Optimisation of heat pipe heat exchangers with deep cooling of boiler flue gas based on entropy generation minimisation method

The paper presents the results of numerical simulations and optimisation of boiler heat exchanger parameters under conditions of deep cooling of combustion products. The specific features of heat and mass transfer calculations, particularly when combustion products are cooled below the dew point with water vapour condensation, are discussed. Experimental results are provided for a large-scale (1.5 m) heat pipe filled with a water/ammonia mixture. These results are utilised in mathematical modelling and optimisation of heat exchanger performance involving heat pipes. The heat transfer process is simulated in a two-stage heat exchanger, where different sections of heat pipes are filled with different working fluids depending on the temperature zones. The optimisation is performed using the minimum entropy production method. The optimal distribution of heat flux density and temperature is determined, considering both design and operational parameters. Numerical examples are provided to demonstrate the performance of the heat recovery system. The results of numerical modelling and optimisation of the design and operating parameters of a two-stage heat exchanger of heat pipes filled with various liquids are presented. The optimal parameters of the heat exchanger are determined using the criterion of minimum entropy production.