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We studied the transient local surface temperature response that occurs when jet impinging of nitrogen gas is applied to the surface to which a pulsating heat flux is applied. After manufacturing a micro jet impinging device, which was composed of three parts—a silicon wafer, a vinyl sticker, and a Pyrex wafer—nitrogen gas was used as working fluid and the velocity was 344.8 m/s, corresponding jet Reynolds was 1467. And the amplitudes of heat fluxes varied from 5.67 to 16.17 W/cm2, the frequency also varied from 0.1 to 1000 Hz, then temperature response of RTDs was acquired. For example, when a heat flux, 8.5 W/cm2, was applied with a frequency of 0.1 Hz, the highest and lowest temperatures of RTD located on the center of the heater and under indirect jets were 30.2 °C and 20.7 °C. From the acquired results, the time constant of the device was estimated between 17 ms and 32 ms, and this result shows the conjugated mode of conduction and convection of heat transfer. To understand the dominant heat transfer, we decoupled the conduction mode and the convective heat transfer mode through analytical calculations and confirmed that the time constants were 11 µs and 128 ms, respectively. In addition, the heat transfer coefficient was inversely calculated through numerical calculation by simulating the RTD manufactured in the device, and through this, it was confirmed that the dimensionless time constant was related to the Nu number and the correlation was developed as τ/t = 3.51Nu−1.067 for f ≤ 1 Hz. [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]

Interest in renewable energy is rapidly growing due to the increase in energy demand, environmental pollution, and depletion of fossil fuels worldwide. One of these sources is solar chimney power plant, which utilizes solar energy to generate electrical energy at a relatively low operating cost. Solar chimneys also do not require cooling, indicating that water resources are not wasted, and they are expected to be widely used in developing countries with abundant solar radiation. This study involved installing a solar chimney with a height of 3 m and a diameter of 3 m for the collector. The experimental results were verified by performing numerical work using ANSYS Fluent, under the scenario that no partition was present in the collector. The exergy efficiencies with I- and C-type partitions were compared with the case without partition. The result showed that the exergy efficiencies were 7.05 % and 7.12 % for the I- and C-type partitions, respectively, and they were higher than the previous 6.44 % for no partition. This study confirmed that the power generation exergy efficiency can be increased by installing partitions in existing solar chimney power plants. [ABSTRACT FROM AUTHOR]