Experimental investigation of boiling heat transfer coefficient in two-phase carbon dioxide in a microchannel with side-view schlieren flow visualization

Carbon dioxide in close vicinity to the critical point has recently drawn interest as a working fluid for thermal management due to its favorable thermophysical properties, dielectric nature, low global warming potential, relative abundance, and low corrosiveness. This study aims to investigate the characteristics and mechanisms affecting near-critical two-phase carbon dioxide flows in microchannels using simultaneous heat transfer measurements and schlieren visualization. By making heat transfer measurements in tandem with side-view visualization, the fluid mechanics of flow boiling in microchannels and their relation to heat transfer are elucidated. The present study developed a novel microfabricated, non-uniformly heated microdevice
$(D_{\mathrm{h}}=500\mu \mathrm{m})$
that allows for simultaneous heat transfer measurements and schlieren imaging of carbon dioxide. Schlieren imaging is a well-established technique for visualization of refractive index gradients in transparent media; however, it has seen limited application in microscale flow boiling studies. The technique operates on the principles of light deflection in inhomogeneous media. When a refractive index gradient is present, light is deflected away from its original path of propagation. This deflected light is then passed through a filter at the focal plane, which creates intensity patterns representing the refractive index gradients present in the fluid. In the case of flow boiling, this refractive index mismatch is brought about by the density change between the two phases as well as temperature gradients within the constituent phases. Moreover, when oriented in side-view relative to a single heated channel wall, schlieren images allow for the visualization of bubble nucleation, liquid thin film formation, and the influence of the piston effect on the bulk flow. The steady-state local heat transfer coefficient is calculated over a short, heated section of the channel in which the flow is hydrodynamically fully developed. Local and average wall temperature measurements made by microfabricated thin-film platinum resistance temperature detectors (RTD) in close proximity to the heated wall allow for precise measurement of wall temperatures and heat transfer coefficients. The heat transfer data and high-speed (8000 fps) schlieren images of the flow are presented for parametrically varied wall heat flux, mass flux, inlet temperature, and pressure.