13th International Heat Transfer Conference,
                                               August 13-18, 2006, Sydney, NS...
1. INTRODUCTION

Impinging flame jets are commonly used in many industrial applications to enhance the heat
transfer to th...
It has been observed from the reviewed literature that the information regarding heat transfer
characteristics of slot fla...
tube burners were of length 300 mm. A pair of packing (each consisting of 7-8 meshes) of fine steel
meshes was used as fla...
impingement surface. Water was supplied to the calorimeter for 15 minutes before the mixture was
ignited to attain tempera...
a)       b)            c)                               a)           b)              c)
                 (View in Y-direct...
5.2 Comparison of heat flux contours for different slot burners

Figure 8-10 show the heat flux contour plots for slot bur...
Figure 9 shows the heat flux distribution on the impingement surface for slot 2. An oval shape heat
flux distribution was ...
200
                                                                                                                      ...
5.3.2 Effect of H/de

Figure 14 shows the heat flux distribution at various non-dimensional separation distances for slot1...
different aspect ratio, slot 3 showed high heat fluxes in stagnation and early wall jet region. In the
later wall jet regi...
3. Baukal, C.E., Gebhart, B., 1995, A Review of Flame Impingement Heat Transfer Part1:
    Experimental Conditions, Combus...
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Paper presented at 13th International Heat Transfer Conference, Sydney

  1. 1. 13th International Heat Transfer Conference, August 13-18, 2006, Sydney, NSW, Australia EFFECT OF SLOT BURNER ASPECT RATIO ON HEAT TRANSFER CHARACTERISTICS FOR METHANE/AIR FLAME IMPINGING ON A FLAT SURFACE Subhash Chander, Abhineesh Das, Arpit Jain and Anjan Ray Department of Mechanical Engineering Indian Institute of Technology Delhi New Delhi -110016 (India) ABSTRACT An experimental study has been conducted to investigate the effect of slot burner aspect ratio (ratio of length to width of slot burner) on heat transfer characteristics for methane/air flame impinging normally on a flat surface. Three aspect ratios were considered, viz., 5.08, 2.77 and 1.54. Equivalent diameter of 12.7 mm was kept same for all slot burners. Results were also compared with tube burner with same exit diameter. Effects of Reynolds number (800-1300) and dimensionless separation distance (H/de = 1.5 -5) on heat transfer characteristics for different slot burners were investigated. It has been observed that the heat transfer characteristics were intimately related to the proximity of the flame inner reaction cone with target surface. Heat flux was more concentrated in the stagnation region for round tube burner and also for low aspect ratio slot burner. For the slot burner itself the heat fluxes in X (along the length of the slot section) and Y (along the width) directions were different. Two off-centered peaks in the heat flux were observed along Y- direction (minor axis). Heating was more uniform in X-direction as compared to Y-direction. Slot burners gave more uniform heat flux distribution on the surface as compared to round tube burner. Amongst the slot burners, one with largest aspect ratio gave most uniform heat flux distribution. Keywords: Flame impingement, slot burner, aspect ratio, convective heat transfer, velocity profile NOMENCLATURE A/F air/fuel ratio Greek Symbols de equivalent diameter of slot burner μ dynamic viscosity (kg-s/m) (mm) ρ density (kg/m3) H distance between the plane of the φ equivalence ratio burner exit and the impingement plate (mm) L length of the slot (mm) Subscripts L/W aspect ratio (AR) actual actual state M molecular weight (kg/kmol) e equivalent Re Reynolds number exit at the exit position W width of the slot i mixture component including fuel X, Y directions along the length and width and air of slot mix air/fuel mixture X/de, dimensionless distances along X and stoic stoichiometric state Y/de Y-directions y molar fraction
