Turbulence modelling is a major issue, affecting the precision of current numerical simulations, particularly for reacting flows. The RANS (Reynolds‐averaged Navier‐Stokes) modelling of turbulence is necessary in the development of advanced combustion systems in the foreseeable future. Therefore, it is important to understand advantages and limitations of these models. In this chapter, six widely used RANS turbulence models are discussed and validated against a comprehensive experimental database from a model combustor. The results indicate that all six models can catch the flow features; however, various degrees of agreement with the experimental data are found. The Reynolds stress model (RSM) gives the best performance, and the Rk‐ε model can provide similar predictions as those from the RSM. The Reynolds analogy used in almost all turbulent reacting flow simulations is also assessed in this chapter and validated against the experimental data. It is found that the turbulent Prandtl/Schmidt number has a significant effect on the temperature field in the combustor. In contrast, its effect on the velocity field is insignificant in the range considered (0.2–0.85). For the present configuration and operating conditions, the optimal turbulent Prandtl/Schmidt number is 0.5, lower than the traditionally used value of 0.6–0.85.
Part of the book: Turbulence Modelling Approaches
The beauty of numerical simulations is its ability to reveal the physics or nature of practical engineering problems in detail, and then, to identify adequate solutions. In this chapter, an excellent example is demonstrated. The rupture of a heavy-duty, high-pressure natural gas heat exchanger is numerically investigated, and the importance of gravity effect is identified, which is often considered as a trivial factor. For the original design, the natural convection in the flow field of the heat exchanger is comparable with the forced convection at the designed operating conditions. These two convections are perpendicular and compete with each other, the flow field is highly unsteady, and high-temperature natural gas is trapped in the upper portion of the vessel, which causes the damage of the exchanger. By vertically mounting the exchanger assembly and locating the outlet pipe on top of the exchanger, the flow parameters become rather uniform at each vertical cross section and the wall temperature of the heat exchanger remains more or less the same as the heated natural gas. The proposed design has been successfully used up to now.
Part of the book: Numerical Simulations in Engineering and Science
The accurate evaluation of the aero-thermodynamic working environments of gas turbine critical components is essential in the development of advanced gas turbine engines, such as cooling flow arrangement and service life assessment. In this chapter, as a technology demonstration, the conjugate heat transfers of the internally air-cooled nozzle guide vane (NGV) and shrouds of a gas turbine engine at flight conditions are numerically examined. The simulations are performed with a high-fidelity CFD model and adequately defined boundary conditions. The effect of the non-dimensional distance from wall surfaces, y+, on the wall temperature is studied. The characteristics of the complex three-dimensional flow and temperature fields are revealed, and the heat fluxes between the hot gas, NGV body, and cooling airflow at selected cross sections are presented and discussed. It is clear that the traditional one-dimensional semiempirical approach is no longer suitable. Generally, the Mach number is higher, and the temperature is lower on the NGV suction side than on the pressure side. It is found that two high-temperature zones occur on the NGV pressure side and the temperature over the middle section is relatively low. These findings are related to where the cooling holes and outlets are located and consistent with the field observation of NGV damages.
Part of the book: Heat Transfer