Transient Heat Transfer Measurements Research Articles

Determination of Local Heat Transfer Coefficients and Friction Factors at Variable Temperature and Velocity Boundary Conditions for Complex Flows

Transient conjugate heat transfer measurements under varying temperature and velocity inlet boundary conditions at incompressible flow conditions were performed for flat plate and ribbed channel geometries. Therefrom, local adiabatic wall temperatures and heat transfer coefficients were determined. The data were analyzed using typical heat transfer correlations, e.g., Nu=CRemPrn, determining the local distributions of C and m. It is shown that they are closely linked. A relationship lnC=A−mB is observed, with A and B as modeling parameters. They could be related to parameters in log-law or power-law representations for turbulent boundary layer flows. The parameter m is shown to have a close link to local pressure gradients and, therewith, near wall streamlines as well as friction factor distributions. A normalization of the C parameter allows one to derive a Reynolds analogy factor and, therefrom, local wall shear stresses.

Optimal design of E-type coaxial thermocouples for transient heat measurements in shock tunnels

Coaxial thermocouples have been widely used for transient heat transfer measurements in high-enthalpy shock tunnels. The one-dimensional semi-infinite heat conduction theory is typically used for temperature data processing. However, lateral heat transfer occurs due to material differences between the two electrodes and the junction, causing a deviation in the heat flux measurement from the prediction results of the one-dimensional semi-infinite heat conduction theory. Thus, the lateral heat conduction effect should be investigated to improve the accuracy and reliability of heat flux measurements. In this paper, the heat transfer in E-type (Chromel-Constantan) coaxial thermocouples was analyzed by numerically solving the two-dimensional axisymmetric heat conduction equation with the Du Fort-Frankel scheme. The maximum temperature point on the surface of the coaxial thermocouples shifted to the positive electrode during the heating process. The numerical simulation indicated that the surface temperature of the coaxial thermocouples and the derived heat flux were larger than the theoretical value. The heat flux measurement error of the coaxial thermocouples can be reduced by increasing the width of the positive electrode. Hence, 13 combinations of the diameters of positive electrode and negative electrode were designed for analysis. With the increase of positive electrode diameter or decrease of negative electrode, the heat flux measurement error kept on decreasing, which can be lower than 0.5% in some cases. The results of this study can provide a reference for the design and optimization of coaxial thermocouples.

An experimental investigation on pressure loss and local heat transfer characteristics in additively manufactured ribbed cooling configurations

Pressure loss and transient heat transfer measurements are performed on real-scale turbine cooling configurations at engine-relevant flow conditions. To determine the influence of rib turbulators and surface roughness on heat transfer and pressure loss, five additively manufactured models and one milled model were investigated, which distinguish in their manufacturing-related surface roughness and/or rib turbulator configurations. The experimentally determined friction factors are compared with data from a computed tomography (CT) scan, which was carried out for one of the additively manufactured models. An inverse discrete volume approach is applied, where the temperature response of the accessible outer wall caused by a sudden change in the temperature of the fluid flowing through the channel is measured. This allows the determination of locally resolved heat transfer coefficients on the non-accessible inner surface and provides information about the internal heat transfer processes. From the variety of experiments performed on the six models, the influence of surface roughness, one-sided high-blockade ribs, combination of these two, and the Reynolds number on friction factor and heat transfer coefficient is determined. The effects of roughness and rib configuration are separated from each other, and the expected measurement uncertainties are estimated by means of a sensitivity analysis.

