Flow boiling in multi microchannels using refrigerant R134a and R1234yf

Abstract

Developing and studying flow boiling in micro channels has been done extensively over the last two decades in pursuit of effective cooling solutions for high power electronic devices. In addition, there is an urgent need for developing cost-effective sources of clean energy as the world faces challenges such as an ever-increasing energy demand and the global warming effect. Some of the renewable energy systems proposed require effective cooling solutions for the electronic controls employed, which again generate sizeable thermal loads over small areas. The current study comprises experimental tests for different test sections with three different aspect ratios of namely 0.56, 1 and 2 and a hydraulic diameter of 0.45 mm. Two different refrigerants were used, namely R134a and R1234yf (R1234yf is to replace the conventionally used R134a). The experiments were conducted under system pressures of 6, 7 and 8 bar, with four different mass fluxes tested at 50, 100, 200 and 300 kg/m2s. The maximum value of the wall heat flux reached was 277 kW/m2 and base heat flux up to 622.4 kW/m2. The exit vapour quality was up to 1. The main contribution of this study is the investigation of the effect of the aspect ratio, system pressure and fluid properties on the flow patterns, pressure drop and flow boiling heat transfer rates. The current study was performed at constant hydraulic diameter and constant surface roughness of the microchannels. Previous studies, which investigated the effect of the aspect ratio, system pressure and fluid properties did not necessarily keep to the same hydraulic diameter and surface roughness. The evaluation of the above effects for the same hydraulic diameter and surface roughness constitutes a unique contribution of the present study. Four types of flow patterns, namely bubbly, slug, churn and annular flow were captured at three different locations along the channel using a high-speed, high resolution camera. Also, the confined bubbles regime was identified. There is a significant effect of the fluid properties on the observed flow patterns. The existing maps in the published literature could not predict accurately the current experimental data, so a universal flow pattern map is still required. In this study, increasing wall heat flux led to an increase of the local flow boiling heat transfer coefficient. However, the mass flux has an insignificant effect on the local flow boiling heat transfer coefficient for all the current experimental results. The heat transfer rates depended on the channel aspect ratio. When the channel aspect ratio increased from 0.56 to 2, the local heat transfer coefficient was increased up to 27.7%. The effect of system pressure is insignificant for the operation system range of 6, 7 and 8 bar. The thermodynamic properties of the two fluids examined have a significant effect on the local heat transfer coefficient results. The current experimental data points were compared with existing correlations and a detailed discussion in included in the thesis. The experimental results demonstrated that the flow boiling pressure drop increased with decreasing channel aspect ratio. At the same time, the flow boiling pressure drop increased with reducing system pressure. The flow boiling pressure drop results of R134a were higher when compared with the R1234yf results. The current results were compared with existing pressure drop correlations with one correlation predicting our results with reasonable agreement. However, and similar to our findings on flow regime maps and heats transfer correlation, further research is required for a universal pressure drop correlation.