Journal of Combustion

Volume 2016, Article ID 8261560, 16 pages

http://dx.doi.org/10.1155/2016/8261560

## Large Eddy Simulation of a Swirl-Stabilized Pilot Combustor from Conventional to Flameless Mode

Department of Applied Mathematics, The Hong Kong Polytechnic University, Kowloon, Hong Kong

Received 19 January 2016; Revised 8 April 2016; Accepted 27 April 2016

Academic Editor: Yiguang Ju

Copyright © 2016 Ehsan Fooladgar and C. K. Chan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper investigates flame and flow structure of a swirl-stabilized pilot combustor in conventional, high temperature, and flameless modes by means of a partially stirred reactor combustion model to provide a better insight into designing lean premixed combustion devices with preheating system. Finite rate chemistry combustion model with one step tuned mechanism and large eddy simulation is used to numerically simulate six cases in these modes. Results show that moving towards high temperature mode by increasing the preheating level, the combustor is prone to formation of thermal with higher risks of flashback. In addition, the flame becomes shorter and thinner with higher turbulent kinetic energies. On the other hand, towards the flameless mode, leaning the preheated mixture leads to almost thermal -free combustion with lower risk of flashback and thicker and longer flames. Simulations also show qualitative agreements with available experiments, indicating that the current combustion model with one step tuned mechanisms is capable of capturing main features of the turbulent flame in a wide range of mixture temperature and equivalence ratios.

#### 1. Introduction

Ever increasing global energy consumption and environmental concerns combined with the lack of energy resources have put the designers of combustion devices to a difficult test in order to come up with new technologies that are more energy efficient and less polluting. One common method to increase energy efficiency in almost all combustion systems including lean premixed (LPM) combustion is increasing the mixture temperature using recovered exhaust heat directly. Employing this method in LPM combustion not only increases the efficiency of the system but also improves combustion stability and flammability limits. As outlined by Huang and Yang [1], LPM combustion is the most promising technology for environmentally friendly combustion systems since operating under fuel lean conditions can have low emissions and high efficiency. Thermal nitric oxide formation is reduced because flame temperature is generally low and, for hydrocarbon fuels which are leaned by excess air, hydrocarbon and carbon monoxide (CO) emissions are reduced due to complete burnout of fuel. Unfortunately, as explained by Dunn-Rankin [2], achieving these improvements and meeting the demands of practical combustion systems are complicated by low reaction rates, extinction, instabilities, mild heat release, and sensitivity to mixing.

Panoutsos et al. [3] studied the effect of preheating of air/methane mixture up to 400°C on local equivalence ratio in a swirl-stabilized model gas turbine combustor using chemiluminescence sensor. They concluded that air preheating was beneficial for the premixing of fuel and air and as the temperature of the combustion air increased from 25°C to 400°C, the flame became shorter and moved upstream until it was stabilized at the boundaries of the inner recirculation zone. Seo [4] and Huang and Yang [5] presented experimental and LES results on flame structure in lean premixed combustors due to increasing inlet temperature. Huang and Yang reported that inlet temperature and equivalence ratio are key parameters determining stability characteristics of their combustor, where slight increase in inlet temperature above a critical value causes abrupt instability in the combustor. Foley et al. [6] investigated experimentally flame shapes for preheat temperatures ranging from 366 K to 533 K with equivalence ratios ranging from 0.40 to 0.70. They found that transition from one flame configuration to another is essentially due to the flame extinction phenomenon and that sensitivity of these transition points to fuel/air ratio and preheat temperature can be reasonably captured with extinction strain rate calculations.

A nonpilot assisted combustion device works in conventional, high temperature combustion (HiTC) [7] and flameless or Moderate or Intense Low-Oxygen Dilution (MILD) combustion modes based on premixture temperature and composition [8]. Despite its benefits, increasing the reactants temperature above a certain level gives rise to thermal formation in conventional and HiTC modes, thereby limiting the potential of this method to produce less . The risk of flashback also grows in these regions. Low heat release ultralean combustion with significant preheat, usually referred to as flameless combustion, could thus be a solution to these problems [9]. However, no significant work has been done to compare the flame and flow evolution of a practical swirl flame in these different combustion modes.

The objective of this paper is to investigate the flame and flow structure of a swirl-stabilized pilot combustor in conventional, HiTC, and flameless mode using finite rate chemistry combustion model and large eddy simulation (LES). In this paper, the flameless mode is achieved by preheating the ultralean premixture indirectly using an external preheater and/or a recoupaerator and is different from the common flameless concept in which the premixture is heated and diluted using exhaust gas recirculation. As experiments in premixed combustion with inlet temperature higher than 700 K is of safety concern, numerical simulation, especially LES, is the preferred choice for studying turbulent premixed combustion in this range of preheating. The present paper also aims to give a better insight to incorporate new technologies like MILD and HiTC in swirl-stabilized combustors.

