Experimental and numerical analysis of water condensation in a condensing economiser for heat recovery

Abstract

Condensing heat exchangers play a key role in industrial processes to enable high efficiency waste heat recovery. Various designs exist and they depend on the primary heat source, pollution level, installation location, etc. The physics involved in these components is very complex and usually difficult to investigate experimentally. Therefore, numerical methods, such as CFD (Computational Fluid Dynamics), prove to be a useful tool for investigating specific phenomena. In particular, the condensation phenomenon is probably the most complex since it implies the co-existence of different phases, their mutual interaction and the variations in concentration of these phases. Focusing on these phenomena, a simplified case study was conducted by considering an infinite pipe geometry and investigated by means of the STAR-CCM + software to develop a novel methodology for the detailed external condensation pro. The geometry considered represents the first tube section of an existing heat exchanger, and the condensation of hot humified air impinging on the cold pipe was analysed using a multiphase multicomponent approach based on VOF (Volume of Fluid Method). A specific optimum mesh was tested with two different flow regimes for the fluid film defined on the condensation surface. Since no condensation regime is known in advance, the Resolved Fluid Film model was used to trigger the condensation on the pipe wall, starting by means of the Fluid Film model, in order to predict the amount of condensate phase and its diffusion into the background region. After that, the VOF condensation model was used to trigger the condensation between the vapor phase and the newly formed water liquid phase. The condensation regime is then controlled by means of three main parameters being the two condensation models (film and VOF) under relaxation factors and the transition threshold, generally raging from 0 to 1 and here fixed to an optimal value. Finally, the total amount of condensate phase was compared with extrapolated values from experimental results. The simulation proved to be a reliable simplified prediction of the average condensation production related to the actual experimental setup, with the spatial distribution showing a net separation between the film and the VOF regimes.