Thin Film Heat Flux Gauges Research Articles

Unsteady relationships between instantaneous surface heat flux, instantaneous surface temperature, and tracked shock wave phenomena

Investigated are unsteady relationships between surface heat flux variations, surface temperature fluctuations, and tracked shock wave phenomena. The level of coherence and time lag between events at different flow locations are used to quantify the associated interactions at different frequencies. With this approach, time-varying surface heat flux and surface temperature fluctuations are used to track events at different flow locations relative to unsteadiness associated with the test section inlet and relative to unsteadiness associated with different shock wave phenomena. Employed for the investigation is a specialty test section with an inlet Mach number of 1.54, which contains a normal shock wave, a lambda foot (which is comprised of an upstream oblique shock wave leg, and a downstream oblique shock wave leg), and a flow separation zone beneath the lambda foot. High and low unsteady Mach wave intensity levels are produced at the test section inlet by different numbers and different amounts of unsteadiness of families of intersecting and interacting Mach waves, as these are located within and just downstream of the test section entrance. When correlated with respect to normal shock wave tracked locations, values and magnitudes of the most highly correlated frequency bands are different depending upon the level of test section inlet unsteady Mach wave intensity, and the location where unsteady flow events originate. With low freestream unsteady Mach wave intensity at the test section inlet, unsteady events are generally present at normal shockwave locations prior to times when their influences are present at thin film temperature and heat flux gage surface locations. The presence of high unsteady Mach wave intensity at the test section inlet produces an environment wherein the low Mach wave intensity inlet flow is overwhelmed by highly active Mach wave fluctuations, when they are present, such that the normal shock wave is no longer a primary originating source of flow unsteadiness.

High-Sensitive Thin Film Heat Flux Gauge With ITO/In2O3 Thermopile on Nickel Alloys for Turbine Blade Applications

Heat flux is key parameter for evaluating the heat dissipation of turbine blade. The accurate measurement of heat flux is vital for the optimizing cooling system and fabrication of turbine blade. The thin film heat flux gauge is usually fabricated on insulated substrate and located away from the measured area of turbine blade. As a result, the changes of heat flux on the surface of turbine blade cannot be reflected quickly and accurately. This paper innovatively designs membrane structure of thin film heat flux gauge on nickel alloys. The sensor is fabricated on the surface of the nickel alloys by thin-film depositing technology to realize in-situ surface heat flux measurement, and ITO/In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thermopile is used to replace precious thermopile to improve sensitivity. The results show that the sensitivity and response frequency of the sensor respectively are measured to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$61.93~\mu \text{V}$ </tex-math></inline-formula> /(kW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and over 14400Hz respectively, which can measure heat flux of 236.4kW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$888^\circ \text{C}$ </tex-math></inline-formula> . Besides, the thin film heat flux gauge is fabricated on the surface of turbine blade to verify functional feasibility with same preparation process of that on nickel alloys, and can obtain reproducible results. This work suggest that the thin film sensor based on ITO/In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thermopile has great potential in applications used as optimizing cooling system of components in aeronautics and astronautics.

Unsteady phenomena at the combustor-turbine interface

The combustor-turbine interface in a gas turbine is characterised by complex, highly unsteady flows. In a combined experimental and large eddy simulation (LES) study including realistic combustor geometry, the standard model of secondary flows in the nozzle guide vanes (NGV) is found to be oversimplified. A swirl core is created in the combustion chamber which convects into the first vane passages. Four main consequences of this are identified: variation in vane loading; unsteady heat transfer on vane surfaces; unsteadiness at the leading edge horseshoe vortex, and variation in the position of the passage vortex. These phenomena occur at relatively low frequencies, from 50–300 Hz. It seems likely that these unsteady phenomena result in non-optimal film cooling, and that by reducing unsteadiness designs with greater cooling efficiency could be achieved. Measurements were performed in a high speed test facility modelling a large industrial gas turbine with can combustors, including nozzle guide vanes and combustion chambers. Vane surfaces and endwalls of a nozzle guide vane were instrumented with 384 high speed thin film heat flux gauges, to measure unsteady heat transfer. The high resolution of measurements was such to allow direct visualisation in time of large scale turbulent structures over the endwalls and vane surfaces. A matching LES simulation was carried out in a domain matching experimental conditions including upstream swirl generators and transition duct. Data reduction allowed time-varying LES data to be recorded for several cycles of the unsteady phenomena observed. The combination of LES and experimental data allows physical explanation and visualisation of flow events.

