Heat transfer regimes of R-125 at supercritical pressures tested in horizontal flow and under organic Rankine cycle conditions.

Organic Rankine cycle (ORC) technology is appropriate for utilizing low-temperature heat sources for electricity generation. Improving the efficiency of an ORC is possible by having a supercritical heat transfer to the working fluid (refrigerant) in the heat exchanger at the hot side. In this work, convective heat transfer measurements to R-125 at supercritical conditions were conducted with heating fluid temperature of 100°C. The test section is a tube-in-tube heat exchanger with a length of 4 m positioned horizontally. The behavior of the fluid at supercritical state was determined by obtaining local temperature measurements. The results show that the effects of the inlet pressure of the working fluid R125 have significant influence to the heat transfer coefficients. Furthermore, the local temperature measurements determine the position where the pseudo-critical temperature reaches its maximum value. This is considerably dominant in the vicinity of the critical point of R125. Determining these effects are of strong essence in deriving a heat transfer correlation.