Session: 11-14-01 Heat Transfer in Gas Turbines

Paper Number: 69458

Start Time: Wednesday, 01:10 PM

69458 - Effects of Turbulence Modeling and Turbulent Schmidt Number on Supersonic Mixing Simulations 

Ensuring proper mixing of fuel with the incoming supersonic air stream is the most challenging problem for the development of scramjet-powered vehicles. The supersonic mixing flow field inside a scramjet combustor is associated with severe turbulence. The interactions of high-speed jets and shock waves further increase the difficulty to analyze the behavior of the flow domain. Hence, choosing the right turbulence model with proper turbulence parameters is crucial for obtaining reliable and reasonable numerical results. The focus of the present work is to guide the selection of a suitable turbulence model with an appropriate turbulent Schmidt number for this type of complex mixing simulations. Accordingly, in this study, a numerical investigation is carried out for a non-reacting supersonic combustor. Gaseous Helium is injected normally into a Mach 2.9 cross-stream from a slot of width 0.559 mm. The flow field is governed by the two-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations. Turbulence contribution is obtained from the Realizable k-ε and the SST k-ω model. Each model is incorporated separately with four different turbulent Schmidt numbers. Starting from the value of 0.50 turbulent Schmidt number is successively reduced until the supersonic mixing behavior reasonably agrees with the experimental findings. During this reduction, the values are clustered towards the lower magnitudes. The value of turbulent Schmidt number 0.50, 0.35, 0.25, and 0.20 is considered for the present analysis.

The calculation is performed by using the commercial simulation package, ANSYS FLUENT. Throughout the entire calculation, the turbulent Schmidt number was kept constant. The results reveal that quantitative measurements of the flow domain greatly depend on the choice of the turbulence model. Each model exhibits its advantages while predicting the characteristics of the flow field. Flow separation occurs earlier in the SST k-ω model than the Realizable k-ε and agrees closely with the experimentally obtained value. The bow shock is predicted steeper in the SST k-ω model while the recompression shock is estimated closer to the wall in the Realizable k-ε model. The effects of the turbulent Schmidt number are observed by analyzing the mass fraction profiles at different upstream and downstream locations of the injector. It is found that diffusion of fuel particles increases with the decrease in turbulent Schmidt number. However, this effect is felt only in the lower concentration regions of fuel. This indicates that a lower value of turbulent Schmidt number would produce higher mixing efficiency. As both of the models are able to predict the general flow features, no model can be concluded superior to the other. However, considering some critical characteristics like upstream flow separation, bow shock prediction, and mass concentration of fuel the SST k-ω model with turbulent Schmidt number 0.20 can be considered for reasonable estimations of supersonic mixing flow fields.

Presenting Author: M. Ruhul Amin Montana State University

Authors:

Md. Navid Chowdhury Bangladesh University of Engineering and Technology
Sarfaraz Aziz Bangladesh University of Engineering and Technology
Shanto Kumar Shingh Bangladesh University of Engineering and Technology
Mohammad Ali Bangladesh University of Engineering and Technology
M. Ruhul Amin Montana State University