Mechanical and Civil Engineering Seminar

High-pressure turbulent species mixing and combustion is ubiquitous in the combustion chambers of virtually all major propulsion devices such as diesel engines, gas turbine engines, and rocket engines. Despite this importance, a fundamental understanding of these processes is only merging. High-pressure species mixing is also a critical feature of the Venus atmosphere near-ground level and addressing the intricacies of this Venus atmosphere mixing will be essential to instrument design, measurements, and data analysis in forthcoming missions. A comprehensive theory of high-pressure multi-species mixing and combustion will be presented. This theory includes the complete form of the species mass- and heat-fluxes, a real-gas equation of state for the mixture, and high-pressure transport properties of the mixture. The equations are solved using a Direct Numerical Simulation (DNS) methodology wherein all scales overwhelmingly responsible for the dissipation are resolved. For ease of interpretation of results, the equations are solved in a temporal mixing layer configuration and a database is created by varying the initial Reynolds number, the free-stream pressure and the composition of the two streams of the mixing layer. In each simulation, the computation is conducted until a time at which the flow exhibits turbulence characteristics. The results show development of regions where the magnitude of the density gradient becomes very large; this is similar to experimental observations obtained at much larger Reynolds number values than achievable in DNS. The trace species can undergo uphill diffusion which may lead to species and/or phase separation. Modeled species-specific effective Schmidt numbers exhibit values exceeding unity in many regions of the flow field, and the modeled effective Prandtl number reaches values similar to those of refrigerants (i.e., 4-5) and even liquid water (i.e., 7). Results from DNS of turbulent combustion indicate that the flame is preponderantly a diffusion type and that uphill diffusion also occurs. The transport model is sufficiently sensitive to distinguish between the effective diffusion behavior of a larger and heavier molecule (e.g., CO2) and that of a smaller and lighter molecule (e.g., H2O) for the same specified initial mass fraction condition. This result is relevant to internal combustion engines having Exhaust Gas Recirculation to minimize formation of pollutants which affect climate change.