the effect of swirl on nox formation in a non-premixed ... - 2185_the...1 university of applied...

Vol. 5(15) Apr. 2015, pp2175-2185

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Article History:

Received Date: xxx Accepted Date: xxxx

Available Online: xxx IJMEC DOI: xxx/xxxxx

The Effect of Swirl on NOX Formation in a Non-Premixed Propane/Air Flame

P. Baziar1, A. Hajipour2*, A. Fahimirad3, M. Mahmoodi Arya4, H.M. Heravi5

1 University of applied science and technology-FZA center, Amol, Iran.

2* Young Researchers and Elite Club, Ayatollah Amoli Branch, Islamic Azad University, Amol, Iran.

3 Iran Heavy Diesel Mfg. Co. (Desa), Amol, Iran.

4 Department of Mechanical Engineering, Faculty of Engineering, Mashhad Branch, Islamic Azad University, Mashhad, Iran.

5 Department of Mechanical Engineering, Faculty of Engineering, Mashhad Branch, Islamic Azad

University, Mashhad, Iran.

*Corresponding Author's E-mail: [email protected]

Abstract

n the present study the effect of swirl on NOx formation in non-premixed propane–air flame is examined. The experiments are conducted in an ax symmetric cylindrical combustion chamber for a

range of equivalence ratio and swirl angle. The numerical simulations are carried out with the CFD code, Fluent. The turbulence k - ε model is employed. Both predictions and experiments show that as the swirl number increase, the temperature and the thermal NOx first slightly increase and then strongly decrease at the exit of the combustion chamber. The predicted results are in good agreement with the measured results.

Keywords: Combustion, Swirl, NOx, Non-premixed.

1. Introduction

Swirl combustion is widely found in various thermal power engineering devices, including furnaces,

boilers, kilns, and gas turbine engines [1,2]. Swirling flows have been studied for decades, with detailed

descriptions in the work of [3-5]. It was found recently that swirl might influence not only combustion

characteristics but also NOx formation [6]. Swirl is an implement for steady flame and heat transfer

control and increased combustion efficiency [7,8]. To meet these requirements, many types of swirl

burners were developed, which employ complicated swirl gas/particle flow to ensure their performance

[9]. For achieving low NOx emission as well, swirl combustion has to be integrated with other combustion

techniques such as staged combustion, flue gas recirculation (FGR), re-burning, and low oxygen

combustion [10]. Some researchers reported an increase in NOx formation with increasing swirl number

I

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P. Baziar ,…/ Vol. 5(15) Apr. 2015, pp2175-2185 IJMEC DOI: 649123/10140

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International Journal of Mechatronics, Electrical and Computer Technology (IJMEC) Universal Scientific Organization, www.aeuso.org

[6], and the others reported a decrease in NOx formation with increasing swirl number [11]. The effect of

swirl on NOx formation is a complex phenomenon. The change of swirl number will affect the flow field,

including the location and size of the recirculation zone, the flame temperature, species concentration

distribution, and turbulence intensity All these changes will affect NOx formation, but it is unclear which

one dominates [6]. For numerical simulation, most authors use the assumed-PDF turbulence- chemistry

model to simulate NOx formation in turbulent flows [12]. Also, effect of Dilution in Premixed CH4-Air

Flame and effect of the Flue Gas Recirculation on NOX Formation in a Premixed Methane/Air Flam are

investigated [13, 14]. In this paper, the experimental and numerical studies on the effect of swirl number

on NOx formation in propane/air turbulent swirling combustion have been investigated. The standard k –

ε model is applied to simulate turbulence flow.

The k – ε model is a two equation eddy viscosity model, which solves for turbulent kinetic energy (k)

and its dissipation rate (ε). Probability density function(PDF) is used to model turbulent combustion. The

application of a tangential velocity component to the flow, gives the flow a rotating component,

represented by the non-dimensional swirl number (S) defined as the ratio of the tangential momentum

flux to axial momentum flux, as [15]:

2 2

0 0/ ,

R R

inS ρUWr dr R ρU rdr

(1)

Here, R denotes the swirler radius. The NOx post-processor is used to predict NOx emissions.

