Horizontal in-tube condensation in the presence of a noncondensable gas.

摘要

A horizontal heat exchanger design has been proposed for the Passive Containment Cooling System (PCCS) condenser of future Boiling Water Reactors (BWR). Compared to the vertical design, it has several advantages, but the challenge is that there is a lack of mechanistic understanding of the heat transfer and fluid flow phenomena occurring in the heat exchanger tubes. There is also the need for mechanistic analysis tools that can assess condenser performance.; This research experimentally investigates the local heat transfer from the condensation of steam in the presence of noncondensable gas in a single horizontal tube. To capture the asymmetrical nature of horizontal two-phase flow and heat transfer, the heat fluxes at the top and bottom of the condenser tube were obtained. A novel thermocouple was designed and fabricated to accurately measure the inside wall temperature from which a local heat flux can be deduced. The calibration technique of the thermocouple pairs was also developed. This local heat flux measurement technique is an original method for data which had previously been unattainable. It is applicable to phase-change heat exchangers of any inclination.; Tests with parameters that cover both design base accident and severe accident conditions have been performed. Analysis showed that the heat transfer at the top of the tube is much better than that at the bottom of the tube, which shows that for most of the conditions the liquid phase distribution has a great effect on the heat transfer. The effect of noncondensable gas has been qualitatively studied both locally and globally from the experimental data.; A mechanistic model was developed independent of the experimental data and was verified by the experimental data. Diffusion layer theory was used to account for the effect of noncondensable gas. For annular flow, the phenomenon inside the tube was assumed to be symmetrical. For wavy and stratified flow, the tube is divided into top and bottom parts peripherally based on the local phase distribution. The heat transfer mode across the liquid film was considered differently for the top and bottom part of the tube. The predictions from the mechanistic model were compared with the experimental data both locally and globally, and the agreement was satisfactory.