Effect of surface roughness on non-equilibrium condensation in a Laval nozzle

The present study focuses on the effects of different surface roughness of the walls of a Laval nozzle on the non-equilibrium condensation of steam. The study describes the result of a numerical investigation of wet steam flow in a low-pressure convergent-divergent nozzle using commercial computational fluid dynamics package ANSYS Fluent 16. A 2D computational domain is considered and is discretized in a structured mesh with finer grid near the nozzle walls to capture the effects of roughness in the supersonic flow of steam. The mathematical model describing the phase change, which involves the formation of liquid droplets in a homogeneous non-equilibrium condensation process, is based on the classical nucleation theory. The Eulerian-Eulerian approach for modeling the wet steam has been adopted. The SST k-ω model has been used for the accurate formulation of the flow physics in the near wall region. The computational results for the case with no surface roughness were validated with the experimental results available in the literature provided by Moses and Stein were found to be in very good agreement. The pressure distribution, nucleation rate, average droplet radius and mass flow rate were compared for different values of surface roughness of nozzle walls. It is found out from the simulations that the parameters studied have a dependence on the surface roughness of the nozzle walls. There is a shift in the point of the incipience of the droplets and also in the nucleation rate. A reduction in the average droplet radius and the nucleation rate with increase in surface roughness along with a reduction in the condensation shock strength of rough nozzles when compared with the nozzle with no surface roughness is observed.