Experimental Observation of Lifted Flames in a Heated and Diluted

Article pubs.acs.org/EF

Experimental Observation of Lifted Flames in a Heated and Diluted Coflow Paul R. Medwell* and Bassam B. Dally School of Mechanical Engineering, The University of Adelaide, S.A. 5005, Australia ABSTRACT: Lifted flames have attracted significant research over many years due to the importance of fundamental understanding of the stabilization mechanism. More recently, lifted flames in a hot coflow have been used to investigate autoignition properties of jet flames. Several experimental and numerical studies to predict the autoignitive liftoff characteristics of a jet in a vitiated coflow have recently appeared in the literature. The configuration of a jet issuing into a heated and diluted oxidant stream can also emulate the fundamental operating conditions of MILD combustion. This paper investigates similarities and differences of the liftoff behavior of jet flames under a wide variety of coflow oxidant conditions, ranging from autoignitive conditions to MILD combustion. The liftoff behavior is observed to not be monotonic with either coflow temperature or oxygen content. The results clearly indicate that there is a fundamental transition in the stabilization mechanism, depending on the oxidant stream properties. apparent is the occurrence of lifted flames between normal flames and MILD combustion. There is, therefore, wide interest in the understanding of lifted flames in a heated oxidant stream for a range of applications. 1.2. Hot and Diluted Coflow Configurations. Practical combustors often employ recirculation of hot combustion products to achieve flame stabilization,4 such as in gas turbines, automotive engines, furnaces and also to achieve MILD combustion.9−12 The complex recirculation patterns within such systems are unsuitable for the development of fundamental understanding. In addition to the more complicated flow patterns, it becomes impossible to separate products produced by the reaction from those recirculated back from a downstream location. There are various approaches that can be used to emulate key features of recirculation, while eliminating the complexities encountered in practical systems. One common approach is with counter-flowing fuel/oxidant streams, where one of the streams is diluted with an inert (e.g., ref 13), which is advantageous for simplified modeling. In the context of turbulence-chemistry interaction, however, jet flows have greater practical significance. Simulating recirculation in a jet flame may be achieved with a fuel jet that issues into a coflowing oxidant stream of combustion products, with a composition resembling that encountered in an actual recirculating system. This configuration has been used for two distinctly different types of studies: one for investigating autoignitive lifted flames and the other for investigating MILD combustion. 1.2.1. Lifted Jet Flames. Lifted jet flames in a hot and diluted coflow (often referred to as a vitiated coflow) has been used for the study of autoignition processes.6,14 If the temperature of the coflow exceeds the autoignition temperature, then this provides a stabilization mechanism for such lifted flames. Other similar

1. INTRODUCTION 1.1. Lifted Flames. Lifted flames have been widely studied because they reveal fundamental understanding of flame stabilization mechanisms. Many theories governing lifted flames have been covered previously, as summarized by Lyons1 and Lawn,2 and the related field of ignition by Mastorakos.3 The study of lifted flames in a heated and diluted (vitiated) coflow, notably by Cabra et al.,4,5 has renewed interest and further developed understanding and modeling of flame stabilization mechanisms under these conditions. Flames under hot and low oxygen conditions exhibit different behavior than those in conventional combustion6 and are of importance for many practical systems that incorporate recirculation of hot combustion products as a means of improving flame stability. Furthermore, these conditions may also be used to emulate MILD combustion.7 Figure 18 illustrates the operating regimes of flames in heated and diluted oxidant conditions, in terms of reactant temperature and oxygen concentration. Clearly

Received: June 18, 2012 Revised: August 23, 2012 Published: September 5, 2012

Figure 1. Combustion regime diagram.8 © 2012 American Chemical Society

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approaches have been used for the study of autoignition, such as with an unavoidably heated fuel jet within a heated air coflow,15 and an equi-temperature coflowing jet of diluted fuel and high temperature air.6 Nonetheless, the jet in a vitiated coflow burner (VCB, shown in Figure 2)4,5 has seen the most significant recent interest in the study of autoignition.

