Date of Award

8-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Mechanical Engineering

Advisor

Saylor, John R

Committee Member

Beasley , Donald

Committee Member

Joseph , Paul

Committee Member

Ma , Lin

Abstract

Laboratory experiments are presented that reveal the effects of surfactant monolayers on natural convection heat transfer and evaporation from bodies of water; more specifically, the situation is studied where the bulk temperature of the water is greater than the temperature of the air so that evaporative convection occurs. Four sets of experiments were performed in a laboratory environment on insulated tanks of different widths and depths for the following surface conditions: 1) clean surface, 2) oleyl alcohol covered surface, 3) stearic acid covered surface, and 4) stearyl alcohol covered surface. An infrared (IR) camera was used to verify the existence of each of these conditions, and to measure the surface temperature during these experiments. The Nusselt-Rayleigh (Nu-Ra) and Sherwood-Rayleigh (Sh-Ra) dimensionless power law parameterizations were used to characterize the efficiency of convective heat transfer and evaporative mass transfer, respectively. This dissertation research presents the first such parameterizations that definitively show how these heat and mass transport processes are affected by surfactant monolayers.
All three of the surfactants reduced Nu by nearly one order of magnitude compared to clean surface conditions at equivalent Ra. This is attributed to the ability of the surfactants to change the hydrodynamic boundary condition at the interface from shear-free for the case of the clean surface to one that supports shear. The convective motion of the water is consequently damped in the presence of the surfactants, and the efficiency of convective heat transfer decreases. Despite this fact, all four surface conditions share essentially the same Nu-Ra power law exponent of 0.360. This result is different from typical Rayleigh-Benard Nu-Ra studies which find a power law exponent close to 1/3. The present results indicate that the rate at which Nu increases with Raw is greater for a free surface condition than for the solid boundary condition of Rayleigh-Benard type studies.
For all four surface conditions, the Sh-Ra parameterizations are found to be very close to obeying a 1/3 exponent which indicates that the mass transfer coefficient is independent of the horizontal extent of the surface W. The oleyl alcohol and stearic acid surface conditions both give the same Sh-Ra result as the clean surface condition; this surprising result reveals that the efficiency of evaporation is unaffected by these two surfactants at equivalent Ra. Stearyl alcohol, however, decreased Sh by approximately 50% compared to all other conditions. This is attributed to the ability of stearyl alcohol to block the transport of water vapor through the surfactant film. An effective relative humidity is defined which allows for the density of water vapor at the surface to be less than the equilibrium saturation value; this newly defined parameter allows for the stearyl alcohol Sh-Ra data to collapse with the other cases.

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