Heat Flux Measurement Techniques Research Articles

Comparison between Direct and Indirect Heat Flux Measurement Techniques: Preliminary Laboratory Tests

Direct and indirect approaches can be employed for estimating the heat flow through components in different application fields. In the building sector, the thermometric method is often applied by professionals for thermal transmittance evaluations. However, miscalculations can derive from inaccurate total heat transfer coefficients, and a consensus regarding the appropriate value to employ remains to be determined. Here, an apparatus was realized for laboratory tests and heat flux measurements were performed following direct and indirect approaches. Data acquired through a common heat flow sensor were compared with those computed through a post-processing based on radiative and convective estimations. The results were affected by the specific correlation adopted for computing the convective coefficients, with the percentage differences ranging from −9.8% to −0.4%. New measurement systems could be designed for automatically computing heat fluxes through indirect approaches, thus providing alternative solutions in the panorama of non-destructive tests for building energy diagnosis.

DEVELOPMENT OF AN EXPERIMENTAL TECHNIQUE FOR THE ESTIMATION OF FATIGUE CRACK EVOLUTION IN TITANIUM ALLOYS BASED ON AN ENERGY APPROACH

This paper presents an experimental study of energy dissipation caused by fatigue crack growth in titanium alloys Ti Grade 2, Ti-1.1Al-0.9Mn, and Ti-4.6Al-1.77V using an original heat flux measurement technique. It is shown that significant structural changes occur in the material under plastic deformation, leading to internal energy evolution. As is known, a large part of the deformation energy is dissipated as heat. The developed experimental technique allows high-accuracy measurements of the heat flux caused by plastic zone development at the crack tip directly in the fatigue experiment. Simultaneous measurements of the crack length and displacements in the stress concentration zone allow estimating the energy balance of the tested specimens. Analysis of the obtained data confirms that the stored strain energy reflecting the structural state of the material can be used as a fracture criterion. A kinetic equation is derived for predicting the fatigue crack growth rate in the Paris regime by energy dissipation rate on the basis of heat flux data.

Highly Time Resolved, Combined Temperature, and Heat Flux Measurement Technique Based on ALTP Sensors

Abstract This study demonstrates a new methodology for the simultaneous direct measurement of temperature and heat flux within the same sensing element of heat flux sensors. The methodology is demonstrated for fast-response atomic layer thermopile (ALTP) sensors and makes use of a temperature–resistance correlation. The new methodology enables an inherent comparison and validation of the heat flux density signal of ALTPs. Good agreement is found based on experiments in the stagnation point of a subsonic hot air jet. The results of the mainly convective heat flux densities are compared against a standard coaxial thermocouple, indicating good agreement of mean values and signal characteristics. The simultaneous direct measurement of the temperature and heat flux allows the time-resolved calculation of local heat transfer coefficients and Nusselt numbers. The spectral analysis of the Nusselt number exhibits an excellent signal-to-noise ratio.

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.

Development of a non-intrusive radiative heat flux measurement for upward flame spread using DSLR camera based two-color pyrometry

A non-intrusive flame to fuel surface radiative heat flux measurement technique is developed using two-color pyrometry of front and side images of upward flame spread along poly(methyl methacrylate) (PMMA). Commercially available Nikon DSLR cameras are radiometrically calibrated to measure soot temperature from the red and green channels of the color images using two-color pyrometry. A 3D ray tracing algorithm is developed to compute radiative heat flux from a sooting flame to the solid fuel surface. Computed heat fluxes are compared to convection corrected Schmidt–Boelter heat flux measurements located at the center of the sample and are shown to be within ± 6.5%. To the authors’ knowledge, this study is the first attempt to develop a non-intrusive radiative heat flux measurement using DSLR cameras and two-color pyrometry. Flame height estimates using the computed heat flux values are in good agreement with visual observations and prior measurements reported in the literature.

