ENGINEERING AND CFD
Numerical Prediction of the Flashback Phenomena in a Swirl-stabilized GT Combustor Operated with Hydrogen in Technically Premixed Conditions
Principal Investigator:
Panagiotis Stathopoulos
Affiliation:
Hermann-Föttinger-Institut, Technische Universität Berlin
Local Project ID:
pr27bo
HPC Platform used:
SuperMUC-NG of LRZ
Date published:
Introduction
The use of hydrogen-enriched fuels is an alternative to reduce pollutant emissions in gas turbine (GT) applications allowing high flexibility of operation and fuel flexibility. However, the presence of hydrogen in fuel mixtures can lead to unpredictable behaviour of combustion systems and undesirable phenomena such as flashback or blow-off. The existence of flashback is one of the main drawbacks on burner-stabilized combustion systems operated with hydrogen, as the H2 content contributes to higher flammability limits and higher burning speeds when compared to traditional hydrocarbon fuels.
Swirling combustors can take advantage of an axial air injection that reduces the velocity deficit along the centerline, thus providing additional resistance against flashback [2]. Axial air injection influences the location of the flame by displacing the stagnation point of the aerodynamically induced recirculation downstream in the combustion chamber, thus increasing burner stability.
An example of this design is the technically-premixed swirl-stabilized presented in the experiments [2], which shows that flashback appears at low equivalence ratios, when this combustor is operated with pure hydrogen. The axial momentum ratio between the fuel jets and the air was found to be the dominant parameter controlling the flame stabilization process and flashback resistance [2] over mixing quality and equivalence ratio fluctuations. This hypothesis motivates the present study where large-eddy simulations (LES) are set with the same axial momentum ratio of the experiments and the influence of fuel/air mixing is removed by a perfectly premixed assumption.
Results and Methods
Methodology
The governing equations describing the reacting flow field correspond to the low-Mach number approximation of the Navier-Stokes equations with the energy equation represented by the total enthalpy in the context of LES with an eddy-viscosity given by the closured proposed by Vreman. The flow conditions considered in this study include the oxidation of a hydrogen/air mixture at preheated conditions in a turbulent flow field at two equivalence ratios using a flamelet method based on the tabulation of laminar premixed flamelets with a presumed-shape Probability Density Function (PDF) with a beta-function [3]. The set of governing equations is integrated in time using a third-order Runge–Kutta explicit method with a second-order low-dissipation low-Mach number scheme. This computational framework is developed into the multiphysics code Alya [4], which is used to run the LES simulations.
Numerical setup
The computational domain is depicted in Fig. 1 and corresponds to a swirl-stabilized technically premixed burner. It consists of plenum, fuel injection, mixing tube and combustion chamber. The numerical simulations have been conducted on a hybrid unstructured mesh including the combustion chamber, mixing tube, plenum and fuel injection system, so the flow distribution across the mixing tube is accounted for. The mesh is composed by prisms, tetrahedrons and pyramids, and locally refined in the regions of interest. While several meshes are considered featuring different resolutions in the combustion chamber, the mesh used of the presented results include a length scale of 0.7 mm in the reacting layer, and 1 mm everywhere else.
Figure 1: Computational domain. © TU Berlin
The three cases have a Reynolds number Re = 75,000 with pre-heated air at Tair = 453 K and hydrogen coming at TH2 = 320 K. The first case corresponds to inert conditions without fuel injection and it is used not only for validation purposes, but also for the evaluation of the impact of heat release on the dynamics of the flow in this burner concept. The other two cases (phi=0.6 and phi=0.4) correspond to reacting flow calculations for which experimental data is available.
Results
The inert calculation revealed a strong characteristic frequency at approximately 1100 Hz that did not appear in the presence of the flames, which has been confirmed by the LES calculations. The characteristic frequency corresponds to the Precessing Vortex Core (PVC), which is a well-known global flow instability arising in swirling flows underlying vortex breakdown, see Fig. 2.
