蒸发器芯层内气液界面的波动原因

毛细抽吸两相回路（CPL）在正常稳态工作时，其蒸发器主要以芯层表面蒸发的方式传热。在此基础上，给出了多孔芯传热的三层模型，并提出等厚气膜薄层模型，分析了气液界面的形状及引起气液界面波动的原因。通过分析得出，蒸气在薄层内的流动阻力主要由芯层本身的特性来决定，而蒸气薄层厚度的精确值对其影响并不大。     

平板型环路热管温度波动现象研究

环路热管(loop heat pipe,LHP)是一种靠蒸发器内毛细芯产生毛细力驱动回路运行,利用工质相变来传递热量的高效传热装置.研制了一套小型平板式蒸发器、风冷式冷凝器的环路热管(mLHP),mLHP的毛细芯为500目不锈钢丝网,工质为丙酮和甲醇.蒸发器、冷凝器以及所有管路均由紫铜制成.着重研究了平板型mLHP在不同热负荷条件下的温度波动特性.实验结果表明,平板式mLHP在某些热流密度区间容易发生温度波动;同时,重点研究了工质对mLHP系统温度波动的影响,并给出相应的合理解释.     

平板型双毛细芯蒸发器LHP的实验研究

实验设计了平板型双毛细芯蒸发器环路热管系统,并针对所设计的系统搭建了实验平台,对其进行启动和变工况特性研究,考虑不同因素如充灌率、重力倾角特对系统性能的影响。实验结果显示:平板型LHP的双毛细芯蒸发器可以提高系统的热传输能力,减小侧壁导热对补偿腔的影响,降低系统的运行温度,改善和提高系统的运行性能。     

双层毛细芯对环路热管传热性能的实验分析

环路热管作为一种高效的相变传热装置,其性能与位于蒸发器和储液槽之间的毛细芯结构密切相关。为了更深入研究双层毛细芯对环路热管传热性能的影响,利用不同颗粒直径铜粉制备双层毛细芯,在毛细芯总厚度为5 mm的条件下,通过调整大粒径和小粒径层的相对厚度来改变毛细芯厚度比,对平板型蒸发器环路热管启动和变工况运行进行实验测试。实验结果表明:在同一工况下,不同厚度比的双层毛细芯启动特性存在显著差异,启动过程中出现小粒径层蒸发效率低引起的温度过冲和环路热管中气液两相流变化导致的温度振荡;同时存在一个较优的双铜层毛细芯厚度比,大粒径(180～280μm)铜层厚度为3 mm可提高蒸发效率,小粒径(56～71μm)铜层厚度为2 mm可提供足够毛细抽吸力保证环路热管稳定运行。搭载该厚度毛细芯的环路热管不仅启动速度快(125 s),而且总热阻和蒸发器壁面温度均最低,最大加热功率达到120 W(21.10 W/cm~2),对应热阻为0.17 K/W。     

纳米流体毛细弯液蒸发界面热质迁移特性分析

基于增广杨拉普拉斯方程的毛细弯液面薄膜蒸发区的传热传质模型,数值分析了过热度和纳米流体工质对毛细弯液面薄膜蒸发区热质迁移特性的影响。结果表明,过热度增大导致薄膜区范围减小,蒸发界面热流密度增大,薄膜区总换热量增大,但同时减弱了薄膜界面的稳定性。在传统流体工质中添加合适的纳米颗粒,纳米流体运动粘性系数随体积分率增大而减小,导热系数随体积分率增大而增大,影响其传热传质效果。较大体积分率的纳米流体,其薄膜厚度更小,薄膜区热流密度和蒸发质量流率更大,但同时蒸发界面的稳定性减弱。不同种类的纳米流体对毛细弯液蒸发界面的影响也较为明显,具有较低运动粘性系数和较高导热系数的纳米流体能够迁移更多的热量。     

毛细蒸发弯月面宏观几何特征变化规律的数值分析

毛细蒸发弯月面的传热传质是毛细相变回路的关键传热过程,其中弯月面接触角和有效半径是分析其热传输极限的重要信息。考虑蒸发弯月面非均匀蒸发特质,提出利用延展弯月面薄膜区传热模型,求解宏观弯月面接触角和有效半径的计算方法,在此基础上,分析了各种传热影响因素对毛细弯月面宏观特征的影响。分析表明,该方法可有效地反映高热流密度下界面蒸发的自调节机制,也可很好地解释装置热传输极限受各种传热因素的影响。     

