Flow observation study along with local heat transfer and pressure drop measurements were conducted during condensation of CFC11 in horizontal microfin tubes. A smooth tube and two microfin tubes with different fin geometry were tested. Flow observation study with use of industrial borescopes revealed that two major flow patterns exist during complete condensation of a vapor. A correlation equation for the local frictional pressure gradient was derived, in which the effect of refrigerant mass velocity was introduced on the basis of the flow regime consideration. The condensing two-phase frictional pressure drop data for the microfin tubes were correlated by the present method to have a mean absolute deviation of 8.3 percent.

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