Surface Junction Thermocouples Research Articles

Investigation of Coaxial Surface Junction Thermocouples Under Different Materials on Thermal Measurement

Coaxial surface junction thermocouples (CSJTs) are commonly applied to transient thermal measurements. Based on the measurement principle of CSJT, one-dimensional analytical inversion is applied for obtaining surface heat fluxes. However, with the thermophysical properties difference between the wall material and CSJT, transverse thermal conduction occurs that leads to a measurement error of surface temperature by CSJT. Hence, conventional temperature data processing methods can lead to surface heat flux measurement error. Therefore, quantitative analysis of lateral structural heat transfer should be carried out in the junction region and the interface between the sensor and the wall. In this paper, the multidimensional transmission of heat between three types of CSJTs and wall material under transient and long duration was investigated by numerical simulations. And the derivation of the axisymmetric heat transfer government equation was obtained by the improved Crank–Nicolson discrete scheme. We found the best wall materials for thermal matching with E-, J-, and K-type CSJTs, respectively. And the stainless steel tube can maintain a stable heat flux curve within a certain period. The conclusions of this study offer practical guidance for the selection of wall material and design improvement of CSJTs.

Technical study of fast-response coil-type coaxial surface junction thermocouple

Technical study of fast-response coil-type coaxial surface junction thermocouple

Fast-Response transient heat flux measurements in a plasma wind tunnel

A fast response heat transfer gauge has been developed to measure the radial distribution and the fluctuations of heat flux in a supersonic arcjet plasma wind tunnel. The heat flux gauge consists of 3.2 mm diameter coaxial surface junction thermocouple which was positioned behind a thin layer of PTFE and mounted in a ESA standard 50 mm diameter flat faced copper probe head. A non-dimensional impulse response for the heat flux gauge was identified using pulsed optical power deposition from a laser. The impulse response was used in combination with reference measurements from a calorimeter at a single location to determine the distribution and fluctuations in heat flux. Eight traverses of the PWK4 jet at the University of Stuttgart confirmed a symmetric Gaussian-like heat flux distribution with a centreline heat flux consistent to within ±6%. These distributions were measured in less than one second, representing a significant gain in productivity when compared to calorimeter-based heat flux measurements where probes must be held stationary at a number of locations in order to resolve spatial distributions. At this particular PWK4 operating condition, which had a flow stagnation enthalpy of approximately 15 MJkg−1, heat flux fluctuations of up to ± 140 kWm-2 (corresponding to a relative variation of 15% of the centreline heat flux) were identified near the vicinity of the nozzle centreline for frequencies from 4 Hz to 1 kHz.

Time-resolved stagnation temperature measurements in hypersonic flows using surface junction thermocouples

Fast-response coaxial surface junction thermocouples have been used to measure time-resolved stagnation temperature of the Mach 6 flow produced by the University of Southern Queensland’s hypersonic wind tunnel, TUSQ. The piston compression and the nozzle expansion of the test gas were found to be approximately isentropic for the first 65 ms of flow. Thereafter, the stagnation temperature reduces from T0≈560K due to the heat lost to the cold barrel, and this process can be modelled based on the measured barrel pressure history to simulate the stagnation temperature in TUSQ to within 2% of the actual value for the first 150 ms of flow. By operating the thermocouples at the flow stagnation temperature, the fluctuations of the flow stagnation temperature were investigated. A 3–4 kHz narrowband stagnation temperature fluctuation appearing after t≈65ms was measured, and found to be correlated with the transition to turbulence of the flow in the barrel.

Effectiveness of coaxial surface junction thermal probe for transient measurements through laser based heat flux assessment

