Thermopile Heat Flux Research Articles

Angle- and Thickness-Dependent Response Characteristics of YBa2Cu3O7−δ-Based Atomic-Layer Thermopile Heat Flux Sensors

Angle- and Thickness-Dependent Response Characteristics of YBa₂Cu₃O_7−δ-Based Atomic-Layer Thermopile Heat Flux Sensors

Conformal Fabrication of High-Sensitivity Embedded Thick Film Thermopile Heat Flux Sensors: Design, Preparation, and Performance

Conformal Fabrication of High-Sensitivity Embedded Thick Film Thermopile Heat Flux Sensors: Design, Preparation, and Performance

Stagnation Point Heat Transfer to an Axisymmetric Impinging Jet at Transition to Turbulence

Abstract A round air jet issuing from a long straight pipe and impinging perpendicularly onto a heated flat plate was investigated experimentally. The Reynolds number (Re) covered the fully laminar, transitional, and fully turbulent regimes—Re = 850–15,400. The main focus of this investigation was the transition regime, which occurred at Re = 2250–3010. Various measurements were recorded during the experiments using a hot-wire anemometer, an infrared thermometer, and a thermopile heat flux sensor; the mass transfer was measured using the naphthalene sublimation technique. The stagnation point heat transfer was correlated to the laminar and turbulent regimes in the form of the stagnation point Nusselt number, Nu0 = CRemPr0.4, where the exponent m = 0.50 and 0.55, respectively. The Nu0–Re relationship exhibited nonmonotonic behavior (decrease) in the transition regime. Two counteracting mechanisms occur during transition—jet core shortening and an increase in velocity fluctuation; the former reduces Nu0, whereas the latter increases it.

Bluntness-Dependent Hypersonic Boundary-Layer Modes’ Excitation

Boundary-layer disturbances on a cone in a hypersonic flow are investigated by heat flux sensors. The cone has a half-angle of 10.9 deg and is situated with a 0 deg angle of attack in a Mach 10 region of a conical nozzle in a shock-tunnel facility. There are three atomic layer thermopile heat flux sensors mounted at different chordwise positions in order to investigate the receptivity to freestream disturbances and the emergence of unstable modes of the boundary layer, that is, Mack first and second modes. The cone’s nose is exchangeable, and there are three differently sized radii to investigate the bluntness effect on transition with respect to entropy/boundary-layer displacement. It is found that each nose type tested shows a different set of amplified spectral bands in the receptivity region of the first two sensors, as well as at the third sensor’s location downstream of the critical point. There is a unique wavelength, scaling with boundary-layer height, that is most receptive to disturbances. For the two sharper cones, this excitation lies in the range of Mack second modes, which are observed in the unstable region. For the blunter cone, this excitation lies near the first mode frequency; as a result, only first modes are amplified and detected at sensor 3, although linear stability theory predicts higher growth rates for second modes.

Calibration of thermopile heat flux gauges using a physically-based equation

A thermopile, in which a number of thermocouple junctions are arranged on either side of a thin layer of insulation, is commonly used to determine the heat flux for steady-state measurements. Gauges using this method are available commercially and a new, generic calibration method is described here. For this purpose, an equation based on physical properties has been derived to determine the theoretical relationship between the measured voltage output of the gauge and the heat flux through it. An experimental rig has been built and used to calibrate gauges under steady-state conditions for heat fluxes between 0.5 and 8 kW/m2. The gauge temperature was controlled between 30 and 110 °C, and voltage-flux correlation – based on the theoretical relationship – was determined using maximum likelihood estimation (MLE). For tests with constant gauge temperature, there was a linear relationship between the voltage and heat flux; owing to the temperature dependency of the Seebeck constants of the thermoelectric materials, the voltage increased with increasing gauge temperature. In all cases, there was very good agreement between the measured and correlated values, and the overall uncertainty of the correlation was estimated to be less than 5% of the measured heat flux.

