Heat Transfer Characteristics in a Double-Pipe Heat Exchanger Equipped with Coiled Circular Wires

An experimental investigation has been carried out to study the enhancement in heat transfer coefficient by inserting coiled wire around the outer surface of the inner tube of the double-pipe heat exchanger. Insulated wires, with a circular cross-section of 2 mm diameter, forming a coil of different pitches (p = 6, 12, and 20 mm), were used as turbulators. The investigation is performed for turbulent water flow in a double-pipe heat exchanger with cold water in the annulus space for both parallel and counter flows. The experiments were performed for Reynolds numbers ranging from 4,000 to 14,000. The experimental results reveal that the use of coiled circular wires leads to a considerable increase in heat transfer coefficients compared with a smooth wall tube for both parallel and counter water flows. The mean Nusselt number increases with Reynolds number and pitch. The convective heat transfer coefficient for a turbulent water flow increases for all coiled wire pitches, with the highest enhancement of about 450% for counter flow and 400% for the parallel flow. New correlations for mean relative Nusselt numbers at different coiled wire pitches are provided.     

Evaluation of Nusselt number and effectiveness for a vertical shell-coiled tube heat exchanger with air bubble injection into shell side

Air bubble injection was employed to increase the heat transfer rate (Nusselt number) of a vertical shell and coiled tube heat exchanger in this article. Hot and cold water flowed into the coil side and shell side of heat exchanger, respectively, and air bubbles were injected inside the shell side of heat exchanger via a memorable method. Bubbles' vertical movement due to buoyancy forcing through the heat exchanger can enhance the heat transfer rate by mixing the thermal boundary layer, increasing the turbulence level of the fluid flow and increasing the shell-side fluid Reynolds number.     

Experimental Heat Transfer Coefficients of Waste Heat Recovery Unit in Detergent Manufacturing Process

The waste heat recovery unit is one of the most important units in the chemical process. It recovers waste heat from the exhaust gas of the process resulting in the reduction of heat loss. In the detergent manufacturing process, the particulate air leaving the spray dryer is the exhaust gas containing the large amount of heat. Therefore, a waste heat recovery unit can significantly reduce heat loss. In this work, heat transfer coefficients of a waste heat recovery unit in the detergent manufacturing process were studied. Waste heat from the particulate air is recovered in the shell and coiled tube heat exchanger. The particulate air flows in the shell side, and water flows in the tube side. Four coiled tubes with different coil pitches were investigated. The results show that the tube-side heat transfer coefficient increases as the coil pitch decreases. Loading ratio also has an important effect on heat transfer coefficients. The increase of loading ratio leads to a lower value of the overall heat transfer coefficient. From 100 experiments, empirical correlations for the prediction of tube-side and shell-side heat transfer coefficients were proposed. The results indicate that the predicted heat transfer coefficients agree well with the experimental values.     

Study on Heat Transfer Performance of Skin Heat Exchanger

Cooling technology is facing new challenges with the increase of electronic equipment power onboard aircraft. The traditional heat sink based on high-altitude bleed air does not satisfy this increase of cooling demands. In this article, an air/air-type skin heat exchanger is studied for cooling aircraft electronic equipment. It uses outside high-altitude cold air rather than bleed air as a heat sink. This cooling technology can effectively remove the heat load of high-power electronic devices without greatly increasing aircraft performance penalty. To assess its high-altitude heat transfer performance, an experimental prototype was designed and made. Some experiments were conducted on a ground experimental test. The heat transfer criteria formulas were obtained for both the side air in the skin heat exchanger and its convective heat transfer coefficients. Based on these experimental analyses, the heat transfer performances of the skin heat exchanger in a high-altitude cruise condition are deduced when it is assumed to be installed at an unfavorable position and a favorable position, separately. This work tries to provide a technical support for its future onboard application.     

