DETERMINATION OF TWO-PHASE FRICTION FACTORS FOR VOID FRACTION AND BOILING HEAT TRANSFER EVALUATION.

摘要

The rotating heat exchanger has been introduced as an effective means of improving heat transfer. Past analysis pertaining to external fluid mechanics and heat transfer characteristics has indicated that the internal resistance governs the transfer of heat in a rotating system. Based on the experimental results, a correlation for the internal heat transfer coefficient was developed. In addition, a mathematical model has also been developed to predict the void fraction. However, the frictional pressure drop must be known in order to use that model. No existing correlations are appropriate for the evaluation of frictional pressure drop in such a boiling system where phase change occurs at the liquid-vapor interface.;For the evaluation of two-phase frictional pressure drop, it is proposed that the boiling system can be simulated by a system for which vapor flows inside a porous tube with uniform mass injection through the porous wall. So far as the pressure drop is concerned, this is considered satisfactory. Thus, the laminar flow in a cylindrical porous tube with uniform mass injection through the porous wall is studied both theoretically and experimentally. The full Navier-Stokes equations are solved numerically by employing some finite-difference schemes. Solutions for velocity profiles and frictional pressure drops have been obtained by the use of vorticity and stream function approach. Three important results have been found: (1) The fully developed velocity profile does exist after a certain distance downstream. (2) The radial velocities near the tube wall are greater than the injection velocity at the tube wall due to the requirement of mass conservation. The assumption of a linear radial velocity profile in some previous analytical studies is incorrect. (3) The effect of a mass injection through the porous tube wall is to increase the frictional pressure drop of the flow. Furthermore, the frictional pressure drop increases with increasing mass injection.;An experimental apparatus for measuring velocity profiles and static pressure drops has been set up. Pitot tubes were used to measure the velocities in terms of dynamic pressures. In order to measure the very low pressure differentials encountered, a new device had to be developed. The device used in the present study is sensitive to pressure differentials of 0.1 micron of water. Experimental results of velocity profiles and static pressure drops for different Reynolds numbers and injection rates have been obtained. The agreement between the experimental data and the numerical solutions is excellent. The numerical solutions are therefore experimentally verified.;In this investigation, attention has been focused on the understanding of boiling phenomenon and the evaluation of frictional pressure drop for refrigerants flowing inside a rotating tube. The experiments performed by the present author include visual and photographic observations of the boiling processes undergone by refrigerants in a cylindrical tube under both stationary and rotating conditions. The results show that there are no bubbles generated for refrigerants heated on the tube surface. This means that the fluid tested goes from convective boiling directly into film boiling without the presence of nucleate boiling. This important finding indicates that the transfer of heat is always enhanced by increasing the centrifugal acceleration.