Time and Space Resolved Heat Transfer - Boiling and Droplet Cooling Studies Using Microheaters. Droplet and Spray Cooling Heat Transfer
Final rept. 1 Jun 1998-1 Apr 2003
MARYLAND UNIV COLLEGE PARK DEPT OF MECHANICAL ENGINEERING
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Heat transfer by phase change has long been an attractive method of cooling since large amounts of heat can be removed with relatively small temperature differences. The current study focused on making detailed measurements of the heat flux and interfacial motion of isolated droplets with and without gas as they impacted an isothermal surface at low and moderate superheats. The heat flux measurements were made using a novel heater array which allowed the local, instantaneous heat flux to be resolved. The results without gas indicate that the vaporization process can be divided into two parts a first part that is characterized by transient effective heat transfer coefficient, and a second part in which the heat transfer coefficient is constant. The results with gas indicate that a gas bubble can nucleate, grow, and merge within the liquid, resulting in an increase in the droplet diameter. The measurements indicate that significantly smaller droplet evaporation times can be achieved as a result of the increase in liquid-vapor surface area during bubble formation. Recent work on the effects of dissolved gas and subcooling on spray cooling is also discussed.
- Physical Chemistry