Mechanical Engineering Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2795

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The effect of surfactant vapor on marangoni convection in absorption and condensation
(2005-11-18) yuan, zhe; Herold, Keith E; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Mass and heat transfer enhancement by the addition of a class of surfactant additives is in common use in absorption machines based on aqueous lithium-bromide (LiBr). It is observed that the addition of on the order of 100 ppm of a surfactant such as 2-ethyl-hexanol (2EH) introduces Marangoni convection on the liquid surface and thereby enhances absorption and condensation rates. The Vapor Surfactant Theory (Kulankara and Herold, 2000) proposed that such Marangoni convection is driven by the surface tension gradient caused by surfactant that circulates through the machine and arrives at the liquid surfaces as a vapor by bulk flow along with water vapor. The objective of this work was to fully understand the vapor surfactant induced enhancement mechanism and to quantify the relationship between the enhancement and the key variables. This goal was achieved by conducting experimental and numerical analyses including the measurement of surface tension with surfactant 2EH in the vapor, determination of 2EH surface concentrations in aqueous LiBr and water, experimental study of Marangoni convection in an absorption and condensation pool with surfactant 2EH in the vapor and modeling of Marangoni convection in the presence of surfactant vapor. The surface tensions were measured with controlled 2EH concentration in the vapor by using the drop volume method. The results show that for both aqueous LiBr and water the surface tensions are reduced with increased 2EH concentration in the vapor. The 2EH concentration in the vapor is a primary variable in determining the surface tension of aqueous LiBr. Calculated surface concentrations show that the presence of LiBr results in a reduction in 2EH solubility, and that the surface concentration of 2EH is more sensitive to surfactant in the vapor than to surfactant in the liquid. Furthermore, the experimental and numerical analyses show that surfactant in the vapor alone can initiate the Marangoni convection; the strength of Marangoni convection is primarily dependent on the 2EH concentration in the vapor. The current studies show that surfactant in the vapor is a necessary condition for significant absorption and condensation enhancement.
EXPERIMENTAL PERFORMANCE EVALUATION OF DESORBER IN A HYBRID ABSORPTION VAPOR COMPRESSION SYSTEM
(2013) Mandel, Bracha; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
A single-effect absorption facility was designed and constructed to experimentally investigate the performance of a 3 kW mini-channel desorber using an R134a/POE32 solution mixture. The facility was fabricated to simulate a hybrid absorption/vapor compression system for an off-grid high temperature application utilizing an air-cooled absorber. Desorber design replicated the utilization of waste heat from a generator source. The effects of temperature, pressure, solution mass flow rate and refrigerant concentration variations on desorber and desorption performance were investigated and analyzed through vapor generation, circulation ratio, poor solution concentration, desorber mean heat transfer coefficient and quality difference. Desorber heat transfer coefficient enhancement was found to be a strong function of solution temperature, rising by up to 75% with a 30°C temperature increase. Due to poor absorber performance, increasing solution temperatures and mass flow rates did not result in a proportional desorber vapor generation enhancement, leading to a reduction in desorber quality difference.