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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]

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]

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]