2. Numerical and Simulation

The simulation equations in cylindrical coordinate based on the continuity, momentum and energy

can be expressed in the following general forms [16]. Continuity equation:

0.i i

i

ρu ρ ux

(2)

Conservation equations:

Γ ,i

j j j i j i j i

i j j j

ρu ρ u ρu u ρ ρu Sx x x x

(3)

Where the fluid density, uj is the velocity component along the coordinate Direction xj, F is the

dependent variable which can be mass, velocity, and enthalpy. Γφ is the diffusion coefficient and Sφ is the

source term.

The CFD code Fluent package [17] is applied to simulate a 2D, steady state and axisymmetric geometry.

Propane is used as fuel. Figure 1 demonstrates grids of mesh having about 15000 cells.

Figure 1: Grids of mesh.

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Fuel and air inlet temperatures are 300 K and pressure is 1 atm. Air inlet velocity is 5 m/s and the wall temperature in simulations is kept at 750 K.

3. Experimental

The experimental setup, including the test section, the air and fuel supply systems, and the

measurement system, is shown in figure 2. The section is a combustor made of a hollow steel tube. The

combustor length is 1000 mm and its inner diameter is 105 mm. The combustor is insulated with glass

fiber of 15mm thickness. Fuel and air are supplied via a nozzle and a central tube which their inner

diameter are 4 and 35 mm respectively, where swirlers with different blade angles are mounted to

create swirling flows with different swirl numbers. The species concentrations were measured using a

Testo 350 XL gas analyser. The velocity was measured using a Lutron YK-2005AM flow meter. Fuel mass

flow rate was measured using a SWP F-06A rota meter. The swirl number is defined as [18]:

3

2

12tan

3 1,r

r

σθS

σ (4)

Where θ is the blade angle and σr is the ratio of swirler inner diameter to outer diameter. The blade

angles used in experiments are: 0, 15, 30, 45, and 60 degrees that swirlers show in figure 3. The swirler

inner diameter is 8 mm, and its outer diameter is 35 mm, so, the swirl numbers are 0, 0.2, 0.4, 0.7 and

1.2, respectively. Experiments are performed in different equivalence ratio.

Figure 2: Experimental setup 1, Propane; 2, Rota meter; 3, Swirler; 4, Test section; 5, gas analyzer; 6, probe; 7, Air

fan; 8, Flow meter.



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Figure 3: Swirlers.

4. Results and Discussions

The overall structure of the gas flow field can be mainly divided into three regions: a central toroidal

recirculation zone (CTRZ), caused by the adverse pressure gradient induced by the swirl, a corner

recirculation zone (CRZ), due to the air stream radial expansion and the wall confinement; and a dual

shear layer (inner layer around the CTRZ and the outer layer around the CTZ). Both recirculation zones

entrain smaller droplets and hot chemically active combustion species from the downstream region of

the flame to the root of the flame to improve its stability. In Figure 4 these zones are shown through

stream vectors by simulation. Results show that with increase of swirl number from 0.4 to 1.2, CTRZ are

formed inside the flow. Forward and backward flow which is created by the vortex increase the fuel and

air mixing and combustion rate. Moreover, the vortex impact NOx emissions and create internal FGR in

close of the flame.

Figure 4: Flow vectors for various swirl numbers.



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It can be seen from the figure 4 that the fuel velocity is high because of the low input diameter. This

would create a high flow velocity which moves toward the end of the combustor. Figure 5 shows the

temperature contours for some swirl number at equivalence ratio of 1.0. As it is demonstrated, for the

low swirl number high temperature region would be found at the end of the combustor. The results

show that the region of the maximum flame temperature is tended to the inlet of combustor with

increase of swirl number. Furthermore, the temperature is declined with increase of swirl number from

0.2 to 1.2 which has great impact on NOx exponentially.

