however, the essential mechanism of the oscilla- tion remains unclear. One of the most significant reasons for this is the lack of information on tur- bulent premixed flame in a high-pressure, high- temperature environment, although a large num- ber of studies on turbulent premixed flames have been performed [1,2]. Combustion oscillation gen- erates high-frequency pressure change up to sev- eral hundred hertz and a change in turbulence characteristics. Therefore, research on the effects of the pressure and temperature of turbulent pre- mixed flames and burning velocity is important for elucidating the oscillation mechanism even for the steady condition of pressure.
The authors of this paper have previously per- formed experimental studies on high-pressure tur- bulent premixed flames using a burner stabilized flame in a high-pressure chamber, keeping the chamber pressure constant, and succeeded in mea- suring turbulent burning velocity and the scale of the smallest wrinkling of the turbulent flame front [3,6]. Moreover, experiments have also been per- formed to measure the scale of wrinkling for a high-temperature premixture at high pressure, and a scale relation model between the smallest scale of flame wrinkling, turbulence length scale, and intrinsic flame instability has been proposed [7]. The aim of the present study was to extend further the measurement of turbulent burning velocity at high pressure and high temperature.
In our previous measurement of turbulent burning velocity, we used an angle method for the mean flame cone determined by laser tomog- raphy using fine seeding particles and a laser sheet [3,5]. However, in the case of turbulent premixed flames, the flame region, i.e., mean flame brush, has a certain thickness, and this thickness varies along the flame cone as is reviewed by Lipatnikov and Chomiak [8]. In this case, a more suitable way to determine the turbulent burning velocity is to use the turbulent flame front determined by employing the mean progress variable, Æcæ. There- fore, measurement of turbulent burning velocity at high-pressure and high-temperature in Æcæ space is the main purpose of this study.
Another purpose of this study was to investi- gate the relationship between the smallest scale of flame wrinkling and turbulent burning velocity. When the flame structure is in the flamelet regime, turbulent burning velocity is basically determined by the total flame area and local laminar burning velocity, meaning that the flame wrinkling scale plays an important role in the determination of burning velocity. In our previous study [7], the above-stated scale relation based on the turbu- lence Reynolds number, Rk, which is based on Taylor microscale, kg, exists at high-pressure but is not well confirmed at ordinary pressure. This is due to the limited range of Rk when a small- scale burner is used. To confirm the general role of Rk for turbulent premixed flames, a large-scale