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The hydraulic performance of the microfin tube BAF312 given in terms of frictional two-phase pressure gradient, which was calculated from the measured total pressure gradient by subtracting the momentum pressure gradient, as:equation(10)dpdzf=dpdztot-dpdzaThe model of Rouhani and Axelsson [20] was used to calculate the void fraction, to account for the momentum pressure drop. The gravitational contribution was neglected because the microfin tube is horizontal.

5. Experimental results

This section reports the experimental results of heat transfer coefficient, vapour quality at the onset of the dryout, and frictional pressure gradient, as a function of the operating test conditions.

The heat flux HF is mold defined as the ratio between the actual heat flow rate and the external area of the tube, whereas the refrigerant mass velocity G is referred to the equivalent cross sectional area of an equivalent smooth tube with an inner diameter equal to the fin tip diameter of the tested microfin tube.