Numerical study of fouling characteristics on two kinds of typical heat exchangers used in the waste heat recovery systems

Abstract

The problem of high exhaust temperature universally exists around the industrial boilers, which causes enormous loss of energy. Different kinds of heat exchangers are widely used in the waste heat recovery systems to recover waste energy from different heat sources. However, considering the heat exchangers mainly work in dusty environment, such as in the tail of large boiler, fly ash fouling on heat exchangers is a major problem in waste heat recovery systems, which can lead to significant efficiency deterioration. Besides, the fouling may also lead to failure of the systems with serious economic consequence. The H-type finned tube heat exchanger is a new type of heat exchanger which have been widely used in waste heat recovery systems in recent years, because of their excellent anti-wear and anti-fouling performance. In this paper, a numerical model was developed to investigate the fouling characteristics of two kinds of H-type finned tube heat exchangers used in the waste heat recovery systems. Firstly, a fouling model which contains a deposition process and a removal process is developed. The deposition process is determined by the energy balance on the particles and the removal process is determined by the the local wall shear stress and the fouling thickness on the fin surface. As the flow field directly affects the particle motion, the fluid flow characteristics of the two kinds of heat exchangers are calculated, and the discrete phase model (DPM) was employed to trace the fly ash particles in the flow field during the fouling process. Since the simulation time is much smaller in magnitude than the real time in fouling process, it is impossible to simulate the whole real fouling time period (hours) with a small time step size. In order to obtain a reliable result, it is necessary to enlarge the simulation time and results to the real time scale. In this paper, a fouling time ratio is proposed by considering the volume difference between the simulated deposit layer and the real deposited particles and the concentration difference of injected particles between the simulation and real situation. According to the fouling time ratio, we can enlarge the simulation time and results to the real time scale. Then the real-time fouling characteristics of the two kind of H-type finned tube heat exchangers are investigated and the effects of inlet velocity, particle diameter on the fouling results are also studied. The results show that, the fouling mainly occurred in the flow stagnation region and the recirculation region of the H-type finned tube. The net fouling mass increases with the operating time, showing an asymptotic characteristic of growth, and finally will reach a stable value, which corresponds to the asymptotic fouling resistance. The increase in inlet velocity and particle diameter would lead to a rapidly decrease in fouling results. The anti-fouling performance of double H-type finned tubes shows slightly better than that of single ones, but the advantages decreases with the increase in inlet velocity and particle diameter.