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DOI: 10.1615/ICHMT.2008.CHT.270
page 20

Ludmila Pustylnik
Afeka, Academic College of Engineering, Tel-Aviv, Israel

Dvora Barnea
School of Mechanical Engineering, Tel-Aviv University, Tel-Aviv

Yehuda Taitel
School of Mechanical Engineering, Tel-Aviv University, Israel


The behavior of two-phase flow in parallel pipes with common inlet and outlet manifolds is quite complex and difficult to predict. The manner in which the phases are distributed among the pipes is a complicated function of the inlet flow rates, their physical properties, pipe inclination and junction geometry.
The motivation for the present work is related to solar power technology based on line concentration of solar radiation using parabolic troughs. At the present, in commercial power plants, oil is the fluid heated in the absorbing pipes and steam is generated in heat exchanges. Replacement of the synthetic oil by direct steam generation (DSG) and eliminating the oil-water heat exchangers would result in lower investment and operating costs and reduction of environmental risks. However, the more efficient process of DSG is not used due to possible uneven flow rate distribution in the parallel pipes and other possible instabilities, related to liquid-vapor flow in parallel pipes.
Although the motivation of this work is concerned with two-phase water-vapor flow, the adiabatic two-phase air-water flow is investigated. Adiabatic flow of water and air was chosen because of safety, easy handing and inexpensive components of the test facility, and the water-air densities ratio is similar to the water-vapor one. In addition the adiabatic air-water flow in parallel pipes with common manifolds has its own value as a basic scientific problem. The experimental system consists of four parallel pipes with common inlet and outlet manifolds. Transparent pipes were used for visualization of the flow distribution characteristics. Experiments were carried out for different flow rates and angles of inclination, and their effect was thoroughly discussed. A simplified theoretical analysis for the determination of flow distribution was conducted using stability analysis. The analysis shows that in the case of (for example) four parallel pipes, the two phase flow mixture from the common inlet manifold could "choose" to flow in one, two, three or in all four pipes depending on the flow rates of the liquid and the gas and on the pipes inclination. For low flow rates of gas and liquid the flow tends to take place only in one line, while stagnant liquid columns are present in the other three pipes. As the flow rates increase, the flow takes place in two, three and finally in four pipes. The experimental results are in reasonable agreement with the theory.
In addition, orifices were inserted at the end of each pipe for the purpose of increasing the range of even flow distribution among the four pipes.

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