Experimental studies of influence of the turbulent flow structure on temperature distribution in a compact heat exchanger

Results of experimental investigation of temperature distribution over the surface of a complex heat exchanger (the Frenkel packing type) are presented. Measurements were carried out in the air flow between two sheets with triangular corrugations directed at 90° to each other. Measurements were carried out by the microthermocouples glued on the heated outer surface. The effect of Reynolds numbers, a gap between corrugated sheets, and substitution of one corrugated sheet by the smooth one on temperature distribution over the heat exchanger surface in the turbulent air flow is analysed. According to the performed experiments, there is a significant effect of a gap and applied perturbations on the type of temperature distribution over the perimeter of a heated cell.     

Experimental study of temperature field evolution in the channel with corrugated wall at a jump-like change in the heat flux

Results of experimental investigation of nonstationary temperature field in the flow and on the surface of the channel with corrugated wall are presented at a jump-like change of heat release in time. The changes were performed at the air flow in a rectangular channel, whose one heated wall was made as a plate with triangular corrugation oriented at 45° relative to the main flow direction. The microthermocouples with the thickness of below 10 microns were used to perform these changes. Investigations revealed typical regularities of temperature evolution in the flow and along the perimeter of the heated rib. Experimental data on time-temperature dependence are approximated well by the exponential function.     

Experimental investigation of local heat transfer in the packing with triangular channels

Results of experimental studies on temperature distribution over the surface of a complex-shape heat exchanger like “Frenkel packing” are presented. Measurements were carried out with the airflow between two corrugated sheets with triangular crimps directed at 90° relative to each other. Microthermocouples glued on the outer surface of the heater were used for measurements. The effect of contact points, Reynolds number, and gap between corrugated sheets on temperature distribution over the heat exchanger surface is analysed under the turbulent mode of airflow. The main attention is paid to temperature distribution over the heating surface in an elementary cell. According to measurements performed, there is insignificant effect of contact points and gap on the type of temperature distribution over the perimeter of heated cell.     

Stagnation point transient heat flux measurement analysis from coaxial thermocouples

The 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.     

Anisotropy of lattice heat conductivity of complex chalcogenides

The present paper gives the results of experimental heat conductivity studies carried out on a series of complex semiconductive AX and ABX₂-type compounds. Measurements were performed over a wide temperature range at different crystallographic directions of the materials studied. In doing so, regularities have been revealed in the variation of the heat conductivity coefficient, depending on crystal orientation and temperature as well as on the substitutions in respective sublattices.     

Numerical investigation of air free convection in a room with heat source

Mathematical modelling of three-dimensional steady free convective incompressible viscous gas flows in the rooms with a heat source is carried out within the framework of the Navier — Stokes equations with the effective viscosity determined on the basis of the κ-ɛ turbulence model. The investigation is carried out for the case of model rooms and heat sources having the form of rectangular parallelepipeds with square bases. The influence of the heat source power and the sizes of the room base on local and averaged values of the air velocity and temperature in the rooms is analysed. The flow pattern in the room is shown to have a torus-like shape. It is found that the variation of sizes of the room base rather than the capacity variation of the heat source is of determining importance for the gas motion character in a closed volume.     

Thermographic analysis of flow distribution in compact heat exchangers for a Formula 1 car

A non-intrusive approach is investigated to calculate the internal flow distribution in heat exchangers. In particular, the liquid flow rate can be determined in each tube of an air–liquid finned-tube heat exchanger. A purposely designed test bench impresses a sudden change of temperature of the liquid flowing through the heat exchanger. The thermal transient that follows is monitored by a thermographic camera. This measures the rise of surface temperature along each tube. The temperature evolution pattern is then correlated to the flow rate in the tube by simple mathematical processing. The heat exchanger is tested in still air. Modification is not required. The approach is tested on heat exchangers for a F1 race car, with encouraging results.     

Specific heat measurements on an Fe Rh alloy

Measurements have been carried out of the specific heat of an Fe_0.49 Rh_0.51 alloy over the temperature range 100–500 K. The entropy change at the antiferromagnetic to ferromagnetic phase transition is found to be 13±1 J kg^-1 K^-1. The measurements are found to be consistent with previously developed theory.     

