Use of fluorescence to measure the lubricant excess surface density during pool boiling

This paper presents what are believed to be the first measurements of the non-adiabatic lubricant excess surface density on a roughened, flat, plain horizontal pool-boiling surface. Pool boiling heat transfer data is given for pure R123 and a R123/lubricant mixture. Lubricant excess surface density data are given for the boiling R123/lubricant mixture. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of a R123/lubricant mixture on a test surface. The fluorescent measurement technique was used to confirm the existence of the lubricant excess layer during refrigerant/lubricant mixture boiling. The refrigerant preferentially boils, thus, concentrating and accumulating the lubricant on the surface in excess of the bulk concentration. The excess lubricant resides in a very thin layer on the surface and influences the boiling performance. Accordingly, the ability to measure the lubricant excess density on the heat transfer surface would lead to a fundamental understanding of the mechanism by which lubricants can degrade or improve boiling performance. In support of this effort, heat transfer data are provided for both pure R123 and an R123/lubricant (1.8% lubricant mass fraction) mixture at 277.6 K. The heat transfer data shows that the lubricant excess causes an average degradation of 12% in the heat flux for a given superheat.

Résumé

This paper presents what are believed to be the first measurements of the non-adiabatic lubricant excess surface density on a roughened, flat, plain horizontal pool-boiling surface. Pool boiling heat transfer data is given for pure R123 and a R123/lubricant mixture. Lubricant excess surface density data are given for the boiling R123/lubricant mixture. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of a R123/lubricant mixture on a test surface. The fluorescent measurement technique was used to confirm the existence of the lubricant excess layer during refrigerant/lubricant mixture boiling. The refrigerant preferentially boils, thus, concentrating and accumulating the lubricant on the surface in excess of the bulk concentration. The excess lubricant resides in a very thin layer on the surface and influences the boiling performance. Accordingly, the ability to measure the lubricant excess density on the heat transfer surface would lead to a fundamental understanding of the mechanism by which lubricants can degrade or improve boiling performance. In support of this effort, heat transfer data are provided for both pure R123 and an R123/lubricant (1.8% lubricant mass fraction) mixture at 277.6 K. The heat transfer data shows that the lubricant excess causes an average degradation of 12% in the heat flux for a given superheat.