Drop Impact Behaviors for Cooling Applications and Experimental Developments for Optical Cavitation

Master of Science, Graduate Program in Mechanical Engineering
University of California, Riverside, June 2015
Dr. Guillermo Aguilar, Chairperson

The cooling behavior of an impacting single droplet and train of droplets on a heated substrate (T = 60oC) for various pool conditions is explored. The effects of several variables such as impact  velocity  (1-4  m/s),  droplet  diameter  (4.8  mm),  pool  depth  (0-34  mm),  and  impact frequency  (0.5-32  Hz) on  the  cooling  dynamics  are  explored.  Fast  response  resistance temperature detectors (RTD) embedded at the surface of the substrate allows for temperature measurement below the droplet impact. A high speed video camera recorded the dynamics of cavity formation and collapse upon impact with the pool surface. Droplet diameter and impact velocity were also measured using the high speed video. The instantaneous heat flux and net heat extraction at the surface were obtained using a finite-time step integration of Duhamel’s theorem.

Heat transfer appears to be maximized within an intermediate region of impact Weber number for the single droplet impacts. At this intermediate Weber number range, the impact crater almost  reached  the  pool  bottom,  suggesting  that  cold  droplet  fluid  made  contact  with  the substrate, maximizing the cooling effect.  Outside this intermediate region of Weber number, the heat flux appears to decrease. At the higher Weber number range, cold droplet fluid is pushed away from the measurement point once the cavity reaches the substrate. Below the optimal range of Weber number, the droplet does not enter the crater formed by the previous droplet, preventing it from reaching the substrate. For a train of droplets, there seems to be several  regions  where  the  heat  flux  is  further  reduced  due  to  collision  of  droplet  with emerging jet. It was also found that dry surface provides better heat flux with the heat flux decreasing with formation of thin film.