DIRECT NUMERICAL SIMULATION OF A TRANSITIONAL JET IN CROSSFLOW WITH MIXING AND CHEMICAL REACTIONS

The paper describes DNS of a jet in crossflow at Re=650 and jet-to-crossflow velocity ratio R=3.3. Laminar boundary conditions are employed which makes the present flow configuration advantageous for benchmarking. Use is made of local grid refinement in the region of transition to turbulence. Comparison with Re=325 is carried out for some appropriate quantities. Nine passive scalars, reacting and non-reacting, are introduced. This allows to systematically study the influence of the Schmidt number and the Damköhler number with first order reactions. The presence of chemical reactions is a particular feature of the present study. Quantitative results for mean flow and turbulent quantities are provided and discussed. Also, the interaction between turbulence and scalar is quantified. Reference data are made available for later benchmarking. INTRODUCTION The configuration of a jet issuing from a pipe into a crossflow (JICF) appears frequently in chemical, pharmaceutical, environmental and combustion engineering, to name but a few application areas. The complex vortical structures of this flow and its good mixing capabilities make it a target of intense investigation for both experimental and numerical groups (Margason 1993). It is hence useful to consider such a situation as a benchmark configuration to assess statistical turbulence models as well as Large Eddy simulations (LES). Earlier work of the present authors was concerned with turbulent inflow conditions of the jet (Fröhlich et al. 2004, Denev et al. 2005a, 2005b). In the present paper, the lowReynolds number regime is considered with laminar pipe flow, where steady boundary conditions can be applied. This is advantageous for two reasons: First, it removes the ambiguity of specifying in a benchmarking simulation the turbulent fluctuations at the inlet. Technical details in this respect may render comparisons of results from different groups difficult, as well as tracing deficiencies in a single computation back to a particular feature of the numerical method. Second, the transition of the jet from laminar to turbulent, now occurring inside the computational domain, provides a challenge for turbulence models (Denev et al. 2006). Kelso et al. (1996) and Lim et al. (2001) performed experiments in the low-Reynolds number regime. They are however mainly concerned with flow visualization and analysis of coherent structures. Recent Direct Numerical Simulations (DNS) are reported by Muppidi and Mahesh (2005a, 2005b, 2006) for a case with velocity ratio R=u∞/wjet=5.7 and Reynolds number Rejet=wjetD/ν=5000, with “∞” indicating crossflow conditions and D the diameter of the pipe. This value of Rejet requires turbulent boundary conditions at some point upstream of the outlet of the jet. In the third of these references, a passive nonreactive scalar with Schmidt number Sc=1.49 was introduced with the jet. The present work aims to supply a reference solution in a similar spirit which differs from the above simulations mainly in the following: (1) The Reynolds number is lower, such that the pipe flow is laminar. (2) Reacting and nonreacting passive scalars are introduced to provide reference data for mixing and chemical reactions. This feature is unique as there is no similar attempt in the literature the authors are aware of. The parameters have been selected such that the transition zone is short which is advantageous when used as benchmark to reduce the computational effort.