Sliding bubble dynamics and the effects on surface heat transfer
An investigation into the effects of a single sliding air bubble on heat transfer from a submerged, inclined surface has been undertaken. Existing literature has shown that both vapour and gas bubbles can increase heat transfer rates from adjacent heated surfaces. However, the mechanisms involved are complex and dynamic and in some cases poorly understood. The present study utilises high speed, high resolution, infrared thermography and video photography to measure two dimensional surface heat transfer and three dimensional bubble position and shape. This provides a unique insight into the complex interactions at the heated surface. Bubbles of volume 0.05, 0.1, 0.2 and 0.4 ml were released onto a surface inclined at 30 degrees to horizontal. Results confirmed that sliding bubbles can enhance heat transfer rates up to a factor of 9 and further insight was gained about the mechanisms behind this phenomenon. The enhancement effects were observed over large areas and persisted for a long duration with the bubble exhibiting complex shape and path oscillations. It is believed that the periodic wake structure present behind the sliding bubble affects the bubble motion and is responsible for the heat transfer effects observed. The nature of this wake is proposed to be that of a chain of horseshoe vortices.
Subcooling effect on boiling heat transfer of inclined downward-facing surface under low flow and pressure
Abstract The present study investigated the physical processes responsible for the variation in the boiling curve and critical heat flux (CHF) caused by liquid subcooling under atmospheric pressure in a rectangular flow channel; the flow channel was oriented 10° upward from the horizon. Bubble dynamics were examined using a high-speed camera and optical fiber microprobes. A solid copper block was utilized as a test heater and mounted above the flow channel to simulate the passive cooling system of an ex-vessel core catcher designed for nuclear power plants. Low mass flux and subcooling conditions ranging from 40–300 kg/m2 s and 5–25 K, respectively, were applied at the inlet of the test section. At the mass flux value of 40 kg/m2 s, large sliding bubbles were attributed to a key criterion for enhanced boiling heat transfer when the liquid subcooling was varied up to 15 K. The results showed that the CHF was weakly dependent on the degree of liquid subcooling, which deviates from the general trend reported by many researchers. A repetitive flow reversal along with a pressure shock appeared, owing to the rapid condensation at the exit, which added complexity to the analysis of the CHF. This study provides physical insights for understanding the subcooled flow boiling heat transfer mechanism (including the CHF) based on sophisticated experimental measurements, such as the visual capture of boiling dynamics using high-speed video and local void fraction.
Effect of temperature on the solvated electron in 1-propanol
Abstract The technique of pulse radiolysis has been used to study the effect of temperature on the solvated electron in 1-propanol. The spectrum of the solvated electron has been measured in the range 140–298 K. The wavelength at maximum absorption, λ max , decreases with decreasing temperature (corresponding to an increase in the energy, E max ) but the spectral width at half height, ΔE 1 2 , changes little with temperature. At temperatures above ≈180 K, the formation of the solvated electron is complete at the end of the pulse and its spectrum shows no change in shape with time. Below ≈170 K, the absorption spectrum is time-dependent in both liquid and glassy phases, the changes being attributed to the solvation process. The reaction(s) leading to solvated electron formation following the pulse show an activation energy of 22±2 kJ mol − and the solvated electron decays with an activation energy of 20±2 kJ mol −1 . The yield of solvated electrons decreases continuously with decreasing temperature in the range 298-77 K and, at all temperatures studied, is less than the total ionization yield measured by scavenging techniques.