Small scale experimentation using particle image velocimetry investigated the effect of the diffusive injection of methane, air, and carbon dioxide on the coherent structures in a swirling flame. The interaction between the high momentum flow region (HMFR) and central recirculation zone (CRZ) of the flame is a potential cause of combustion induced vortex breakdown (CIVB) and occurs when the HMFR squeezes the CRZ, resulting in upstream propagation. The diffusive introduction of methane or carbon dioxide through a central injector increased the size and velocity of the CRZ relative to the HMFR whilst maintaining flame stability, reducing the likelihood of CIVB occurring. The diffusive injection of air had an opposing effect, reducing the size and velocity of the CRZ prior to eradicating it completely. This would also prevent combustion induced vortex breakdown CIVB occurring as a CRZ is fundamental to the process; however, without recirculation it would create an inherently unstable flame.
The depletion of fossil fuels and concern about the climate change have led to the development of new technologies to meet power generation demand, whilst maintaining security of supply and decreasing the environmental impact. The use of gas turbines, a well-developed technology, fired on nontraditional fuels, is an increasingly viable method for producing energy in the short to medium term. Fuels that can be used for this purpose range from those based on highly enriched hydrogenated blends to those that are produced from biomaterials [
In order to reduce the emission of nitrogen oxides (
Two of these mechanisms, that is, boundary layer flame propagation and upstream propagation of coherent structures, have been studied by several groups using natural gas. However, the use of unconventional fuels can be extremely detrimental to the control of these phenomena, and very little literature is available on this subject. High turbulence levels, one of the very useful features of swirling flow because of mixing potential, affect flashback limits detrimentally due to effects on turbulent flame speed (
Some authors [
Dam et al. [
In terms of unconventional fuels, the primary goal of introducing CO2 into the gas turbine combustor is to reduce the emissions of
Lee et al. [
This study focuses on, but is certainly not limited to, flames that require a pilot to maintain stability. The injection of CO2 is expected to augment the size and intensity of coherent structures whilst increasing mass flow rate through the exit nozzle, in much the same way methane pilot does, in such a way that CIVB is inhibited. The effect that the diffusive injection of air on stability will also be investigated and compared.
A swirl burner constructed from stainless steel was used to examine the flame structure at atmospheric conditions (1bar, 293 K) at Cardiff University’s Gas Turbine Research Centre (GTRC). External and sectioned views of the generic burner are presented in Figure
(a) External view and (b) sectioned schematic of the generic burner.
A single tangential inlet (a) feeds the premixed air and fuel to an outer plenum chamber (b) which uniformly distributes the gas to the slot type radial tangential inlets (c) which impart the swirling momentum on the premixed flow. Swirling premixed air and fuel then pass into the swirl chamber (d) and then into the exit nozzle (e). The central diffusion fuel injector (f), through which nonpremixed gases are introduced to the combustion zone, extends centrally through the combustor body to the exhaust.
The geometric swirl number (
During these trials the geometry was kept constant, so the only change in geometric swirl number was caused by the injection of gases through the pilot injector.
The system was fed using compressed air through flexible hoses and a Coriolis meter for flow rate metering. Bottled methane was used as main fuel at a constant flow rate during the trials, fed through flexible hoses passing through another Coriolis meter. A Dantec PIV (particle image velocimetry) system consisting of a dual cavity Nd: YAG Litron Laser of 532 nm capable of operating at 15 Hz, a Dantec Dynamics laser sheet optics (
After acquisition of the PIV data, a frame-to-frame adaptive correlation technique was then carried out with a minimum interrogation area of
The cross sectional area and velocities (
Designation of areas within the flame.
Varying amounts of carbon dioxide, air, and methane were injected through the diffusive pilot of the AGSB to assess the effect on flame structure. The three gases were selected for their differing combustion properties: methane which is often used as a pilot fuel in gas turbines and will increase the global equivalence ratio of the flame, global equivalence ratio being that of the premixed air and methane, and the carbon dioxide which does not affect oxidant-to-fuel ratio but has been shown in previous studies to alter flame conditions [ air which will reduce the flames global equivalence ratio.
In order to allow comparison between results, premix flow rates of 0.2 g/s methane and 3.44 g/s air were kept constant; these equate to a 10 kW, stoichiometric mixture. Details of the test points are displayed in Table
Details of AGSB test points 1–26.