  2. 2. 1. INTRODUCTION Impinging flame jets are commonly used in many industrial applications to enhance the heat transfer to the impingement surface. Typical applications include melting and shaping of glass and melting of scrap metal. Heat transfer from flame to the substrate depends upon different factors like; jet inlet velocity/Reynolds number, separation distance between the burner rim and the impingement surface, air/fuel ratio, burner geometry and others (Viskanta (1998), Baukal and Gebhart (1995). It has been seen that little attention has been given to burner geometry/exit velocity profile. Most of the studies available in the literature have been carried out with round burners. Very little work has been done with slot burners and studies related to other burner shapes are nearly non-existing. It was evident from the literature that the studies related to slot jet impingement were mostly pertaining to isothermal jets. Wadsworth and Mudawar (1990) found that use of two dimensional slot air jet, in place of round jet, provided a large impingement zone and ensured uniform coolant rejection from the surface after impingement. Benefits like, cooling uniformity and high effectiveness (high average heat fluxes) were attained with slot jets. Heiningen et al. (1976) investigated numerically the effects of uniform suction and nozzle exit velocity profile on the flow and heat transfer characteristics of semi-confined laminar impinging slot jet. It was observed that there was significant effect of exit velocity profile on the heat transfer characteristics over the plate. Chou and Hung (1994) developed local Nusselt number correlations in the wall jet region of confined slot jet flow under different jet exit velocity profiles. Al-Sanea (1992) found numerically that fully developed parabolic profile enhanced the rate of heat transfer at the impingement region well above those produced by uniform velocity profile. Sezai and Mohamad (1999) investigated numerically the flow and heat transfer characteristics of impinging laminar slot air jets of different aspect ratios. Two off- centered velocity peaks were observed for the rectangular jet whereas non- uniform velocity profile with single peak at the centre was found for square jet. These off-centered velocity peaks resulted in off-centered Nusselt number peaks on the impingement surface. It was also observed that the drop in Nusselt number was more gradual for jet with higher aspect ratios but decreased sharply for square nozzles. The Nusselt number was higher in the direction of minor axis of the jet as the flow velocities were higher in that direction. Ashforth-Frost et al. (1997) investigated experimentally velocity and turbulence characteristics of a semi-confined impinging slot jet. Measurements show that the potential core of the jet is longer for the semi-confined configuration when compared with the unconfined, owing to limited entrainment and spreading of the jet. Sahoo and Sharif (2004) investigated numerically the flow and heat transfer characteristics in the cooling of a heated surface by impinging slot jets. It is observed that for a given domain aspect ratio and Richardson number, the average Nusselt number at the hot surface increased with increasing jet exit Reynolds number. Gao and Sunden (2003) investigated experimentally the heat transfer characteristics between the confined impinging slot jets and a flat surface. Lin et al. (1997) studied experimentally the heat transfer behavior of a confined impinging slot jet. Most of the studies related to single flame jet impinging on a flat surface pertain to round jet (Milson and Chigier (1973), Hargrave et. al (1987), Rigby and Webb (1995), Baukal and Gebhart (1997; 1998) Dong et al. (2001), Kleijn (2001) Hou and Ko (2005) and Chander and Ray (2006)). Very few numbers of studies were found in the literature for slot flame jet impinging on a flat surface. Dong et al. (2002) and Kwok et al. (2003) investigated heat transfer characteristics from impinging butane/air round and slot flame jets with same cross-sectional area. On comparing the radial heat flux distributions, it was observed that in the impingement region, the round jet showed a concentrated peak heat flux whose value was higher than that obtained in the case of slot jet. Slot jet produced more uniform heat flux profile and large averaged heat fluxes than the circular flame jets.
