Multi-Variable Cascade Control of Flow Rates in a Row of Evaporative Cold Plates

To contribute stable operations of a mechanically pumped two-phase fluid loop, evaporative cold plates, developed in NASDA/Toshiba, were hydraulically tested in a coupled row before system integration. A significant result of such pretests was that the two-row arrangement is so highly interfering as to make difficult the exit vapor quality control. A flow control means other than used before has thus become necessary to suppress plate-to-plate operational interferences. Proposed is the multi-variable cascade control method, upon which flow rates are fittingly modulated so that the cold plate exit void fractions would constantly be kept as specified. The system identification, indispensable to practical use of this method, was experimentally parametrically done to provide us with a mathematical model of the coupled cold plates. The model is expressed in matrix representation readily applicable to state predictions. A digital controller, composed of a predictor and an instructor, was newly designed and fabricated for automatic loop operations. The predictor, acting on that model, is a LQR (Linear Quadratic Regulator); which yields a discrete-time series of modified reference void fractions. Manipulated variables are the valve opening degrees and the pump speed ratio. The instructor calculates them from modified to measured exit void fraction differences in accordance with empirically determined relations. A comparison on control responses was also made between LQR and PIR (Proportional Integral Regulator) to demonstrate availability of the LQR. The controller was then integrated into that loop to perform operations tests. Test cases are thirteen in all; including those of heat load change, of uneven heat load, of nonuniform heat load, of reference quality change, and of setpoint temperature change. Almost all test results were operationally quite satisfactory.