Figure 3. Jet in hot coflow (JHC) burner of Dally et al.35−37

3) to emulate MILD combustion, enabling a range of combustion parameters to be varied independently and also decoupling the flow from the chemical kinetics. The JHC burner has been used for both experiments35−38 and modeling.39−47 A similar burner has also been used for combined experiments48,49 and modeling.50 Under certain MILD conditions, the combustion reaction can occur with no (or low) flame luminosity (hence another term for MILD combustion is flameless oxidation, FLOX51). In the context of the JHC configuration, when the jet flame occurs in the confines of the coflow, it is also possible to produce a reaction that is not visible. Beyond the controlled conditions of the coflow, entrainment of surrounding ambient air causes the flame appearance to begin to resemble conventional conditions. This combination of an ‘invisible’ zone followed by a typical luminous flame can make a flame appear lifted, though this may not necessarily be the case. Furthermore, low concentration of the OH radical in the MILD region can also indicate a lifted flame, though precursor reactions have been identified providing evidence that a reaction does occur, albeit different to conventional flames.37,52,53 As a result of the precursor reactions and an OH tail that is seen to extend to the jet exit plane, Medwell et al.37 referred to these flames as transitional rather than lifted. Importantly, “liftoff” height in these transitional flames has been observed to decrease with an increase in jet velocity,37,48,49 a trend that provides further evidence that these flames are inherently very different to conventional lifted flames. The transitional behavior of MILD combustion is particularly important in the establishment of this regime. In the transition from conventional combustion to MILD combustion, an unstable regime is encountered, referred to as region B by Wünning and Wünning.51 Furthermore, the transition from MILD combustion to no combustion is gradual, and so, it can be difficult to visually identify the difference between the MILD regime and no reaction without in situ measurements.13 It is this transition to the MILD regime (either from conventional combustion or from no combustion) that remains relatively poorly understood, and where unexpected flame behavior is observed. A key parameter in MILD combustion is known to be the level of mixing. In fact, Kumar et al.25 claim that flame liftoff is an essential requirement for MILD combustion. Similarly, Choi et al.6 reported that in MILD combustion the flame does not attach at low jet velocities. It is important, however, to note that these claims are critically dependent on the design of the

Figure 2. Vitiated coflow burner (VCB) of Cabra et al.4,5

The VCB emulates the coupling of turbulent mixing and chemical kinetics in reacting recirculating flows,4 such as those encountered in gas turbines and furnaces where there is a recirculation of hot combustion products.16 Conventional lifted flames in ambient temperature air may be classified as bimodal: the flame is either lifted or not lifted. In contrast, for hydrogen flames in a heated coflow, it is possible to have conditions ranging from mixing only to fully reacting.4 For methane/air in the jet, a bimodal behavior has been reported;5 importantly, however, this difference is believed to be an artifact of the measurement technique (such that the transient reaction zone intersects the probe volume differently due to the differences in the magnitude of the fluctuation of the liftoff height), rather than a fundamental difference due to the fuel type.5 From these observations, it has been postulated that the absence of a bimodal behavior is suggestive of either a distributed reaction zone or autoignition.5 Subsequent work by Gordon et al.14,17,18 in the VCB burner has indicated that autoignition is the main stabilization mechanism of lifted flames in this configuration, as indicated by a build-up of radicals prior to ignition. This conclusion, however, was based on a limited range of operating conditions. A particularly interesting feature observed in the VCB configuration is an extreme sensitivity of the liftoff height to the coflow temperature. For hydrogen flames, a 1% (10 K) change in the coflow temperature can double the liftoff height.16 For methane/air flames, a 5% coflow temperature change was observed to roughly double the liftoff height.5 As a result of this extreme sensitivity, this configuration has been extensively studied as a modeling “test case”;9,14,16−34 however, in all cases, the coflow conditions have been limited to the narrow range encountered in the VCB experimental data. 1.2.2. MILD Combustion. The hot and diluted oxidant coflow of the VCB resembles, though does not fully achieve, the conditions required for MILD combustion. Dally et al.35 developed the jet in hot coflow burner (JHC, shown in Figure 5520


