Spray cooling is among the most promising techniques for lowering temperatures of high heat flux electronics. In vapor assisted cooling, the droplet spray becomes more effective when smaller sized water droplets are formed and propelled by a vapor phase in the direction of the target surface. For the ideal spray evaporative cooling regime, the droplets impact, spreads into a very thin film and then vaporizes. If a carrier vapor is present, the vapor phase might affect the droplet dynamics by accelerating the droplet and by imposing hydrodynamic forces on the free surface during spreading and receding. The physics behind this performance increase is yet to be fully understood and has undergone little examination. This has triggered numerous studies on the impact of a water droplet propelled by an air stream onto a smooth, dry and unheated surface.
Research conducted by Dr. Mahsa Ebrahim and Professor Alfonso Ortega from the Laboratory for Advanced Thermal and Fluid Systems at Villanova University in the United States sought to experimentally examine the impact of a water droplet driven by an air stream onto a smooth, dry and unheated surface. Their main aim was to extend the range of the droplet impact Weber number and identify regimes of impact, spreading and receding. Their work is now published in Experimental Thermal and Fluid Science.
A water droplet transported within a pipe flow was expelled from the pipe exit by an impinging gas jet towards a smooth, dry unheated silicon surface. An impinging gas flow was availed at the point of impact of the droplet which imposes shear and pressure forces on the droplet free surface during impact spreading and receding in addition to its basic functions. The researchers then examined the effects of impact velocity on the droplet dynamics using a varying range of impact Weber numbers (900 < We < 6000) with water droplets on silicon at room temperature. Impact dynamics on surfaces of changing roughness were also examined.
The two researchers observed that the higher impact Weber numbers led to a bigger diameter and thinner film thickness. This helped understand that the gas stream had minimal influence on the spreading phase. The propellant gas was observed to decrease the droplet receding phase, delay the start of droplet splashing and increase the upper Weber number limit for the droplet spreading regime.
Droplet impact velocities can be greatly increased by a gas propellant. The increasing droplet impact velocities increase both the extent and rate of droplet spreading which scales with impact Weber number. When gas propulsion is used, extremely high Weber numbers are achieved without noticeable droplet disintegration. Therefore, an analytic model that employs the energy balance approach has been developed to predict dynamic and instantaneous droplet diameter. The model is in agreement with the experimental observations within a 5% rate in the spreading phase, but poorer agreement is observed in the receding phase due to the assumption of constant receding contact angle.
Time elapsed images of droplet impingement at We = 1000
Mahsa Ebrahim, Alfonso Ortega. Identification of the impact regimes of a liquid droplet propelled by a gas stream impinging onto a dry surface at moderate to high Weber number. Experimental Thermal and Fluid Science, volume 80 (2017) pages 168–180
Laboratory for Advanced Thermal and Fluid Systems, Villanova University, Villanova, PA 19085, USA.Go To Experimental Thermal and Fluid Science