Numerical investigations of the lateral heat transfer in coaxial thermocouples

Coaxial thermocouples have been widely used for transient heat transfer measurements in high-enthalpy shock tunnels. The one-dimensional semi-infinite heat conduction theory is typically used for temperature data processing. However, due to the material difference between the two electrodes and the junction, lateral heat transfer occurs, causing a deviation in the heat flux measurement from the prediction results of the one-dimensional semi-infinite heat conduction theory. Thus, the lateral heat transfer effect has to be investigated to improve the accuracy and reliability of heat flux measurements. In this article, the heat transfer in E-type (chromel-constantan) coaxial thermocouples was analyzed by numerically solving the two-dimensional axisymmetric heat conduction equation with the Du Fort-Frankel scheme. During the heating process, the maximum temperature point on the surface of the coaxial thermocouples moved to the positive electrode over time. The numerical simulation indicated that the surface temperature of the coaxial thermocouples and the derived heat flux were larger than the theoretical value. The heat flux measurement error of the coaxial thermocouples can be reduced by increasing the width of the positive electrode. The results of this study can provide a reference for the design and manufacture of coaxial thermocouples.

An experimental approach to determine local heat transfer characteristics in additively manufactured cooling configurations

Transient heat transfer measurements are performed on an additively manufactured real-scale cooling configuration with ribs on one side. A test rig is presented that operates at elevated pressure and provides a sudden temperature step in the fluid to heat the model walls. It enables the establishment of application-relevant flow conditions under which the local heat transfer characteristics are investigated while the manufacturing-related wall roughness can be preserved. The small dimensions prevent a direct temperature measurement in the cooling channels and a direct observation of the heat transfer surface is not possible. Thus, the fluid temperature is measured upstream and downstream of the model and the temperature response on the outer surface is measured by infrared thermography. Analytical modeling of the fluid temperature and a numerical method for calculating the 3D temperature propagation inside the model wall enable the determination of local heat transfer coefficients on the inner heat transfer surface. The crucial temperature data filtering and its effect on the results is described. A sensitivity analysis is performed to determine the best-suited evaluation times locally. The results of the developed method are compared to results from former measurements applying a well-established evaluation method on an up-scaled model with similar geometry.

Film Cooling Performance of a Fully Cooled Vane at High Subsonic Conditions

This work focuses on the experimental study of the heat transfer and film cooling characteristics of a fully cooled vane under high subsonic conditions; the transient heat transfer measurements with thermocouples were employed. The inlet Reynolds numbers of the cascade ranged from to and the exit Mach number was 0.8. Two freestream turbulence intensities (1.3% and 14.7%) and three mass flow ratios (5.5%, 8.4%, and 12.5%) were tested. The averaged film cooling effectiveness of the current vane with shaped holes was found to be increased by 15%–43% more than that of the vane only with cylindrical holes, and elevated turbulence intensity reduced the averaged film cooling effectiveness by 3%–26%. The heat transfer augmentation of the current vane was lower than that of the vane only with cylindrical holes. With the mass flow ratio increasing, the heat transfer augmentation gradually increased on the pressure side, whereas it was nearly kept invariable on the suction side. The net heat flux reduction results showed that the vane with shaped holes significantly improved the cooling performance, whereas elevated turbulence intensity significantly deteriorated the cooling performance.

Measurement of transient heat transfer in a vessel during filling steam and Nitrogen gas

Measurement of transient heat transfer in a vessel during filling steam and Nitrogen gas

Heated thin film gauge arrangements to reduce uncertainty in transient heat transfer measurements

This paper describes the development of heated double-sided thin film gauge configurations for transient heat transfer measurements. By heating the substrate it is possible to measure the heat flux over a range of surface temperatures and deduce the adiabatic wall temperature and the external heat transfer coefficient. The accuracy of the measurement depends on the stability of the regression of heat flux against wall temperature and can be improved by extending the range of wall temperature over which the regression is performed. In this paper we compare two methods of local heating: double-sided gauges with an underside thin film heater and self-heating double-sided gauges. Both arrangements have been used in the Oxford Turbine Research Facility to measure the heat transfer on the uncooled turbine shroud of the MT1 high-pressure turbine stage at engine-representative conditions. These measurements yield improved regressions compared to conventional techniques to determine the adiabatic wall temperature and the heat transfer coefficient.

