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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]

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]

Detailed flow field and heat transfer characteristics developed in a row of three co-rotating dual swirling impinging flames have been investigated experimentally as well as numerically. Impingement heat transfer and pressure distribution has been studied experimentally at different separation distances and inter-jet spacings. Inverse heat conduction procedure (IHCP) has been used for estimating heat fluxes on the front side of impingement plate. Turbulence induced mixing results in strong interactions amongst adjacent flames causing deflections of inner flames of the burners situated at sides of central burner. Numerical simulation predicted formation of asymmetric recirculation zones for side flames. Symmetric interactions taking place for central flame produced two equal recirculation lobes for central flame. Behavior of central inner flame has been observed to be dependent on the value of inter-jet spacing used. Suppression of central inner flame tend to occur at higher inter-jet spacings due to recirculating products. Impingement pressure distribution is observed to be consistent with the heat flux distribution. Averaged heat fluxes registered at the impingement plate due to the central flame are higher in magnitude to those pertaining to each of the side flames. [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]

Experimental and numerical studies have been conducted to investigate the flow field and heat transfer characteristics of a pair of dual interacting swirling flames impinging on a flat surface. Commercial computational fluid dynamics (CFD) code (FLUENT®) has been used to simulate the interacting isothermal swirling impinging jets. Inverse heat conduction procedure (IHCP) has been used to calculate the impingement heat fluxes from the surface temperatures captured by Infra-red camera. Effect of separation distance (H/D h  = 2.5, 4, 6 and 8) and inter-jet spacings (S/D h  = 4, 6, 8 and 10) have been studied at various Reynolds numbers ( Re (o) = 7000, 9000, 11000, 13,000 and Re (i) = 700, 1000, 1300) under stoichiometric conditions. Strong interactions between adjacent dual swirling flames result in high heat transfer due to increased mixing and turbulence in the interaction region. The inner non-swirling flames are seen to deflect towards interacting side due to asymmetric interactions. Numerical simulation predicted this deflection to be primarily due to large recirculation bubble developed from asymmetric interactions. Tilted cross-flow, emerging from interaction region has been observed due to momentum exchange taking place between cross-flow and swirling flames (jets). Area weighted average of local heat flux and relative deviation from averaged value has been calculated at various H/D h and S/D h . High average heat fluxes are obtained at smallest H/D h and S/D h . It has been concluded that for a system of burners considered for the present study, H/D h  = 2.5 and S/D h  = 8 is the optimum configuration on the basis of minimum relative deviation. [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]

An experimental study has been conducted to evaluate the heat transfer characteristics of CNG/air dual flame (outer swirling and inner non-swirling flame) impinging on a flat surface. Effects of variation of outer swirling flame Reynolds number (Re(o) = 3000-11,000), inner non-swirling flame Reynolds number (Re(i) = 900-1700), dimensionless separation distance (H/Dh = 1-5) and equivalence ratio of outer swirling flame (ϕ(o) = 0.8-1.2) on heat transfer characteristics have been investigated at fixed inner flame equivalence ratio (ϕ(i) = 1.0) and swirling insert helix angle of 40°. A significant effect on shape and size of the inner non-swirling flame has been observed due to central recirculation zone established by the outer swirling flame and vice versa. Impingement pressure distribution shows that the pressure peaks are the locations of peak heat fluxes on the impingement surface. Effect of presence of inner flame on heating characteristics is considerably less at larger separation distances (H/Dh ≥ 5) due to dominating effect of recirculation zone over the central axial flow. Heating characteristics showed that the heat flux distribution lines corresponding to inner flame were pushed inward towards burner axis with increase in outer swirling flame Re(o) at fixed Re(i). On the other hand, the inner boundary of the outer flame had shifted outwards with increase in Re(i) at fixed Re(o). Average heat flux imparted to the target surface increases with decrease in separation distance and increase in Re(o). There has not been much gain in the average heat transfer to the impingement surface beyond certain value of Re(o) because the rate of increase of average heat flux decreases with increase in Re(o) due to entrainment effect. Relative deviation in heat flux distribution on the impingement surface decreases with increase in separation distance. In most of the cases it again starts increasing for H/Dh > 2. In many cases, with further increase in separation distance beyond H/Dh of 3, the deviation starts continuously decreasing. Thus, it is concluded that for a dual flame impinging on a flat surface, best operating conditions prevail at moderate Re(o) (7000-9000) and H/Dh of 2. [ABSTRACT FROM AUTHOR]

