Keywords

1 Introduction

Accurate predictions of combustion and pollutant emissions are necessary to improve new engine gas turbine designs or to make prospective analysis of how a particular engine parameter may engine performance. Pollutants are strongly coupled to chemical kinetic effects, and therefore demand a high number of species and reactions to be quantified, which may result in too high computational efforts for typical Computational Fluid Dynamics (CFD) calculations. Reactor networks offer an optimum approach with a good accuracy in terms of pollutant predictions but at a much lower computational demand [1,2,3,4,5]. The present paper reports a modelling effort aimed at the development of a reactor network with Cantera [6], an open-source software tool, to enable predictions of the engine emissions indexes. This introduction is followed by a methodology section, where the main ideas around the network construction are described. After that, the main results for two engine operating conditions are presented and main conclusions are derived.

2 Methodology

The modelling workflow starts from the aeroengine geometrical and thermodynamic data, from which a zero- and one-dimensional (0D/1D) network is built. For every reactor, inlet temperature, pressure and composition must be defined, as well as the reactor volume for 0D reactors, or reaction area and length for 1D reactors. Both types are considered as ideal gas, constant pressure systems, with 0D being modelled as Perfectly Stirred Reactors (PSR) and 1D modelled as Plug Flow Reactors (PFR). Evolution of species is quantified by means of the corresponding balance equations, which include chemical kinetic terms. In the present case, Luche [7] chemical mechanism has been used to model the reaction rates, and the surrogate fuel is a mixture of decane, propyl-bencene and cyclo-hexane.

PSR and PFR reactors are the modules upon which the reactor network is built to emulate the combustion chamber of the aeroengine. Figure 1 shows a schematic of an annular combustion chamber of a conventional gas turbine engine, upon which the corresponding zones, which will be later considered as reactors, are superimposed. The center part of the chamber volume, which will be referred to as the chamber core, is mainly divided into three parts, namely Primary Zone (PZ), Intermediate Zone (IZ) and Dilution Zone (DZ), with corresponding rich, lean and very lean equivalence ratio ϕ values. To improve the description of the combustion phenomena, two additional zones are defined. The first one, still at the core, is the Flame Front (FF), with a similar cross-section to that of PZ and a close-to-stoichiometric equivalence ratio. The second one is the Wall Zone, with an important length and small cross-section, which is located around the core and mainly transports air. The latter zone is divided into two parts (WZ and WZ2) to better model the interaction between wall and core flows.

Fig. 1.
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Combustion chamber schematic for a generic aviation engine (a) and reactor network schematic for CFM56 7B27/B1F aeroengine (b).

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Figure 1 shows the layout of the developed reactor network. Reactors where equivalence ratio is lean have been modelled as PFR, to better capture CO evolution. Zones where equivalence ratio is rich and stoichiometric, where NOx are formed due to the high temperatures, are modelled by means of PSR reactors. The air addition into the burner, which occurs by means of the side orifices, is emulated in the reactor network by means of air flow additions to every reactor, which control the corresponding equivalence ratio value.

The model has been developed for the CFM56 7B27/B1F engine similarly to [8]. The present study focuses on engine operating conditions from the ICAO database [9], for which corresponding combustion chamber inlet conditions are summarized in Table 1. Fuel temperature has been assumed to be constant and equal to 400 K, while PZ is fed with a stream resulting from the adiabatic mixing of both fresh air and fuel. Geometrical data of each reactor has been summarized in Table 2, where the area of the core and wall reactors are shown to correspond to 80 and 20% of the total area, respectively. Lengths and areas for every zone are derived from the burner geometry except for the length of FF, which has been considered as a calibration parameter.

Table 1. CFM56 7B27/B1F engine operating conditions.
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Table 2. Reactor Network geometrical data.
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Table 3. Reactor Network calibration results.
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3 Results

The developed network is calibrated by optimizing some of the model parameters so that emission indices match the corresponding ICAO LTO test values at take-off conditions. These parameters are the fraction of air flowing into each of the reactor zones \({F}_{A,i}\), the relative flow split \({REL}_{\dot{m},PZFF}\) at the exit of PZ between the core and wall, and the relative flow split \({REL}_{\dot{m},WZIZ}\) at the exit of WZ1 between DZ and WZ2, and the length of the flamefront zone \({L}_{FF}\). Optimized results are reported in Table 3.

