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Emission of impinging swirling and non-swirling inverse diffusion flames
Applied Energy 88 (2011) 1629–1634
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Emission of impinging swirling and non-swirling inverse diffusion flames H.S. Zhen, C.W. Leung ⇑, C.S. Cheung Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
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Article history: Received 30 August 2010 Received in revised form 1 November 2010 Accepted 23 November 2010 Available online 21 December 2010 Keywords: Inverse diffusion flame CO/NOx emissions Fame impingement
a b s t r a c t The overall pollutants emission from impinging swirling and non-swirling inverse diffusion flames (IDFs) was evaluated quantitatively by the ‘hood’ method. The results of in-flame volumetric concentrations of CO and NOx and overall pollutants emission of CO and NOx in terms of emission index were reported. The in-flame volumetric concentrations of CO and NOx were measured through a small hole drilled on the impingement plate. In comparison with the corresponding open flame, the CO and NOx concentrations for the impinging swirling IDF are greatly lowered due to the entrainment of much more ambient air which is related to the increased flame surface area. For the swirling and non-swirling IDFs, the EINOx increases as the nozzle-to-plate distance (H) increases because more space is available for the development of the high-temperature zone in the free jet portion of the impinging flame, which favors the thermal NO formation. The variation of EICO with H is different for the impinging swirling and non-swirling IDFs because they have different flame structures. For both flames, the EICO is high when their main reaction zone or inner reaction cone is impinged and quenched by the copper plate. The parameters of air jet Reynolds number, overall equivalence ratio and nozzle-to-plate distance have significant influence on the overall pollutants emission of the impinging swirling and non-swirling IDFs and the comparison shows that the swirling IDF emits less NOx and CO under most of the experimental conditions tested. Furthermore, it is found that compared with the open flames, the impinging flames emit lower level of NOx and higher level of CO. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Direct gas flame impingement is employed in a wide range of industrial and domestic heating or drying processes, and there has been continued effort in the investigation of the thermal characteristics of the gas flame impingement system. Extensive reviews of literatures on impinging flame systems have been given by Viskanta [1], Baukal and Gebhart [2,3] and Chander and Ray [4]. By impinging the flame on the target, forced convection greatly enhances the heat transfer rate. So, significant attention has been paid to the heat transfer characteristics of the impinging flame system. The flame impingement heat transfer characteristics of a single impinging premixed flame have been studied by many researchers [5–7]. The heat transfer behaviors of a slot impinging premixed flame have been investigated by Dong et al. [8]. They also extended their investigations to an array of two and three impinging premixed flames [9,10]. Huang et al. [11] examined the heat transfer characteristics of an impinging premixed flame with induced swirl. Non-premixed flames have also been used and investigated for impingement heat transfer. Rigey and Webb [12] investigated the
⇑ Corresponding author. Fax: +852 23654703. E-mail address: [email protected] (C.W. Leung). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.11.036
flame structure and heat transfer characteristics of an impinging diffusion flame. More recently, the impingement heat transfer from inverse diffusion flames have been examined by several researchers [13–15]. Zhen et al. [16] studied the impingement heat transfer from inverse diffusion flames with induced swirl. The pollutants emission characteristics of the impinging flame system have been less attacked though they are important. Mishra [17] carried out an investigation of the emissions from a premixed flame impinging onto a flat cold surface. The effects of burner-toplate spacing, equivalence ratio and Reynolds number on the measured volumetric content of the pollutants in the flue gas were examined. Saha et al. [18] made a study of both the heat transfer and emission characteristics of an impinging rich premixed flame. They also examined the effects of Reynolds number, equivalence ratio and burner-to-plate distance on the volumetric content of CO and NOx in the flue gas. Mohr et al. [19] made an exhaust gas analysis of an impinging non-premixed flame. The exhaust gases were collected through a quartz probe placed to the outer edge of the impingement plate and in contact with the plate surface. They also obtained volumetric concentrations of CO and NOx, and compared the pollution formation characteristics of the two flames considered in their study. In the examination of the impinging inverse diffusion flame, Sze et al. [13] sampled the combustion products through a small hole drilled on the impingement plate. They used the measured radial distributions of O2, CO, CO2 and NO con-
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Nomenclature Re U
air jet Reynolds number overall equivalence ratio
H r
centrations to infer the combustion conditions inside the flame for the purpose of better understanding the heat transfer behaviors. The pollutants emission from an impinging flame system is a very important behavior of the flame and the emission characteristics of an impinging flame may totally differ from the emission characteristics of the same flame in the open environment. However, there are only a few studies conducted in this field and the previous studies only examined the local volumetric concentrations of the pollutants inside the flame or the exhaust gas without measuring the overall pollutants emission from the impinging flame system. Therefore, the objective of this study is to quantitatively evaluate the overall pollutants emission from an impinging inverse diffusion flame (IDF) system in terms of emission index. The effects of air jet Reynolds number, overall equivalence ratio and nozzle-to-plate distance on the emission index of the pollutants of CO and NOx will be studied in details. The flames under consideration include swirling and non-swirling IDFs, and hence comparisons of the emission index are made both between the impinging swirling IDF and open swirling IDF, and between the impinging swirling IDF and impinging non-swirling IDF. 2. Experimental setup and method The experimental apparatus is schematically shown in Fig. 1. The flame from the burner impinges vertically normal to a flat surface. The flat surface is a circular copper plate with a radius of 200 mm and 10 mm thick. The copper plate is evenly cooled on the backside by a cooling water jacket and the temperature of cooling water is maintained at 38 °C by a thermostat to avoid condensation. In the center of the copper plate, there is a small hole of 1 mm diameter, which was used as a probe for sampling in-flame local gases from the flame layer adjacent to the flat surface. For
nozzle-to-plate distance, m raidal distance from flame centerline, m
quantifying the overall pollutants emission, the hood method proposed by Butcher et al. [20] and validated by Tremeer and Jawurek [21] was used. A conical stainless steel hood of 600 mm base-diameter and 600 mm in height was placed over the impingement plate to collect the exhaust products. After the flame impinges onto the copper plate, it develops radially outwards along the surface of the plate. When the radially outgoing exhaust gases go beyond the edge of the copper plate, they turn upwards due to buoyancy into the conical hood. The cover at the outlet of the hood is adjustable so as to control the flow rate inside the hood. The flow rate is controlled such that no flue gases run away from the base of the hood to guarantee that the hood collects all the gases from the combusting flame. When the flow inside the hood achieves equilibrium, the flue gases collected in the hood become well-mixed mixtures at the upper portion of the hood. And at several different openings at the outlet of the hood, as shown in Fig. 1, both the gas temperature and pollutants concentration are uniform. The emission measurements were performed in two series. One is for in-flame local pollutants concentration measurement, and the other is for the overall pollutants emission measurement. In the former case, the gases sampled by the 1-mm hole on the copper plate go through an 1-m long stainless steel tube for cooling down below 60 °C and then enter the NO/NOx analyzer (California Instruments Corporation, Model 400) and the CO/CO2 analyzer (California Instruments Corporation, Model 300) to obtain the volumetric concentration of gaseous species including CO and NOx. In the latter case, the 1-m long stainless steel tube was connected to one of those openings at the outlet of the hood. Water vapor is condensed and removed from the samples before they enter the same NO/NOx analyzer (CLA) and CO/CO2 (NDIR) analyzer. Zero and span calibrations were performed before and after each measurement.
