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Heat transfer characteristics of household burner flames impinging on a flat surface have been investigated experimentally. This experimental study employed the analytical inverse heat conduction procedure (IHCP) concept for obtaining the local heat flux distribution. The heat flux distribution has been studied for various operating parameters such as firing rates, separation distance, burner-cap port diameter and equivalence ratios. The average heat flux distribution for three different burner caps has been compared at similar operating conditions. The separation distance between the burner tip and target surface has significant importance in heat transfer and stability viewpoint. Results of present experimental study revealed that the maximum heat flux values occur at separation distance corresponding to 15 mm. The firing rate has been observed to significantly influence the heat flux distribution on the impinging surface. The heat flux distribution has been found to be more uniform at higher separation distances and higher firing rates. [ABSTRACT FROM AUTHOR]

Experimental investigations have been carried out for impingement heat transfer characteristics of an enclosed dual swirling flame for various enclosure configurations. Effects of the size of the enclosure (DE = 70 mm, 90 mm, 90 mm, 110 mm and 150 mm) and the level of gap between the top of the enclosure and the target surface (G = 12, 16, 20, 24 and 28 mm) have been examined at fixed other operating conditions (Re(o), H/Dh and S). In addition to this, the effects of variation in outer swirling flame Reynolds numbers (Re(o) = 7000–13,000), dimensionless separation distances (H/Dh = 2–5) and the swirl number (S = 0.86, 1.54 and 2.4) have also been examined. Impingement heat transfer characteristics strongly depend on the size of the enclosure (DE) and the level of gap (G). Larger sized enclosures (DE of 110 and 150 mm) with smaller gap levels (G of 12 and 16 mm) perform much better than small sized enclosures (DE of 70 and 80 mm) with larger gap levels (G of 24 and 28 mm). Heat transfer performance continuously increases with an increase in Re(o) in presence of an enclosure in contrast to unenclosed flames where performance deteriorates at high Re(o). The heat transfer enhancement effect in presence of enclosure is more pronounced at larger separation distances (H/Dh of 4 and 5) because of the formation of the outer recirculation zone (ORZ). The effect of the presence of enclosure on impingement heat transfer enhancement becomes more significant at higher levels of swirl. [ABSTRACT FROM AUTHOR]

Detailed flow field and heat transfer characteristics of an array of three corotating dual swirling impinging flames arranged in a triangular configuration have been investigated experimentally as well as numerically. Effect of varying separation distance and inter-jet spacing on interactions and impingement heat transfer has been studied at fixed Reynolds numbers under stoichiometric conditions. Adjacent co-swirling flames developed strong interactions resulting in high heat transfer due to increased mixing and turbulence. Bending of inner non-swirling flames takes place toward geometric center of the array due to asymmetric interactions. Numerical simulation also predicted asymmetric interactions resulting in the formation of large distorted recirculation bubble in each dual-swirling flame. Interacting co-swirling flames resulted in the formation of circulating patterns of flow at the geometric center of array. The effect of inter-jet spacing on average heat flux is observed to be more significant than separation distance. [ABSTRACT FROM AUTHOR]

Experimental and numerical studies have been conducted to investigate the flow field and heat transfer characteristics of a pair of counter rotating dual-swirling flames impinging on a flat surface. Effect of separation distance (H/D h = 3, 4, 6 and 8), inter-jet spacings (S/D h = 4, 7 and 10) and Reynolds number on flow interactions and impingement heat transfer have been studied under stoichiometric conditions. The inner non-swirling flames are seen to bend towards interacting side due to asymmetric interactions developed in a pair of dual-swirling flames. Commercial computational fluid dynamics (CFD) code (FLUENT®) has been used to simulate the interacting dual-swirling impinging flames. Numerical simulation predicted the deflection of inner flames to be primarily due to large distorted RCZ developed from asymmetric interactions. Comparisons of co-and-counter swirling configurations suggest that the presence of plate has a stronger influence on flow structure of impinging flames than sign of rotation. High resolution heat fluxes have been obtained using steady state 1-D heat balance from temperature readings obtained on rear side of quartz impingement plate through IR thermal camera. 1-D heat balance has been corrected for lateral conduction taking place inside the impingement plate using an iterative procedure. For counter swirl case, cross-flow leaves as a straight flow along the transverse direction due to the nature of interactions. Cross-flow causes heat flux contours in the interaction region to be asymmetric and preferential heating is observed along the transverse direction. The increase in Re (o) causes wider flame spread and severe interactions result in increased tendency for preferential heating of the impingement plate. High average heat fluxes are obtained at smallest H/D h and S/D h whereas minimum relative deviation is observed at S/D h = 10 for H/D h = 4. [ABSTRACT FROM AUTHOR]