The transient temperature measurements and subsequent prediction of heat flux are prime requirements to quantify instantaneous heat transfer characteristics in short duration unsteady flow phenomena. Coaxial surface junction thermocouples (CSJT) are efficient laboratory tools that can cater both the requirements of measuring continuous as well as instantaneous temperatures. In addition, they have the capability of fast response behaviour for the measurement of transient temperature even in harsh environments. Ease of fabrication process, robustness and cost effectiveness are some of the advantages of CSJTs over its counterparts. The present investigations mainly focus on CSJT as a potential “heat flux sensor” for short duration experiments. For this purpose, an E-type probe (3.25 mm diameter and 10 mm long) was prepared in-house. The fabrication process involves intentional plastic deformation between the two metallic thermo-elements of the probe to achieve a sensing junction of 20 μm. The characterization and quality of the sensing surface is supported through Electron Discharge X-ray (EDX) and Field Emission Scanning Electron Microscope (FESEM) studies. The probe was exposed to a continuous wave laser source of known wattage (in the range of 0.2 W–0.5 W) which acts as a source for step heat load. Transient temperatures recorded from the probe are further processed for heat flux computation through analytical and numerical methods. As a term of inference during the test window of 0.4 s, it is observed that temperature as well as heat flux values have a nice match in trend as well as magnitude with an uncertainty band of ±5%.

Transient surface heat flux measurement for short duration using K-type, E-type and J-type of coaxial thermocouples for internal combustion engine

The surface heat flux measurement in the internal combustion engine is imperious work, because the heat transfer rate is changing very quickly during combustion process in the internal combustion engine and has very transitory heat transfer in nature. Due to this, a fast response measuring thermal sensor is required to trace the heating rate in the internal combustion engine. The coaxial thermocouples are appropriate to measure extremely transitory surface heat flux due to having the response time in the range of in millisecond or less and can invariantly use for short time observation facilities due to their meticulousness measurements. The transient heat flux recovered from transient temperature is one of the conventional techniques and used in many engineering application such as internal combustion engine, aerodynamic vehicles cooling device, etc. In this existent exploration, the surface heat flux has measured of a four-stroke diesel engine at cylinder surface. In this backdrop, three (K-type, E-type and J-type) types of coaxial thermocouples have been contrived and calibrated to measure transient surface heat flux of engine cylinder. A field emission scanning electron microscopy (FESME) technique has been used for microstructural analysis of measuring the surface of these handmade coaxial thermocouples and an energy dispersive X-ray analysis (EDXA) technique used to qualitatively appraise of the coaxial surface junction thermocouples (CSJTs) materials composition. Using these coaxial thermocouples, the transient surface temperature has been recorded at four different experimental shot for short duration as well as its heat flux has been recovered using one-dimensional heat conduction modelling for a semi-infinite body. The average value of surface heat flux recovered for all experiment have a reasonable deviation of 3% to each other and have 0.075% for transient surface temperature. This investigation states that these hand-made coaxial thermocouples are the efficient candidate to measure surface heat flux in the application internal combustion engine.

Stagnation point transient heat flux measurement analysis from coaxial thermocouples

ABSTRACTThe transient heat flux measurement at stagnation point is a significant solicitation at highly compressed flow field environment. In aerodynamics surface heating point of view, the estimation of stagnation heat fluxes at the tip of a blunt body is very imperative. When the blunt body is exposed to high-speed flow field environments, at the stagnation point heat transfer would be maximum. The coaxial surface junction thermocouples (CSJTs) are convenient for short duration time scale due to the fast response in the range of millisecond or less (̴0.1 ms). These robust CSJTs have the tractability of intensifying them directly on any type of surface and can be used for routine measurement in ground-based impulse amenities as a temperature measuring devices where rapid heat loads are anticipated. In this work, three different types of coaxial thermocouples K-type, E-type, and J-type have been designed and contrived. The microstructural analysis of measuring surface property has been carried out to see the surface morphology using field emission scanning electron microscopy (FESME) and chemical characterization of these CSJTs materials using energy dispersive X-ray analysis (EDXA) technique is used to verify qualitatively appraise the CSJTs materials composition. The thermal coefficient of resistance (TCR) and sensitivity (S) of each coaxial thermocouple have been determined by using oil-bath calibration technique with the linear variation of resistance corresponding to the variation of temperature and found that these coaxial thermocouples are highly sensitive and suitable for highly transient heat transfer measurements. For this purpose, these three types of CSJTs have been tested under highly compressed heated air 310 K temperature for 100 ms at pressure 6.1 bar with Mach number unity (M = 1) using compressor test rig. Numerical simulation has also been carried out with these three RTDs to satisfy the experimental parameters using Ansys Fluent 15.0 and typical transient temperature recorded. Surface heat fluxes recovered from experimental and numerical transient temperatures histories using semi-infinite heat conduction modeling having good agreement with accuracy ±3% or less. This study divulges the expertise of these handmade coaxial thermocouples for transient surface heat flux measurement for short durations at highly compressed air facilities.

