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The numerical simulation of the combustion process is a common practice during the combustor design. The accurate simulation of lean flames is a problem of great technical relevance. At these conditions, the flames can be susceptible to strong instabilities as well as local quenching phenomena, which can also alter significantly the flame shape. In this framework, Scale Resolving Simulations (SRS) can provide accurate predictions of the main fluid-dynamic quantities as well as the pollutant concentrations in the turbulent flow-field. In this work, the TECFLAM swirl burner, a premixed unconfined methane-air flame, is simulated with Large Eddy Simulation (LES) using ANSYS Fluent. The turbulent combustion is modelled using the Flamelet Generated Manifold (FGM) formulation both with an Extended Turbulent Flame Closure (ETFC) model to introduce the stretch and heat loss effects on the flame and with the standard Zimont’s Turbulent Flame Closure. A generalized implementation of the ETFC model using a look-up table for the laminar flame speed enables the use of the model for arbitrary conditions. The detailed experimental data allowed a detailed validation of the numerical results, showing that the ETFC model improves significantly the flame shape prediction with respect to the Zimont model. [ABSTRACT FROM AUTHOR]

Turbine entry temperature has seen a steady increase over the years as it is directly linked to the overall efficiency of a gas turbine engine, thus determining fuel consumption and ultimately CO2 emissions. An accurate estimation of the heat loads is of paramount importance to design the cooling system and determine the life-span of the component, therefore it is mandatory to assess the capability of CFD to predict the involved phenomena. Surface roughness plays a key role in determining the heat transfer and becomes relevant especially in case of in-service degradation and additive-manufactured components. It is seen that for a turbine blade in service for about a thousand hours, the surface roughness has similar effects as turbulence on the heat transfer. This paper is focused on the study of the influence of roughness, turbulence and Reynolds number on the aerodynamics and heat transfer by using commercial code ANSYS Fluent. Transition was accounted exploiting the Transition-SST model, whereas roughness was modeled with an equivalent sand grain approach. Numerical results have been compared against experimental data from the literature. [ABSTRACT FROM AUTHOR]

1-D codes are still much utilized regarding the preliminary design and the design phase of a single component. In particular, combustors is one of the most critical component in gas turbine engine and its design mainly affects all other components, starting from the turbine. During the initial phases of the design process, parameters are known approximately; during this phase, critical for the definition of a starting set of design parameters, there is little point in doing high fidelity Computational Fluid Dynamics (CFD) analyses. On the contrary, the exploration of the whole space is extremely important to better understand the behaviour of the system and to focus on the design objectives. Uncertainty quantification (UQ), mainly developed in recent years and applied in many fields, can be really helpful in the preliminary design phase and also as a support during the whole design process. This work comes with the idea to estimate the main source of geometrical uncertainties in the design phase of a combustor. The test case is based on a full annular lean burn combustor, tested at Central Institute of Aviation Motors (CIAM) during the LEMCOTEC (Low EMissions COre-engine TEChnologies) European project. Among the test points investigated in the experimental campaign, the Approach condition is analysed. The inner liner is considered to analyse the metal temperature. Therm-1D, a 1-D in-house simulation code, is used to model the combustor. After having built the baseline case of the combustor, several uncertainty analyses are investigated. In particular a classical Monte Carlo approach is compared with 4 innovative polynomial-chaos approaches for each group: Gauss quadrature, total order with Latin Hypercube sampling (LHS), probabilistic collocation and stochastic collocation. The analyses proved how the last two methods give the same results with a sensible lower amount of simulation (depending on the number of input variables). An additional sensitivity analysis to the order of the polynomial is conducted and results show how, with a 3rd order polynomial approximation, results can be very similar to those ones of the Monte Carlo simulation. Lastly, results are compared with experimental data to achieve a better understanding of the most relevant input parameters and the propagation of their uncertainty on the results. [ABSTRACT FROM AUTHOR]

The constant growth of renewable energy sources has changed the working condition of the modern power plants, where a greater flexibility of the conventional units is required. Large steam turbines, for instance, have to be capable of partial load operation as well as quick starts in order to ensure a stable functioning of the power grid. This reflects on the design of the steam turbine and in particular on exhaust hoods which have a significant impact on the overall turbine performance, recovering the energy left and therefore affecting the power output. Exhaust hoods have to convert kinetic energy at the turbine last stage exit into static pressure at the condenser by decreasing the flow speed. In axial-radial hoods the flow must turn by 90 degrees in a very short distance, therefore, if such components are not well designed, this may lead to flow separation phenomena which reduce the hood recovery performance. Due to the nowadays operational requirements, it is mandatory to carefully design the steam turbine exhaust system for a wide range of operating conditions, in order to ensure high efficiencies also at partial loads. This work concerns a numerical investigation on a low-pressure steam turbine exhaust hood through computational fluid dynamic simulations, assessing several numerical methods to couple the exhaust casing with the turbine rear stage. The numerical outcomes were compared in terms of flow field distribution and pressure recovery coefficient, by allowing to define a numerical setup which is a good trade-off between results accuracy and computational effort. [ABSTRACT FROM AUTHOR]

Nowadays, the growing need for advanced solutions to limit the environmental impact in thermal power generation has given rise to significant changes in steam turbines design and operability. In this context, improvement in efficiency and more flexibility in different operating conditions are the major challenges. An increased flexibility is related to rapid load changes, fast start-up and frequent shutdown that could lead to high material thermal stresses in the unit. Hence, in order to achieve the rise in efficiency and to preserve the lifetime of the turbine, a detailed knowledge of the thermal behavior of the turbine is required. This work is focused on the investigation of flow and heat transfer phenomena in the steam chambers between stator blade carriers. Correlative 1D models are usually used in the industrial practice to estimate heat transfer coefficients to be used as boundary conditions in detailed FEM simulation of the Whole Engine Modelling (WEM). However, the flow pattern in this chamber is complex and this approach could represent an oversimplification, therefore computational fluid dynamic (CFD) analyses have been performed with the aim to obtain a more accurate estimation. All the analyses have been carried out with ANSYS CFX with a quasi-2D axis-symmetric approach. This methodology, despite the need for some simplifications and assumptions to reduce the problem from 3D to quasi-2D, ensures less expensive simulations in terms of computational resources when compared to a fully 3D. After a numerical setup validation in terms of mesh sensitivity, a parametric CAD has been generated and tested. The goal is to analyze how geometrical parameters (such as the axial distance between the stator blade carriers or the height of the chamber) of the chamber will influence heat transfer coefficients, considering different operating conditions. A design of experiment (DOE) with ANSYS Workbench platform has been carried out in order to perform automatically a set of simulations. Finally, the results obtained have been post-processed to extract a sufficiently accurate correlation for the average Nusselt number applicable for almost all the steam chambers in the turbine even in different operating conditions. [ABSTRACT FROM AUTHOR]