For lean-premixed combustion with increasing fuel reactivity, lean blow out (LBO) limits are extended, offering excellent low-NOx potential. Simultaneously, FB disposition is increased. FB denotes the upstream propagation of a flame in a combustible mixture into regions not designed for flame holding and constitutes an operability limit for gas turbine combustors [21]. Aspects limiting the operational range of swirl-stabilized combustors are intensively detailed in the work by Huang and Yang [47] and Lieuwen [48]. The increased FB tendency of various mixtures with increasing hydrogen content has been investigated in numerous studies [33, 49, 50, 51]. Detailed investigations regarding the effect of inlet or outlet conditions by Syred et al.[52], swirl number by Sayad et al. [53], or a hydrodynamic instability by Schönborn et al. [54] on the stability limits of high-hydrogen content fuels have also been reported.
For hydrogen–air combustion basically two inherently different combustion systems have experienced relevant development progress in the past decades. On the one hand, micro-mix combustors, where multiple compact flames are generated at multiple fuel injection locations aiming to distribute the heat release to prevent hot spots and minimize residence time. In these concepts, minimal premixing times in conjunction with high main flow velocities are utilized to suppress FB, which is why these systems are also referred to as lean direct injection (LDI) systems.
1.5 Flashback in Premixed, Swirling Combustor Flow 23
On the other hand, swirl-stabilized premix combustors which are state-of-the-art in modern gas turbines firing natural gas, have experienced a push to extend their operational range to high reactivity fuels. These systems employ a strongly swirling flow to achieve a high degree of premixing which makes their premixing section particularly susceptible to FB when firing high reactivity fuels.
1.5.1 Minimal Premixing: LDI / Micro-Mix Combustors
Already in the 1970s, NASA examined new fuel injector designs for potential hydrogen gas turbine engines. Anderson [22] investigated a LDI concept, which basically consisted of a perforated plate flame holder, where 80 smaller flames were stabilized downstream of small passages through a plate. Fuel was injected via a jet in cross flow configuration inside the small passages. In the early 2000s, the experimental work on LDI injectors by Anderson was utilized to validate a CFD code which was then used for preliminary combustor design purposes by Shih et al. [55]. Schefer et al. [56] conducted OH-PLIF investigations with a conceptual fuel nozzle, that was similar to the LDI injector used by Anderson, and provided insight into the flame stabilization and flame structure of hydrogen–air flames. These efforts by NASA were concluded by an investigation of Marek et al. [57] who compared several perforated plate designs for their ability to minimize the FB risk in hydrogen–air combustion. The authors report FB limits and NOx emissions at elevated air preheat temperatures and pressures up to 7 bar. They conclude, that the best investigated configuration yields satisfying FB characteristics and NOx emissions comparable to state of the art LDI combustor concepts firing kerosene. However, they also report difficulties to achieve uniform fuel distribution to the numerous injection ports and massive cooling problems due the the hydrogen flame anchoring close to the injectors, leading to failure during test execution.
Relevant research on a low NOx combustion system was contributed by Ziemann et al. [11] in the context of the Cryoplane project [58]. They conducted a screening on various combustor designs with respect to NOx reduction potential and wide operational range. They investigated, amongst others, concepts of micro-mix and premixed swirl-stabilized combustors. For these two concepts, they report the lowest NOx emissions of all investigated concepts. However, they abandoned the premixed swirl concept and continued further tests only with the micro-mix concept. Dahl and Suttrop [12] proved the technical feasibility of the micro-mix hydrogen combustor when they replaced the conventional kerosene combustion system of a GTCP 36-600 auxiliary power unit with such a micro-mix combustor and achieved significantly reduced NOx emissions.
The next development step, beyond the basic micro-mix concept of a perforated plate with a fuel jets in cross flow, was suggested by Hernandez et al. [59] and Lee et al. [60]. The new concept applies internal fuel and air staging within the 6–12 mm micro-mix injectors
which are designed to achieve compact flames aiming to minimize residence times at the high flame temperatures. It is suggested, that when applied to a gas turbine, a full-scale combustor will contain 30-60 closely-packed micro-mix injectors for every megawatt of thermal power. The authors report single injector tests at ambient [59] and elevated temperature and pressure conditions up to 5 bar [60]. Two categories of injectors were distinguished, radial inflow and axial flow injection geometries. While the radial inflow injectors achieved lower emissions, they proved robust to FB for pressures up to 3 bar.
Above 3 bar the radial injector was reportedly prone to FB. The axial injection concept, that achieves higher axial velocities at the injector outlet, exhibits a higher FB resistance, while it does not achieve the mixing quality and low emissions of the radial concept.