Base heat flux measurement techniques in impulse hypersonic test facilities

AbstractSafe and efficient hypersonic missions require exhaustive testing to capture the surface heat flux used in optimizing thermal protection systems. Though low, base heat transfer rates are still relevant and their estimation is of the utmost importance. Impulse hypersonic test facilities like the shock tunnel are commonly used for such estimation tests. The present paper compares contemporary heat flux measurement techniques, namely, E‐type coaxial thermocouples, Pt‐thin films, and atomic layer thermopiles in a hypersonic shock tunnel at the base of a scaled‐down re‐entry capsule. Base flow establishment is ensured using pressure measurements. The measurements indicated that the base‐stagnation point heat flux was approximately 2% to 3% of the forebody stagnation heat flux and it stabilized at an approximate nondimensional establishment time of 43.2 ± 2.4.

Heat transfer coefficient: a review of measurement techniques

Heat transfer coefficient is a basic parameter used in the calculation of convective heat transfer problems. Due to the importance of the experimental measurements for the development of convective heat transfer, this review identifies, classifies and describes the experimental methods used for the measurement of heat transfer coefficient. The methods were classified into five major groups: (1) direct method, (2) transient method, (3) Wilson method, (4) heat/momentum/mass transfer analogy method and (5) boundary layer thickness method. Their applications, limitations and the reported accuracy were evaluated in the context of new developments in temperature and heat flux measurement techniques. Finally, this review provides criteria for the selection of the most suitable technique for measurements of heat transfer coefficient according to the aspects of spatial resolution, geometric scale, intrusiveness, fluid type, response time and accuracy.

Design of the Purdue Experimental Turbine Aerothermal Laboratory for Optical and Surface Aerothermal Measurements

Following three decades of research in short duration facilities, Purdue University has developed an alternative turbine facility in view of the modern technology in computational fluid mechanics, structural analysis, manufacturing, heating, control, and electronics. The proposed turbine facility can operate continuously and also perform transients, suited for precise heat flux, efficiency, and optical measurement techniques to advance turbine aerothermo-structural engineering. The facility has two different test sections, linear and annular, to service both fundamental and applied research. The linear test section is completely transparent for optical imaging and spectroscopy, aimed at technology readiness levels (TRLs) of 1–2. The annular test section was designed with optical access to perform proof of concepts as well as validation of turbine component performance for relevant nondimensional parameters at TRLs of 3–4. The large mass flow rate (28 kg/s) combined with a minimum hub to tip ratio of 0.85 allows high spatial resolution. The Reynolds number (Re) extends from 60,000 to 3,000,000, based on the vane outlet flow properties with an axial chord of 0.06 m and a turning angle of 72 deg. The pressure ratio can be independently adjusted, enabling testing from low subsonic to Mach 3.2. This paper provides a detailed description of the sequential design methodology from zero-dimensional to three-dimensional (3D) unsteady analysis as well as of the measurement techniques available in this turbine facility.

Experimental Validation of a Boundary Layer Convective Heat Flux Measurement Technique

If a steady thermal boundary layer is sufficiently thick, wall heat fluxes and associated convective heat transfer coefficients can be directly calculated from measured temperature distributions taken within it using a traversing thermocouple probe. The boundary layer can be laminar, turbulent, or transitional and on a surface of arbitrary surface temperature distribution and geometry. Herein, this technique is presented and validated in a steady, turbulent, two-dimensional boundary layer on a flat, uniform-heat-flux wall. Care is taken to properly account for radiation from the wall and conduction within the thermocouple wire. In the same setting, heat flux measurements are made for verification purposes using an energy balance on a segment of the test wall carefully designed to minimize and include radiation and conduction effects. Heat flux values measured by the boundary layer measurement technique and by the energy balance measurement agree to within 4.4% and the difference between the two lie completely within their respective measurement uncertainties of 5.74% and 0.6%.

