基于格子Boltzmann方法的微通道受热表面析晶污垢模拟

针对现有的污垢析晶沉积模型不能有效模拟真实污垢生长的问题,建立了一种引入析晶沉积动力学模型的多物理场耦合数值模型。模型基于格子Boltzmann方法和有限差分方法,模拟了微通道非等温热表面上近壁面处的沉积物溶质质量浓度分布和污垢生长过程,研究了流速、壁温和沉积物溶质质量浓度对微通道热表面污垢析晶沉积的影响。结果表明：沉积初始时刻流速和壁温对近壁面沉积物溶质质量浓度分布具有不同程度的影响,随着污垢不断生长,污垢-流体界面处的析晶沉积速率减小;相比于流速,沉积物溶质质量浓度对污垢热阻的影响更为显著。     

Void Fraction Variations in a Fractal-Like Branching Microchannel Network

Based on predictions of lower pressure drop penalties in fractal-like branching channels compared to parallel channels, an experimental investigation of two-phase void fraction variations was performed. The flow network, mimicking flow networks found in nature, was designed with a self-similar bifurcating channel configuration and etched 150 μ m into a 38.1 mm diameter silicon disk. A Pyrex® cover was anodically bonded to the silicon disk to allow for flow visualization. The length and width scale ratios between channels on either side of a bifurcation are fixed. The channel widths range in size from 100 μ m to 400 μm over a total channel length of approximately 17 mm. Experimental results of flow boiling are presented for a heater energy input power of 66 W and an inlet water flow rate of 45 g/min at a fixed inlet fluid temperature of 88°C. High-speed, high-resolution imaging was used to visualize the flow and quantify void fraction values in several channels within a branching structure. Both time-averaged and instantaneous two-dimensional void fraction data are presented, showing a correlation between channels at the same bifurcation level and between channels at different bifurcation levels.     

CHF in a Circumferentially Nonuniformly Heated Tube Under Low-Pressure and Low-Mass-Flux Condition (Influence of Inclined Angles Under High-Heat-Flux Condition)

Critical heat flux (CHF) is an important design factor for boiling two-phase flow equipment, such as boilers and others. In actual boiling systems, the water tube suffers from the nonuniform heating and/or tube inclinations. The objective of this investigation is to understand the influence of tube inclination on CHF characteristics under such high-heat-flux conditions. The experimental investigation was conducted with a forced convective boiling system by using a uniformly heated tube and a nonuniformly heated tube set at arbitrary inclination angles ?. The obtained CHF was strongly influenced by the circumferential location of local maximum heat flux point and tube inclination. In the case of the normal tube, the CHF always occurred by the liquid film dryout at the top of the tube. In the case of the nonuniformly heated tube, the influence of the inclination on the CHF characteristics strongly depended on the circumferential heat flux distribution. When the the heat flux at the bottom was higher than that at the top, two types of CHF mechanism, namely, low-quality CHF upstream of the test section under high-mass-flux condition, and liquid film dryout at the tube exit under low-mass-flux condition, were observed. When the heat flux at the top was higher than that at the bottom, intermittent dryout was observed as the dryout mechanism. These CHF characteristics could be categorized by using the CHF ratio against the value of the vertical upward flow with the modified Froude number, which corresponded to the influence factor of disturbance wave.     

Hotspot Analysis of Double-Layer Microchannel Heat Sinks

ABSTRACT

The performance of double-layer microchannel heat sinks are evaluated comparatively for the parallel flow, counter flow, and transverse flow configurations with and without hotspots as heating condition. Conjugate heat transfer analysis is performed by solving three-dimensional Navier–Stokes and energy equations using a finite volume solver. The flow is considered to be steady, incompressible, and laminar. Functional relations between the thermophysical properties of water and temperature are developed and used for numerical calculations with variable fluid properties. The thermal resistances, maximum temperature increase at the hotspots, temperature variation among the hotspots, and pressure drops are evaluated for the three heat-sink designs with two hotspot schemes (single hotspot at the center of the heat sink and multiple hotspots distributed uniformly at six peripheral locations). For the single-hotspot case, the parallel flow heat sink exhibited the lowest thermal resistance and temperature rise at the hotspot. For all the six multiple hotspot cases, the transverse flow heat sink exhibited the lowest thermal resistance and temperature variation among the hotspots.     