Application of 3-omega method for thin-film heat flux gauge calibration

Double-sided thin-film resistance temperature detector (RTD) heat flux gauges (HFGs) are commonly used to characterize heat transfer rates in high-heat flux environments with complex flow features. These gauges comprise two thin-film RTDs on opposing sides of a dielectric. To deduce accurate heat flux, the RTDs must be properly calibrated and the material properties of the dielectric must be characterized. This study presents a complete gauge characterization method for sensors of this type by applying standard calibration procedures with specially-designed RTDs capable of utilizing the 3-omega method. The 3-omega method quantifies the thermal conductivity and thermal product of a material by measuring the response of a specially designed heater/thermometer deposited on the substrate. This study shows the 3-omega method enables RTD calibrations and thermal property determination over a range of temperatures for individual gauges, reducing the uncertainty in calculated heat flux. Although the method is quite general, this study utilized platinum RTDs with a polyimide dielectric, which is common in turbomachinery applications. The thermal properties obtained through this method agree with previous characterization efforts; however, discrete characterization of seven gauges shows that gauge-to-gauge variation in the dielectric could influence measured heat flux by as much as 30%. This study also builds the framework to characterize the thermal conductivity of the adhesive layer beneath the gauge which is necessary to mount the sensors to the test article. Although often uncharacterized, the adhesive thermal conductivity has a significant impact on matching experimental measurements to simulations. Additionally, this study found that if the thermal conductivity of the dielectric is constant (an assumption that holds for the present study), an in-situ RTD calibration can be performed. In-situ RTD calibration and traditional method RTD calibration agreed to within 0.1%. Overall, this work has practical implications in obtaining high quality measurements from HFGs of this type.

PHANTOM COOLING EFFECTS ON ROTOR BLADE SURFACE HEAT FLUX IN A TRANSONIC FULL SCALE 1+1/2 STAGE ROTATING TURBINE

Abstract An experimental and numerical investigation of phantom cooling effects on cooled and uncooled rotating high pressure turbine blades in a full scale 1+1/2 stage turbine test is carried out. Objectives set to capture, separate, and quantify the effects of upstream vane film-cooling and leakage flows on the downstream rotor blade surface heat flux. Multiple series of tests were carried out in the Air Force Research Laboratory, Turbine Research Facility, at Wright-Patterson Air Force Base, Ohio. A non-proprietary research turbine test article is uniquely instrumented with high frequency double-sided thin film heat flux gauges custom made at AFRL. High bandwidth, time resolved surface heat flux is measured on multiple film-cooled and non-film-cooled HPT rotor blades downstream of both film-cooled and non-film-cooled vane sectors. Upstream wake passing and heat flux is characterized on both rotor pressure and suction side surfaces, along with quantifying rotor phantom cooling effects from non-uniform 1st stage vane film cooling and leakage flows. Fast response heat flux measurements quantify how rotor phantom cooling impacts the blade pressure side greatest; increasing along the pressure side towards the trailing edge. It is discovered upstream vane film-cooling alone can account for 50% of the rotor blade cooling effect, and even outweigh the rotor blade film cooling effect far from the blade showerhead holes. Added unsteady numerical simulation demonstrates how variations in inlet total temperature and incidence angle can also contribute to circumferentially non-uniform rotor heat flux.