不同加热方式下环路热管蒸发器可视化研究

针对环路热管内部工质相变及流动换热问题,设计了环路热管蒸发器中心通道可视化实验平台,研究了不同加热方式对热管内工质状态和传热特性的影响。结果表明：加热方式直接影响热管10W启动过程,双面加热启动速度最快。相同热载荷时,不同加热方式下环路热管热阻及蒸发器中心通道内液面高度和成核情况存在差异。10W - 40W热载荷时,随着热载荷的增大,三种加热方式的传热热阻均在减小。40W-50W热载荷时,顶部加热方式下的热管性能出现恶化,底部加热传热性能出现停滞,仅双面加热性能稳定并有提高趋势。随着热负荷的增加,蒸发器中心通道内气液界面升高、气泡的产生变得更加剧烈,蒸发器通过吸液芯向储液器的漏热量增加,进而影响环路热管的性能。     

海水淡化中水平管降膜蒸发器传热传质的研究现状

水平管降膜蒸发器广泛应用于制冷、食品和海水淡化领域,其传热传质过程直接影响到整个蒸发器的性能,因而受到各国研究者的重视。介绍了水平管降膜蒸发器的传热传质过程及影响因素,通过对传热温差、蒸发温度、热通量、喷淋密度、管束布置方式等对液膜厚度、液膜流动状态、热阻、液体黏度、表面张力等的影响分析进而得出其对传热传质过程的影响。     

制冷系统水平管降膜蒸发器换热性能的影响因素分析

在理论上对制冷系统中降膜蒸发器进行基本分析,介绍降膜蒸发器的发展背景、发展进程以及工作原理和结构,分析降膜蒸发器的蒸发管上方制冷剂分布器的分配效果和结构、液膜流动特性、制冷剂流量、蒸发器内部的管阵布置方式、蒸发管的表面特性、管束规格和几何排列、气流扰动情况和润滑油等影响因素对降膜蒸发器传热性能的影响,比较降膜蒸发器与其他蒸发器的特点,介绍降膜蒸发的某些技术难题。     

ORC系统管翅式蒸发器管侧工质相变传热特性分析

基于CFD方法对有机朗肯循环(ORC)系统中管翅式蒸发器管侧进行数值模拟,根据发动机-ORC联合系统试验台架的实验数据,验证仿真模型的准确性,并分析工质R245fa在管侧的流动和相变传热特性,通过改变发动机转速和蒸发压力以及管侧工质进口速度,对比分析不同工况下工质的流动和传热特性。结果表明:发动机转速、蒸发压力和管侧进口速度均对工质的蒸发有较大影响。发动机转速对管侧进出口压降影响较小,蒸发压力和工质进口速度对管侧进出口压降影响较大。     

Internal heat generation in a discrete heat source: Conjugate heat transfer analysis

In the present work, we conduct an asymptotic and numerical analysis for the cooling process of a discrete heat source, which is placed in a rectangular-channel laminar cooling flow. In our physical model, the heated strip is embedded in a substrate, generating continuously a uniform volumetric heat rate. We assume that this heat-generation mechanism is due to an electrical current in the heat source. Hence, heat losses to the cooling fluid and to the substrate material during this process are presented. The governing equations of the cooling flow and the participating solid are reduced to an integro-differential equation that predicts the temperature variations of the heat source. We show that the conjugate heat transfer process is controlled by a conjugate nondimensional parameter, here denoted by α, which determines the basic heat transfer regimes between the cooling flow and the discrete heat source.     

Wall-particles heat transfer in rotating heat exchangers

The wall-particles heat transfer coefficient has been measured in small-scale rotary drum heat exchangers. Experiments have been conducted with nine granular materials of different nature, with particle diameters ranging from 194 μm to 4mm. The effects of rotational speed (1–40 rev min⁻¹), filling degree (4–17%) and drum diameter (0.25 and 0.485 m) have been investigated. The experimental data have been correlated by a semi-empirical relationship, that includes a contact resistance at the wall, the heat capacity of the particles immediately adjacent to the wall and the heat penetration resistance of the bulk of the particle bed. The contact resistance is shown to be due to the roughness of the particles. A mean roughness height of 12 μm provides a good fit to the measured coefficients.     

A review of the applications of heat pipe heat exchangers for heat recovery

The waste heat recovery by heat pipes is accepted as an excellent way of saving energy and preventing global warming. This paper is a literature review of the application of heat pipes heat exchangers for the heat recovery that is focused on the energy saving and the enhanced effectiveness of the conventional heat pipe (CHP), two-phase closed thermosyphon (TPCT) and oscillating heat pipe (OHP) heat exchangers. The relevant papers were allocated into three main categories, and the experimental studies were summarized. These research papers were analyzed to support future works. Finally, the parameters of effectiveness of the CHP, TPCT and OHP heat exchangers were described. This review article provides additional information for the design of heat pipe heat exchangers with optimum conditions in the heat recovery system.     