Figure 5: Temperature contour for swirl number 0, 0.4 and 1.2 and equivalence ratio of 1.0.

In figure 6, the simulated temperature graph align the combustor axis is shown for equivalence ratios

of 0.9, 1 and 1.1 in various swirl numbers. As the numerical results show, the temperature is grown with

increase of swirl number up to 0.2 then it decreases with growth of swirl number from 0.2 to 1.2. The

simulations show that the maximum temperature is found at swirl number of 0.2.

Figure 7 shows NOx emission for equivalence ratios of 0.9, 1.0 and 1.1 with various swirl numbers along

combustor axis. NOx is formed at 25 cm from the inlet of combustors where the combustion occurs. NOx

emission is increased with increase of swirl number from 0 to 0.2 then it is declined. As it can be seen

from the figures that temperature is affected NOx emissions.



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Figure 6: Temperature profiles in long of combustor in various equivalence ratios for all swirl number.



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Figure 7: NOx profiles in long of combustor in various equivalence ratio for all swirl number.

Figure 8 shows the Numerical and experimental results of the effect of swirl number on NOx

emissions for different equivalence ratios. NOx is increased with growth of swirl number around 0.2 and

then it is declined. The lowest levels of NOx is occurred at swirl number of 1.2. The trend of NOx emission

is similar to temperature which shows the influence of temperature on NOx. At high swirl number,

central toroidal recirculation zone is grown near the combustor wall, so most of the reactants have gone

far away from the reaction zone and the combustion temperature and NOx emission have reduced. The

experimental and numerical results are in good agreement. The results also compared with experimental

results of Choi et al.[8] and show good agreement.

Figure 9 shows the experimental results of NOx emission and temperature at swirl number 1.2 for a

range of equivalence ratio. The results show that both temperature and the NOx emission are increased

up to around stoichiometric equivalence ratio and then decreased.



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φ=0.8

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1 1.2

S

NO

x (

pp

m)

Simulation result

Exprimental result

Choi exprimental result

φ=1.0

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1 1.2

S

NO

x (

pp

m)

Simulation result

Exprimental result

φ=1.2

0

10

20

30

40

50

60

70

0 0.2 0.4 0.6 0.8 1 1.2

S

NO

x (

ppm

)

Simulation result

Exprimental result

Figure 8: Comparison of experimental and numerical results of NOx.

Figure 9: The NOx and temperature measurements at the exit of the combustor.



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5. CONCLUSION

The effects of swirl flow on NOx emission in non-premixed flames have been studied numerically and

experimentally. It concludes that:

With use of swirl, flow region is divided to the central toroidal recirculation zone and corner

recirculation zone and CTZ

The maximum flame temperature tends to the inlet of combustor as the swirl number increases.

The temperature and the levels of NOx emissions are increased up to swirl number of 0.2 then

strongly decreased at the exit of combustor.

The experimental and numerical results are in good agreement.

6. References

[1] Yang, W. and Zhang, J., “Simulation of methane turbulent swirling flame in the TECFLAM combustor”, Applied Mathematical Modelling, Vol. 33(6), (2009), 2818-2830.

[2] Shang, Q. and Zhang, J., “Simulation of gas-particle turbulent combustion in a pulverized coal-fired swirl combustor”, Fuel Vol. 88(1), (2009), 31-39.

[3] Beer, J.M., ‘Low NOx Burners for Boilers, Furnaces and Gas Turbines: Drive towards the Low Bounds of NOx Emissions’, Third International Conference on Combustion and Technology for a Clean Environment, Lisbon, (1995).

[4] Syred, N., ‘A review of oscillation mechanisms and the role of the precessing vortex core (PVC) in swirl combustion systems’,Progress in Energy and Combustion Systems, Vol. 32(2), (2006), 93-161.

