Spontaneous condensation in boundary layers

It is a well known effect that accumulation of non condensable gas causes a high heat transfer resistance during direct contact condensation in binary steam- nitrogen mixtures. But especially with high pressures and low water temperatures a second effect reduces heat transfer additionally. Fog forms within in the steam-nitrogen boundary layer and the steam condenses at the water droplets of the fog layer and reaches the cooling water interface no longer. The convective mass transfer to the cooling water interface diminishes and no heating up of the water layer takes place. This effect was observed with experiments at stratified two phase flow run with pressures up to 2.0 MPa. The interface temperature has proved to be the most important parameter for fog formation. The paper explains this effect by means of film theory. It offers correlations to quantify the effect and to calculate the minimum interface temperature to avoid spontaneous condensation. Received on 29 May 1998