Cover Another opening Condenser Outlet of the hood
Hood
Pump Flowmeter Copper plate
CO/CO2 analyser
1-mm hole
NO/NOx analyser
Flame & Burner
Thermostat
Zero gas Fig. 1. Experimental apparatus.
Span gas
Span gas
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3. Results and discussions 3.1. In-flame local pollutants emission When the flame impinges upwards normal to the copper plate, it develops radially outwards along the surface of the copper plate. The small hole drilled at the center of the copper plate drew gases from the layer of the flame touching the copper plate, and the inflame pollutants emission was measured and reported in this section. Figs. 2 and 3 illustrate the radial distributions of volumetric concentrations of CO and NOx, respectively, at different nozzleto-plate distances (H) for the swirling IDF at Re = 8000 and U = 1.5. From Fig. 2, it is seen that the CO concentration decreases with increasing radial distance (r) from the flame centerline. It is known that in the central region of the impinging flame is an internal recirculation zone (IRZ) which is formed by the inward flow diverged by the copper plate, and that high CO creation occurs at the
14000 H = 12 mm H = 18 mm H = 24 mm H = 36 mm H = 48 mm H = 60 mm H = 84 mm H =108 mm H =132 mm
CO concentratoin (ppm)
12000 10000 8000 6000 4000 2000 0 0
20
40
60
80
100
120
140
160
180
Radial distance from flame centerline r (mm) Fig. 2. In-flame CO concentration distribution for the impinging swirling IDF at Re = 8000 and U = 1.5.
18 16
NOx concentration (ppm)
The burners used in the present study are the same as those described in Ref. [22]. The flames considered are inverse diffusion flames either with or without swirl, and their emission characteristics are compared and reported in this paper. A geometric swirl number computed from the same equation as used in Ref. [22] shows a value of 9.12 for the swirling IDF. The burner was mounted on a 3-D positioner. Screen meshes were used to enclose the flame for minimization of the disturbance from the surrounding air. The fuel used is standard liquefied petroleum gas (LPG) available in Hong Kong. Calibrated flowmeters were used to control the flow rates of supplied air and fuel. Following the convention of Ref. [22], Re refers to the air jet Reynolds number, the overall equivalence ratio U is adopted and the nozzle-to-plate distance H is the vertical distance from the burner rim to the copper plate. The experiments were conducted to study the effects of Re, U and H on both the local and overall emission behaviors of the swirling IDF. The overall emission of the non-swirling IDF was also measured for comparison with the swirling IDF. The operational conditions for the flames were chosen to be at Re = 8000 and U = 1.5 while H increases from 12, 18, 24, 36, 48, 60, 84, 108 and 132 mm; at Re = 8000 and H = 18 mm while U increases from 1.0, 1.2, 1.4, 1.6, 1.8 to 2.0; and at U = 1.5 and H = 18 mm while Re increases from 2000, 4000, 6000, 8000 to 10,000. An uncertainty analysis is conducted with the method suggested by Kline and McClintock [23]. Using a 95% confidence level, the uncertainties are 9.1% in CO concentration, 8.4% in CO2 concentration and 10.3% in NOx concentration.
14 12 10 8 6
H= H= H= H= H=
4 2
12 mm 18 mm 24 mm 36 mm 48 mm
H = 60 mm H = 84 mm H = 108 mm H = 132 mm
0 0
20
40
60
80
100
120
140
160
180
r (mm) Fig. 3. In-flame NOx concentration distribution for the impinging swirling IDF at Re = 8000 and U = 1.5.
