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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

# Detailed Examination of a Modified Two-Stage Micro Gas Turbine Combustor

[+] Author and Article Information
Andreas Schwärzle

Institute of Combustion Technology,
German Aerospace Center (DLR),
Pfaffenwaldring 38-40,
Stuttgart 70569, Germany
e-mail: andreas.schwaerzle@dlr.de

Thomas O. Monz, Andreas Huber, Manfred Aigner

Institute of Combustion Technology,
German Aerospace Center (DLR),
Pfaffenwaldring 38-40,
Stuttgart 70569, Germany

1Corresponding author.

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 5, 2017; final manuscript received July 18, 2017; published online October 3, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 140(2), 021501 (Oct 03, 2017) (10 pages) Paper No: GTP-17-1283; doi: 10.1115/1.4037749 History: Received July 05, 2017; Revised July 18, 2017

## Abstract

Jet-stabilized combustion is a promising technology for fuel flexible, reliable, highly efficient combustion systems. The aim of this work is a reduction of NOx emissions of a previously published two-stage micro gas turbine (MGT) combustor (Zanger et al., 2015, “Experimental Investigation of the Combustion Characteristics of a Double-Staged FLOX-Based Combustor on an Atmospheric and a Micro Gas Turbine Test Rig,” ASME Paper No. GT2015-42313 and Schwärzle et al., 2016, “Detailed Examination of Two-Stage Micro Gas Turbine Combustor,” ASME Paper No. GT2016-57730), where the pilot stage (PS) of the combustor was identified as the main contributor to NOx emissions. The geometry optimization was carried out regarding the shape of the pilot dome and the interface between PS and main stage (MS) in order to prevent the formation of high-temperature recirculation zones. Both stages have been run separately to allow a detailed understanding of the flame stabilization within the combustor, its range of stable combustion, the interaction between both stages, and the influence of the modified geometry. All experiments were conducted at atmospheric pressure and an air preheat temperature of 650 $°C$. The flame was analyzed in terms of shape, length, and lift-off height, using OH* chemiluminescence (OH-CL) images. Emission measurements for NOx, CO, and unburned hydrocarbons (UHC) emissions were carried out. At a global air number of λ = 2, a fuel split variation was carried out from 0 (only PS) to 1 (only MS). The modification of the geometry leads to a decrease in NOx and CO emissions throughout the fuel split variation in comparison with the previous design. Regarding CO emissions, the PS operations are beneficial for a fuel split above 0.8. The local maximum in NOx emissions observed for the previous combustor design at a fuel split of 0.78 was not apparent for the modified design. NOx emissions were increasing, when the local air number of the PS was below the global air number. In order to evaluate the influence of the modified design on the flow field and identify the origin of the emission reduction compared to the previous design, unsteady Reynolds-averaged Navier–Stokes simulations were carried out for both geometries at fuel splits of 0.93 and 0.78, respectively, using the DLR (German Aerospace Center) in-house code turbulent heat release extension of the tau code (theta) with the k–ω shear stress transport turbulence model and the DRM22 (Kazakov and Frenklach, 1995, “DRM22,” University of California at Berkeley, Berkeley, CA, accessed Sept. 21, 2017, http://www.me.berkeley.edu/drm/) detailed reaction mechanism. The numerical results showed a strong influence of the recirculation zones on the PS reaction zone.

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## Figures

Fig. 1

Schematic of a jet-stabilized MGT combustor including mixing air holes (dilution) and flow path. About 33% of the air is fed to the pilot and the MS of the combustor. The larger amount of air is used for dilution.

Fig. 2

Modified version of the two-stage combustor with a 20 nozzle jet-stabilized MS and a ten nozzle swirl-stabilized PS. The dashed line indicates the shape of the original PS.

