Tip Opening of Burner-Stabilized Flames - American Chemical Society

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On the Tip Opening of Burner Stabilized Flames Abdul Naseer Mohammed, Edacheri Veetil Jithin, Dineshkumar L., Velamati Ratna Kishore, and Akram Mohammad Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02858 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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On the Tip Opening of Burner Stabilized Flames Abdul Naseer Mohammed1, Edacheri Veetil Jithin2, Dineshkumar L2, V. Ratna Kishore2, Akram Mohammad1,* 1

Department of Aeronautical Engineering, King Abdulaziz University, Jeddah, Saudi Arabia. 2

Department of Mechanical Engineering, Amrita School of Engineering, Amrita Vishwa

Vidyapeetham, Coimbatore, India.

*Corresponding author: Akram Mohammad, Department of Aeronautical Engineering, Faculty of Engineering, King Abdulaziz University Jeddah, Saudi Arabia, 21589 Tel: +966502478398 Fax: +966-12-2641686 Email: [email protected], [email protected] Abstract The tip opening mechanism of burner stabilized flames is investigated computationally using premixed propane+air mixtures. The temperature, net production rate, and reaction rates are investigated for rich mixtures. The flame tip structure was analyzed based on reaction rates to understand the conditions of equivalence ratio at which the tip opening phenomenon occurs. Numerical predictions of tip opening are in good agreement with experimental observations. The study revealed that tip opening phenomenon starts at φ = 1.4. As the mixture becomes rich, the tip opening was found to increase. When flame tip opens, volumetric heat release rate at the tip was found to be less than 50% of the heat release rate at the flame shoulder. There observed an increase in the flame tip thickness around 30%, from equivalence ratio 1.3 to 1.4. The effect of temperature on the propane burner flame structure is studied by performing simulations at three different mixture inlet temperatures 300 K, 350 K and 400 K. When the mixture unburnt gas temperature increases, the propane-air tube burner flame tip

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opening begins at more rich fuel condition compared to that of the mixture at ambient temperature. A detailed sensitivity analysis was carried out to identify the reactions having large sensitivities.

Keywords: Tip opening flames, burner stabilized flames, propane

1. Introduction Premixed combustion finds many applications spanning from household heating, cooking to internal combustion engines, gas turbines, and jet engine propulsion. The fuel and oxidizer are mixed prior to combustion in premixed flames. Because of this, premixed flames produce high temperature with less pollutant. Premixed flames are prone to instabilities because of prior mixing. The tip opening of the laminar premixed flame is a typical phenomenon which is predominantly related to some fundamental aspects such as preferential diffusion and stretch rate. Basically, tip opening resembles the local extinction of the flame-front at the portion where the curvature is high. This occurs with either lean hydrogen/air or highly rich hydrocarbon+air mixtures. It is evident that the understanding and quantification of this phenomenon has both fundamental and practical interest. This helps in the understanding of flame phenomena such as flame propagation, flame-front instabilities for flames exhibiting a strong curvature. This also gives an insight into local flamelet properties in turbulent flame. The detailed numerical analysis gives a clear understanding of the local flow field, reaction rates and flame structure leading to various instabilities occurring in a premixed flame. There is a large amount of work available for two-dimensional flame simulations with different configurations like partially premixed flames, cup burners, triple flames etc. In the present work, the focus is going to be on two-dimensional tube/Bunsen burner for premixed flames. de Lange and de Goey [1] developed a flame code for twodimensional premixed methane+air flame using vorticity-stream function formulation. They used the locally refined grid for resolving steep gradients in the flame and onestep global reaction chemistry. Katta and Roquemore [2] investigated premixed, H2-Air Bunsen flames numerically with a detailed reaction mechanism (11 species and 40 reactions). They had predicted open tip behavior for fuel-lean conditions. While under


