Condensation occurs when the temperature of vapor is reduced below its saturation temperature. Presence of even a small amount of Non-condensable gas (e. g. air, N2, H2, He, etc.) in the condensing vapor leads to a significant reduction in heat transfer during condensation. The buildup of non­condensable gases near the condensate film inhibits the diffusion of vapor from the bulk mixture to the liquid film. The net effect is to reduce the effective driving force for heat and mass transfer. This phenomenon is the concern of industrial applications and nuclear reactor systems.

In nuclear plants, the condensation of steam in the presence of non-condensable gas becomes an important phenomenon during LOCA (loss of coolant accident) when steam released from the coolant system mixes with the containment air. Besides this, nitrogen gas in accumulators is a source of non­condensable gas, which can affect the condensation heat transfer inside the steam generator tubes of nuclear power plants, and may effect the core make-up tank performance. The effect of non­condensable gases on condensation heat transfer is also relevant to certain decay heat removal systems in advanced reactor designs, such as passive containment cooling systems. [4]

The effect of non-condensable gases on the condensation of steam has been extensively studied for both natural and forced convection flows. In each of them, geometries of interest (e. g. tubes, plates, annulus, etc.) and the flow orientation (horizontal, vertical) can be different for various applications. The condensation heat transfer is affected by parameters such as mass fraction of non-condensable gas, system pressure, gas/vapor mixture Reynolds number, orientations of surface, interfacial shear, Prandtl number of condensate, etc. Multi-component non-condensable gases can be present.