Shigeru Nozu

Condensation of a refrigerant CFC11 in horizontal microfin tubes (Proposal of a correlation equation for frictional pressure gradient)

Local heat transfer and pressure drop measurements were made during condensation of CFC11 in horizontal microfin tubes. A smooth tube and two microfin tubes with different fin dimensions were tested. Flow observation study with use of an industrial borescope revealed that the condensate flowed along the grooves, and a thick condensate film covered fins in the lower part of the tube in the low quality region. Static pressure gradients in the microfin tubes were up to 70% larger than that in the smooth tube. 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 measured frictional pressure gradient data were found by the present method to have a mean absolute deviation of 8.3%.

Condensation of a zeotropic CFC 114-CFC 113 refrigerant mixture in the annulus of a double-tube coil with an enhanced inner tube

Abstract Local heat transfer and pressure drop measurements were made during condensation of a zeotropic CFC114-CFCll3 refrigerant mixture in the annulus of a double-tube coil consisting of three U-bends and four straight lengths. The inner tube is a 19.1-mm O.D. corrugated copper tube with wire fins soldered onto the outer surface and the inner diameter of the outer duct is 25.0 mm. The vapor-phase mass transfer coefficient exhibited a sawtooth behavior with the U-bends showing higher coefficients than the straight lengths. The frictional pressure gradient data agreed well with a previously developed empirical equation for the condensation of pure refrigerants. A prediction method for the condensation heat transfer rate was proposed on the basis of the correlations of the vapor-phase mass transfer coefficient and heat transfer coefficient of the condensate film. The heat transfer data were correlated by the present method to a mean absolute deviation of 12.9%.

Condensation of refrigerants CFC11 and CFC113 in the annulus of a double-tube coil with an enhanced inner tube

Abstract Local heat transfer and pressure drop measurements were made during condensation of pure refrigerants CFC11 and CFC113 in the annulus of a double-tube coil consisting of three U-bends and four straight lengths. The inner tube is a 19.1-mm O.D. corrugated tube with wire fins soldered onto the outer surface, and the inner diameter of the outer duct is 25.0 mm. The mass velocity of the test fluids ranged from 100 to 235 kg/(m 2 s). The local heat transfer coefficient decreased along the tube length, with the U-bends showing higher coefficients than the straight lengths. The amplitude of the coefficient variation decreased as the condensation proceeded and was less remarkable at a lower vapor velocity. The experimental data were compared with the previous results for the straight annuli obtained using the same inner tube as the present study. Empirical equations for the local heat transfer coefficient and frictional pressure gradient were developed for both the U-bends and the straight lengths.