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Highlights • Local, transient flow boiling in a microchannel with a pin fin was experimentally studied. • The temperature measurements were synchronized with high-speed images to analyze boiling event. • Local heat transfer coefficient was calculated and compared with available models in the literature. Abstract Flow boiling in a microchannel with a pin fin was experimentally studied. Micro resistance temperature detectors (RTD) array was integrated inside a microchannel to enable local transient temperature measurement. The temperature measurements were synchronized with high-speed images to analyze boiling event and the corresponding transient temperature characteristics. Three distinct two-phase flow regimes were observed: bubbly flow, slug flow, and attached vapor cavity. Cyclic temperature fluctuations were observed during slug flow and it was related to local periodic vapor slug activities. In a single cycle, three distinct stages were observed: liquid wetting, film evaporation, and local dry out. These three processes were decomposed to calculate local heat transfer coefficient based on the experimental observation, as h x , y = τ l h l + τ film h film + τ v h v and the result was compared with available models in the literature. [ABSTRACT FROM AUTHOR]

Local single-phase flow heat transfer downstream a single pin fin in a microchannel was experimentally and numerically studied. Three distinct flow regimes, depending on the Reynolds number, were characterized, namely: laminar flow with steady wake, laminar flow with unsteady wake, and turbulent flow. Local temperature measurements with high spatial resolution were obtained by incorporating an array of micro resistance temperature detectors (RTDs) (∼55 µm × 55 µm) on the internal microchannel surface. Local surface temperatures were related to the flow structures under different flow regimes. An enhanced local heat transfer coefficient at the trailing edge of the wake region downstream the pillar was observed. It is believed to be a result of vortex shedding and large-scale flow mixing triggered by flow instability at high Reynolds number. The numerical model enabled a full conduction/convection conjugate analysis of the entire system including heat conduction within the solid substrates and heat losses to the surrounding environment. Local heat transfer coefficient downstream the pin fin at each Reynolds number was obtained. [ABSTRACT FROM AUTHOR]

Local temperature measurements were made in a microchannel jet impingement cooling system with a single slot jet ( D h  = 68 µm and standoff distance of 210 µm). A 40%/60% solution of propylene glycol in deionized water was used as the working fluid. Resistance temperature detectors ( RTD s) were fabricated over a rectangular heater of size 1500 µm × 400 µm allowing local temperature measurements. Nominal heat fluxes ranged between 50 W/cm 2 and 150 W/cm 2 , and jet Reynolds numbers were in the range of 122–435. A three-dimensional conduction/convection conjugated numerical model with laminar and turbulent variants was developed to predict the jet hydrodynamics and heat transfer process. Good agreement was achieved between the model and the experimental data in terms of flow coefficients and local wall temperatures. Furthermore, a generalized Nusselt number dependence on Reynolds number was formulated, taking into account the temperature-dependent viscosity of the working fluid. The results provide valuable information about local and surface-averaged heat transfer due to a flow field generated by an impinging micro slot jet. [ABSTRACT FROM AUTHOR]

Local heat transfer downstream a single pin fin in a microchannel was experimentally studied by incorporating an array of micro resistance temperature detectors (RTD) (∼55 μm × 55 μm) on the internal microchannel surface. Local temperature distribution with spatial resolution as high as 150 μm was obtained and was superimposed onto the velocity field to reveal the interaction between the flow structure and the local heat transfer at different regions downstream the pin fin. Initial result in which the surface temperature inside the steady wake region was lower than in the regions outside the recirculation zone was explained and linked to an interplay of fluid convection and solid substrate conduction. Ignoring this local interplay and processing the data without careful consideration to the conduction process resulted in misinterpretation of the heat transfer processes. To address this issue numerical thermal and fluid model of the entire device was simulated to provide local heat flux distribution. This in turn allowed to resolve the local heat transfer coefficient in the vicinity, and outside the region, of the pin fin. [ABSTRACT FROM AUTHOR]