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.
Highlights
With a further increase in world population and people’s pursuit for convenience and wealth, the world energy demand is ever increasing [4]
There is a strong influence on the combustion behavior on the pilot stage due to the large inner recirculation zone (IRZ) caused by the high momentum jets at an air inlet temperature of 650°C
The introduction of a cone-shaped pilot stage was a first step to the optimization of the pilot stage emissions with the goal of removing the outer recirculation zone (ORZ)
Summary
With a further increase in world population and people’s pursuit for convenience and wealth, the world energy demand is ever increasing [4]. At the DLR Institute for Combustion, a jet-stabilized combustor was developed for the Turbec T100, 100 kWel class micro gas turbine [5, 6]. In order to operate the combustor from a cold start up to full load MGT conditions, a swirl stabilized pilot stage was developed and incorporated in the combustor design [1]. The primary combustion zone is characterized by a large inner recirculation zone, driven by the high momentum jets of the main stage. A detailed examination of the combustor was carried out by Schwarzle et al [2] that identified the swirl pilot stage as main contributor to NOx emissions
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