was found to be capable of generating a turbulent wedge within an otherwise laminar boundary layer at a unit Reynolds number of 2.6 x 10(6) m(-1) at the 6-MJ/kg condition. A tripping strip, located 100 mm from the leading edge and consisting of a line 37 teeth of 2 rum height equally spaced and spanning the test surface, was also found to be capable of advancing the transition location at the same condition and at the higher enthalpy condition.

Turbulent spots in boundary layers in a free-piston shock-tunnel flow

In this paper, experiments to detect turbulent spots in the transitional boundary layers, formed on a flat plate in a free-piston shock tunnel flow, are reported. Experiments indicate that thin-film heat-transfer gauges are suitable for identifying turbulent-spot activity and can be used to identify parameters such as the convection rate of spots and the intermittency of turbulence.

Measurement of turbulent spot convection rates in a transitional boundary layer

A new technique, based on the analysis of the signature of turbulent spots on the heat flux at the wall, is used to estimate the propagation rates of naturally occurring turbulent spots in a zero pressure gradient transitional boundary layer. Experiments were performed in a compression-heated transient aerodynamic facility using a flat plate with 32 thin-film heat transfer gauges along its centerline. Simultaneous wide-bandwidth heat transfer measurements at a number of streamwise locations were used to track the coherent transitional activity along the plate. The free-stream turbulence intensity was about 0.5%, and tests were performed at unit Reynolds numbers in the range 1.8 × 106−4.2 × 106 m −1. Special data reduction techniques were used to calculate the convection velocities of the leading and trailing edges of the turbulent spots, and these results compared well with those for artificially generated spots. The intermittency distribution in the transition zone showed good agreement with the “universal” distribution of Narasimha, confirming the reliability of this technique.

A method for the accurate determination of the thermal product (ρck)1/2for thin film heat transfer or surface thermocouple gauges

Describes a novel method for determining the thermal product ( rho ck)1/2 for thin film or surface thermocouple type heat transfer gauges. The method utilises optical technique using a known power laser. The values of ( rho ck)1/2 so obtained compare favourably with those obtained by previous investigators.

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.

多層薄膜熱流束センサによる斜め衝撃波と乱流境界層との干渉域における熱伝達係数の測定

A new type of thin film heat transfer gauge is applied to the measurements of heat-transfer coefficients in the interaction regions of oblique shock waves and fully developed turbulent boundary layers. It has been developed for the measurements of heat flux with high spatial resolution and fast response for wind tunnels with long flow duration. Experiments have been performed under the conditions of Mach number=4, total pressure=1.2MPa, Tw/T0=0.59-0.65, Reynolds number=1.3-1.5×107, and incident shock angles from 17.8 degrees to 22.8 degrees. Flow fields are visualized by the Schlieren technique. Both distributions of surface pressure and heat-transfer coefficient are measured throughout the interaction regions by scanning the shock generator parallel to the free stream. Owing to the high spatial resolution of the new sensor, complicated features of the aerodynamic heating in the interaction regions are revealed.

多層薄膜熱流束センサの試作とそれによる新しい熱流束測定法に関する研究

A new method of measuring heat transfer rate with high spatial resolution and fast response is developed to be used in wind tunnels with long flow duration. For the development of the new method a multi-layered thin film heat transfer gauge is proposed. The principle of the sensor is based on measuring the temperature gradient across a thin, heat resistant layer of SiO with two, thin film resistance thermometers on its upper and lower surfaces. These thin films and the heat resistant layer are deposited by vacuum evaporation technique. The design consideration, the accuracy of the sensor, frequency response, and the calibration technique are discussed. The sensor is applied to the measurement of the heat transfer rate in turbulent boundary layer on a flat plate at Mach number 4, Tw/T0= 0.64, and Re=1.4×107, and shows high sensitivity and fast response. It also shows excellent durability.