Noninvasive Method to Measure Thermal Energy Flow Rate in a Pipe

Abstract A noninvasive, thermal energy flowrate sensor based on a combination of heat flux and temperature measurements is developed for measuring the volume flowrate and the fluid temperature in a pipe. The sensor is covered by a thin-film heater and clamped onto the outer surface of the pipe. Two types of thin-film thermocouple elements are compared to minimize the thermal contact resistance R″ between the thermocouple and the surface of the pipe. A thin, flexible thermopile heat flux sensor (PHFS) is mounted over the thermocouples. A one-dimensional transient thermal model is applied before and during activation of the external heater to provide estimates of the fluid heat transfer coefficient h. The results are correlated with the volume flowrate Q and the fluid temperature Twc. Several different parameter estimation codes are used to estimate the optimal parameters by using the minimum root-mean-square (rms) error between the analytical and experimental sensor temperature values. The experiments are completed over a range of volume flowrates—1.3 gallons/min to 14.5 gallons/min. Encouraging measurement results give good correlation, repeatability, and sensitivity between the heat transfer coefficient h and the volume flowrate Q with an accurate estimation of the fluid temperature Twc. The resulting noninvasive thermal energy flowrate sensor can be used to estimate the volume flowrate and the fluid temperature in a variety of applications.

Radical recombination sensor based on dual probe thermopile heat flux sensors

Hydrogen radicals play an important role in, e.g., the cleaning of extreme ultraviolet reflective mirrors. Therefore, there is a need to quantify the surface radical flux in the various (plasma) setups where these effects are studied. In this paper, a catalytic radical sensor is presented, based on the measurement of the recombination heat of radicals on a surface, using dual probe thermopile heat flux sensors (HFSs). The first HFS1 has a high recombination (probability) coefficient coating, e.g., Pt. The second HFS2 has a low recombination coefficient coating, e.g., Al2O3. Signal subtraction largely eliminates common mode heat losses/gains such as conduction/convection and IR radiation, the net result representing the radical recombination heat. The signal can be improved by switching the radical source on/off at regular intervals. Radical recombination rates were measured in a remote microwave plasma chamber (38 Pa H2) over the range 1018−1021 atH/(m2 s), with nearly linear response as a function of plasma power setting. The sensor full scale limit is ∼1023 atH/(m2 s) and is dictated by the maximum allowable sensor surface temperature (<250 °C).

A molecular dynamics simulation on the static calibration test of a revised thin-film thermopile heat-flux sensor

A revised thin-film thermopile heat-flux sensor with cooper-based heat sink is calibrated at a static calibration system that provides a stable heat flux source in the range of 0.1–10.0 MW/m2. The calibration results show that sensitivity coefficients (SCs) of the revised heat-flux sensor are quadratically dependent on surface temperatures, which is different from the linear SCs of original heat-flux sensors. A molecular dynamics simulation is applied to predict the thermal conductivity (TC) of thin-film thermal resistance layer and the interfacial thermal resistivity at the interface with film thickness and substrate material being considered. With the aid of the molecular dynamics simulation and finite element method, the reason for such a difference is analyzed to be the coupled effect of temperature-dependent thermal conductivity (TC) of thin-film thermal resistance layer and the interfacial thermal impedance at the interface.

WITHDRAWN: Measurement Issues Associated with Surface Mounting of Thermopile Heat Flux Sensors

WITHDRAWN: Measurement Issues Associated with Surface Mounting of Thermopile Heat Flux Sensors

Measurement issues associated with surface mounting of thermopile heat flux sensors

Thermopile is one of the most commonly used heat flux sensor, usually attached to the surface of interest for measurement of wall heat flux. One of the major drawbacks of such surface mounted sensors is that they potentially disturb the very thermo-physical phenomena responsible for heat transfer, altering the heat transport through the medium on which it is mounted, resulting in erroneous measurements. This paper closely examines the intrusion due to surface mounting of such sensors. Parametric simulations have been performed for commonly employed sensor mounting configurations. Furthermore, the attenuation in heat transfer through the plate as a function of its thermophysical properties (absolute as well as relative to the sensor material) is evaluated. It is found that matching the impedance of sensor and heat conduction medium is important for minimizing the intrusive effects. Simulations are substantiated by a case-study involving heat flux measurement during filmwise condensation of steam on a flat plate in the presence of air. The experimental design ratifies the recommendations of the simulation study, minimizing the intrusive effects after proper impedance matching and in situ calibration. The resulting heat transfer coefficient estimated using the measured wall heat flux matches well with those obtained using Terasaka-Makita correlation.

Convective heat transfer coefficient in compartment fires

Heat transfer to compartment surfaces was measured in fully developed fire experiments. The experiments involved scaled compartments ranging from 1/8th to 3/8th with full-scale height of 2.54 m. Gas temperatures reached 1000°C, and total surface heat flux could reach 200 kW/m2, with convection accounting for 25% of the total. A combination of thermopile heat flux gage, metal plate sensor, and gas and wall thermocouples was used to separate the convective and radiative components. The convective heat transfer coefficients were resolved experimentally. Convective heat transfer coefficient was correlated against temperature rise within the compartment for both flaming and after extinction phases.