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the fuel-rich condition, they found that the tip of the flame burns intensely. They studied the impact of local Lewis number on the premixed flame structure. Altay et al. [3] performed two-dimensional simulations of steady, laminar, perforated plate stabilized methane+air flames. They studied the effect of equivalence ratio, mean inlet velocity, hole to hole distance and the plate thermal conductivity on flame structure, stand-off distance and heat loss to the plate. Jithin et al. [4, 5] performed three-dimensional numerical simulations to understand the influence of thermo-physical properties on the steady-state combustion characteristics of the perforated burner flame. In the literature, most of the works on multi-dimensional premixed flame simulations with detailed chemistry were focused on methane-air flames. The effects of various fundamental characteristics causing tip opening have been studied. Law et al. [6] experimentally investigated the local extinction of Bunsen flame tips and edges of hydrocarbon+air mixtures. They observed that for both the propane+air and butane+air mixtures the flame tip opening commence for highly rich mixture at the equivalence ratio of 1.44. Ishizuka et al. [7] studied the flame structure and stabilization of propane+air mixtures in the stagnation flow field. It was shown that the flame front instability arises for rich mixtures due to the preferential diffusion considerations. Mizomoto et al. [8] studied the simultaneous effects of preferential diffusion and flame stretch on the combustion intensity of lean and rich mixtures. They observed that for rich propane-air mixtures (Le < 1), the tip locally extinguishes. Mizomoto and Yoshida [9] experimentally investigated the effect of the mixture transport properties on the combustion intensity of Bunsen flames. Yamamoto and Ishizuka [10] described the dependency of flame temperature on stretch effects for different mixtures with the Lewis number considerations. The flame-front curvature is an important geometrical factor that influences significantly the premixed flames propagation. The planar and curved flames possess significantly different stretch rates. The dynamics of these curved fronts can influence flame-front instability and extinction. Choi and Puri [11] conducted experimental studies on the effects of flame stretch on a two-dimensional premixed ‘regular’ and ‘inverted’ flames of methane and propane. It was observed that the flame speed increases highly along the curved region on the ‘regular’ flame. Choi and Puri [12] also


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described the effect of geometrical configurations on tip opening by conducting experiments on two different burner configurations. Kozlovsky and Sivashinsky [13] performed a numerical study on Bunsen burner flame tip. It was shown that the flame tip opening exists for the low Lewis number mixtures does not necessarily involve local extinction and fuel leakage through the tip. Ern and Vincent [14] performed a numerical investigation on the influence of thermal diffusion on the structure of hydrogen-air and methane-air flames. They presented a new algorithm for the accurate approximations of the transport coefficients in the mixture. Wang et al. [15] conducted a numerical investigation on the combustion characteristics of the H2-air mixture in a microcombustor with wall cavities. During this study, it was observed that the flame splits at the inner wall, because of which there exist two high-temperature regions and double temperature peaks at the outer wall. Also, they found out that the increase in equivalence ratio of the mixture and the length-depth ratio of the cavity increases the flame splitting limits. Wang et al. [16] experimentally and numerically investigated the effects of preferential diffusion and stretch on tip opening of laminar premixed syngas (H2-CO) flame. The results showed that the strong negative stretch at tip intensifies the preferential diffusion at the flame tip, which leads to tip opening at lean mixture conditions. Vu et al. [17] observed that tip opening for methane+air and propane+air occurs at a constant equivalence ratio and is independent of the jet velocity of the mixture. They inferred that the local Karlovitz number offers a reasonably good understanding of local extinction of the negatively stretched premixed flame tip. Recently, Yang et al. [18] analyzed the effect of the inlet temperature of the mixture on the flame tip opening of the H2-air mixture, in a micro-combustor with cavities. This study revealed that there is a significant improvement in tip opening phenomenon with an increase in inlet mixture temperature. Laminar premixed flames occur in many industrial and household applications. Liquified petroleum gas (LPG) is the most widely used conventional burner fuel in Asian countries [19]. Propane is one of the major constituents of LPG and autogas (green fuel). It has the ability to liquidize under moderate pressure conditions, thus enabling easier handling with onboard storage. Propane fuel also finds many domestic and industrial applications. Hence, it will be interesting to study the fundamental flame properties such as tip opening of premixed propane-air flames. Unfortunately, the


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fundamental combustion characteristics of propane-air flames are less understood because of the complex multi-physics nature of the propane combustion reaction. The objective of this work is to analyze computationally the steady state open premixed laminar flames of the Propane-air mixture, stabilized on tube burner. This kind of premixed laminar flame occurs in domestic burners. In the present study, FLUENT, a commercial CFD software, is used to model premixed laminar flames with a detailed reaction mechanism.