  3. 3. It has been observed from the reviewed literature that the information regarding heat transfer characteristics of slot flame jet impinging on the flat surface is scarce. Authors could not find any study related to slot flame jet impinging on a flat surface where the effect of jet aspect ratio was investigated. So the present study deals with investigation of slot jet aspect ratio on heat transfer characteristics for methane/air flame impinging on a flat surface. All slot jets were of same equivalent diameter. Results were also compared with the tube burner of same diameter. 2. EXPERIMENTAL SETUP Figure 1 shows the schematic of flame impingement setup. The experimental setup has two major sections; one to facilitate the heat generation part (burners) and other for heat absorption (calorimeter). 16 16 2 3 17 24 15 6 4 14 18 5 21 4 12 13 7 19 1 9 22 10 11 3 20 23 8 1.Water Tank 2. Inlet Water supply 3. Pump 4. Flow Control Valve 5. Heating Element 6. Thermostat 7. Insulated Container 8. 3D Positioning Traverse 9. Calorimeter Stand 10. Burner Stand 11. Burner 12. Flame 13. Heat Flux Sensor 14. Calorimeter 15. Signal Conditioning Amplifier 16. Multi-meters 17. Temperature Readout 18. Water Outlet to Waste Tank 19. Mixing Tube 20. Needle Valves 21. Gas Rotameters 22. Air from Cylinder 23. Methane from Cylinder 24. Water Rotameter Figure 1 Schematic for Single Flame Jet Impingement Setup Figure 2 Direct photographs of the slot burners and tube burner Heat generation part comprised of different slot burners of different aspect ratios and a tube burner. The exit dimensions of slot 1, 2 and 3 were 25.4 mm × 5 mm , 18.6 mm × 6.7 mm and 14 mm × 9.1 mm and corresponding aspect ratios were 5.08, 2.77 and 1.54 respectively. The effective diameter (as given by equation1 (Dong et al. (2002); Kwok et al. (2003)) of 12.7 was kept same in all burners. 1 ⎛ 4WL ⎞ 2 de = ⎜ ⎟ (1) ⎝ π ⎠ Results were compared with a tube burner with inner diameter same as equivalent diameter. Figure 2 shows the direct photographs of the different burners used in this study. Each slot burner was and
  4. 4. tube burners were of length 300 mm. A pair of packing (each consisting of 7-8 meshes) of fine steel meshes was used as flame arrestor and was placed at the tail of the burner. Material used for all the burners was brass. 3 4 2 1 5 6 7 8 9 O r 10 11 1. Outer Jacket Insulation 2. Water Inlet 3. Cork 4. T-Type Thermocouple 5. Transparent Cover 6. Water Outlet 7. Copper Plate 8. K-Type Thermocouples 9. Sensor 10. Inner Fixture 11. Outer Fixture Figure 3 Flat plate calorimeter Figure 3 shows the schematic for flat plate calorimeter. Copper plate of 8 mm thickness and 300 mm diameter was used as impingement surface. The surface of the plate was smooth in fabrication and also it was uncoated. There was no soot deposition on the surface; still as precaution it was periodically cleaned. A water jacket was provided at the rear of the copper plate to evenly cool the plate from the backside. A perspex sheet of 10 mm thickness was used as transparent cover to the cooling jacket to visualize the flow of cooling water. Water flows into the calorimeter at the center and comes out from the calorimeter through the two exits provided at diametrically opposite points. Inlet and outlet temperatures of the water are measured with T-type thermocouple. A row of equally spaced K-type thermocouples, in one radial direction, is used to measure the surface temperature. All the thermocouples are inserted, at 10 mm distance apart, from the rear of the impingement plate by drilling blind holes up to 1mm from the impingement side. The local heat flux on the impingement surface was measured with a single heat flux micro-sensor (HFM) of 6.35 mm diameter (Vatell Corporation, HFM-7E/H). This heat flux sensor unit also has a provision for surface temperature measurement. The sensitivity of HFM-7E/H was 150±10 μV/W/cm2. The HFM-7E/H was secured to specially designed copper fixtures. The outer and the inner fixtures were threaded in to the mounting base of the impingement copper plate at the centre of the plate with the heat flux sensor flush with the impingement surface. The millivolt (mV) output from the sensor was sensed by a Vatell Corporation Model AMP-6 signal conditioning amplifier and directed to a Hewlett Packard Model 34401A Multi-meter. Based on the calibration of HFM-7E/H sensor the appropriate heat flux was calculated from the output of AMP-6 by the software, Hfcompv4, provided by the company. Different commercial rotameters (accuracy ±2%FSD) are used for measuring the flow rates of air and fuel respectively. Rotameters are calibrated with DryCal DC- Lite primary gas flow meter (±1% of reading). Commercially available methane with 99.99% purity is used and is burnt with synthetic bottled air (Volume %: 21% O2 and 79% N2). A 3D positioning mechanism (with ±0.05 mm, ±0.05 mm and ±1 mm least counts in X, Y and H directions respectively) was used for the positioning of the burner. 3. EXPERIMENTAL PROCEDURE Measured quantities of air and fuel were mixed in the mixing tube and mixture was fed to the burner. Equivalence ratio (φ = 1.0) was kept constant for all experiments. Hot water was supplied to the calorimeter to avoid the condensation of water vapors present in the combustion products at the