flame front and the flame front is in an annular region [16,22]. Therefore, a portion of the CO formed at the flame front is pushed inwardly into the IRZ in the central region of the flame where the effect of dilution is relatively weak, thus leading to the high CO in the central region of the flame. With increasing radial distance, the effect of dilution results in gradual decrease in the CO concentration. The effect of H on the CO concentration can be observed in Fig. 2. That is, the CO concentration monotonically drops at higher H. The reason is that at small H, the flame is quenched by the cold copper plate, thus the production rate of incomplete combustion product of CO is high. As H increases, more space is available for the flame development under the copper plate and hence the free jet portion of the impinging flame becomes longer. Consequently, the combustion becomes more complete and the quenching effect exerted by the copper plate also decays with increasing H, both of which lead the CO concentration to decrease with H. The effect of H on CO concentration is consistent with Refs. [18,24]. The NOx concentration distributions are shown in Fig. 3, which tells that the NOx profiles can be divided into three groups. The first group corresponds to H = 12–24 mm, the second group to H = 36– 108 mm and the third group to H = 132 mm. For the first group, the nozzle-to-plate distance is small and the flame front touches the impingement plate [16]. Therefore, the intense combustion occurs in both the stagnation region and wall jet region, leading to a rather uniform radial distribution of the NOx concentration. As H increases, there is more space available for the establishment of the flame between the burner and copper plate. Hence the free jet region of the impinging flame becomes larger. Thus, for the second group of H = 36–108 mm, the intense combustion mainly occurs in the free jet region of the flame and as a result of the larger size of the free jet region, an increase in the NOx concentration is observed in the stagnation region. The NOx concentration in the wall jet region shows a decrease due to the absence of intense combustion and due to the dilution of the transported NOx by entrained ambient air. At H = 132 mm, the impinging flame resembles the appearance of the corresponding open flame, and the additional expansion of the impinging flame disappears. The additional expansion of the flame has been reported in Ref. [16] which stated that there exists an enlarged cross-sectional area of the free jet portion of the impinging swirling IDF in comparison to the corresponding open flame, and this additional expansion is a result of faster entrainment of ambient air. Therefore, the surge in the NOx concentration at H = 132 mm is the result of decreased dilution of the flue gas caused by the disappearance of the additional expansion of the flame. On the other hand, at H = 132 mm, there is no visible layer of the flame developing along the copper plate
H.S. Zhen et al. / Applied Energy 88 (2011) 1629–1634
(a)
3.0 2.5
EINOx (g/kg)
and it is just flue gases with no chemical reactions that impinge on the copper plate in the stagnation region. At this situation, the NOx concentration is significantly influenced by dilution of entrained ambient air, being higher in the stagnation region than that in the wall jet region. It is interesting to compare the in-flame CO and NOx concentrations for the open and impinging swirling IDFs. At Re = 8000 and U = 1.5, the in-flame CO and NOx concentrations for the open flame, as reported in Ref. [22], have typical values above 20,000 ppm and 120 ppm for CO and NOx, respectively in the region close to the flame centerline. In contrast, the in-flame CO and NOx concentrations for the impinging flame are generally below 13,000 ppm and 20 ppm for CO and NOx, respectively. This comparison indicates that either CO or NOx concentration in the impinging flame is much lower than that in the open flame. It is because the configuration of radially spreading of the impinging flame along the surface of the copper plate increases the flame surface, i.e. the contact area between the flame and atmospheric air. The consequence of increased flame surface is that the flame mixes with more ambient air compared with the open flame, and much more ambient air is entrained into the impinging flame layer adjacent to the copper plate, which leads to lower pollutants concentration in the impinging flame. However, the comparison of in-flame local pollutants concentration cannot tell whose pollutants emission is heavier or which flame is cleaner. Therefore, it is significant to quantify and thus compare the overall pollutants emission of the open and impinging flames in terms of emission index, which is presented in the next section.
2.0 1.5 1.0 Swirling IDF Non-swirling IDF
0.5 0.0 0
20
40
60
80
100
120
140
80
100
120
140
H
(b) EICO (g/kg)
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50 40 30 20 10 0 0
20
40
60
H Fig. 4. Variation in EINOx (a) and EICO (b) against H at Re = 8000 and U = 1.5.
3.2. Overall pollutants emission: emission index By means of the ‘hood’ method, simultaneous measurements of the concentrations of CO2 and pollutants of NOx and CO were made. The emission index for both NOx and CO was calculated from the obtained data. Fig. 4 shows the values of EINOx and EICO for the impinging swirling IDF at Re = 8000, U = 1.5 and different H. Fig. 4 illustrates that the EINOx increases monotonically with H. This is simply because that at larger H, more space is available for the development of the flame between the burner and the copper plate, hence the free jet region of the impinging flame becomes longer, and hence the high-temperature zone is larger, which favors the thermal NO formation. The EICO is the highest at 30 g/ kg at H = 12 mm where the main reaction zone of the swirling IDF directly impinges on the plate. The cold copper plate receives heat from the impinging flame, thus quenching the chemical reactions and resulting in high CO emission. As H increases from 12 to 36 mm, the EICO quickly drops below 10 g/kg, because the main reaction zone, which is about 24 mm in height [16], becomes gradually established in the free jet region of the impinging flame and hence evades the effect of quenching exerted by the copper plate. There is a slight increase in the EICO from H = 36 mm to H = 84 mm and beyond H = 84 mm, the EICO decreases again. The explanation of this can be related to the structures of the impinging swirling IDF. As reported in Ref. [16] which stated that there exists an enlarged cross-sectional area of the free jet portion of the impinging swirling IDF in comparison to the corresponding open flame, and this additional expansion is a result of faster entrainment of ambient air. Therefore, excess dilution and cooling of the flame occurs in association with the occurrence of the additional expansion, which provides a low-temperature environment unfavorable for CO oxidation. The increase in EICO starts from H = 36 to 84 mm and then a decrease in EICO ensues at H > 84 mm, probably following the variation of the strength and size of the additional expansion. The EICO and EINOx of the non-swirling IDF are also shown in Fig. 4. The EINOx increases slightly with H, being higher at around H < 80 mm and lower at H > 80 mm than that of the swirling IDF.
The swirling IDF has lower EINOx at low H probably due to the lower flame temperature because the flame is cooled during the process of additional expansion of the flame. The higher EINOx at high H is because the disappearance of the additional expansion increases the flame temperature [16], and hence the thermal NO production is enhanced. The EICO of the non-swirling IDF drops quickly as H increases from 12 to 36 mm, because of the lessening of the quenching effect caused by the copper plate. The potential core of the non-swirling IDF is below the copper plate at H > 36 mm and it is mainly the inner reaction cone of the flame that is in contact with and quenched by the copper plate. So the EICO shows an obvious increase from H = 36 mm to H = 84 mm which is the height of the tip of the inner reaction cone. At H > 84 mm, the post-combustion region of the non-swirling IDF impinges on the copper plate, and the CO formed is sufficiently burned out into CO2, leading to a decreasing EICO. Fig. 4 shows that the variation of EICO with H for the impinging IDFs with and without swirl is different because their structures of the impinging flames are distinctly different. One common point is that the EICO is high when the main reaction zone or inner reaction cone of the impinging flame touches and hence is quenched by the impingement plate, and as the quenching effect lessens at higher H, the EICO gradually drops. Fig. 4 also shows that compared with the swirling IDF, the non-swirling IDF emits more CO at all H tested in this study, because the stronger mixing between the supplied fuel and air in the case of the swirling IDF makes the oxidation of CO into CO2 more efficiently. Fig. 5 examines the influence of Re on the EINOx of the impinging swirling and non-swirling IDFs at U = 1.5 and H = 18 mm. The EINOx increases when Re increases from 2000 to 4000 in both cases and then decreases slightly with further increase in Re. The value of the EINOx, being between 1.0 g/kg and 1.5 g/kg, actually means that there is a rather constant NOx emission with variation in Re. The EINOx of the non-swirling IDF is higher than that of the swirling IDF probably because there is more NOx formation in the layer
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(a)
3.0
Swirling IDF Non-swirling IDF
2.0 1.5 1.0
3.0
Swirling IDF Non-swirling IDF
2.5
EINOx (g/kg)
EINOx (g/kg)
2.5
(a)
2.0 1.5 1.0
0.5 0.5 0.0 2000
4000
6000
8000
10000
Re
(b)
(b)
50
1.2
1.4
1
1.8
2.0
1.0
1.2
1.4
1.6
1.8
2.0
350 300
40
EICO (g/kg)
EICO (g/kg)
45
1.0
35 30 25 20
250 200 150 100 50
15 2000
4000
6000
8000
0
10000
Re Fig. 5. Variation in EINOx (a) and EICO (b) against Re at U = 1.5 and H = 18 mm.
