Modeling and analysis of steam-water two-phase flow distribution and wall temperature distribution in parallel heated pipes with different manifold types

Abstract

In solar tower receiver, boiler water-wall and various heat exchangers, the uneven distribution of two-phase flow often occurs among parallel heated pipes, which seriously reduces the operating efficiency and may further induce heat transfer deterioration and soaring wall temperature. In this paper, a coupling model for the flow distribution and heat transfer process of steam-water two-phase flow in parallel heated pipes was established based on the discrete methods. In the present model, two main improvements were proposed as below. On one hand, the calculated equations for the pressure drop and the phase distribution of two phase fluid in both the combining T-junction and dividing T-junction were established to consider the coupling effects of the inlet manifold and outlet manifold on the flow distribution characteristics. On the other hand, the calculation module for the heat transfer and wall temperature along the branch pipes was also coupled in the present model with considering of different heat transfer regions (saturated nucleate boiling region, liquid deficient region, and superheated steam region) along the pipe. The present model was then verified and used to investigate the effects of system pressure, inlet fluid flow, inlet steam quality and manifold type on the flow distribution characteristics of high temperature and high pressure steam-water in parallel vertical upward pipes, and the effect of the uneven distribution of the gas-liquid two phases on the system safety was analyzed and evaluated. It was found that gas phase preferentially enters the first branch pipe near the inlet of the dividing manifold affected by buoyancy, and this results that the inlet steam quality of the corresponding branch pipe is always obviously higher than that of the subsequent pipes. As the system pressure decreases, the outlet fluid enthalpy distribution and outlet wall temperature distribution among branch pipes become more uneven. Moreover, the outlet fluid enthalpy and outlet wall temperature of the branch pipes closer to the inlet of the dividing manifold increase sharply with the decrease of system pressure. In the U-type parallel pipe system, the branch pipes away from the inlet of the dividing manifold are more prone to wall overheating at the outlet, while the opposite is true for the Z-type parallel pipe system.