  5. 5. impingement surface. Water was supplied to the calorimeter for 15 minutes before the mixture was ignited to attain temperature uniformity in the experimental setup. All the readings were taken under steady state when the temperature of the outlet water became constant. First set of experiments were conducted at constant Re = 1000 and fixed H/de =3, for all three slot burners and tube burner. Second set of experiments were conducted for different Re (800-1300) for different slot burners. In the Third set of experiments H/de was varied from 1.5 to 5 at fixed Re of 1000 for all burners. Figure 4 shows the co-ordinate system used for heat flux measurements for slot jet experiments. Measurements were made along both X (major axis/length of slot) and Y-directions (minor axis/width of slot). The flame jets exit Reynolds number was calculated based on cold fuel/air mixture gases (Dong et al. (2002)); Re = uexit d e ρ mix and μ mix = ∑(μ yi i Mi ) μ mix ∑( y i Mi ) Here the equivalence ratio is defined as: ( F / A)actual φ= ( F / A)stoic To ascertain the influence of calorimeter cooling rate on the results obtained, the water flow rate in the calorimeter was varied over a wide range to examine the sensitivity of heat flux and temperature distributions to the cooling rate of the copper plate. The resulting variations observed in heat flux distributions were not quite significant. Y-direction Slot L W X-direction O (centre) Impingement surface Figure 4 Co-ordinates for heat flux measurement over the impingement surface 4. UNCERTAINTY ANALYSIS Uncertainty analysis was carried out as per the procedure given by Kline and Mclintock (1953). Maximum uncertainty in equivalence ratio and Reynolds number measurement was 5.35% and 4.9% respectively. Heat flux measurements were done with heat flux sensor of accuracy ±5%. 5. RESULTS AND DISCUSSION 5.1 Impinging flame shapes In case of flame impingement studies the heat transfer to the impingement surface is intimately related to the proximity of the inner reaction zone with surface (Milson and Chigier (1973); Rigby and Webb (1995), Dong et al. (2000; 2001), Hou and Ko (2005) and Chander and Ray (2006)). Direct photographs of impinging flames were taken with digital camera. All the flames comprised of blue inner reaction zone and light blue outer diffusion layer.
  6. 6. a) b) c) a) b) c) (View in Y-direction) (View in X-direction) Figure 5 Direct photographs of impinging flames at various Reynolds numbers (H/de = 3 and φ = 1.0) for Slot 1 a) Re = 800, b) Re = 1000 and c) 1300 a) b) c) a) b) c) (View in Y-direction) (View in X-direction) Figure 6 Direct photographs of impinging flames at various dimensionless separation distances for Re = 1000 and φ = 1.0 a) H/de = 1.5, b) H/de = 3 and c) H/de = 5 Figure 5-7 show different impinging flames for different slot burners under different operating conditions. It has been evident from the previous literature that heat transfer to the surface was significantly affected with the proximity of the inner reaction zone with surface. Figure 5 shows the impinging flame shapes for slot 1 for different Reynolds numbers. The dimensionless separation distance was kept constant (H/de = 3). The flame shapes were nearly rectangular along the length of the slot (X-direction) and triangular along the width of the burner (Y-direction). With increase in Re the length of the inner reaction zone was increased because of increase in average velocity of flow. At low Re, a dip in the inner reaction cone was observed along the length at the centre. This can be attributed to the velocity profile for the slot jet (Sezai and Mohamad (1999)). At higher Re there was no central dip in the flame reaction zone at the centre. In Y- direction i.e. along the width of the flame, height of triangular reaction zone increased with Re. Figure 6 shows the impinging flame shapes at different separation (H/de = 1-5) distances for slot 1 for Re = 1000, and φ = 1.0. There was wider spread of the outer diffusion layer on the impingement surface at small separation distances. At separation distance of H/de = 1.5, the flame inner reaction zone was closet to the impingement surface. Figure 7 shows the impinging flame shapes corresponding to identical operating conditions (Re = 1000 and φ = 1.0 and H/de = 3) for slot burners of different aspect ratios and a tube burner. The flame inner reaction cone length increased with decrease in aspect ratio. Longest flame length was observed for the tube burner. The different flame shapes for these burner geometries can be attributed to different exit velocity profiles (Chander and Ray (2006)). The flame shapes along the length were rectangular for slot 1 and 2 whereas it was nearly triangular for slot 3. a) b) c) d) a) b) c) d) (View in Y-direction) (View in X-direction) Figure 7 Comparison of flame shapes of a) slot1, b) slot 2, c) slot 3 and d) tube at Re = 1000, φ = 1.0 and H/de = 3