Fig. 6. Variation in EINOx (a) and EICO (b) against U at Re = 8000 and H = 18 mm.
of the flame along the copper plate where there is relatively more unburned fuel due to relatively weaker fuel/air mixing in the nonswirling IDF. Fig. 5 also illustrates the effect of Re on the EICO of the two flames. For the non-swirling IDF, the small nozzle-to-plate distance of H = 18 mm impedes the development of the inner reaction cone in the free jet region, and the combustion mainly occurs in the diffusion mode due to the poor fuel/air mixing in the layer of the flame developing along the copper plate. Therefore, an increase in Re which means an increase in the fuel supply tends to increase the EICO. While in comparison with the non-swirling IDF, the swirling IDF achieves relatively more complete combustion at small nozzle-to-plate distance due to strong fuel/air mixing [16], and an increase in Re induces stronger fuel/air mixing and higher turbulence level, and thus the EICO decreases at higher Re. The effects of U on the emission indices of the impinging flames at Re = 8000 and H = 18 mm are shown in Fig. 6. An increase in U generally leads to more chemical reactions and more heat release from the impinging flame, which result in an increase in the NO production and hence the EINOx. The EINOx is generally lower except at U = 2.0 in the case of the swirling IDF, showing that smaller U and strong fuel/air mixing reduce the NOx formation, similar to the open flames where strong swirl and lean combustion are key conditions for reducing NOx emission [22]. The effect of U on the EICO shows that the non-swirling IDF emits more CO at higher U, indicating that the increased supply of fuel cannot be sufficiently oxidized. In contrast, the EICO of the swirling IDF exhibits a ‘U’ shape with the minimum value at U = 1.4. When the supply of fuel is small at U = 1.0, the flame may be under lean combustion, and an increase in U leads to the combustion towards stoichiometric and thus reducing the EICO. As the flame becomes rich at higher U, there is more supplied fuel and chemical reactions in the layer of the flame touching the copper plate, and the effect of quenching causes the EICO to increase with U. The comparison of the EICO shows that the swirling IDF has lower level of CO emission because the strong fuel/air mixing inside the flame helps the oxidization of CO into CO2.
The EINOx and EICO for the open swirling and non-swirling IDF have been reported in Ref. [22]. So a direct comparison of the overall pollutants emission behaviors of the open and impinging IDFs can be made. The open swirling and non-swirling IDFs have typical values of EINOx and EICO at around 2.5 g/kg and below 14 g/kg for NOx and CO, respectively. While the impinging swirling and nonswirling IDFs have values of EINOx and EICO below 2.5 g/kg and above 14 g/kg in most of the experimental conditions tested in this study. Therefore the conclusion is that the impinging flame tends to emit less NOx and more CO when compared with the open flame. 4. Conclusion The ‘hood’ method was used to quantify the overall pollutants emission from impinging swirling and non-swirling inverse diffusion flames (IDFs). The exhaust gas emitted from the flame impingement system was collected by a hood and the emission index was calculated from the simultaneous measurement of the concentrations of CO2 and pollutants of CO and NOx. The results of in-flame volumetric concentrations of CO and NOx and overall pollutants emission of CO and NOx in terms of emission index were reported. The in-flame local pollutants emission of the swirling IDF, in terms of volumetric concentrations of CO and NOx, was measured through a small hole drilled on the impingement plate. In comparison with the corresponding open flame, the CO and NOx concentrations for the impinging swirling IDF are greatly lowered due to the entrainment of much more ambient air which is related to the increased flame surface area. For the swirling and non-swirling IDFs, the EINOx increases as the nozzle-to-plate distance (H) increases because more space is available for the development of the high-temperature zone in the free jet portion of the impinging flame, which favors the thermal NO formation. The variation of EICO with H is different for the impinging swirling and non-swirling IDFs because they have different flame structures. For both flames, the EICO is high when
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their main reaction zone or inner reaction cone is impinged and quenched by the copper plate. The parameters of air jet Reynolds number, overall equivalence ratio and nozzle-to-plate distance have significant influence on the overall pollutants emission of the impinging swirling and nonswirling IDFs and the comparison shows that the swirling IDF emits less NOx and CO under most of the experimental conditions tested. Furthermore, it is found that compared with the open flames, the impinging flames emit lower level of NOx and higher level of CO. Acknowledgement The Authors wish to thank for the fully financial support from the Research Grants Council of The Hong Kong SAR to the present Project (polyU 5223/08E B-Q14X). References [1] Viskanta R. Heat transfer to impinging isothermal gas and flame jets. Exp Therm Fluid Sci 1993;6:111–34. [2] Baukal CE, Gebhart B. A review of flame impingement heat transfer studies– Part 1: experimental conditions. Combust Sci Technol 1995;104:339–57. [3] Baukal CE, Gebhart B. A review of flame impingement heat transfer studies– Part 2: measurements. Combust Sci Technol 1995;104:359–85. [4] Chander S, Ray A. Flame impinging heat transfer, a review. Energy Convers Manage 2005;46:2803–37. [5] Kilham JK, Purvis MRI. Heat transfer from normally impinging flames. Combust Sci Technol 1978;18:81–90. [6] Conolly R, Davies RM. A study of convective heat transfer from flames. Int J Heat Mass Transfer 1973;15:2155–72. [7] Milson A, Chigier NA. Studies of methane and methane-air flames impinging on a cold plate. Combust Flame 1973;21:295–305. [8] Dong LL, Leung CW, Cheung CS. Heat transfer from an impinging premixed butane/air slot flame jet. Int J Heat Mass Transfer 2002;45:979–92. [9] Dong LL, Leung CW, Cheung CS. Heat transfer and wall pressure characteristics of a twin premixed butane/air flame jets. Int J Heat Mass Transfer 2004;47:489–500.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Emission of impinging swirling and non-swirling inverse diffusion flames H.S. Zhen, C.W. Leung ⇑, C.S. Cheung Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 30 August 2010 Received in revised form 1 November 2010 Accepted 23 November 2010 Available online 21 December 2010 Keywords: Inverse diffusion flame CO/NOx emissions Fame impingement