2. Numerical Modeling 2.1 Governing equations Two-dimensional steady-state Navier-Stokes equations in cylindrical (r, z) coordinates are solved along with the energy and species conservation equations, using FLUENT 15 [20]. The effect of the gravitational field is also considered with the inclusion of a body force term. Variable thermo-physical properties are incorporated in the model. Based on the above assumptions, the governing equations can be modeled as follows. ∂ ∂ ρ vr (1) (ρ vz ) + (ρ vr ) + =0 r ∂z ∂r rr ∇.( ρ vv) = −∇p + ∇.(τ ) + ρ g (2) Where, the stress tensor is related to strain by



r

rT

2

r 

τ = µ (∇v + ∇v ) − δ∇.vI  3   r r r   ∇.(v ( ρ E + p)) = ∇  k ∇ T − ∑ h j j j + (τ .v )  + S h j  

(3) (4)

Where z, r, vz and vr are the axial coordinate, radial coordinate, axial velocity and the radial velocity respectively. p, ρ , g, µ , I and k are the static pressure, density, acceleration due to gravity, molecular viscosity, unit tensor and the thermal conductivity respectively. The conservation equations for chemical species under steady state conditions take the form,

r uur ∇⋅ (ρ v Yi ) = −∇⋅ Ji + Ri


(5)

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r

Where j i , Yi and Ri are the diffusion flux, the mass fraction, the net rate of production of ith species respectively.

Spatial discretization of the governing equations is carried out using second-order upwind scheme. An implicit method with double-precision segregated solver is employed. The pressure and velocity terms are coupled with the SIMPLE algorithm.

2.2 Computational domain and grid

The computational domain and grid along with various boundary conditions for tube

Figure 1. Computational grid taken in the computations for two-dimensional premixed flame stabilized with tube burner burner stabilized premixed flame is shown in Figure 1. The physical domain is of 16 × 25 mm. The exterior boundaries in the r and z directions are positioned sufficiently far. Initial grid for the entire domain consists of 7540 cells having 0.1 mm × 0.12 mm grid in the flame region. The flame region in the computational domain got adapted three


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times, after which the total number of cells went above 60000 with the flame region having a grid size of 12.5 µm × 15 µm.

2.3 Boundary conditions The computational domain for the two-dimensional tube burner shown in Figure 1 also shows all the boundary conditions around the domain. The parabolic velocity profile is imposed at the mixture inlet boundary. The Reynolds number for the present flow condition is 136.92. The burner (rim) wall temperature is constant at 300K. In open flames, the flame is induced by flow in its surroundings. Due to this, a co-flow of cold air is induced from bottom and sides where pressure inlet boundaries were prescribed. The pressure outlet boundary is placed at such distance from the burner inlet, that all the variables are nearly constant so that the perpendicular gradients become nearly zero. The centerline of the burner is defined as symmetry.

2.4 Chemical reaction modeling A detailed chemical kinetics model consisting 30 species and 192 reactions given by Qin et al. [21] has been used to describe the propane-air combustion. The chemical reaction mechanism is imported in CHEMKIN format. Thermodynamic database for the species was imported in NASA piecewise-polynomial format for two temperature ranges. Transport properties for the species are calculated using Lennard-Jones kinetic theory parameters, σ and ε/kB from a transport database. A finite-rate laminar species model along with a stiff chemistry solver is employed for solving volumetric reactions.