Ultrasonic drying of foodstuff in a fluidized bed: Parametric study

The application of high power ultrasound for dehydration of porous materials may be very effective in processes in which heat-sensitive materials such as foodstuffs have to be treated. In fact, high-intensity ultrasonic vibrations are capable of increasing heat and mass transfer processes in materials. The application of ultrasonic energy can be made alone or in combination with other kind of energy such as hot-air. In this case, ultrasound helps in reducing temperature or treatment time. The aim of this work is to study the effect of air flow rate, ultrasonic power and mass loading on hot-air drying assisted by a new power ultrasonic system. The drying chamber is an aluminium vibrating cylinder, which is able to create a high intensity ultrasonic field in the gas medium. To that purpose the chamber is driven at its centre by a power ultrasonic vibrator at 21.8 kHz. Drying kinetics of carrot cubes and lemon peel cylinders were carried out at 40 degrees C for different air velocities, with and without ultrasound. The results show that the effect of ultrasound on drying rate is affected by air flow rate, ultrasonic power and mass loading. In fact, at high air velocities the acoustic field inside the chamber is disturbed and the effect of ultrasound on drying kinetics diminishes.     

半干旱草原下垫面动量和感热总体输送系数参数化关系研究

本文利用锡林郭勒草原2008年春季近地层涡旋相关系统和铁塔的风、 温平均梯度观测资料, 分析了总体输送系数随梯度Richardson数的变化特征, 建立了动量总体输送系数随大气稳定度、近地层风速以及感热总体输送系数随大气稳定度和近地层气温的关系. 中性条件下, 半干旱草原植被下垫面动量总体输送系数与近地层大气动力状态之间存在明显的相互作用, 总体输送系数与近地面层风速之间满足二次曲线拟合关系; 风速较小时, 大气动力特征对地表粗糙度长度的改变不是很明显, 动量总体输送系数随气流增强而增大; 而当风速较大时, 强风速会使植被高度发生改变, 动量总体输送系数随气流增强而减小. 另外, 感热总体输送系数与近地层气温之间也存在二次曲线关系. 动量总体输送系数与近地层风速之间的关系、感热总体输送系数与近地层气温之间关系的建立为总体输送系数参数化提供了重要途径, 同时该方案避免了对动力学粗糙度长度和热力学粗糙度长度的求解. 关键词： 总体输送系数 参数化 湍流通量 相似性函数     

Modelling of methanol-to-hydrogen steam reforming with a heat flux distributed along a microchannel

The flow of reacting mixture of methanol and steam in a 2D microslot was studied numerically at activation of the reactions on the channel wall. This modelling was carried out in the framework of Navier — Stokes equations for a laminar flow of multicomponent compressible gas. Correlations between thermal, diffusion, and physical-chemical processes were studied under the conditions of intense endothermic reaction and external heat supply distributed along the channel. It is shown that not only the amount of heat supplied to the reaction zone is essential, but also the mode of heat supply along the channel length is important, which allows optimization of the compact reactor for hydrogen production.     

TURBULENT NATURAL-CONVECTION FLOW OVER A VERTICAL FORWARD-FACING STEP

Measurements of turbulent natural-convection flow over a two-dimensional, vertical, forward-facing step are reported. Laser Doppler velocimeter and cold-wire anemometer were used to, respectively, measure simultaneously the time-mean turbulent velocity and temperature distributions and their turbulent fluctuation intensities. The upstream and downstream walls and the step itself were heated to a uniform and constant temperature. The experiment was carried out for a step height of 22 mm and a temperature difference between the heated walls and the free stream (ambient air), j T, of 30°C (corresponding to a local Grashof number Gr xi = 6.45 2 10 10 ). Measurements of the time-mean velocities and temperature distributions along with their turbulent fluctuation intensities are presented at various locations upstream and downstream of the forward-facing step. The present results reveal that the maximum local Nusselt number occurs in the vicinity of the reattachment region and it is approximately two-and-a-half times that of a flat-plate value at similar flow and thermal conditions. These measurements can serve as a test case for turbulence models in predicting numerically the flow and heat transfer characteristics in this geometry and these experimental conditions.     

The effect of channel orientation on heat transfer and wall shear stress in the bubbly flow

Results of experimental investigation of heat transfer and wall friction in the upward bubble flow in a flat inclined channel are presented. Measurements were carried out in the range of superficial liquid velocities of 0.3–1.1 m/s and different values of volumetric void fraction. The hydrodynamic structure was measured by means of the electrochemical method with the use of miniature friction sensors. The values of average friction and heat transfer at different channel orientation were determined. It is shown that in the bubble gas-liquid flow we can observe a significant dependence of friction and heat transfer on the angle of channel inclination.     

超临界LNG在错列S形翅片微通道的流动传热特性研究

作为一种新型微通道换热器,印刷电路板式换热器(Printed Circuit Heat Exchanger,PCHE)因比表面积大、耐高压和低温、海上适应性强以及便于模块化等特点,近年来逐渐成为浮式LNG接收站和浮式储存及再气化装置的主低温换热器首选.针对改进后的错列S型翅片,对超临界LNG在翅片通道内的流动传热特性展...     