The influence of spatial arrangement of endpoints on output characteristics of ITO/In2O3 heat flux gauge

Thermopile is a common test structure, in which the distribution of hot and cold endpoints has a high impact on its performance especially for applications in thin-film heat flux gauge. In this work, the influence of spatial distribution of endpoints is investigated on the output characteristics of the thin film heat flux gauge based on ITO/In2O3 thermopile. Three different structures of the heat flux gauges, which are horizontal, vertical and stepped, are designed and fabricated via sputtering according to different spatial arrangement of endpoints. The results show that the heat flux gauges based on vertical distributed structures have higher sensitivities than the gauges based on the other two structures on a convection heat source calibration platform. The output sensitivities of the gauges based on stepped distributed structure is the highest on laser radiation calibration platform, which reach 74.9 μV/(kW/m2). The thin-film heat flux gauge based on horizontal distribution structure has the best dynamic characteristic on a laser heat source calibration system. The operating temperature of the gauge is close to 800 °C, and the dynamic frequency reaches 5.2 kHz.

Numerical modelling of thin film gauge for short duration thermal measurement

Abstract The numerical study of direct heat conduction through thin film heat flux gauge is carried out to see how closely it resembles with short duration experimental thermal data. The thermal data obtained from transient impulse facilities at Von Karman Institute, Belgium (case1) and Oxford University (case2) is taken from literature as a reference for validation purpose. The numerically simulated temperature data is compared with experimental data. A good agreement is observed. Further, heat flux is recovered from temperature data using the cubic spline technique for comparative study. The peak deviation from experimental heat flux data for case 1 and 2 is found to be 6% and 7.2% respectively. All these deviations are in the acceptable range of 8%. This deviation is also accounted by the fact that direct heat condition modelling is used instead of numerical modelling of impulse facilities. Thus, direct heat conduction modelling is acceptable approach to predict the transient thermal data with admissible errors.

Comparison of Thin Film Heat Flux Gauge Technologies Emphasizing Continuous-Duration Operation

Abstract Thin-film heat flux gauges (HFGs) have been used for decades to measure surface temperatures and heat flux in test turbines with the majority being used in facilities that are short-duration. These gauges are typically composed of two resistive temperature devices deposited on opposing sides of a dielectric. However, because these sensors have been traditionally applied for measurements in transient-type facilities, the challenges facing adaptation of this technology for a steady facility warrant investigation. These challenges are highlighted, and the solutions are presented throughout the paper. This paper describes the nanofabrication process for heat flux gauges and a new calibration method to address the potential deterioration of gauges over long runtimes in continuous-duration facilities. Because the primary uncertainty of these sensors arises from the ambiguity of the thermal properties, the emphasis is placed on the property determination. Also, this paper presents a discussion on the use of impulse response theory to process the data showing the feasibility of the method for steady-duration facilities after an initial settling time. The latter portion of the paper focuses on comparing well-established heat flux gauges developed for short-duration turbine test facilities to recently developed gauges fabricated using modern nanofabrication techniques for a continuous turbine test facility. The gauges were compared using the test case of an impinging jet over a range of Reynolds numbers. The comparison between the PSU gauge and the reference device indicated agreement within 14%, and similar results were achieved through comparison with established sensors from partner institutions.

Prediction and Measurement of the Heat Transfer Coefficient in a Direct Oil-Cooled Electrical Machine With Segmented Stator

The heat transfer coefficient (HTC) is a critical parameter that is required for accurate thermal modeling of electrical machines. This is often achieved from empirical correlations of ideal geometries or computational fluid dynamics (CFD) simulations. This paper presents a novel technique using double-sided thin film heat flux gauges for measuring the HTC from a direct oil-cooled electrical machine with segmented stator. While thin film gauges are often used in transient measurements of the HTC on gas turbine components, their application to electrical machines has been largely unexplored. This is the topic of this paper. Due to the large viscosity of the coolant, the transient technique was found to be inadequate and a steady-state adaptation for oil-cooled machines was developed. This paper explores the challenges linked with this measurement technique when applied to oil-cooled machines, and develops new nondimensional correlations of the Nusselt number with Reynolds number. These correlations are applicable to machines with different geometries, flow, and coolant properties. The experimental results were compared to CFD simulations and existing pipe flow correlations. It is shown that these underpredict the HTC by approximately 60% at Re = 20. The discrepancy gradually decreases to around 10% at Re = 200.