Comparison of heat flux measurement techniques during the DIII-D metal ring campaign

The heat fluxes expected in the ITER divertor raise concerns about the damage tolerances of tungsten, especially due to thermal transients caused by edge localized modes (ELMs) as well as frequent temperature cycling from high to low extremes. Therefore we are motivated to understand the heat flux conditions that can cause not only enhanced erosion but also bulk thermo-mechanical damage to a tungsten divertor. For the metal ring campaign in DIII-D, tungsten-coated TZM tile inserts were installed making two toroidal arrays of metal tile inserts in the lower divertor. This study examines the deposited heat flux on these rings with embedded thermocouples (TCs) sampling at 10 kHz and compares them to Langmuir probe (LP) and infrared thermography (IRTV) heat flux measurements. We see agreement of the TC, LP, and IRTV data within 20% of the heat flux averaged over the entire discharge, and that all three diagnostics suggest parallel heat flux at the OSP location increases linearly with input heating power. The TC and LP heat flux time traces during the discharge trend together during large changes to the average heat flux. By subtracting the LP measured inter-ELM heat flux from TC data, using a rectangular ELM energy pulse shape, and taking the relative size and duration of each ELM from measurements, we extract the ELM heat fluxes from TC data. This over-estimates the IRTV measured ELM heat fluxes by a factor of 1.9, and could be due to the simplicity of the TC heat flux model and the assumed ELM energy pulse shape. ELM heat fluxes deposited on the inserts are used to model tungsten erosion in this campaign. These TC ELM heat flux estimates are used in addition to IRTV, especially in cases where the IRTV view to the metal ring is obstructed. We observe that some metal inserts were deformed due to exposed leading edges. The thermal conditions on these inserts are investigated with the thermal modeling code ABAQUS using our heat flux measurements when these edges were exposed. We discuss how the thermal cycling on the ends of the inserts caused this deformation.

Temperature and heat flux measurement techniques for aeroengine fire test: a review

This review is made of studies whereby some types of fire test measuring instrument were compared based on their mode of operation, sensing ability, temperature resistance and their calibration mode used for aero-engine applications. The study discusses issues affecting temperature and heat flux measurement, methods of measurement, calibration and uncertainties that occur in the fire test. It is found that the temperature and heat flux measurements of the flame from the standard burner need to be corrected and taken into account for radiation heat loss. Methods for temperature and heat flux measurements, as well as uncertainties analysis, were also discussed.

High-resolution hot-film measurement of surface heat flux to an impinging jet

To investigate the complex coupling between surface heat transfer and local fluid velocity in convective heat transfer, advanced techniques are required to measure the surface heat flux at high spatial and temporal resolution. Several established flow velocity techniques such as laser Doppler anemometry, particle image velocimetry and hot wire anemometry can measure fluid velocities at high spatial resolution (µm) and have a high-frequency response (up to 100 kHz) characteristic. Equivalent advanced surface heat transfer measurement techniques, however, are not available; even the latest advances in high speed thermal imaging do not offer equivalent data capture rates. The current research presents a method of measuring point surface heat flux with a hot film that is flush mounted on a heated flat surface. The film works in conjunction with a constant temperature anemometer which has a bandwidth of 100 kHz. The bandwidth of this technique therefore is likely to be in excess of more established surface heat flux measurement techniques. Although the frequency response of the sensor is not reported here, it is expected to be significantly less than 100 kHz due to its physical size and capacitance. To demonstrate the efficacy of the technique, a cooling impinging air jet is directed at the heated surface, and the power required to maintain the hot-film temperature is related to the local heat flux to the fluid air flow. The technique is validated experimentally using a more established surface heat flux measurement technique. The thermal performance of the sensor is also investigated numerically. It has been shown that, with some limitations, the measurement technique accurately measures the surface heat transfer to an impinging air jet with improved spatial resolution for a wide range of experimental parameters.

Subcooled boiling incipience on a highly smooth microheater

Subcooled boiling incipience on a highly smooth microscale heater (270 μm × 270 μm) submerged in FC-72 liquid is investigated. Using high-speed imaging and a transient heat flux measurement technique, the mechanics of homogeneous nucleation on the heater are elucidated. Bubble incipience on the microheater was observed to be an explosive process. It is found that the superheat limit of boiling liquid is required for bubble incipience. It is concluded that boiling incipience on the microheater is a homogeneous liquid–vapor phase change process. This is in contrast to recent observations of low-superheat heterogeneous nucleation on metallic surfaces of rms roughness ranging from 4 to 28 nm [T.G. Theofanous, J.P. Tu, A.T. Dinh, T.N. Dinh, The boiling crisis phenomenon part I: nucleation and nucleate boiling heat transfer, Exp. Therm. Fluid Sci. 26 (2002) 775–792; Y. Qi, J.F. Klausner, Comparison of gas nucleation and pool boiling site densities, J. Heat Transfer 128 (2005) 13–20; Y. Qi, J.F. Klausner, Heterogeneous nucleation with artificial cavities, J. Heat Transfer 127 (2005) 1189–1196]. Following the explosive bubble incipience, the boiling process on the microheater can be maintained at much lower superheats. This is mainly due to the necking during bubble departure that leaves an embryo from which the next-generation bubbles grow.

