Structure and Lean Extinction of Premixed Flames ... - ACS Publications

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Structure and lean extinction of premixed flames stabilized on conductive perforated plates Ahmed M Gamal, Abdelmaged H. Ibrahim, Elsayed-Mahdi M Ali, Fawzy M Elmahallawy, Ahmed Abdelhafez, Medhat Ahmed Nemitallah, Sherif S. Rashwan, and Mohamed A. M. Habib Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02874 • Publication Date (Web): 16 Jan 2017 Downloaded from http://pubs.acs.org on January 17, 2017

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Structure and lean extinction of premixed flames stabilized on conductive perforated plates Ahmed M. Gamal*, Abdelmaged H. Ibrahim*, Elsayed-Mahdi M. Ali*, Fawzy M. Elmahallawy*, Ahmed Abdelhafez†§, Medhat A. Nemitallah¶†, Sherif S. Rashwan†, and Mohamed A. Habib† *

Mechanical Power Engineering Dept., Faculty of Engineering, Cairo University, Giza, Egypt

†

KACST-TIC on CCS and Mechanical Engineering Department, Faculty of Engineering, King

Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia ¶

Mechanical Engineering Department, Faculty of Engineering, Alexandria University,

Alexandria 21544, Egypt KEYWORDS: Perforated plate; Premixed flames; Burner heat transfer; Lean extinction; Flame structure

The structure and lean extinction of premixed LPG-air flames seated on conductive perforated plates were examined experimentally, with focus on the effects of plate material, thickness, and hole diameter. The lean extinction limit was determined by gradually reducing the fuel flow rate for a given air flow rate, until extinction occurred. Flame structure was quantified by mapping the local mean temperature and species concentrations and by imaging the average visual length of flame plume. Pyrometer measurements of the temperature of upper plate surface were made to §

Corresponding author. Email: [email protected], Tel: +966-13-860-7869. KFUPM Box 2095, Dhahran 31261, KSA.

ACS Paragon Plus Environment

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estimate the heat transfer through the plate. It was found that the flames stabilized on plates with higher thermal conductivity were shorter and more stable (i.e., have lower lean extinction limits). This was attributed to preheating of fresh reactant mixture by greater heat transfer through the plate. Increasing the hole diameter (percentage open area) was found to enhance flame stability by reducing the reactant jet velocity for a given flow rate of reactant mixture. Heat transfer through the plate deteriorated with increasing hole size. However, the positive effect of smaller jet velocity on flame stability overpowered the negative effect of reduced heat transfer, and the net result was enhanced stability with larger hole sizes. Plate thickness, on the other hand, was found to have a weak effect on flame stability and structure. Thicker plates showed slightly better stability characteristics because of greater heat transfer through them. Nonetheless, plate heat transfer did not affect flame stability as significantly as jet velocity did.

1. Introduction Gas-turbine engine manufacturers on both the utility and aviation scales have traditionally utilized diffusion-flame combustion technology, because it offers reliable performance and simple load control [1,2]. This technology, however, generates excessively high levels of NOx emissions that do not meet the progressively stricter environmental regulations anymore [3-5]. This, accordingly, led to the development of lean-premixed (LPM) combustion technology [6,7], where fuel is premixed in lean proportions with part of compressor-discharge air upstream of reaction zone. No stoichiometric zones are, thus, formed within the combustor, which significantly reduces thermal NOx [8,9]. Operating under lean conditions, however, poses the challenge of firing the combustor near its lean blowout limit, which triggers the need for seeking efficient flame-stabilization techniques [10-12].