Fig. 3

Computational fluid dynamics (CFD) simulation of the standard (top) and cone (bottom) combustor in the atmospheric test rig at m˙air = 60 g/s a fuel split of Sf = 0.93 and 650 °C, showing the averaged absolute velocity including streamline patterns

Fig. 4

Atmospheric combustor test rig with exemplary OH* chemiluminescence (OH-CL) image including air flow path (dashed arrows) and baffle [9]. —air inlet, —simulation: air inlet, —location of static pressure and preheat temperature gauges in combustor plenum, —location of exhaust gas probe for emission measurements, and —simulation: outlet.

Fig. 5

Volume for OH* imaging, seen by the intensified charge-coupled device camera (a) and definition of the HAB and the FL for an OH* image (b) [9]

Fig. 6

Grid of the 36 deg wedge of the standard (a) and cone (b) simulated domain. (a) Standard, 1,037,815 cells, 458,472 points and (b) cone, 1,162,935 cells, 494,240 points.

Fig. 7

OH-CL for five different air numbers λp of the PS at m˙air = 60 g/s for the standard and the cone combustor: (a) standard and (b) cone

Fig. 8

Normalized NOx emissions versus λp at m˙air = 60 g/s for the standard and the cone combustor

Fig. 9

Normalized CO emissions versus λp at m˙air = 60 g/s for the standard and the cone combustor

Fig. 10

OH-CL for five different air numbers λm of the MS at m˙air = 60 g/s for the standard and the cone combustor: (a) standard and (b) cone

Fig. 11

FL and HAB versus λm at m˙air = 60 g/s for the standard and cone combustor

Fig. 12

Normalized NOx emissions versus λm at m˙air = 60 g/s for the standard and cone combustor

Fig. 13

Normalized CO und UHC emissions versus λm at m˙air = 60 g/s for the standard and the cone combustor

Fig. 14

OH-CL for a fuel split variation from Sf = 0 (only PS) to 1 (only MS) at a global air number λg = 2.0 and m˙air = 60 g/s for the standard and the cone combustor: (a) standard and (b) cone

Fig. 15

Normalized, uncorrected NOx emissions versus (theoretical) fuel split for pilot only, main only, and combined operation for the standard and the cone combustor

Fig. 16

Normalized, uncorrected CO and UHC emissions versus (theoretical) fuel split for pilot only, main only, and combined operation for the standard and the cone combustor

Fig. 17

Recirculation rate R along the axial location x for the reactive and nonreactive numerical simulations of the standard and cone combustor

Fig. 18

CFD simulation of the standard (top) and cone (bottom) combustor in the atmospheric test rig at m˙air = 60 g/s a fuel split of Sf = 0.78 and 650 °C, showing the averaged heat release rate. The isolines attached to the reaction zones indicate the local air number λ = 2. The isolines marked with ORZ or IRZ, respectively, indicate an axial velocity of u = 0.

Fig. 19

CFD simulation of the standard (top) and cone (bottom) combustor in the atmospheric test rig at m˙air = 60 g/s a fuel split of Sf = 0.93 and 650 °C, showing the averaged heat release rate. The isolines attached to the reaction zones indicate the local air number λ = 2. The isolines marked with ORZ or IRZ, respectively, indicate an axial velocity of u = 0.

Fig. 20

CFD simulation of the standard (top) and cone (bottom) combustor in the atmospheric test rig at m˙air = 60 g/s a fuel split of Sf = 0.93 and 650 °C, showing the averaged temperature

Fig. 21

CFD simulation of the standard (a) and (c) and cone (b) and (d) combustor in the atmospheric test rig at m˙air = 60 g/s a fuel split of Sf = 0.78 and 650 °C. (a) and (b) show streamlines colored by averaged temperature. (c) and (d) show an isosurface of constant radius close to the outer wall colored by the circumferential velocity θ and isolines of constant averaged temperature.

Fig. 22

OH-CL for a global air number variation λg at a thermal load of Qth = 100 kW and constant fuel split Sf = 0.93 for the standard and the cone combustor: (a) standard and (b) cone

Fig. 23

Normalized NOx, CO, and UHC emissions versus global air number for the standard and the cone combustor

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