Application of Film Cooling to an Unshrouded High-Pressure Turbine Casing

In this paper, we describe the design, modeling, and experimental testing of a film cooling scheme employed on an unshrouded high-pressure (HP) rotor casing. The casing region has high thermal loads at both low and high frequency, with the flow being dominated by the potential field of the rotor and over-tip leakage flows. Increasingly high turbine entry temperatures necessitate internal and film cooling of the casing to ensure satisfactory service life and performance. There are, however, very few published studies presenting computational fluid dynamics (CFD) and experimental data for cooled rotor casings. Experimental testing was performed on a film-cooled rotor casing in the Oxford Turbine Research Facility (OTRF)—a rotating transonic facility of engine scale. Unsteady CFD of an HP rotor blade row with a film-cooled casing was undertaken, uniquely with a domain utilizing a sliding interface in the tip gap. A high density array of thin film heat flux gauges (TFHFGs) was used to obtain time-resolved and time-mean results of adiabatic wall temperature and film cooling effectiveness on the film-cooled rotor casing between −30% and +125% rotor tip axial chord. Results are compared to CFD predictions, and mechanisms for interaction of the coolant with the rotor tip are proposed and discussed. Acoustic effects within casing coolant holes due to the passing of the rotor are demonstrated on a 3D CFD geometry, supporting conclusions drawn in earlier work by the authors on the importance of this effect in a casing film cooling system.

Improved Methodologies for Time-Resolved Heat Transfer Measurements, Demonstrated on an Unshrouded Transonic Turbine Casing

The high pressure (HP) rotor tip and over-tip casing are often life-limiting features in the turbine stages of current gas turbine engines. This is due to the high thermal load and high temperature cycling at both low and high frequencies. In the last few years, there have been numerous studies of turbine tip heat transfer. Comparatively fewer studies have considered the over-tip casing heat transfer. This is in part, no doubt, due to the more onerous test facility requirements to validate computational simulations. Because the casing potential field is dominated by the passing rotor, to perform representative over-tip measurements a rotating experiment is an essential requirement. This paper details the measurements taken on the Oxford turbine research facility (OTRF), an engine-scale rotating turbine facility which replicates engine-representative conditions of Mach number, Reynolds number, and gas-to-wall temperature ratio. High density arrays of miniature thin-film heat-flux gauges were used with a spatial resolution of 0.8 mm and temporal resolution of ∼120 kHz. The small size of the gauges, the high frequency response, and the improved processing methods allowed very detailed measurements of the heat transfer in this region. Time-resolved measurements of TAW and Nu are presented for the casing region (−30% to +125% CAX) and compared to other results in the literature. The results provide an almost unique data set for calibrating computational fluid dynamics (CFD) tools for heat transfer prediction in this highly unsteady environment dominated by the rotor over-tip flow.

New technique for the fabrication of miniature thin film heat flux gauges

This paper details the improvements made to the design and fabrication of thin-film heat flux gauges at Oxford. These improvements have been driven by the desire to improve measurement accuracy and resolution in short duration wind-tunnel experiments.A thin-film heat flux gauge (TFHFG) measures heat flux by recording the temperature history of thin film resistive temperature sensors sputtered onto an insulating substrate. The heat flux can then be calculated using Fourier’s law of heat conduction.A new fabrication process utilising technology from the manufacture of flexible printed circuit boards is outlined, which enables the production of significantly smaller and more robust gauges than those previously used.