Short Duration Measurements of Heat-Transfer Rate to a Gas Turbine Rotor Blade

The paper describes the results of an experimental study of the effect of Mach number, Reynolds number, inlet flow angle, and free-stream turbulence level on heat transfer rate to a gas turbine rotor blade. The measurements were made in the VKI short-duration isentropic light piston tunnel using thin film heat transfer gages painted on a machinable ceramic blade of 80 mm chord and 100 mm height. The tests were performed for three cascade inlet Mach numbers: 0.62, 0.92, 1.15; inlet unit Reynolds number was varied from 0.3 × 107 m−1 to 1.2 × 107 m−1; the inlet flow angle from 30 to 45 deg (for an inlet blade angle of 30 deg); the turbulence level from 0.8 percent to approximately 5 percent. The effect of changing these parameters on boundary layer transition and separation, on leading edge and average heat transfer to the blade was examined. For typical situations, experimental blade heat distributions were compared with boundary layer predictions using a two-equation closure model.

Measurement of Heat Flux and Pressure in a Turbine Stage

Selected portions of the first stage stationary inlet nozzle, shroud, and rotor of the AiResearch TFE 731-2 turbine were instrumented with thin-film heat-transfer gages and heat-flux measurements were performed using a shock tunnel as a source of high-temperature, high-pressure gas. Experiments were performed over a range of Reynolds numbers, based on mid-annular stator chord, from 1.6 × 105 to 3.1 × 105 and corrected speeds from approximately 70–106 percent. The full-stage heat-flux results are compared to our previous measurements obtained with a stator only, in the absence of a rotor. The previous results are in good agreement with the full-stage data for the tip end-wall region, but they are significantly less than the full-stage results for the stator airfoil. Pressure measurements were obtained throughout the model and these results are shown to be in excellent agreement with the steady-state rig data supplied to us by AiResearch for this turbine. Heat-flux measurements are also presented for the stationary shroud as a function of rotor mid-annular chord. The shroud heat-flux data are shown to be in excess of the rotor blade results. Rotor-tip heat-flux measurements are shown to be slightly greater than the shroud results.

Measurement of Heat-Transfer Rate to a Gas Turbine Stator

A sector of the first stage stationary inlet nozzle of the AiResearch TFE-731-2 engine was instrumented with thin-film heat-transfer gages and experiments were performed to obtain detailed heat-transfer rate distributions. It is shown that the experimental apparatus can be used to study total-pressure losses in cascades. The experimental apparatus consists of a helium-driven shock tube, as a short-duration source of high-temperature high-pressure gas, driving a nozzle-test-section mounted near the exit of a primary shock-tunnel nozzle and extending into the shock-tunnel receiver tank. The nozzle-test-section device consists of a forward transition section with a circular opening facing the supersonic primary nozzle flow and with the external shape of a frustum of a cone. Internal contouring is provided to transform the circular-section subsonic intake flow into one filling a 176 deg annular segment having a geometry approximating that of the entrance to the turbine stator stage in a turbojet.

Shock tube study of boundary layer instability

An experimental study of the spatio-temporal evolution of the transition in a shock-tube wall boundary-layer has been carried out with thin film heat transfer gauges placed all along the tube. The results show that the transition appears, for small initial pressures, under the form of turbulent spots which are periodically created: these spots grow and are transported roughly at a velocity slightly smaller than the inviscid flow, regressing towards the contact surface. For higher initial pressures, a continuous transitional front is created which moves at about the same velocity as the shock-wave. These results seem to explain previous and often contradictory interpretations based on quasi-punctual measurements but the fact remains that the characterization criteria of the transition in unsteady boundary layers are still to be studied.

Development of a hypersonic shock tunnel

The design, development and calibration of a shock tube/tunnel is described in this paper. Its main features are: (1) test section size: 30 cm dia jet enclosed in a 30 cm × 30 cm square test section; (2) diameter and overall length of shock tube: 50 mm and 5.63 m respectively; (3) test section Mach number range: 4 to 13; (4) stagnation temperature simulation: upto 8000°K. This facility has been operated in a ‘straight-through’ as well as ‘reflected’ mode. Instrumentation like piezo-electric pressure gauges, thin film heat transfer gauges along with electric analogue net works have been used extensively to calibrate the facility as well as to measure the flow variables inside the shock tube and in the test section. Re-entry heating has also been simulated.