Experimental Investigations and Modelling of Condensation in Plate Heat Exchangers

Abstract

The use of Plate Heat Exchanger (PHE) as condensers and evaporators increases every year. The easy and modular construction and the high thermal performance at very compact size make the PHE to an ideal heat exchanger for a wide range of applications. Due to the missing of authoritative models for the design of PHEs used as condensers or as evaporators they will typically be constructed larger than needed. For a physically based model more detailed knowledge is needed to take into account the flow and heat interactions during the condensation or evaporation process inside a PHE. In this work the two-phase flow in a PHE is analysed. For this purpose one gap of a PHE was built with two transparent polyurethane plates with the same geometrical parameter than the stainless steel plates of a commercial used PHE. Demineralized-water and air are used to define an exact 2-phase-flow. Measurements of the contact angles and surface energies of the stainless steel plates and the transparent polyurethane plates with different working fluids have been made to show the relation between the wettability and the surface energies of the plates. These measurements show a significant influence of the surface energies on the wettability of the steel and polyurethane plates and therefore it is necassary to take into account the plate material properties for the visualization experiments. A flow pattern map is developed and the typical flow patterns are presented and explained. The defined flow regimes converge with comparable data from the literature. The flow pattern of bubbly flow, irregular bubbly flow, film flow and slug flow can be identified. The film flow occurs over the whole range of the air flow rate and at superficial liquid velocities below 0.1 m/s. Above this limit at low flow rates of the gaseous phase a bubbly flow occurs and with increasing air flow rate the flow pattern changes at a superficial air velocity of 1.5 m/s to an irregular bubbly flow. With further rising of the gas phase fraction the irregular bubbly flow shifts at gas velocities nearly 4 m/s into a transition regime, which is marked by the mixed behaviour of the irregular bubbly and the slug flow regime. Slug flow can be identified when the superficial velocity of the air rises up to 7 m/s.