蒸汽双效溴化锂吸收式制冷机的新流程

针对目前国产蒸汽双效溴化锂吸收式制冷机体积大、性能系数（ＣＯＰ）低、铜材耗量多、制造成本高的特点,在认真分析传统蒸汽双效溴化锂吸收式制冷机流程的基础上,经过大量对比计算,提出了两种新溶液循环流程系统,新循环流程方案与传统流程方案相比,传热面积可大大减小,性能系数（ＣＯＰ）可大幅度提高,不但能降低制造成本,还可节省运行费用。     

Experimental Evaluation of One,Two, and Three Stage Evaporative Cooling Systems

The thermal performance of four different arrangements of evaporative air coolers have been evaluated experimentally during the hot summer of Kuwait. The systems include one-stage direct evaporative cooling (DEC), one-stage indirect evaporative cooler (IEC) linked to an external cooling tower, two-stage indirect/direct evaporative coolers (IEC/DEC), and three-stage system of evaporative cooling and mechanical vapor compression (IEC/DEC-MVC). Two variables are used to evaluate the system thermal performance, the thermal effectiveness and the energy efficiency ratio (EER). The data show that the IEC/DEC has the highest EER, followed by the DEC, IEC/DEC-MVC, and IEC. The DEC has the lowest effectiveness, succeeded by the DEC/IEC, IEC, and IEC/DEC-MVC. Coupling MVC with IEC/DEC extends the cooling range and can cool the ambient air dry bulb temperature over a range of 40°C to 15°C at any value of ambient air wet bulb temperature. Two experimental correlations have been developed for each one of the tested systems. The first one relates the effectiveness to water to air mass flow ratio (L/G) or Reynolds number (Re), while the second correlation relates the EER with effectiveness and L/G and/or Re. These relationships are very important in designing and optimizing the studied evaporative cooling units.     

Two-Phase Analysis of A Helical Microchannel Heat Sink Using Nanofluids

A three-dimensional helical microchannel heat sink (HMCHS) model is developed to investigate the heat transfer characteristics using Al₂O₃–water-based nanofluid. The two-phase mixture model with modified effective thermal conductivity and viscosity equations is employed for solving the problem numerically. The model developed is validated by comparing the results of Nusselt number with available experimental and numerical data for a wide range of Reynolds number. The detailed results of the thermal field are reported for the effects of helix radius (0.15–0.30 mm), pitch (0.5–2.0 mm), number of turns (7–10), and aspect ratio (1.5–3.0). The analysis presents a unique fundamental insight into the complex secondary flow pattern in the channel due to curvature effects.     

耦合太阳能集热的MVR蒸发结晶系统性能分析

针对高浓度含盐废水蒸发结晶过程中机械式蒸汽再压缩（mechanical vapor recompression, MVR）系统能效显著降低等问题,提出耦合太阳能集热的MVR蒸发结晶系统,并基于数值模型对系统关键运行参数进行了仿真分析。结果表明：针对质量浓度为2%的NaCl废水,耦合太阳能集热的MVR蒸发结晶系统压缩机耗功显著降低,系统性能系数 （coefficient of performance, COP） 达到24.96;随蒸发器浓缩倍率由4升高至12,压缩机耗功增加71.5%,集热面积减少72.9%;低压闪蒸有利于降低系统温度并提高物料处理量,但压缩机耗功随之增大。     

低雷诺数下液体的微尺度流动与传热

针对低雷诺数下单相水流经矩形微通道时的流动特性和换热特性进行了试验和理论研究。试验工质为去离子水，微通道宽度为50μm，高度为200μm，雷诺数2．3～15．6。试验结果表明，在低雷诺数下的流动特性和传统理论存在偏差：矩形微通道实验压降值与阻力系数均大于传统理论值的预测，阻力系数的最大偏差为7．9％；Poiseuille数比传统理论预测值大6％；但当热流密度为2．5～15W／cm^2时，换热特性的试验结果与传统理论较为吻合。     

Forced Convection Analysis of Discrete Heated Porous Convergent Channel

This study investigates numerically forced convection heat transfer and flow analyses of a passive heat exchanger for nonporous and partially filled porous channels with varying exit height (1, 0.5, and 0.25). Four discrete heat sources with uniform heat flux are simulated on the channel bottom wall. The partially filled porous channels are tested at two different porous block heights (0.5 and 1). The flow field and thermal analyses inside the channels are investigated across a wide range of Reynolds and Darcy numbers for Prandtl number of 0.71. The results reveal that the porous block and the exit height affect substantially the flow and heat transfer characteristics inside the tested channels. The Nusselt number is enhanced by 20–40% for the partially filled porous convergent channel (exit height = 0.25 and porous block height = 1) compared to the nonporous channel. Consequently, the heat exchanger size can be reduced by 37.5%. Moreover, the overall heat transfer performance parameter is enhanced with further increase in Darcy number at low Reynolds number. As a result, compact heat exchangers that provide superior heat transfer coefficients lead to development of macro- and microelectronic devices.     

