It has been recognized that a turbulent flame is regarded as an ensemble of laminar diffusion flamelets and that the flamelet approach is practically valid for many turbulent flames. This point has been the motivation for extensive theoretical and experimental studies on the dynamics of steady strained laminar flame and significant advances have been made in understanding the behavior of laminar flamelets. These studies have been mainly addressed to the counterflow diffusion flame based on the quasi-steady concept and have been applied to the modeling of turbulent flames. Although the steady state assumption constitutes a convenient approximation, which facilitates the practical application to the turbulent combustion, the importance of understanding a transient flame response has been recognized for several decades. Since each flamelet only spends a finite residence time in a given region of the turbulent field, it may not be able to adjust to the local condition to reach a steady state structure. The experimental results of turbulent reaction flames also present unsteadiness as a bimodal behavior in the turbulent diffusion flame.
Since the early unsteady theoretical study was performed on the droplet-burning process at the stagnation point in a forced convection field, there have been many studies on the diffusion flame responses to the unsteady flow field. However, although a large number of studies provided an understanding of unsteady behavior of diffusion flames, experimental investigations are relatively limited. Beside the experiments of sinusoidal varying strain rate (Saitou and Otsuka, 1976; Egolfopoulos and Campbell, 1996), Rolon et al. (1995) experimented the flame-vortex interaction by employing a counterflow burner with an impinging jet, which has been widely used for vortex interaction as an unsteady flame response (Thevenin et al., 1998; Yoshida and Takaki, 1998). Park and shin (1997) proposed that the flame behavior near the jet tip depends on the time history of strain rate.
In this work, the unsteady behavior of diffusion flames is experimentally studied with an evolving jet diffusion flame where the fuel jet is injected downward. This experiment has some advantages compared with other studies. One is that the evolving jet flame enables us to observe the long term behavior of vortex than the counterflow including impulsive vortex (Rolon et al., 1997). And the other is that downward injection, in contract to upward (Park and Shin, 1997), provides that the flame behavior can be obtained with good reproducibility because the buoyancy-induced plume is minimized and various time histories of diffusion flame are observed near the flame extinction. At first, the characteristics of non-reacting jet are examined according to the evolving directions. The results show that the transient behaviors, developing distance and growing diameter, are almost the same in both cases, which means that the buoyance effect of transient region is smaller than that of a steady prediction. To investigate the unsteady flame response to the flow field, maximum flame temperature is used as a representative scalar variable. A new method is explored to determine the time constant of maximum flame temperature in a moving flame sheet. Experiments, in which the time constants are determined as a function of maximum flame temperature and velocity, are conducted. It is consequently found that the time constants are invariant for a certain velocity range and exponentially decrease according to the maximum flame temperature. Through the measurement of maximum flame temperature, the experimental results suggest that the time history of strain rate has a strong influence on the flame behavior not only in the extinction region but also in a relatively small strain rate in the unsteady laminar diffusion flame. Finally, a large activation energy asymptotic method is introduced to confirm and complement the current experimental results. We also have investigated the flame response to the flow unsteadiness by imposing a linearly varying strain rate, based on the present experimental results.