Abstract: In this investigation, thermo-hydraulic performance of rectangular ducts roughened with a new configuration of ‘V-down rib having gap’ on one wide wall is determined. Small symmetrical gap equal to rib height is created at the centre of both legs of V of continuous V-down rib. The duct has aspect ratio (AR) of 12 and the Reynolds number (Re) ranged from 3000 to 15,000. To simulate the indoor testing of solar air heater, the roughened side of rectangular duct is heated with constant heat flux electric heater while the other sides are insulated. The roughness has relative roughness height of 0.043 and relative roughness pitch of 8. Five rib roughened plates having flow-attack-angle (α) from 30° to 75° have been tested. The thermo-hydraulic performance parameter based on equal pumping power (η), friction factor (f) and Nusselt number (Nu), were found to be highest for α =60°. The results obtained at α =60° were compared with those of continuous V-down rib for same rib-roughness parameters. [Copyright &y& Elsevier]

Abstract: An experimental study has been conducted to investigate the heat transfer characteristics of compressed natural gas (CNG)/air swirling flames impinging on a flat surface. Qualitative flame structures have been studied by taking direct photographs of impinging flames. The radial temperature distribution of swirling free and impinging flames is presented at different axial heights under fixed operating conditions. Effects of dimensionless separation distance (1–6), Reynolds number (3500–6000), equivalence ratio (1–1.5) and helical vane swirler angle (0–60°) on heat transfer characteristics have been investigated. The heat transfer characteristics of swirling and non-swirling flames have been compared under similar operating conditions. A dip in the heat flux at and around stagnation point was observed in almost all cases which could be the main cause of non-uniformity even in case of heating with swirling impinging flames. The dip in heat flux becomes more pronounced at higher Reynolds number. The heat flux distribution on the impingement plate was more uniform with swirl as compared to without swirl. Also, heat flux distribution was observed to be more uniform at moderate separation distance and at larger helical vane swirl angle. Stagnation point heat flux variation remains almost constant at higher separation distances for different values of Reynolds number and equivalence ratios. There was significant decrease in average heat flux with increase in separation distance. It was further observed that average heat flux increases sharply with Reynolds number whereas the variation was not that steep with change in equivalence ratio. [Copyright &y& Elsevier]

Abstract: A combined experimental and numerical study has been conducted to investigate the occurrence of off-stagnation peak for laminar methane/air flame impinging on a flat surface. Experiments were conducted for three tube burners of internal diameter 8mm, 9.7mm and 12mm. Radial heat flux distributions were compared (experimentally) for different burner diameters under identical operating conditions (with firing rates of 0.25kW, 0.40kW and 0.50kW, ϕ =1 and H =40mm). An off-stagnation point peak in heat flux was observed for some of the configurations in the present study which is in accordance with the previous findings. This off-stagnation point peak is a function of stand-off distance between the exit plane of the burner and the plate and also the distance between flame-tip and the plate. A satisfactory explanation is presented to explain the existence of this off-stagnation peak with the help of results of numerical simulation carried out with commercial CFD code FLUENT. It is concluded that this off-stagnation peak in heat flux is primarily due to the peak in the axial velocity profile close to the impingement surface. [Copyright &y& Elsevier]

Abstract: A combined experimental and numerical study has been conducted to determine the stagnation point heat transfer for laminar methane/air flame impinging on a flat surface. Effects of Reynolds number, equivalence ratio and burner diameter on stagnation point heat flux were examined experimentally at different separation heights. Maximum stagnation point heat flux was obtained when the flat surface was closest to the tip of the inner premixed reaction zone. Heat flux decreased along the axial direction when the separation distance was further increased from the tip of inner reaction zone. There was a secondary rise in heat flux at the stagnation point at larger separation distances. Correlations were developed for stagnation point Nusselt number. Numerical simulations were carried out using a commercial CFD code (FLUENT) for laminar methane/air flame impinging on a flat surface for various separation distances. Results were compared with those found experimentally. The reason for conducting the simulations was to (a) gain more insight into how the presence of the plate affects the flame and the flow and temperature fields and (b) to explain the reason for high heat flux when the tip of the inner reaction zone was very close to the stagnation point. [Copyright &y& Elsevier]

Abstract: An experimental study has been conducted to determine the heat transfer characteristics of methane/air laminar flames impinging normal to a cylindrical surface. Effects of variations in the values of Reynolds number (Re =600–1300), equivalence ratio (ϕ =0.8–1.3), dimensionless separation distance (H/d =1–5), and burner diameter to cylinder diameter ratio (d/D =0.0538–0.1076) have been investigated. Three important configurations, viz., flame inner reaction zone far away, just touching and intercepted by the impingement surface, were examined in detail. High stagnation point heat fluxes were obtained when tip of the flame inner reaction zone just touched the target surface. Stagnation point heat fluxes were either zero or negative when the inner reaction zone was intercepted by the impingement surface. An off-stagnation peak in heat flux was obtained at moderate separation distances above the flame tip. Both stagnation point and peak heat fluxes increased with Re when the inner reaction zone length was less than the separation distance. Heat fluxes in the wall-jet region were high at high Re. Maximum heat fluxes were obtained for initially fuel-rich mixture conditions due to entrainment of the surrounding air. Smaller burner diameters produced high heat flux at the stagnation region for fixed Reynolds number and opposite trends were seen in the wall-jet region. A secondary rise in stagnation point heat flux was obtained at larger separation distances. This secondary rise in heat flux was quite significant for larger burner diameters and at low flow rates. Correlations were developed for stagnation point heat flux. Results were also compared with flat plate under identical operating conditions. [Copyright &y& Elsevier]

