Simulation of acoustic enhancement of heat transfer in a droplet heat exchanger

Enhancement of heat transfer from a droplet exposed to acoustic fields is investigated. Investigation is part of a research project in enhancing the heat transfer in direct contact heat exchangers. Adding high intensity sound to Droplet Heat Exchanger (DHX) design produces relative gas motion around droplets otherwise entrained in the main flow field. Particles do not get fully entrained in the high frequency acoustic field giving rise to relative velocity. This enhances the heat transfer from droplets. Further benefits could be obtained by acoustic agglomeration of small droplets. DHXs have high contact area, no interface losses, low pressure drop and superior heat transfer characteristics compared to standard heat exchangers. With further enhancement of heat transfer by high intensity acoustic field application makes DHXs very attractive in many industrial applications such as droplet/particle reactors, humidifiers, gas scrubbers as well as ground based power generating gas turbines. In this paper, results of simulations of a single droplet exposed to acoustic fields of a range of sound intensity level (SPL) and frequency are presented. Spherical droplets are exposed to high intensity acoustic fields up to 175 dB with frequencies 25-2000Hz. Droplet size considered here is 100μm. Three dimensional (3-D) simulation of an oscillating flow field around a spherical droplet are carried out using FLUENT code. First, simulation results of space-averaged Nusselt numbers for steady flow around a single droplet are compared with available experimental data. Results were within 1-5% of each other. Simulations with acoustic field with and without steady velocity component were carried out and the results were compared with previous two dimensional studies as well as experimental and correlations of the same phenomena. The current simulation results are on average 22% higher than the 2D simulation results indicating the 3-D nature of the flow. Space and time-averaged Nusselt numbers were more than 400% higher than the ones obtained without the acoustic field for acoustic Reynolds number 100 and frequency 50Hz and 30% higher than 2D simulation results. Finally, entrainment of droplets in the oscillating flow field was also considered. The result showed insignificant reduction (< 1%) in heat transfer rate compared to the case with no entrainment at all ranges of frequency (50 - 2000Hz).