2.5

Grid independence

The initial mesh size along r and z directions were 0.1 mm × 0.12 mm respectively. The coarse initial grid for the entire domain consisted of 7540 cells. Once the flame is stabilized, the grid near the flame region was refined to third levels by region adaptation. After three levels of adaptation, the total number of cells in the domain is above 60000.To test the grid convergence, temperature and OH mole fraction in the flame region are compared for different grids (Propane-air case, φ =1.3, T=300K). The temperature profiles in Figure 2(a) show that the first, second and third level adaptations are in good agreement. A similar trend was observed for OH mole fraction contours


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also, as shown in Figure 2(b). Hence, all the results presented in this paper were adapted to the third level.

(a)

(b)

Figure 2. Comparison of temperature and OH mole fraction profiles for coarse, first-, second and third-level adaptations.

3. Validation Vu et al. [17] had conducted an experimental study on the structure of propane-air tube flame. The experimental set up consisted of a co-flow burner having a central stainless steel tube of inner diameter 7.53 mm and wall thickness 1 mm. The total length of the tube was set to be 470 mm, to ensure the mixture flow inside the tube to be fully developed with an average velocity of 1 m/s. These conditions were used to simulate propane-air flame with a detailed chemical kinetics model consisting 30 species and 192 reactions given by Qin et al. [21]. In Figure 3 (a), the temperature measurements of Vu et al. [17] for φ = 1 and 1.5 are plotted along with present predictions using FLUENT, along the vertical axis. The figure shows that the numerical results of temperature are exactly matching with that of experimental results in most of the regions in the domain. Even though, there is a significant variation observed in the result after 10mm distance in the case of φ = 1.0. Also, the temperature comparison show variation between experimental and predicted values between 10mm and 20 mm distance for the case of φ = 1.5. These variations in results might be because of the expected uncertainties in the experimental results. In Figure 3(b) and (c), the velocity profiles obtained from the experimental measurements of Vu et al. [17] for φ = 1 and 1.5 are compared with the corresponding results from the present numerical predictions. The figures show that the predicted velocity profiles follow similar trends with that of the experimental results in


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both the equivalence ratio conditions. Taking in to account the experimental uncertainties, the temperature predictions are in reasonable agreement. This shows the accuracy of the reacting module and methodology used in the present study.

(a)

(b)

(c)

Figure 3. (a) Predicted temperature profile along the symmetric axis compared with measured temperature by Vu et al. [17], 2014. (b) and (c) shows the predicted velocity profile along the symmetric axis compared with the measured velocity by Vu et al. [17], 2014, for φ = 1 and 1.5.

4. Results and Discussion In this section computations of two-dimensional flames stabilized on a burner are presented. The computations were performed for propane-air mixture with equivalence


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ratio varying from 1.3 to 1.6 in steps of 0.1. Law et al. [6] have found out that the local extinction of propane-air Bunsen flame tips (tip opening) and edges occurs at a constant equivalence ratio of 1.44. The fuel equivalence ratio at which tip opening occurs was independent of flow rate, inlet velocity profile (i.e parabolic/uniform) and configuration of the burner (tube/tube nozzle/rectangular nozzle). Mizomoto and Yoshida [9] had studied the simultaneous influence of preferential diffusion and flame stretch on the burning intensity of lean and rich mixtures of methane, propane, butane, and hydrogen with air. For propane-air, the flame was exhibiting tip opening at φ ≈ 1.53. Based on the experimental results, the flame structure of computed flames for equivalence ratio 1.3, 1.4, 1.5 and 1.6 are presented. The effect of temperature on propane burner flame structure is studied by performing simulations at three different mixture inlet temperatures 300 K, 350K and 400K.

