Experimental investigation of the heat transfer of supercritical R134a in a horizontal micro-fin tube

The heat transfer of supercritical R134a was investigated experimentally in a horizontal micro-fin tube for conditions related to the supercritical heater in trans-critical ORC systems, including the effects of heat flux, pressure, and mass flux. The results showed that Nu(top) was smaller than Nu(bottom), due to the buoyancy. The effects of pressure and heat flux on Nu were found to differ in different bulk enthalpy regions and tube positions. Large mass fluxes increase Nu while buoyancy has a stronger effect at low mass fluxes. Previous correlations were evaluated with new correlations developed for Nu in a micro-fin tube. The Bishop correlation gave reasonable predictions of Nu(top), but none of the correlations could accurately predict Nu(bottom). A pair of more accurate correlations were developed for the top and bottom Nusselt numbers using the dimensionless number pi(c) to account for the effects of the drastic property variations. The new correlations have a mean deviation of 10.3% for Nu(top), and 17.8% for Nu(bottom), with 96.1% of the experimental data for Nu(top) and 83.8% of the data for Nu(bottom) well predicted with deviations smaller than 30%.