Identification of Microchannel Flow Boiling Regimes With Pt Thin-Film Impedance Sensors

Knowledge of the flow regime is critical to predicting heat transfer during two-phase flow boiling in microchannel heat sinks, as each flow regime has different heat transfer characteristics. One way to determine the flow boiling regime is to measure the electrical impedance of the boiling flow using the difference in electrical permittivity and conductivity of the liquid and vapor phases. To accomplish this, the article presents two innovative 200-nm thin platinum (Pt) thin-film impedance sensor designs. Both the sensor designs provided noninvasive and accurate real-time absolute impedance measurements for flow regime detection. The sensors were deployed on Pyrex glass lids for easy integration and interchangeability in microchannel heat sinks. They were positioned as the top wall over a stainless steel microchannel with a hydraulic diameter of
$850~\mu $
m, allowing for optical observation of the boiling phenomenon. All impedimetric measurements were synchronized with a high-speed camera validating the sensor output by optical observation of the boiling flow in the microchannel. The absolute impedance measurements were performed in the liquid-vapor two-phase flow of deionized (DI) water, taking advantage of the direct contact of the Pt structures with the DI water flowing in the microchannel. Both the impedance sensor designs successfully detected fully liquid, slug, slug-annular transition and annular flow by the location of the maxima of kernel density estimations of the absolute impedance measurement. The study’s simple approach could pave the way for imageless flow regime control in commercial microchannel heat exchangers. However, the impedance sensors could not detect critical heat flux (CHF).