4.1 Effect of equivalence ratio on flame structure In this section the structure of rich propane-air burner flames anchored on a tube burner is analyzed by performing numerical simulations at equivalence ratios varying from 1.3 to 1.6 in steps of 0.1, with average inlet velocity (Uavg) 0.5 m/s and unburnt mixture temperature (Tu) 300 K. In Figure 4, contours of temperature and heat release rate of propane-air tube flame for various equivalence ratios are shown. Although the calculations were made only for one side, the entire flame region is shown by using mirror-image data. The inner cone of the flame is visible in the temperature contours plotted on the left-hand side of Figure 4. Close observation of the temperature contours shows a sharp inner cone tip for φ = 1.3, compared to that of other equivalence ratio conditions. This indicates that the flame burns intensely at the tip for φ = 1.3. Beyond φ = 1.4, the inner cone at the tip is found to diffuse enormously, which may be due to tip opening. The volumetric heat release rate contours shown on the right-hand side of Figure 4 substantiate these findings. The volumetric heat release at the tip shows very low value beyond equivalence ratio 1.4. This indicates that the intensity of combustion is very low at the tip of these equivalence ratios and hence tip is open. The flame thickness was calculated as the interval of the steepest tangent to the temperature profile between the unburnt and adiabatic temperature for all the cases [22]. For φ = 1.3, 1.4 and 1.5, the flame thickness was obtained as 0.380 mm, 0.491 mm and 0.642 mm


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respectively. The increase in flame thickness beyond φ = 1.4 indicates that the flame tip is open. The maximum temperature along the axis for each equivalence ratio case was compared with the corresponding adiabatic flame temperature. The flame temperature near the tip was found to be considerably lower than the adiabatic flame temperature for the mixture up to φ = 1.4. This reduction in the burning intensity at the flame tip, leading to tip opening may be due to the increase in negative stretch at the flame tip, in accordance with the findings of Mizomoto et al. [8] The mole fraction contours of CO and OH radicals of propane-air tube flame for various equivalence ratios are shown in Figure 5.The mole fraction contours of CO are shown on the left-hand side of Figure 5. The results show that the consumption of CO at the flame tip is very low beyond φ = 1.4, indicating lower combustion intensity at the tip. The OH concentration shows a maximum at high temperature. The mole fraction contours of OH are shown on the right-hand side of Figure 5. From the figure, it is understood that the intensity of OH near the tip region reduces with increase in equivalence ratio. The low concentration of OH beyond φ = 1.4 is a sign of tip opening. The net reaction rate of CO and OH is shown in Figure 6. It is observed from the figure that the net reaction rates of both the species near the tip region are decreasing with increase in equivalence ratio. For φ = 1.4 and 1.5, the net reaction rate of CO along the axis was found to be 93.32 kg/m3s and 71.35 kg/m3s respectively whereas the net reaction rate of CO along the flame shoulder was found to be 170.75 kg/m3s and 149.02 kg/m3s respectively. It can be seen that when flame tip opens, the net reaction rate of CO at the tip is reduced to around 50% of the net reaction rate at the flame shoulder. The net reaction rate of OH at the tip for both φ = 1.4 and 1.5 are comparable with the values at the unburnt region, which indicates there is no OH radical reaction at the tip when the tip is open. At the same time, the net reaction rate of OH at the flame shoulder for φ = 1.4 and 1.5 was found to be 12.31 kg/m3s and 10.63 kg/m3s respectively, indicating that the combustion intensity at the flame shoulder is high in comparison with that at the tip when the tip is open.


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Figure 4. Computational results showing contours of temperature (left) and heat release rate (right) of propane-air tube flame for various equivalence ratios, at Uavg= 0.5m/s and Tu=300 K. The one-dimensional simulation was performed using PREMIX to identify the reactions having large sensitivities. Out of the 192 reactions, the following reactions were found to have the largest sensitivities. O+H2H+OH

(R1)

H+O2O+OH

(R2)

OH+H2H+H2O

(R3)

OH+COH+CO2

(R4)

HO2+CH3OH+CH3O

(R5)

C3H6+HC2H4+CH3

(R6)

In Figure 7, bar charts showing the maximum value of the kinetic rate of reaction along the axis, for the above-mentioned reactions are presented for various equivalence ratios.