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One of the earliest and most fundamental techniques is jet stabilization in Bunsen-type [13] and perforated-plate burners, which are widely used in many domestic and industrial combustion applications. Although jet stabilized flames have been extensively investigated [14], recent studies still provide useful insights into their structure and stabilization and blowout mechanisms. Chen et al. [15] examined highly-stretched turbulent stoichiometric premixed CH4air flames on a piloted Bunsen burner at three different mean exit velocities. The large-diameter annular pilot flame ran on CH4 and air, premixed in stoichiometric proportions as well. LDA, combined with 2D Rayleigh thermometry and line Raman/Rayleigh laser-induced predissociation fluorescence (LIPF)-OH, were used to study the flame in detail. The three examined central flames represented the entire range of distributed reaction zones [16]. It was found that instantaneous local temperatures within the early mixing layer between the unburnt central methane-air jet and the annular pilot flame are significantly lower than the adiabatic flame temperature, which was attributed to the short residence time and to heat loss to the burner. With increasing residence time in the axial direction, the mean temperature was found to increase. Cabra et al. [17,18] experimentally and numerically studied a non-premixed jet-in-hot-coflow (JHC) burner configuration with a lifted turbulent H2/N2 jet flame in a hot vitiated coflow of combustion products from a lean H2/air flame (0.25 equivalence ratio, 1045 K). Rayleigh and Raman scattering, combined with LIF, were used to perform simultaneous measurements of temperature and concentrations of OH and NO species. A thickened turbulent reaction zone was observed at flame base. Both the experimental and numerical results supported the possibility of propagation of turbulent premixed flame by small-scale recirculation (~ flame thickness) and mixing of hot products and reactants, followed by rapid ignition of the mixture. Numerical models were also developed to predict the experimental lift-off height.


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Medwell et al. [19] also investigated a JHC burner geometry, where C2H4/H2, C2H4/air, and C2H4/N2 fuel mixtures were burned in 1100-K coflows of highly-diluted artificial air (3% and 9% O2) to mimic combustion with moderate or intense low oxygen dilution (MILD). Ethylene was chosen because it is a key species in the oxidation mechanisms of heavier hydrocarbon fuels and in the mechanism of soot formation. Soot was observed to be suppressed due to the hightemperature of coflow. Imaging of the hydroxyl radical (OH) and formaldehyde species (H2CO) was performed to visualize the reaction zone. Although the examined flames appeared lifted, OH and H2CO were observed within the lift-off region down to the jet exit plane. Transition from weak to strong OH was observed at the lift-off height as expected. It was thus concluded that pre-ignition reactions occur upstream of the main flame front at the lift-off height. Dunn et al. [20] designed a piloted premixed jet burner (PPJB) to examine the effects of finiterate chemistry in highly turbulent LPM flames. Their axisymmetric design comprised a LPM high-velocity central jet, surrounded by a coaxial stoichiometric premixed low-velocity pilot that acts as an ignition source for the central jet. Both central and pilot jets are further surrounded by a large-diameter coflow of H2-air combustion products to prevent the quenching and dilution effects of ambient-air entrainment from affecting the central combustion process. Such configuration of a LPM central jet piloted by a stoichiometric annular flame mimics LPM gasturbine combustors utilizing dual annular counter-rotating swirlers (DACRS) premixers [21-24], but without the complications of swirl and flow recirculation. Two-dimensional Rayleigh-PLIF (OH) and LDV measurements were conducted to demonstrate that the PPJB design provides efficient stabilization for flames with intense shearing and significant finite-rate chemistry, like the conditions encountered in highly turbulent LPM combustion systems.


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On the industrial scale, the principles of premixed jet stabilization on perforated plates were used to develop low-emissions micromixer nozzles [25], which have demonstrated reliable operation in gas-turbine combustors [26-28]. The geometry of micromixer faceplate (jet spacing and diameter and number of jets) is examined to optimize emissions and control combustor operability [29]. The airflow pressure drop across micromixer and the operational equivalence ratio are both chosen carefully to prevent flame flashback into the micromixer [30]. This study aims at examining premixed flames stabilized on conductive perforated plates. The effects of geometry (hole diameter, number of holes, and plate thickness) are investigated experimentally. Plate thermal conductivity is also examined to highlight its key role in controlling heat transfer through the plate. Conductive perforated plates were studied by Altay et al. [31], who numerically examined the effects of operating conditions and plate geometry. Conductivity values up to 10 Wm-1K-1 were considered. They reported that the flame consists of multiple cone or Gaussian-shaped flamelets that anchor downstream of the plate holes; these flamelets are connected by U-shaped flame fronts that stabilize downstream of stagnation regions at the plate surface between holes. It was found that burning velocity and flame structure are affected significantly by the equivalence ratio and mean jet velocity of the reactant mixture as well as by plate thermal conductivity and hole spacing. In the extreme case of an adiabatic plate, the flamelets take a conical shape anchored near the corner of hole exit. This shape changes to Gaussian with finite heat transfer to the plate, and the flamelets stabilize at a stand-off distance that grows with plate conductivity. It was, thus, claimed that heat flux to the plate declines at higher plate conductivity, which reduces its topsurface temperature. No conductivity values beyond 10 Wm-1K-1 were examined by Altay et al. [31], which considerably limited the magnitude of heat transferred through the plate to preheat