Test point | Flow rates (g/s) | Φ |
Total power |
Isothermal |
Bulk exit velocity |
Re |
AFT | ||
---|---|---|---|---|---|---|---|---|---|
Pilot |
total | ||||||||
Methane | 1 | 0.000 | 3.640 | 1.00 | 10.0 | 1.05 | 4.88 | 4361 | 1953 |
2 | 0.015 | 3.655 | 1.07 | 10.7 | 1.04 | 4.90 | 4379 | 1928 | |
3 | 0.030 | 3.670 | 1.15 | 11.5 | 1.02 | 4.92 | 4397 | 1875 | |
4 | 0.045 | 3.685 | 1.22 | 12.2 | 1.01 | 4.94 | 4415 | 1816 | |
5 | 0.059 | 3.699 | 1.30 | 13.0 | 0.99 | 4.96 | 4433 | 1760 | |
6 | 0.074 | 3.714 | 1.37 | 13.7 | 0.98 | 4.98 | 4451 | 1702 | |
7 | 0.089 | 3.729 | 1.45 | 14.5 | 0.97 | 5.00 | 4468 | 1645 | |
8 | 0.104 | 3.744 | 1.52 | 15.2 | 0.95 | 5.02 | 4486 | 1589 | |
|
|||||||||
Air | 9 | 0.000 | 3.640 | 1.00 | 10.0 | 1.05 | 4.88 | 4361 | 1953 |
10 | 0.100 | 3.740 | 0.97 | 10.0 | 1.00 | 5.02 | 4482 | 1924 | |
11 | 0.201 | 3.841 | 0.94 | 10.0 | 0.95 | 5.15 | 4602 | 1902 | |
12 | 0.301 | 3.941 | 0.92 | 10.0 | 0.90 | 5.29 | 4722 | 1875 | |
13 | 0.402 | 4.042 | 0.90 | 10.0 | 0.86 | 5.42 | 4843 | 1846 | |
14 | 0.502 | 4.142 | 0.87 | 10.0 | 0.82 | 5.56 | 4963 | 1817 | |
|
|||||||||
Carbon dioxide | 15 | 0.000 | 3.640 | 1.00 | 10.0 | 1.05 | 4.88 | 4361 | 1953 |
16 | 0.025 | 3.665 | 1.00 | 10.0 | 1.04 | 4.92 | 4391 | 1931 | |
17 | 0.050 | 3.690 | 1.00 | 10.0 | 1.03 | 4.95 | 4421 | 1923 | |
18 | 0.074 | 3.714 | 1.00 | 10.0 | 1.03 | 4.98 | 4451 | 1914 | |
19 | 0.099 | 3.739 | 1.00 | 10.0 | 1.02 | 5.02 | 4481 | 1906 | |
20 | 0.124 | 3.764 | 1.00 | 10.0 | 1.01 | 5.05 | 4510 | 1897 | |
21 | 0.149 | 3.789 | 1.00 | 10.0 | 1.00 | 5.08 | 4540 | 1889 | |
22 | 0.174 | 3.814 | 1.00 | 10.0 | 0.99 | 5.12 | 4570 | 1881 |
The experimental method was the same for all three gases: the flame was lit and 150 images were recorded with the PIV system, and the flow rate of diffusion gases was then increased in regular intervals, with a further 150 images recorded at each interval, until the flame reached a point of quasistability. In order to maintain inlet and outlet conditions between test points, the flame was left alight.
The PIV results in Figure
Scalar plots of TP 1 to TP 8 in the AGSB as detailed in Table
The flow in test point (TP) 1 has the highest velocities at 0.25
TP 1 has a very narrow, low velocity, CRZ. Within the recirculation zone the maximum value of
The alteration in shape becomes further exaggerated as the flow rate of pilot methane increases through test points 3 to 8. With each increase in the flow rate of diffusive methane the width of the reverse flow region increases, as do the negative axial and total velocities of the gases within them. The injection of methane reduces the peak velocity of the reacting flows in the HMFR; however, the shape and overall velocity profiles in these regions remain largely unaffected.
The diffusive injection of air has a markedly different effect on the flow structure than was observed with methane. PIV images in Figure
Scalar plots of TP 9 to TP 14 in the AGSB as detailed in Table
The flow structure and velocities of TP 9 are very similar to those observed in TP 1; this is to be expected as the premixed flow rates are the same. The structure of TP 10 however is very different to that of TP 2. Rather than increasing with diffusive flow rate, the width of the recirculation zone is unaltered from TP 9. Although total velocity is also unaltered, negative axial velocity is actually reduced.
