Fig. 4 - Maximum temperature vs compressor ratio
A big effort has been made to properly modulate flow rates, velocities, momentum and minimum size of the combustion chamber. It’s clear that inlets placement plays a key role in the formation of a energetic vortex which can be able to properly dilute the reactants and create sufficient residence time that favorites complete combustion. The resulting configuration establishes a perfect balance between the action of the tangential flows, which tend to generate the vortex and the action of the vertical flows that tend to destroy it (fig. 2).
In this sense, it is worth pointing out that it is especially the tangential flow further from the outlet which is the most effective. The negative effects on vortex location and size resulting from a reduction of its strength, compared with the other inlets, have been evident. Even the distance between the two vertical hole rows has been properly adjusted. In fact, the upper tangential stream flows between the two vertical rows on the opposed side and, if the available space is insufficient, it doesn’t remain adherent to the wall and the vortex is destroyed. In principle, a significant presence a very reactive hydrogen, can produce elevated temperatures and fast reaction, especially near fuel inlets. For this reason, inlet velocities are sufficiently high to generate a fast rotating vortex and then a rapid mixing. Further increase in fuel injection velocity has a negative influence on vortex temperature peaks. The aim of the LES simulation has been to analyze the unsteadiness of the system. The vortex appears very stable in the cavity, i.e. trapped. No vortex shedding has been noted. Compared to axial combustors, the minimum space required for the vortex results in an increase in volume and a reduction in power density. On the other hand, carbon monoxide content, with its slow chemical kinetic rate compared to natural gas, requires longer residence time and a bigger volume.
Fig. 5 - EICO (CO emission index) vs compressor ratio
It can be observed that high temperature zones (fig. 2) are concentrated in the vortex heart. If the hydrogen content is rapidly consumed, the carbon monoxide lasts longer and accumulates given that the amount of primary air is less than the stoichiometric value. The mean LES and the steady RANS fields have provided very similar results. It is interesting to evaluate the residence time inside the chamber. In the central part of the chamber, the residence time ranges from 0.02 to 0.04 sec. The establishment of a MILD combustion regime depends especially on a sufficient internal exhaust gas recirculation, which associated to a good degree of mixing with reactants, represents a necessary condition for MILD combustion. In order to quantify the degree of mixing, the following variable has been mapped:
MIX=|H2-H2mean|+|H2OH2Omean|+| CO2-CO2mean|+|O2- O2mean|+|CO-COmean|+|N2-N2mean|
If all the species involved were perfectly mixed, MIX should be zero everywhere. In practice, the more MIX tends to zero, the more reactants and products are well mixed. If the zones immediately downstream the inlets are neglected, MIX assumes very low values in the chamber. This supports the fact that the vortex is able to produce the expected results. The recirculation factor, i.e. the ratio between exhaust recirculated and fresh mixture introduced, is about 0.87, while exhaust composition is: 0.19% CO2, 0.05% H2O, 0.005% CO, 0.039% O2, 0.713% N2.
In order to determine where reactions are concentrated, it is useful to analyze radical species distribution, such as OH. The fact that radicals are not concentrated in a thin flame front, but well distributed in the volume (fig. 3), represents an evidence of a volumetric reaction regime.
Fig. 6 - EINOx (NOx emission index) vs compressor ratio
A sensitivity analysis has been conducted varying the boundary conditions around the references previousy reported in table 1. The quality of the different cases was judged in terms of pollutants emission indices (g pollutant/kg fuel), in particular for CO and NOx. CO is an indicator for incomplete combustion. Its presence in the exhaust is favored by low temperature and lack of oxygen. On the contrary, NOx are favored by high temperature and oxygen abundance. A 30% increment of the tangential flow velocity causes a faster rotation of the vortex, but the recirculation ratio increases only to 0.92. A 30% decrement of the equivalence ratio (leaner combustion) causes a reduction in temperature and a subsequent increment in unburnt species, especially CO. NOx emissions are in general unimportant for all the cases mentioned.
If the operating pressure is augmented from 1 bar to 10-20 bar at constant geometry and inlet velocities, the mass flow rates and then the burner power increases by about 10-20 times, even if the specific power density (MW/m3 bar) remains constant. The temperature and pollutants trends for those cases are reported in figures 4-6. The maximum and average temperatures in the chamber increase almost linearly, while EICO increases and EINOx decreases as pressure increases. For each operating pressure, it is then possible to identify an optimal equivalence ratio condition, at the intersection of the two curves in figure 7, where the major pollutants are kept down at the same time.