Investigation Of Pressure Oscillation In A Microchannel Two-Phase Flow Loop

Rapid progress in electronic information technology,space exploration and military applications has promoted the evolution of electronic systems.In the next generation electronic systems,the chips can be more powerful within more compact sizes.The higher and higher power density of these chips challenges the original thermal management technologies.Advanced reliable and effective cooling technologies are particularly desirable.As one of the most potential cooling technologies,microchannel evaporators seem to be an ideal selection to meet these challenges.In the present dissertation,flow boiling in a microchannel is studied by numerical simulation.Experimental investigations and theoretical analysis are performed to study the flow boiling instability in microchannel evaporators.Besides,methods to suppress the pressure oscillation in the evaporator are also investigated.The main contents of this dissertation are as follows:1.Numerical simulation investigations of flow boiling in a microchannel.A numerical model to describe the evaporation at phase interface is established.And the bubble nucleation model is improved.Numerical simulation investigations of flow boiling are conducted.Pressure in the microchannel rises rapidly due to bubble growth.The bubble growth and venting leads to the high-frequency pressure oscillation.2.Theoretical analysis of low-frequency pressure oscillation in microchannel evaporators.Starting from the pressure drop analysis,the trigger mechanism of low-frequency pressure oscillation: working at the negative region of pressure drop-flow rate curve with compressible volume upstream the channels,is studied.This pressure oscillation is identified as a kind of self-excited vibration(SEV).Characteristics of oscillation amplitude and period are studied theoretically.The compressible volume to sustain the oscillation cannot come the internal compressibility of microchannels.Two typical stages in the oscillation are identified: flow boiling stage and single liquid phase flow stage.3.Experimental studies of flow boiling instability in microchannel evaporator.Flow boiling instability experiments in microchannels are conducted.Both water and ammonia are studied.Pressure drop and wall temperature oscillations are measured.And two-phase flow patterns are observed.Flow boiling and single liquid phase flow occur alternatively during pressure oscillation.Two kinds of pressure oscillation modes are identified according to wavelet analysis.4.Theoretical analysis of parallel channel instability.The trigger mechanism of maldistribution between parallel microchannels is identified.The prior boiling channels suppress flow boiling in the remained channels by using the flow rate maldistribution.This maldistribution may lead to increasement of the flow resistance in the microchannel evaporator.Apart from frictional pressure drop,accelerational pressure drop and gravitational pressure drop,the transient pressure drop cannot be neglected in microchannels.Bubbles in parallel channels grow out of phase lead to the high-frequency pressure oscillation.5.Suppression of pressure oscillation.Based on the self-excited vibration analysis,two kinds of schemes to suppress the low-frequency pressure oscillation: changing the spatial arrangement of the elements in the two-phase flow loop,changing the microchannel cross section shape or adding an inlet restrictor,are analyzed.