Per Fahlén

An Experimental Study of Liquid-Phase Heat Transfer in Multiport Minichannel Tubes

Heat transfer has been evaluated experimentally on the tube side of three different minichannel tubes, so-called multiport extruded (MPE) tubes, with different cross-section geometry. The objective was to find out which correlations, models, and relations should be used when predicting the performance of complete heat exchangers consisting of such tubes. The geometry of the tube channels or ducts was rectangular with different aspect ratios and one of the tubes was equipped with surface enlargements inside the channels. Evaluations were performed with four different secondary refrigerant liquids: water, propylene glycol, Hycool 20, and Temper-20. The laminar flow regime was the main focus of the investigations. Despite the shape of the minichannels, best agreement was found with a model presented by Gnielinski for laminar flow, developing temperature profile, and fully developed velocity profile in circular tubes with the constant wall temperature boundary condition.

Heat Transfer and Pressure Drop Under Dry and Humid Conditions in Flat-Tube Heat Exchangers With Plain Fins

Flat-tube heat exchangers could be an interesting alternative to make indirect cooling of display cabinets more energy-efficient. This application involves low air velocities in combination with condensation of water vapor on the air side, so plain fins could be suitable. Two different heat exchangers having flat tubes and plain fins on the air side were evaluated experimentally. One of the heat exchangers had continuous plate fins, and the other had serpentine fins. The performances during dry and wet test conditions were compared and related to theoretical predictions for different assumptions. The influence of air velocity, air humidity, and inclination angle was investigated. The results show that, in most cases, the heat transfer performance is somewhat reduced under wet conditions in comparison with dry test conditions, and that wet heat transfer surfaces lead to an increased pressure drop. At the lower air velocity range that was investigated, the heat exchanger having continuous plate fins drained better than the one with serpentine fins.