Haruo Nakata

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 refrigerants R-11 and R-113 in the annuli of horizontal double-tube condensers with an enhanced inner tube

Abstract Experiments were performed to study the flow and heat transfer characteristics during condensation of R-11 and R-113 in the annuli of horizontal double-tube test condensers. The condensers are made up of a 19.1 mm O.D. corrugated inner tube with wire fins soldered on the outer surface and three outer smooth tubes of 24.8, 27.2 and 29.9 mm I.D. The mass velocity of the test fluids ranged from 50 to 300 kg/(m 2 s), and the condensation temperature difference ranged from 0.7 to 20 K. The frictional pressure gradient was correlated fairly well by using the Lockhart-Martinelli parameters. The local heat transfer coefficient was 2 to 13 times as large as those for horizontal annuli with smooth and corrugated inner tubes. An empirical equation for the local heat transfer coefficient was developed, in which the dimensionless parameters based on the surface tension controlled flow and the vapor shear controlled flow models were introduced for the low and high vapor velocity regimes, respectively.

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.

Condensation of Refrigerant CFC 11 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 CFC 11 in microfin tubes. A smooth tube and two microfin tubes with different fin dimensions were used. Flow observation study with use of an industrial bore-scope revealed that the condensate swirled 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 percent larger than that in a 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 percent.

Condensation of refrigerants Rll and R113 in the annulus of a double-tube coil with an enhanced inner tube.

Local heat transfer and pressure drop measurements were made during condensation of refrigerants R11 and R113 in the annulus of a double-tube coil consisting of four straight sections and three U-bends. The inner tube was a corrugated copper tube with soldered wire fins. The local heat transfer coefficient in the U-bend was higher than that in the straight section by a maximum of 35%, whereas the pressure drop in the former was about twice as large as that in the latter. The results were compared with the previous work for straight double tubes with the same inner tube as the present ones. Empirical equations for the local heat transfer coefficient and the local frictional pressure gradient were developed for both the U-bend and straight sections.