International Journal of Advanced Thermofluid Research Vol. 3, No. 1, 2017 ISSN 2455-1368 (Online) Published by: International Research Establishment for Energy and Environment (IREEE), Kerala, India. (www.ijatr.org; www.ireee.net) 16 Experimental and Numerical Investigation of LPG Fuelled Inverse Diffusion Flame in a Coaxial Burner Vipul Patel and Rupesh Shah * Department of Mechanical Engineering, Sardar Vallabhbhai National Institute of Technology, Surat 395007, India * Corresponding Author: E-mail: [email protected]Phone: +91-9824172452 Abstract: An experimental and numerical investigation of unconfined liquefied petroleum gas (LPG) fuelled inverse diffusion flame (IDF) is presented. The effect of air-fuel velocity ratio on the appearance of the LPG IDF are investigated. A fixed air jet velocity of Va = 11.31 m/s is chosen for the analysis, while the fuel jet velocity is varied from 0.014 m/s to 0.083 m/s. The observations indicate that the air-fuel jet velocity ratio has significant effect on the flame appearance of LPG IDF. It is found that the flame length increases with increase in fuel jet flow. Actual flame in burner is highly complex in nature as it involves momentum, mass and energy transfer in highly turbulent flow regime. To extract detailed information of flow physics, numerical analysis of LPG IDF is performed. It is observed that temperature along the centerline of IDF increases as distance from burner exit (Z) increases. This nature of temperature distribution indicates that the central cold air in the air jet is gradually heated toward the downstream of the IDF. The highest flame temperature is noticed at Z = 30 mm. The highest flame temperature predicted numerically at axial height of 30 mm is central portion of the blue zone of the flame which is main reaction zone. Keywords: Inverse diffusion flame; Flame appearance; Flame length; Coaxial burner. 1. Introduction Domestic and commercial combustion systems generally employ premixed flame due to its relatively clean and rapid combustion characteristics. Premixed flame has narrow flammable limits hence limited stability. Inverse diffusion flame (IDF) is non-premixed diffusion flame. It is a unique kind of flame where high mass flow rate of inner air jet is surrounded by low mass flow rate of outer fuel jet. High momentum air jet ensures the entrainment of fuel into air jet and enhances mixing of fuel and air compared to normal diffusion flame (NDF). IDF possesses characteristics of wide range of stability, better operational safety and less emissions compared to NDF. Wu and Essenhigh (1984) described six different regimes of methane IDF, based on air-fuel velocities, visible appearance and temperature characteristics. Makofski et al. (2004) studied the effect of air flow variation on flame structure of methane and ethylene IDF using hydroxyl laser induced fluorescence with theoretical scale estimation. Zhen et al. (2011a) investigated the effect of nozzle length in multi jet fuel burner. They reported that the potential core and flame base were shorter with short nozzles compared to long nozzles. The effects of addition of natural gas and air on inverse flame structure were investigated by Sobiesiak and Wenzell (2005). Mahesh and
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International Journal of Advanced Thermofluid Research Vol. 3, No. 1, 2017 ISSN 2455-1368 (Online)
Published by: International Research Establishment for Energy and Environment (IREEE), Kerala, India. (www.ijatr.org; www.ireee.net)
16
Experimental and Numerical Investigation of LPG Fuelled Inverse Diffusion Flame in a Coaxial Burner
Vipul Patel and Rupesh Shah*
Department of Mechanical Engineering, Sardar Vallabhbhai National Institute of Technology, Surat 395007, India
Abstract: An experimental and numerical investigation of unconfined liquefied petroleum gas (LPG) fuelled inverse diffusion flame (IDF) is presented. The effect of air-fuel velocity ratio on the appearance of the LPG IDF are investigated. A fixed air jet velocity of Va = 11.31 m/s is chosen for the analysis, while the fuel jet velocity is varied from 0.014 m/s to 0.083 m/s. The observations indicate that the air-fuel jet velocity ratio has significant effect on the flame appearance of LPG IDF. It is found that the flame length increases with increase in fuel jet flow. Actual flame in burner is highly complex in nature as it involves momentum, mass and energy transfer in highly turbulent flow regime. To extract detailed information of flow physics, numerical analysis of LPG IDF is performed. It is observed that temperature along the centerline of IDF increases as distance from burner exit (Z) increases. This nature of temperature distribution indicates that the central cold air in the air jet is gradually heated toward the downstream of the IDF. The highest flame temperature is noticed at Z = 30 mm. The highest flame temperature predicted numerically at axial height of 30 mm is central portion of the blue zone of the flame which is main reaction zone. Keywords: Inverse diffusion flame; Flame appearance; Flame length; Coaxial burner.
