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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]

In this study, the nonlinear dynamic behavior of an asymmetric Wave Energy Converter (WEC) was studied on a 1/11-scale model based on a numerical and experimental method in a regular wave field. The numerical analysis involved both frequency-domain and time-domain solutions. The frequency-domain solution was based on linear potential flow theory using the WAMIT® model whereas the time-domain solution was based on the Reynolds-Averaged Navier-Stokes (RANS) equation using the OpenFOAM® model. The pitch response amplitude operators obtained from the numerical results were compared with experimental data. Additionally, the nonlinear dynamic behavior of the asymmetric WEC due to various wave heights and periods obtained through the experiments were compared with the OpenFOAM results. Wave excitation moments and hydrodynamic coefficients based on the linear solution approach were separately obtained for the fixed asymmetric WEC induced by waves and the forced oscillating motion with OpenFOAM. Moreover, frequency-domain solutions based on linear potential flow theory were obtained in order to ascertain the nonlinear effects observed in the OpenFOAM results. It was found that a higher motion amplitude could be attributed to nonlinear hydrodynamic coefficients on the asymmetric WEC calculated by the forced oscillation motion of the RANS-based solution. [ABSTRACT FROM AUTHOR]

• Forced-flow cooling with helium gas is required for HTS fusion magnets at 20–30 K. • Temperature distribution is calculated with thermal interaction between conductors. • There exists an internal peak temperature, which may well be predicted by NTU. • Thermal insulation in axial contact can significantly improve cryogenic stability. A key design feature is presented to improve the cryogenic stability of forced-flow gas-cooled HTS conductors for fusion magnets. In order to realize the compact and economic fusion system, a variety of conductors are under development with stacked REBCO tapes so that the high-field magnets can operate at 20–30 K with the internal flow of helium gas. Thermal-hydraulic analysis is typically based on 1-D models under a prescribed thermal load along the conductors. In this study, the effect of thermal interaction between adjacent conductors in a winding pack is rigorously considered for the analysis. In a double pancake with single cooling loop, it is evidently verified that the conductor temperature may rise to a peak at inner layers, seriously affecting current sharing and thermal stability. The analytical results are useful in predicting the temperature and location of hot spot in terms of a dimensionless parameter, NTU. In theory, the gas-cooled magnet can be thermally stable by either infinitely increasing or infinitesimally decreasing the thermal interaction between conductors. In practice, however, the thermal insulation to minimize the contact between inbound and outbound flows in a double pancake is proposed as an effective way to improve the cryogenic stability. [ABSTRACT FROM AUTHOR]

The thermal and hydraulic phenomena in an inclined semicircular channel through which nitrogen changes to supercritical state flows were studied through numerical analysis. The geometry of the flow channel was semicircular in cross-section with a diameter of 2 mm and a length of 350 mm. The heat fluxes applied to all sides of the semicircle were 20, 40, and 60 kW/m2; the angles of the inclined flow channel were 30°, 45°, 60°, 90°, and −90°, and one more case was analyzed under zero gravity as a reference case. Under the given conditions, nitrogen changed to a supercritical state owing to the heat flux applied while flowing through the channel, and the pseudo-critical point differed depending on the size of the heat flux. However, with a change in the inclination angle, physical properties of the heated working fluid, such as the density, viscosity, and c p , changed rapidly, and the buoyancy effect and acceleration parameter values changed accordingly. As a result, a secondary flow appeared, and the change in the heat transfer performance was explained by the heat transfer coefficient. By calculating the change in the Nusselt number and the change in each variable according to the location of the channel, we developed correlations based on the change in the inclination angle and heat flux. [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]

An experimental study of hydrodynamic cavitation was conducted in a rectangular microchannel with a pillar. Distilled water was used as working fluid in an open fluid loop, and cavitation was obtained by applying a range of pressure differences between inlet and outlet tanks. High speed camera captured the flow patterns from inception to fully developed cavitating flow. A minimum delay of 10 min in the formation of cavitation was recorded in all experiments, which is due to the stochastic nature of phenomenon. Cavitation inception conditions were obtained in terms of the cavitation numbers, and a flow map was developed for subsequent cavitation flow. By analyzing time series of gray scale intensity of pixels inside the cavity, dominant frequencies were identified. Transient single phase numerical simulations were performed to gain a better understanding of the flow field in the microchannel, verify pressure measurements, and to relate the separation angle to the attached cavitation angle around the pillar. Emphasis was placed on characterizing the wake region downstream the pillar as it is closely related to the occurrence of the cavitation phenomena. [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]