An experimental investigation has been conducted on the heat transfer characteristics of compressed natural gas (CNG)/air dual flame (with co-swirling inner and outer flames) impinging on a flat surface. Effects of various parameters such as outer vane swirler angle ( θ o = 30°, 40° and 50°), inner vane swirler angle ( θ i = 0°, 15°, 22.5° and 30°), outer swirling flame Reynolds number ( Re o = 5000–9000), and dimensionless separation distance ( H / D h = 1–4) on heat transfer characteristics have been evaluated. The inner flame Reynolds number ( Re i ) was kept constant to 1300. A significant difference in the heat transfer characteristics has been observed in the stagnation region for different values of the θ o and θ i at different H / D h . High local and average heat fluxes have been observed at smaller separation distances. It has also been observed that the distribution of heat flux on the impingement surface was more uniform for θ o of 40° at H / D h of 2 and 3. Inducing swirl also to the inner flame has made the heat flux distribution further more uniform in the stagnation region and the effect was most favorable at low swirls ( θ i = 15°). Comparisons were also made for average heat fluxes at different radial extent of integration areas for different operating conditions. It has been found that radial extent of integration area correspond to r / D h of 5 covers most of the effective heating area of the dual impinging flame. Finally, it has been inferred from the present study that a combination of θ o of 40°, θ i of 15°, H / D h of 2 and Re o of 7000 produces the lowest percentage relative deviation and significantly high average heat transfer to the target surface for dual flame impinging on a flat surface. [ABSTRACT FROM AUTHOR]

• Inner flame plays a crucial role in flame stabilization. • Impinging flame stability declines with increase in the separation distance. • The dual flame presents improved resistance to lean blow-off. • Excessive swirl levels and higher Reynolds number promotes flame destabilization. • Lean blow-off limits improve with increase in the number and diameter of ports. An experimental study has been carried out to investigate the lean blow-off (LBO) characteristics of a tangential entry type dual swirling flame under different operating and geometrical parameters. The amount of swirl is controlled by changing the ratio of tangential to axial in-flows (T:A). The qualitative flame structure is accessed by taking the direct flame images and the radial distribution of static impingement pressure. The flame structure has also been illustrated using commercial CFD code ANSYS Fluent®. The results showed that the flame behavior and the mixing between the reactants are strongly influenced by the outer flame Reynolds number (Re(o)) and the intensity of swirl (S). Significant transitions in flame shapes were observed owing to these flow alterations. These transitions in flame shapes strongly govern the stability near blow-off conditions. It has been seen that LBO limits are considerably improved with the presence of an inner supporting flame and also in presence of the impingement surface. The inner conical flame significantly supports the outer swirling flames for stability at high flow rates and under fuel-lean operating conditions. The outer flame could be sustained stably up to ϕ(o) of 0.1 with inner stoichiometric flame and both inner and outer flames could be stabilized for simultaneous variation of equivalence ratio up to ϕ of 0.6 (S = 0.86). Geometrical design parameters of the burner showed a major impact on improving the LBO limits. Thus, the dual flame presents significant improvements to lean blow-off limits as compared to the single outer swirling flame. [ABSTRACT FROM AUTHOR]

