An experimental study investigated the characteristics of a stretched cylindrical diffusion flame, with a convex curvature with respect to the air stream, in response to periodic air flow velocity oscillation. The fue...An experimental study investigated the characteristics of a stretched cylindrical diffusion flame, with a convex curvature with respect to the air stream, in response to periodic air flow velocity oscillation. The fuel was methane diluted with nitrogen, and the oxidizer air. The oscillation frequency was varied from 5 to 250 Hz. The results are summarized as follows. Though the fluctuation amplitude of the air stream velocity gradient was constant with respect to the frequency, the amplitude of the fuel stream increased. The fluctuation amplitude of the flame radius changed quasi-steadily from 5 to 25 Hz, and decreased with increasing frequency in the frequency range greater than 50 Hz. The flame luminosity did not respond quasi-steadily at 5 Hz, and the oscillation amplitude of flame luminosity was less than that of a steady flame, over the same velocity fluctuation range. The oscillation amplitude of luminosity peaked at 50 Hz, and was greater than that of a steady flame. It is considered that this complex change in flame luminosity with respect to frequency was closely related to the phase difference in the respective time variations in the ratio of flame thickness to radius, the velocity gradients of the air and fuel streams, and the magnitude of these values, with the ratio of flame thickness to radius related to the flame curvature effect, the velocity gradient of the air stream correlated to the flame stretch effect, and the velocity gradient of the fuel stream impacting the fuel transportation.展开更多
In the current work,we investigated hydrogen/air flame propagation under supergravity conditions.Results show that when gravity is in the same/opposite direction as flame propagation,it leads to acceleration/decelerat...In the current work,we investigated hydrogen/air flame propagation under supergravity conditions.Results show that when gravity is in the same/opposite direction as flame propagation,it leads to acceleration/deceleration of the flame,and that such an effect could substantially modify the flame propagation and structure at high gravity levels.Furthermore,for the absolute and relative flame propagation speeds,the gravity-affected flame speed shows opposite trends as the absolute flame speed is more affected by the local induced flow field,while the relative flame speeds are controlled by the super-adiabatic or sub-adiabatic flame temperature.The gravity-affected thermal and chemical flame structures are also examined through the influence of the mixture equivalence ratio,pressure,and flame stretch.展开更多
A small cubic closed vessel with schlieren measurement technique combined with high-speed video camera were used to study limit flame properties under microgravity conditions at atmospheric pressure and room temperatu...A small cubic closed vessel with schlieren measurement technique combined with high-speed video camera were used to study limit flame properties under microgravity conditions at atmospheric pressure and room temperature.The rich flammability limit of C3H8/air was determined to be 9.2% C3H8.Stretched flame propagation speeds,stretched laminar burning velocities and unstretched laminar burning velocities near rich C3H8/air flammability limits were measured at different equivalence ratios.Outwardly propagating spherical flames were used to study the sensitivities of the flame propagation speeds and laminar burning velocities to flame stretch using Markstein lengths.Unstretched laminar burning velocity at rich flammability limit was determined to be 1.09cm/s.Lewis numbers were less than unity in rich C3H8/air and negative Markstein lengths were concluded.Absolute values of Markstein lengths were found to decrease linearly with equivalence ratios increase.展开更多
文摘An experimental study investigated the characteristics of a stretched cylindrical diffusion flame, with a convex curvature with respect to the air stream, in response to periodic air flow velocity oscillation. The fuel was methane diluted with nitrogen, and the oxidizer air. The oscillation frequency was varied from 5 to 250 Hz. The results are summarized as follows. Though the fluctuation amplitude of the air stream velocity gradient was constant with respect to the frequency, the amplitude of the fuel stream increased. The fluctuation amplitude of the flame radius changed quasi-steadily from 5 to 25 Hz, and decreased with increasing frequency in the frequency range greater than 50 Hz. The flame luminosity did not respond quasi-steadily at 5 Hz, and the oscillation amplitude of flame luminosity was less than that of a steady flame, over the same velocity fluctuation range. The oscillation amplitude of luminosity peaked at 50 Hz, and was greater than that of a steady flame. It is considered that this complex change in flame luminosity with respect to frequency was closely related to the phase difference in the respective time variations in the ratio of flame thickness to radius, the velocity gradients of the air and fuel streams, and the magnitude of these values, with the ratio of flame thickness to radius related to the flame curvature effect, the velocity gradient of the air stream correlated to the flame stretch effect, and the velocity gradient of the fuel stream impacting the fuel transportation.
基金supported by Beijing Natural Science Foundation(Grant No.3244041).
文摘In the current work,we investigated hydrogen/air flame propagation under supergravity conditions.Results show that when gravity is in the same/opposite direction as flame propagation,it leads to acceleration/deceleration of the flame,and that such an effect could substantially modify the flame propagation and structure at high gravity levels.Furthermore,for the absolute and relative flame propagation speeds,the gravity-affected flame speed shows opposite trends as the absolute flame speed is more affected by the local induced flow field,while the relative flame speeds are controlled by the super-adiabatic or sub-adiabatic flame temperature.The gravity-affected thermal and chemical flame structures are also examined through the influence of the mixture equivalence ratio,pressure,and flame stretch.
基金Supported by the Research Foundation of Beijing Institute of Technology(20070242004)
文摘A small cubic closed vessel with schlieren measurement technique combined with high-speed video camera were used to study limit flame properties under microgravity conditions at atmospheric pressure and room temperature.The rich flammability limit of C3H8/air was determined to be 9.2% C3H8.Stretched flame propagation speeds,stretched laminar burning velocities and unstretched laminar burning velocities near rich C3H8/air flammability limits were measured at different equivalence ratios.Outwardly propagating spherical flames were used to study the sensitivities of the flame propagation speeds and laminar burning velocities to flame stretch using Markstein lengths.Unstretched laminar burning velocity at rich flammability limit was determined to be 1.09cm/s.Lewis numbers were less than unity in rich C3H8/air and negative Markstein lengths were concluded.Absolute values of Markstein lengths were found to decrease linearly with equivalence ratios increase.