a b s t r a c t The overall pollutants emission from impinging swirling and non-swirling inverse diffusion flames (IDFs) was evaluated quantitatively by the ‘hood’ method. The results of in-flame volumetric concentrations of CO and NOx and overall pollutants emission of CO and NOx in terms of emission index were reported. The in-flame volumetric concentrations of CO and NOx were measured through a small hole drilled on the impingement plate. In comparison with the corresponding open flame, the CO and NOx concentrations for the impinging swirling IDF are greatly lowered due to the entrainment of much more ambient air which is related to the increased flame surface area. For the swirling and non-swirling IDFs, the EINOx increases as the nozzle-to-plate distance (H) increases because more space is available for the development of the high-temperature zone in the free jet portion of the impinging flame, which favors the thermal NO formation. The variation of EICO with H is different for the impinging swirling and non-swirling IDFs because they have different flame structures. For both flames, the EICO is high when their main reaction zone or inner reaction cone is impinged and quenched by the copper plate. The parameters of air jet Reynolds number, overall equivalence ratio and nozzle-to-plate distance have significant influence on the overall pollutants emission of the impinging swirling and non-swirling IDFs and the comparison shows that the swirling IDF emits less NOx and CO under most of the experimental conditions tested. Furthermore, it is found that compared with the open flames, the impinging flames emit lower level of NOx and higher level of CO. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Direct gas flame impingement is employed in a wide range of industrial and domestic heating or drying processes, and there has been continued effort in the investigation of the thermal characteristics of the gas flame impingement system. Extensive reviews of literatures on impinging flame systems have been given by Viskanta [1], Baukal and Gebhart [2,3] and Chander and Ray [4]. By impinging the flame on the target, forced convection greatly enhances the heat transfer rate. So, significant attention has been paid to the heat transfer characteristics of the impinging flame system. The flame impingement heat transfer characteristics of a single impinging premixed flame have been studied by many researchers [5–7]. The heat transfer behaviors of a slot impinging premixed flame have been investigated by Dong et al. [8]. They also extended their investigations to an array of two and three impinging premixed flames [9,10]. Huang et al. [11] examined the heat transfer characteristics of an impinging premixed flame with induced swirl. Non-premixed flames have also been used and investigated for impingement heat transfer. Rigey and Webb [12] investigated the
⇑ Corresponding author. Fax: +852 23654703. E-mail address: [email protected] (C.W. Leung). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.11.036
flame structure and heat transfer characteristics of an impinging diffusion flame. More recently, the impingement heat transfer from inverse diffusion flames have been examined by several researchers [13–15]. Zhen et al. [16] studied the impingement heat transfer from inverse diffusion flames with induced swirl. The pollutants emission characteristics of the impinging flame system have been less attacked though they are important. Mishra [17] carried out an investigation of the emissions from a premixed flame impinging onto a flat cold surface. The effects of burner-toplate spacing, equivalence ratio and Reynolds number on the measured volumetric content of the pollutants in the flue gas were examined. Saha et al. [18] made a study of both the heat transfer and emission characteristics of an impinging rich premixed flame. They also examined the effects of Reynolds number, equivalence ratio and burner-to-plate distance on the volumetric content of CO and NOx in the flue gas. Mohr et al. [19] made an exhaust gas analysis of an impinging non-premixed flame. The exhaust gases were collected through a quartz probe placed to the outer edge of the impingement plate and in contact with the plate surface. They also obtained volumetric concentrations of CO and NOx, and compared the pollution formation characteristics of the two flames considered in their study. In the examination of the impinging inverse diffusion flame, Sze et al. [13] sampled the combustion products through a small hole drilled on the impingement plate. They used the measured radial distributions of O2, CO, CO2 and NO con-
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Nomenclature Re U
air jet Reynolds number overall equivalence ratio
H r
centrations to infer the combustion conditions inside the flame for the purpose of better understanding the heat transfer behaviors. The pollutants emission from an impinging flame system is a very important behavior of the flame and the emission characteristics of an impinging flame may totally differ from the emission characteristics of the same flame in the open environment. However, there are only a few studies conducted in this field and the previous studies only examined the local volumetric concentrations of the pollutants inside the flame or the exhaust gas without measuring the overall pollutants emission from the impinging flame system. Therefore, the objective of this study is to quantitatively evaluate the overall pollutants emission from an impinging inverse diffusion flame (IDF) system in terms of emission index. The effects of air jet Reynolds number, overall equivalence ratio and nozzle-to-plate distance on the emission index of the pollutants of CO and NOx will be studied in details. The flames under consideration include swirling and non-swirling IDFs, and hence comparisons of the emission index are made both between the impinging swirling IDF and open swirling IDF, and between the impinging swirling IDF and impinging non-swirling IDF. 2. Experimental setup and method The experimental apparatus is schematically shown in Fig. 1. The flame from the burner impinges vertically normal to a flat surface. The flat surface is a circular copper plate with a radius of 200 mm and 10 mm thick. The copper plate is evenly cooled on the backside by a cooling water jacket and the temperature of cooling water is maintained at 38 °C by a thermostat to avoid condensation. In the center of the copper plate, there is a small hole of 1 mm diameter, which was used as a probe for sampling in-flame local gases from the flame layer adjacent to the flat surface. For
nozzle-to-plate distance, m raidal distance from flame centerline, m
quantifying the overall pollutants emission, the hood method proposed by Butcher et al. [20] and validated by Tremeer and Jawurek [21] was used. A conical stainless steel hood of 600 mm base-diameter and 600 mm in height was placed over the impingement plate to collect the exhaust products. After the flame impinges onto the copper plate, it develops radially outwards along the surface of the plate. When the radially outgoing exhaust gases go beyond the edge of the copper plate, they turn upwards due to buoyancy into the conical hood. The cover at the outlet of the hood is adjustable so as to control the flow rate inside the hood. The flow rate is controlled such that no flue gases run away from the base of the hood to guarantee that the hood collects all the gases from the combusting flame. When the flow inside the hood achieves equilibrium, the flue gases collected in the hood become well-mixed mixtures at the upper portion of the hood. And at several different openings at the outlet of the hood, as shown in Fig. 1, both the gas temperature and pollutants concentration are uniform. The emission measurements were performed in two series. One is for in-flame local pollutants concentration measurement, and the other is for the overall pollutants emission measurement. In the former case, the gases sampled by the 1-mm hole on the copper plate go through an 1-m long stainless steel tube for cooling down below 60 °C and then enter the NO/NOx analyzer (California Instruments Corporation, Model 400) and the CO/CO2 analyzer (California Instruments Corporation, Model 300) to obtain the volumetric concentration of gaseous species including CO and NOx. In the latter case, the 1-m long stainless steel tube was connected to one of those openings at the outlet of the hood. Water vapor is condensed and removed from the samples before they enter the same NO/NOx analyzer (CLA) and CO/CO2 (NDIR) analyzer. Zero and span calibrations were performed before and after each measurement.