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and Gordon et al.14,18) and those under MILD combustion conditions (such as Medwell et al.36−38 and Oldenhof et al.48,49). For the VCB burner experiments that showed an extreme sensitivity of liftoff height to temperature, the coflow temperature and oxygen concentration were dependent on the fuel, but in all cases, they were higher than those for MILD combustion. A list of some of the key experiments with a jet issuing into a heated coflow are in Table 1 (jet conditions) and

burner. Adequate mixing may be achieved via other techniques, which do not rely on high jet velocities. Nonetheless, it is clear that effective mixing is an important parameter, and that flame liftoff may play an important role in establishing MILD combustion. 1.2.3. Counter-flow Flames. The counter-flow configuration provides a simplified geometry for developing fundamental understanding of the phenomenon governing both lifted jet flames and MILD combustion. The counter-flow configuration has been employed extensively for the investigation of autoignition of flames in a hot environment.54−63 Fundamental understanding of MILD combustion has also been reported in the counter-flow configuration, for example a widening of the flammability limits when the oxidant stream temperature is sufficiently high.64 Highlighting the capabilities of this configuration, it is further reported that the characteristic S-shaped curve typically associated with ignition/extinction becomes monotonic under MILD conditions.13,64,65 A related observation is that, under conventional combustion conditions, extinction is sudden when the oxygen mole fraction or temperature is decreased below a critical value, whereas above a certain oxidant temperature extinction does not occur, even for very low fuel concentrations.64 These findings can be directly translated into observations in MILD combustion systems, but they can be analyzed in greater detail from the counter-flow studies. 1.2.4. Autoignition Studies. The various configurations presented thus far (viz. lifted jet flames in the VCB burner, MILD combustion in the JHC burner, and counter-flow flames) have incorporated aspects of autoignition. Markides et al.15 have specifically designed a ‘confined turbulent hot coflow’ (CTHC) burner for the study of autoignition.15,31,66−70 This burner is particularly well suited to investigating autoignition, and for model validation, though other configurations may be more suitable for examining other combustion features. Nonetheless, this configuration is similar to that used for lifted flame studies.6,71 1.3. Lifted Flame Summary. Combustion of fuel in a hot environment (typically with a reduced oxygen concentration) is practically significant as a mechanism for increased flame stability. A high temperature oxidant stream greatly increases the stability, so much so that, despite the loss of oxygen associated with hot product dilutions, strain rates can be sustained at least an order of magnitude higher than with unheated and undiluted reactants.64 The ignition of a fuel is more dependent on the temperature conditions, rather than the reactants,56 hence the increased stability under heated conditions. In the heated oxidant stream configuration, autoignition is the dominating stabilization mechanism. Gordon et al.18 proposes a three-step process in lifted flames: build-up of precursor pool (such as CH2O), initiation of reaction (peak of OH), and finally a steady flame (consumption of CH2O, steady OH). This is very similar to that observed under MILD conditions, with some subtle but significant differences. In the imaging work of Medwell et al.,37 it was also noted that CH2O is located well ahead of the flame-front, but in contrast, the OH radical was measured all the way to the jet exit plane, including in the “lifted” region. Thus, it is not clear whether it is appropriate to describe flames under MILD combustion conditions as lifted in the conventional sense. It is important to note the differences in the oxidant stream properties for the lifted flame studies (such as Cabra et al.4,5

Table 1. Summary of Jet Flames in a Heated and Diluted Coflow Experiments 35

Dally et al. 2002 Cabra et al. 20024 Cabra et al. 20055 Medwell et al. 200736 Medwell et al. 200837 Oldenhof et al. 201048 Oldenhof et al. 201149

fuel (jet)

Rejet

djet (mm)

CH4/H2 H2/N2 CH4/air natural gas/H2 C2H4/N2/H2/air natural gas natural gas

9482 23 600 28 800 5000−15 000 10 000 3000−9500 2500−8800

4.25 4.57 4.57 4.6 4.6 4.5 4.5

Figure 4. Summary of coflow conditions of jet flames in a heated and diluted coflow experiments.

