Thomas E. Diller

High-Frequency Heat Flux Sensor Calibration and Modeling

A new method of in-situ heat flux gage calibration is evaluated for use in convective facilities with high heat transfer and fast time response. A Heat Flux Microsensor (HFM) was used in a shock tunnel to simultaneously measure time-resolved surface heat flux and temperature from two sensors fabricated on the same substrate. A method is demonstrated for estimating gage sensitivity and frequency response from the data generated during normal transient test runs. To verify heat flux sensitivity, shock tunnel data are processed according to a one-dimensional semi-infinite conduction model based on measured thermal properties for the gage substrate. Heat flux signals are converted to temperature, and vice versa. Comparing measured and calculated temperatures allows an independent calibration of sensitivity for each data set. The results match gage calibrations performed in convection at the stagnation point of a free jet and done by the manufacturer using radiation. In addition, a finite-difference model of the transient behavior of the heat flux sensor is presented to demonstrate the first-order response to a step input in heat flux. Results are compared with shock passing data from the shock tunnel. The Heat Flux Microsensor recorded the heat flux response with an estimated time constant of 6 μs, which demonstrates a frequency response covering DC to above 100 kHz.

Development of methodologies for the estimation of blood perfusion using a minimally invasive thermal probe

Parameter estimation techniques have been utilized in the development of methodologies for the noninvasive determination of blood perfusion using measurements from a new thermal surface probe. The basic concept behind this work is that heat flux and temperature measurements from the probe are combined with results from a mathematical model of the probe and tissue in an estimation procedure for the determination of the blood perfusion. The key element of the probe is a thin sensor, which is placed in contact with the tissue and provides time-resolved signals representing heat flux and temperature while the probe is cooled by air jets. This probe has been newly modified to enhance performance. Parameter estimation techniques were developed which incorporate measured heat flux and/or temperature data and corresponding calculated data from the model to estimate blood perfusion and also the thermal contact resistance between the probe and the tissue. The sensitivity coefficients associated with heat flux were found to be much higher than those associated with temperature such that the heat flux measurements were the most influential in the estimation of the parameters. Simultaneous estimates of blood perfusion and contact resistance were successfully obtained using the Gauss minimization method. The resulting estimates of blood perfusion were consistent with the range of values found in the literature.

An Investigation of Heat Transfer in a Film Cooled Transonic Turbine Cascade: Part I — Steady Heat Transfer

Experiments were performed in a transonic cascade wind tunnel to investigate the film effectiveness and heat transfer coefficient on the suction side of a high-turning turbine rotor blade. The coolant scheme consisted of six rows of staggered, discrete cooling holes on and near the leading edge of the blade in a showerhead configuration. Air was cooled in order to match the density ratios found under engine conditions. Six high-frequency heat flux gauges were installed downstream of the cooling holes on the suction side of the blade. Experiments were performed with and without film and the coolant to freestream total pressure ratio was varied from 1.02 to 1.19. In order to simulate real engine flow conditions, the exit Mach number was set to 1.2 and the exit Reynolds number was set to 5×106. The freestream turbulence was approximately 1%. The heat transfer coefficient was found to increase with the addition of film cooling an average of 14% overall and to a maximum of 26% at the first gauge location. The average film cooling effectiveness over the gauge locations was 25%. Both the heat transfer coefficient and the film cooling effectiveness were found to have only a weak dependence upon the coolant to freestream total pressure ratio at the gauge locations used in this study.© 2000 ASME

Durable Heat Flux Sensor for Extreme Temperature and Heat Flux Environments

This paper reports on the development and evaluation of a novel heat flux sensor, the high-temperature heat flux sensor, tested at temperatures and heat flux levels in excess of 1000°C and 10-13 W/cm 2 , respectively. The current sensor configuration uses type-K thermocouple materials in a durable welded thermopile arrangement contained within a surface-mountable high-temperature housing. The steady-state sensitivity of the design is predicted using a simplified one-dimensional thermal-resistance model. The design performance of a prototype sensor is validated using both conduction and convection heat transfer calibration at low temperature. The average experimental values of the sensitivity are 623 ± 33 mV/W/cm 2 and 579 ± 29 mV/W/cm 2 in conduction and convection, respectively. These calibration results compare very well with the predicted room-temperature sensitivity of 559 μV/W/cm 2 . Minimal dependence on heat transfer coefficient is found in convection. Prolonged thermal cycling of the sensor using a high-temperature kiln and a propane torch apparatus demonstrates survivability near the maximum temperature of the thermoelectric materials with negligible oxidation or loss of calibration.

A Hybrid Method for Measuring Heat Flux

The development and evaluation of a novel hybrid method for obtaining heat flux measurements is presented. By combining the spatial and temporal temperature measurements of a heat flux sensor, the time response, accuracy, and versatility of the sensor is improved. Sensors utilizing the hybrid method are able to make heat flux measurements on both high and low conductivity materials. It is shown that changing the thermal conductivity of the backing material four orders of magnitude causes only an 11% change in sensor response. The hybrid method also increases the time response of heat flux sensors. The temporal response is shown to increase by up to a factor of 28 compared with a standard spatial sensor. The hybrid method is tested both numerically and experimentally on both high and low conductivity materials and demonstrates significant improvement compared with operating the sensor as a spatial or temporal sensor alone.

