开孔泡沫金属微细通道蒸发器两相流动换热研究

An open-cell metal foam mini-channel evaporator (OMFME) is proposed. The wall of the minichannel is metal foam with open-cell. It possesses the following advantages: 1) the capillary force of porous metal is capable of containing cooling medium, which benefits the delay of dry-out at higher quality; 2) the pores in the foam produces large amount of nuclei of boiling and enhances the boiling; 3) the pressure difference of different channels can be reduced due to the pores in the walls between the channels, which can lower and even remove the reverse flow at outlets due to the unbalance pressures of different channels in the evaporator with solid wall and improve the stability; 4) the fabrication of OMFME is simple and it is easier to make a large area OMFME than other high heat flux evaporator; 5) the vapor is expelled out through channels and hence the pressure drop is much smaller than the evaporator crammed with metal foam. The initial experiments of OMFME demonstrated that the heat transfer coefficient increased with increasing vapor quality and heat flux in the experimental scope. The heat transfer coefficient of OMFME was 1.5 times of that with solid wall and its dependence on flow rate was small. The dry-out was delayed in OMFME compared with the evaporator with solid wall. This project investigates the effects of the channel and the wall width and depth, flow rate and heat flux, on the bubble flow pattern, flow stability, resistance, heat transfer coefficient and critical heat flux with R134a, etc., as the coolants. The experimental correlations will be derived based on the theory of boiling and the experimental data.

提出了一种以开孔泡沫金属为通道间壁面的金属泡沫微细通道蒸发器，它具有如下优点：1）泡沫金属壁面的毛细力具有含蓄工质和铺展输运工质的能力，有利于延缓通道烧干；2）泡沫孔增加了汽化核心，可提高换热速率；3）泡沫孔可传递通道间横向压力，能减小不同通道间压力不平衡造成的回流，改善蒸发器的稳定性；4）制造简单，易于制作大面积蒸发冷板；5）产生的蒸气通过通道排除，流动阻力远低于整体填充泡沫金属的蒸发器。泡沫铜蒸发器的初步实验表明，在实验范围内，换热系数在大部分范围内随干度的增大而增大，随热流增加而增大，受流量的影响较小；换热系数是实心间隔壁面微细通道蒸发器的1.5倍；明显延缓了蒸发器下游的烧干。本项目以R134a等为工质，研究通道的高度、宽度，以及流量和热流密度对气泡流型、流动稳定性、阻力、换热系数、临界热流密度的影响规律，并结合沸腾换热理论和实验数据，分析物理机制，最终获得流动和传热的经验式。

针对高热流密度器件散热问题，提出了一种以开孔泡沫铜为肋的微细通道蒸发器（热沉）（FFMC）的方案，开孔泡沫肋有如下作用：毛细力吸附工质延缓烧干，增加汽化核心，增加通道间压力横向传递减缓通道间压力不平衡造成的回流。该设计易于大面积的蒸发器冷板制造。.通过研究获得了R134a和水为工质在FFMC中流动沸腾的流型随流量、热流密度、干度的变化规律，对比了FFMC和实体肋微小通道(SFMC)中的流型的变化规律与影响机理；获得了阻力、换热特性随流动参数、肋宽、孔隙率的变化，获得了R134a的换热关联式；发现R134a工质在某些工况下FFMC能够同时增强换热并减小压降。.在在实验条件范围内，以水为工质的FFMC比SFMC的传热系数提高80%，临界热流密提高25%，壁面温度波动和压降波动明显减小；主要是由于回流的缓解、表面再湿润频率的增加和翅片的毛细效应；在泡沫翅片中，Bo数较小时以核状沸腾为主，Bo数中等或较大时以薄膜蒸发为主；通道内始终以薄膜蒸发为主；Bo数较大时出现局部烧干。由于液膜较薄，FFMC的传热系数在初始烧干热流以下随着热流增加而增加，超过初始烧干热流后随热流增加而降低；在中、低质量流量下质量流量对传热系数影响显著，在中、高质量流量下影响较小。FFMC的压降随热流(干度)的增加几乎呈线性增加，压降是SFMC的1.2 ~ 2倍。.与R143a相比，水的液-气密度比更大，更容易出现蒸汽逆流现象。.数值构造了周期性的FFMC，采用孔隙尺度方法模拟得到单相流动下的速度、温度分布、泊肃叶数关联式和努塞尔数（Nu）变化规律。在周期性充分发展时,流量集中在通道区，温度较低，进入流体的净热流集中在靠近通道的肋区，在肋区内部较低。在相同通道高度下，泊肃叶数受雷诺数的影响很小，随肋宽和孔隙率增大而减小，随通道宽度增大而增大，Nu随雷诺数和肋宽增大而增大、随孔隙率和通道宽度的增大而减小。