Experimental Investigation of Flow Boiling Instability in a Single Horizontal Microtube With and Without Inlet Restriction

An experimental study is conducted to investigate the effects of inlet restriction (orifice) on flow boiling instability in a single horizontal microtube. The test-section is composed of a stainless steel tube with an inner diameter of 889 μm, and a length of 150 mm. Experiments are performed for three different orifice configurations with 20%, 35%, and 50% area ratio. Mass flux is varied from 700 to 3000 kg/m² · s, whereas the heat flux is varied from 6 to 27 W/cm². The dielectric coolant FC-72 is selected as the working fluid. In the absence of an orifice at the inlet, four oscillation types are observed at the onset of flow instability; it is also noticed that the frequency of the oscillations increases with increasing heat flux, while the amplitude remains constant. The addition of an orifice at the inlet helps stabilizing the flow without generating significant pressure drop at the same operating condition as the microtube without orifice. The 20% area ratio orifice shows better performance at low mass fluxes (<1000 kg/m² · s). Whereas, at high mass fluxes (>2000 kg/m² · s), 50% and 35% area ratio orifices are efficient in stabilizing the flow or delaying the onset of flow instability. Therefore, selecting the area ratio of the orifice depends on the operating condition. A small area ratio orifice is preferably used at low mass fluxes, whereas a large area ratio orifice is more suitable for high mass fluxes.