Heat Transfer for the Blade of a Cooled Stage and One-Half High-Pressure Turbine—Part I: Influence of Cooling Variation

Heat-flux measurements are presented for a one-and-one-half stage high-pressure turbine operating at design-corrected conditions with modulated cooling flows in the presence of different inlet temperature profiles. Coolant is supplied from a heavily film-cooled vane and the purge cavity (between the rotor disk and the upstream vane) but not from the rotor blades, which are solid metal. Thin-film heat-flux gauges are located on the uncooled blade pressure and suction surface (at multiple span locations), on the blade tip, on the blade platform, and on the disk and vane sides of the purge cavity. These measurements provide a comprehensive picture of the effect of varying cooling flow rates on surface heat transfer to the turbine blade for uniform and radial inlet temperature profiles. Part I of this paper examines the macroscopic influence of varying all cooling flows together, while Part II investigates the individual regions of influence of the vane outer and purge cooling circuits in more detail. The heat-flux gauges are able to track the cooling flow over the suction surface of the airfoil as it wraps upwards along the base of the airfoil for the uniform vane inlet temperature profile. A similar comparison for the radial profile shows the same coolant behavior but with less pronounced changes. From these comparisons, it is clear that cooling impacts each temperature profile similarly. Nearly all of the cooling influence is limited to the blade suction surface, but small changes are observed for the pressure surface. In addition to the cooling study, a novel method of calculating the adiabatic wall temperature is demonstrated. The derived adiabatic wall temperature distribution shows very similar trends to the Stanton number distribution on the blade.

Aerodynamic drag reduction by heat addition into the shock layer for a large angle blunt cone in hypersonic flow

Reduction in aerodynamic drag for a large angle blunt cone flying at hypersonic Mach number by heat addition into the shock layer is demonstrated in HST2 hypersonic shock tunnel. The heat addition is achieved by the exothermic reaction of chromium atoms ablated from the stagnation region of the chromium coated blunt cone with the atomic oxygen behind the shock wave. The measurements show about 47% reduction in the drag coefficient for a 60° apex angle blunt cone in a Mach 8 flow of 3.4MJ∕kg specific enthalpy. The reduction in drag is measured using the accelerometer based force balance system and the heat addition into the shock layer is identified by the surface mounted thin film heat flux gauges and the corresponding movement of the shock wave is visualized by schlieren pictures.

4105 衝撃風洞を用いたラバルノズルスロート部における伝熱特性に関する研究(G19 宇宙工学に関する基盤研究,G19 宇宙工学)

The adequate cooling of nozzles of rocket engines requires design information for predicting a local heat flux. The heat flux has highest value at a nozzle throat area. The purpose of this study is to reveal experimental heat transfer data in the nozzle throat by using a shock tunnel. The shock tunnel was built with a shock tube and instrumented nozzle at the end of the shock tube. In the case of shock tunnel, duration time of supersonic flow is so short that high response instrumentation or methods are necessary for measuring the transient heat flux and temperature. We applied platinum thin film heat flux gauge. Open literatures suggest several experimental equations that may be applicable for predicting the heat transfer coefficients in the nozzle. Bartz's equation is one of them and is a well-known equation. This paper will present comparison between experimental measurement data by using the developed gauges and estimation by Bartz's equation.

Characterization of Thin-Film Heat-Flux Gauges

Characterization of Thin-Film Heat-Flux Gauges

A Fast-Response High Spatial Resolution Total Temperature Probe Using a Pulsed Heating Technique

This paper discusses the operation of a fast-response total temperature probe based on transient thin film heat flux gage technology. The probe utilizes two thin film gages located close to the stagnation point of a hemispherically blunted fused quartz cylinder. Development of the present total temperature probe was motivated by the need for a fast-response device with a high spatial resolution. The diameter of the probe was 2.8 mm and the two films were separated by a distance of less than 1 mm. Measurement of the flow total temperature requires the films to operate at different temperatures. In the present work, the temperature difference was generated using a current pulse (approximately 70 mA with a duration of around 1 s) to heat one of the thin film resistance gages. With this technique, temperature differences between the hot and cold films of around 120 K were achieved. The interpretation of the transient surface temperature measurements is discussed, and the validity and utility of the technique are demonstrated with reference to total temperature and convective heat transfer coefficient measurements in a compressible free jet. The results demonstrate that accurate total temperature and convective heat transfer coefficient measurements with high spatial and temporal resolution can be obtained with the present device.