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The bar chart clearly demonstrates that the kinetic reaction rates (kg mole/m3s) of all these reactions reduce with increase in equivalence ratio. The enormous reduction in reaction rates of the major contributing reactions can be seen beyond φ = 1.4. This reduction in reaction rate may be due to the low intensity of combustion at the tip as a result of tip opening. The volumetric rate of heat released (W/m3) at the tip is also calculated. It is found that the volumetric rate of heat released by the above-given reactions contributes to more than 60% of the total volumetric heat released at the tip. For φ = 1.4 and 1.5, the maximum volumetric heat release along the axis was found to be 1.4×109 and 8.8×108 w/m3 respectively whereas maximum volumetric heat release along the flame shoulder was found to be 2.7×109 and 2.3×109 W/m3 respectively. When flame tip opens, volumetric heat release rate at the tip was found to be less than 50% of the heat release rate at the flame shoulder.

Figure 5. Computational results showing mole fraction contours of CO (left) and OH (right) of propane-air tube flame for various equivalence ratios, at Uavg= 0.5m/s and Tu =


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300 K.

Figure 6. Computational results showing contours of CO (left) and OH (right) reaction rates of propane-air tube flame for various equivalence ratios, at Uavg= 0.5 m/s and Tu= 300 K.


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Figure 7. Kinetic rate of reaction for the elementary reactions which contribute the largest proportion of total heat release rate.

The tip opening phenomenon occurs due to the combined effects of preferential diffusion and the flame curvature at the tip. The preferential diffusion is characterized by the effective Lewis number (Leeff). For the present study, the effective Lewis number is calculated from burnt gas temperature using the following expression [23].

 1  Tb 1 =1 + Ka  − 1 ⇒ Leeff = 0   Tb (1/ Ka )(Tb / Tb0 − 1) + 1  Leeff  Ka- Karlovitz number. Tb- Actual burnt gas temperature. Tb0- adiabatic flame temperature. Table 1 shows the effective Lewis number values obtained for φ = 1.3, 1.4 and 1.5. The results show that as the equivalence ratio increases towards the tip opening condition (φ = 1.4), the effective Lewis number (Leeff) becomes less than 1. At φ = 1.5, Leeff is further reduced. This signifies the effect of preferential diffusion on the tip opening, between φ = 1.4 and 1.5. The effect of flame curvature on the tip opening phenomenon is characterized by the negative stretch at the tip, given by κ = 4U / R [17] and the Karlovitz number (Ka). The Karlovitz number for the present study is calculated for different equivalence ratios using the following expressions.


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Ka =

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4U / R [17] S L2,0 / α 0

The radius of curvature, R is calculated by extracting data along an ellipse which best fits to the tip region and fitting a function f(x) = ax2+bx+c to the data. The radius of curvature R = 1/2|a| [17] The results shown in Table 1 gives an idea of the effect of flame curvature on the beginning of tip opening for propane-air mixture at φ = 1.4. From the table, it is identified that the Karlovitz number increases towards the equivalence ratio where the flame tip is expected to open. The Karlovitz number remains reasonably constant with respect to equivalence ratio, once the tip is open. The negative flame stretch also shows increasing trend with equivalence ratio. In order to get further understanding on the local extinction of the flame at the tip at this condition, the local equivalence ratio of the mixture is calculated for φ = 1.3, 1.4 and 1.5, as shown in Figure 8 (a-c). From these contours, it can be identified that the local equivalence ratio at the tip region increases due to preferential diffusion. Hence, locally the burning velocity becomes less compared to that of the inlet mixture composition. Since the volumetric heat release rate is proportional to laminar burning velocity, the flame tip opens in the presence of simultaneous stretch and preferential diffusion.

Table 1 φ = 1.3 Leeff Ka κ (1/s)

φ = 1.4

φ = 1.5

1.01

0.99

2.89 -16851.1

4.79 -13967.6

0.84 4.98 -4866.4


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Figure 8. Computational results showing contours of local equivalence ratio for (a) φ = 1.3, (b) 1.4 and (c) 1.5 at 300 K.