Figure 4 (coflow conditions). It is reported that with a higher temperature (≳1300 K) and oxygen concentration flames become lifted, whereas under MILD conditions (characterized by low oxygen), the flames tend to stabilize at the jet exit plane. From Figure 4, it is observed that no single study has yet broadened the operating conditions to enable comparison and transition from one experiment to another. Furthermore, within each operating window, different phenomenon are observed. For example, Oldenhof et al.48,49 and Medwell et al.37 measure a reduction in liftoff height with an increase in jet Reynolds number, though this was not observed by Cabra et al.4,5 Oldenhof et al.48 identify a clear distinction between lifted flames in cold air and flames in a vitiated coflow, which have been identified as transitional by Medwell et al.37 To address this issue, Oldenhof et al.48,49 analyzed liftoff in terms of appearance of ignition kernels and showed that the mean liftoff height is related to the probability of burning. Among the existing studies, there are significant differences in the fuel types and Reynolds number (Table 1), and there is no overlap in the 5521


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coflow properties (such as temperature or oxygen level); it is therefore impossible to make any conclusive comments about the transition to MILD combustion and the general behavior of flames under these conditions. The aim of the current study is to extend the operating regime of the coflow to independently assess the effects of the coflow temperature and oxygen concentration, using a single burner configuration. This study, therefore, provides a first step toward providing greater understanding of the transition between lifted flames and MILD combustion and bridging the gaps between the existing discrete studies in the operating domain.

2. EXPERIMENTAL OUTLINE Figure 5. Flame photographs for (a) short exposure time and (b) long exposure time. (c) CH* chemiluminescence image, shown as inverseintensity (i.e., black coloration indicates highest CH* emission). Images represent approximately 300 mm and 150 mm above the jet exit plane for the photograph and CH* images, respectively.

The burner in this study is a jet in hot coflow (JHC) burner, used previously.35−37 In brief, the JHC burner consists of a central insulated fuel jet (i.d., 4.6 mm) within an annular coflow (i.d., 82 mm) of hot exhaust products from a premixed secondary burner mounted upstream of the jet exit plane. In this study, the coflow temperature and O2 level are controlled independently by varying the inlet composition to the secondary burner (a mixture of natural gas, hydrogen, air, and nitrogen). The O2 level of the coflow is defined as the excess O2 from the secondary burner. It is known that the extent of the coflow is ∼120 mm downstream of the exit plane.35,36 In this region, the jet oxidant stream is controlled entirely by the excess O2 level from the secondary burner. Beyond ∼120 mm downstream, the jet and coflow mix with the surrounding air. Unless stated otherwise, the coflow velocity remains constant at 2.8 m/s, and the volumetric ratio of hydrogen to natural gas in the fuel stream of the secondary burner is 1.3:1. The O2 level in the coflow is controlled by balancing the ratio of air and nitrogen while maintaining constant fuel flow rates. In this way, the concentrations (molar basis) of H2O and CO2 are fixed at 10% and 3%, respectively. The temperature distribution across the coflow stream is uniform, as demonstrated previously.36,37 For each coflow condition, the jet Reynolds number is varied. The minimum jet Reynolds number is ∼100. The flame liftoff height is measured using both flame photographs taken with a digital still camera (with a range of exposure times) and CH* chemiluminescence imaged with an intensified CCD (ICCD) camera through a 430 nm (10 nm bandwidth) optical filter. It should be noted that in this study the liftoff height is based on visual observation and analysis of the photographs and CH* chemiluminescence, rather than an arbitrary threshold as used in previous studies (such as in ref 4). Although this approach is subject to some interpretation, due to the wide range of flame conditions encountered, it is not possible to define a single value related to luminosity that accurately delineates the flame liftoff location. Since different methods are used to estimate the liftoff height and there is some degree of uncertainty, the results presented in this paper show a range of liftoff heights (presented as errorbars). This enables a clear set of trends to be observed that could otherwise be masked, or inaccurately interpreted, if only a single threshold was used for determining the liftoff height. Despite the low luminosity of flames in a hot and diluted coflow oxidant, it is still possible to distinguish a reaction. While there is typically a point in the flame where the reaction zone becomes very clearly apparent, upstream there is an identifiable reaction that takes place with low luminosity. For this reason, we have previously described such flames as transitional rather than lifted. Depending on the measurement approach, the precise location of the transition point is difficult: nonetheless, there is a clear change that occurs at this transition height. In contrast, below a certain point in some of the flames, there is no evidence of any reaction. The reported liftoff height covers the range between the point where the flame is unequivocally visible and the lowest point where a reaction zone can be identified, as indicated in Figure 5.

