Effects of inlet/outlet configurations on flow boiling instability in parallel microchannels

A simultaneous visualization and measurement study has been carried out to investigate effects of inlet/outlet configurations on flow boiling instabilities in parallel microchannels, having a length of 30 mm and a hydraulic diameter of 186 μm. Three types of inlet/outlet configurations were investigated. Fluid flow entering to and exiting from the microchannels with the Type-A connection was restricted because the inlet and outlet conduits were perpendicular to the microchannels. The fluid flow had no restriction in entering to and existing from the microchannels with the Type-B connection. In the Type-C connection, fluid flow was restricted in entering each microchannel but was not restricted in exiting from the microchannels. It is found that amplitudes of temperature and pressure oscillations in the Type-B connection are much smaller than those in the Type-A connection under the same heat flux and mass flux conditions. On the other hand, nearly steady flow boiling exists in the parallel microchannels with the Type-C connection under the experimental conditions. Therefore, this configuration is recommended for high-heat-flux microchannel applications. As predicted, the stability threshold is determined by the minimum in the pressure-drop-versus-flow-rate curve. The pressure drop and heat transfer coefficient versus vapor quality for flow boiling in microchannels with the Type-C connection are presented. It is found that experimental data of pressure drop are higher and heat transfer coefficients are lower for boiling flow at high vapor quality in microchannels than those predicted from correlation equations for boiling flow in macrochannels, due to local dryout.