颗粒帘换热器中气粒两相换热特性实验研究

根据某1 t/h燃煤工业锅炉空气预热器的热力参数,设计并搭建了一套颗粒帘空气预热器模拟实验系统,研究了不同进气温度Tg0(150~300℃)、进气速度Vg0(0.9~1.5m/s)、颗粒帘进口厚度b0 (60~180 mm)、颗粒粒径dp(150~212μm)以及不同颗粒质量流量ms (550~2150 g/s)工况条件下热空气与进口温度tp0=20℃的硅砂颗粒帘间的换热特性。结果表明:影响颗粒帘换热器中气粒两相换热特性因素的重要性次序为进气温度、进气速度、颗粒质量流量、颗粒粒径、颗粒帘进口厚度;换热端差最低可至4.5℃,最大可达87℃;颗粒帘及颗粒帘出口气流的温度沿颗粒下落方向在前期上升迅速(186~475℃/m)而后期上升比较缓慢(60~108℃/m),并且在0~0.5 m和0.5~1.0 m的高度范围可分别用线性和对数方程来描述。     

水平螺旋管内超临界CO2冷却换热的数值模拟

采用RNG k-ε 湍流模型对超临界CO₂流体在内径为4 mm, 长度2000 mm, 节距为10 mm, 曲率为0.1的水平螺旋管内的冷却换热进行了数值模拟.研究了质量流量、热流量以及压力对换热系数的影响, 并和超临界CO₂在水平直管内的冷却换热进行了对比.研究结果表明, 超临界CO₂在水平螺旋管内流动产生的二次流强于水平直管内的二次流, 前者的换热系数大于后者; 换热系数随质量流量的增加而增大; 在似气体区, 换热系数随着热流量的增加而增大, 而在似液体区, 热流量对换热系数几乎没有影响; 换热系数峰值点随着压力的升高而下降, 并向高温区偏移.     

Empirical Study of an Air-to-Air Heat Pipe Heat Exchanger in Tropical Climates

Abstract

This article presents the empirical study of a heat pipe heat exchanger that was directly experiencing the ambient tropical air in its evaporator section. The performance of the heat pipe heat exchanger was monitored during two weeks of operation to determine the performance curves. The temperature of return air was controlled at about 22°C as the representative temperature of inside air, and a face velocity of 2 m/s on the heat pipe heat exchanger coil was established for this purpose. It was found that for the present investigation, the heat pipe heat exchanger can pay for itself in 1.5 years.     

Study of Automatic Transmission Fluid in a Serpentine Minichannel Heat Exchanger: An Experimental Approach

An experimental investigation was conducted on automatic transmission fluid cooling in a minichannel heat exchanger using a closed-loop integrated thermal wind tunnel test facility. Effects of automatic transmission fluid Reynolds number (Re_L) on heat transfer coefficient and Nusselt number were examined within the Re_L of 3–30 for air-flow Re of 1,450–5,200. Effects of serpentine on heat transfer enhancement and flow characteristics were evaluated through Dean number analysis. The analysis of Eckert number and Brinkman number showed a contribution to the viscous heating even for a low Re_L in the minichannel. The study showed enhanced heat transfer characterizations of the multi-port minichannel heat exchanger.     

Thermo-Hydraulic Behavior of Microchannel Heat Exchanger System

This article presents an experimental study of thermo-hydrodynamic phenomena in a microchannel heat exchanger system. The aim of this investigation is to develop correlations between flow/thermal characteristics in the manifolds and the heat transfer performance of the microchannel. A rectangular microchannel fabricated by a laser-machining technique with channel width and hydraulic diameter of 87 μm and 0.17 mm, respectively, and a trapezoidal-shaped manifold are used in this study. The heat sink is subjected to iso-flux heating condition with liquid convective cooling through the channels. The temporal and spatial evolutions of temperature as well as total pressure drop across the system are monitored using appropriate sensors. Data obtained from this study were used to establish relationships between parameters such as longitudinal wall conduction factor, residence and switching time, and thermal spreading resistance with Reynolds number. Result shows that there exist an optimum Reynolds number and conditions for the microchannel heat exchanger system to result in maximum heat transfer performance. The condition in which the inlet manifold temperature surpasses the exit fluid temperature results in lower junction temperature. It further shows that for a high Reynolds number, the longitudinal wall conduction parameter is greater than unity and that the fluid has sufficient dwelling time to absorb heat from the wall of the manifold, leading to high thermal performance.     

