Infrared thermography measurement of two-phase boiling flow heat transfer in a microchannel

In this study, a non-intrusive method to directly measure the fluid temperature and two-phase flow patterns in micro-scale system was developed. To achieve this goal, an adequate calibration process of infrared (IR) thermography measurement and an experimental design of IR transparent convective flow in a microchannel were established. The temperature distribution of the fluid along the microchannel was measured by IR thermography through a germanium window of thickness 5 mm. The transparent germanium window was used to facilitate the transmission of long-wavelength IR rays. The infrared images were obtained at a frame rate of 200 fps, which enabled observation of the transient temperature behavior during flow boiling in the microchannel. A semi-transparent liquid, namely ethanol, was used as the working fluid, with a mass flux of 20.3 kg/m²s and heat flux range of 3.1-244.1 kW/m². The experimental results confirmed that IR thermography could be used to capture the transient single- and two-phase flow patterns as well as the fluid temperature along the channel. To the best of our knowledge, this paper is the first presentation of IR visualization of two-phase flow patterns in a microchannel. The trends of the local heat transfer coefficients with respect to the two-phase flow patterns are discussed, and the experimentally determined coefficients are compared with those calculated by commonly used equations.