Studies on heat transfer characteristics of heat pipe heat exchanger natural convection

Abstract

An analytical model has been developed to evaluate the thermal performance of a heat pipe heat exchanger (HPHE) under natural convection by adopting thermal resistance approach. The model evaluates the rate of heat transport and pressure drop across evaporator system of the HPHE under natural convection. The model computes various thermal resistances of the heat pipe at the external surface of evaporator and condenser as well as at the internal surface of the heat pipe based on the correlations available in the literature. The rate of heat transport has been calculated by converting the model into a computer programme, whose solution is based on an iterative procedure. A test rig has been developed for thermal performance evaluation of a single heat pipe under natural convective conditions as well as to do the validation of the analytical model for the same. Natural convective experimental studies have been carried out on the heat pipe at different tilt angles from the horizontal, i.e., 15 o , 20 o , 25 o , 30 o , 35 o , and 90 o and various temperatures of the heating fluid, i.e., 40 o C, 50 o C, 60 o C, and 70 o C at a constant flow rate of the heating fluid (laminar flow regime) in the evaporator section at an ambient temperature of about 1 1 o C. The experimental observations reveal that the tilt angle and the heating fluid temperature influence the heat transport rate of the heat pipe. The heat transport rate increases as the tilt angle of the heat pipe increases from 15 o to 25 o , and it starts decreasing beyond 25 o . The maximum heat transport rate of the heat pipe has been obtained at a tilt angle of 25 o and 70.3 o C heating fluid temperature, which may be due to the lowest overall thermal resistance at this operating condition, which is caused by the minimal external condenser thermal resistance of the finned condenser. Some typical experimental results on different individual heat transfer coefficients have been compared with the analytical model for a single heat pipe at 25 o tilt angle.