周期性波纹通道内换热与流动的数值研究

采用曲线坐标系下压力与速度耦合的SIMPLER算法,数值研究了一种紧凑换热器中波纹通道内周期性充分发展的层流流动与换热情况,流动Re数的范围为100~1100,Pr数为0.7.计算考察了不同波纹高度、波纹间距对流动与换热的影响,并对模型参数进行了性能评价.计算结果表明,整体Nu数及fRe数随着流动Re数的增加而增加.随着波纹高度的增加或波纹间距的减小,换热增强,特别是在高Re数下波纹高度的增加更加强化换热.最佳波纹高度和间距分别为1.15 mm和13 mm.     

油基钻屑螺纹推进式换热器流动传热特性

对油基钻屑在螺纹推进式换热器内的流动换热过程进行了数值模拟,研究了螺杆转速、油基钻屑雷诺数Re和螺纹截面形状对流动换热的影响.结果表明:随着螺杆转速增加,传热系数、油基钻屑出口温度均增大;同时发现,当雷诺数Re＜250时,壳侧Nusselt数随雷诺数Re增大而迅速增大,此后雷诺数对Nusselt数影响较小;Nussel...     

立式螺旋管汽液两相流沸腾传热恶化实验研究

本文实验研究了低质量流速(500～800 kg·m~(-2)·s~(-1))下的立式螺旋管临界热负荷,对沸腾传热恶化壁温特性和螺旋管临界热负荷影响因素进行分析.并同前人工作汇总,以流量,压力和临界干度为参数,建立了基于局部条件假设的立式螺旋管临界热负荷查询表(Look-Up-Table),共272个点.压力范围6.5～21 MPa,质量流速范围500～1800 kg·m~(-2)·s~(-1).     

Comprehensive Study of the Effect of the Addition of Four Drag Reducing Macromolecules on the Pressure Drop and Heat Transfer Performance of Water in a Finned Tube Heat Exchanger

Abstract

In the present study the effects of the addition of four drag reducing agents (DRA), including carboxy methyl cellulose with high molecular weight (DRA1) and medium molecular weight (DRA2), polyacrylamide (DRA3) and the natural polymer, xanthan gum (DRA4), to water on the pressure drop and heat transfer performance in a finned tube-heat exchanger were compared. Laminar flow (Reynolds number (Re) <1400) was studied to transfer heat between water and air in the finned tube heat exchanger. The results showed that DRA1, with a maximum %DR of 26%, and DRA4, with a maximum %DR of 5%, were the highest and the lowest obtained results, respectively. In the case of heat transfer reduction percentage (%HTR), DRA4, having more than 34.5%, was the highest, and DRA1, with about 13.7%, was the lowest result for the concentration range of 0-100?ppm and temperature range of 40–65?°C.     

非均匀热流边界条件下螺旋管内换热特性

对均匀和非均匀热流边界条件下螺旋管内湍流换热进行了数值模拟,结果表明:当螺旋管表面加热功率一定时,相同Re数下均匀热流边界条件时螺旋管截面周向局部Nu数高于非均匀热流边界条件;非均匀热流边界下充分发展段的平均Nu数小于均匀热流边界;相同的De数下,曲率较小的螺旋管换热系数大。     

Numerical simulation of laminar forced convection of water-CuO nanofluid inside a triangular duct

In this article, distilled water and CuO particles with volume fraction of 1%, 2% and 4% are numerically studied. The steady state flow regime is considered laminar with Reynolds number of 100, and nano-particles diameters are assumed 20 nm and 80 nm. The hydraulic diameter and the length of equilateral triangular channel are 8 mm and 1000 mm, respectively. The problem is solved for two different boundary conditions; firstly, constant heat flux for all sides as a validation approach; and secondly, constant heat flux for two sides and constant temperature for one side (hot plate). Convective heat transfer coefficient, Nusselt number, pressure loss through the channel, velocity distribution in cross section and temperature distribution on walls are investigated in detail. The fluid flow is supposed to be one-phase flow. It can be observed that nano-fluid leads to a remarkable enhancement on heat transfer coefficient. Furthermore, CuO particles increase pressure loss through the channel and velocity distribution in fully developed cross section of channel, as well. The computations reveal that the size of nano-particles has no significant influence on heat transfer properties. Besides, the study shows a good agreement between provided outcomes and experimental data available in the literature.     