Steady-State and Frequency Response of a Thin-Film Heat Flux Gauge

Anewand simplerdesign ofthin-e lm heate ux gaugehasbeendeveloped forusein high-heat-e ux environments. Heat e ux gauges of the same design were fabricated on three different substrates and tested. The heat e ux gauge comprises a thermopile and a thermocouple junction, which measures the surface temperature. The thermopile has 40 pairs of S-type thermocouples and is covered by two thermal resistance layers. Calibration and testing of thesegaugesweree rst carriedoutin an arc-lamp calibrationfacility.Sensitivityofthegaugewasdiscussed in terms of the relative conductivity and surface temperature. The heat e ux calculated from the gauge output was in good agreement with the precalibrated standard sensor. The steady-state and the transient response characteristics of the heat e ux gauge were also investigated using a carbon dioxide pulse laser as a heat source. The dynamic frequency response was evaluated in terms of the nondimensional amplitude ratio with respect to the frequency spectrum of a chopped laser beam. The frequency response of the gauge was determined to be about 3 kHz. The temperature proe les in the thin-e lm heat e ux gauge were obtained numerically in steady-state conditions using FLUENT and compared with the experimental results. Nomenclature d = thickness of thermal resistance layer, m Es = thermopile voltage output, mV K = thermal conductivity, W/m C K = relative conductivity, W/m C N = number of thermocouple pairs in the thermopile Q = heat e ux, W/m 2 S T = absolute thermoelectric power at temperature T, mV/ C T = temperature, C t = time, s = absorptivity d = thickness difference in thermal resistance layers between 1 and 2 Subscripts 1, 2 = thermal resistance layers 1 and 2, respectively

Investigation of the Heat-Island Effect for Heat-Flux Measurements in Short-Duration Facilities

This paper presents the results of an experimental investigation designed to determine the possible influence of a surface-temperature discontinuity on the heat-flux history inferred from the temperature history of a miniature thin-film heat-flux gage (painted on an insulator) embedded in the surface of a metallic component. The resulting surface material discontinuity has the potential of causing a heat-island effect. Simultaneous measurements of the heat-flux history at selected locations along the midspan of a representative turbine airfoil (metal) were performed using continuous strip-type contoured inserts (without temperature discontinuity) and button-type inserts (with temperature discontinuity). Measurements are reported for two Reynolds number conditions. The experimental results suggest that for the experimental conditions of interest to gas turbine research in short-duration facilities, the influence of a potential heat-island on the measured heat flux is insignificant.

An experimental study of a radially arranged thin-film heat-flux gauge

A new thin-film heat-flux gauge was designed and fabricated on three different substrate materials. Forty pairs of Pt - Pt/10% Rh thermocouple junctions were deposited in a circular pattern on the same plane of the substrate. Over the thermocouples, 5 and 10 m thick thermal resistance layers were deposited to create a temperature gradient across those layers. Calibration and testing of these gauges were carried out in an arc-lamp calibration facility. The heat flux calculated from the gauge output is in good agreement with the value obtained from the pre-calibrated standard sensor. A laser was also used to test the steady-state and dynamic responses of the heat-flux gauge. During the steady-state test, the time constant for the heating period was 30 s. The frequency response of the heat-flux gauge was measured in the frequency domain using a laser and a chopper. The responses from an infrared detector and the heat-flux gauge were measured simultaneously and compared. It was found that the thin-film heat-flux gauge has a dynamic frequency response of 3 kHz.