The Woodruff School

SUMMARY

Electrospray (ES) is a phenomenon which has been studied for over 100 years. The present work considers the utilization of an evaporation cooling system where the liquid is delivered to the heated surface by means of ES. Electrospray is a potentially effective means for evaporation cooling since it has been shown to produce aerosol spray plumes consisting of highly disperse sub-picoliter droplets, and is also a highly energy-efficient means of liquid delivery. Based on these benefits, the present work seeks to investigate ES-driven evaporative cooling behavior.

Cooling performance is determined by relevant heat transfer resistances within the system. For an ES-based evaporation cooling scheme, those can be broadly placed into two categories; the thermal resistance across a liquid film lying on the heated surface and the interrelated convection and mass transfer resistances in the gas phase. There appear to be no studies which look at the formation of a continuous volatile liquid film lying on a heated surface which is experiencing simultaneous evaporation and commensurate replenishment by ES. This leaves a gap in the understanding of the possible film morphologies, behaviors, and potential heat transfer performance. Importantly, the convection and mass transfer resistances in the gas at the evaporating liquid-vapor interface are a strong function of the gas velocity, an aspect of ES that has not been well studied. Despite the potential for ES droplets to impart momentum to the surrounding gas, the velocity field resulting from this momentum exchange has been largely neglected due to determinations of its lack of impact on conventional electrospray. A more thorough study is necessary to determine whether this neglect is valid for nanoelectrospray (nES – electrospray at flow rates of ~nL/s), particularly with respect to the role that such gas motion would play in advection of evaporating vapor. Preliminary work on nES cooling indicates that the induced gas flow in such sprays has a significant and beneficial effect on the heat transfer performance. The proposed work builds on this finding to characterize and understand the behavior and morphology of liquid films formed by nES, the motion and topology of airflow resulting from nES, and the performance of nES-based evaporative cooling schemes, with the larger objective to fill knowledge gaps in the understanding of overall nES behavior.

SUBJECT:	Ph.D. Proposal Presentation

BY:	Joel Chapman

TIME:	Monday, April 22, 2019, 1:00 p.m.

PLACE:	Love Building, 210

TITLE:	Heat and Mass Transfer Enhancement by Entrained Gas Jets in Electrospray Aerosol Plumes

COMMITTEE:	Dr. Andrei Fedorov, Co-Chair (ME) Dr. Samuel Graham, Co-Chair (ME) Dr. Peter Kottke (ME) Dr. S. Mostafa Ghiaasiaan (ME) Dr. Muhannad Bakir (ECE) Jeffrey Didion (NASA)

SUMMARY Electrospray (ES) is a phenomenon which has been studied for over 100 years. The present work considers the utilization of an evaporation cooling system where the liquid is delivered to the heated surface by means of ES. Electrospray is a potentially effective means for evaporation cooling since it has been shown to produce aerosol spray plumes consisting of highly disperse sub-picoliter droplets, and is also a highly energy-efficient means of liquid delivery. Based on these benefits, the present work seeks to investigate ES-driven evaporative cooling behavior. Cooling performance is determined by relevant heat transfer resistances within the system. For an ES-based evaporation cooling scheme, those can be broadly placed into two categories; the thermal resistance across a liquid film lying on the heated surface and the interrelated convection and mass transfer resistances in the gas phase. There appear to be no studies which look at the formation of a continuous volatile liquid film lying on a heated surface which is experiencing simultaneous evaporation and commensurate replenishment by ES. This leaves a gap in the understanding of the possible film morphologies, behaviors, and potential heat transfer performance. Importantly, the convection and mass transfer resistances in the gas at the evaporating liquid-vapor interface are a strong function of the gas velocity, an aspect of ES that has not been well studied. Despite the potential for ES droplets to impart momentum to the surrounding gas, the velocity field resulting from this momentum exchange has been largely neglected due to determinations of its lack of impact on conventional electrospray. A more thorough study is necessary to determine whether this neglect is valid for nanoelectrospray (nES – electrospray at flow rates of ~nL/s), particularly with respect to the role that such gas motion would play in advection of evaporating vapor. Preliminary work on nES cooling indicates that the induced gas flow in such sprays has a significant and beneficial effect on the heat transfer performance. The proposed work builds on this finding to characterize and understand the behavior and morphology of liquid films formed by nES, the motion and topology of airflow resulting from nES, and the performance of nES-based evaporative cooling schemes, with the larger objective to fill knowledge gaps in the understanding of overall nES behavior.