Steam Condensation in the Presence of a Noncondensable Gas in a Horizontal Tube

Abstract

Perhaps the most common flow configuration in which a convective condensation occurs is a flow in a horizontal circular tube. This configuration is encountered in air-conditioning and refrigeration condensers as well as condensers in Rankine power cycles. Although a convective condensation is also sometimes contrived to occur in a co-current vertical downward flow, a horizontal flow is often preferred because the flow can be repeatedly passed through the heat exchanger core in a serpentine fashion without trapping liquid or vapor in the return bends. (Carey, 1992) Horizontal heat exchangers are also widely used in the nuclear industry. Recently, a horizontal heat exchanger design has been proposed for a passive containment cooling system (PCCS) of future light water reactors. Current PCCS designs typically employ a vertical condenser. The horizontal design is proposed because horizontal heat exchangers have a potentially higher heat removal capability than vertical heat exchangers. (Wu & Vierow, 2006b) As well as, horizontal heat exchangers have less tube fouling, higher structural earthquake resistance which will improve the reliability of the safety system, and a large economic benefit because the shorter coolant pool allows for reduction in the containment height and volume. In spite of these advantages, there is a lack of mechanistic understanding of the heat transfer and fluid flow phenomena occurring in the heat exchanger tubes. This is mainly due to the fact that the phenomena are more complicated compared to the case of vertical heat exchangers. In vertical tubes the phenomena is mainly laminar or turbulent film condensation, whereas in horizontal tubes, the phenomena is complicated by strong asymmetry and flow regime transitions, which causes transitions in heat and mass transfer mechanisms. There is also the need for mechanistic analysis tools that can assess condenser performance. (Wu, 2005) There were many investigations for the condensation phenomena inside horizontal tubes to study the horizontal heat exchangers. However, almost all of them obtained tube sectionaveraged data without a noncondensable gas. Recently, Wu and Vierow (2006a, 2006b) studied experimentally the condensation of steam in a horizontal heat exchanger with air present, as shown in Fig. 1. In order to measure the condenser tube inner surface temperatures and to calculate the local heat fluxes, they developed an innovative thermocouple design that allowed for nonintrusive measurements. The experimental results show that the top of the condenser tube is a much better heat transfer surface. At any tube