Frosting of heat pump with heat recovery facility

This paper aims to prolong the heat pump frost time and reduce its growth with heat recovery facility, which should mix the exhausted indoor and outdoor air before entering the evaporator. An ideal mathematic model is developed for heat transfer, frost generation and airside pressure drop. The properties of the mixture would be obtained by solving the mass and energy conservation equations. A parametric analysis is performed to investigate the effects of air inlet temperature, relative humidity and air mass flow rate on total heat transfer coefficient, frost thickness and airside pressure drop, respectively. The results show that rationalizing the ratio of indoor and outdoor air could prolong frosting time and reduce the frost thickness greatly. The total heat transfer coefficient, frost thickness and airside pressure drop increase monotonically with time going, but are not proportional. Decreasing the mixture inlet air temperature and relative humidity could essentially reduce frost growth on the tube surfaces. This can also be observed when increasing the air mass flow rate.     

Waste heat recovery using heat pipe heat exchanger for heating automobile using exhaust gas

The feasibility of using heat pipe heat exchangers for heating applying automotive exhaust gas is studied and the calculation method is developed. Practical heat pipe heat exchanger is set up for heating HS663, a large bus. Simple experiments are carried out to examine the performance of the heat exchanger. It is shown that the experimental results, which indicate the benefit of exhaust gas heating, are in good agreement with numerical results.     

地源热泵的套管式地下换热器传热研究

地源热泵是一种节能、对环境无害的绿色空调设备,可成为下个世纪冷暖技术的核心,采用能量平衡,建立了地下浅埋套管式换热器传热平衡式,进行求解,并分析了影响传热的各关联因子,提出了需研究的强化地下换热的措施,根据求解结果,给出了相应的函数关系图。对实验设计和工程实践有一定的指导作用。     

Upgrading heat by a reversed absorption heat pump

Analysis of a reversed absorption heat pump's performance for upgrading low temperature waste-heat from industrial processes and solar heat from a flat-plate solar collector has been carried out. In the case of upgrading waste-heat, the ratio of output to input heat decreases as the temperature at which the output heat is needed increases. For water heating, the performance of the conventional solar water heating system is better than that of the heat pump system because in the latter case, heat is rejected from the condenser.     

一种新颖简易的太阳能热动力转换装置--明托热机

明托热机(Minto Engine),俗称明托转轮,它 是通过液体活塞在转轮中不停地变换位置,产生不 同的动力矩,从而驱动转轮旋转的机器。明托热机 最早于1975年被维利·明托(Wally Minto)发明。 早期的明托热机结构十分简单,效率也很低,但由 于它能将低品位热能转变成高品位的动力。因而一 下子吸引了许多科学家的注意。之后,许多学者对 明托热机进行了研究,提出了许多改进的方向和应 用领域,推动了明托热机的发展。     

Forced convection heat transfer in microchannel heat sinks

This paper presents an analysis of forced convection heat transfer in microchannel heat sinks for electronic system cooling. In view of the small dimensions of the microstructures, the microchannel is modeled as a fluid-saturated porous medium. Numerical solutions are obtained based on the Forchheimer–Brinkman-extended Darcy equation for the fluid flow and the two-equation model for heat transfer between the solid and fluid phases. The velocity field in the microchannel is first solved by a finite-difference scheme, and then the energy equations governing the solid and fluid phases are solved simultaneously for the temperature distributions. Also, analytical expressions for the velocity and temperature profiles are presented for a simpler flow model, i.e., the Brinkman-extended Darcy model. This work attempts to perform a systematic study on the effects of major parameters on the flow and heat transfer characteristics of forced convection in the microchannel heat sink. The governing parameters of engineering importance include the channel aspect ratio (α_s), inertial force parameter (Γ), porosity (ε), and the effective thermal conductivity ratio (k_r). The velocity profiles of the fluid in the microchannel, the temperature distributions of the solid and fluid phases, and the overall Nusselt number are illustrated for various values of the problem parameters. It is found that the fluid inertia force alters noticeably the dimensionless velocity distribution and the fluid temperature distribution, while the solid temperature distribution is almost insensitive to the fluid inertia. Moreover, the overall Nusselt number increases with increasing the values of α_s and ε, while it decreases with increasing k_r.