3. RESULTS Figure 5a and b shows typical photographs of the flames studied in this paper. Specifically, Figure 5 presents a natural gas flame (Rejet = 15 000) issuing into a 1600 K coflow at 3% O2 concentration. From the photographs, it is apparent that the flame luminosity is very low. Importantly, the effects of the coflow stream are known to persist for a height above the jet exit plane, z ≲120 mm. Within this range, the oxidant stream composition is well-controlled by the products of the secondary premixed burner. Beyond z ≳120 mm, mixing with surrounding air occurs, and thus, the composition of the oxidant stream is no longer controlled. The liftoff height is determined from the distance between the identifiable base of the flame above the jet exit plane. Due to the very low luminosity, it is somewhat difficult to accurately identify the visual liftoff height. This is particularly problematic in the low temperature and/or low oxygen cases. In these flames, the jet flame luminosity is very low and of a similar order to the equilibrium products from the coflow. In such cases, it is very difficult to say whether or not the flame is or is not lifted. Furthermore, the features identified in flame photographs are dependent on the exposure. Parts a and b of Figure 5 are taken with exposure times of 0.5 and 4 s, respectively. A brief look at Figure 5a may indicate a much larger liftoff height than the actual liftoff height as apparent from Figures 5b, where it is not completely clear that the flame is lifted at all. As such, it is clear that there is some level of ambiguity over the determination of the exact liftoff height. Nevertheless, it is possible to identify a range over which the flame base clearly exists. To assist in the identification of the flame base beyond visual observations, CH* chemiluminescence images, such as those shown in Figure 5c, are also used. Even with the assistance of the intensified camera used to collect the CH* images, there is some variation in what may be considered the liftoff height. Nonetheless, the results from either the photographs or CH* images tend to both indicate very similar liftoff heights. As a result of the variation in the identifiable liftoff height, a range of liftoff height estimates is obtained from both the photographs and the CH* images. This variation is shown in the plots of liftoff height as errorbars, though they should be considered a range of uncertainty and not an inherent error with the measurements. Based on the range of liftoff height 5522


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estimates from both the photograph and CH* images, the mean liftoff height is plotted. The actual liftoff height is likely closer to the lower estimate of the range presented. The upper range presented indicates the unequivocal maximum liftoff height. Defining a range of identifiable liftoff height is preferred over the previous approaches where a (somewhat arbitrary) cutoff is applied because it takes into consideration the smooth transition that these particular flames possess. As discussed previously,37 flames issuing into a heated and diluted coflow are not lifted in the conventional sense: applying conventional thresholding techniques are therefore not considered appropriate. 3.1. Effect of Coflow TemperatureNatural Gas Flames. 3.1.1. 3% O2 Coflow Results. Figure 6 shows the Figure 7. Liftoff height as a function of jet Reynolds number for natural gas jet flames within a 6% O2 coflow at various temperatures.