1.5.2 High Degree of Premixing: Swirl-Stabilized Combustors
Similar to other modern premixer concepts [25], in the current thesis, a cylindrical mixing tube without centerbody is used to ensure sufficient mixing. The FB mechanisms prevailing in this type of combustor have been discussed by Lieuwen et al. [34]. They distinguish between four generally different types of FB which may lead to fast upstream flame propagation. Type 1–3 rely on mechanisms that are driven by the competition between the flame speed and local flow velocities. These types of FB can generally occur in both, swirling and non-swirling flows. Type 4 is the result of the interaction between a swirling flow and heat release from the flame and this type’s occurrence is, thus, limited to swirling flows. Additionally, reports in the literature exist for FB events caused by autoignition, representing type 5. Thus, the FB types are categorized as follows:
1. Flashback in the core flow
2. Flashback due to combustion instabilities 3. Wall boundary layer flashback
4. Combustion-induced vortex breakdown 5. Flashback due to autoignition
The first type, FB due to flame propagation in the core flow occurs when the turbulent burning velocity in the premixing section exceeds the flow velocity as discussed in the previous section. A conservative estimate of turbulence intensities in a gas turbine combustor of 20% of the main flow u0rms= 0.2u0 yields for the critical condition at FB, when the bulk flow velocityu0 matches the turbulent flame speed ST
ST/u0rms fb
= 5. (1.7)
1.5 Flashback in Premixed, Swirling Combustor Flow 25
Utilizing the simple relationship for the turbulent flame speed from Eq. 1.4, the laminar flame speed at the combustor inlet condition of an intensely recuperated combustor and conservative estimates of the turbulence intensity Lieuwen et al. [34] derive a worst-case ratio of
ST/u0rms
worst case
<1.3.
Since this value is substantially lower than required for flame propagation against the main flow velocity (Eq. 1.7), there is no indication that this type of FB is the most critical.
However, in case of poorly conditioned combustor flow fields, e.g., vortex breakdown in the mixing tube, this FB type may still occur. For experimental studies on swirling flames with a low reactivity fuel, here natural gas, Blesinger et al. [61] describe a flow configuration where already at isothermal conditions the axial location of vortex breakdown was located inside the mixing tube. Due to the comparably low reactivity of the fuel, for low equivalence ratios the flame was still anchored downstream of the mixing tube, inside the combustion chamber. Increasing the equivalence ratio above a critical value, allowed the turbulent burning velocity of the mixture to exceed the local flow velocity and lead to FB in the core flow, along the vortex axis.
The second type is FB due to combustion instabilities. Such instabilities manifest in high pressure fluctuations which are associated with velocity fluctuations. The fluctuating velocity can cause the local, instantaneous flow velocity to fall below the burning velocity of the combustible mixture. Given a sufficiently low frequency of the velocity fluctuations this mechanism will lead to FB. FB occurrence due to this mechanism was previously reported for both, a generic backward-facing step by Keller et al. [62] and more recently in a model combustor setup by Laperey et al. [63]. However, this FB type will not be considered throughout this thesis, since combustion instabilities have to be avoided for other reasons and FB due to this mechanism did not occur during regular, stable combustor operations assured during test execution.
The third type is FB in the wall boundary layer investigated by Lewis and von Elbe [64]. The wall-parallel flow velocity continuously decreases towards the wall due to the no-slip condition. Only flame quenching prevents upstream flame propagation along the wall to occur in any case. The chemical reactions cannot sustain within a certain distance from the wall due to heat loss and third body recombination reactions. This distance is referred to as quenching distanceδq. However, FB occurs when the burning velocity exceeds the flow velocities outside of the quenching distance, i.e., when a critical velocity gradient suggested by Lewis and von Elbe [64] is undercut. In laminar flows, FB limits correlate with the velocity gradient at the wall [64]. This lead to the concept of a critical velocity gradientgf, below which FB occurs. The critical velocity gradient gf correlates
with the laminar burning velocity SL and the quenching distance δq.
gf ∼SL/δq (1.8)
Taking into account the laminar burning velocities SL and quenching distances δq of hydrogen and methane (Table 1.1) yields
gfH2
gf CH4 = 20.5.
The required velocity gradient of hydrogen is approximately one order of magnitude higher than that for natural gas which underlines the elevated risk of boundary layer FB for hydrogen combustion.
The fourth type is FB due to combustion-induced vortex breakdown (CIVB). This mechanism describes that, even if at isothermal conditions the vortex breakdown is located downstream of the sudden expansion of the mixing tube, the chemical reaction can nevertheless lead to a further upstream breakdown of the flow, resulting in an upstream flame propagation. This effect was first identified by Fritz et al. [65]. Konle and Sattelmayer [66] reported time-resolved data of the flow field in the mixing tube and the upstream flame front during CIVB. They reveal that typically during the transient process of upstream flame propagation, the vortex breakdown, i.e. the low velocity region in the flow, travels upstream first and the flame follows. However, the initiation of upstream flame propagation is caused by an interaction between vortex breakdown and heat release. This interaction leads to a negative azimuthal vorticity gradient in axial direction which causes streamline divergence and, thus a declining axial velocity on the axis of rotation (Fig. 1.10). This declined axial velocity results in an upstream shift of VB. Once the VB is located inside the mixing tube, this effect is reinforced due to the high volume specific heat release resulting in the upstream propagation of the flame. Thus, the further downstream the initial location of VB, the higher is the resistance of the flow field against this type of FB. The interaction of heat release and the flow field was modeled by Duwig and Fuchs [67]. They also reported flame stability to benefit from a vortex breakdown location well downstream of the mixing tube, since under these conditions they observe a decoupling of the flame and hydrodynamic flow instabilities.
The fifth type of FB is observed, if the residence time in the premixing section exceeds the autoignition delay time associated with the combustible mixture at the current combustor inlet conditions. In this case, premature ignition of the mixture leads to flame holding in the premixing section as was previously reported by Sayad et al. [40].