An experimental investigation towards calibration of a shock tube and stagnation heat flux determination

This paper aims at comprehensive investigations towards design and installation of a moderate size shock tube (7 m long having an inner diameter of 55 mm, 20 mm thickness) and its experimental calibration through comparative parametric study. Necessary instrumentations are incorporated for measuring shock speeds and pressure rise across primary as well as reflected shock, obtained using nitrogen/helium as driver gas. The experimental evidence shows reasonable agreement between theoretical (one-dimensional shock tube relations) results. Further, an E-type coaxial surface junction thermocouple (CSJT) has been prepared in the laboratory and oil-bath calibration experiment gives its 'sensitivity'. Measurement of instantaneous surface temperature rise and subsequent stagnation heat flux are obtained using CSJT mounted on a specially-designed end-flange. The thermocouple noted a maximum rise of temperature of 7,800 K, marked as a characteristic constant of the sensor; since it is found to be independent of the magnitude of the step change in temperature.

Comparative performance assessments of surface junction probes for stagnation heat flux estimation in a hypersonic shock tunnel

The aerodynamic environment in the test section of a hypersonic shock tunnel is very hostile as the high enthalpy flows prevail over the test model for a very short test time (∼1ms). It calls for the requirement of the high-speed thermal sensors to survive in this harsh environment. Coaxial surface junction thermocouples (CSJTs) have the characteristics features of very high response times that can be used to capture highly transient temperature data during short duration hypersonic flows in the shock tunnel. The present study aims at the comparative performance assessment studies of three different surface junction probes (E, J and T-type) in a hypersonic shock tunnel at a flow Mach number of 8.2. All of them are mounted simultaneously in a rake along with a pitot-probe, in the test section of the tunnel where they experience a step heat load through a slug of test gas prevailing for 1ms flow duration. Subsequently, the transient temperature histories are recorded at stagnation point of the junction probe and surface heat fluxes are predicted through one-dimensional heat conduction modeling. Side by side, stagnation heat flux is calculated independently through numerical simulations and analytical methods under same experimental flow conditions. The surface heat flux is recovered within a reasonable accuracy for E and T-type probes when experimental results are compared with numerical simulation and analytical solutions. In contrast, substantial deviations in surface heat flux value are observed for a J-type probe, which is mainly due to inconsistency in “thermal product (TP)” value of CSJT. However, they are quite sensitive in acquiring transient data during short time scale of measurements prevailing in the shock tunnel. The performance indicators such as “sensitivity and response time” for E-type CSJTs are found to be 58.96μV/°C and 21μs, respectively as compared to 43.82μV/°C and 29μs for J-type and 28.47μV/°C and 24μs for T-type, respectively.

Experimental techniques for thermal product determination of coaxial surface junction thermocouples during short duration transient measurements

The measurement of transient surface heat flux is a crucial parameter for short duration aerodynamic experiments. They are generally obtained from temperature histories by mounting calorimetric gauges (such as thin film sensors and coaxial surface junction thermocouples) on the aerodynamic surfaces. While recovering the surface heat fluxes from transient temperatures, the appropriate form of one-dimensional heat conduction modeling is employed. However, such predictions of surface heat fluxes mainly depend on the correctness of thermal properties commonly known as, “thermal product (TP)” of the sensing surface and the accuracy in surface temperature history. Many a times, the TP values do change due to the nature/type of materials and during the fabrication of the gauge. In the present study, it is intended to evaluate the thermal product values of the in-house fabricated coaxial surface junction thermocouples (CSJT: E and J-types) for short duration experiments. These CSJTs are in-situ designed, fabricated and calibrated in the laboratory. For estimating thermal product in millisecond time scales, two laboratory experiments are designed viz., “water droplet technique and water plunging technique” by which impulse heat loads are applied for 8ms and the transient temperature responses are acquired from CSJTs. The experimental evaluations of TP values are compared with the corresponding theoretical estimates for both types of CSJTs. Subsequently, the effects of TP values on surface heat fluxes are analyzed by comparing them with peak and average heat loads. It is observed that surface temperature histories and average heat flux for all the experiments are in very good agreement. The experimental determination of TP values for E-type CSJTs are in close resemblance (within ±3% accuracy) while a significant under-prediction of about 29% is noticed for experimentally determined TP values with respect to its theoretical estimates for J-type CSJT during “water-plunging” experiments. In turn, it affects the peak heat flux predictions from surface temperature histories. Based on the results of experiments, the E-type CSJTs are found to be better in comparison to J-type CSJTs in terms of its sensitivity and consistency in predicting surface heat flux accurately.

