[Show abstract][Hide abstract] ABSTRACT: Binary mixtures of highly concentrated emulsions (HCE) with three droplet size ratios and different compositions were prepared. It was found that by the proper selection of droplet size ratio and composition of binary mixtures, the shear modulus, viscosity, yield stress, and yield strain can be dropped lower than mixing rules and even primary HCE. This effect is similar to what is known for dispersions with volume fraction less than 0.7 but has not been described for HCE. For such formulations, the caged mechanism of droplets dynamics is not dominant due to the provided free volume that can be occupied by smaller droplets during flow. This is originated from the increase in maximum closest packing and thus more efficient spatial packing. By studying the scaling behavior of shear modulus and yield stress, the significance of interdroplet interaction was distinguished.
Journal of Rheology 09/2012; 56(5):1299-. DOI:10.1122/1.4736556 · 3.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study investigates flow boiling heat transfer and pressure drop characteristics of mixtures of R-134a and an ester based lubricant. Nominal oil concentrations of 1 %, 3%, and 5% are tested. Experimental heat transfer coefficients and pressure drops are collected over a wide range of flow conditions. The two-phase flow regime is observed visually. The oil has a significant impact on both heat transfer and pressure drop. Small concentrations of oil enhance the heat transfer coefficient. The enhancement is observed for all oil concentrations tested; however, the enhancement is decreasing at an oil concentration of 5%. The enhancement is attributed to several factors including the promotion of an annular-type flow pattern at low to moderate flow rates, the degradation of the nucleate boiling contribution at the same flow rates, and foaming at higher flow rates. The presence of small amounts of oil also increases the pressure drop. This increase is seen at all oil concentrations tested. The magnitude of the increase continues to rise with oil concentration. Factors influencing the pressure drop are the increase in the mixture viscosity, the promotion of an annular-type flow pattern, and foaming. Mixture properties are used in both a heat transfer correlation and a pressure drop correlation to determine the effects of the thermophysical property changes of the mixture. The results indicate that in order to accurately predict refrigerant-oil data, effects other than just property variations must be taken into account. General correlations must properly account for the flow regime effects and foaming. Air Conditioning and Refrigeration Center Project 37
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