Large eddy simulation of swirling flows in a non-reacting trapped-vortex combustor

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Abstract

Numerical investigation of swirling flows in a trapped-vortex combustor (TVC) is carried out using large eddy simulation (LES). A multi-block flow solver is employed to solve three-dimensional filtered compressible Navier-Stokes equations in generalized coordinate system through the use of high-order compact differencing schemes. The effect of swirl strength on the behavior of swirling flow features including vortex-breakdown, precessing-vortex core (PVC) and shear layer is practiced for a TVC with circumferential cavity and annular inlet mainstream. The axial curved-vane swirlers are designed to generate flows with swirl numbers of 0 and 0.34 (low-swirl), 0.6 and 0.75 (medium-swirl), and 1.03 and 1.34 (high-swirl). Evaluation of the mean flow field reveals that in the medium-swirl range, a central recirculation zone (CRZ) is established due to the vortex-breakdown mechanism apart from the bluff-body recirculation zone, while no CRZ is observed for low and high swirl ranges. The case with S = 0.6 in the medium-swirl range demonstrates a better performance by generating a larger central recirculation zone. The PVC is located in periphery of central recirculation zone and its precession frequency linearly increases with swirl number. The strain rate and overall pressure drop are invoked for further analysis of the flow field. The strain rate is sharp and intense in the low-swirl range and becomes diluted as the swirl number increases with its maximum point moving towards the cavity. Moreover, an overall pressure drop of under 5% is achieved for the low and medium-swirl ranges. Finally, turbulent kinetic energy (TKE) and local turbulence intensity as potential indicators of higher mixing in combustion systems are investigated. High levels of TKE are found to occur in all cases. For medium-swirl range, the high-TKE region is located at the proximity of shear layer between mainstream jet and cavity recirculation zone. Furthermore, for medium-swirl numbers the local turbulence intensity near the central recirculation zone boundary is between 50 and 100%.

Introduction

Establishing stable combustion has been a major issue for combustion chamber designers especially in high speed flow conditions. Trapped-vortex combustor (TVC) as a new concept for flame stabilization in gas turbine engines was suggested by Hsu et al. [1]. In TVC, the flame stability is achieved by using hot reacting vortices trapped inside a cavity which provide a continuous source of ignition for the incoming fuel and air. The advantages and capabilities of trapped-vortex combustor for flame-stabilization were demonstrated in the early researches [1], [2], [3], [4], [5], [6], [7]. Due to its unsophisticated design, small pressure loss, more stable combustion, low lean-blow-out limits, reduced NOx emissions and high combustion efficiency, TVC attracted increasing attention in the successive years. Like conventional swirl-stabilized combustors, investigation of flow field and its effects on combustion became an engrossing subject for TVC researchers. Kumar and Mishra [8] carried out a numerical investigation to unravel the impact of momentum flux ratio (MFR) on flow and flame structures in an axisymmetric TVC. MFR is defined as the ratio between sum of the cavity jet momentums to the mainstream momentum. Their results revealed that the vortical flow structure for the lower MFR case (MFR∼0.57) is dominated by a single vortex inside the cavity whereas several vortices are formed within the cavity in higher MFR cases (MFR∼0.82 and 1.8). Jin et al. [9] conducted a numerical investigation in order to develop an insight into flow field and vortical structure of a planar trapped vortex combustor (TVC) with radial struts. The results revealed existence of two typical flow structures inside the cavity: a dual-vortex pattern is observed in a plane between two radial struts, while a single-vortex structure is seen in a plane along a radial strut. Radial struts are believed to improve mixing in spanwise direction due to the creation of wake regions and streamwise vortices. Zeng et al. [10] carried out an unsteady RANS simulation of a 2D twin TVC. They devised a new geometrical structure including flow guide vane and blunt body to enhance combustion efficiency. They studied the effect of a blunt body on the flow field through unsteady turbulent flow simulation. Their results showed that values and the distribution area of turbulent kinetic energy (TKE) are promoted with the inclusion of a blunt body enhancing heat and mass transfer in the combustion chamber. Dual-vortex cavity flow structure, regardless of its extent and position, is not affected by mainstream and blunt body.

Krishna and Ravikrishna [11] conducted an investigation on fuel-air mixing and vortex structure in a planar trapped-vortex combustor. The fuel–air momentum flux ratio is found to be an important parameter to characterize the fuel-air mixing and combustion. Experiments showed that the fuel–air mixing in the cavity is weak at high momentum flux ratios. As the momentum flux ratio reduces, a desirable vortex is formed inside the cavity and the mixing is improved. Zeng et al. [12] numerically obtained the optimum geometrical parameters of blunt body and guide vane for a planar TVC. Also, their findings showed that higher inlet temperatures and lower inlet velocities result in lower pressure drops, whereas the equivalence ratio has little impact on it. Sharifzadeh and Afshari [13], [14], [15] conducted large-eddy simulation (LES) studies in order to identify the best arrangement of fuel and air injection in a planar single-cavity TVC for isothermal and reacting conditions. They employed the hybrid scheme of LES/Filtered mass density function to resolve the vector and scalar fields. To assess fuel/air mixing quality for different injection strategies, various quantitative criterions including mean cavity and near stoichiometric equivalence ratios, global fuel distribution, mixing efficiency curves, temperature distribution and flame structure were utilized. They found that the configurations in which both air and fuel jets are issued adjacent to the cavity inferior wall result in a more homogenous in-cavity mixture, more uniform temperature distribution, more contained flame and lower maximum temperature. They explained their results by means of vortical structure and mixing quality analysis. Zhu et al. [16] investigated the impacts of position and angle of primary injection on the flow field in a non-reacting planar TVC. The cavity flow structure is found to be dominated by two different flow patterns, which are referred as “broken” and “closed”. The “broken” flow structure occurs when the primary jet injection location becomes gradually far from the cavity inferior wall while the injection angle of the primary jet remains unchanged. They showed that if the injection location is kept constant and angle of injection is increased, the cavity flow structure experiences a transition from “broken” mode to “closed” mode.

