Applied Sciences  
Article  
Pressure Drop of a Refrigerant Flowing Vertically Upward and Downward in Small Circular, Rectangular, and Triangular Tubes  
[PERSON]  
Department of Mechanical and Intelligent System Engineering, The University of Electro-Communications,  
1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan; [EMAIL_ADDRESS]; Tel.: +81-42-443-5337  
[PERSON] Pressure Drop of a Refrigerant Flowing Vertically Upward and Downward in Small Circular, Rectangular, and Triangular Tubes.  
for higher mass fluxes using [PERSON]'s parameter \(C=21\). However, [PERSON]'s parameter needs to improve accuracy at a lower mass fluxes condition, because most of the data lie between \(C=5\)-\(20\).  
[PERSON] and co-workers (1993, 1996) [7,8] reported air-water mixtures flowing vertically in circular and rectangular tubes with diameters ranging from 1 to 5 mm. They reported that if the inner diameter is less than 5 mm, [PERSON]'s parameter \(C\) should consider the effect of the inner diameter, because they noted that [PERSON]'s parameter changed from 21 to 0 as the hydraulic diameter decreased.  
[PERSON] et al. (2001) [9] reported the characteristics of the two-phase frictional pressure drop of R22, R407C, and R410A flowing horizontally in circular tubes. The inner diameter of test tubes was 3, 5, 7, and 9 mm, the frictional pressure drop could not be reproduced by [PERSON]'s correlation, the mean deviation of the correlation was as high as 117.6%, and considerable over-predictions were observed at the low mass flux condition.  
[PERSON] and [PERSON] (2007) [10] measured the water-air two-phase pressure drop flowing horizontal rectangular channel under the atmospheric pressure. The channels width was fixed to 20 mm, but the height ranges changed from 0.4 to 4 mm. They reported the mass flux and channel size affect the Chisholm parameter C.  
[PERSON] et al. (2007) [11] performed an experiment of HFE-7100 flowing vertically upward in 40 parallel mini-channels with a rectangular cross-section of 0.5 2.6 mm and a hydraulic diameter of 0.84 mm. They found that a homogeneous model accurately predicted the frictional pressure drop, which depended on the vapor quality.  
The purpose of this study is to comprehensively clarify the effects related to the cross-sectional shape and hydraulic diameter of the small tubes, and the effect of flow directionality using the alternative freon refrigerant R410A under the saturation pressure (temperature) at 1.09 MPa (10 \({}^{\circ}\)C), which is actually widely used and condition in the heat exchanger. In order to clarify these, the experiments were performed to examine the characteristics of the two-phase frictional pressure drop of an R410A refrigerant flowing vertically upward and downward. The cross-sections of copper test tubes were 0.5, 0.7, 1.0, 1.5, and 2.0 mm circular tubes, and rectangular and triangular tubes with hydraulic diameters of 1.04 and 0.88 mm, respectively. The characteristics of the measured pressure drops were clarified by different inner diameter tubes, cross-section shapes, and flow directions. In addition, comparing [PERSON]'s parameter and various modified Chisholm's parameters in Lockhart-Martinelli correlation for small tubes, I examined whether or not they reproduced our measurement data under the physical property conditions of the refrigerant in actual use.