  7. 7. 5.2 Comparison of heat flux contours for different slot burners Figure 8-10 show the heat flux contour plots for slot burners with different aspect ratios under identical operating conditions (Re = 1000, H/de = 3 and φ = 1.0). Flame inner reaction zone were away from the impingement surface in all cases. Figure 8 depicts the contour plot for slot1. A nearly rectangular shape of the contour was observed for this geometry. Heat flux was nearly uniform in the impingement region and decreased monotonically in the wall jet region along the X- direction (along the length of the slot). Two off-centered peaks of heat flux were observed along the Y-direction (along minor axis). As the aerodynamics of the single flame jets is similar to those of the isothermal jets (Viskanta (1998)), so the explanation for these two off-centered peaks can be given from the studies on isothermal impinging jets. Sezai and Mohamad (1999) observed off- centered Nusselt number peaks on the impingement surface in their numerical study for investigation of flow and heat transfer characteristics of impinging laminar slot air jets of different aspect ratios. There, those off centered peaks were attributed the velocity profile at the nozzle exit. It was also reported that these two peaks have been found to form at an elevation where wall jet formation started along the minor axis of the jet. Higher heat fluxes (Figure 1) in Y-direction (minor axis) can be attributed to higher jet velocities in that direction (Sezai and Mohamad (1999)). It has also been observed that the peak heat fluxes were distributed over a large region over the impingement surface. Figure 8 Heat flux (kW/m2) contour plot for slot 1 at Re = 1000, H/de = 3 and φ = 1.0 Figure 9 Heat flux (kW/m2) contour plot for slot 2 at Re = 1000, H/de = 3 and φ = 1.0
  8. 8. Figure 9 shows the heat flux distribution on the impingement surface for slot 2. An oval shape heat flux distribution was observed. Because of longer flame length, peak heat flux was slightly higher than slot 1 but was concentrated over a smaller area as compared to slot 1. Different heat fluxes were observed in X and Y -directions (Dong et al. (2002) and Kwok et al. (2003)). In this case also two off-centered peaks in heat fluxes were observed. Figure 10 shows the heat flux contours for slot 3. This burner was corresponding to least aspect ratio. In this case heat fluxes were concentrated at the stagnation region and were gradually decreasing in both X and Y- direction. There were no off-centered peaks in the heat flux along the minor axis. Heat flux contours were nearly circular in shape. Highest magnitude of heat fluxes were observed in this case. On comparing the heat flux contours for above discussed three slot burners it was observed that the peak heat fluxes were distributed on a large area over the impingement surface for slot burner with largest aspect ratio (slot 1). Figure 10 Heat flux (kW/m2) contour plot for slot 3 at Re = 1000, H/de = 3 and φ = 1.0 5.3 Comparison of heat flux distribution 5.3.1 Effect of Re Figures 11-13 show the comparison of heat flux profiles for all three slot burners along X and Y- direction for different Re. Re was varied from 800 to 1300 and dimensionless separation distance (H/de = 3) was kept constant. Figure 11 shows the heat flux profiles for slot 1. Heat fluxes were increased with Re both in stagnation and wall jet regions. This can be attributed to higher flow velocities at higher Re resulting in higher convective heat transfer at the surface. This was in agreement with the previous studied of Mison and Chigier (1973), Dong et al. (2002), Kwok et al. (2003) and Chander and Ray (2006). Corresponding to any flow condition heat fluxes were higher along the Y- direction (minor axis) as compared to X-direction. This can be attributed to higher flow velocities in that direction (Sezai and Mohamad (1999)). An off-centered peak in the heat fluxes were observed along the minor axis corresponding to all Re. A uniform heat flux distribution was observed along the X-direction at Re = 800 and 1000. At Re = 1300, there was rise in heat flux at the centre and was monotonically decreasing in X-direction. It has also been observed that heat flux profile was much flatter in X-direction at lowest Re which ensures uniform heating at the surface in that direction.