The total and axial velocities in the HMFR of TP 11 are considerably lower than in TPs 9 and 10. The flame is also displaying significant asymmetry, with the low velocity recirculation zone positioned left of of the central axis of the burner. The increased addition of air continues to reduce the velocity of the flow within the CRZ and the HMFR in TP 12, whilst the asymmetry becomes more exagerated. The diffusive flow rate in TP 13 has caused an almost complete reduction of the CRZ, although a region exists where axial velocities are just below 0. In TP 14 the CRZ no longer exists, with no flow in the negative axial direction. Instead the CRZ is replaced by a central region of high positive axial velocity, on the right of the central burner axis as shown. The likely cause of the CRZ destruction is combustion inside of the CRZ, causing an expansion of gases along the central axis of the burner. This gradually reduces the pressure differential that is required in order to induce recirculation.
The effect of reduced swirl number may also contribute to the results seen in test points 13 and 14, where the
Carbon dioxide has a very similar effect of flow structure to methane; as a result, the progression caused by its diffusive injection between TP 15 to TP 22, which is shown in Figure
Scalar plots of TP 15 to TP 22 in the AGSB as detailed in Table
Test points 15 through 22 are displayed in Figure
In TP 16 it can be seen that the CRZ increases in width as carbon dioxide is diffusively injected, and the total and axial velocities within the CRZ are also increased. With methane this increase in velocity coincided with an elongation of the recirculation, with the region of reverse flow approaching the burner exit; with carbon dioxide this was not observed. A reduction in velocity in the HMFR is evident between TP 15 and TP 16.
Between TP 16 and TP 22, the increase in diffusive CO2 flow rate results in an increase in the size of the CRZ, and axial and total velocities within it also increase, with the reverse flow region propagating slightly upstream. This reduction is expected as the dilution effect of the CO2 will reduce flame temperatures and the expansion of the gas. The observed reduction in axial velocity is less pronounced than the reduction in total velocity, which suggests that radial velocity is reduced significantly.
When CO2 is diffusively injected, the low velocity boundary that exists between the CRZ and the HMFR appears to be wider when CH4 is injected. This is particularly evident when comparing vector plots of TP 5, with methane injection and TP 19, with carbon dioxide injection, which are shown in Figure
Vector plots of TP 5 and TP 19 in the AGSB with altered premix air flow rate and pilot CO2 flow rate as detailed in Table
When all gases entering the burner are introduced tangentially, they are considered to be fully mixed prior to entering the combustion zone. With diffusive injection, mixing is forced to occur in the region of the burner exit, resulting in unequal mixtures within the combustion zone.
Simulation with CHEMKIN using the GRIMech 3.0 mechanism [
The increase is in both size and mean velocity caused by CO2 injection are not as prominent as with methane, and the CRZ has a less well-defined shape as an irregular flame front develops at the boundary between it and the HMFR, as is reflected in the level of the turbulent intensity. Figure
Comparison of turbulent intensity between flames in TP 5 and TP 19.
And
The flame with diffusive methane injection has a very low turbulent intensity CRZ
The velocities in the methane injected flame are greater than those of the carbon dioxide injected flame, which has to be considered when comparing turbulent intensity, which is dependent on local velocity. When comparing the root-mean-square of the turbulent velocity fluctuations in the axial-radial plane, the mean turbulent kinetic energy levels in the CRZ boundaries of the methane and carbon dioxide flames are actually very similar,
The axial velocity profiles at
Axial velocities at
However, the axial velocity profiles of the flames with CO2 injection, test points 15 to 22, display the same behaviour as observed with the injection of CH4. The diffusive injection causes the positive axial velocities in the HMFR to decrease, whilst the negative axial velocities in the CRZ increase. There is also evidence of the CRZ becoming wider as the value of
The amount of stabilisation provided by the CRZ can be assessed by the reverse flow momentum (RFM) of the gases within it. Momentum is a product of velocity and mass, as the temperature, and as a result, density of the gases within the CRZ and the reverse flow momentum are unknown. The volume of a particular measurement point is fixed; therefore, if variations in density are ignored, it is possible to compare the flow momentum by comparing velocity. As such, changes in reverse flow momentum (RFM) at
The RFM of each test point is plotted against the isothermal geometric swirl number in Figure
The (a) reverse and (b) positive flow momentum for all test points at
The increased diffusive injection of methane, which reduces
The complete alteration in structures caused by the introduction of air means that there are two effects on the PFM. The gradual destruction of the CRZ means that the main reaction zone expands, and the velocity in the HMFR reduces. Conversely, the destruction of the CRZ means that the regions of positive flow increases; the velocity in these regions will also increase due to the axial momentum of the injected air.
