Fouling characteristics analysis and morphology prediction of heat exchangers with a particulate fouling model considering deposition and removal mechanisms

- A Fouling model considering deposition and removal mechanisms is established.
- An asymptotic behavior of fouling growth with removal mechanism is observed.
- Effects of flue gas velocities and particle diameters on fouling are discussed.
- Fouling morphology is predicted by dynamic mesh method and user-defined functions.
- Effects of fouling morphology on heat transfer performance are evaluated.

Fouling of heat exchangers in industrial waste heat recovery is a problem which needs to be solved urgently. In this paper, a numerical investigation on fly-ash deposition and removal processes in tube bundle heat exchangers was conducted for industrial waste heat recovery. This study using a comprehensive fouling model integrated with the CFD framework was carried out with a discrete phase model and a dynamic mesh model in ANSYS FLUENT. First of all, the fly-ash deposition and removal algorithms were developed to establish the judgment criterion of particulate fouling based on the energy balance model and the critical moment theory. Based on this model, then, fouling characteristics with and without removal mechanism were compared. Then fouling processes for different particle diameters and flue gas velocities were simulated. At last, fouling morphology and the effect of fouling on heat transfer performance were predicted. The results indicated that the fouling mass increased almost linearly with time without removal mechanism, but an asymptotic behavior of fouling mass was observed when the removal mechanism was considered. When the number of particles injected from the inlet was specified, smaller particles showed a higher fouling rate. Also, the deposit growth rate decreased with the increase of flue gas velocity. For the particle diameter of 5 μm and the flue gas velocity of 6 m/s, the thermal effectiveness number was reduced by about 10% after 15 min. The paper highlights the fly-ash deposition and removal model and the application of the dynamic mesh method.