Flow distribution and heat transfer in minigap and minichannel heat exchangers during flow boiling

The topic of boiling heat transfer in miniscale geometries has focused the ever increasing interest of researchers in recent years. However, most of the works are related to mini- and microchannels and much less to minigaps. Meanwhile, minigaps allow for more comprehensive experimental studies, i.e. flow visualisations due to the flat, two-dimensional configuration of the flow. The results of the experimental investigations of a model plate heat exchanger, composed of a single plate with a heat exchange surface of 0.1 m x 0.2 m, made of brass are presented. The working fluid is pure ethanol. Between the plate and the cover of the exchanger, 0.5 mm and 1.0 mm thick minigaps are arranged. The 0.5 mm minigap has been compared to an equivalent minichannel structure (a set of 50 parallel minichannels) with an equal cross-sectional area (rectangular 1 mm x 1 mm). An attempt to intensify the heat exchange by modifying the minigap wall was made. Two variants were tested to check the potential increase in the heat transfer coefficient and flow resistance. The section cover is transparent in order to simultaneously register the flow structures with the measurements. The heat is supplied using a water circuit, where the temperature and flow of the water can be regulated in a constant temperature water tank. The flow configuration of the exchanger is countercurrent, with the vertical flow of ethanol from the bottom to the top. The inlet and outlet manifolds are arranged as trapezoidal (Z-type design). The authors have analysed the efficiency of heat transport and pressure drop depending on the system operating parameters. The visualisations are thoroughly analysed to better understand the process. The main advantages of the presented work are a direct comparison of minigap and minichannel structures while standardising all other parameters; an interesting application of a simple, passive heat transfer intensification method which resulted in the unification of the velocity fields in the minigap and reduced maldistribution.