A Comparison of the Linear and Nonlinear k–ε Turbulence Models in Combustors

In swirling turbulent flows, the structure of turbulence is nonhomogeneous and anisotropic and it has been observed that the assumptions leading to the formulation of the k-ε model, which is used very often in many engineering applications, are inadequate for highly swirling flows. Furthermore, even with the various modifications made to the k-ε model, it is still not capable of describing secondary flows in noncircular ducts and it cannot predict non-zero normal-Reynolds-stress differences. Recently Speziale (1987) has developed a nonlinar k-ε model, which extends the range of validity of the standard k-ε model while maintaining most of the interesting features of the k-ε model; for example, the ease of application in existing Computational Fluid Dynamics (CFD) codes. In this work, we will use the nonlinear k-ε closure to model the turbulence in combustors. The particular combustor geometries selected for this study are (i) the flow in a round pipe entering an expansion into another coaxial round pipe, and (ii) the flow in two confined co-axial swirling jets. The results show that there are no significant differences in the performance of the two models. It is speculated that the inlet conditions for k and ε may play as crucial a role in achieving predicted accuracy as turbulence modeling details. Also it is possible that weaknesses in the performance of the modeled equations for k and ε may have masked differences in the two models.