A Fast-Response Calorimeter with Dynamic Corrections for Transient Heat Transfer Measurements

Robust fast-response transient calorimeters with novel calorimeter elements have attracted the attention of researchers as new synthetic materials have been developed. This sensor uses diamonds as the calorimeter element, and a platinum film resistance is sputtered on the back to measure the temperature. The surface heat flux is obtained based on the calorimetric principle. The sensor has the advantages of high sensitivity and not being prone to erosion. However, non-ideal conditions, such as heat dissipation from the calorimeter element to the surroundings, can lead to measurement deviation and result in challenges for sensor miniaturization. In this study, a novel transient calorimeter (NTC) with two different sizes was developed using air or epoxy as the back-filling material. Numerical simulations were conducted to explain the complex heat exchange between the calorimeter element and its surroundings, which showed that it deviated from the assumption of an ideal calorimeter sensor. Accordingly, a dynamic correction method was proposed to compensate for the energy loss from the backside of the calorimeter element. The numerical results showed that the dynamic correction method significantly improved the measurement deviation, and the relative error was within 2.3% if the test time was smaller than 12 ms in the simulated cases. Detonation shock tunnel experiments confirmed the results of the dynamic correction method and demonstrated a practical method to obtain the dynamic correction coefficient. The accuracy and feasibility of the dynamic correction method were verified in a single detonation shock tunnel and under shock tube conditions. The NTC calorimeter exhibited good repeatability in all experiments.

Measurement of three-dimensional flow structure and transient heat transfer on curved surface impinged by round jet

In this study, the transient heat transfer characteristics of a circular cylinder surface cooled by an impinging air jet were measured through thermographic phosphor thermometry. Time-resolved surface temperature fields were visualized under constant heat flux conditions. After the jet impingement on the circular cylinder, the curved three-dimensional (3D) wall jet developed toward both spanwise and streamwise directions, followed by most of the fluid moving along the curved surface due to the Coanda effect. Therefore, the high heat transfer region was elongated toward the streamwise direction. The impinging angle was established to be a dominant factor in heat transfer characteristics. The heat transfer was improved with an increase in the impinging angle, as the cooling area expanded toward the streamwise direction. To discuss the transient heat transfer characteristics, time-resolved two-dimensional Nusselt number distributions were obtained according to the varied impinging angles and Reynolds numbers. The secondary peak of heat transfer was not clearly observed due to the stabilizing effect of curvature on the wall jet. To understand the heat transfer phenomenon, the 3D flow structures of the jet on the curved surface were obtained using time-resolved volumetric particle tracking velocimetry (PTV). The velocity vectors and streamwise vortices were visualized according to the impinging angle and analyzed in three-dimensions. The 3D curved wall jet possessed a big counter-rotating streamwise vortex structure in the upper region of the attached wall jet on the cylinder wall. With an increase in the impingement angle, the streamwise vortex developed longer at the same Reynolds number. The 3D flow structure and heat transfer distribution were found to demonstrate similar tendencies.

Transient heat transfer measurements on pulsating and oscillating flows in a shock tunnel

Flow field over high-speed vehicles can be changed drastically with the addition of a spike. These spiked bodies are associated with pulsation and oscillation unsteady flow phenomena on a spiked-body configuration. The flow separation leads to fluctuations in pressure and heat transfer of high amplitude and frequency. An investigation is made in this study, to measure the heat flux using platinum thin film sensors for these two unsteady flow characteristics, which would provide additional insights and may lead to better understanding of such type of flows. Experiments are carried out on a sharp-tipped spike attached to a flat face cylinder in a shock tunnel operating at Mach 5.7. Provisions are made to vary the spike length to diameter ratio (L/D), ranging from L/D = 0.75, 1, 1.25 and 1.5 for current investigation. The measured heat transfer varied from 100 W/cm2 to 10 W/cm2 for a pulsation case (L/D = 0.75) and whereas for the oscillation case (except for the corner gauges) the sensors indicate a heat transfer variation between 50 W/cm2 to 5 W/cm2. In additions, the corner gauge shows a heat transfer of around 120 W/cm2 to 10 W/cm2. This indicates the region of shock-shock interaction. The results from schlieren technique, used for visualization of the flow field is compliments the quantitative results.