  9. 9. 200 Re = 800 (X) Re = 1000 (X) Re = 1300 (X) 150 Heat Flux (kW/m2) Re = 800 (Y) Re = 1000 (Y) Re = 1300 (Y) 100 50 Slot1, H/de = 3, φ =1.0 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 X/de or Y/de Figure 11 Heat flux distribution on the surface for slot 1 in X and Y direction for various Reynolds numbers Figure 12 shows the heat flux profiles for slot 2. Similar to slot 1 the higher heat fluxes were observed along Y-direction. Off- centered peak in heat fluxes were observed along Y-direction at Re =800 and 1000 but at Re =1300 the peak in the heat flux was shifted to centre of the slot. This is because of position of the proximity of inner reaction with the impingement surface. At this Re, the tip of the flame was closer to the impingement surface resulting in the higher heat fluxes at the centre (Kleijn (2001)). It has also been observed that the heat flux profiles at Re = 800 and 1000 along X-direction were less flat as compared to slot1 resulting in less uniformity in the heating. 200 180 Re = 800 (X) 250 Re = 800 (X) Re = 1000 (X) Re = 1000 (X) 160 Re = 1300 (X) 200 Re = 1300 (X) Heat Flux (kW/m2) Heat Flux (kW/m2) 140 Re = 800 (Y) Re = 800 (Y) 120 Re = 1000 (Y) Re = 1000 (Y) Re = 1300 (Y) 150 100 Re = 1300 (Y) 80 100 60 40 50 20 Slot 2, H/de = 3, φ =1.0 Slot 3, H/de = 3, φ =1.0 0 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 X/de or Y/de X/de or Y/de Figure 12 Heat flux distribution on the surface for slot Figure 13 Heat flux distribution on the surface for slot 2 in X and Y direction for various Reynolds numbers 3 in X and Y direction for various Reynolds numbers Figure 13 shows the comparison of heat flux distribution in X and Y -direction for slot 3. Comparatively longer flame lengths were observed for this slot burner as compared to earlier slot burners. At lower Re = 800 and 1000, the heat flux distribution in X and Y direction was same in the impingement region. In the wall jet region lesser heat fluxes were observed along X-direction. At Re = 1300, flame inner reaction cone was intercepted by the plate and there was impingement of cool un-burnt mixture directly on the impingement surface. This was resulting in fall in heat flux at the centre to zero (Chander and Ray (2006)). Peak in the heat flux was observed at the point on the surface where the inner reaction zone comes actually in contact with the surface. A highly non- uniform heating on the surface was conserved in the stagnation region. At this Re (Re =1300), heat fluxes were higher in X-direction in early wall jet region, reverse took place in later wall region. Again compared to slot1 the heating was more- uniform as the heat flux profiles were not as flatter as in case of slot 1 was.
  10. 10. 5.3.2 Effect of H/de Figure 14 shows the heat flux distribution at various non-dimensional separation distances for slot1. H/de was varied from 1.5 to 5. At H/de = 1.5, the tip of the inner reaction was closer (Figure 6) to the impingement surface resulting in higher heat fluxes at the centre. Heat flux decreased monotonically along Y-direction with peak at the centre. Higher heat fluxes were observed at the stagnation region along X-direction. This can be attributed to the fact that in case of slot burner there is stagnation line in comparison to stagnation point of round burner. There was cross over of heat flux lines in the later wall jet region. 300 H/de = 1.5 (X) 250 H/de = 3 (X) Heat Flux (kW/m2) H/de = 5 (X) 200 H/de = 1.5 (Y) 150 H/de = 3 (Y) H/d = 5 (Y) e 100 50 Slot 1, Re = 1000, φ =1.0 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 X/de or Y/de Figure 14 Heat flux distribution on the surface for slot 1 in X and Y direction for various non-dimensional separation distances at Re = 1000 At H/de = 3, the peak heat fluxes were very much lower than that at H/de = 1.5. This was because of position of the inner reaction zone with impingement surface. Higher heat fluxes were observed along Y-direction as compared to X-direction in both stagnation and wall jet region. Centre point heat flux was less and an off- centred peak in heat flux was observed. At H/de = 5, flame inner reaction zone went away from the impingement surface. A higher heat flux at the centre was observed. This can be attributed to buoyancy effect at large separation distance which resulted in increased heat flux (Hargrave et al. (1987), Chander and Ray (2006)). At this separation distance, the heat flux along X-direction decreased gradually whereas an off- centered peak in heat flux was formed along Y-direction. Lowest heat fluxes were observed in the wall jet region along both X and Y –direction. 