Flow and heat transfer characteristics of r22 and ethanol at supercritical pressures

This paper presents an experimental investigation of the flow and convection heat transfer characteristics of R22 and ethanol at supercritical pressures in a vertical small tube with an inner diameter of 1.004 mm. The heat flux ranges from 1.1 x 10(5) W m(-2) to 1.8 x 10(6) W m(-2), the fluid inlet Reynolds number ranges from 3.5 x 10(3) to 2.4 x 10(4), and the pressure ranges from 5.5 MPa to 10 MPa. The results show that for supercritical R22, the frictional pressure drop increases significantly with the heat flux. At p = 5.5 MPa, Re-in = 12,000 and a heat flux of 10(6) W m(-2), the local heat transfer is greatly reduced due to the low density fluid near the high temperature wall. Both buoyancy and flow acceleration have little effect on the heat transfer. For supercritical ethanol, the frictional pressure drop variation with the heat flux is insignificant, while the local heat transfer coefficient increases as the enthalpy increases. Ethanol gives better flow and heat transfer performance than R22 at supercritical pressures from 7.3 M Pa to 10 MPa for heat fluxes of 1.1 x 10(5)-1.8 x 10(6) W m(-2). (C) 2012 Elsevier B.V. All rights reserved.