Heat transfer and pressure drop during supercritical cooling of HFC refrigerant blends

Cooling of refrigerant blends R404A and R410A at critical and supercritical pressures in horizontal tubes with 0.76 < D < 9.4 mm is investigated. Local heat transfer coefficients and pressure drops are measured at P_r = 1.0, 1.1 and 1.2, mass fluxes of 200 < G < 800 kg m^-2 s^-1, and temperatures ranging from 30-110 °C. A technique that simultaneously allows accurate measurement of low local supercritical gas cooling heat duties and deduction of the heat transfer coefficient from the measured overall resistance is used. The sharp variations in the fluid properties in the vicinity of the critical point are found to have a substantial effect on heat transfer coefficients. Due to these abrupt property variations, the heat transfer coefficients show sharp peaks near the critical temperature. Models for predicting f and Nu are developed based on the prevailing flow regime (liquid-like, pseudo-critical transition and gas-like).