A local analysis of the reactor state is shown in Fig. 2 for two operating conditions, namely take-off and idle, where pollutant predictions are most and least accurate, respectively. Agreement for climb and approach (not shown for the sake of brevity) are similar to take-off. For both cases plot lines mimic the two paths followed by the flow along the combustion chamber, i.e. the CORE path including the main flow reactors PZ, FZ, IZ and DZ, and the WALL path including PZ, WZ, WZ2 and DZ. Results in Fig. 2 show that for take-off condition the equivalence ratio distribution is in agreement with the initial definition of the reactors, i.e. PZ corresponds to a rich equivalence ratio followed by a close-to-stoichiometric equivalence ratio at the FF, where the main reaction zone is located, and further downstream lean values for the remaining reactors are found. Corresponding reactor temperatures show that peak values are reached for stoichiometric conditions, i.e. at FF. For idle conditions, the larger air quantity reduces the global equivalence ratio, as well as those of the local reactors, even for PZ and FF. This behaviour brings down temperatures and has a major influence on pollutant formation, as will be discussed below. On the other hand, residence time values are lower for idle than for take-off conditions, due to the lower mass flow through the burner. Maximum residence times are found for WZ, due to the very small mass flow compared to the reactor size (only 1% of the total mass flow follows the wall path according to the calibration results in Table 3).

The previous combustion description has a direct influence on pollutant formation along the reactor network, which is analysed in Fig. 3 for both take-off and idle in terms of mass fraction of unburnt hydrocarbons (UHC), carbon monoxide (CO) and nitrogen oxides (NOx). DZ values are compared to the experimental values from ICAO [9]. Model predictions are in good agreement for take-off, as well as for climb and approach (not reported), while discrepancies are quite evident at idle. Results show that, for take-off condition, both UHC and CO appear in the first two reactors PZ, FF, with rich-stoichoimetric equivalence ratio values, and then disappear (note the very low numerical values) because oxidation is completed due to the high enough combustion temperatures and long enough residence times. As for the wall path, both species drop abruptly at WZ due to the combination of a lean equivalence ratio and long residence time, which makes it possible to reach an essentially zero value, preventing from the formation of these species at WZ2. Note that for take-off condition, with the engine at maximum power, numerical values agree with the reported experimental ones, i.e. essentially zero UHC,CO emissions. As for NOx, Fig. 3 shows that they are mainly formed at FF, with local equivalence ratio closest to stoichiometric values and peak reactor temperatures. This result highlights the governing role of the thermal mechanism in NOx pollutant production. Further downstream along the core path, NOx drops due to the addition of fresh air, which has both a dilution and cooling effect. The concurrent contribution of low equivalence ratio and combustion temperatures justifies the null contribution of wall reactors to NOx formation. Model predictions are pretty accurate in all three pollutant species.

For the idle operating condition UHC and CO have a similar evolution to that of take-off, with higher values at the PZ and FF that a later drop along the core path towards the final DZ value. Wall path also produces virtually no UHC and very low CO amounts. Prediction of both pollutants at DZ is higher for idle than for the take-off condition, in agreement with ICAO cycles, but experimental values at idle are not accurately reproduced by the model. At idle, all the reactors are operated under lean conditions, which results in lower UHC and CO formation, but also lower combustion temperatures occur. This prevents from a complete oxidation, especially when compared to take-off. Therefore, resulting values for both UHC and CO at the last reactor are higher than for take-off condition, where the balance between formation and oxidation is governed by the second process due to the high temperatures. Finally, NOx production at idle shows a very different behaviour compared to take-off. Production occurs at PZ, which corresponds to the closest to stoichiometric reactor and peak temperatures. Reduction process, however, freezes due to the lower combustion temperatures, and burner outlet values are slightly higher if compared to experiments.

Fig. 2.
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Local equivalence ratio, residence time and temperature for each of the reactors at two operating conditions, take-off (a, b, c) and idle (d, e, f).

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Fig. 3.
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Pollutant mass fraction (UHC, CO and NOx) for each of the reactors at two operating conditions, take-off (a, b, c) and idle (d, e, f). Experimental values from ICAO LTO cycle are shown on top of the DZ modelling results.

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4 Conclusions

A reactor network model has been developed for a gas turbine engine to predict pollutant emissions with chemical kinetic approach. The model creates a number of zones with different equivalence ratios corresponding to the burner primary zone, flamefront, intermediate, dilution and wall zones. It has been calibrated for the CFM56 7B27/B1F engine. Predictions of mission indices match experiments from ICAO LTO cycle for UHC, CO and NOx, over the whole power range. Divergences are evident only for idle conditions, where the model underpredicts both UHC and CO, while NOx is overpredicted. This behaviour hints at an excess of energy conversion to final products by the model, which does not occur in the actual engine operation, most probably due to the highly simplified approach. Discrepancies could also be due to the chemistry simplification or even the surrogate fuel formulation, which may be limiting for conditions where combustion rate diminishes due to the lower temperatures. Still, the model can be used to assess the engine operation with good accuracy and a reduced computational cost.