Analysis and active control of pressure-drop flow instabilities in boiling microchannel systems
Pressure-drop oscillations are one of the most severe dynamic instabilities for boiling flow especially in microchannel systems. This paper presents a systematic framework for the transient analysis and active control of microchannel flow oscillations at a system-level view. To quantify the upstream compressibility and the associated oscillatory transients in an experimental microchannel boiling system, a lumped oscillator model is derived from the momentum balance equation, and both analytical and numerical nonlinear parameter identification methods are proposed. The predictions from the flow oscillation model agree well with the experimental pressure-drop observations across a flow meter and a microchannel heat sink. Based on the identified nonlinear oscillator model, a virtual state observer is designed to estimate the mass flow acceleration from the mass flowrate measurement in the boiling channel. Then a family of state and dynamic output-feedback active flow controllers is developed and evaluated for the dynamic pressure-drop instability suppression. Further analysis and simulations show the flow oscillation amplitude can be regulated to whatever level is desired under certain conditions.