Conference Year
2016
Keywords
pool boiling, lubricant, viscosity, surface tension, miscibility
Abstract
   For typical vapor compression processes, lubricant oil is very essential for lubricating and sealing the sliding parts and the lubricant also takes part in cushioning cylinder valves. However lubricants may migrate to the evaporator to alter the heat transfer characteristics. This is can be made clear from the viscosity and surface tension of lubricant since the viscosity of lubricant oil is about two to three orders higher than that of refrigerant whereas the corresponding surface tension of lubricant is approximately one order higher. Typically, the presence of lubricant may deteriorate heat transfer performance, yet this phenomenon becomes more severe when the lubricant mass fraction is higher. However, some previous literatures had clearly showed that the presence of lubricant oil may favor the heat transfer performance at a low lubricant fraction and the heat transfer performance may peak at a specific oil concentration. In this study, the authors aim at clarifying this phenomenon subject to pool boiling condition. Various parameters affecting the heat transfer coefficient, such as viscosity, surface tension, critical solution temperature and other thermodynamic and transport properties will be examined.    During pool boiling process, the lubricant accumulates on the surface since the refrigerant is preferential to evaporate. Hence, excess lubricant enrichment on the surface results in a thin lubricant excess layer and a thermal boundary layer, which influence the heat transfer performance, either enhancement or degradation. The excess layer may bring about a liquid-solid surface energy reduction which increases site density and reduces the bubble departure diameter, causing enhancement and degradation in heat transfer performance, respectively. However, the effect of the bubble departure diameter normally surpasses the influence of site density. This may be the crucial reason that gives rise to an occurrence of the plateau of heat transfer coefficient and followed by an apparent decline of heat transfer coefficient with a further increase of lubricant concentration.    Moreover, with the preferential evaporation of the refrigerant, a surface tension gradient is formed, which induces the Marangoni effect through which refrigerant/lubricant mixtures is supplied toward the contact line. From the phase equilibrium diagram, the maximum of the Marangoni number may occur at the low lubricant concentration with a maximum temperature difference. Hence, the presence of Marangoni effect may also be the favor the heat transfer accordingly. Also, a small fraction of lubricant will increase a larger viscosity that provide a thicker thermal boundary layer which may activate more site density, and enhances the heat transfer performance. Furthermore, miscibility may also play a crucial factor that affects the pool boiling heat transfer performance. The fluid with a smaller difference between the bulk fluid temperature and critical solution temperature may yield a better heat transfer performance by drawing superheated liquid onto the bubble sides. Â