5.3.3 Comparison of different slot burners with tube Figure 15 and Figure 16 show the comparison of heat flux profiles along X and Y-direction for different slot burners and tube burner under identical operating conditions (Re = 1000, H/de = 3 and φ = 1.0). Figure 15 shows the heat flux distribution along X- direction. Slot 1 gave least heart fluxes in both impingement and wall jet regions. Impingement region heat flux increased with decrease in the value of aspect ratio. For slot 3 heat fluxes were highest both in stagnation and wall jet region. Highest heat fluxes were observed for the tube burner. This can be attributed to the longest flame length found in case of tube burner resulting in decrease in tip to surface distance and higher stagnation point heat fluxes (Kleijn (2001)). It was evident from the Figure 15 that although heat fluxes were least in case of largest aspect ratio slot burner but the heat flux was most uniformly distributed in this direction. Figure 16 shows the comparison of heat flux profiles along Y-direction. For all slot burners, an off- centered peak in heat flux was observed. In the stagnation region, highest heat flux was observed for tube burner but a reverse trend was observed in the wall jet region. Amongst slot burners of
  11. 11. different aspect ratio, slot 3 showed high heat fluxes in stagnation and early wall jet region. In the later wall jet region, highest heat fluxes were observed for slot 2 and slot 1 showed lowest heat fluxes at all regions. So it was observed that tube burner gave highly non-uniform heat flux distribution as compared to slot burner. 140 140 Slot 1 Slot 1 120 Slot 2 120 Slot 2 Slot 3 Slot 3 Heat Flux (kW/m2) Heat Flux (kW/m2) 100 100 Tube Tube 80 80 60 60 40 40 20 Re =1000, φ = 1.0, H/de =3 20 Re =1000, H/de = 3, φ =1.0 0 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 X/de Y/de Figure 15 Heat flux distribution on the surface for Figure 16 Heat flux distribution on the surface for various slot burners and tube under identical operating various slot burners and tube under identical operating conditions in X direction conditions in Y direction 6. CONCLUSIONS An experimental study has been conducted to investigate the effect of slot jet aspect ratio on heat transfer characteristics for methane/air flame impinging on a flat surface. All the burners were having same equivalent diameter (de = 12.7 mm) and results were compared with tube burner of same diameter. Following conclusions were made from the study. 1. There were different heat fluxes in X and Y-directions for the slot burners. Two off- centered peaks in heat flux was observed for slot burners along the minor axis (Y-direction). Heat flux distribution was more uniform in X-direction for slot with largest aspect ratio. Lesser magnitude of peak heat flux was observed along X-direction for slot with largest aspect ratio. 2. In comparison to tube burner slot burner gave more uniform heat flux distribution but peak heat fluxes were more in case of tube. 3. At small separation distances highly non-uniform heating was observed even in the case of slot burners when, either flame inner reaction zone was intercepting or close to the surface. 4. At large separation distances, a second peak in heat flux was observed due to buoyancy effect. 5. With increase in Re heat flux increased both in stagnation and wall jet regions and also the heat flux in the stagnation region was intimately related to proximity of inner reaction zone with surface. 7. REFERENCES 1. Al-Sanea, S., 1992, A Numerical study of the Flow and Heat-Transfer Characteristics of an Impinging Laminar Slot-Jet Including Cross Flow Effects, Int. J. Heat Mass Transfer, 35, 2501–2513. 2. Ashforth-Frost, S., Jambunathan, K. and Whitney, C. F., 1997, Velocity and Turbulence Characteristics of a Semi-confined Orthogonally Impinging Slot Jet, Experimental Thermal and Fluid Science, 14, 60-67.