In situ High Temperature Heat Flux Sensor Calibration

Recent advances in heat flux measurement have resulted in the development of a robust thermopile heat flux sensor intended for use in extreme thermal environments. The High Temperature Heat Flux Sensor (HTHFS) is capable of simultaneously measuring thermopile surface temperature and heat flux at sensor temperatures up to 1000 °C. The need for high temperature heat flux calibration of the HTHFS has resulted in the development of a new wide angle radiation calibration system, which operates with the sensor at elevated temperatures. The temperature dependence of the sensor output over the range of 100–900 °C has been successfully characterized with acceptable uncertainty limits. The calibrated HTHFS sensitivity agrees well with a theoretical sensitivity model, suggesting that the primary cause for the sensor’s output temperature dependence is due to the change in thermal conductivity of the sensor elements with temperature.

Novel Method for Measurement of Total Hemispherical Emissivity

Abstract : This report was developed under a SBIR contract. This paper describes a heat flux-based method for measuring emissivity of a surface. In this method the emissivity of a surface is calculated using direct measurement of the heat flux passing through the surface. Unlike storage-based calorimetric methods, this method does not require application of known amounts of heat to the surface or the temperature history of a known amount of thermal mass to calculate the surface emissivity. Application and operation of this method is much simpler than calorimetric methods as it does not require careful thermal insulation of the heat radiating body from the surroundings. This technique allows emissivity measurements of the newly developed variable emissivity surfaces with significantly lighter and energy efficient measurement equipment that can operate for long term space missions. In this study, a commercially available thermopile heat flux sensor was used to measure the emissivity of a black paint and a variable emissivity surface, Electrostatic Switched Radiator (ESR). This paper details the concept, experimental setup, and the experiment results.

Experimental In-Cavity Radiative Calibration of High Heat-Flux Meters

The accuracy of the calibration of heat-flux meters up to 500 kW · m -2 irradiance levels by insertion into the cavity of a high-temperature blackbody is examined. The experimental study consists of measurements on two thermopile heat-flux meters using two graphite-tube variable-temperature blackbodies. To provide a baseline comparison, the meters were first calibrated up to 50 kW · m -2 outside the blackbody cavity with the irradiance level determined using a well-characterized electrical substitution radiometer. In contrast to the outside calibration method, in-cavity calibration covers the full heat-flux range of the meters. The measured radiance distribution along the cavity axis, corroborated by Monte Carlo simulations, shows that the optimum location for the meter is approximately one cavity radius from the closed end of the cavity. The in-cavity measured responsivity values corrected for the effective emissivity agree with the outside-cavity values within the measurement uncertainty. The results demonstrate the feasibility of using in-cavity techniques to calibrate heat-flux meters at high irradiance levels, limited primarily by the blackbody operating temperature.

The Skin-Layer Ocean Heat Flux Instrument (SOHFI). Part I: Design and Laboratory Characterization

Abstract An untended instrument to measure ocean surface heat flux has been developed for use in support of field experiments and the investigation of heat flux parameterization techniques. The sensing component of the Skin-Layer Ocean Heat Flux Instrument (SOHFI) consists of two simple thermopile heat flux sensors suspended by a fiberglass mesh mounted inside a ring-shaped surface float. These sensors make direct measurements within the conduction layer, where they are held in place by a balance between surface tension and float buoyancy. The two sensors are designed with differing solar absorption properties so that surface heat flux can be distinguished from direct solar irradiance. Under laboratory conditions, the SOHFI measurements agree well with calorimetric measurements (generally to within 10%). Performance in freshwater and ocean environments is discussed in a companion paper.

Thin Disk On A Convectively Cooled Plate—Application To Heat Flux Measurement Errors

An analysis is made of the steady-state thermal processes associated with a thin disk affixed to a convectively cooled solid plate. The disk represents a thermopile heat flux gauge. In the first part of the paper, the heat conduction problem of the plate is solved for the simplified condition where the heat flow through the disk is axially one-dimensional. It was found that the local divergence of the heat flux field within the plate owing to the presence of the disk may result in a gauge reading that underestimates the heat transfer rate. Also, a sizeable local plate temperature augmentation can occur. Furthermore, the analysis yields dimensional estimates of the region where the temperature and heat flux fields within the plate are significantly altered by the presence of the disk. An adjunct to the foregoing analysis develops a one-dimensional conduction factor which is useful in determining when the heat flow through the disk can be considered to be axially one-dimensional.