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On the Combustion of Syngas in Intersecting Burners using Interferometry Method Mehrdad Kiani, Ehsan Houshfar, Amir Ehsan Niaraki Asli, and Mehdi Ashjaee Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01612 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017
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Energy & Fuels
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On the Combustion of Syngas in Intersecting
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Burners using Interferometry Method
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Mehrdad Kiani, Ehsan Houshfar*, Amir Ehsan Niaraki Asli, Mehdi Ashjaee
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School of Mechanical Engineering, College of Engineering, University of Tehran, P.O. Box
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11155-4563, Tehran, Iran
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ABSTRACT
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This paper presents the results of interferometry of flames and combustion properties by
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conducting extensive experimental measurements and numerical simulations of syngas flame in
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an intersecting burner setup. The experiments were performed to analyze the effects of key
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parameters on the flame structure and the temperature field of the intersecting slot burners in a
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dual flame syngas combustion setup using Mach-Zehnder interferometry technique. Reynolds
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number, equivalence ratio, intersecting angle of burners, and the jet-to-jet spacing between the
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burners were studied and validated with an acceptable accuracy. It was shown that, although
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increasing Reynolds number from 100 to 200 does not change the maximum flame temperature
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more than 125 K, it has a significant influence on the flame structure. In addition, the maximum
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temperature is directly related to the intersecting angle and has an inverse relation with the
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equivalence ratio and jet-to-jet spacing. The maximum temperature of 2211 K was observed for
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θ=100°. Results from the numerical simulations confirmed the data obtained from the
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interferometry and showed that NOx formation increases as the maximum observed temperature
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increases.
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KEYWORDS: Syngas, Intersecting slot burners, Flame structure, Equivalence ratio, NOx
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1
Introduction
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Synthesis gas or syngas, which is nowadays regarded as an alternative option to fossil fuels,
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can naturally be produced under specific circumstances by decomposition of wood, corn, coal,
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and generally by the conversion of various solid and liquid fuels. It, primarily, consists of
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hydrogen (H2) and carbon monoxide (CO), with lesser quantities of methane (CH4), and other
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hydrocarbons (CxHy).1 The ratio of hydrogen to carbon monoxide in syngas determines the
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energy release rate which plays a vital role in its thermal efficiency.2 On the other hand, type and
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shape of burners have a significant influence on the combustion quality.3 Premixed gas burners
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have several applications in industrial and residential processes, e.g., in melting sectors,
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industrial furnaces, and domestic consumption. Various types of multiple-burners such as
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parallel, counterflow, and intersecting burners are among the most common approaches to
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enhance the heat transfer rate.4
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Dong et al.,5 investigated the effects of distance between the nozzle and the impingement plate
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and Reynolds on the flame characteristics of two-parallel burners, with slot and circular nozzles.
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It was reported that, in comparison with circular nozzles, greater heat flux and a more stable
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flame are produced by the ones with slot cross section. Najafian et al.6 studied the effect of
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Reynolds number and jet-to-jet spacing in parallel slot burners, using Mach-Zehnder
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interferometry. Their results showed that the fluctuations of maximum flame temperature are
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barely related to the changes in Reynolds number. Moreover, they have reported that an optimum
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jet-to-jet spacing exists for parallel burners at each Reynolds number. In a recent cross-sectional
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study, Askari and Ashjaee7 investigated the effect of pressure on the laminar burning velocity
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and flammability limits of landfill gas, where their results indicate that reducing pressure
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increase laminar burning velocity. Studies over the past years have provided important
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information on the effects of mixing conditions for various fuels, experimentally8,9 and
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numerically10.
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The structure of laminar opposed-flow diffusion flames with syngas fuel was studied by Drake
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and Blint11 in a wide range of stretch conditions (α=0.1–5000 s-1). Their extensive research has
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shown that even in very high mixing time such as α=0.1 s-1, non-equilibrium effects were
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observable. These effects tend to grow in higher stretches. The flame modes can also alter upon
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manipulation of double concentric jets of disc bluff-body.12,13 Disimile et al.14 reported that for
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enhancing the fuel mixing, burners’ flow can collide through an angle rather than impinging
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concentrically. Impinging setups (also known as intersecting burners) are attractive for rocket
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engines as well.15 The effect of intersecting burners with a 45-degree angle between the slot
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nozzles (V-shaped burners) with propane fuel is also investigated by Li et al.16 using Particle
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Image Velocimetry (PIV) measurement technique. Data from their study on V-shaped burners
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suggest that flames produced by these burners are more stable in comparison with the flat-type
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configurations. The effect of adding hydrogen and carbon monoxide on flame characteristics of
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propane was studied by Chen et al.17 using PIV experiments. Their results indicate that
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increasing the burning velocity changes the flame structure from hill-type to M-type. In a
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numerical study, Ghiti et al.18 showed that the NOx emission formation rate is dependent on the
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flame temperature and fuel-air mixing efficiency.
