Pressure drop characteristics and instabilities during flow boiling in parallel and oblique finned microchannels - a comparative study

Abstract

Experiments on flow boiling were conducted in parallel and oblique finned microchannels. The test vehicles were made from copper with a footprint area of 2.5cm × 2.5cm. The microchannels were formed by wire cut Electro Discharge Machining process and have surface roughness (Ra) of about 2.0 µm. Tests were performed on channels having width of 500 µm and a nominal aspect ratio of 3 over different mass velocity range and inlet temperature of 90°C. The impact of mass flux and heat flux on two-phase flow pressure drop and flow instability for deionized water is presented in this work for both parallel and oblique finned microchannels. It is observed that the pressure drop for a particular microchannel increases with the increase in applied heat flux for a fixed mass flux. In the oblique finned microchannel, for single phase fluid flow, the breakage of continuous fin into oblique sections causes the thermal boundary layers to be re-initialized at the leading edge of each oblique fin and reduces the boundary-layer thickness. This regeneration of the entrance effect causes the flow to be always in a developing state. In addition, the presence of the smaller oblique channels causes a fraction of the flow to branch into the adjacent main channels. Flow reversals caused by flow boiling instability in microchannel were compared for both the geometries. This work presents a first glimpse of the pressure drop performance of the oblique finned microchannel under flow boiling conditions and also explores its potential to mitigate or minimize the flow boiling instabilities in microchannel heat sinks.