Cover Another opening Condenser Outlet of the hood
Hood
Pump Flowmeter Copper plate
CO/CO2 analyser
1-mm hole
NO/NOx analyser
Flame & Burner
Thermostat
Zero gas Fig. 1. Experimental apparatus.
Span gas
Span gas
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3. Results and discussions 3.1. In-flame local pollutants emission When the flame impinges upwards normal to the copper plate, it develops radially outwards along the surface of the copper plate. The small hole drilled at the center of the copper plate drew gases from the layer of the flame touching the copper plate, and the inflame pollutants emission was measured and reported in this section. Figs. 2 and 3 illustrate the radial distributions of volumetric concentrations of CO and NOx, respectively, at different nozzleto-plate distances (H) for the swirling IDF at Re = 8000 and U = 1.5. From Fig. 2, it is seen that the CO concentration decreases with increasing radial distance (r) from the flame centerline. It is known that in the central region of the impinging flame is an internal recirculation zone (IRZ) which is formed by the inward flow diverged by the copper plate, and that high CO creation occurs at the
14000 H = 12 mm H = 18 mm H = 24 mm H = 36 mm H = 48 mm H = 60 mm H = 84 mm H =108 mm H =132 mm
CO concentratoin (ppm)
12000 10000 8000 6000 4000 2000 0 0
20
40
60
80
100
120
140
160
180
Radial distance from flame centerline r (mm) Fig. 2. In-flame CO concentration distribution for the impinging swirling IDF at Re = 8000 and U = 1.5.
18 16
NOx concentration (ppm)
The burners used in the present study are the same as those described in Ref. [22]. The flames considered are inverse diffusion flames either with or without swirl, and their emission characteristics are compared and reported in this paper. A geometric swirl number computed from the same equation as used in Ref. [22] shows a value of 9.12 for the swirling IDF. The burner was mounted on a 3-D positioner. Screen meshes were used to enclose the flame for minimization of the disturbance from the surrounding air. The fuel used is standard liquefied petroleum gas (LPG) available in Hong Kong. Calibrated flowmeters were used to control the flow rates of supplied air and fuel. Following the convention of Ref. [22], Re refers to the air jet Reynolds number, the overall equivalence ratio U is adopted and the nozzle-to-plate distance H is the vertical distance from the burner rim to the copper plate. The experiments were conducted to study the effects of Re, U and H on both the local and overall emission behaviors of the swirling IDF. The overall emission of the non-swirling IDF was also measured for comparison with the swirling IDF. The operational conditions for the flames were chosen to be at Re = 8000 and U = 1.5 while H increases from 12, 18, 24, 36, 48, 60, 84, 108 and 132 mm; at Re = 8000 and H = 18 mm while U increases from 1.0, 1.2, 1.4, 1.6, 1.8 to 2.0; and at U = 1.5 and H = 18 mm while Re increases from 2000, 4000, 6000, 8000 to 10,000. An uncertainty analysis is conducted with the method suggested by Kline and McClintock [23]. Using a 95% confidence level, the uncertainties are 9.1% in CO concentration, 8.4% in CO2 concentration and 10.3% in NOx concentration.
14 12 10 8 6
H= H= H= H= H=
4 2
12 mm 18 mm 24 mm 36 mm 48 mm
H = 60 mm H = 84 mm H = 108 mm H = 132 mm
0 0
20
40
60
80
100
120
140
160
180
r (mm) Fig. 3. In-flame NOx concentration distribution for the impinging swirling IDF at Re = 8000 and U = 1.5.