temperatures with the O2 level in the coflow to 6% (as compared to 3% for Figure 6). In the 3% O2 coflow case, the minimum temperature required to stabilize a natural gas flame was 1300 K. Increasing to 6% O2 enables a stable flame at 1100 K. Nonetheless, as was observed at 3% O2, increasing the temperature beyond the minimum leads to an increase in the liftoff height. At 6% O2, the liftoff height is maximum for a coflow temperature around 1300−1400 K. Further increasing the temperature subsequently reduces the liftoff height. In both Figures 6 and 7, the effects of the Reynolds number are generally relatively minor. The 6% O2, the 1300 K flame shows a steep change in liftoff at around Rejet = 4000. At 6% O2, 1300 K corresponds to the greatest liftoff height, suggesting the “least reactive” flame. The transition to turbulence at around the Rejet = 4000 seems to trigger a marked increase in the liftoff height for this flame. Indeed, the sharpest increase in the liftoff height with Reynolds number for the 3% O2 flame in Figure 6 also occurred for the “least stable” flame, and also at around the Reynolds number corresponding to a transition from laminar to turbulent. It is also observed that the most highly lifted flames, in both coflow O2 levels, show a slight decrease in liftoff height with increasing jet Reynolds number (jet velocity). Although unusual, this behavior has been observed previously.37,48,49 3.1.3. 9% O2 Coflow Results. In Figure 8 the coflow O2 level is increased to 9%. It was not possible to achieve a coflow temperature above 1400 K with 9% excess O2 from the

Figure 6. Liftoff height as a function of jet Reynolds number for natural gas jet flames within a 3% O2 coflow at various temperatures.

estimated liftoff height for natural gas flames, over a range of Reynolds numbers, for four coflow temperatures (1300−1600 K) with 3% O2 in the coflow. The minimum temperature that sustained a flame was 1300 K, below which no flame could be identified. At 1300 K coflow temperature, the flame luminosity is very low. A faint reaction zone can be identified to extend to the jet exit plane, though a definite flame is not apparent until z ≈ 20 mm. Interestingly, increasing the temperature of the coflow from 1300 K to 1400 K leads to an increase in the liftoff height. It could be expected that increasing the temperature of the oxidant stream would increase the reactivity, and hence reduce the liftoff height. At 1300 K the flame is likely attached (or lifted at most by 20 mm) up to Rejet = 5000; however, when the coflow temperature is increased to 1400 K, the flame is definitely lifted, by at least 30 mm, and likely more. Further increasing the coflow temperature subsequently leads to a reduction in the liftoff height. Even up to a coflow temperature of 1600 K, the flame remains lifted: it only appears attached at the lowest coflow temperature (1300 K). Despite the range of uncertainty in the data due to the low luminosity of these flames, there is a clear trend seen in 3% O2 coflow flames presented in Figure 6. As the temperature is increased from the minimum required to sustain a visible flame (i.e., 1300 K) to 1400 K, the flame liftoff height increases. A subsequent increase in coflow temperature leads to a reduction in the liftoff height but never back to the attached flame of the 1300 K case. 3.1.2. 6% O2 Coflow Results. Figure 7 shows the effect of Reynolds number on the liftoff height for a range of coflow

Figure 8. Liftoff height as a function of jet Reynolds number for natural gas jet flames within a 9% O2 coflow at various temperatures. 5523


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furnace temperature and determined by varying the recirculation rate (hence the O2 level). The ‘most unstable’ region identified in the data presented in Figures 6−8 was for a fixed O2 level and changing the temperature. To draw a better comparison to the Wünning and Wünning work, Figure 9 presents the liftoff height over a range of jet Reynolds numbers, for a fixed coflow temperature of 1300 K, for various coflow O2 levels.

