Visualization and modelling of flow pattern transitions in a cross-corrugated plate heat exchanger channel with uniform two-phase distribution

Abstract

Plate heat exchangers are often used for two-phase applications such as evaporation or absorption. In order to predict heat and mass transfer and pressure drop more accurately, knowledge of flow patterns is essential.Most of the published flow pattern maps for plate heat exchangers are biased by non-uniform inlet conditions. The objective of this study is to eliminate inlet effects, to which end, a uniform and steady two-phase distribution is implemented.An adiabatic air-water mixture is visualized in a transparent cross-corrugated single channel. The flow direction is varied between horizontal, vertical upward and downward. Both phases are injected directly into the channel via multiple nozzles. All superficial velocities are determined for the actual operating conditions.Flow patterns are mostly not affected by flow direction. Instead, the centrifugal force often dominates over buoyancy due to the swirling flow induced by the corrugations. A corrugation Froude number is defined to explain the findings. So-called slug flow is not observed, although being reported by other authors. This is attributed to the inlet design.Simple model assumptions regarding flow pattern transitions in tubes are adapted to the cross-corrugated channel. An additional model is proposed to account for the gradual transition of flow patterns at higher void fractions. The minimum kinetic energy for bubble breakage is also modelled to explain the gas accumulation observed at slow down-flow.