Recently, there are interests in studying TVC's performance while subjected to swirling flows. Merlin et al. [17] performed a large eddy simulation of cold and reacting flows in an axisymmetric TVC with an internal cavity. In their study the effects of the main flow rate, the cavity geometry and inflow swirl intensity were investigated. Their results revealed that adding the swirl motion is an appropriate choice because it suppresses strong pressure fluctuations arising from combustion instabilities. Chen et al. [18] carried out a Reynolds averaged simulation of a swirl TVC to assess the impacts of high-spinning motion on combustion characteristics, such as fuel–air mixing and vortex dynamics. They showed that adding spinning motion results in strong three-dimensional flow and improves fuel–air mixing so that a stronger in-cavity pilot flame could be established. In continuation of previous research, Chen and Zhao [19] studied a swirling trapped vortex combustor in cold flow conditions. They studied transient flow field while abruptly changing swirl number from 0.6 to 0.98. Their findings showed that the vortex is well trapped in the cavity and highly resistant against disturbances at different swirl numbers. They demonstrated that adding the swirling motion into the flow in axisymmetric trapped vortex combustors can lead to an increase in turbulence intensity and turbulent kinetic energy which improves fuel–air mixing and TVC's performance.

Developing new approaches is inevitable for designing combustors with higher performance characteristics. There have been a vast number of remarkable researches dedicated to flow field features of isothermal swirling flow in dump combustors [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. Moreover, investigation of cold flow field in non-swirling TVC's has attracted much attention so far [7], [11], [13], [19], [33], [34], [35], [36], [37], [38]. However, employing swirl motion in TVCs has been investigated by a very limited number of studies [17], [18], [19] but appears to be a promising approach to improve efficiency of non-swirling axisymmetric TVC's. It provides main advantages of both the trapped-vortex and swirl-stabilized combustors, simultaneously. Imposing swirl motion can enhance the turbulence levels and create central recirculation zone (CRZ) which assists the mixing inside the combustor. So far, the vast majority of trapped-vortex combustor researches have been devoted to planar TVCs and only few researchers have studied axisymmetric TVCs, among which incorporation of swirl motion is a rare topic. However non-reacting flow conclusions cannot be extended to reacting flows, but investigation of fundamental isothermal flow characteristics of swirling-TVC can be insightful prior to studying reacting cases. Hence, conducting a comprehensive research in order to understand the combined effects of swirling flow and cavity flow in isothermal conditions seems inevitable. Investigating the isothermal flow can help us to adopt a better approach to study reacting cases by carefully utilizing the general perception provided by clod flow simulations. In addition, it provides some other advantages such as: (1) decoupling the effect of the geometry of the combustor from the effect of combustion on the flow field, (2) resembling the ideal distributed reactor, which has uniform temperature field, (3) providing qualitative understanding of the flow field in regions of the flow for which experimental data is not available, (4) providing a comprehensive picture of the flow field and understanding the modifications induced by the combustion process. Thus, the main objective of current study is to investigate the fundamental flow field features of a typical swirling TVC in non-reacting conditions. Three salient flow structures are observed in swirling flows: central recirculation zone created downstream of dump plane induced by vortex-breakdown, precessing vortex core (PVC) created around the central recirculation zone, and shear layers originating from the outer edge of the inlet mainstream and stretching between central and cavity recirculation zones. The focus of this study is placed on three structures and their interaction with TVC and flow in-cavity. Pressure distribution and turbulent quantities are also investigated to enhance the understanding of flow field inside swirling TVC's.

Section snippets

Governing equations

The three-dimensional Navier–Stokes equations for unsteady compressible turbulent flows are employed. A filtering function H with the filter width Δ is applied to a flow variable f(x,t) to obtain a filtered flow variable f(x,t),f˜(x,t)=f(x,t)=+f(x,t)H(x,x)dx

Where x is the position vector [39]. For compressible flows it is customary to employ the Favre-filtered value, f(x,t)L=ρf/ρ. By applying the filtering procedure, the governing equations convert to the set of

Validation

Simulation of a dump combustor with an inlet swirl is carried out to examine the capability of the flow solver for predicting swirling flows. The benchmark data is provided by two-dimensional particle image velocimetry (PIV) measurements of Strakey and Yip [20]. Their experimental configuration is an axisymmetric abrupt expansion with an annular inlet displayed in the Fig. 1.

This configuration is comprised of an inlet section followed by combustor and exhaust sections. The inlet section is an

Results and discussion

In this section, first a swirling TVC is designed and the flow configuration, boundary conditions and grid resolution are discussed. Then the flow characteristics are investigated under different swirling intensities.

Concluding remarks

Non-reacting swirling flow in an axisymmetric TVC with annular inlet mainstream was numerically investigated using large eddy simulation. The profiles of mean velocity components and the rms velocity predicted by the employed numerical scheme were compared well with the available experimental benchmark data. The impact of swirl intensity on swirling flow characteristics including vortex breakdown-induced central recirculation zone, precessing spiral vortex core and shear layer regions was

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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