Heat transfer intensification in emergency cooling heat exchanger of nuclear power plant using air-water mist flow

Abstract

Advanced nuclear power plants are equipped with passive systems for emergency decay heat removal from reactor equipment (PEHRSs) in case of development of accidents accompanied with primary cooling circuit leakage and for transferring heat to the final heat absorber (ambient air). Here, intensity of heat dissipation to air from the heat exchanger outer surface achieved by buoyance induced natural convection is extremely low, which necessitates the need to expand heat conductivity surfaces and to apply different types of heat transfer intensifiers (grooves, ribs and extended surfaces, positioning at higher altitudes, etc.). Intensity of heat removal is also strongly dependent on the ambient air temperature (disposable temperature head). Construction of nuclear power plants in countries with high ambient temperatures (Iran, Bangladesh, Egypt, Saudi Arabia and others) with characteristic high level of ambient air temperature imposes additional requirements on the expansion of heat exchange surfaces. Results of experimental investigation of intensification of heat exchange by low energy-intensity ultrasound supply of super-small liquid droplets (size ~3 mm) in the cooling air are provided in the present paper. In such case, transfer of heat between the cooled surface and cooling airflow involves the following three physical effects: convection, conductive heat exchange and evaporation of water droplets. The latter two effects weakly depend on the ambient air temperature and ensure active heat removal in any type of situation. Investigation was performed using high-precision calorimeter with controlled rate of heat supply (between 7800 and 12831 W/m2) imitating heated surface within the range of Reynolds numbers from 2500 to 55000 and liquid (water) flow rates from 23.39 to 111.68 kg/m2·h-1. The studies demonstrated that presence of finely dispersed water results in significant increase of heat transfer compared with the case of application of purely air-cooling. With fixed heat flow energy efficiency increases with increasing concentration of water reaching the values in excess of 600 W/m2·degree-1, which is 2.8 times higher than for the case of air-cooling. Application of the suggested technology for intensification of heat exchange in dry cooling towers of nuclear and thermal power plants used in the conditions of hot and extreme continental climate is possible subject to further investigation for the purpose of specification of optimal ranges of heat exchange intensification. © 2019 Obninsk Institute for Nuclear Power Engineering, National Research Nuclear University 'MEPhI'. All rights reserved.