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the incoming reactant mixture. The effect of significant reactant preheating on flame stability was thus missed. This motivated the current study to examine conductive copper plates (385 Wm-1K-1). Altay et al. [31] also found that the flame stand-off distance grows with mean jet velocity of reactant mixture but shrinks at higher equivalence ratios raising the plate temperature. Greater hole spacing was found to increase heat transfer to the plate with a corresponding drop in burning velocity of the U-shaped flames between holes. The temperature of top plate surface increases because of such greater heat transfer to the plate, which shrinks the flame stand-off distance. The flame base was found to have high concentrations of intermediate species, like CO. In a similar study, Kedia and Ghoniem [32] numerically investigated the mechanisms of stabilization and blowout of a laminar premixed flame seated on a heat-conducting perforated plate. Conductivity was again limited to 1.5 Wm-1K-1, and heat transfer to the plate was treated as “heat loss” that lowers the burning velocity. Nonetheless, useful insights were provided into the stabilization and blowout mechanisms as the reactant jet velocity is increased at constant equivalence ratio. As jet velocity increases, the recirculation zones between flamelets grow and the stagnation points move downstream. The radius of curvature of flame base decreases to accommodate the growing but confined flame length, which reduces flame strength, lowers the burning velocity at the base, and brings the flame there closer to the stagnation point. At even higher jet velocity, the radius of curvature of flame base approaches its physical minimum (comparable to flame thickness) and remains nearly constant until blowout. Earlier studies of perforated-plate burners focused on their combustion dynamics, which depend on the convoluted effects of flame interactions with acoustic waves and combustor walls [33,34]. Thus, the geometry and thermal properties of perforated plate have significant effects on the steady flame structure and unsteady flame dynamics. The thermal interaction between flame and


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plate top surface was found to generate significant oscillations in burning velocity [35,36]. Such interaction forces the flame to stabilize at a finite stand-off distance away from the plate [37]. Earlier models [38] of perforated-plate-seated flames solved the uncoupled, unsteady conservation equations in one dimension only, while using the flame surface equation to describe the flame motion. Such models are based on the limitation of planar flame assumption, which forces the mean burning velocity to match the mean reactant jet velocity. At higher jet velocities, however, the flame is not flat, and the effect of oscillations in flame area needs to be modeled. 2. Experimental setup and approach The aim of this experimental study is to examine different factors affecting the structure and lean extinction of premixed flames stabilized on conductive perforated plates. Factors examined are plate material, thickness, and hole diameter. The latter controls the percentage open area, which is defined here as the ratio of total hole cross-sectional area to plate cross-sectional area (based on plate outer diameter of 42 mm). Figure 1 shows a schematic of the experimental setup. The vertical burner used comprises a cylindrical heat-treated iron tube, with an inner diameter of 42 mm and a length of 320 mm, as shown in Figure 2. The tube consists of two parts that thread together to hold a ceramic foam (50 ppi porosity) that acts as a flame trap and mixing device. A threaded cap at burner exit secures the tested perforated plate. Seven different plates were examined, as specified in Table 1. The plates were designed and categorized into three test groups to target the three factors of interest, namely plate material (group I), hole diameter (group II), and plate thickness (group III).


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Figure 1. Schematic diagram of the test facility.

Figure 2. Premixing burner used in the present study.


