Study of the Combined Contributions of Porous Coverings and Mixture Surface Energies in Enhancing Pool Boiling

2006 ◽  
Author(s):  
Eric Siqueiros ◽  
Rene Reyes
Keyword(s):  
Heat Transfer ◽  
Binary Mixture ◽  
Convective Heat Transfer Coefficient ◽  
Boiling Water ◽  
Lauryl Sulfate ◽  
Surface Energies ◽  
Enhancing Heat Transfer ◽  
Aqueous Mixture ◽  
Covering Design ◽  
Comparison Of The Results

Factors as the boiling fluid surface tension and the characteristics of the solid surface where the heat transfer takes place could be modulated for increasing the boiling heat flux. In this work was observed the increase in the boiling convective heat-transfer coefficient (h) from the participation of: (a) the use of a binary mixture at its critical micelle concentration (16% w/w ethanol-water); (b) the addition of the surfactant sodium-lauryl-sulfate (SLS) to this aqueous mixture; and (c) the use of a porous covering fabricated from stainless steel bands with void volume 0.25, pore diameter 0.8 mm and covering thickness 8 mm. The sequence of results allowed the calculation of the relative participation of these factors in h (and the related values of excess temperature), for power supply from 100 to 1000 W on the same heater cartridge for all the experiments. For boiling water on the bare heater, hmax bare heater = 8.27 W/cm2 K; for boiling water on the porous covering, hmax covering = 19.36 W/cm2 K; the boiling of the water-ethanol (16%) mixture on the porous covering produced hmax covering+cmc = 31.72 W/cm2 K; and the binary mixture with 100 ppm of SLS, hmax covering+cmc+surfactant = 38.07 W/cm2 K. Considering this value of hmax covering+cmc+surfactant as the sum of the contributions, the relative participation of the mechanical forces breaking the escaping bubbles through the covering is 29.13%; the surface energies associated to the formation of micelle structures 32.47%; and the surface energies from the surfactant 16.67%. Thus, the search of enhancing heat transfer should consider the boiling mixture composition as well as the porous covering design. A comparison of the results obtained with the covering developed in this work with some coverings developed in a previous work reveals that the geometry of the covering material could be the base for heat transfer enhancement.

Synergism of Binary Mixtures Wettability and Cover Porosity on Enhanced Pool Boiling Heat Transfer

2004 ◽  
Author(s):  
Elva Mele´ndez ◽  
Rene´ Reyes
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Interfacial Energy ◽  
Boiling Heat Transfer ◽  
Convective Heat Transfer Coefficient ◽  
Water Mixture ◽  
Lauryl Sulfate ◽  
Maximum Heat ◽  
Maximum Heat Transfer

The wettability of the system in capillary covers is an important element to increase the boiling heat transfer on the coverings. The sessile drop methodology allows the evaluation of either the surface energy of solids or the interfacial energy of liquids, and from both the system’s wettability. This methodology was tested with an experimental set up built for this study. The surface energies calculated for solids and metal foils used for construction of capillary coverings were in accordance with previous experimental results. The same methodology is used for measuring interfacial energies of the liquids used for increasing boiling heat transfer like ethanol-water mixtures. The mixture with 16% ethanol by weight had the lowest contact angle (associated to the lowest interfacial energy) and produced the highest convective heat transfer coefficient, h. Thus, the maximum for h correlates with an increase in the wettability of this system. This behavior is related to that observed as the critical micelial concentration (cmc) for surfactants, that produce the lowest interfacial energy of the liquid. Thus a set of experiments was developed to correlate the binary mixture behavior around the concentration with maximum heat transfer coefficient with the cmc boiling behavior. The surfactant sodium lauryl sulfate (SLS) produced an increase of the wettability of the solid surface with the addition of 100 ppm (or less) that is its cmc. The h values increase with the addition of SLS up to 100 ppm but do not change if the concentration of surfactant is higher than that value. The maximum heat transfer coefficient is obtained with the cmc of SLS in water, and with the 16% by weight ethanol-water mixture, both having the highest wettability. Porous coverings were tested with two covering’s thickness. A synergistic effect is found for the appropriate cover thickness combined with either a 16% by weight ethanol-water mixture or water with the cmc of SLS.

Correlation of Surface and Interfacial Energies on Enhanced Pool Boiling Heat Transfer

Volume 3 ◽  
2004 ◽  
Author(s):  
Elva Mele´ndez ◽  
Rene´ Reyes
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Surface Energy ◽  
Interfacial Energy ◽  
Boiling Heat Transfer ◽  
Contact Angles ◽  
Lauryl Sulfate ◽  
Surface Energies ◽  
Interfacial Energies ◽  
Set Up

The surface energy of the material used in the construction of capillary covers is an important element to increase the boiling heat transfer on the coverings. There are a variety of methodologies for measuring the surface energy of solids, but few could be used with the construction materials tested. The sessile drop methodology allows the evaluation of either the surface energy of solids or the interfacial energy of liquids. The methodology uses an image digitalization system for measuring the contact angle of liquids on the solid’s surface. The contact angles thus measured are used to calculate the superficial and interfacial energies. This methodology was tested with an experimental set up built for this study. The accuracy of the set up was obtained with clean and greased surfaces of high heat conductivity metals. The surface energies calculated were in accordance with previous experimental results. The surface energies of metal foils used for construction of capillary coverings were similar to the values calculated for the parental solid metal. The surfaces with different grease thickness get values of surface energy close to the value for the adhered hydrocarbons. The same methodology is used for measuring interfacial energies of pure and mixtures of liquids. The liquids studied include those used for increasing boiling heat transfer. Ethanol-water mixtures were analyzed. The mixture with 16% ethanol by weight had the lowest contact angle (associated to the lowest interfacial energy) and produced the highest convective heat transfer coefficient, h. A minimum in the value of the contact angle around the 16% weight ethanol mixtures follows the maximum in the value of h around this composition, and a maximum in the wettability. Similarly, the surfactant sodium-lauryl-sulfate (SLS) produced an increment of the wettability of the mixture on the solid surface. The reduction of the contact angle is obtained with the addition of 100 ppm of SLS or less, depending on the base metal, but above this concentration, the surfactant does not modify the value of the contact angle. The h values increased with the addition of surfactant up to 100 ppm but do not change if the concentration of surfactant is higher than that value.

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