The effect of ionization of spray on the wetting characteristics of an adiabatically cooled heat exchanger

[摘要] ENGLISH ABSTRACT: An investigation was made into the use of pre-cooling of air with evaporative coolingas a means of improving the performance of air-cooled heat exchangers (finned tube)under conditions of ambient temperatures above maximum design values and duringtimes of increased load. A review of previous research on this subject indicated thatthe concept is theoretically sound, but that practical application thereof is still limited.It was found that one of the major areas of concern is the wetting of the heatexchanger finned surface and subsequent corrosion.Mathematical models were derived for the behavior of liquid droplets in free airstream conditions and droplets that have penetrated a laminar hydrodynamic boundarylayer formed on a flat plate. These two models were combined to determine thebehavior of a liquid droplet for its entire lifetime. It was found that evaporation ofdroplets in a boundary layer resulted in major improvements in heat transfer. In anattempt to prevent droplets from impacting and wetting the finned tube heatexchanger, the use of electrostatically charged water spray was investigated.Experiments were performed to determine the charging performance of a capacitiveelectrostatic nozzle. It was found that this type of nozzle successfully charged dropletsin a spray. Experiments were then performed whereby electrostatic spray was sprayedon to a heat exchanger with a similar electric charge as the droplets. It was found thatdroplet deposition decreased significantly as the charge on the droplets was increased.However, total prevention of deposition could not be achieved, since the equipmentused could not produce high enough voltages. This concept shows some promise, andit is recommended that further research be performed on it. At this stage, no reliablemethod of evaporative precooling of air has yet been found. The only viable optionfor cooling capacity shortages at present is the construction of large air-cooled heatexchangers or the addition of wet cooling towers.

[发布机构] Stellenbosch University