Studies on two-phase flow in small scale pipes have become more important, because of the application of mini-scale devices in several engineering fields including, high heat-flux compact heat exchangers, and cooling systems of various types of equipment. In a mini pipe the behavior of two phase flow is not the same as flow in conventional pipes. The difference is caused by different effective forces; for e. g. inside a mini pipe capillary forces are more important in comparison with gravitational forces. This paper is devoted to numerical simulation of gas-liquid two phase flow in a vertical mini pipe. Prediction of bubble shape and the effects of gas and liquid velocities on flow characteristics are considered. Also simulation involves prediction of changes in average void fraction along pipe axis. Numerical simulations in this paper are performed by a designed and developed CFD package which is based on Eulerian-Eulerian approach. The governing equations which are solved in the CFD package are momentum, continuity and Fractional Volume of Fluid (VOF) function equations. The fluid is assumed to be viscous and incompressible. The pressure-velocity coupling is obtained using the SIMPLEC algorithm. The geometry, which have been studied in this paper, is a D = 1.02 mm pipe, with 500 mm height. Bubble shape and the distribution of void fraction in a mini pipe are related to many parameters such as: gas and liquid velocities, pressure losses and etc. Since these mechanisms vary over time, time-average value of void fraction is used. Comparisons between Numerical results and experimental work which performed by hibiki et al. [1] indicated good agreement. Also results have shown that the present model is capable to simulate the behavior of nitrogen-water two phase flow in a mini pipe with acceptable accuracy. Furthermore, the results indicates that average void fraction along the pipe axis is related to the height and nitrogen superficial velocity. Also it is observed that at constant nitrogen superficial velocities, average void fraction decreases with water superficial velocity increments.

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