Title

Thermal oscillations in a single channel upflow boiling system

Date of Award

1990

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Mechanical Engineering

First Committee Member

T. Nejat Veziroglu, Committee Chair

Second Committee Member

Sadik Kakac, Committee Member

Abstract

Investigations on three two-phase flow instability phenomena--namely, thermal and pressure-drop oscillations and Ledinegg instability--are presented.In the first part of the dissertation, the experimental set-up and experimental procedure are described. Some typical results for steady-state pressure-drop versus mass flow rate characteristics and the pressure-drop and thermal oscillations are presented and discussed. A scaling analysis for the period of the thermal and pressure-drop oscillations is performed, and it is shown that the scale of the period can be represented as a simple function of the surge-tank (compressible) volume and operating mass flow rate.Next, details of finite-difference analysis of the oscillations are presented. The one-dimensional partial differential equations of the drift-flux model are solved for predicting steady-state characteristics, and reasonably good results are obtained. The pressure-drop and thermal oscillations are simulated by making use of the steady-state results and a quasi-steady-state approximation. Good agreement between the theory and experiments is shown.In the third part of the thesis, some analytical results are derived for pressure-drop oscillations and the Ledinegg instability. An integral model is developed, which results in a three-dimensional, nonlinear, coupled set of ordinary differential equations. Predictions of this simple model about the behavior of the system are in good agreement with experimental results. Also, correct parametric behavior with respect to the compressible volume and the mass flow rate is exhibited.A bifurcation analysis is carried out next, and it is shown that the pressure-drop oscillation limit-cycles are a consequence of a super-critical Hopf bifurcation that takes place in the dynamics of the system as heat input is increased. It is also shown that the Ledinegg instability is a result of a saddle-node bifurcation, which occurs with further addition of heat. Thus, these two instability mechanisms are treated on a common ground. Finally, the bifurcation set for the two-phase flow system is developed, and all possible behaviors of the system are outlined.

Keywords

Engineering, Mechanical

Link to Full Text

http://access.library.miami.edu/login?url=http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:9114814