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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]

Brazing bonding of iron-chromium-aluminum alloy (Fecralloy) with nickel based filler metal (MBF20) were performed at different maximum temperatures, 1000 °C and 1100 °C by varying the rate of temperature rise, 5 °C/min and 10 °C/min. And for comparison, diffusion bonding of Fecralloy-Fecralloy was also done with a condition, 1000 °C and 10 °C/min. For testing the tensile strength, the prepared tensile strength specimens were just bonded for diffusion bonding, and filler metal was inserted between the half specimens for brazing bonding. And in order to visually investigate about the bonding conditions, channeled heat plates were also prepared. The dimension of Fecralloy plates was 49 mm × 24 mm × 2 mm thick with three 2 mm diameter channels and 60 urn thick MBF20 was used as filler metal. The cross section parts of the test specimens were analyzed with SEM (XRD, EDS, EPMA, BSE) images, which were image processed to quantify the penetration depth of both filler and base metals to counter materials and components distribution using image processing. And such quantified results were compared with the tensile strength. From the both analysis, the tensile strength results were correlated with the change of thickness of filler metal as well as the distribution of components at the different bonding conditions. [ABSTRACT FROM AUTHOR]

Micro heat exchangers have been widely used in both of academic and industrial fields for controlling heat for decades. Especially, the printed circuit heat exchanger (PCHE) is applicable to high pressure and high temperature cases since the manufacturing process, diffusion bonding, gives high durability. However, flow does not occur uniformly in multiple channels, which causes a heat transfer difference between the channels that reduces the effectiveness of the system. Also, it makes the total pressure drop larger than when it is uniformly distributed. Flow distribution has been studied intensively for decades, but the mechanism has not been clarified yet, since flow in the header is much too complicated. As a step to reveal the mechanism, we focus on maldistribution and quantitatively investigate the phenomena. We prepared experimental devices, including a set of parallel micro-channels, inlet and outlet. We used water as a working fluid, and water soluble red ink for visualization. We measured the interface location to derive the flow speed at each channel. We focused on the dependency of channel length and flow speed. In the theoretical part, we derived the flow speed profile of the entire channels that shows good agreement with the experimental results. This study provides a theoretical basis on resolving the maldistribution problem. [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]