Evaluation of rate-determining step of methane hydrate decomposition by measurement of transient heat and mass transfer near solid–gas interface

The transient heat and mass transfer near a methane hydrate decomposition interface were quantitatively measured to evaluate the rate-determining factors of dissociation with high spatiotemporal resolution. In this study, we proposed a measurement system for the dissociation phenomenon near the hydrate interface, and the rate-determining step of the dissociation phenomenon was discussed in terms of the experimental results and numerical simulation using a dissociation model. The hydrate was generated around a water-like film under temperature of 274 K and pressure of 4.8 MPa of mixed gas comprising methane and helium in a temperature and pressure controlled system. A high-speed phase-shifting interferometer was used to measure the interfacial transient heat and mass transfer. In the present study, transient variations in the optical path length difference caused by the variation in the density field near the hydrate interface were visualized with temporal and spatial resolutions of 1 ms and 3.88 μm/pixel, respectively. The measured values were compared with the results of numerical simulations, assuming that the mass flux of methane was a function of the activation energy of dissociation. The comparison showed that the experimentally measured values exhibited similar tendency as the values estimated using the activation energy of approximately 60,000 J/mol. This result was also confirmed from the interface shape variation of the hydrate. Finally, the dissociation phenomenon of the hydrate was estimated as the rate-determining step of the reaction by using our special interferometer system.

Influence of test model material on the accuracy of transient heat transfer measurements in impulse facilities

The development of suitable heat-resistant materials and appropriate thermal structure design for a hypersonic aircraft requires highly precise heat transfer predictions. Unfortunately, existing techniques for measuring the transient heat flux by thermal sensors in impulse facilities are overly complex; any slight deviation from ideal conditions may lead to inaccuracy. In this study, the influence of different model materials leading to lateral heat conduction between model and sensor on the accuracy of heat flux measurements using type-E coaxial thermocouples was investigated. The behavior of the materials results to a deviation from the assumption of one-dimensional heat conduction more or less. The materials examined were stainless steel, aluminum, carbon steel, and polyamide, which are frequently used as model materials in ground tests. The influence of the model materials was estimated by comparing the heat flux derived from the junction temperature with the actual heat flux loading. Particular attention was paid to aluminum, which is extensively used as wind tunnel model material. An engineering-based approach is also presented to conduct high accuracy measurements. The results show that the difference in the thermal properties between the sensor and the model materials creates complicated lateral heat conduction between them; stainless steel 304 is suggest as the use of model material whenever high-accuracy heat transfer measurements are desired due to its similarity of the thermal properties to type-E sensors. The use of polyamide PA6 material resulted in a larger heat flux due to its smaller thermal effusivity and the aluminum and carbon steel led to lower heat fluxes due to their larger thermal effusivity. The deviation of either material increases over testing time.

Application of the Transient Heat Transfer Measurement Technique Using Thermochromic Liquid Crystals in a Network Configuration With Intersecting Circular Passages

The present study deals with the application of the transient thermochromic liquid crystal (TLC) technique in a flow network of intersecting circular passages as a potential internal turbine component cooling geometry. The investigated network consists of six circular passages with a diameter d = 20 mm that intersect coplanar at an angle θ = 40 deg, the innermost in three, the outermost in one intersection level. Two additional nonintersecting passages serve as references. Such a flow network entails specific characteristics associated with the transient TLC method that have to be accounted for in the evaluation process: the strongly curved surfaces, the mixing and mass flow redistribution at each intersection point, and the resulting gradients between the wall and passage centerline temperatures. All this impedes the choice of a representative fluid reference temperature, which results in deviations using established evaluation methods. An alternative evaluation approach is introduced, which is supported by computational results obtained from steady-state three-dimensional (3D) Reynolds-averaged Navier–Stokes equations (RANS) simulations using the shear-stress transport (SST) turbulence model. The presented analysis uncouples local heat transfer (HT) coefficients from actually measured local temperatures but uses the time information of the thermocouples (TC) instead that represents the fluid temperature step change and evolution along the passages. This experimental time information is transferred to the steady-state numerical bulk temperatures, which are finally used as local references to evaluate the transient TLC experiments. As effective local mass flow rates in the passage sections are considered, the approach eventually allows for a conclusion whether HT is locally enhanced due to higher mass flow rates or the intersection effects.