Radiative furnaces pose significant thermal inertia and single impinging flames have been observed to cause occurrence of hotspots on the target surface. Multiple burners arranged in suitable array configuration represent one of the plausible solutions for more uniform heat transfer. In this study, computational fluid dynamics (CFD) simulations have been carried out for multiple swirling impinging flames arranged in a hexagonal array configuration. The turbulence chemistry interactions in the flame field are solved numerically using renormalization group (RNG) based k-e/eddy dissipation model (EDM) framework. Comparison of co-and-counter-swirling configurations has been studied for interactions and spent gas release mechanism. Multiple swirling impinging flames undergo strong interactions resulting in distortions of recirculation zones (RCZ) for all the surrounding except central flame. Co-swirling flames result in development of higher turbulence in the interaction regions as compared to counter-swirl case. Results indicate that some flames in counter-swirl case are underutilized due to the fluid dynamics developed in the system and co-swirling hexagonal array configuration is a better arrangement for effective heating of target surface. Effect of interjet spacing (S/Dh=5, 7, and 9) and separation distance (H/Dh=3, 5, 7, and 9) studied for co-swirl case revealed that peak heat fluxes decreased with increasing interjet spacing and separation distance. Central flame represented a region of low heat flux and this region has been noticed to expand in size for increasing interjet spacings. Suppression of central flame has been observed to be maximum for minimum separation distance. [ABSTRACT FROM AUTHOR]

Flame jet impingement has been studied as an indispensable technique for direct heating applications suitable for metal processing industries. The current experimental and numerical investigation pursues to unfold the fluid dynamics and heat transfer characteristics for an obliquely impinging co-axial swirling flame jet. Co-axial flame used in the present work contains outer swirling flame together with an inner non-swirling flame. Steady state heat balance technique using one dimensional energy balance across the impingement plate has been employed to determine the local heat flux distribution. Due to obliquely impinging swirling jet, development of an asymmetric thermal boundary layer takes place on the target surface. The geometry being studied here develops a complex flow field which has also been investigated numerically using RANS based 'k-e' RNG turbulence model. The effect of various operating parameters such as inclination (θ = 0°, 15°, 30° and 45°), impingement distance (Z/D h = 3, 5, 7 and 9), Reynolds number (Re(s) = 2500, 4000, 5500, 7000 and 8500) and equivalence ratio (ϕ = 0.9, 1.0, 1.1 and 1.2) have been studied on the flow field development and resulting heat transfer distribution. • Obliquely impinging co-axial swirling flame jet with inner non-swirling flame. • Asymmetric heat flux patterns have been observed for inclined impingement plate. • Deflection of inner flame does not only depend on the plate inclination. • Deflection of inner flame is a stronger function of co-axial swirling flame behavior. • Structure of recirculation zone depends on inclination angle of plate. • Asymmetric recirculation lobes have been noticed for co-axial swirling flame. [ABSTRACT FROM AUTHOR]

Experimental and numerical study of linear arrays of co-axial flames impinging on a flat quartz plate have been performed and interactions and heat transfer characteristics are analyzed. High resolution inverse heat conduction procedure (IHCP) has been employed for determining local heat flux distribution. Strong interactions developed amidst adjacent flames led to asymmetric interactions. Combustion simulations revealed differences in the flow structures of four and five element arrays. Asymmetric interactions resulted in bending of all of the inner flames in four element array whereas central inner flame remains undeflected in five element array. Same has also been observed for six and seven element arrays too. Tilted columnar flows have been identified in even as well as odd numbered linear array of co-axial flames. These tilted flow structures resulted in distortion of recirculation zones (RCZ) of all the flames. Impingement heat flux contours are studied for various impingement spacings (H/D h = 3, 5 and 7). At smallest impingement spacing, these contour maps revealed footprints corresponding to central inner flame due to symmetrical interactions. Thermal efficiency and heat transfer uniformity of four element array has been observed to be higher than five element array at all impingement spacings. [ABSTRACT FROM AUTHOR]