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Table 1. Specifications of tested perforated plates. Plate #

Plate material

Test group

Hole diameter [mm]

Plate thickness [mm]

Hole spacing [mm]

Hole edge-toedge distance [mm]

No. of holes

% open area

0

Ceramic

I

2

12

3

1

180

40.8

1

Copper

I

2

12

3

1

180

40.8

2

Copper

II

1

6

2

1

400

22.7

3

Copper

II

2

6

3

1

180

40.8

4

Copper

II, III

4

6

5

1

70

63.5

5

Copper

III

4

12

5

1

70

63.5

6

Copper

III

4

24

5

1

70

63.5

The fuel used in this study is commercial liquefied petroleum gas (LPG), which is a mixture of propane and butane (~50/50% by volume), sourced from the Egyptian Petrogas Company. Fuel and air enter the burner at its lower end and mix upstream of ceramic foam. Rotameters were used to meter the flow rates of fuel and air with an accuracy of ± 4% of measured value. The obtained readings were corrected for non-atmospheric flow pressure at the exit of each rotameter, as specified by the manufacturer. Fuel flow readings were also corrected for fuel specific gravity, since the fuel rotameter came pre-calibrated for air flow. Since this study examines flames stabilized on conductive perforated plates, heat transfer from the flame to upper plate surface then through the plate itself to the fresh incoming reactant mixture is expected to play a significant role in flame stability and structure. To analyze such heat transfer, the temperature of plate surface was measured by means of a pyrometer. After 15 minutes of continuous steady run time, the air and fuel flows were rapidly stopped, and the


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temperature of plate surface was measured immediately after shutdown. It was found iteratively that 15 minutes is a long enough time to reach steady state, i.e., longer run times prior to shutdown did not yield higher temperature readings. Representative emissivity values of 0.05 and 0.90 [39] were chosen for the copper and ceramic surfaces, respectively. It should be noted, however, that the obtained surface temperatures are only as accurate as those emissivity values. Use of this data was thus limited to conducting semi-quantitative comparisons between plates based on plate heat transfer. The lean extinction limits were determined for all tested plates by implementing the following approach: a) Burner light-off and adjustment of air and fuel flow rates to attain a stable flame with an equivalence ratio of 0.9, b) 15-min wait to ensure that steady-state conditions have been reached, and c) Gradual reduction of fuel flow for the same air flow rate, until extinction occurs. Seven different air flow rates were examined for each plate, namely 5.3, 6.5, 7.8, 9.3, 10.8, 12.4, and 14.3 kg/h air. The performance curve of each plate (critical equivalence ratio for extinction vs. air flow rate) was thus compared to other plates within the same test group, in order to highlight the effects of plate material, thickness, and hole diameter on lean extinction. Flame structure was quantified by mapping the local mean temperature and species concentrations. A precision traversing mechanism was used for this purpose with 0.05-mm and 1-mm accuracies in the radial and axial directions, respectively. Temperature was measured at steady state (after 15 min) by means of a bare-junction type-B thermocouple (Pt-6%Rh vs. Pt30%Rh) with 200-µm junction diameter and 20-cm length. The thermocouple wires were protected inside a 2-hole ceramic tube of 1.5-mm outer diameter for structural strength and electrical insulation.


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Local mean temperatures were obtained by averaging local instantaneous readings. A dataacquisition system was designed to sample continuously at a rate of 500 Hz while averaging the latest 10,000 instantaneous readings (20 seconds) into one mean value. A data point of local mean temperature was recorded only after that mean value was noticed to fluctuate about a steady value. Consequently, a new data point was usually recorded after 25-30 seconds of traversing the thermocouple. The data-acquisition system consists of a DAQ card installed in a personal computer and programmed via LabVIEW. An I/O connector block amplifies the thermocouple EMF by a gain factor of 100 to be accurately detectable by the DAQ card. The recorded voltage data was converted to temperature values using the calibration tables provided by thermocouple manufacturer. The temperature readings were finally corrected per the model of Brohez et al. [40] for the radiation error associated with bare-junction measurements in an open flame. All corrected temperature data was, nonetheless, still used to perform qualitative comparisons only, because the surface of thermocouple junction was not treated with chemically-inert coating to neutralize the catalytic effect of platinum. Species concentrations were quantified at steady state (after 15 min) using a Land LANCOM 6500A portable electrochemical gas analyzer. Local gas samples were withdrawn from the flame by means of a water-cooled sampling probe. Three concentric stainless-steel tubes of 1.5-mm, 4mm, and 6-mm outside diameters and 20-cm length were used to construct the probe. Flame-structure mapping (temperature and species concentrations) was performed at constant jet equivalence ratio (φ = 0.9) and constant fuel and air flow rates of 0.54 and 9.3 kg/h, respectively, which corresponds to a heat load of 6.7 kW. Measurements of species concentrations were limited to the burner axis (R = 0) to avoid the effect of ambient-air entrainment at the boundary of such open flames.


