Arvind Pattamatta

In the present work, pool boiling experiments are performed on copper minichannel and flat surfaces at atmospheric pressure with water as the working fluid. The pool boiling experiments are conducted for liquid subcooling of 0K (saturated), 10K, and 20K at atmospheric pressure. Minichannel-1 and Minichannel-2 have a square cross-section with side lengths 1 mm and 2 mm, respectively. The thermal performance of the boiling surface is characterized by the critical heat flux and heat transfer coefficient. The critical heat flux (CHF) is increased by 36-45 % for minichannel-2 and 15-17 % for minichannel-1 compared to a flat surface at all subcoolings. CHF can be increased as high as 287.54 W/cm2 by employing minichannel-2 and 20 K subcooling. Minichannel-1 and minichannel-2 enhanced the heat transfer coefficient as high as 25.21% and 68.59 %, respectively, compared to the flat surface. It is observed that the increase in surface area is the dominant factor in the enhancement of pool boiling heat transfer on minichannel surfaces.

This paper intends to study the effect of air turbulence on the burning characteristics of fiber suspended n-heptane droplets without and with addition of nanoparticles. Dilute concentration of Alumina nanoparticles (mass loading from 0.3% to 1%) in conjunction with the oleic acid (as surfactant) are added to the base fuel followed by ultra-sonication such that a stable suspension of the nano-fuel is obtained. Time resolved images of the burning droplets are captured using a high speed camera. In order to study the effect of turbulence alone (in absence of mean flow) on droplet burning, the experiments were conducted in a so called 'box of turbulence', where zero-mean isotropic turbulence is achieved at the central region of the box. The turbulence was characterized by the application of LDV/PIV technique. The burning of pure heptane and heptane plus surfactant droplets (without nanoparticles) are also studied in the absence/presence of air turbulence. Interestingly, for pure heptane the burning rate was found to decrease for higher turbulent intensity. In case of nano-fuel droplet burning three distinct stages of droplet burning (classical combustion following d2-law, bulging of droplet followed by explosion, surfactant flame) were observed. However, the low intensity explosions are primarily attributed to the difference in the boiling point of the surfactant and base fuel. The addition of nanoparticles promotes such explosions and increases the burning rate in comparison to that for pure fuel droplet.