STUDY ON PERFORMANCE OF PASSIVE AIR COOLED HEAT EXCHANGERS AS A SAFETY SYSTEM FOR NUCLEAR POWER PLANTS
The Passive Heat Removal System (PHRS) is used at Kudankulam Nuclear Power Plant KKNPP (VVER-1000) as a safety system which is an air cooled heat exchanger for the removal of residual heat released from the Reactor Core in case of beyond-standard accidents with total AC power supply loss. The system consists of natural air circulation for cooling. The system consists of four independent circuits of natural circulation, one per steam generator (SG) of the Nuclear power plant. Each circuit includes heat-exchanging modules where steam flows in the tubes from the steam generator which is cooled by air on the duct side and the condensate is returned to SG.
CFD simulations using Fluent 6.2.16 were performed to study the performance of PHRS system under varying process conditions such as different steam load, air inlet temperature and inlet velocity. Grid generation resulted in formation of 5880 cells and 6093 nodes. The 2D, laminar and turbulent simulations of the PHRS was obtained by solving the governing equations using Fluent 6.2.16. Model with density, viscosity and thermal conductivity variation of air with temperature change was used for simulations. The CFD results were validated using the theoretical correlations such as Zukauskas model for flow over tube bundle. Flow and temperature field surrounding the heat exchanger were observed using the developed CFD model. Mathematical modelling of fins were also carried out to study effectiveness of fins on tubes in such air cooled heat exchangers.
Heat transfer coefficients at different inlet air velocity and tube surface temperature were observed. The modeled system predicts value of heat transfer coefficient within average error of 8% from the theoretically calculated value which validates the model. Heat transfer coefficient varies with a lot of parameters such as air inlet velocity, properties of air, orientation of tubes, flow field, etc., thus modeling heat transfer coefficient to such an accuracy level is widely acceptable.
It was observed that heat transfer coefficient increases with increase in inlet air velocity. The increase is very rapid with increase in inlet air velocity for low values of initial velocity. At higher velocity the increase in heat transfer coefficient is not very rapid. Percentage decrease in heat removed with increase in air inlet temperature was also observed. Maximum heat transfer decrease of 12% was observed when the air inlet temperature in the heat exchanger was increased from 30°C to 50 °C. The average fin efficiency and fin effectiveness at different velocities is around 0.95 and 20.14 respectively. Thus presence of fins over the heated tube is justified.