flame front and the flame front is in an annular region [16,22]. Therefore, a portion of the CO formed at the flame front is pushed inwardly into the IRZ in the central region of the flame where the effect of dilution is relatively weak, thus leading to the high CO in the central region of the flame. With increasing radial distance, the effect of dilution results in gradual decrease in the CO concentration. The effect of H on the CO concentration can be observed in Fig. 2. That is, the CO concentration monotonically drops at higher H. The reason is that at small H, the flame is quenched by the cold copper plate, thus the production rate of incomplete combustion product of CO is high. As H increases, more space is available for the flame development under the copper plate and hence the free jet portion of the impinging flame becomes longer. Consequently, the combustion becomes more complete and the quenching effect exerted by the copper plate also decays with increasing H, both of which lead the CO concentration to decrease with H. The effect of H on CO concentration is consistent with Refs. [18,24]. The NOx concentration distributions are shown in Fig. 3, which tells that the NOx profiles can be divided into three groups. The first group corresponds to H = 12–24 mm, the second group to H = 36– 108 mm and the third group to H = 132 mm. For the first group, the nozzle-to-plate distance is small and the flame front touches the impingement plate [16]. Therefore, the intense combustion occurs in both the stagnation region and wall jet region, leading to a rather uniform radial distribution of the NOx concentration. As H increases, there is more space available for the establishment of the flame between the burner and copper plate. Hence the free jet region of the impinging flame becomes larger. Thus, for the second group of H = 36–108 mm, the intense combustion mainly occurs in the free jet region of the flame and as a result of the larger size of the free jet region, an increase in the NOx concentration is observed in the stagnation region. The NOx concentration in the wall jet region shows a decrease due to the absence of intense combustion and due to the dilution of the transported NOx by entrained ambient air. At H = 132 mm, the impinging flame resembles the appearance of the corresponding open flame, and the additional expansion of the impinging flame disappears. The additional expansion of the flame has been reported in Ref. [16] which stated that there exists an enlarged cross-sectional area of the free jet portion of the impinging swirling IDF in comparison to the corresponding open flame, and this additional expansion is a result of faster entrainment of ambient air. Therefore, the surge in the NOx concentration at H = 132 mm is the result of decreased dilution of the flue gas caused by the disappearance of the additional expansion of the flame. On the other hand, at H = 132 mm, there is no visible layer of the flame developing along the copper plate
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(a)
3.0 2.5
EINOx (g/kg)
and it is just flue gases with no chemical reactions that impinge on the copper plate in the stagnation region. At this situation, the NOx concentration is significantly influenced by dilution of entrained ambient air, being higher in the stagnation region than that in the wall jet region. It is interesting to compare the in-flame CO and NOx concentrations for the open and impinging swirling IDFs. At Re = 8000 and U = 1.5, the in-flame CO and NOx concentrations for the open flame, as reported in Ref. [22], have typical values above 20,000 ppm and 120 ppm for CO and NOx, respectively in the region close to the flame centerline. In contrast, the in-flame CO and NOx concentrations for the impinging flame are generally below 13,000 ppm and 20 ppm for CO and NOx, respectively. This comparison indicates that either CO or NOx concentration in the impinging flame is much lower than that in the open flame. It is because the configuration of radially spreading of the impinging flame along the surface of the copper plate increases the flame surface, i.e. the contact area between the flame and atmospheric air. The consequence of increased flame surface is that the flame mixes with more ambient air compared with the open flame, and much more ambient air is entrained into the impinging flame layer adjacent to the copper plate, which leads to lower pollutants concentration in the impinging flame. However, the comparison of in-flame local pollutants concentration cannot tell whose pollutants emission is heavier or which flame is cleaner. Therefore, it is significant to quantify and thus compare the overall pollutants emission of the open and impinging flames in terms of emission index, which is presented in the next section.
2.0 1.5 1.0 Swirling IDF Non-swirling IDF
0.5 0.0 0
20
40
60
80
100
120
140
80
100
120
140
H
(b) EICO (g/kg)
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50 40 30 20 10 0 0
20
40
60
H Fig. 4. Variation in EINOx (a) and EICO (b) against H at Re = 8000 and U = 1.5.
3.2. Overall pollutants emission: emission index By means of the ‘hood’ method, simultaneous measurements of the concentrations of CO2 and pollutants of NOx and CO were made. The emission index for both NOx and CO was calculated from the obtained data. Fig. 4 shows the values of EINOx and EICO for the impinging swirling IDF at Re = 8000, U = 1.5 and different H. Fig. 4 illustrates that the EINOx increases monotonically with H. This is simply because that at larger H, more space is available for the development of the flame between the burner and the copper plate, hence the free jet region of the impinging flame becomes longer, and hence the high-temperature zone is larger, which favors the thermal NO formation. The EICO is the highest at 30 g/ kg at H = 12 mm where the main reaction zone of the swirling IDF directly impinges on the plate. The cold copper plate receives heat from the impinging flame, thus quenching the chemical reactions and resulting in high CO emission. As H increases from 12 to 36 mm, the EICO quickly drops below 10 g/kg, because the main reaction zone, which is about 24 mm in height [16], becomes gradually established in the free jet region of the impinging flame and hence evades the effect of quenching exerted by the copper plate. There is a slight increase in the EICO from H = 36 mm to H = 84 mm and beyond H = 84 mm, the EICO decreases again. The explanation of this can be related to the structures of the impinging swirling IDF. As reported in Ref. [16] which stated that there exists an enlarged cross-sectional area of the free jet portion of the impinging swirling IDF in comparison to the corresponding open flame, and this additional expansion is a result of faster entrainment of ambient air. Therefore, excess dilution and cooling of the flame occurs in association with the occurrence of the additional expansion, which provides a low-temperature environment unfavorable for CO oxidation. The increase in EICO starts from H = 36 to 84 mm and then a decrease in EICO ensues at H > 84 mm, probably following the variation of the strength and size of the additional expansion. The EICO and EINOx of the non-swirling IDF are also shown in Fig. 4. The EINOx increases slightly with H, being higher at around H < 80 mm and lower at H > 80 mm than that of the swirling IDF.
The swirling IDF has lower EINOx at low H probably due to the lower flame temperature because the flame is cooled during the process of additional expansion of the flame. The higher EINOx at high H is because the disappearance of the additional expansion increases the flame temperature [16], and hence the thermal NO production is enhanced. The EICO of the non-swirling IDF drops quickly as H increases from 12 to 36 mm, because of the lessening of the quenching effect caused by the copper plate. The potential core of the non-swirling IDF is below the copper plate at H > 36 mm and it is mainly the inner reaction cone of the flame that is in contact with and quenched by the copper plate. So the EICO shows an obvious increase from H = 36 mm to H = 84 mm which is the height of the tip of the inner reaction cone. At H > 84 mm, the post-combustion region of the non-swirling IDF impinges on the copper plate, and the CO formed is sufficiently burned out into CO2, leading to a decreasing EICO. Fig. 4 shows that the variation of EICO with H for the impinging IDFs with and without swirl is different because their structures of the impinging flames are distinctly different. One common point is that the EICO is high when the main reaction zone or inner reaction cone of the impinging flame touches and hence is quenched by the impingement plate, and as the quenching effect lessens at higher H, the EICO gradually drops. Fig. 4 also shows that compared with the swirling IDF, the non-swirling IDF emits more CO at all H tested in this study, because the stronger mixing between the supplied fuel and air in the case of the swirling IDF makes the oxidation of CO into CO2 more efficiently. Fig. 5 examines the influence of Re on the EINOx of the impinging swirling and non-swirling IDFs at U = 1.5 and H = 18 mm. The EINOx increases when Re increases from 2000 to 4000 in both cases and then decreases slightly with further increase in Re. The value of the EINOx, being between 1.0 g/kg and 1.5 g/kg, actually means that there is a rather constant NOx emission with variation in Re. The EINOx of the non-swirling IDF is higher than that of the swirling IDF probably because there is more NOx formation in the layer
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(a)
3.0
Swirling IDF Non-swirling IDF
2.0 1.5 1.0
3.0
Swirling IDF Non-swirling IDF
2.5
EINOx (g/kg)
EINOx (g/kg)
2.5
(a)
2.0 1.5 1.0
0.5 0.5 0.0 2000
4000
6000
8000
10000
Re
(b)
(b)
50
1.2
1.4
1
1.8
2.0
1.0
1.2
1.4
1.6
1.8
2.0
350 300
40
EICO (g/kg)
EICO (g/kg)
45
1.0
35 30 25 20
250 200 150 100 50
15 2000
4000
6000
8000
0
10000
Re Fig. 5. Variation in EINOx (a) and EICO (b) against Re at U = 1.5 and H = 18 mm.
