Browsing by Author "Tsa, SS"

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Flame stabilization over a Tsuji burner by four-step chemical reaction
(2003-11-01)
Tsa, SS
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Chen, CH
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機械工程學系
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Department of Mechanical Engineering
This investigation applies a four-step chemical kinetics mechanism to implement the original combustion model developed by Chen and Weng (Combustion Science and Technology, Vol. 73, pp. 427-446, 1990). Comparing the blowoff curve of Tsuji (Progress in Energy and Combustion Science, Vol. 8, p. 93, 1982) with that of Chen and Weng reveals that this study yields a much better prediction than that in the latter reference. Also, the data in this study has an excellent agreement with the measured data of Dreier et al. (Berichtc der Bunsen-Gesellschaft-Physical Chemistry, Vol. 90, pp. 1010-1015, 1986). The interested parameter is the in flow velocity U-in. As U-in increases, the envelope diffusion flame, wake flame, liftoff flame, and another wake flame appear in that order before complete extinction. The formal wake flame is transformed from the envelope diffusion flame and the other is from the liftoff flame. The existence of a liftoff flame is verified by a corresponding experimental observation (Tsa et al., 2003). The maximal liftoff height is 1.7 D when U-in is 1.05 m/s, and this height is maintained up to U-in=1.09 m/s. Then the height declines gradually as the in flow velocity increases, which is a process that can be regarded as flashback. No recirculation flow exists behind the cylindrical burner for these liftoff flames. A transition from liftoff to wake flame occurs between 1.13 to 1.15 m/s. The wake flame reappears at U-in=1.16 m/s. Finally, the flame is extinguished completely when U-in>2.12 m/s. The flame's lifting and dropping back is explained.
Interaction between counterflow flames over binary Tsuji burners arranged in side-by-side
(2005-10-01)
Chen, DD
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Tsa, SS
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Chen, CH
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機械工程學系
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Department of Mechanical Engineering
This numerical study modifies Tsa and Chen's preliminary combustion model (2003) by using a multi-block grid to investigate further the interaction of flames over binary Tsuji burners. The effects of inter-cylinder spacing (L) and inflow velocity (U-in) are investigated. A wider inter-cylinder spacing generally corresponds to lower flame transition velocity, associated with the transformation of the envelope flame into the wake flame. However, the combustion efficiency increases with L. The twin envelope diffusion flames merge into a larger envelope diffusion flame completely when L is equal to or less than 1.5D. Only one vortex is present behind each burner when L = 1.5D or 2D. However, no vortex is present when L = 1.2D. When L is equal to or greater than 3.5D, no interference occurs between the two flames. The mechanism of control of the interaction between twin counterflow diffusion flames involves oxygen deficiency between the dual flames. In the case of varying U-in at fixed L 3D, the dual envelope diffusion flames transform into dual wake flames as U-in increases to 0.79m/sec. The dual wake flamesare extinguished as U-in increases further to 1.96m/sec. Increasing the inflow velocity can enhance the interaction between dual envelope flames. A larger inflow velocity yields a lower flame temperature due to the flame stretch effect. Three vortices are present behind each cylinder when the flames are at the near-extinction velocity. For a fixed inter-cylinder spacing, the dual flames tend to attract each other normally. However, the dual flames repel each other as the inflow velocity increases to the near-extinction limit.
Numerical study for interaction between water mist and counterflow diffusion flame over Tsuji burner
(2001)
Tsa, SS
;
Liu, ET
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Chen, CH
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機械工程學系
;
Department of Mechanical Engineering
The interaction between a water mist and a counterflow diffusion flame over a Tsuji burner is studied numerically. The governing system is comprised of two parts. The gas phase combustion model adopts the one developed by Chen and Weng (1990). For droplets, their motions are described by Newton's second law. The corresponding droplet heat and mass transfer are considered by using empirical correlations. The PSI-Cell model is used to manage the two-phase coupling. The evaporation of droplets through heat absorption from the flame can generate vapor in the flow field. The vapor dilutes both the oxidizer and fuel concentrations, and the heat absorption by phase change lowers the flame temperature, For 50 pm droplets, the envelope flame is extinguished from the forward stagnation area and becomes a wake flame as the mist flow rate increases to 7%. The wake flame no longer exists when the mist flow rate is greater than 15%. The critical values for transition of the envelope flame to a wake flame and complete flame extinction for 80 mum droplets were 4.5% and 10% separately. Under the same mist flow rate, the smaller the droplet sizes, the stronger the flame. The effect of droplet size is studied by changing the droplet size and the corresponding mist flows with the number flow rate constant. The critical sizes for transition of the envelope flame to a wake flame and for complete flame extinction were 55 pm and 75 mum respectively. Future work is also discussed.