Flow boiling and flow regimes in small diameter tubes

Boiling heat transfer in small diameter tubes has been experimentally investigated using R134a as the working fluid. The heat transfer experiments were conducted with two stainless steel tubes of internal diameter 4.26 and 2.01 mm respectively. Other parameters were varied in the range: mass flux 100–500 kg/m2 s; pressure 8–12 bar; quality up to 0.9; heat flux 13–150 kW/m2. It was found that the nucleate boiling is dominant when the vapour quality is less than about 40–50% for the 4.26 mm tube and 20–30% for the 2.01 mm tube. Above these quality values, heat transfer coefficients decrease with vapour quality. Furthermore, this decrease occurs for the entire quality range at high heat flux values. Flow visualization experiments were carried out using the same experimental facility with Pyrex glass tubes. A flow pattern map was obtained at a system pressure of 10 bar and tube diameter of 4.26 mm.