4.2 Effect of temperature on flame structure In this section, the effect of temperature on of rich propane-air burner flames anchored on a tube burner is analyzed by performing numerical simulations at three different unburnt mixture temperatures viz. 300K, 350Kand 400K for φ = 1.4 and 1.5 with average inlet velocity (Uavg) 0.5 m/s. In Figure 8, contours of temperature and heat release rate of propane-air tube flame for 300K, 350 K and 400 K are shown for φ = 1.4 and 1.5.The temperature contours are plotted on the left-hand side of Figure 9. As explained in the previous section, the flame tip of propane-air tube burner flame begins to open at 300K for φ = 1.4 and the tip is completely open at 300K for φ = 1.5 as shown in Figure 9 (a) & (b). But, for the same equivalence ratio, the inner cone of the flame seems to be sharp at the tip when the unburnt temperature of the mixture is increased to 350K, indicating that the flame tip opening has not yet started (see Figure 9 (c)). In the temperature contour shown in Figure 9 (d), the propane-air tube flame inner cone looks more diffused at φ = 1.5 when 350 K, which shows the flame tip is open at this


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condition. The flame thickness at 350 K, for φ = 1.4 and 1.5 was obtained as 0.463 mm and 0.612 mm respectively. The increased flame thickness at φ = 1.5 is an indication of tip opening. The volumetric heat release rate contours shown at the right hand side of Figure 9 (a) &(c) indicates that for φ = 1.4 combustion is significant at the propane burner flame tip at 350 K, unlike the condition at 300 K. These results denote that the equivalence ratio at which the propane-air tube burner flame tip opening begins is shifted from φ = 1.4 to a higher value when the unburnt mixture temperature is increased from 300K to 350 K. Also, the tip seems to be completely open for 350K unburnt mixture temperature, at equivalence ratio φ = 1.5. Figure 9 (e) & (f) shows the contours of temperature (left-hand side) and volumetric heat release rate (right-hand side) propane-air tube burner flame for φ = 1.4 and φ = 1.5 respectively at 400K mixture unburnt temperature. The temperature contour shows the inner cone of the flame is sharp at φ = 1.4 compared to that at φ = 1.5. The flame thickness was found to be 0.456mm, 0.575 mm and 0.709mm respectively for φ = 1.4, φ = 1.5 and 1.6 at 400 K mixture unburnt temperature. This indicates that the flame tip opening begins at φ = 1.5 and the flame tip is completely open at φ = 1.6. The volumetric heat release rate is shown in Figure 9 (e) and (f) also signifies the above-mentioned predictions. The volumetric heat release rate release contour for φ = 1.4 at 400 K shows that there is significant combustion occurring at the flame tip. But for φ = 1.5, the contour shows that the volumetric heat release rate is very low at the flame tip compared to that at φ = 1.4. These results indicate that, when the mixture unburnt temperature is increased from 350 K to 400 K, the flame tip begins to open at φ = 1.5. The mole fraction contours of CO and OH radicals of propane-air tube flame for equivalence ratiosφ = 1.4 and φ = 1.5 at 300 K, 350 K and 400 K are shown in Figure 10. The variations in mole fraction contours of CO (left-hand side) and OH (right-hand side) shown in Figure 10 also signifies that the tip opening of propane-air tube burner flame occurs at higher equivalence ratio when the mixture unburnt temperature is increased. A comparison of CO mole fraction contours for φ = 1.4 at mixture unburnt temperature 300 K and 350 K (Figure 10 (a) & (c)) show that the consumption of CO at the flame tip is very high at 350 Κ, compared to that at 300 K. This indicates that the flame tip burns intensely at 350 K without having tip opening, unlike 300 K condition


