Dynamic simulation of the pressure-drop type instability in an upflow two-phase boiling system

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Mechanical Engineering

First Committee Member

Sadik Kakac, Committee Chair


This work has theoretically studied the steady state and transient characteristics of an electrically heated single channel two-phase boiling system susceptible of pressure-drop type instability under certain operating conditions. Freon-11 flows upwards inside the channel.Before solving the steady state characteristic of the system, the analysis of thermal non-equilibrium existing between the two phases has been first carried out. The enthalpy profile satisfying the boundary conditions at net vapor generation point and the point at infinity away from the inlet of the boiling channel in the flow direction is assumed in the boiling region. Secondly, a mathematical model correlating the two-phase pressure drop as function of mass flow rate and vapor mass quality has been derived and verified by the experimental data on the same system.Using the finite difference method, the conservation equations of mass and momentum, along with the drift-flux model and other constitutive equations, are solved numerically for the steady state characteristics of the system at different power inputs and fluid inlet temperatures. For comparison purpose, the steady state curves obtained from the non-equilibrium and equilibrium models have been shown in the pressure drop and mass flow rate plane, together with the experimental measurements. It has been discovered that the theoretical models successfully predict the pressure drop variation with the mass flow rate at steady states, and the non-equilibrium model matches the experiments with more satisfaction.The dynamic response of the system including the effect of thermal non-equilibrium has been studied by introducing small perturbation in the surge tank pressure. A dynamic model comprising the lumped momentum equations upstream and downstream of the surge tank, and the dynamic equation of the surge tank itself has been presented. The system stability boundaries in the steady state pressure drop versus mass flow rate plane, as well as the dynamic simulations of the surge tank pressure and mass flow rate when pressure-drop type instability occurs, are obtained by solving the dynamic model numerically. The effects of non-equilibrium on the stability boundaries and oscillating periods of the pressure-drop type oscillation have been illustrated and analyzed. When compared with the equilibrium theory, the results of non-equilibrium model approach closer to the experimental recordings.


Engineering, Mechanical

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