Abstract
A mathematical model is developed to study the stability characteristics of a single channel, electrically heated, forced convection upflow system. The model is based on the assumptions of homogeneous two-phase flow and thermodynamic equilibrium of the phases. The effects of the two-phase viscosity and heat transfer, gravitational forces, and the compressibility of the two-phase region are included in the formulation. The model consists of a set of non-linear hyperbolic partial differential equations. These equations are solved by explicit finite-difference techniques on a high speed digital computer. Comparison with experiments showed that the model is satisfactory in simulating the density-wave type oscillations. The model is found to be sufficiently accurate to predict the stability-instability boundaries at various flow conditions. With further simplification the model is also used to simulate, successfully, the low frequency oscillations, i.e., pressure-drop type oscillations.The model is applied at various inlet conditions and power inputs. The variations in heat transfer characteristics are also simulated; it is concluded that it has an important role in sustaining the oscillations. The theoretical analysis has verified that the compressible air-vapor mixture located at the upstream side of the system is equally important in sustaining the pressure-drop type oscillations. In the case of density-wave oscillations, the compressibility of the two-phase mixture at the downstream side is found to be an important factor in sustaining the oscillations.