secondary burner. Nonetheless, Figure 8 again shows similar trends of an initial increase then decrease of the liftoff height with increasing temperature. More apparent in this figure (Figure 8 compared with Figures 6 and 7) is that at low Reynolds numbers the highest temperature case has the greatest liftoff height. Nonetheless, after the transition to turbulence (Rejet ∼ 5000), the 1400 K case is lifted by a lesser amount than the 1200 and 1300 K coflow case, which both exhibit a significant increase in liftoff height. 3.1.4. DiscussionEffect of Coflow Temperature. Irrespective of the coflow O2 level, there is a trend of an increase in the liftoff height with temperature, followed by a subsequent decrease. The increased liftoff height indicates a reduction in the flame reactivity, despite what would be expected as an increase in reactivity due to the higher temperature oxidant stream. This increase in liftoff height is analogous to a reduction in flame stability; that is, less stable flames occur in the transition between more stable combustion regimes. The transition observed in the JHC burner seems consistent with the behavior in furnaces operating in the regime between conventional conditions and MILD combustion.51 Worth noting, however, is that for Rejet ≳ 5000 for the 3% and 6% O2 cases (Figures 6 and 7) there is a scarcity of data since these flames could not be sustained. Nonetheless, at 9% O2 coflow, the observed trends are valid into the fully turbulent regime. In the early MILD combustion (or FLOX) study presented by Wünning and Wünning,51 an unstable combustion regime was identified between the stable combustion modes seen in either conventional or MILD conditions. Similar reductions in flame stability in the transition regime has been observed elsewhere, too.72 The trend in the liftoff height with different oxidant stream temperatures (for a range of coflow O2 levels) shows a clear trend consistent with that associated with the transition from conventional to MILD combustion, including the intermediate unstable regime. The results reflected in the depleted O2 environments of Figures 6−8 also seem to suggest a ‘less stable’ intermediate combustion regime between the lower temperature and higher temperature conditions. The sensitivity of the flame stability to the coflow temperature and composition is of particular interest to the understanding of the flame stabilization mechanism under these conditions. In particular, the non-monotonic relationship of liftoff height with coflow temperature requires significant future study beyond the analysis presented in the present paper. The importance of temperature is more significant than simply the sensitivity of liftoff height over the narrow temperature range reported in the VCB burner. It seems that the impact of the temperature change on the chemical kinetics and/or mixing rate between the coflow and the fuel becomes critical at these conditions. The result is a fundamental change in the flame behavior. While an analysis of the ignition kernels under some of these conditions has been analyzed in term of statistics and appearance by Oldenhof et al.,48,49 results over a wider range of operating conditions are required for a complete understanding of the governing physics. Beyond chemistry effects, a further factor that may contribute to the observed trends in liftoff height is the differences in mixing due to laminarisation that can occur due to the different temperatures, both from the coflow and the reaction zone heat release.73 3.2. Effect of Coflow O2 Level. 3.2.1. Natural Gas Flames. The unstable combustion regime, referred to as region B by Wünning and Wünning51 was located between the conventional and MILD regimes; however, it was for a fixed

Figure 9. Liftoff height as a function of jet Reynolds number for natural gas jet flames within a 1300 K coflow at various oxygen concentrations.

In Figure 9, the low oxygen levels (3 and 4.5% O2) give a flame that visually appears to be in the MILD combustion regime. Increasing the O2 level to 6% gives a more luminous flame and leads to a significant increase in the liftoff height. Increasing the O2 level beyond 6% reduces the liftoff height, that is to say, leads to a more stable flame. The general trend of an increase and then decrease in the liftoff height with increasing the O2 level (with constant temperature) is essentially the same trend that was seen with the increase in the temperature (with a constant O2 level). The increase in liftoff height with increased coflow O2 concentration is consistent with the laminarization effects that affect mixing in the shear layer observed previously.73 From Figures 6−8 it may be noted that the flames in the 1400 K coflow typically are lifted the most. Furthermore, the results at this temperature consistently show a trend of decrease in liftoff height with increased Reynolds number, as has been observed previously.37,48,49 For the conditions considered in the current paper, it seems that this effect is most pronounced for a temperature of 1400 K and is less influenced by the O2 level. This transition of a reduction in liftoff height occurs at a Reynolds number corresponding to a transition to turbulence. The importance of this transition is most critical in the flames that have the highest liftoff height, notably at 1400 K (Figure 10). 3.2.2. Ethylene Flames. Figure 11 presents the effect of coflow temperature for ethylene flames, within a coflow temperature of 1300 K. The low luminosity of the flames at the lowest O2 level, coupled with relatively small liftoff heights, makes accurate determination of the liftoff height difficult, resulting in large uncertainty. As was observed for the natural gas flames, ethylene fuel also shows a similar trend of an initial increase in liftoff height, and then subsequent decrease in liftoff height, with an increase in coflow O2 concentration. The other trends that were noted for 5524


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Figure 12. Ethylene jet flame within a 3% O2 coflow, for different coflow bulk velocity and ratio of NG/H2 in premixed secondary burner (thus affecting the coflow composition ratio of H2O/CO2 and minor species).