A Local Heat Flux Measurement Technique for Inclined Heat Exchanger Tubes

This article presents the design, fabrication, and calibration of thermocouple pairs for local heat flux measurement. The intended application of the thermocouple pairs is on the tubes of phase-change heat exchangers experiencing heat fluxes on the order of 105 W/m2. Particular advantages of this technique are that it is accurate even for thin-wall tubes, there are no restrictions on the tube inclination, and it is economical. A thermocouple pair consists of a sheathed thermocouple which measures the tube outer surface temperature and a novel plug-type thermocouple which measures the tube inner surface temperature without interrupting the heat transfer phenomena. A calibration was performed by imposing a series of known heat fluxes on a condenser tube that had been outfitted with a number of thermocouple pairs and measuring the temperature difference between the thermocouples. The maximum uncertainty associated with the calibration constants was ±9.5%. This tube was then used as the condenser tube of a double-pipe heat exchanger to obtain local heat flux data that had previously been unattainable around a horizontal tube.

The dual thin-film probe for high-frequency flow temperature measurements

In turbomachines, the flow total temperature determination requires probes that are not only robust and withstand high temperatures, but also able to correctly determine their fluctuations. To obtain accurate measurements, three dual thin-film probes based on a transient heat flux measurement technique have been constructed for time-averaged and time-resolved quantities. The design of the probes is first discussed. The probes, placed behind a cascade in the von Karman Institute (VKI) compression tube facility CT-2, are validated through a comparison with thermocouples. Measurements are also successfully performed behind the rotor and the second stator of the 1.5 stage high-speed turbine in the VKI compression tube facility CT-3. The comparison is made with a cold wire probe. The maximum-minimum temperature fluctuations measured behind the rotor and the second stator are 24 and 7°, respectively.

401 衝撃波実験における熱流束計測 : 薄膜ゲージによる温度・熱流束計測法の開発

401 衝撃波実験における熱流束計測 : 薄膜ゲージによる温度・熱流束計測法の開発

Heat flux measurement techniques

Heat flux measurement techniques

Heat flux measurement techniques

Heat flux measurement is used in the field of fluid mechanics and heat transfer to quantify the transfer of heat within systems. Several techniques are in common use, including: differential temperature sensors such as thermopile, layered resistance temperature devices or thermocouples and Gardon gauges; calorimetric methods involving a heat balance analysis and transient monitoring of a representative temperature, using, for example, thin-film temperature sensors or temperature sensitive liquid crystals; energy supply or removal methods using, for example, a heater to generate a thermal balance; and, finally, by measurement of mass transfer which can be linked to heat transfer using the analogy between the two. No one method is suitable to all applications because of the differing considerations of accuracy, sensitivity, size, cost and robustness.Recent developments including the widespread availability and application of thin-film deposition techniques for metals and ceramics, allied with advances in microtechnology, have expanded the range of devices available for heat flux measurement. This paper reviews the various types of heat flux sensors available, as well as unique designs for specific applications. Critical to the use of a heat flux measurement technique is accurate calibration. Use of unmatched materials disturbs the local heat flux and also the local convective boundary layer, producing a potential error that must be compensated for. The various techniques in common use for calibration are described. A guide to the appropriate selection of a heat flux measurement technique is provided according to the demands of response, sensitivity, temperature of operation, heat flux intensity, manufacturing constraints, commercial availability, cost, thermal disturbance and acceleration capability for vibrating, rotating and reciprocating applications.