The boiling heat transfer of refrigerant R-134a flow in horizontal small-diameter tubes with inner diameter of 0.51, 1.12, 2.0 and 3.1 mm was experimentally investigated. Local heat transfer coefficient and pressure drop were measured at a heat flux ranging from 5 to 39 kW/m2, mass flux from 100 to 450 kg/m2s, inlet vapor quality from 0 to 0.2, and evaporating pressure of 0.49 MPa, 3.0 and 3.7 MPa. Results showed that the local heat transfer coefficient tends to decrease at lower vapor quality with the decrease in tube diameter. The effect of heat flux on local heat transfer coefficient becomes significant with the decrease in tube diameter, while the effect of mass flux is weak especially for small diameter tube. With decreasing tube diameter, the flow inside it approached homogeneous flow, and the contribution of forced convective evaporation to the boiling heat transfer decreases. With the increase in pressure near the critical pressure (3.0 to 3.7 MPa), the heat transfer coefficient increased, and the effect of mass flux on the heat transfer coefficient became weak. These results implied that the nucleate boiling was dominant under high pressure conditions. A modified Chen-type correlation taking into account the effect of tube diameter was proposed for the prediction of boiling heat transfer of R-134a in horizontal tube. The effect of tube diameter on flow boiling heat transfer coefficient was characterized by the Weber number in gas phase. Comparison with experimental results showed that this correlation could be applied to a wide range of tube diameters (0.5 to 11 mm) and pressure conditions (reduced pressure from 0.1 to 0.9).

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