The paper presents a numerical simulation of the effects of fouling on a representative section of the vertical boiler tubes in a pulverised coal boiler for a thermoelectric power plant. The fouling, which in the real case is produced by agglomeration of sticky, low-melting ash deposits, unburned particulate and incombustibles on the gas side of the tube wall, is here simulated by a uniform coating of solid homogeneous material with average thermal properties derived by experimental data. Both heat transfer modes (convection and radiation) are simulated in the numerical calculation, that has been performed via a commercial finite-element code, FIDAP of FDI Inc.
After a two-dimensional numerical test, necessary to calibrate the simulation parameters and to determine a suitable mesh, a fully three dimensional simulation was carried out. Full account has been taken of the properties of the real fluid (the hot gas is assumed to be viscous, Newtonian and homogeneous) and of the real characteristics of its flow (fully turbulent and subject to strong radiation effects). The heat transfer in the coating and in the tube wall has been calculated without introducing heat exchange coefficients, but imposing instead a well-established heat transfer correlation for the boiling water side. Turbulent effects have been simulated using a modified k-ε model. Periodic flow in the vertical direction and forced velocity on the core side have been imposed as boundary conditions.
The results of the numerical computation are compared with those of an extensive experimental campaign conducted in 1992 on an industrial boiler for a pulverised coal, 240 MW power plant: the steam conditions at design point are 17 MPa and 540°/540°C. It is shown that the numerical results agree very well with the experimental data for the selected test section: it is therefore proposed to extend the generality of the numerical simulation to implement and complete the results of future tests on the effects of fouling.