Computer simulation of heat and mass transfer processes during water vapor condensation from natural gas combustion products on smooth cylindrical tubes

Abstract

The results of numerical simulation of heat and mass transfer processes during the condensation of water vapor from natural gas combustion products on bundles of smooth horizontal cylindrical tubes are presented. An empirical mathematical model of condensation of water vapor from a gas-steam mixture with a high content of non-condensable gases has been developed based on experimental data. The proposed mathematical model includes jointly solvable equations of thermal energy, momentum and mass conservation, while the equation of conservation of mass takes into account the species transport due to convection, molecular and turbulent diffusion. The phase change is taken into account in the source terms of the mass conservation equation; both condensation in the volume as the mixture passes through the dew point and local surface condensation on the cooling tubes are taken into account. To describe condensation in the volume, the return to saturation temperature model is used, and for surface condensation an algebraic empirical model was developed based on the analysis of experimental data. The advantage of the chosen approach is that there is no need to calculate the hydrodynamics of droplets and condensate films as a separate continuous one due to the influence of these factors on heat and mass transfer in the experimental coefficients, which significantly reduces the computational complexity of the problem and allows engineering calculations to be carried out in a coupled formulation. The structure of the developed mathematical model ensures easy integration with common commercial and freely available CFD codes. Based on experimental data, the coefficient of the developed condensation model was determined. It is shown that when adjusting the coefficient using one base point, the model ensures agreement with experimental data for other modes with a deviation not exceeding the experimental error. Using a verified model, a section of a condensation heat exchanger for gas turbine unit exhaust gases with a staggered bundle of smooth pipes in a coupled formulation was simulated, and the numerical value of increasing cooling water heat perception due to the utilization of latent heat of condensation was determined. The obtained modeling data and the developed model of condensation of water vapor from natural gas combustion products can be used in the calculations and design of condensing heat exchangers as well as condensing boilers.

Keywords

• condensation of water vapor
• non-condensable gases
• drip condensation
• condensation heat exchangers
• exhaust heat recovery
• heat and mass transfer processes
• computational fluid dynamics