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According to the above literature survey, the flame structure of various burners is affected by
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the operational and geometrical parameters. Although the multiple burners’ flame behavior is
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well studied in the previous investigations, the effects of various parameters such as Reynolds
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number, Equivalence Ratio of Fuel to Air (ERFA), and configuration of burners (angle and their
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distance) on the temperature field of intersecting slot burners have not been investigated yet.
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Overall, these studies highlight the need for further research on the structure of flame in an
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intersecting burner. The thorough understanding of these practical parameters is the main criteria
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for designing the efficient burners in combustion systems. In the current work, the effect of the
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intersecting angle of the burners and jet-to-jet spacing were studied in a range of θ=60°–100° and
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structure were also identified. Therefore, this project provided an important opportunity to
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advance the understanding of syngas combustion in the intersecting burners. Reynolds number
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was changed in a range of 100 to 200, and ERFA was varied from 0.8 to 2.5. This article
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discusses the effect of four mentioned parameters on the temperature; while the flame structure is
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regarded as the flames’ different zones (recirculation zone, flashback zone etc.) based on the
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temperature behavior of the flame. No mass fraction and velocity field investigation have taken
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place in our experiments. The effects of these parameters were investigated experimentally by
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Mach-Zehnder interferometry method. To validate the experimental case, a numerical case was
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defined identical to the experiments as well as using thermocouples.
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of 3.8 to 9.6, respectively. The effects of Reynolds number and equivalence ratio on the flame
Experiments
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The impinging burners’ setup was designed adjustable for burners’ heights, spacing, and
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intersecting angle, as shown in Figure 1 (a). Each slot burner was made of two carved stainless-
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steel planes with 60 mm depth, sealed with copper tapes with a thickness of 0.1 mm, creating a
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cross-section of 0.7×20 mm2. To achieve a uniform and fully-developed flow of syngas at the
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nozzles’ outlet, a perforated steel bar was located in middle of the nozzles (see Figure 1 (b)).
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(a) 89
(b)
Figure 1. (a) Holding and adjusting mechanism and (b) slot burner
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For conveying the fuel, a very long tube was used between the mixer and the slot burners to
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guarantee that the fuel is fully homogeneous before reaching the nozzle. Syngas was produced
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by gas mixing, where hydrogen and carbon monoxide were transferred from two capsules to the
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mixing chambers through two Fischer rotameters. The pressure and the purity of hydrogen
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container were 100 bar and 99.99%; while carbon monoxide capsule had a pressure of 180 bar
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and the same purity of 99.99%. In the mixing chamber, the fuel was mixed with injected air,
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supplied by a compressor working at 5 bar pressure, and the air flow rate was being metered by a
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Dwyer rotameter. After the mixing chamber, another two Fischer rotameters was utilized to
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divide the fuel into two equal portions for the two burners, illustrated in Figure 2. A type K
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thermocouple was utilized for validating the measured data from the interferometry method.
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Pressure and relative humidity of the laboratory environment were recorded during all the
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experiments.
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As a common method, thermocouples are being inserted at different points of the flame to
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capture the temperature field of the entire flame domain. Since inserting these thermocouples can
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influence the flow regime, non-contact evaluation methods such as interferometry are
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recommended.19 Interferometry methods are able to instantly measure the temperature field. In
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this method, the temperature at each point can be calculated based on the changes in the
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refractive index of the flame. In this study, Mach-Zehnder interferometry method was used for
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capturing refraction index of the combustion products. More information about the Mach-
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Zehnder method is presented elsewhere.20,21 As demonstrated in Figure 3, the light beam emitted
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from a Helium-Neon laser source with a maximum power of 5 mW and wave length of 632.8
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nm, passes the micro lens. By passing through the compiler, the beam splitter divides the
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compiled beam into two equal beams. While the first half of the splitted beam advances to the
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second mirror, the second half passes through the test case before reaching the second mirror. In
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the second compiler, the two beams eventually reach the CCD camera which is capable of
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capturing 30 frames per second and get delivered to the computing system. All the experiments
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are conducted in a controlled condition at the pressure of 0.87 bar and the temperature of 300 K.
