Experimental Study of Heat Transfer and Pressure Drop Characteristics of Air/Water and Air-Steam/Water Heat Exchange in a Polymer Compact Heat Exchanger

Abstract

Experiments of heat transfer and pressure drop in a polymer compact heat exchanger made of PolyVinyliDene-Fluoride were conducted under various conditions for air/water heat exchange and air-steam/water heat exchange, respectively. The overall heat transfer coefficients of air-steam/water heat exchange (150 to 600 W/m²K) are much greater than that of air/water heat exchange (80 to 130 W/m²K). The pressure drops of air-steam are also greater than that of dry air in the test range. The gas-side heat transfer coefficients were obtained with the Wilson plot technique. The heat transfer coefficients of dry air range from 80 to 150 W/m²K, which are in the same magnitude grade as the overall heat transfer coefficients. It shows that the major heat resistance is in the gas side. The heat transfer coefficients of air-steam range from 400 to 24000 W/m²K at various inlet conditions. The inlet temperatures and humidities have a great effect on the heat transfer coefficients. In reality, it is the inlet steam mass fractions that have a great effect on the air-steam heat transfer coefficients. However, very high heat transfer coefficients (up to 24000 W/m²K) have been obtained in the existence of very high noncondensable gas (air) contents. This is because high efficient dropwise condensation heat transfer can be achieved in the polymer compact heat exchanger. This has been confirmed by the visualization of the condensation process. A physical explanation of the phenomenon has been given. The possible reason is the effect of small channels on the dropwise condensation process at high noncondensable gas contents. Further study on this aspect is suggested.

ACKNOWLEDGMENTS

The experiments reported in this article were conducted at the Department of Mechanical Engineering of Eindhoven University of Technology. The authors wish to express their thanks for the suggestion and careful reading provided by Professor T. G. Karayiannis of School of Engineering at South Bank University.