Experimental study of an ultrasonic mist generator as an evaporative cooler

Abstract

This paper presents an ultrasonic mist generator used as an evaporative precooler for condensers in air conditioning applications. Ultrasonic mist generators eliminate pressure loss at the inlet air stream to the condenser and allow controlling the characteristics of the water atomized droplets. A water mist generation unit has been designed, built and tested to assess its thermal performance and its water mist production capacity in terms of mass ow rate of atomized water and size distribution of the droplets generated. To evaluate the performance and cooling capacity of the water mist produced by the ultrasonic mist generator, a set of tests has been conducted on a test bench consisting mainly of a subsonic wind tunnel equipped with instrumentation and control devices to modify the operating conditions. A Sauter mean diameter D3;2 = 13:2 m has been determined for the size distribution of the generated droplets and the range of water mist flow rates that the system can produce is between 0:11 10􀀀3 and 0:52 10􀀀3 kg/s. It has been found that, under many operating conditions, the evaporative cooling process is not homogeneous throughout the air flow, so a novel performance indicator called "LCP (local cooling performance) has been de ned to speciffically evaluate this phenomenon. A maximum direct evaporative cooling effciency "DEC = 83:7% is obtained for a water-to-air flow ratio rw = 0:35 10􀀀3 and air flow rate 630 m3/h, and a maximum average evaporative cooling efficiency of "AEC = 65% for rw = 2:41 10􀀀3 and air flow rate 630 m3/h.