Challenges and possibilities in cooling electronics equipment

Abstract

Flow boiling patterns of R245fa in a 1.1 mm diameter copper tube were used to evaluate flow pattern maps in small to micro tubes. The flow boiling experiments were conducted over an experimental range of mass flux 100-400 kg/m2s, heat flux 3-25 kW/m2, inlet pressure of 1.85 and 2.45 bar and inlet subcooling of 5 K. The test section was heated directly using DC current. The effect of hysteresis was also investigated by increasing and decreasing the heat flux. A borosilicate glass observation section at the test section outlet was used for flow visualization. Hysteresis was evident across the whole experimental range, with obvious changes in the flow patterns between increasing and decreasing heat flux. The four main flow patterns were bubbly, slug, churn and annular flow. Confined flow was also evident. For increasing heat flux, only annular flow was evident but all the flow patterns were evident with decreasing heat flux. Therefore, the evaluation of flow pattern maps carried out in the present study was based on the decreasing heat flux data, as this covered the full range of flow patterns.