ON THE IMPORTANCE OF CHEMISTRY / TURBULENCE INTERACTIONS IN SPRAY COMPUTATIONS

Abstract

This article presents results from spray computations performed as a part of the National Combustion Code (NCC) development activity. The NCC is being developed with the aim of advancing the prediction tools used in the design of advanced technology combustors based on multidimensional computational methods. The solution procedure combines the novelty of the application of the scalar Monte Carlo probability density function (PDF) method to the modeling of turbulent spray flames with the ability to perform the computations on unstructured grids with parallel computing. A validation summary is provided for two different cases: one is a reacting spray without swirl, and the other is a similar spray but non-reacting. The comparisons involving both gas-phase and droplet velocities, droplet size distributions, and gas-phase temperatures show reasonable agreement with the available experimental data. The comparisons involve both the results obtained from the use of the Monte Carlo PDF method as well as those obtained from the conventional computational fluid dynamics (CFD) solution. The detailed comparisons clearly highlight the importance of chemistry / turbulence interactions in the modeling of reacting sprays. The results from the PDF and non-PDF methods were found to be markedly different, and the PDF solution is closer to the reported experimental data. The PDF computations predict that most of the combustion occurs in a predominantly diffusion-flame environment and the rest in a predominantly premixed-flame environment. However, the non-PDF solution predicts incorrectly that the combustion occurs in a predominantly vaporization-controlled regime. The Monte Carlo temperature distribution shows that the functional form of the PDF for the temperature fluctuations varies substantially from point to point. The results bring to the fore some of the deficiencies associated with the use of assumed-shape PDF methods in spray computations. Finally, we end the article by demonstrating the computational viability of the present solution procedure for its use in 3-D combustor calculations by summarizing the results of parallel performance for a 3-D test case with periodic boundary conditions. For the 3-D case, the parallel performance of all the three solvers (CFD, PDF, and spray) has been found to be good when the computations were performed on a 24-processor SGI Origin workstation.