Applications of two-phase flow and heat transfer in compact heat exchangers

Abstract

Three applications of two-phase flow and heat transfer in plate-fin heat exchangers have been studied. A dephlegmator is a heat exchanger in which reflux condensation of a vapour mixture occurs, and plate-fln versions have importance in cryogenic gas separation processes. Numerical calculations for different binary mixtures show that the number of transfer units can be expressed as a simple function of the inlet vapour state and flow rate, heat load, and channel geometry. The calculations also show that the vapour and liquid exit compositions tend to limiting values as the number of transfer units increase. These limiting values correspond to liquid leaving the exchanger in equilibrium with the vapour entering. The effect of surface tension on liquid running down the rectangular passages of plate-fin exchangers is to draw it to the corners leaving less liquid on the walls and possible dry-out. A comparison of two CFD calculations with and without surface tension shows that effect can be significant. For a liquid with a surface tension only 1% that of water, about 50% more mass flows down the corner region of a square duct of side 0.944 mm. This transfer of liquid to the corner takes place in the first millimetre of flow downwards. Experimental measurements of pressure drop were taken for air and air-water flow through eight different plate-fin pads. The two-phase pressure drops for the serrated fin were two to five times larger than those for the plain fin. However, the effect of increasing the water flow rate at a fixed air flow rate was to increase the pressure drop by up to 75% in both cases. Over 200 two-phase pressure drops measurements were made, nearly all of the data were predicted to within 20% by the Lockhart and Martinelli (1949) correlation with C factor of 12.