[5] Gupta, A.K., Lilley, D.J. and Syred N., “Swirl Flows”, Abacus Press, Tunbridge Wells, United Kingdom. (1984). [6] Zhou, L.X., Chen, X.L. and Zhang, J., ‘Studies on the effect of swirl on no formation in methane/air turbulent combustion’,

Proceedings of the Combustion Institute, Vol. 29, (2002), 2235-2242. [7] Chen, Z., Li, Z., Jing, J., Chen, L., Wu, S. and Yao, Y. ‘Gas/particle flow characteristics of two swirl burners’, Energy

Conversion and Management, Vol. 50(5), (2009), 1180–1191. [8] Choi, C.R. and Kim, C.N., ‘Numerical investigation on the flow, combustion and NOx emission characteristics in a

500 MWe tangentially fired pulverized-coal boiler’, Fuel, Vol. 88(9), (2009). [9] Kurose, R., Ikeda, M., Makino, H., Kimoto, M. and Miyazaki, T., ‘Pulverized coal combustion characteristics of high-fuel-ratio

coals’, Fuel, Vol. 83(13), (2004), 1777–1785. [10] Peng, L. and Zhang, J., ‘Simulation of turbulent combustion and NO formation in a swirl combustor’, Chemical Engineering

Science, Vol. 64(12), (2009) 2903 – 2914. [11] Ishak, A., Shaiful, M., Jaafar, M. and Nazri, M., ‘The Effect of Swirl Number on Reducing Emissions from Liquid Fuel Burner

System’, Jurnal Mekanikal, Vol. 19, (2005) 48 – 56. [12] Zhou, L.X., ‘Advances in numercal modeling of turbulent reaction rate of NOx formation’, Adv. Mech., (China), 30(1), (2000),

77–82. [13] Mahmoodi Arya, M., Fahimirad, A., Baziar, P., Heravi, M.H. and Hajipour, A., “Experimental and Numerical Study on the

Effect of Dilution in Premixed CH4-Air Flame”, International Journal of Mechatronics, Electrical and Computer Technology, Vol. 4(12), (2014), 1155-1174.

[14] Fahimirad, A., Mahmoodi Arya, M., Hajipour, A., Baziar, P. and Heravi, M.H., “The Effect of the Flue Gas Recirculation on

NOX Formation in a Premixed Methane/Air Flam”, International Journal of Mechatronics, Electrical and Computer Technology, Vol. 4(12), (2014), 1214-1231.

[15] Khelil, A., Naji, H., Loukarfi, L. and Mompean, G., ‘Prediction of a high swirled natural gas diffusion flame using a PDF mode’, Fuel Vol. 88, (2009), 374-381.

[16] Versteeg, H.K. and Malasekera, W., ‘An introduction to computational fluid dynamics The finite volume method ’, Longman Scientific & Technical (1996).

[17] FLUENT. Fluent Inc., Fluent 6.3.26 User Manual, 2007. [18] Kwark, J., Jeong, Y.K., Jeon,C.H. and Chang, Y.J., ‘Effect of swirl intensity on the flow and combustion of a turbulent

non-premixed flat flame’. Flow, Turbulence and Combustion, Vol. 73(3-4), (2004), 231–257.



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Authors

Peyman Baziar (P. Baziar) is born in Amol, Iran in 1983. He received his B.Sc. in Mechanical Engineering from Islamic Azad University of Sari in 2006 and received his M.Sc. in Mechanical Engineering from Islamic Azad University of Mashhad in 2009. He published 2 journal papers and 13 conference papers. His current research interest includes Thermodynamics and Combustion. He is a member of Iranian Society of Mechanical Engineering (ISME). He is currently works in Total Productive Maintenance (TPM) unit of Kalleh Dairy Co., Amol, Iran and as a lecturer at University of applied science and technology-FZA center, Amol, Iran. E-mail: [email protected]

