Figure 4. Temperature contour map in the reactor wherein the leading edge of the subsurface layer has a tapered shape that corresponds to the angle of cross-bars in the top-most layer so that fluids from the bulk flow path are not trapped beneath the top-most layer.
The sensible enthalpy contour map in the reactor is plotted in Figure 5 wherein the leading edge of the subsurface layer has a tapered shape that corresponds to the angle of cross-bars in the top-most layer so that fluids from the bulk flow path are not trapped beneath the top-most layer. So far, boiling in microchannels has not been used in the thermal management and control of the microchannel chemical reaction processes due to various postulated or practical technical issues. Flow boiling in microchannels is associated with the flow patterns different from that found in the ordinary flow channels where vapor bubbles are smaller than the channel diameters and the channel wall is generally well wetted by the liquid. The hydraulic diameter of microchannels is usually smaller than the characteristic diameter of the vapor bubbles so that due to capillary effect vapor slugs and liquid slugs consecutively flow by a fixed location of the channel. The prediction methods and design criteria for this flow pattern are not well established. The other desired flow patterns such as bubbly flow and annular flow may only be possible in a very narrow flow parameter range or limited operation conditions or may be absent. Due to the existence of vapor slugs, local hot spot of the wall and in turn the temperature non-uniformity may occur due to the low vapor-wall heat transfer rate. Due to the existence of vapor slugs, severe flow and pressure oscillation may occur in microchannel boiling. Instability of the entire cooling system may instantly occur. The heat transfer crisis can occur even at low heat duty due to the large difference between the heat transfer coefficients by evaporation and by single-phase vapor convection. This is characterized by the critical heat flux that may be very low and lead to non-isothermal heat transfer. The flow distribution and manifolding are difficult in microchannel arrays with two-phase flow, while a large number of integrated microchannels is usually needed for the desired process capacity. The present process makes it possible to make use of flow boiling in microchannels integrated in unit operations to realize a stable isothermal boundary condition for the exothermal reaction. The reaction process can be thus thermally controlled to operate in an optimal condition. The local quality of the convicting flow is needed to estimate the pressure drop in a channel. Knowing the void fraction and vapor quality variation along the channel length, the two-phase pressure drop in the channel can be calculated using a separated flow model. The term critical heat flux is the local heat flux at which wall temperature cannot be maintained due to heat transfer mechanism change from boiling to vapor convection. This results in the formation of a localized hot spot.