A New Model for Two-Phase Flow Boiling Heat Transfer of Refrigerant and Nanolubricant Mixtures in Smooth Tubes

摘要

Nanolubricants, that is, nanoparticks dispersed in the non-volatile component of a refrigerant and oil mixture, have shown potential to augment heat transfer in the evaporators of refrigeration systems. The two-phase flow boiling heat transfer coefficient superposition models, available in the literature, used Dittus-Boelter or Gnielinski correlations to predict convective heat transfer rates, and Forster-Zuber or Cooper correlations to estimate the nucleate boiling heat transfer rates. These correlations do not account for the presence of nanoparticks in the two-phase flow and cannot predict the heat transfer enhancements, or sometimes degradation, observed during flow boiling experiments of refrigerant and nanoparticle laden lubricant mixtures. A new comprehensive model was developed by modifying and integrating existing convective heat transfer models originally developed for nanofluids and pool boiling models for nanolubricants. The newly developed model accounts for the interactions of nanoparticks with the base fluids in terms of increasing the heat transfer by transfer of momentum from the nanoparticks to the bubbles, and introducing slip velocity effects at the interface between the nanoparticks and the base fluid. Experimental heat transfer data of the saturated two-phase flow boiling of R410A with two nanolubricants in a smooth copper tube were used to validate the new superposition model. The two nanolubricants had non-spherical ZnO nanoparticks and spherical y-Al2O3 nanoparticks dispersed in Polyolester (POE) lubricant. The model results followed the data trends and confirmed that the presence of low concentrations of nanoparticks in the turbulent flow increased the laminar sublayer thickness. This phenomenon was responsible for the observed decrease of the two-phase flow evaporative heat transfer. However, the model predicted potential enhancements in heat transfer when constraining the nanoparticks within the laminar sublayer of the flow.