The radial velocity profiles at
Radial velocities at
Coupled with the decrease in axial velocities these results suggest that the level of swirl has increased, which is why the RFM has increased relative to the PFM. The different effect is observed with the diffusive injection of CO2; the radial plots for test points 15 to 22 are shown in Figure
Figure
The normalised effect of the injection of carbon dioxide, methane, and air on (a) cross sectional area and (b) mean velocity of the entire CRZ and (c) mean velocity of the CRZ centre.
This increased velocity is demonstrated in Figures
Another structure in a swirling flame is the high momentum flow region; this is high velocity region which contains the main reaction zone of the flame. In this study, it is defined as being the region where axial velocity exceeds 3 m/s.
The effects of pilot injection on the size and mean velocity in this region are shown in Figure
The normalised effect of the injection of carbon dioxide and methane on (a) the cross sectional area and (b) mean velocity of the HMFR.
Between
There are four mechanisms that may result in flame flashback [
Previous studies have demonstrated that CIVB can be the result of the CRZ being “squeezed” by the HMFR, which causes the CRZ to be displaced, upstream, ultimately surrounding a central body in the combustor [
Regardless of equivalence ratio changes, the interactions between the CRZ and HMFR flow structures are important factors relating to the initiation of CIVB, and this interaction may be altered by the diffusive injection of gases.
For the results taken in these trials the interaction between the CRZ and HMFR is assessed as described in (
The full size of the HMFR is cropped by the measurement area. However, local changes in the measured area are indicative of global changes. The increasing value of
Normalised effect of the injection of carbon dioxide and methane on the ratio between RFM and PFM.
The differing effects of CO2 and CH4 on the CRZ, and in particular the HMFR, compensate for each other to produce a very similar response on the flame structure as a whole. The introduction of methane causes a large initial rise in
Carbon dioxide produces a near linear response over the range of results, with the highest value of
Experimentation using a generic swirl burner and PIV system demonstrated how the diffusive injection of different gases into a premixed methane flame altered its coherent structures.
The injection of both methane and carbon dioxide resulted in an increase in cross sectional area and velocity of gases within the central recirculation zone whilst maintaining flame stability. For the unconfined flame, diffusive injection of methane increased the burning rates and temperatures within the swirling flows, effecting flame propagation and hot gas expansion. Radial momentum increased whilst axial momentum decreased, increasing the volume of the recirculated gases when methane was injected. This leads to an expansion of the recirculation zone near the burner exit and a stable, well-defined boundary between the CRZ and high momentum flow region. The injection of carbon dioxide reduced the burning rate and temperature of gases within the flame, inhibiting flame propagation and reducing the hot gas expansion. Both axial and radial momentums were proportionally reduced, so the increased mass flow rate increased the size and velocity of the recirculation zone.
A differing effect on the high momentum flow region of the flame was seen. When flow rates of the diffusively injected gases produced isothermal geometric swirl numbers above 1.02, the size and velocity of the HMFR decreased, due to CH4 increasing the level of swirl and CO2 decreasing flame temperatures. When methane flow rates resulted in
Despite differing effects of the level of swirl and flame temperature, the actual effect on the RFM of the CRZ compared to the PFM of the HMFR was the same. The expansion of the CRZ compared to the HMFR opposes the changes in flame structure that result in combustion induced vortex breakdown; therefore, the diffusive injection of either CH4 or CO2 could be used as a measure to mitigate CIVB.
Tests were also performed with the diffusive injection of air. Its effect on the flow structures heavily contrasted those of CO2 and CH4, reducing the size and velocity of the CRZ before completely preventing recirculation occurring. Although the overall result would almost certainly reduce the likelihood of CIVB occurring, the destruction of the CRZ would result in an inherently unstable flame that would be highly susceptible to blow off.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was partly funded and supported via the European Union FP7 Project H2IGCC, and the ERDF funded Low Carbon Research Institute.