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where tip opening begins. At the same time, at 350 K mixture unburnt temperature, the consumption of CO is very low for equivalence ratio φ = 1.5, which shows the tip is open (see Figure 10 (d)). CO mole fraction contours for φ = 1.4 and φ = 1.5 respectively, at mixture unburnt temperature 400 K are shown at left-hand side of Figure 10 (e) & (f) These contours show that the consumption of CO at flame tip is high at φ = 1.4, compared to that at φ = 1.5, indicating a significant combustion at the flame tip at φ = 1.4. Thus, it can be seen that the flame tip opening starts only at φ = 1.5 when mixture unburnt temperature is 400 K. The OH mole fraction contours are shown at the right-hand side of Figure 10. It is observed from this figure that the intensity of OH is very low near the flame tip region, for the conditions at which the flame tip opening is expected. Figure 11 shows the net reaction rates of CO (left) and OH (right) of propaneair tube flame for φ = 1.4 and φ = 1.5 at 300 K, 350 K and 400 K with Uavg= 0.5 m/s. It is observed from the figure that the net reaction rates of both the species are very low near the flame tip, for the conditions at which the tip opening is expected. In Figure 12, bar charts showing the maximum value of the kinetic rate of reaction along the axis, for the major contributing reactions are presented at 300 K, 350 K and 400 K mixture inlet temperatures, for φ = 1.4, 1.5 and 1.6. The bar chart clearly indicates that the kinetic reaction rates of all the reactions at the flame tip increase with an increase in unburnt mixture temperature. This causes the increase in equivalence ratio at which tip opening occurs, with the increase in mixture unburnt temperature. As discussed in the previous section, to the effect of stretch and preferential diffusion on tip opening, the local Lewis number and the negative stretch are analyzed for φ = 1.4 and 1.5 at 350 K. The results showed that the Lewis number for φ = 1.4 was close to unity at 350 K, whereas the effective Lewis number reduces at φ = 1.5 for 350 K. This show that the tip opening starts at φ = 1.5 for 350 K, mainly due to the preferential diffusion effect. When the unburnt mixture temperature increased from 300 K to 350 K for φ = 1.4, the negative stretch rate at the tip decreases. Because of this, the tip opening phenomenon occurs at equivalence ratio, >1.4, for 350 K unburnt mixture temperature.


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Figure 9. Computational results showing contours of temperature (left) and heat release rate (right) of propane-air tube flame forφ = 1.4 and φ = 1.5 at 300 K, 350 K and 400 K with Uavg= 0.5m/s.


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Figure 10. Computational results showing mole fraction contours of CO (left) and OH (right) of propane-air tube flame for φ = 1.4 and φ = 1.5 at 300 K, 350 K and 400 K with Uavg= 0.5 m/s.


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Figure 11. Computational results showing contours of CO (left) and OH (right) reaction rates of propane-air tube flame forφ = 1.4 and φ = 1.5 at 300 K, 350 K and 400 K with Uavg= 0.5 m/s.


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(b)

(a)

(c)

Figure 12. Kinetic rate of reaction for the elementary reactions which contribute the largest proportion of total heat release rate (a) at 300K, (b) at 350 K and (c) at 400 K mixture inlet temperatures.

4. Conclusions The flame structures propane-air gaseous fuel mixture have been investigated computationally. FLUENT has been used for computations of two-dimensional tube burner flames. The effect of equivalence ratio on the opening of propane-air burner flame tip is investigated. The tip opening phenomenon is predicted to start at φ = 1.4 for propane-air burner flames anchored on a tube burner at 300 K. Based on the detailed flame structure tip opening phenomenon is explained. The one-dimensional simulation was performed using PREMIX to identify the reactions having large sensitivities. When flame tip opens, volumetric heat release rate (by the major contributing reactions) at the tip was found to be less than 50% of flame shoulder heat release rate. At the point of the beginning of tip opening (φ = 1.4), an increase in flame tip thickness around 30% was observed. The effect of unburnt mixture temperature on the flame structures propane-air tube burner flame has been investigated. These results denote that the equivalence ratio at which the propane-air tube burner flame tip opening begins is shifted from φ = 1.4 to a higher value when the unburnt mixture temperature is increased from 300K to 350 K. Also, the tip seems to be completely open for 350K unburnt mixture temperature, at equivalence ratio φ = 1.5. The combined effect of stretch and preferential diffusion on the local extinction behavior is also discussed.


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Acknowledgement

All computations were performed on Aziz Supercomputer at King Abdulaziz University's High-Performance Computing Center (http://hpc.kau.edu.sa/). The authors would like to acknowledge the computer time and technical support provided by the center.

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Tip Opening of Burner-Stabilized Flames - American Chemical Society

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