Performance assessment of thermal sensors during short-duration convective surface heating measurements

The determination of convective surface heating is a very crucial parameter in high speed flow environment. Most of the ground based facilities in this domain have short duration experimental time scale (~milliseconds) of measurements. In these facilities, the calorimetric heat transfer sensors such as thin film gauges (TFGs) and coaxial surface junction thermocouple (CSJT) are quite effective temperature detectors. They have thickness in the range of few microns and have capability of responding in microsecond time scale. The temperature coefficient of resistance (TCR) and the sensitivity are calibration parameter indicators that show the linear change in the resistance of the gauge as a function of temperature. In the present investigation, three of types of heat transfer gauges are fabricated in the laboratory namely, TFG made out of platinum, TFG made out of platinum mixed with CNT and chromel–alumel surface junction coaxial thermocouple (K-type). The calibration parameters of the gauges are determined though oil-bath experiments. The average value TCR and sensitivity of platinum TFG is found to be 0.0024 K−1 and 465 μV/K, while similar values of CSJT are obtained as, 0.064 K−1 and 40.5 μV/K, respectively. The TFG made out of platinum mixed with CNT (5 % by mass) shows the enhancement of TCR as well as sensitivity and the corresponding values are 0.0034 K−1 and 735 μV/K, respectively. The relative performances of heat transfer gauges are compared in a simple laboratory scale experiment in which the gauges are exposed to a sudden step heat load in convection mode for the time duration of 200 ms. The surface heat fluxes are predicted from the temperature history through one dimensional heat conduction modeling. While comparing the experimental results, it is seen that prediction of surface heat flux from all the heat transfer gauges are within the range of ±4 %.

A coaxial thermocouple for shock tunnel applications

A chromel-constantan coaxial surface junction thermocouple has been designed, fabricated, calibrated, and tested to measure the temperature-time history on the surface of a body in a hypersonic freestream of Mach 8 in a shock tunnel. The coaxial thermocouple with a diameter of 3.25 mm was flush mounted in the surface of a hemisphere of 25 mm diameter. The hypersonic freestream was of a very low temperature and density, and had a flow time of about a millisecond. Preliminary test results indicate that the thermocouple is quite sensitive to low temperature-rarefied freestreams, and also has a response time of a few microseconds (≈5 μs) to meet the requirements of short duration transient measurements. The sensor developed is accurate, robust, reproducible, and is highly inexpensive.

Design and fabrication of coaxial surface junction thermocouples for transient heat transfer measurements

Low cost coaxial surface junction thermocouples (CSJTs') have been fabricated in-house and calibrated to measure the transient surface temperature rise within a UNITEN's shock tube wall facility, consisting of K-type coaxial thermocouple elements. The aim of this paper is to explain the design technique of the CSJTs' and the difficulties that have occurred during the fabrication process. The microstructural analysis and the chemical characterization for these types of thermocouples have also been carried out to verify the surface morphology and to qualitatively evaluate the CSJT materials composition. The preliminary testing was performed to demonstrate the performance of these thermocouples to be used for measuring the surface temperatures and heat transfer rates under transient conditions. The preliminary results from shock tube tests have shown that these thermocouples have a time response on the order of microseconds and were suitable for making heat transfer measurements in highly transient conditions. It was concluded that the current construction technique produced gauges that were reliable, reproducible, rugged and inexpensive.