Experimental Measurements and Modeling of the Effects of Large-Scale Freestream Turbulence on Heat Transfer

The influence of freestream turbulence representative of the flow downstream of a modern gas turbine combustor and first stage vane on turbine blade heat transfer has been measured and analytically modeled in a linear, transonic turbine cascade. High intensity, large length-scale freestream turbulence was generated using a passive turbulence-generating grid to simulate the turbulence generated in modern combustors after passing through the first stage vane row. The grid produced freestream turbulence with intensity of approximately 10–12% and an integral length scale of 2 cm (Λx /c = 0.15) near the entrance of the cascade passages. Mean heat transfer results with high turbulence showed an increase in heat transfer coefficient over the baseline low turbulence case of approximately 8% on the suction surface of the blade, with increases on the pressure surface of approximately 17%. Time-resolved surface heat transfer and passage velocity measurements demonstrate strong coherence in velocity and heat flux at a frequency correlating with the most energetic eddies in the turbulence flow field (the integral length-scale). An analytical model was developed to predict increases in surface heat transfer due to freestream turbulence based on local measurements of turbulent velocity fluctuations and length-scale. The model was shown to predict measured increases in heat flux on both blade surfaces in the current data. The model also successfully predicted the increases in heat transfer measured in other work in the literature, encompassing different geometries (flat plate, cylinder, turbine vane and turbine blade) and boundary layer conditions.Copyright

The Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence

Experiments have been performed in a water tunnel facility to examine the physical mechanism of heat transfer augmentation by freestream turbulence in classical Hiemenz flow. A unique experimental approach to studying the problem is developed and demonstrated herein. Time-resolved digital particle image velocimetry (TRDPIV) and a new variety of thin-film heat flux sensor called the heat flux array (HFA) are used simultaneously to measure the spatiotemporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation surface. Laminar flow and heat transfer at low levels of freestream turbulence (Tu x =0.5-1.0%) are examined to provide baseline flow characteristics and heat transfer coefficients. Similar experiments using a turbulence grid are preformed to examine the effects of turbulence with mean streamwise turbulence intensity of Tu x =5.0% and an integral length scale of Λ x = 3.25 cm. At a Reynolds number of Re D = U ∞ D/v=21, 000, an average increase in the mean heat transfer coefficient of 64% above the laminar level was observed. Experimental studies confirm that coherent structures play a dominant role in the augmentation of heat transfer in the stagnation region. Calculation and examination of the transient physical properties for coherent structures (i.e., circulation, area averaged vorticity integral length scale, and proximity to the surface) shows that freestream turbulence is stretched and vorticity is amplified as it is convected toward the stagnation surface. The resulting stagnation flow is dominated by dynamic, counter-rotating vortex pairs. Heat transfer augmentation occurs when the rotational motion of coherent structures sweeps cooler freestream fluid into the laminar momentum and thermal boundary layers into close proximity of the heated stagnation surface. Evidence in support of this mechanism is provided through validation of a new mechanistic model, which incorporates the transient physical properties of tracked coherent structures. The model performs well in capturing the essential dynamics of the interaction and in the prediction of the experimentally measured transient and time-averaged turbulent heat transfer coefficients.

Experimental measurements and modeling of the effects of large-scale freestream turbulence on heat transfer

The influence of freestream turbulence representative of the flow downstream of a modern gas turbine combustor and first stage vane on turbine blade heat transfer has been measured and analytically modeled in a linear, transonic turbine cascade. High-intensity, large length-scale freestream turbulence was generated using a passive turbulence-generating grid to simulate the turbulence generated in modern combustors after passing through the first stage vane row. The grid produced freestream turbulence with intensity of approximately 10-12% and an integral length scale of 2 cm (Λ x /c=0.15) near the entrance of the cascade passages. Mean heat transfer results with high turbulence showed an increase in heat transfer coefficient over the baseline low turbulence case of approximately 8% on the suction surface of the blade, with increases on the pressure surface of approximately 17%. Time-resolved surface heat transfer and passage velocity measurements demonstrate strong coherence in velocity and heat flux at a frequency correlating with the most energetic eddies in the turbulence flow field (the integral length scale). An analytical model was developed to predict increases in surface heat transfer due to freestream turbulence based on local measurements of turbulent velocity fluctuations and length scale. The model was shown to predict measured increases in heat flux on both blade surfaces in the current data. The model also successfully predicted the increases in heat transfer measured in other work in the literature, encompassing different geometries (flat plate, cylinder turbine vane, and turbine blade) and boundary layer conditions.

On the organization of flow and heat transfer in the near wake of a circular cylinder in steady and pulsed flow

Skin friction, pressure, and heat transfer gages are employed to monitor the flow and heat transfer field along the periphery of a circular cylinder in steady and pulsed flow at Reynolds numbers, Re = 23,000 to 50,000. Averaged distributions, RMS, and power spectra of all measurements are displayed. Special attention is directed at the organisation of the near wake, as detected by the three types of surface gages. The response of the wake to pulsing of the oncoming stream is also examined. It is found that when the wake is locked on the driving frequency, the basic character of the flow is not changed, but the organised motion stands out more clearly. Moreover, the signals become cleaner and background noise in the spectra is reduced. Skin friction and heat transfer gages are shown to respond to local variations of the corresponding quantities, whereas pressure gages respond to local variations of the corresponding quantities, whereas pressure gages respond to global characteristics of the flow.

A direct-measurement thin-film heat flux sensor array

A new thin-film heat flux array (HFA) was designed and fabricated using a series of nickel/copper differential thermocouples deposited onto a thin Kapton® polyimide film. A special bank of amplifiers was designed and built to measure the signal from the HFA. Calibrations were performed to determine the gages sensitivity and temporal response. The HFA produced signals of 42 µV (W cm−2)−1 with a measured first-order response time of 32 ms. The apparent thermal conductivity of the Kapton used was larger than what is usually reported. The design methodology, construction techniques, steady-state and transient calibrations, and a test case are all discussed.