천이 열전달 측정 실험에서 3차원 전도 효과의 보정

Accurate heat transfer coefficient measurement of gas turbine component is very important for heat transfer design of components exposed to high temperature. Transient heat transfer measurement method with one-dimensional semi-infinite solid model assumption has been widely applied to gas turbine heat transfer studies for the last decades because it dose not require complex boundary conditions. But there are several defect in the technique caused by the 1D heat transfer assumption. In this paper, various analysis were performed to investigate the 3D conduction effect in each technique and introduced better model which can minimize the 3D conduction effect.

Experimental investigation of heat transfer characteristics on turbine endwall with full coverage film cooling

Transient heat transfer measurement by Thermochromic Liquid Crystal (TLC) is applied in this paper to investigate heat transfer characteristics of the nozzle endwall with full coverage film cooling. The endwall heat transfer coefficient (h) with different blowing ratios (M) ranging from 0.7 to 4.0 at a constant mainstream Reynolds Number 1.63×105 is measured and analysed. The experiment results show that the heat transfer characteristics of the endwall are significantly affected by the film cooling. However, this effect changes with blowing ratio and varies in different regions of the endwall. At low M (M=0.7), coolant jets get into the boundary layer of the secondary flow, and enhance heat transfer, but lead to highly uneven h on the endwall. With the increase of M, coolant jets start to detach from the endwall while the turbulent mixing begins to enhance the heat transfer of the endwall downstream. Therefore h on the endwall first decreases and then increases with M. However, different turning points (the cases of M = 1 or M = 1.5) and increasing trends with M (staying constant or increasing monotonically) appear in different regions of the endwall.

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.

Evaluation of Rate-limiting Process of Methane Hydrate Dissociation by Transient Interfacial Heat and Mass Transfer Measurement

Evaluation of Rate-limiting Process of Methane Hydrate Dissociation by Transient Interfacial Heat and Mass Transfer Measurement

Measurement of transient heat transfer in vicinity of gas–liquid interface using high-speed phase-shifting interferometer

The heat transfer process and initial stage of coupled convection at a gas–liquid interface are observed with high temporal and spatial resolutions in view of understanding phase transition dynamics such as evaporation or condensation for energy technologies. A high-speed phase-shifting interferometer is used to precisely measure the transient heat conduction and convection processes near the gas–liquid interface of a small water droplet. In the present study, the transient heat conduction around a water droplet interface during the adiabatic expansion process before the appearance of convection is visualized and examined. In the visualization experiment, transient density variations due to heat conduction in the vicinity of the gas–liquid interface are observed with temporal and spatial resolutions of 1ms and 8.83μm/pixel, respectively. It is determined that convection appears at approximately t=0.25s in a fast depressurization process, while transitions in both temperature and pressure are observed. In addition, the transient density variations and distributions of the gas phase before convection are compared with numerical simulations as an optical path length difference, and there is good agreement between the simulations and experimental results. The measurement methods developed in this study can be applied in the measurement of interfacial heat and mass transfers with high temporal and spatial resolutions.

Transient contact heat transfer measurements based on high-speed IR-thermography

This paper presents measurements of transient contact heat transfer between specimens made out of several ferrous alloys for varying loads. The temporal resolution of the results allows for a deeper insight into the contact behaviour of the paired surfaces. At the same time, the application of high-speed infrared thermography increases the accuracy of the temperature measurements compared to the K-type thermocouples commonly used.The experimental data show a positive correlation between applied load and contact heat transfer, and the significance of the softer material's hardness, both agreeing with existing literature. With increasing load, the load-related gradient of contact heat transfer approaches zero, suggesting a limiting load, at which the maximum area of contact and therefore the maximum contact heat transfer coefficient is reached. In addition, the results suggest, that the transient approach allows for a post-experimental classification of the investigated contact.