Large eddy simulation of unsteady turbulent flow in a semi-industrial size spray dryer

Abstract

This paper is concerned with the numerical simulation of gas flow inside spray dryers, which is known to be highly transient. Experimental evidence presented in the literature to date is acquired either at a scale much smaller than industrial or at a limited number of locations in the system (e.g. the central axis). Alternatively, computational fluid dynamics (CFD) simulation, in particular methods based on the Reynolds-averaged Navier–Stokes (RANS) equations, may provide the information of the complete flow field at industrial scales. However, RANS methods to some extent are hampered by the limitations of turbulence modelling. In this study, it has been investigated whether the current limitations of experiments and RANS CFD simulations can be omitted by the use of large eddy simulation (LES). The spray dryer studied was of semi-industrial size with a volume of 60 m3. Via grid size and time step refinement studies, the minimum numerical requirements for obtaining converged time averaged velocity profiles could be identified. Spectral analysis and estimates of the Kolmogorov scales demonstrated that LES resolves scales well into the inertial subrange of turbulence scales. The LES calculation shows a complex precession of the jet. The jet moves around in the drying chamber seemingly with no reoccurring modes. Even with very long sampling times, no typical frequencies could be found. This work demonstrates the feasibility of LES for a spray dryer of semi-industrial scale and the ability of LES to predict large-scale motions. Future work will include the further validation of the LES via comparison with laser Doppler anemometry measurements and, subsequently, the comparison of RANS, unsteady RANS and LES simulations.