Thermo-flow Characteristics Of Hybrid Cooling System Based On Numerical Study

Due to evaporated cooling the wet cooling system needs large supplement of fresh water,as a result,the natural draft dry cooling system has been increasingly developed and widely used especially in arid places where cooling water is not available but coal is abundant because the air-cooled technology consume less fresh water quantities.However,the natural draft dry cooling towers will suffer a significant reduction of performance during the high ambient temperature period especially in summer.As a result of that study,the hybrid cooling system,jointing effects of wet cooling technology and air-cooled technology,will lead the optimal technology for the cold-end of power plants.It is quit necessary to study the thermo-flow and fields characteristics of hybrid cooling system which is directly related to the safe operation and economic benefits of power plants.In present thesis,the computational models of air-side fluid flows and heat transfer of cooling air coupled with energy balances of circulating water and exhaust steam in condenser for a 600 MW power plant in both natural dry cooling system and natural hybrid cooling system are developed.Base on those computational models,the operating modes,parameters and ambient conditions are investigated for the hybrid cooling system comparing to the dry cooling system.For operating parameters optimization of hybrid cooling system,the variable fields of cooling air,the mass flow rate and heat rejection of each cooling section,the outlet water temperature of each cooling section and cooling system,and the corresponding turbine back pressures are obtained.The results show that the buoyancy forces will decline because of the lower both heat rejection and outlet air-flows temperature of wet cooling section while the droplet diameter is too big or the water flow rate of wet cooling section is too low.The decrease of buoyancy forces will deteriorate the thermos-flow performances of hybrid cooling system even worse than dry cooling system.In order to analyze the thermos-flow and fields characteristics of hybrid cooling system and dry cooling system under different ambient conditions,the variable fields of cooling air,the mass flow rate and heat rejection of each cooling section,the outlet water temperature of each cooling section and each cooling system,and the corresponding turbine back pressures are calculated for air dry bulb temperatures,relative humidity values,and crosswinds.The results indicated that hybrid cooling system could provide lower outlet water temperatures and lower turbine back pressures than dry cooling system even under higher air dry bulb temperatures and higher relative humidity values because of higher air flow rate of hybrid cooling system than dry cooling system thanks to the two air inlets of hybrid cooling system.In the absence of winds or at low wind speeds,hybrid cooling system perform better thermo-flow than dry cooling system while buoyancy forces still play dominant roles in the airflows of each cooling system.On the contrary,at medium wind speeds,under the effects of crosswinds the outlet airflows of wet cooling section will depress the backward air-cooled heat exchanger which cause dramatically decline of air flow rates of hybrid cooling system.When the wind speed exceeds 12m/s,since the airflows from both windward air-cooled heat exchanger and wet cooling section through the backward air-cooled heat exchanger,hybrid cooling system perform better than dry cooling system again.