Paper presented at 13th International Heat Transfer Conference, Sydney
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
Subhash Chander, Abhineesh Das, Arpit Jain and Anjan Ray
Department of Mechanical Engineering
Indian Institute of Technology Delhi
New Delhi -110016 (India)
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
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
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 molar fraction
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
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
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
4 12 13
3 20 23
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.
⎛ 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
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.
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
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));
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
( 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.
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.
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
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
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
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.
Re = 800 (X)
Re = 1000 (X)
Re = 1300 (X)
Heat Flux (kW/m2)
Re = 800 (Y)
Re = 1000 (Y)
Re = 1300 (Y)
Slot1, H/de = 3, φ =1.0
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.
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)
20 Slot 2, H/de = 3, φ =1.0 Slot 3, H/de = 3, φ =1.0
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.
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.
H/de = 1.5 (X)
250 H/de = 3 (X)
Heat Flux (kW/m2)
H/de = 5 (X)
H/de = 1.5 (Y)
150 H/de = 3 (Y)
H/d = 5 (Y)
Slot 1, Re = 1000, φ =1.0
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
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
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.
Slot 1 Slot 1
120 Slot 2 120 Slot 2
Slot 3 Slot 3
Heat Flux (kW/m2)
Heat Flux (kW/m2)
20 Re =1000, φ = 1.0, H/de =3 20 Re =1000, H/de = 3, φ =1.0
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
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
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
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
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
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