#### 2. Numerical Procedure

Reacting flows are governed by the balance equations of mass, momentum, species, and energy. The basic idea of LES is resolving the larger turbulent motions in a flow field and modelling only the effects of the small ones. The resolved contribution is obtained by applying the spatial LES filter to instantaneous variables . Filtering the instantaneous governing equations and introducing the Favre filtered variable, , where over-bars denote spatial filtering, leading to the following equations:where is the velocity, is the density, is the pressure, is the viscous tensor, is the species mass fraction, is the sensible enthalpy, is the thermal conductivity, is the temperature, is the species diffusivity, is the species reaction rate, and is the species formation enthalpies. The viscous heating term and radiation sink term are neglected in (4) as they are negligible compared to the combustion source term. The species mass flux is described by Fick’s law in the first term on the right hand side of (3). may therefore be expressed as the ratio of kinematic viscosity to Schmidt number *,* which is assumed to be unity for all species in this paper. The dynamic mixture viscosity and thermal conductivity are modeled by Sutherland’s law and considering the mixture as a Newtonian fluid, , where is the deviatoric part of filtered strain tensor defined as .

In this system of equations, the second terms in the brackets result from filtering the convective terms and contain subgrid flow physics. These unclosed quantities are modeled using one-equation eddy viscosity model in which the unresolved subgrid scale (SGS) stresses are modeled as , unresolved species fluxes as and unresolved enthalpy fluxes as where and are turbulent Schmidt and Prandtl numbers, respectively. The subgrid viscosity is defined as , where is the filter size calculated as the cube root of the local cell volume and is provided by an equation for the subgrid kinetic energy [10] such thatwith dimensionless constants of and as proposed by Berglund et al. [11]. Finally, the filtered species reaction rate is determined via the Arrhenius expression and is modeled as described in Section 3.

The CFD code used in this study is OpenFoam which was introduced by Weller et al. [12] and has been validated in a number of studies, including nonreacting flows [13] and reacting flows [14, 15], with generally good agreement with experiments. The code is compressible and employs an unstructured collocated Finite Volume method of Grinstein et al. [13] in which discretization is based on Gauss theorem together with a semi-implicit time-integration scheme. The filtered governing equations, (1)–(5), are discretised using 2nd-order spatial and first order temporal schemes. Time integration is performed implicitly using PISO method, in which pressure and velocity fields are decoupled and solved iteratively, with three PISO correctors. The equations are solved sequentially, with iteration over the nonlinear source terms to obtain rapid convergence, with a maximum CFL number of 0.2.

The computational domain of this paper is based on the reduced-scale swirl-stabilized combustor developed by Nogenmyr et al. [16]. In order to create swirling flow, the burner is designed with a radial swirler with tangential inlets enabling a wide range of swirl numbers. Premixed fuel is injected through a premixing system just before both tangential and axial inlets. The benefit of a reduced-scale combustor from a numerical simulation point of view is that the filter size, which is the mesh size in implicit filtering, could be as small as the laminar flame thickness. Geometry of the combustor is given by Nogenmyr et al. [16] and an overview of the domain and mesh used in this paper is shown in Figure 1. Essentially, axial flow of reactants is supplied via a central tube of 16 mm diameter and 160 mm in length. Swirling flow of reactants is induced by four tangential ducts supplied via a plenum with two lateral entries of 9.53 mm diameter tubes to assure flow symmetry. Each tangential duct has a rectangular cross-section of 8 mm in height aligned in the axial direction and 4 mm in width to introduce angular momentum to the axial flow through the burner pipe. To assure a uniform axial velocity distribution as well as generating turbulence, a perforated plate with circular holes of 1.62 mm diameter is placed just upstream of the tangential ducts. Figure 1(b) also outlines the overall flame location in the combustor by showing the mean fuel mass fraction isosurface at 0.01 for the benchmark case as described in Section 4. Computationally, the mesh consists of 1.7 M hexahedral cells, which are body fitted inside the domain. The resolution inside the burner and combustor is 0.5 mm per cell, yielding 32 cells over the diameter of the axial inlet. The computational domain consists of the swirler, the combustor, and a dome shaped region downstream of the combustor contraction to take into account a part of the outside atmosphere. Detailed dimensions of the computational set up are given by Nogenmyr et al. [16].