Fig. 6. Variation in EINOx (a) and EICO (b) against U at Re = 8000 and H = 18 mm.
of the flame along the copper plate where there is relatively more unburned fuel due to relatively weaker fuel/air mixing in the nonswirling IDF. Fig. 5 also illustrates the effect of Re on the EICO of the two flames. For the non-swirling IDF, the small nozzle-to-plate distance of H = 18 mm impedes the development of the inner reaction cone in the free jet region, and the combustion mainly occurs in the diffusion mode due to the poor fuel/air mixing in the layer of the flame developing along the copper plate. Therefore, an increase in Re which means an increase in the fuel supply tends to increase the EICO. While in comparison with the non-swirling IDF, the swirling IDF achieves relatively more complete combustion at small nozzle-to-plate distance due to strong fuel/air mixing [16], and an increase in Re induces stronger fuel/air mixing and higher turbulence level, and thus the EICO decreases at higher Re. The effects of U on the emission indices of the impinging flames at Re = 8000 and H = 18 mm are shown in Fig. 6. An increase in U generally leads to more chemical reactions and more heat release from the impinging flame, which result in an increase in the NO production and hence the EINOx. The EINOx is generally lower except at U = 2.0 in the case of the swirling IDF, showing that smaller U and strong fuel/air mixing reduce the NOx formation, similar to the open flames where strong swirl and lean combustion are key conditions for reducing NOx emission [22]. The effect of U on the EICO shows that the non-swirling IDF emits more CO at higher U, indicating that the increased supply of fuel cannot be sufficiently oxidized. In contrast, the EICO of the swirling IDF exhibits a ‘U’ shape with the minimum value at U = 1.4. When the supply of fuel is small at U = 1.0, the flame may be under lean combustion, and an increase in U leads to the combustion towards stoichiometric and thus reducing the EICO. As the flame becomes rich at higher U, there is more supplied fuel and chemical reactions in the layer of the flame touching the copper plate, and the effect of quenching causes the EICO to increase with U. The comparison of the EICO shows that the swirling IDF has lower level of CO emission because the strong fuel/air mixing inside the flame helps the oxidization of CO into CO2.
The EINOx and EICO for the open swirling and non-swirling IDF have been reported in Ref. [22]. So a direct comparison of the overall pollutants emission behaviors of the open and impinging IDFs can be made. The open swirling and non-swirling IDFs have typical values of EINOx and EICO at around 2.5 g/kg and below 14 g/kg for NOx and CO, respectively. While the impinging swirling and nonswirling IDFs have values of EINOx and EICO below 2.5 g/kg and above 14 g/kg in most of the experimental conditions tested in this study. Therefore the conclusion is that the impinging flame tends to emit less NOx and more CO when compared with the open flame. 4. Conclusion The ‘hood’ method was used to quantify the overall pollutants emission from impinging swirling and non-swirling inverse diffusion flames (IDFs). The exhaust gas emitted from the flame impingement system was collected by a hood and the emission index was calculated from the simultaneous measurement of the concentrations of CO2 and pollutants of CO and NOx. The results of in-flame volumetric concentrations of CO and NOx and overall pollutants emission of CO and NOx in terms of emission index were reported. The in-flame local pollutants emission of the swirling IDF, in terms of volumetric concentrations of CO and NOx, was measured through a small hole drilled on the impingement plate. In comparison with the corresponding open flame, the CO and NOx concentrations for the impinging swirling IDF are greatly lowered due to the entrainment of much more ambient air which is related to the increased flame surface area. For the swirling and non-swirling IDFs, the EINOx increases as the nozzle-to-plate distance (H) increases because more space is available for the development of the high-temperature zone in the free jet portion of the impinging flame, which favors the thermal NO formation. The variation of EICO with H is different for the impinging swirling and non-swirling IDFs because they have different flame structures. For both flames, the EICO is high when
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their main reaction zone or inner reaction cone is impinged and quenched by the copper plate. The parameters of air jet Reynolds number, overall equivalence ratio and nozzle-to-plate distance have significant influence on the overall pollutants emission of the impinging swirling and nonswirling IDFs and the comparison shows that the swirling IDF emits less NOx and CO under most of the experimental conditions tested. Furthermore, it is found that compared with the open flames, the impinging flames emit lower level of NOx and higher level of CO. Acknowledgement The Authors wish to thank for the fully financial support from the Research Grants Council of The Hong Kong SAR to the present Project (polyU 5223/08E B-Q14X). References [1] Viskanta R. Heat transfer to impinging isothermal gas and flame jets. Exp Therm Fluid Sci 1993;6:111–34. [2] Baukal CE, Gebhart B. A review of flame impingement heat transfer studies– Part 1: experimental conditions. Combust Sci Technol 1995;104:339–57. [3] Baukal CE, Gebhart B. A review of flame impingement heat transfer studies– Part 2: measurements. Combust Sci Technol 1995;104:359–85. [4] Chander S, Ray A. Flame impinging heat transfer, a review. Energy Convers Manage 2005;46:2803–37. [5] Kilham JK, Purvis MRI. Heat transfer from normally impinging flames. Combust Sci Technol 1978;18:81–90. [6] Conolly R, Davies RM. A study of convective heat transfer from flames. Int J Heat Mass Transfer 1973;15:2155–72. [7] Milson A, Chigier NA. Studies of methane and methane-air flames impinging on a cold plate. Combust Flame 1973;21:295–305. [8] Dong LL, Leung CW, Cheung CS. Heat transfer from an impinging premixed butane/air slot flame jet. Int J Heat Mass Transfer 2002;45:979–92. [9] Dong LL, Leung CW, Cheung CS. Heat transfer and wall pressure characteristics of a twin premixed butane/air flame jets. Int J Heat Mass Transfer 2004;47:489–500.
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