Microchannel two-phase flow oscillation control with an adjustable inlet orifice

This work describes the experimental setup, method, and results of utilizing a micrometer to move an adjustable orifice immediately in front of an array of microchannels. Research by others indicates potential for significant improvement in delaying critical heat flux and increasing heat transfer coefficients when placing an orifice in front of each individual channel of a microchannel array. The experimental setup in this work allows incremental orifice size changes. This ability allows the experimentalist to find which orifice size provides enough pressure drop immediately in front of the channels to reduce oscillations. The design also allows for rapid change of orifice size without having to remove and replace any components of the test setup. Signal analysis methods were used to identify frequency and amplitude of pressure and temperature oscillations. Low mass flux experiments (300 kg m ^-2 s ^-1 and 600 kg m ^-2 s ^-1 of R134a in a pumped loop) showed reduced channel wall temperatures with smaller orifice sizes. The orifice concept was found to be effective at reducing oscillations for the higher 600 kg m ^-2 s ^-1 flow rate, but the data indicate that wall temperature reduction with inlet orifice use is not solely due to elimination of oscillations. Signal analysis was an effective method of identifying oscillations without the availability of pictorial representation of flow patterns in the channels.