Introduction

Internal-flow condensation is encountered in refrigeration and air-conditioning systems and during some accident scenarios in nuclear reactor coolant systems. Internal-flow condensation leads to a two-phase flow with some complex flow patterns. The condensing two-phase flows have some characteristics that are different from other commonly encountered two-phase flows. Empirical correlations are available for pure vapors condensing in some simple basic geometries (e.g., horizontal circular channels). Heat transfer (condensation rate) and hydrodynamics are strongly coupled and are sensitive to the two-phase flow regime. The two-phase flow regimes themselves depend on the orientation of the flow passage with respect to gravity.

Internal–flow condenser passages are usually designed to support vertical downward flow, inclined downward flow, or horizontal flow. Configurations that can lead to unfavorable hydrodynamics (e.g., countercurrent flow limitation and loop seal effect) are avoided in these systems. As a result, most of the published experimental studies and analytical models cover vertical downflow, and horizontal flow. Condensation in unfavorable configurations can be encountered during off–normal and accident conditions of many systems, however.

Shell-side phenomena in shell-and-tube-type condensers will not be discussed in this chapter. Complex three–dimensional flow is encountered in large power plant condensers. In these condensers the condensing fluid (steam) typically flows in the shell side of the shell–and–tube-type heat exchangers, with the secondary coolant flowing inside the tubes. The shell-side flow and condensation processes have certain common features with both internal and external condensing flows. Marto (1984, 1988) has written some useful reviews of these condensers.