Heat transfer from laminar premixed flames to cylindrical walls: Experimental studies using thermographic phosphors and micro-thermocouples
In industrial and domestic heating processes, components of various geometrical configura-tions have to be heated. Impinging flame jets are widely used for these heating processes be-cause of their high heat transfer rates. The heat transfer from the flame to the target body de-pends on many physical and chemical parameters. Whereas the interaction between the unburned gas velocity, the flame speed, and heat transfer to the burner is well understood for laminar flame jets impinging on flat surfaces, limited work has been done on cylindrical sur-faces. The curved surfaces can be found in many technical applications. Since the flow fields in axial and radial directions differ between the curved and flat surfaces, the heat transfer characteristics change significantly. Therefore, understanding the interaction between the combustion process and the heat transfer to the cylindrical wall is important for effective con-trol of heating process. This work focuses on determining the sensitive parameters influencing the heat transfer to cylindrical surfaces. In this study, an experimental investigation was conducted to heat flux distribution on a heat-ed cylindrical surface by the laminar flame jets. The heat flux from the flame jet to the cylin-drical surface was determined by measuring the temperature gradients across the cylindrical wall and in the gas phase close to the target surface. For the surface temperature measurement, phosphor thermometry was adopted because the method is superior against interferences from flame radiation and is insensitive to changes in target surface emissivity. The temperature on the surface was obtained by measuring the temperature-dependent phosphorescence lifetimes of chromium-doped aluminium oxide after excitation by a pulsed light-emitting diode. The temperature profiles of the flames were also measured using a micro-thermocouple. The effects of the following parameters on the stagnation point heat flux were examined. The unburned gas velocity was varied from 0.1 m/s to 0.6 m/s; the equivalence ratio was varied in the range of Ф = 0.85 - 1.2, the burner to cylindrical surface spacing in the range of H = 15 - 60 mm, and the target surface diameter in the range of D = 20 - 60 mm. Furthermore, the fuel dependence on heat transfer was studied by comparing the gaseous fuels, methane (CH4), propane (C3H8), and dimethyl ether (CH3OCH3). Methane and propane are conventional fuels used for heating applications, and dimethyl ether can be produced as a biofuel from renewable sources. The findings show that the heat flux at the stagnation point is strongly influenced by the flame stabilization mechanism, which changes from burner to wall stabilization when the un-burned gas velocity is higher than the laminar flame speed. Decreasing the spacing between the burner and the cylindrical surface increases the axial velocity of the flow, which in turn increases the heat flux at the stagnation point. The highest heat flux is measured in the stoichi-ometric flame and lowest in the lean flame as the unburned gas velocity approaches the lami-nar flame speed. In comparison between a cylindrical and a flat surface, higher heat flux at the stagnation is measured on the former surface than on the latter surface. Besides, the heat flux increases as the tube diameter is reduced due to the movement of the stagnation point closer to the target surface. Comparing the three gaseous fuels, slightly more heat, 3 % to 6 %, is transferred to the cylindrical surface from the propane flame than from the methane flame, while about 13 % to 17 % more heat is transferred to the target surface from the dimethyl ether flame than from the methane flame. The significant difference in the heat transfer from dimethyl ether flames compared to the other two fuels, indicate that dimethyl ether can be used as sole fuel or as an additive to achieve enhanced heating on the surface.
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