CFD analysis of Gypsum crystallization fouling in 2D plate heat exchangers

Abstract

The deposition of material present in the flowing fluids onto the surfaces of various internal parts of heat exchangers is the main reason for thermal efficiency reduction. This phenomenon of deposition is known as fouling. The fouling phenomenon could be understood prominently by experimental and numerical investigations. In the current work, computational fluid dynamics (CFD) has been used to develop numerical models to simulate the crystallization fouling, and a correlation of an optimum operating conditions for minimum fouling effect has been achieved. The development of numerical models required many stages, and these stages involved a critical evaluation of all the processes in crystallization fouling. In the first stage, the numerical solutions for the crystallization flux are validated with corresponding experimental fouling resistance distributions. The validation provides an insight into factors determining the likelihood of the fouling mechanisms. Then, the impact of deposit growth on fouling is determined by calculating the fouling resistance and monitoring its temporal variation. Afterward, the numerical values of the fouling rate are calculated from the resulting fouling curves. The simulation results show that fouling includes both time and space dependency. Also, the fouling thickness increases up to 1.4 mm in approximately 200hrs time passage for a given heat flux of 54 kW/m2 and concentration of 2.42 g/l at different inlet velocities. Similarly, the fouling resistance increases up to 0.003 K.m2 /W under the same operating conditions. Almost the same trends have been obtained at the same inlet velocity of 0.2 m/s and different heat transfer rates. In the context of space dependencies of fouling phenomenon, the mass deposition rate of fouling increases from 0.005gm/m2 s to 0.035 gm/m2 s along the length of the plate from 380 mm to 600 mm at a given heat flux of 54 kW/m2 and concentration of 2.42 g/l at different inlet velocity. And, hence the correlation of an optimum operating condition of minimum fouling effect was achieved for a two dimensional (2D) plate heat exchanger. The results obtained from the CFD simulation indicate that multiple factors affect the fouling phenomenon including fouling thickness, fouling resistance, heat transfer coefficient, velocity, heat flux input, and so on at the local level. Further, the fouling phenomenon of heat exchangers has been accessed with the help of various related innovative ideas of modeling in CFD. The research objectives of predicting the fouling behavior using CFD simulation have been achieved, and various exciting results have been found.