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Figure 2. Fuel supplement schematic map
1. Helium-Neon laser 2. Micro lens 3. Compiler 1 4. Beam splitter 1 5. Test section 6. Mirror 1 7. Mirror 2 8. Beam splitter 2 9. Compiler 2 10. CCD camera
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Figure 3. Interferometry table
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Data Analysis
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Once the light emitted by the He-Ne laser source arrives at the beam splitter No. 1, it will split
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into two distinct beams. While one beam passes through a path without any refractive index
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variation, the other one is transmitted through the combustion products. Refractive index
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difference between the combustion products and air causes a significant phase difference
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between the two beams which results in dark and bright lines in the Fringe image (Figure 4).
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Figure 4. Fringe map of Syngas flame in an intersecting burner.
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The relation between the phase difference and refractive index is expressed by Equation 1.6
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Considering the origin of the coordinate system on the symmetry line at the same height of the
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nozzles’ outlet, ∆ø , 1 , , 2
(1)
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In Equation 1, ∆ø states the phase difference, is the wave length of the laser light, L is the
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length of the test section (which is equal to the length of the slot burners), n∞ is refractive index
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of the flame’s ambient, and n is the refractive index of air which varies by the changes in the
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location. The effect of the aspect ratio of the nozzles on the accuracy of the interferometry
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method was studied by Qi et al. 3. They have argued that by increasing the slot aspect ratio, the
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effect of the edges on the flame structure decreases. Thus, nozzles with higher aspect ratios are
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more appropriate for our purpose. In this study, the aspect ratio of the burners’ outlet was
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designed large enough, to compensate for the flames’ bending effect at the burners’ front and
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back. As a result, the refractive index along the laser’s path can be assumed constant.
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Consequently, the obtained pattern is consistent for the entire length of the burner. Thus, by
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neglecting the variation of refractive index along the beam’s path, Equation 1 can be simplified
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to Equation 2: ∆ø , , 2
(2)
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Each dark and bright line (flame Fringe) in the photograph has the same phase information. As
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a consequence, for each line, a non-negative integer number (Fringe Number, FN) is dedicated,
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as defined in Equation 3.6 While the integer numbers represent the bright lines, the other
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numbers stand for the dark lines. FN=0 is used for ambient gas while the first line has the
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FN=0.5. Using MATLAB image processing program,22 the distribution of the fringe numbers is
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determined. On the other hand, the Fringe number is in relation to the refraction index presented
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in Equation 4.6
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1 , 1, 2, 3, … 2
(3)
∆ø , 2
(4)
The 2D relation between the Fringe number and local refractive index in the xy plane is expressed by Equation 5.6
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(5)
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Refractive index is a function of temperature. So, by applying Gladstone-Dale
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approximations,6 the temperature field can be extracted by Equation (6). This Equation is valid
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for temperatures up to 6000 K.6 # , $
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157
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159
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the ambient temperature of the flame. To ease comparability of the obtained data of the slot nozzles with the other studies, a hydraulic diameter was defined, as in Equation 7.6
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4 ) 2 + )
calculated by Equation 8. -./0 120/3 &' 4./0
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(8)
Where ρmix is the density of the gaseous mixture, Vexit is the exit velocity, µmix is the dynamic viscosity of the mixture given by Equation 9. 4./0
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(7)
Where L and t are the slot’s length and width, respectively. Reynolds number can now be
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(6)
In Equation 6, T(x,y) is the local temperature at a given point in any specific plane and T∞ is
&' 160
1 %# , 1
∑4/ 6/ 78/ ∑6/ 78/
(9)
where µi is the dynamic viscosity of component i in the mixture, Yi is the mole fraction of each component and 8/ is the molecular weight.
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Energy & Fuels
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Mathematical Modelling and Numerical Simulation
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The schematic diagram of the impinging jets, burners and the computational domain is shown
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in Figure 5. According to the configuration of the experimental setup, the width of slot burners
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has been set to 0.7 mm. The intersecting angle and the jet-to-jet spacing of the burners are
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considered as the key parameters, and their effects on the flame temperature, structure, and NOx
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emission of a steady and turbulent two-dimensional syngas flow were investigated. In this
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model, the burners have a negligible thickness and are in stationary mode.
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Figure 5. Numerical domain and boundary conditions
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Due to the symmetry of the case configurations, only one-half of the computational domain is
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simulated. The working fluids (air-fuel mixture and combustion products) are Newtonian and
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incompressible ideal gases. The effects of free convection, radiation heat transfer, and viscose
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dissipation are neglected in the energy Equation. In view of the above assumptions, the mass-
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weighted Equations of mass, momentum and energy conservation for the turbulent reacting flow
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can be defined as:23 9-̅ 9 + ; ? 0 9) 9 ;
(10)
9 9 9AB 9 + =DEE =FEE ? + -̅ H; ; ? + / =>; ?