Experimental and numerical study of heat transfer enhancement in a turbulent bubbly flow in a sudden pipe expansion

Results of an experimental and numerical simulation of heat transfer in an upward bubbly flowin a sudden pipe expansion are presented. The experimental study of the heat transfer has been performed using infrared thermography. The measurements of the bubble size before the pipe expansion area were carried out by the shadow photographymethod. The numerical simulation of the bubbly flow structure in the sudden pipe expansion has been performed using the Eulerian approach in the presence of heat transfer between the two-phase flow and the wall surface. The model uses the system of Reynolds-averaged Navier–Stokes equations in an axisymmetric approximation, written with consideration of the back effect of bubbles on the averaged and pulsation characteristics of the flow. It has been experimentally and numerically shown that addition of air bubbles causes a significant (up to 3-fold) increase in the heat transfer intensity, these effects growing with bubble concentration. The largest rise in the heat transfer has been revealed in the region of flow relaxation downstream of the flow attachment point.     

Convection heat transfer in a Maxwell fluid at a non-isothermal surface

Analysis is carried out to study the convection heat transfer in an upper convected Maxwell fluid at a non-isothermal stretching surface. This is a generalization of the paper by Sadeghy et al. [21] to study the effects of free convection currents, variable thermal conductivity and the variable temperature at the stretching surface. Unlike in Sadeghy et al., here the governing nonlinear partial differential equations are coupled. These coupled equations are transformed in to a system of nonlinear ordinary differential equations and are solved numerically by a finite difference scheme (known as the Keller-Box method) and the numerical results are presented through graphs and tables for a wide range of governing parameters. The results obtained for the flow and heat transfer characteristics reveal many interesting behaviors that warrant further study of nonlinear convection heat transfer.     

新型沟槽道微热管散热性能实验研究

微热管以其效率高、响应快且无能耗,在高功率集成微电子散热方面应用广泛。针对电子器件的小型化、高能耗发展趋势,本文提出一种新型沟槽道微热管结构,对该沟槽道微热管进行稳态和瞬态热性能实验研究,研究了风速、角度、加热功率等因素对该新型热管的热性能影响规律。结果表明,该微热管在整个散热器传热上起主导作用,性能比达到0.88,冷凝端温差为0.8℃,具有良好的均温性,该微热管加热功率为140 W,空气流速1.5 m/s时,换热系数可达2 359 W/(m^2·℃),热阻为0.27℃/W;高功率状态下可保持良好的热扩散性能,有效避免微热管的热应力集中,有望高效解决集成电子器件的散热问题。     

Investigation of heat transfer from a lead heat carrier to a tube streamlined longitudinally

Results of experimental studies on the local characteristics of heat transfer from a lead heat carrier to the surface of a cooled tube in an annular gap are shown at control and alteration of oxygen admixture content under the conditions of power circuits with heavy liquid-metal heat-transfer agent. This work is aimed at obtaining the grounded formulas for engineering calculations of heat transfer surfaces. Investigations were carried out at the lead temperature of 400–500 °C, the average velocity of heat-transfer agent of 0.1–1.5 m/s, the range of Prandtl number of 0.0123–0.0211 and Peclet numbers of 500–7000. The heat flux changed within 50–160 kW/m². Controllable changing content of oxygen admixture changed from the value of thermodynamic activity of oxygen from 10⁻⁵-10⁰ to saturation and higher with deposition of lead oxides near the heat-transferring surface.     

Experimental investigation of cooling of a plate by ionic wind from a corona-forming wire electrode

We report on the results of experimental investigations of the kinematic structure of ionic wind from a wire electrode placed near a heated plate, which plays the role of the earthed electrode. Experiments are carried out in a wide range of voltages for different polarities of the wire for several values of the electrode gap. We compare the structures of the flows emerging as a result of natural convection in open air for different positions of the plate and in the presence of a fast ionic wind jet that considerably intensifies heat transfer in the boundary layer at the heated planar electrode. Local temperature distributions over the plate surface are obtained, as well as the integral dependences of the effective heat removal on the electric parameters of the corona discharge. The velocity of air flows with ionic wind reaches 4 m/s, and the heat power removed from the plate for fixed overheating increases ninefold compared to the situation with natural convection.     

Characteristics of a gravity-assisted heat pipe-based heat exchanger

An experimental study of the performance of an air-to-air thermosyphon-based heat exchanger utilizing R-22 as the working fluid has been carried out to investigate its behavior under different operating conditions. A test installation has been developed to model a variety of HVAC real life applications. The results reported in this article describe the influence of various parameters such as: supply and exhaust air stream mass flow rates, stream temperatures and exhaust stream moisture content on the effectiveness of the heat exchangers. Heat exchanger heat flow hysteresis has been recognized. Some optimization criteria are presented.