间接蒸发冷却板式换热器的(火用)效率分析

在考虑壁面两侧空气温度实际变化的前提下,对间接蒸发冷却传热过程中影响yong效率的主要因素,如一次空气进口温度、一、二次空气流速之比,二次空气相对湿度,一、二次通道宽度之比以及壁面润湿率进行了分析研究,为实际换热器设计提供理论依据。     

Optimization of Water Consumption in Hybrid Evaporative Cooling Air Conditioning Systems for Data Center Cooling Applications

In the light of its potential to reduce energy consumption when coupled to the condenser of existing vapor compression systems, evaporative cooling technology still faces questions relating to its water consumption pattern and the effect of water quality on its performance. This paper presents an experimental investigation of the water consumption characteristics in a split air-conditioning system employing evaporative precooling of air flowing over the condenser coils. The effect of the cooling pad thickness on the coefficient of performance of the system is investigated by varying the pad thickness from 5 to 15 cm in step size of 5 cm. Condition of the exit air from the pad shows that evaporative cooling can be employed as a standalone method for cooling of data centers, with adequate humidity control systems in place. Results obtained on water consumption shows that the volume of water consumed does not increase proportionally with time, rather it increases by a factor of 1.8, 2.5, and 3.2, for 2, 3, and 4 hours of operation respectively, relative to 1hour of operation. Two parameters; salinity and turbidity were used to study the effect of water quality on the evaporative cooling process. Additionally, a method of optimizing the pump operation is introduced in this paper. Running the pump intermittently could cut the energy consumed by the water pump in such a hybrid system by up to 20%.     

锅炉受热面管子腐蚀成因分析

通过对锅炉失效受热面管子的宏观检查、化学成分分析、金相分析、电子探针分析，认为受热面管于失效的主要原因是凹坑腐蚀。     

Performance of Evaporative Condensers

Experimental investigation is conducted to study the performance of evaporative condensers/coolers. The analysis includes development of correlations for the external heat transfer coefficient and the system efficiency. The evaporative condenser includes two finned-tube heat exchangers. The system is designed to allow for operation of a single condenser, two condensers in parallel, and two condensers in series. The analysis is performed as a function of the water-to-air mass flow rate ratio (L/G) and the steam temperature. Also, comparison is made between the performance of the evaporative condenser and same device as an air-cooled condenser. Analysis of the collected data shows that the system efficiency increases at lower L/G ratios and higher steam temperatures. The system efficiency for various configurations for the evaporative condenser varies between 97% and 99%. Lower efficiencies are obtained for the air-cooled condenser, with values between 88% and 92%. The highest efficiency is found for the two condensers in series, followed by two condensers in parallel and then the single condenser. The parallel condenser configuration can handle a larger amount of inlet steam and can provide the required system efficiency and degree of subcooling. The correlation for the system efficiency gives a simple tool for preliminary system design. The correlation developed for the external heat transfer coefficient is found to be consistent with the available literature data.     

Comparative Analysis of Microchannel Heat Sink Configurations Subject to a Pressure Constraint

Leakage losses and ever-increasing power dissipation in the microprocessor are causing significant thermal, mechanical, and reliability problems. Conventional cooling methods are reaching their practical limits, and new methods of lowering the operating temperature of microprocessors are being explored. Microfluidics-based cooling schemes are one approach being considered. The implementation of microchannels for forced convection at the chip level shows much promise, as the effective heat transfer surface area and attainable heat flux are very favorable. A major design limitation to such an implementation is the pressure developed within such micro-flows and the stresses that could result. In this study, multiple discrete microchannel heat sink configurations are analyzed computationally and compared in a cooling capability sense, while total pressure drop across the flows is carefully considered. A single cooling channel over an energy source is split into two smaller channels, and so on, while total pressure drop is maintained constant, and specified such that all flows remain in the laminar regime. It is shown that for the configurations analyzed, there exists multiple-dependence optimum cooling configurations. In addition, it is shown that a slimmer design may be implemented with a relatively small effect on cooling capability. Furthermore, cooling capability dependence on total pressure drop of the flows is shown to be minimal for high-performing microchannel configurations.     