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The average visual length of flame plume was quantified by capturing photographic images of the visual plume appearance next to a reference length scale. All shots were taken using a 10Mega-pixel camera operated in night-vision mode with 1/60-s shutter speed, 5.6 f-stop, and 1600 ISO to capture the average plume shape. An approximate length of visual plume was then determined from the photos by comparing the plume shape to the reference scale. To eliminate the effect of ambient-air entrainment in unconfined flames, imaging of visual plume length was repeated for confined flames in a quartz tube of 10-cm diameter and 55-cm length. Flame temperature and species concentrations were not mapped inside the confined flames. 3. Results and discussions This study experimentally examines the structure and lean extinction of premixed flames seated on conductive perforated plates. Figure 3 shows an image and a descriptive schematic of flame structure. Two distinct reaction zones can be identified; a primary zone formed of multiple coneshaped flamelets (similar to the findings of Altay et al. [31] and Kedia and Ghoniem [32]), and a secondary homogeneous zone (plume) downstream of the primary one. Each flamelet within the primary zone is seated at the exit of a hole in the perforated plate. The cone half-angle, and consequently cone height, depend on the balance between flame speed and jet velocity of fresh combustible mixture exiting the hole. A region of relatively lower velocities and base flow recirculation exists at the plate surface between the flamelets. Such recirculation is the main factor affecting heat transfer from flamelet zone to the perforated plate. Heat absorbed by the plate preheats the fresh combustible mixture as it flows through the holes, which significantly affects the structure and lean extinction of the flames under investigation. The focus of this study is, thus, to quantify how plate material, thickness, and hole diameter affect heat transfer through the plate and, consequently, flame structure and stability.


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Secondary reaction zone (plume)

Primary (flamelet) reaction zone

Figure 3. Structure of flame seated on perforated plate.

To shed more light on the characteristics of examined flamelets, the regime diagram of Peters [16] for premixed flames was consulted, see Figure 4. This diagram classifies the premixed flame structure based on the RMS of velocity fluctuations (u’), normalized by the laminar flame speed (sL), and the integral length scale (l), normalized by the thermal flame length (lF). A common approximation for u’ is 10% of the average jet velocity of reactant mixture (Ujet), which was calculated using the following equation:

(1)

Where is the air mass flow rate at a given test point, is the corresponding fuel mass flow rate at the equivalence ratio of test point, is the density of air-fuel mixture (approximated at 1 bar and 300 K), is the cross-sectional area of one hole, and N is the number of holes in perforated plate (see Table 1).


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Figure 4. Regime diagram of Peters [16] for premixed flames. The red dots represent flamelets at 9.3 kg/h air and φ = 0.9 for different hole diameters and plate materials.

The laminar flame speed (sL) for LPG fuel was approximated to 37.9 cm/s as the mass-weighted average of the corresponding laminar flame speeds of propane and butane, namely 39.6 and 36.6 cm/s, respectively. The integral length scale (l) is defined as the radial distance from flame centerline to the point of maximum temperature, which roughly corresponds to the hole radius in this case, since the flamelets are conical in shape with a base diameter approximately equal to the hole diameter. The thermal flame length (lF), on the other hand, is defined as the axial distance measured along the flame centerline from flame base to the point of maximum temperature, which corresponds here to flamelet height. If the analysis of Peters’ diagram is performed on plates #0, 1, 2, and 4 in Table 1 to examine different plate materials and hole diameters at 9.3 kg/h air and an equivalence ratio of 0.9, it can be seen from Figure 4 that all tested flamelets have laminar plane flame fronts. It is also


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interesting to highlight here that the Reynolds number of a reactant-mixture jet exiting a hole was found to vary from 270 to 1050 under cold-flow (non-reacting) conditions, which shows that all flames examined in this study have laminar jets exiting the holes. From a Biot-number perspective, all examined copper plates can be treated as lumped capacitances with Bi

Structure and Lean Extinction of Premixed Flames ... - ACS Publications

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