Alireza Hajipour (A. Hajipour) is born in Amol, Iran in 1984. He received his BSc. degree in Mechanical Engineering – Heat and Fluids from Islamic Azad University of Sari in 2007 and received MSc. degree in Mechanical Engineering- Energy Conversion from Science and Research Branch of Islamic Azad University in 2013. He is a PhD. student in Mechanical Engineering – Energy Conversion at Central Tehran Branch of Islamic Azad University (IAUCTB), Tehran, Iran. His current research interest includes Thermodynamics, Air-Standard Cycles, Internal Combustion Engines, Energy and Exergy. Alireza Hajipour is a member of editorial

board of International Journal of Mechatronics, Electrical and Computer Technology (ISC); also he is a reviewer of Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering (ISI), International Journal of Engineering (Scopus) and International Journal of Mechatronics, Electrical and Computer Technology (ISC). He was a member of Executive Committee of International Conference on Nonlinear Modeling & Optimization (ICMNO), Aug 2012, Amol, member of scientific committee of First Regional Conference on Municipalities and Electronic Citizen, Dec 2013, Amol, The 2nd Regional Conference on New Achievements in Electrical and Computer Engineering, November 2014, Jouybar, Iran and member of scientific and executive committee of 2nd International Conference on Nonlinear Modeling & Optimization (ICNMO) & 1st International Conference on Knowledge-Based Engineering Science, May 2015, Dubai, UAE. He published one book: An Introduction to Matlab (2014), (339 pages), (in Persian), 10 journal papers (3 of them are in Scopus) and 5 conference papers. He is a member of Iranian Society of Mechanical Engineering (ISME), Supreme Association of Iranian Elites – Mazandaran and the Young Researchers and Elite Club of Ayatollah Amoli Branch, Islamic Azad University, Amol, Iran. He is currently as a lecturer at Islamic Azad University of Mahmoudabad and Khazar Institute of Higher Education, Mahmoudabad, Iran. E-mail: [email protected]

Afshin Fahimirad (A. Fahimirad) is born in Amol, Iran in 1984. He received his B.Sc. degree in Mechanical Engineering from Islamic Azad University of Sari in 2006 and received his M.Sc. degree in Mechanical Engineering from Islamic Azad University of Mashhad in 2009. His current research interest includes Thermodynamics and Combustion. He published 4 journal papers and 30 conference papers. He is member of Iranian Society of Mechanical Engineering (ISME). He is currently works as manager of Research and Development (R&D) of Iran Heavy Diesel Mfg. Co. (Desa), Amol, Iran and as lecturer at Islamic Azad University of Mahmoudabad, Iran. E-mail: [email protected]






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Mohammad Mahmoodi Arya (M. Mahmoodi Arya) is born in Mashhad, Iran in 1983. He received his B.Sc. degree in Mechanical Engineering from Industrial University of Shahrood in 2006 and received his M.Sc. degree in Mechanical Engineering from Islamic Azad University of Mashhad. His current research interest includes Thermodynamics and Combustion. He published 3 journal papers and 18 conference papers. He is member of Iranian Society of Mechanical Engineering (ISME). E-mail: [email protected]

Hamid Momahedi Heravi (H. M. Heravi) received his PhD degree in Mechanical Engineering from Cardiff University. He currently serves as an adjunct instructor at Temple University, Philadelphia. He was an Associate Professor of Mechanical Engineering at Islamic Azad University, Mashhad Branch, Iran. His Principal research interests lie in the field of Combustion and Pollution both modeling and experimentally and include: 1. Modeling of Premixed and Non Premixed Combustion, 2. Burning Velocity, 3. Bio Fuel Production, Combustion and Pollution and 4. The Strategies of NOx Reduction. He has authored and co-authored over fifty

peer-reviewed research articles published in the leading engineering journals and prominent conferences. He is member of Association of Nanotechnology, Association of Combustion of Iran and Iranian Society of Mechanical Engineering (ISME). E-mail: [email protected]

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