Investigation Of Heat Transfer Characteristics Of Spray Cooling On Micro-grooved Surfaces

Spray cooling technology has many advantages, such as higher heat transfer, smaller heattransfer temperature, less fluid inventory, smaller heat resistance and no hysteresis of boiling,which makes it become the best choice for cooling high heat flux electronics in the near future.Investigations for flat surface have demonstrated that heat transfer mechanisms of spraycooling include single phase convection, thin film evaporation, nucleate boiling andsecondary nucleation. Enhancement heat transfer of spray cooling is focusing onmicro-structured surfaces at present, but studies on this are not adequately, and mechanism ofenhancement of heat transfer is not well understood. In this paper, experimental study,theoretical analysis and numerical simulation are adopted to investigate the heat transfercharacteristics of spray cooling on micro-grooved surfaces. The main contents andconclusions are the following.1. The influence of spray characteristics on heat transfer was investigated. Results showthat droplet velocity, droplet diameter and droplet number flux all affect the heat transfer. Heatflux increases with the increasing of droplet velocity and droplet number flux, while thedroplet diameter has small influence on heat flux. Spray volume flux is the best parameterwhich can be bused to distinguish heat transfer characteristics, because it not only possess thedimension of velocity, but also is a parameter combination of droplet velocity, dropletdiameter and droplet number flux. It is found that the bigger the spray volume flux is, thehigher the heat flux is.2. The influence of surface structure on heat transfer was investigated. It is found that thestraight-grooved surface has the largest heat flux enhancement relative to the flat surface in allmicro-structured surfaces having the same groove depth and flow area, because the straightgroove is in favor of film flowing in it. On this basis, heat transfer was further studied underdifferent structure parameters (groove depth, groove width and groove pitch) of straight-grooved surfaces. The corresponding relationship between volume flux and theoptimal surface structure parameter is found. At last, the reason that the best heat transferoccurs is discussed through the force analysis of droplet.3. Mechanism of enhancement of heat transfer about micro-structured surface wasinvestigated. Results show that evaporation heat transfer and convective heat transfer ofmicro-structured surface are all increased, but the degree of enhancement depends on volumeflux. For low volume flux (ranged from0.938to3.479L/(m2·s)), evaporation heat transferincreases faster, which is main reason of enhancing heat transfer for micro-structured surface.While for high volume flux (9.479L/(m2·s)), convective heat transfer and evaporation heattransfer increase as the same proportion, which contribute to enhanced heat transfer similarly.4. The changes of critical heat flux (CHF) were investigated. When reaching CHF, thecoolant film ruptures, which causes droplet contacting directly with hot surface.Micro-grooved surfaces can prevent droplet from leaving surface and increase contact time,which make CHF increase. It is found that the denser the groove and the longer thethree-phase contact line are, the higher the CHF are. There is an optimal groove depth, wherecritical heat flux reaches the maximum. CHF increases as spray volume flux is raised.5. The influence of the type of working fluid on heat transfer was investigated.Compared with distilled water under the same mass flux, the convective heat transfer ofethanol is obviously low. But in view of its lower boiling point and specific heat, theevaporation heat transfer of ethanol is stronger than that of distilled water. For this reason,ethanol has higher heat transfer capacity.6. A model used to calculate straight-grooved surface heat flux was obtained for the firsttime. Heat flux is calculated under different volume flux and surface structure parameter inusing the established model. By comparing results between calculation and experiment, it isknown that the two results coincide with each other well, which certifies the feasibility andvalidity of the idea of modeling.7. The heat transfer correlations of spray cooling about micro-grooves and flat surfacewere derived individually, in which the influence of surface temperature, volume flux andstructure parameters of micro-grooved surfaces on heat transfer were considered. Comparedto the former, the correlation referred in the paper can exactly reflect heat transfer situation, and are simple and convenient for use.8. The influence of spray parameters on heat transfer was investigated in using VOF(Volume of Fluid) model. Results show that droplet velocity and pitch have stronger influenceon heat transfer, heat transfer coeficient increase remarkably with increasing droplet velocityand reducing droplet pitch. Liquid film thickness also affects heat transfer, the thicker theliquid film is, the less the heat transfer coefficient is. While droplet diameter has a littleinfluence on heat transfer, heat transfer coefficient increase slightly with the reduction ofdroplet diameter.