Determination of quantitative relationships between the non-dimensional axial length of an annular pre-turbine mixing chamber and the integral characteristics of temperature field uniformity

Abstract

The object of the research is the annular mixing chamber of the pre-turbine duct of a three-stream aircraft turbofan engine, designed to enforce the mixing of hot and cold flows within a confined axial geometry, with particular emphasis on the gas-dynamic and thermal processes occurring within it. The research problem addressed in this research was the determination of the minimally required relative length of the mixing chamber. This length must ensure an acceptable level of temperature uniformity at the turbine inlet without increasing the overall dimensions of the propulsion system. This condition is particularly critical for compact engines, including propulsion systems for unmanned aerial vehicles. A series of three-dimensional CFD simulations was performed using ANSYS Fluent with a Reynolds stress model (RSM) for turbulence. The modeling was conducted for an annular chamber with fixed geometric parameters (D = 1107 mm, d = 492 mm) within a range of relative lengths L* = 0.42–2.11. The research covered four different engine operating conditions, varying in bypass ratio. The results revealed a clear nonlinear dependence of the temperature non-uniformity coefficient θ on the relative chamber length L*. It was found that for L* < 1.2, the mixing process remains incomplete, accompanied by a significant increase in temperature non-uniformity. In contrast, within the range L* = 1.2–1.7, nearly complete temperature equalization is achieved (θ ≤ 0.1). These results can be explained by the dominance of the turbulent mixing mechanism, as confirmed by low Richardson numbers (R_i << 1) and the minor influence of operating parameters compared to geometric factors. The findings can be applied in the design of compact mixing chambers for aircraft gas turbine engines, especially under strict constraints on their axial dimensions. This is particularly relevant for propulsion systems of unmanned aerial vehicles.