| Recent breakthroughs of many pioneering technologies rely increasingly on their high-heat-flux removal capabilities,which arouses attentions from both the commercial and defense industries.Jet impingement cooling is one of the most promising solutions.In this dissertation,local and average heat transfer characteristics of confined and submerged jet array impingement are investigated along with boiling visualization images so that nonuniformity of the wall temperature distribution is explained.Deionized and degassed water flows through a row of 7 holes with diameters of 1mm,impinges onto the heater and is dispensed out of the chamber through the outlets at two sides.The heater is made by depositing aluminum film onto silicon wafers,back of which are installed streamwise 7 thermocouples to monitor local temperatures.An innovative design of orifice plate facilitates observation of various boiling phenomena with a high speed camera at small orifice-to-heater distances.The main contents and results of this investigation include:1.Heat transfer characteristics of confined jet array impingement.Wall temperatures and surface heat transfer coefficients at different locations show only minor difference.Heat transfer characteristics curves are found to pose similar shapes and trends.Boiling curves shift leftwards and h-q curves ascend with increasing flowrate.Orifice-to-heater spacings within 1-3 orifice diameters have negligible influence on heat transfer characteristics whereas stronger negative effects when raised to 5D.Boiling curves move left and h-q curves descend when liquid subcoolings are heightened.Boiling curves with the wall temperature as abscissa nearly overlap but boiling begins at the same superheat rather than wall temperature when only test section outlet pressures are different.A new dimensionless parameter constructed by using maximum and minimum wall temperatures is proposed to evaluate nonuniformity of the wall temperature distribution,whose trends coincide with those of heat transfer characteristics curves and boiling phenomena,and can be used to infer boiling status on the heating surface when visualization is either absent or infeasible.2.Boiling visualization of confined jet array impingement.The boiling phenomenon displays an easily recognizable pattern,i.e.,nucleation is difficult within the stagnation region and nearby bubbles are tiny while violent nucleation happens easily where two adjacent wall jets collide,which can hopefully be used to decrease the temperature of local hotspots.As heat flux rises,the number of nucleation zones,bubble diameters and the nucleation density all increase,with boiling areas expanding progressively towards the stagnation point.At a low flowrate,bubbles first appear in a single nucleation zone;while nucleate simultaneously at multiple zones when the flowrate is high.Meanwhile,bubbles grow smaller,live shorter and nucleation becomes unsteady.Boiling is more furious at the early stage of the multi-zones nucleation distribution when the orifice-to-heater spacing is raised.Under the same heat flux,nucleation areas become larger with increasing inlet temperatures.Although outlet pressures of the test section are different,boiling phenomena look similar when wall temperature superheats are the same.Bubble break-ups are observed under low heat fluxes and temperature superheats,indicating the potential to further enhance the nucleate boiling heat transfer. |