1. Introduction
Domestic and commercial combustion systems generally employ premixed flame due to its relatively clean and rapid combustion characteristics. Premixed flame has narrow flammable limits hence limited stability. Inverse diffusion flame (IDF) is non-premixed diffusion flame. It is a unique kind of flame where high mass flow rate of inner air jet is surrounded by low mass flow rate of outer fuel jet. High momentum air jet ensures the entrainment of fuel into air jet and enhances mixing of fuel and air compared to normal diffusion flame (NDF). IDF possesses characteristics of wide range of stability, better operational safety and less emissions compared to NDF. Wu and Essenhigh (1984) described six different regimes of methane IDF, based on air-fuel velocities, visible appearance and temperature characteristics. Makofski et al. (2004) studied the effect of air flow variation on flame structure of methane and ethylene IDF using hydroxyl laser induced fluorescence with theoretical scale estimation. Zhen et al. (2011a) investigated the effect of nozzle length in multi jet fuel burner. They reported that the potential core and flame base were shorter with short nozzles compared to long nozzles. The effects of addition of natural gas and air on inverse flame structure were investigated by Sobiesiak and Wenzell (2005). Mahesh and
International Journal of Advanced Thermofluid Research Vol. 3, No. 1, 2017 ISSN 2455-1368 (Online)
Published by: International Research Establishment for Energy and Environment (IREEE), Kerala, India. (www.ijatr.org; www.ireee.net)
17
Mishra (2008 & 2010) carried out experiments on liquefied petroleum gas (LPG) IDF stabilized in backstep burner with different air-fuel ratios. Elbaz and Roberts (2014a) investigated the role of entrainment and mixing in methane IDF. Stelzner et al. (2013) performed experimental and numerical investigations of structure of rich methane IDF. Makofski et al. (2006) measured flame height of ethylene and methane IDF with planar laser-induced fluorescence of hydroxyl radicals. They observed a greater flame height of luminous zone than the reaction zone due to presence of luminous soot. Mahesh and Mishra (2011) compared LPG IDF in coaxial and recessed backstep burner. They observed more compact flame shape in the backstep burner. Elbaz and Roberts (2014b) characterized methane IDF, based on temperature distribution, flame height and emission characteristics. Sze et al. (2004) experimentally investigated heat flux and temperature distribution in butane IDF. Bhatia et al. (2012) numerically analyzed the effect of oxygen enrichment and gravity variation on flame structure of the IDF and NDF. Barakat et al. (2013) investigated the effects of jet dynamics and geometrical parameters of burner in LPG IDF. Sze et al. (2006) revealed that intense air-fuel mixing causes higher flame temperature in circumferentially arranged ports compared to coaxial arranged ports. Takagi et al. (1996) numerically analyzed the influence of preferential diffusion on the temperature characteristics of NDF and IDF. Dong et al. (2013) investigated heat transfer characteristics in butane IDF with different air-port diameters. Zhen et al. (2011b) investigated the influence of swirling on heat transfer characteristic in LPG IDF. Porous medium (PM) combustion is also an immersing technique for domestic and industrial burners due to higher burning rates with less pollutant emissions. Muhad et al. (2011) worked on enhancement of a partially premixed PM combustor. Abdul Mujeebu et al. (2013) carried out experimental and numerical studies on surface combustion in PM and developed a premixed PM burner which is suitable for normal household applications.
