Flow patterns and pressure drop of downward two-phase flow in a capsule-type plate heat exchanger

Capsule-type plate heat exchanger has wide industrial applications for its low flow resistance and less deposition and fouling. To broaden its application in phase-change heat transfer processes, the flow behaviors including flow patterns and pressure drop were experimentally studied in a vertical downward transparent capsule-type plate channel. Three main flow regimes are identified, which are film flow, plug flow and churn flow. Flow pattern transition criteria for downward two-phase flow in plate channel have been developed. Liquid block forms when the shear stress on the wavy film interface is much larger than the surface tension, which indicates the transition to plug or churn flow from film flow. And plug flow transits to churn flow when the liquid block is over aerated due to the flow turbulence. The transition boundries proposed for capsule-type plate channel show good agreement with the corresponding boundries of chevron-type plate channels. To predict the friction pressure drop, a new correlation containing the mixture Reynolds number is proposed to describe the law between the Lockhart-Martinelli parameter and liquid two-phase multiplier, which can reflect the effect of mass flux. The comparison of predicted data and experimental results shows good agreement by 82.4% of the data in ±15% error bands.