Page 39 - חוברת תזות הנדסה ירוקה 2023

Increasing the heat transfer coefficient in convection from a surface by changing the surface
Zil Shoikhetbrod1, shohat.z@gmail.com
Dr. Alex Koyfman2, Dr. Gedalya Mazor2
1Green Engineering Department, Sami Shamoon (SCE), Be’er Sheva, Israel 2Mechanical Engineering Department, Sami Shamoon (SCE), Be’er Sheva, Israel
It is a known fact that a drop falling on a very hot surface )above the Leidenfrost temperature( will evaporate very slowly, thus the surface loses heat at small rate. This fact makes spray cooling an inefficient method. However, in some cases, spray cooling is the only possible means for removing heat. Among these cases are when putting out accidental fires that occur in industrial facilities, where many metal surfaces reach very high temperatures and. of course. during LOCA/LOFA )loss of coolant / loss of flow( accidents, such as what happened in the Fukushima nuclear disaster. In the latter case, Japanese firefighters pumped sea- water into the reactor, to prevent its core from melting. It was the incident in Fukushima that motivated this work--the study of the influence of salt concentrations in water solutions on heat transfer coefficients during the spray cooling of hot surfaces.
To understand the impact of salt concentrations on heat transfer coefficients, the influence of various salt concentrations on properties, such as surface tension and wetting angle, were examined. These two properties are known to affect heat transfer characteristics. The investigation was performed experimentally. The experiments were conducted by spraying two types of salt-water solution on hot surfaces and measuring the temperature drop rate. The solutions used were water-magnesium sulfate )( and water-calcium chloride )( at different salt concentration ratios. These experiments showed that higher salt concentrations have better heat transfer performance.
From the surface tension experiments performed on the magnesium sulfate and calcium chloride solutions, we found that the value of the surface tension slightly decreases with the increase of salt concentration in both solutions. However, in the experiments for cooling hot surfaces, we found that surface heat flux removal increases with an increase in the solution’s salt concentration. As such, it may be assumed that there is an inverse connection between surface tension and heat flux. Our experiments showed that, for solutions with a high concentration of salt, the surface tension decreases, whereas heat flux removal increases. The results of this study also indicate that magnesium sulfate solutionwith a salt concentration of 15% has a heat flux improvement factor 2.5 times greater than that of distilled water. Furthermore, from the experiments, we saw that the formation of homogeneous crystalline structures on the surface significantly increase the rate of heat removal from the surface and increase surface heat flux.
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