Fluid Dynamics and Heat Transfer Analysis of Three Dimensional Microchannel Flows with Microstructures

The subsonic gas flows through straight rectangular cross-sectional microchannel with patterned microstructures was simulated using the direct simulation Monte Carlo (DSMC) method. An implicit treatment for low-speed inflow and outflow boundaries for the DSMC of the flows in microelectromechanical systems (MEMS) is employed. The 3-D microchannel flows are simulated with the cross-section aspect ratio ranging between 1 and 5. The comparison between 3-D cases and 2-D case shows that when the aspect ratio < 3, the two extra side-walls in the 3-D case have significant effects on the heat transfer and flow properties. When the aspect ratio increases, the flow pattern and heat transfer characteristics tend to approach those of 2-D results. The 2-D simplification is found to be reasonable when the cross-section aspect ratio is larger than 5. The microchannel flows with microstructures are also calculated with three different Knudsen numbers regime cases, and each case is calculated with three different microstructure temperatures, 273 K, 323 K, and 373 K. One Knudsen numbers regime ranges between 0.72 and 1.8, another regime ranges between 0.24 and 0.6 and the other regime ranges between 0.08 and 0.2. The computations show that the cooling and heating effects of the microstructure temperature on flow properties are enhanced with decreasing Knudsen number, and the higher microstructure temperature accelerates the velocity of the flow at the locations above the microstructures.     

火力发电厂受热面安全性能检验重点分析

根据规定,电站锅炉在安装前应进行安全性能检验工作,对近年来受热面的安全性能检验过程中发现的典型问题进行了分析总结,并对产生缺陷的原因进行了理论分析,根据经验,提出火力发电厂受热面安全性能检验中应注意的事项。     

一种小型蒸发式车用液暖燃油加热器性能试验分析

对一小型蒸发式车用液暖燃油加热器,分别做了燃烧室温度场测量,电磁泵不同供油频率(油量)性能对比;水流量对热效率的影响及循环水泵性能试验等。通过以上试验,对该型号加热器性能有了一个比较全面的认识。     

Numerical Analysis of Flow and Thermal Performance of Liquid-Cooling Microchannel Heat Sinks with Bifurcation

Single-phase liquid-cooling microchannels have received great attention to remove the gradually increased heat loads of heat sinks. Proper changes of the flow path and/or heat transfer surface can result in much better thermal performance of microchannel heat sinks. In this study, a kind of rectangular straight microchannel heat sink with bifurcation flow arrangement has been designed, and the corresponding laminar flow and heat transfer have been investigated numerically. Four different configurations are considered. The effects of the bifurcation ratio (the initial channel number over the bifurcating channel number) and length ratio (the channel length before bifurcation over the bifurcation channel length) on laminar heat transfer, pressure drop, and thermal resistance are considered and compared with those of the traditional straight microchannel heat sink without bifurcation flow. The overall thermal resistances subjected to inlet Reynolds number and pumping power are compared for the five microchannel heat sinks. Results show that the thermal performance of the microchannel heat sink with bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sinks with larger bifurcation ratio and length ratio provide much better thermal performance. It is suggested to employ bifurcation flow path in the liquid-cooling microchannel heat sinks to improve the overall thermal performance by proper design of the bifurcation position and number of channels.     

Enhancement in Evaporative Effectiveness of an Evaporative Tubular Heat Dissipator Using Experimental Design Approach

An experimental investigation of evaporative effectiveness and mass transfer coefficient on a bundle of tubes of an evaporative tubular heat dissipator is presented. Based on the experiments, correlations of evaporative effectiveness and mass transfer coefficient are derived using multiple regression analysis. A statistical model is developed to correlate the operating variables using design of experiment approach by selecting central composite design of a response surface methodology. Results shown in this article indicate that as the cooling film flow rate increases, evaporative effectiveness and mass transfer coefficient increases provided that the air flow rate is constant which is flowing from underneath the tubes of the evaporative tubular heat dissipator. Derived correlations are helpful in improvement of the design of heat transfer devices and many other engineering applications. Consideration of relative humidity of upstreaming air as one of the operating variables leads to the contribution to heat and mass transfer study of evaporative tubular heat dissipators in the present investigation.