Generally, it is observed that all configurations exhibiting a high FB resistance also reveal a high value of xVB. However, due to the contribution of dilatation due to heat addition (cp. Sec. 89), configurations inherently imminent to FB occurrence are found to exhibit similarly high values of xVB. Thus, large values ofxVB are identified as a necessary, but not sufficient conditions for FB resistance.
Meanwhile, the upstream flame front xf is reported to reliably exhibit large values in case of high FB resistance and to always approach xf/D= 0 prior to FB occurrence. This is confirmed over a wide range of operating conditions including both, particularly FB resistant and FB prone configurations (Fig. 11, p. 81). Thus, values above xf/D= 0.25 are identified as a sufficient condition for FB resistance.
3.3 Main Drivers for Increased Flashback Resistance 93
was identified as an excellent trade-off, capable of fully suppressing FB on the investigated operational range and still achieving single digit NOx emissions. For the choice of the initial swirl number, also the LHV of the fuel needs to be taken into account: The lower the LHV, the more fuel momentum is introduced into the premixing section at the same power level, resulting in further reduction of the resulting swirl number.
Effect on upstream flame front
An increasing AI rate shifts xf downstream. This effect of AI on the upstream flame front is best expressed by Fig. 12 (p. 69). The investigated medium and high AI rates exhibit a strong increase with increasing momentum ratioJ with a nearly identical slope for both AI rates. However, between the two AI rates the correlation exhibits a constant offset.
Thus, increasing the AI rate increases xf, independent of the simultaneously prevailing momentum ratioJ and, thus, increases FB resistance independent of fuel momentum.
Effect on stability limits
AI significantly extends FB limits, while LBO limits remain unaffected. In the absence of axial air injection, the operational range with neat hydrogen was very limited and FB would occur almost immediately for most conditions. However, already a medium amount of axial air injection substantially extended the stability limits, allowing to operate the burner at stoichiometric conditions. FB would occur at the lower limit. Further increasing the AI rate again extended the operational range of all configurations and fully suppressed FB for three of the four investigated configurations (Fig.7, p. 42).
Effect on precessing vortex core
In agreement with previous results reported in literature, the axial non-swirling jet suppresses a hydrodynamic instability referred to as PVC. This suppression along with the flow field changes positively affect the temporal fuel-air mixing quality and pose one mechanism that contributes to the excellent overall fuel–air mixing quality observed even for high rates of AI.
Effect on fuel–air mixing and NOx emissions
The obvious question arises, to what extend is the non-swirling axial air jet detrimental to the fuel-air mixing and, therefore, to the NOx emissions? The results revealed that for the most suitable geometry, spatial unmixedness remained unaffected while the temporal unmixedness was even improved (Fig. 10a and Fig. 10b, p. 44). Whereas the first observation is attributed to the leaner core due to the central air jet, the latter observation was shown to originate from the damping of the PVC. Three out of the four investigated configurations
delivered single-digit NOx emissions up to adiabatic flame temperatures of almost 2000K (Fig. 13, p. 47).
3.3.2 Fuel Momentum
In case of hydrogen, significant additional momentum is introduced into the premixing section that has the potential to alter the flow field in both, premixing section and combustion chamber. The detailed effect on the flow field strongly depends on geometry and alignment of fuel injectors. Fuel should be allowed into the system as such, that sufficient fuel-air mixing is maintained and the fuel momentum constructively contributes to the flow field features desired for FB resistance: a plug-flow shaped axial velocity distribution at the mixing tube outlet, and a VB which is at isothermal conditions located sufficiently downstream of the mixing tube outlet.
The momentum of the fuel that is introduced into the combustion system relative to the main air stream is quantified by the fuel–air momentum ratio J. The momentum ratio J is identified as the governing parameter for the downstream shift of the flame front with increasing fuel momentum over a wide parameter space, e.g., varied bulk flow velocities, combustor powers, and air preheat temperatures. For calculating the momentum ratio at given operating conditions, the fuel temperature is required. To this end, a fuel temperature model was developed and validated for the current geometry. This allows to obtain the fuel temperature as a function of air preheat temperature, air mass flow, equivalence ratio and fuel type.
Effect on flow field
An increase in momentum ratio above J = 2 significantly alters the flow field (Fig. 5, p. 65). Such high momentum ratio values are not achieved for methane or natural gas.
However, in case of high hydrogen-content syn gases or neat hydrogen, J = 2 is already exceeded at moderate equivalence ratios of about φ= 0.5 and an air–fuel temperature ratio of Tin/Tfuel = 1.8, representative of typical engine conditions (Fig. 2, p. 63). A proper fuel injection geometry allows to exploit the additional fuel momentum as such, that it supports the formation of a plug flow-like axial velocity distribution at the mixing tube outlet. To comply with this requirement, the alignment of the fuel injectors is a crucial parameter. Fuel injector alignment in flow direction, as in the current study, reduces the resulting swirl number for increasing equivalence ratios similar to AI. This results in the desired plug flow-like axial velocity distribution. Note, that an alignment of fuel injection ports perpendicular to the main flow would instead increase the resulting swirl number and presumably oppose the effect of AI.