Heat Transfer Characteristics of Boiling Two-Phase Flow in a Plate Heat Exchanger

Abstract

This study deals with the effect of flow direction, vertically upward and downward, on the boiling heat transfer performance in a single channel commercial heat exchanger. HCFC 123 was used as the working fluid, and was heated by hot water from the both sides. From the experimental results for inlet subcooling condition (ΔT subin=13K) and inlet saturated liquid condition, it was shown that heat transfer coefficient of downward flow was higher than that of upward flow. On the other hand, in the case that the inlet flow was gas-liquid mixture condition (x=0.05), the effect of the flow direction was little. On the assumption that flow behaviors of gas-liquid two-phase flow strongly affects on heat and flow characteristics, air-water adiabatic two-phase flows in the same single channel commercial plate heat exchanger were visualized by neutron radiography. It was shown from the results that flow pattern of vertically upward flow was quite different from that of downward flow, especially at low liquid volumetric flux. Void fraction at the center was higher for upward flow, and was lower for downward flow. In spite of the difference in the flow pattern, there was little difference in the average void fraction. It can be said that the difference in flow pattern led to the difference in heat transfer performance.