Figure 10. Liftoff height as a function of jet Reynolds number for natural gas jet flames within a 1400 K coflow at various oxygen concentrations.

noting that both the major species (CO2/H2O) and minor species will be affected by these changes in the coflow burner. This result has implications for achieving MILD combustion conditions, where it is often believed that temperature and O2 are the two dominant factors for achieving MILD combustion. Furthermore, in other studies where the coflow temperature is controlled only through the equivalence ratio of the secondary burner, the subsequent changes in the coflow composition will also have significant effects. When moving outside of the low O2 levels associated with MILD combustion, Figure 13

Figure 11. Liftoff height as a function of jet Reynolds number for ethylene jet flames within a 1300 K coflow at various oxygen concentrations.

natural gas in Figures 6−8 also apply for ethylene fuel (not presented for brevity). As the jet Reynolds number is increased, it is interesting to note that the liftoff height seems to converge to a constant height for each O2 level. This observation is more clearly identified in Figure 11 than the natural gas flames (Figures 6−8), predominately due to the higher Reynolds number that can be sustained prior to flame blowoff. 3.3. Effect of Coflow Composition. The data presented thus-far has all been for a coflow bulk velocity of 2.8 m/s and the ratio of natural gas (NG) to H2 in the premixed secondary burner of 1:1.3. Figure 12 shows the effect on an ethylene flame of changing the coflow bulk velocity (Ucf = 1.4 or 2.8 m/s) and coflow fuel ratio (volumetric ratio of hydrogen to natural gas, H2/NG = 1.3 or 2), for a constant 3% O2 concentration and temperature of 1300 K. Figure 12 indicates that decreasing the coflow bulk velocity increases the liftoff height, likely a result of the increased shear layer due to the increased velocity gradient. Increasing the ratio of H2 to NG (which will change the composition of the jet oxidant stream) leads to an increase in liftoff height, indicating that the liftoff height behavior is indeed sensitive to the composition of the coflow, and not only the temperature and O2 level. The role of minor species on ignition chemistry has previously been reported (e.g., refs 74,75), but it has been largely ignored in flames with a hot coflow, where the coflow properties have been controlled by changing the stoichiometry of the premixed secondary burner. It is worth

Figure 13. Ethylene jet flame within a 12% O2 coflow, for different coflow bulk velocity and ratio of NG/H2 in premixed secondary burner (thus affecting the coflow composition ratio of H2O/CO2 and minor species).

indicates that at 12% O2, the effects of the coflow velocity and O2 level are less pronounced, however. This highlights some of the observed differences between the VCB studies and what is seen under MILD combustion conditions with the JHC burner.

4. CONCLUSIONS A jet flame in a hot and diluted coflow is a common configuration for experimental studies, ranging from autoignition in lifted flames to controlled MILD combustion. In this paper, the liftoff heights of natural gas and ethylene flames were determined from both photographs and CH* chemiluminescence for various coflow temperature and oxygen levels. For the 5525


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combination of low temperature and oxygen, the flames show low luminosity but appear attached (or lifted slightly). Such flames typify MILD combustion. Contrary to what may be expected, an increase in either the temperature or oxygen concentration causes the flame to become highly lifted. Further increases in the temperature or oxygen subsequently reduce the liftoff height. The increase in liftoff height (which is analogous to a reduction in flame stability) with an increase in the oxygen and/or temperature level of the coflow is attributed to the departure from MILD combustion. Claims of such a transition from MILD to conventional conditions have been reported in furnaces and are now replicated under the controlled conditions of a jet in hot coflow. It is also noted that the ignition chemistry is sensitive to changes in the composition of the coflow, most notably under MILD combustion conditions, indicating that the source and composition of the diluent gases may affect the flame stabilization, which could impact burner design.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Experimental Observation of Lifted Flames in a Heated and Diluted

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