Effect of aspect ratios and Bond number on internal flow patterns of Closed End Oscillating Heat Pipe at critical state

摘要

Flow visualization of internal flow patterns of a closed-end oscillating heat pipe (CEOHP) at critical state at several evaporator lengths and internal diameters was conducted in order to confirm the hypothesis used in our previous quantitative experiments. The CEOHPs used employed the Pyrex glass tube with inside diameters of 1 mm and 2 mm. The lengths of evaporator, adiabatic and condenser sections were equally varied as 50 and 150 mm. Number of turn was 10 turns. The working fluid used was R123 with a filling ratio of 50% of the total volume of the tube. The CEOHPs operated at bottom heat mode. The heat was supplied to evaporator section by electric heater and the condenser section was cooled by using distilled water passing through cooling jacket. The photographs of flow patterns of evaporator section at specified times were recorded by a digital camera when the critical state was achieved. It was found that, the major cause of dryout occurred in CEOHP is flooding at the entrance of evaporator section. Critical state of the CEOHP is consequently occurs. Aspect ratios affect the internal flow pattern such that, the higher the aspect ratio, the internal flow pattern changes from churn flow to annular flow. Flooding likely occurs in annular flow resulting in lower critical heat flux. Bond number affects the internal flow pattern such that, the higher Bond number, the internal flow patterns change from annular flow to churn flow. Thickness of liquid film inside of the tube is higher, causing higher critical heat flux.