Abstract: An experimental study has been conducted for three interacting methane/air flame jets (arranged in a triangular configuration) impinging normally on a flat surface. Surface heat flux distributions have been determined for various dimensionless inter-jet spacings (S/d =3, 4, 6 and 7.58) and separation distances between the exit plane of the burners and the target plate (H/d =2, 2.6, 5 and 7). All experiments were conducted for stoichiometric mixture at a Reynolds number of 800. The surface heat flux distributions were intimately related to flame shapes. For small inter-jet spacings and small separation distances, flames were deflected outward from the centroid of the triangular arrangement due to strong interaction between the jets. The heating was quite non-uniform at very large inter-jet spacings. Zones of low heat flux were obtained when the tip of inner reaction zones were intercepted by the plate (H/d =2). There were sharp peaks in the heat flux distribution when the tips of the inner reaction zones just touched the impingement surface (H/d =2.6). Heat flux distribution was non-uniform at small separation distances (H/d =2 and 2.6). For the system of flame jets under consideration, the optimum configuration, considering the magnitude of the average heat flux and the uniformity in the heat flux distribution, was corresponding to H/d =5 and S/d =3. [Copyright &y& Elsevier]

The air/water mist jet impingement phenomenon on a flat target with constant heat flux has been studied experimentally and numerically. The effects of various operational variables on the heat transfer characteristics of the mist jet have been analyzed through an experimental study. These variables are the air Reynolds number (Re a = 5305–10297), the mist loading percentage (f = 0 %–1.5 %), and the nozzle-to-plate spacing ratio (H/D o = 20–40). Additionally, the computational study investigates the effects of the nozzle ratio parameter (D i /D o = 0.1–0.3) and droplet diameter (2 μm–100 μm) on mist jet impingement. A comparison between Eulerian-Eulerian and Eulerian-Lagrangian modeling techniques for simulating the air/water mist jet has been provided. The current computational findings based on the Eulerian-Eulerian method accord well with the experimental data, with an accuracy of 15 %; however, numerical predictions using the Eulerian-Lagrangian approach deviate significantly from the experimental results, with errors of up to 65 %. The increase in Re a results in a greater improvement in the stagnation point Nusselt number, up to 85.32 %, compared to 34.99 % with an increase in f. Furthermore, a rise in f causes a greater spread of the mist jet flow in the domain and on the target impingement surface than an increase in Re a. When D i /D o increases from 0.1 to 0.3, the stagnation point Nusselt number increases by approximately 55.67 %. In addition, the spread of the mist jet flow in the domain increases as the D i /D o ratio increases. Experimental correlations have been developed based on studied parameters for calculating the Nusselt number values on the flat target. [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]

• Interactions result in clustering of high heat flux regions. • Flow circulation observed along the inner periphery of annulus. • Significant thermal dilution due to entrainment along the outer periphery of annulus. • Asymmetric interactions under impinging conditions deflected the inner flames. • Optimum performance observed at S/D h of 4 and H/D h of 6. Performance of gas burners can be optimized by improvements in their design and proper selection of operating parameters associated with combustion and heat transfer mechanisms. Array of multiple swirl burners is one such configuration where uniformity in heat transfer can be increased significantly. The current study presents experimental and numerical investigations conducted for flow patterns and impingement heat transfer developed in an annular arrangement of eight co-rotating dual swirling impinging flames. Impingement heat flux distribution has been studied experimentally using analytical inverse heat conduction procedure (IHCP). Effect of change in separation distance and inter-jet spacing has been studied for interactions and subsequent impingement heat transfer characteristics. Turbulent co-swirling flames portray intense mixing developed at the interaction regions due to strong interactions. Presence of impingement plate causes large scale changes in the flow-field. Numerical simulation conducted under reacting conditions predicted formation of asymmetric and distorted recirculation zones. Inner flames have been observed to deflect from their straight upright positions. Adjacent co-swirling flows merge together and develop circulation of flow at the inner periphery of the annulus. High heat fluxes observed at the target surface are corresponding to interaction regions and tend to cluster around the geometric centre of the array. Reversed downward flow developed at the geometric centre represents region devoid of heating. Averaged heat fluxes registered at the impingement plate decreased with increasing inter-jet spacing and separation distance. Maximum uniformity in the heat transfer has been observed at inter-jet spacing, S/D h of 4 and separation distance, H/D h of 6. [ABSTRACT FROM AUTHOR]