Enhancement mechanism of high alcohol surfactant on spray cooling: Experimental study

The addition of surfactant is considered as an important method for heat transfer enhancement in spray cooling based on numerous advantages such as less demand, significant effect and stable performance. Recent studies have found that high alcohol surfactant has a significant effect on the heat transfer in spray cooling. In this study, the effect of four kinds of high alcohol surfactants, 1-heptanol, 1-octanol, isooctanol and n-decanol, on spray cooling heat transfer using water as the working medium was studied experimentally. It was found that there exists an optimal concentration for each surfactant to achieve the best heat transfer performance. Under the experimental conditions, the optimal value of 1-heptanol was 0.3‰, heat dissipation flux reached 180.9 W/cm2, and the heat transfer increased by 20.9%; the best value of 1-octanol was 0.3‰, heat dissipation flux reached 200.8 W/cm2, and the heat transfer increased by 34.2%; the best value of isooctanol was 0.5‰, heat dissipation flux reached 185 W/cm2, and the heat transfer increased by 23.7%; the best value of n-decanol was 0.1‰, heat dissipation flux reached 180.7 W/cm2, and the heat transfer increased by 20.8%. To further analyze the enhancement mechanism, this paper also studied the influence of surfactant concentration on physical properties of working fluid and the spray characteristics. The experimental results showed that the effect of surfactant on saturated vapor pressure was small, and the change became obvious at higher temperature. The tension changed obviously with the surfactant concentration, and the surface tension decreased rapidly at lower concentration. The dynamic viscosity did not vary much with the type and concentration of the surfactant. The relationship between physical properties and concentration was also obtained in experiments. After adding surfactant, droplet number was lower than that of water, the distribution of droplets at different heights was more even. Droplet diameter reduced in certain area, however it increased in the edge of the area due to local impact and fusion. The change of droplet velocity was not obvious. This paper also put forward relations between the physical properties of different working fluids and their concentration and correlations which can predict diameter and velocity of spray droplet.