The objective of the present work is to examine the influence of air-fuel velocity ratio on the visible appearance of LPG IDF in a coaxial burner. In this respect the flame length, flame shape and structure is evaluated at different air-fuel velocity ratios. The experimental results are used in combination with numerical results of temperature profiles and velocity profiles to analyze the flame appearance and flame length.
2. Experimental Setup The coaxial burner used in the present work is shown in Figure 1 and Figure 2. The burner has concentric tubes with diameter ratio of 2.16. The burner consists of two coaxial tubes with inner tube of 13 mm diameter and outer tube of 28 mm diameter. With separately supplied air and fuel, the central air jet is surrounded by annular fuel jet. Figure 3 shows the photograph of the experimental setup. The air is supplied through the blower, while the fuel is supplied from LPG cylinder. The fuel and air flow rates are measured by calibrated rotameters (accuracy ± 2.0 % of full scale). LPG used as a fuel, contains 69% C4H10 (butane), 30% C3H8 (propane) and other trace of gases by volume. To prevent any disturbance from outside air, an enclosure with cross section 50 cm × 50 cm and height of 100 cm is used to surround the burner. The flame appearance of IDF is observed through a glass window provided at front side of the enclosure. Flame photographs are recorded by high resolution digital camera (24.2 megapixel). Ten photographs are taken for each IDF configuration. The
International Journal of Advanced Thermofluid Research Vol. 3, No. 1, 2017 ISSN 2455-1368 (Online)
Published by: International Research Establishment for Energy and Environment (IREEE), Kerala, India. (www.ijatr.org; www.ireee.net)
18
length of the flame is measured by an image processing software ImageJ (version 1.50i). Final flame length is determined by averaging of flame length measured from ten photographs. The flame length is defined as the distance between burner-exit to flame tip along the vertical centerline of IDF.
Air
Fuel
Outer tube
Inner tube
150 mm
Figure 1. Schematic of the coaxial burner Figure 2. Photograph of the coaxial burner
Figure 3. Experimental test setup.
3. Numerical investigation
Actual flame in burner is complex in nature as it involves momentum, mass and energy transfer in highly turbulent flow regime. To extract detailed information about temperature contours, temperature profiles and velocity profiles, numerical analysis is carried out. Three–dimensional domain of dimensions 0.5 m × 0.5 m × 1.5 m above burner exit is considered for numerical investigation (Figure 4 (a)). Unstructured mesh is generated in
International Journal of Advanced Thermofluid Research Vol. 3, No. 1, 2017 ISSN 2455-1368 (Online)
Published by: International Research Establishment for Energy and Environment (IREEE), Kerala, India. (www.ijatr.org; www.ireee.net)
19
this domain as shown in Figure 4(b). Mesh generation is accomplished with pre-processor GAMBIT. The air and fuel separately enter into the domain through bottom of the domain and products of combustion exit from the top of the domain. Air and fuel entry surfaces are defined as velocity inlets and outlet is define as pressure outlet (zero gauge pressure). Grid independence study is separately done to derive the grid independent size. The variation in the turbulent kinetic energy along the axis of the burner is considered as grid sensitivity parameter. Grid with total 1122083 elements is achieved as grid independent size. All reactive flow analysis is performed with this grid-independent size. FLUENT (ANSYS version 15.0.0) is used as processor for the simulation of the burner geometry. Continuity equation, momentum equations, equations related to k-ε turbulence model and Favre mean mixture fraction equation are used in solving present combustion problem numerically. The PDF (non-premixed combustion model) approach is used for the reaction modelling. PDF table is created for LPG as a fuel. In reactive flow analysis LPG is used as a fuel and the ratio of mole fractions of butane: propane taken is 0.7:0.3. In order to study variations in velocity and temperature, various stations have been considered at deferent axial locations in the domain.
(a) The geomety of domain (b) Meshed simulation model