Figure 2: Q-vorticity visualization and power spectra density of the inert case including PIV and LES. © TU Berlin
A flow visualization of the reacting flow field is shown in Fig. 3 by a volumetric rendering of the density gradient. The flame topology of a swirl-stabilized flame can be distinguished in this plot.
Figure 3: Q-vorticity visualization and power spectra density of the inert case including PIV and LES. © TU Berlin
The distribution of axial velocity and temperature, during stable operation at phi = 0.6 is also shown in Fig. 4. The effects of heat release also influence the vortex-breakdown mechanism. The inert case exhibits a narrow central recirculation with an axial velocity deficit at the nozzle exit plane, which can be identified as bubble-type vortex-breakdown. The heat release causes a wider opening angle with a correction of the velocity deficit at the nozzle exit that changes the stabilization mechanism to cone type vortex-breakdown as shown in Fig. 4.
Figure 4: Axial velocity and temperature snapshots of the flow field during stable operation at phi=0.6. © TU Berlin
The LES results are able to predict this transition of the vortex breakdown in the reacting case, which is a fundamental aspect for flashback safety in this burner (Reichel et al. 2015). The propagation of the vortex breakdown into the mixing tube is fundamentally induced by the density stratification caused by the flame heat release, and suggests the burner is prone to combustion-induced vortex breakdown flashback [5].
An analysis of the spectra and POD modes indicate that the PVC is attenuated due to the increase in axial momentum and is ultimately suppressed in the reacting flow field. The LES is capable to reproduce both damping effects, which are also in agreement with the experimental data [3].
The analysis of the flames has shown certain dynamics as the flashback point is approached. As the axial momentum ratio is reduced, the flashback propensity of the burner increases due to an intensification in the velocity deficit of the incoming mixture. Moreover, the recirculation region is shifted upstream, the central recirculation is altered and the flame position is displaced towards the reactants. The study of instabilities confirms there is no instability at the onset of flashback as it is suppressed by the density stratification.
Computational cost
The reacting flow simulations were run using 1280 CPUs for 96 hours to obtain fully-averaged flow fields and extract first and second order statistics. The inert calculations were much faster and required 36 hours. In order to compute the POD analysis, many snapshots were stored requiring about 0.8 Terabytes of data for each case.
On-going Research / Outlook
Thanks to SuperMUC-NG, we were able to run these simulations and perform data analysis and visualizations in the cluster. Our next steps are to include a detailed study of the mixing process between fuel and air.
References and Links
[1] n/a
[2] Reichel, T.G., Goeckeler, K., Paschereit, O.: Investigation of lean premixed swirl-stabilized hydrogen burner with axial air injection using oh-plif imaging. J. Eng. Gas Turbines Power 137, 111513 (2015).
[3] Mira, D., Lehmkuhl, O., Both, A. et al. Numerical Characterization of a Premixed Hydrogen Flame Under Conditions Close to Flashback. Flow Turbulence Combust 104, 479–507 (2020).
[4] Vazquez, et al., Multiphysics engineering simulation toward exascale. J. Comput. Sci. 14, 15–27 (2016).
[5] Oberleithner, K., Stöhr, M., Im, S.H., Arndt, C.M., Steinberg, A.M.: Formation and ame-induced sup- pression of the precessing vortex core in a swirl combustor: experiments and linear stability analy- sis. Combust. Flame 162(8), 3100–3114 (2015).
Research Team
Oriol Lehmkuhl2, Daniel Mira2, Panagiotis Stathopoulos1 (PI), Tom Tanneberger1
1Hermann-Föttinger-Institut, Technische Universität Berlin
2Barcelona Supercomputing Center
Scientific Contact
Prof. Dr. sc. P. Stathopoulos
Technische Universität Berlin
Chair of Fluid Dynamics
Department: Unsteady Thermodynamics in Gas Turbine Processes
Müller-Breslau-Straße 8, D-10623 Berlin (Germany)
e-mail: stathopoulos [@] tu-berlin.de
Local project ID: pr27bo
March 2021