The influence of non-equilibrium dissociation on the flow produced by shock impingement on a blunt body

We describe an investigation of the effects of non-equilibrium thermochemistry on the interaction between a weak oblique shock and the strong bow shock formed by a blunt body in hypersonic flow. This type of shock-on-shock interaction, also known as an Edney type IV interaction, causes locally intense enhancement of the surface heat transfer rate. A supersonic jet is formed by the nonlinear interaction that occurs between the two shock waves and elevated heat transfer rates and surface pressures are produced by the impingement of the supersonic jet on the body. The current paper is motivated by previous studies suggesting that real gas effects would significantly increase the severity of the phenomenon. Experiments are described in which a free-piston shock tunnel is used to produce shock interaction flows with significant gas dissociation. Surprisingly, the data that are obtained show no significant stagnation enthalpy dependence of the ratio of the peak heat transfer rates with and without shock interaction, in contrast to existing belief. The geometry investigated is the nominally two-dimensional flow about a cylinder with coplanar impinging shock wave. Holographic interferometry is used to visualize the flow field and to quantify increases in the stagnation density caused by shock interaction. Time-resolved heat transfer measurements are obtained from surface junction thermocouples about the model forebody. An improved model is developed to elucidate the finite-rate thermochemical processes occurring in the interaction region. It is shown that severe heat transfer intensification is a result of a jet shock structure that minimizes the entropy rise of the supersonic jet fluid whereas strong thermochemical effects are promoted by conditions that maximize the entropy rise (and hence temperature). This dichotomy underlies the smaller than anticipated influence of real gas effects on the heat transfer intensification. The model accurately predicts the measured heat transfer rates. Improved understanding of the influence of real gas effects on the shock interaction phenomenon reduces a significant element of risk in the design of hypersonic vehicles. The peak heat transfer rate for the Edney type IV interaction is shown to be well-correlated, in the weak impinging shock regime, by an expression of the form [equation] for use in practical design calculations.

Heat transfer during transient compression: Measurements and simulations

Experiments have been performed to assess the utility of unsteady one-dimensional heat conduction modelling for the calculation of heat losses during a free piston compression process. Heat transfer measurements have been obtained within a gun tunnel barrel using surface junction thermocouple instrumentation. The gun tunnel was operated with a relatively heavy piston such that the shock waves induced by the piston motion were weak. Peak heat transfer values are estimated reasonably well by the unsteady one-dimensional model. However, overall quantitative agreement between the measurements and calculations has not been achieved at this stage, principally because the development of turbulent heat transport was not properly modelled.

Transient heat flux measurement using a surface junction thermocouple

A new form of surface junction thermocouple sensor has been developed and tested. The novel feature of the design is the use of a tapered fit between two coaxial thermocouple elements to form a thin, robust junction. The gauge has a response time on the order of 1 μs and is suitable for measuring large transient heat fluxes in hypervelocity wind tunnels. Asymptotic analysis is used to demonstrate the operating principles and to assess the errors associated with the finite thickness of the surface junction. Spectral deconvolution methods are used to infer a mean square optimal estimate of the surface heat flux from time resolved surface temperature measurements. This improved signal processing method is applicable to transient heat flux gauges of all types. Potential reducible error sources and other systematic errors are described. Measurements of the heat flux about the forebody of a cylindrical body in a hypervelocity flow demonstrate the functioning of the gauge and are used to obtain statistical estimates of the repeatability of the technique. The measured heat fluxes are compared with established theoretical predictions.

Assessment of effective thermal product of surface junction thermocouples on millisecond and microsecond time scales

Surface junction thermocouples are used extensively for transient heat flux measurements, but their accuracy is dependent on the effective thermal product (TP) of the gauge and this can be a function of the time scale of interest. In the present work the response of surface junction k-type thermocouples was investigated experimentally using a water droplet calibration technique (for millisecond times scales) and a small shock tube (for microsecond time scales). Different junctions formed by scalpel blade scratches and abrasive paper were investigated. When scratches from scalpel blades were used to form the junction, the TP identified from the water droplet calibrations consistently differs by approximately 20% depending on whether the junction was made on the chromel or alumel substrate, in accord with existing thermal properties data. However, the shock tube calibrations indicate that for scalpel-scratched junctions there is considerable variability in thermocouple response time due to effective junction depth variations produced during construction. In contrast, junctions formed with abrasive paper produced rise times consistently less than 1s, but the water droplet and shock tube experiments both indicated significant variability in the effective TP for these gauges. The consistency in TP for scalpel-scratched junctions for millisecond time scales and the variability for junctions created with abrasive grit for both the millisecond and microsecond time scales is attributed to the differences in the effective proximity of the junction to the insulation between chromel and alumel substrates. For junctions created with abrasive grit, the effective TP is approximately 30% smaller for microsecond time scales than it is for millisecond time scales.