Two-phase flow patterns and heat transfer characteristics of R134a refrigerant during flow boiling in a single rectangular micro-channel

• There are six different flow patterns occurring during boiling of R134a.
• At low heat flux, mass flux has no significant effect on the heat transfer coefficient.
• The heat transfer coefficient is greater at higher saturation temperatures.
• At very high saturation temperature, a partial dry-out occurs.

In this paper, flow patterns and heat transfer characteristics of R134a refrigerant during flow boiling in a single rectangular micro-channel with 0.68 mm hydraulic diameter are investigated. In this experiment, heat transfer coefficients were studied at a heat flux range of 7.63–49.46 kW/m2, mass flux range of 600–1400 kg/m2 s, and saturation temperature range of 23–31 °C. Results show that there are six different flow patterns occurring during boiling of R134a refrigerants: bubbly flow, bubbly-slug flow, slug flow, throat-annular flow, churn flow, and annular flow. It is also found that flow pattern has a significant relation to heat transfer coefficients. In low heat flux range, variation of mass flux has no significant effect on the heat transfer coefficient. It corresponds to the nucleate boiling heat transfer mechanism in which its flow patterns are bubbly flow, bubbly-slug flow, slug flow, throat-annular flow, and churn flow. The heat transfer coefficient at higher saturation temperatures is greater than that of lower saturation temperatures. In high heat flux ranges, the heat transfer coefficient increases with increasing mass flux. It corresponds to the convective boiling heat transfer mechanism in which its flow pattern is annular flow. Increase of saturation temperature causes the heat transfer coefficient to decrease. At very high saturation temperature, a partial dry-out occurs that results in a lower heat transfer coefficient.