Unsteady simulation paradigms for trailing edge ejection

Tucker

Unsteady numerical study of wet steam flow in a low pressure steam turbine

In steam power plants condensation already starts in the flow path of the low pressure part of the steam turbine, which leads to a complex three-dimensional two-phase flow. Wetness losses are caused due to thermodynamic and mechanical relaxation processes during condensation and droplet transport. The present investigation focuses on the unsteady effects due to rotor-stator interaction on the droplet formation process. Results of unsteady three dimensional flow simulations of a two-stage steam turbine are presented, whereby this is the first time that non-equilibrium condensation is considered in such simulations. The numerical approach is based on RANS equations, which are extended by a wet steam specific nucleation and droplet growth model. Despite the use of a high performance cluster the unsteady simulation has a considerably high simulation time of approximately 60 days by use of 48 CPUs. © Springer-Verlag Berlin Heidelberg 2012.

Comment and reply on the 'assessment of combustion and submodels for turbulent nonpremixed hydrocarbon flames'

In deriving the flamelet model for nonpremixed combustion certain terms, but not the unsteady term, are assumed to be negligible. This results in a relation between all reacting scalars and the mixture fraction as independent variable. An ideal test of the flamelet assumption can be based on direct numerical simulation (DNS) data, if all reacting scalars are conditioned on mixture fraction and conditional moments are evaluated. The fundamental assumption of the flamelet model are unwillingly justified. The unsteady and steady formulations of the same equations are compared and found that unsteadiness is important in an unsteady simulation.

Secondary flows and loss caused by blade row interaction in a turbine stage

A study of the three-dimensional stator-rotor interaction in a turbine stage is presented. Experimental data reveal vortices downstream of the rotor which are stationary in the absolute frame - indicating that they are caused by the stator exit flowfield. Evidence of the rotor hub passage vortices is seen, but additional vortical structures away from the endwalls, which would not be present if the rotor were tested in isolation, are also identified. An unsteady computation of the rotor row is performed using the measured stator exit flowfield as the inlet boundary condition. The strength and location of the vortices at rotor exit are predicted. A formation mechanism is proposed whereby stator wake fluid with steep spanwise gradients of absolute total pressure is responsible for all but one of the rotor exit vortices. This mechanism is then verified computationally using a passive-scalar tracking technique. The predicted loss generation through the rotor row is then presented and a comparison made with a steady calculation where the inlet flow has been mixed out to pitchwise uniformity. The loss produced in the steady simulation, even allowing for the mixing loss at inlet, is 10% less than that produced in the unsteady simulation. This difference highlights the importance of the time-accurate calculation as a tool of the turbomachine designer.

The time-resolved flow within an unsteady ejector

The flow field within an unsteady ejector has been investigated using experimental and computational techniques. The experimental results show a peak thrust augmentation of 1.4; numerical simulation gives a value of 1.37. It is shown that the vortex ring dominates the flow field. At optimal thrust augmentation the vortex ring acts like a fluid piston accelerating the fluid inside the ejector. A model is proposed for the operation of unsteady ejectors, based on the vortex ring acting like a fluid piston. Control volume analysis is presented showing that mass entrainment is responsible for thrust augmentation. It is proposed that the spacing of successive vortex rings determines the mass entrainment and therefore thrust augmentation. The efficiency of unsteady ejectors was found to vary between 28% and 32% depending on the L/D ratio of the unsteady jet source. Copyright © 2008 by J H Heffer.

Airfoil flow and noise computation using monotonically integrated large eddy simulation and acoustic analogy: Effect of the grid resolution.

A new scalable Monotonically Integrated Large Eddy Simulation (MILES) method based on the Compact Accurately Boundary-Adjusting high-REsolution Technique (CABARET) has been applied for the simulation of unsteady flow around NACA0012 airfoil at Re = 400,000 and M = 0.058. The flow solution is coupled with the Ffowcs Williams-Hawkings formulation for far-field noise prediction. The computational modeling results are presented for several computational grid resolutions: 8, 16, and 32 million grid cells and compared with the experimental data available.

A flame surface density approach to large-eddy simulation of premixed turbulent combustion

The flame surface density approach to the modeling of premixed turbulent combustion is well established in the context of Reynolds-averaged simulations. For the future, it is necessary to consider large-eddy simulation (LES), which is likely to offer major advantages in terms of physical accuracy, particularly for unsteady combustion problems. LES relies on spatial filtering for the removal of unresolved phenomena whose characteristic length scales are smaller than the computational grid scale. Thus, there is a need for soundly based physical modeling at the subgrid scales. The aim of this paper is to explore the usefulness of the flame surface density concept as a basis for LES modeling of premixed turbulent combustion. A transport equation for the filtered flame surface density is presented, and models are proposed for unclosed terms. Comparison with Reynolds-averaged modeling is shown to reveal some interesting similarities and differences. These were exploited together with known physics and statistical results from experiment and from direct numerical stimulation in order to gain insight and refine the modeling. The model has been implemented in a combustion LES code together with standard models for scalar and momentum transport. Computational results were obtained for a simple three-dimensional flame propagation test problem, and the relative importance of contributing terms in the modeled equation for flame surface density was assessed. Straining and curvature are shown to have a major influence at both the resolved and subgrid levels.