波纹板式空冷器阻力与传热特性实验研究

在可改变风量和热水流量的实验条件下,对波纹板式空冷器的阻力与传热特性进行实验研究。得到了空气侧的阻力降关联式以及两侧的对流换热系数关联式,其适用于热水雷诺数在2000-8000之间、空气雷诺数在2000-10000之间。在相同工况下,比较了波纹板式、光管式和翅片管式空冷器的性能指标,结果表明:迎面风速在2.45-4.1 m/s之间,波纹板式空冷器传热系数达到100-160 W/m2/℃;约比光管式提高70%,但只有以管束外表面为基准的翅片管式传热系数的六分之一;板式空冷器单位体积换热量约是翅片管式空冷器的1.5倍,是光管式的15倍;板式空冷器单位功耗换热量约是光管式空冷器的5.5倍,而翅片管式空冷器与光管式空冷器则相差不大。     

微细多孔介质中流动及换热实验研究

本文对空气流过烧结微细多孔介质的流动和对流换热进行了实验研究。分析了不同颗粒直径条件下摩擦阻力系数与等效雷诺数的关系,并通过实验数据拟和得到了渗透率K和惯性常数F。结果表明:当颗粒直径比较大时,摩擦阻力系数的实验结果与计算关联式基本吻合;随着颗粒直径的减小,二者的差别增大。通过实验得到了微细烧结多孔介质内部体积平均对流换热系数,并与已有关联式的计算结果基本吻合。     

Implementation of Liquid Crystal Thermography to Determine Wall Temperatures and Heat Transfer Coefficients in a Tube-in-tube Heat Exchanger

Liquid crystal thermography was used in a water-operated concentric tube-in-tube heat exchanger to determine local annular heat transfer coefficients at the inlet region. An annular diameter ratio of 0.54 was considered with the inlet and outlet orientated perpendicularly to the axial flow direction. Both heated and cooled cases were considered at annular Reynolds numbers ranging from 1,000 to 13,800. Wall temperature distributions were directly measured by means of a coating of thermo-chromic liquid crystals. Local heat transfer coefficients at the inlet were higher than those predicted by most correlations, but good agreement was obtained with some literature.     

温度和雷诺数对表面活性剂溶液传热的影响

对不同温度和雷诺数下阳性离子表面活性剂十六烷基三甲基氯化铵(CTAC)溶液在循环回路中的传热特性进行了实验研究。在水中加入表面活性剂后溶液的传热特性明显降低,在不同的温度工况下均存在一个临界雷诺数,随着溶液温度的改变而发生变化。在不同的实验浓度条件下均存在一个临界温度,在临界温度以下时,临界雷诺数随温度升高而增加;在临界温度以上时,临界雷诺数随温度升高而急剧下降。分析该表面活性剂溶液阻力减小和传热性能降低之间的关系,提出了通过对溶液温度的控制来改变减阻流体传热特性的方法。     

Experimental Investigation of Transient Forced Convection of Liquid Methane in a Channel at High Heat Flux Conditions

Transient heat transfer of liquid methane under forced convection in a 1.8 mm × 1.8 mm asymmetrically heated square channel was investigated. This study is aimed at understanding the heat transfer behavior of cryogenic propellant in cooling channels of a regeneratively cooled rocket engine at the start-up condition. To simulate high heat load conditions representative of regeneratively cooled rocket engines, a high heat flux test facility with cryogenic liquid handing capabilities was developed at the Center for Space Exploration Technology Research. The time history of inlet and outlet fluid temperatures and test section channel wall temperatures were measured at high heat flux conditions (from 1.19 to 3.80 MW/m²) and a Reynolds number (Re) range of 1.88 × 10⁵ to 3.45 × 10⁵. The measured wall temperature data point toward possible film boiling within the test section during certain tests, particularly with higher heat fluxes and lower Reynolds number conditions that resulted in higher wall temperatures. The transient average Nusselt numbers (Nu_L) of the channel obtained from the experimental measurements are lower than those calculated from the Sieder–Tate correlation (Nu_O); however, the ratio (Nu_L/Nu_O) increases with the increase in Reynolds number. The ratio is around 0.25 at the lower end of Re and then increases to 0.7 at the maximum Re studied in the present investigation.