A Study of Discrepancies in Flow Boiling Results in Small to Micro Diameter Metallic Tubes

There is a disagreement in the reports on flow boiling heat transfer on the dependence of the local heat transfer coefficient on local vapour quality, mass and heat flux and system pressure. As a result, various conclusions were reported about the dominant heat transfer mechanism(s) in small to micro diameter tubes. Yet, the reasons behind this large disagreement are not clear. The current study investigated experimentally two important parameters that may contribute in explaining the scatter in the published heat transfer results. The first parameter is the tube inner surface characteristics whereas the second is the length of the heated section. The surface effect was experimentally investigated through examining two stainless steel tubes manufactured by two different methods. The first tube is a seamless cold drawn tube whilst the second is a welded tube. The two tubes have similar design and dimensions and were investigated at 8 bar system pressure and 300 kg/m² s mass flux. The inner surface of the two tubes was examined using a scanning electron microscope (SEM) and was found to be completely different. The heat transfer results demonstrated that the trend of the local heat transfer coefficient versus local vapour quality in the seamless cold drawn tube is completely different from that in the welded tube. Three heated lengths were investigated for a seamless cold drawn tube with an inner diameter of 1.1 mm over a wide range of experimental conditions; mass flux range of 200–500 kg/m² s, system pressure of 6–10 bar, inlet sub-cooling value of about 5K and exit quality up to about 0.95. The results indicated that the heated length strongly influences the magnitude as well as the local behaviour of the heat transfer coefficient. There is a progression from nucleate boiling to convective boiling as the heated length increases. The variation in the heat transfer coefficient due to differences in the heated length may also influence the performance of the existing micro scale heat transfer correlations. The flow patterns observed at the exit of each test section are also presented.