  12. 12. 3. Baukal, C.E., Gebhart, B., 1995, A Review of Flame Impingement Heat Transfer Part1: Experimental Conditions, Combustion Science and Technology, 104, 339-357. 4. Baukal, C.E., Gebhart, B., 1997, Surface Condition Effects on Flame Impingement Heat Transfer, Exp. Thermal Fluid Sci., 15, 323–335. 5. Chander, S., Ray, A., 2006, Influence of Burner Geometry on Heat Transfer Characteristics of Methane/Air Flame Impinging on Flat Surface. Experimental Heat Transfer, 19(1), 15- 38. 6. Chou, Y. J. and Hung, Y. H.,1994, Impingement Cooling of an Isothermally Heated Surface with a Confined Slot Jet, J. Heat Transfer, 116, 479–482. 7. Dong, L.L., Cheung, C.S., Leung, C.W., 2001, Heat Transfer of an Impinging Butane/Air Flame Jet of Low Reynolds Number, Experimental Heat Transfer, 14, 265-282. 8. Dong, L.L., Cheung, C.S., Leung, C.W., 2002, Heat Transfer from an Impinging Pre-Mixed Butane/Air Slot Flame Jet, Int. J. Heat Mass Transfer, 45, 979-992. 9. Dong, L.L., Cheung, C.S., Leung, C.W., 2003, Combustion Optimization of a Slot Flame Jet Impingement System, Journal of Institute of Energy, 76, 80-88. 10. Gao, X. and Sundén, B., 2003, Experimental Investigation of The Heat Transfer Characteristics of Confined Impinging Slot Jets, Experimental Heat Transfer, 16, 1–18. 11. Hargrave, G.K., Fairweather, M., Kilham, J. K., (1987), Forced Convection Heat Transfer from Impinging Flames. Part II: Impingement Heat Transfer, Int. J. Heat Mass Transfer, 8, 132-138. 12. Hou, S.S. and Ko, Y.C., (2005) Influence of Oblique Angle and Heating Height on Flame Structure, Temperature Field and Efficiency of an Impinging Laminar Jet Flame, Energy Conversion and Management, 46(6), 941-958. 13. Kleijn, C.R., Heat Transfer from Laminar Impinging Methane Air Flames, PVP-Vol.424-1, Computational Technologies for Fluid/Thermal/Structural/Chemical Systems with Industrial Applications, Eds., V. Kudriavtsev, S. Kawano, and C.R. Kleijn, Book no. G1173A-2001, 259-269. 14. Kline, S.J., McClintock, F.A., 1953, Describing Uncertainties in Single Sample Experiments, Mech. Engg., 75, 3-8. 15. Kwok, L.C., Cheung, C.S., Leung, C.W., 2003, Heat Transfer Characteristics of Slot and Round Pre-mixed Impinging Butane/air Flame Jet, Experimental Heat Transfer, 16, 111- 137. 16. Lin, Z. H., Chou, Y. J. and Hung, Y. H., 1997, Heat Transfer Behaviors of a Confined Slot Jet Impingement, Int. J. Heat Mass Transfer, 40(5), 1095-1107. 17. Milson, A., Chigier, N.A., 1973, Studies of Methane and Methane/ Air Flames Impinging on Cold Plate, Combustion and Flame, 21, 295-305. 18. Rigby, J.R., Webb, B.W., 1995, An Experimental Investigation of Diffusion Flame Jet Impinging Heat Transfer, in ASME/JSME Thermal Engineering Conference, 3, 117-126. 19. Sahoo, D and Sharif, M. A. R., 2004, Mixed-Convective Cooling of an Isothermal Hot Surface by Confined Slot Jet Impingement, Numerical Heat Transfer: Part A, 45, 887–909. 20. Sezai and A. A. Mohamad, 1999, Three-Dimensional Simulation of Laminar Rectangular Impinging Jets, Flow Structure, and Heat Transfer, J. Heat Transfer, 121, 50–56. 21. Turns, S.R., 2000, An Introduction to Combustion. Second Edition, McGraw-Hill, Boston, MA, 255-256. 22. van Heiningen, ARP, Majumdar, AS and Douglas, WJM, 1976, Numerical Prediction of Flow Field and Impingement Heat Transfer Caused by Laminar Slot Jet, Journal of Heat Transfer,654-658. 23. Viskanta, R., 1998, Convective and Radiative Flame Jet Impingement Heat Transfer, International J. Transport Phenomenon, 1, 1-15. 24. Wadsworth, D. C. and Mudawar, I.,1990, Cooling of a Multi-chip Electronic Module by Means of Confined Two-Dimensional Jets of Dielectric Liquid, J. Heat Transfer, 112, 891– 898.

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