Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.
In an earlier paper the results of a simulation of unstable flow phenomena in vertical condensing flows were presented. The horizontal condensing flow model was extended to include gravitational effects. The simulation of the non-linear model revealed existence of limit-cycle type of oscillations of large amplitude, including possibilities of flow reversals, as were observed on horizontal flows. The simulations also revealed that the gravitational effects have an attenuating effect on the oscillatory behavior in downward flows, and an enhancing effect on the upward flows. Matlab/Simulink tools were used for the system simulations. In the present paper the non-linear model was linearized, leading to an identification of the stable and unstable domains of operation for vertical condensing flows in both upward and downward flow directions. The stability boundary for the horizontal condensing flows lies in between the stability boundaries for vertical flows. The downward condensing flow is more stable and has a narrower unstable region than the horizontal flow. By comparison the vertical condensing upward flow is more unstable than both horizontal and downward flows. These observations are in correspondence with those of the non-linear model. A linear stability criterion including effects of gravity for vertical flows, that is an extension of such a criterion for horizontal flows, is presented.