Investigations Of Boiling Cooling Applied To Cylinder Head Based On Bubble Behaviors

Boiling heat transfer can be applied in engine cylinder head cooling to satisfy thestringent need for thermal load in modern high power density engine, due to its advantagesin high heat transfer efficiency and low volume ratio. In this paper, we used experimental,theoretical and numerical methods to study the mechanism and application of boiling heattransfer in cylinder head based on bubble behaviors, considering various problems ofboiling cooling in engine applications. Our work mainly includes:An experimental device for boiling heat transfer was established to observe bubblebehaviors, considering features of boiling cooling applied in engine cylinder head. Thecorrelation was suggested between bubble waiting time and factors including wall flux anddeparture frequency. The accuracy of bubble departure diameter prediction model wasvalidated based on our experiments. Boiling cooling mechanism was analyzed in view ofbubble behaviors, with the alteration of intrinsic rules in different phases explained, basedon bubble observation and heat flux measurement. It was concluded that boiling cooling indifferent phases was the coupling consequence from the “surface quenching” effect and the“heat resistance” effect, which were caused by boiling bubble departure and attachedbubble growth, respectively.Feasible domain and control strategy in engine cylinder head application of boilingcooling were established, based on the deep study into bubble disappearance, bubblecoalesce, and cooling design principle in bridge zone. The more accurate model for bubbledisappearance than conventional ones was suggested, based on the observed distortion ruleof boiling bubble surface. The feasible domain of boiling cooling in cylinder head wasdetermined as the zone between onset of boiling and the upper limit in boiling control area.The upper limit in boiling control area was suggested StControl=89×Pe-0.74, considering thespecial heated state and structural features in bridge zone, together with boiling heattransfer efficiency and bubble disappearance rate. Furthermore, the design principle ofboiling cooling parameters in cylinder head bridge zone was established.Pressure fluctuation in boiling-state flow field inside cylinder head was experimentally studied. The pressure signals in dangerous boiling state were concluded as (i) largefluctuation amplification and (ii) randomization in frequent spectrum without apparentorders, based on pressure signal comparisons among partially developed boiling state, fullydeveloped boiling state, and dangerous boiling state. It was also found that the peak powerspectral density and bubble family behavior happened in the same frequency, which waslower than300Hz, after the analysis in relation between pressure fluctuation signals andbubble behaviors. Finally, the discernment methods of dangerous boiling state frompressure fluctuation signals was suggested in view of two-point autocorrelation coefficientsand frequency-domain features. The methods were (i) two-point autocorrelation coefficientswere lower than0.82in20-500Hz, and (ii) the peak power spectral density was higher than2×10-3kPa2/Hz in400-500Hz.The bubble mean diameter and the boiling criterion were needed when boilingphenomenon was numerically calculated with Eulerian multiphase methods, since boththese two quantities varied with different boiling states. We suggested an iteration-basedmethod for multiphase flow simulation considering the bubble effect, after the study inrelations between bubble departure diameter and boiling state, between boiling criterionand boiling state, and between bubble mean diameter and bubble lift-off diameter. Ourmethod was validated with experimental data.The design principles of boiling heat transfer in engine cylinder head was suggestedbased on studies above. We redesigned the related parameters in the1015engine,considering the application of boiling cooling. The inlet coolant temperature wasdetermined91℃, and inlet velocity2m/s, based on the way of converse flow in cylinderhead. The newly designed cylinder head got a more uniform temperature field, with peaktemperature reduced7K, minimum temperature increased4.5K, and flux lowered nearly40%, compared with original one.