# Analysis of Pressure Effect on Three-Dimensional Flame Surface Density Estimation
## 2 Experimental Methodology  
_Experimental Setup and Measurements_  
The high-pressure experiment was conducted in a high-pressure facility, which can provide a pressure up to 3.0 MPa, and has been introduced elsewhere [18; 19]. To maintain a high ambient pressure in the chamber, a large amount of highly compressed fresh air is charged from the bottom of the chamber, and the internal pressure is regulated manually through a pressure gauge. The chamber is equipped with several quartz windows, allowing for laser diagnostics to be conducted. A small portion of compressed air is quantitatively controlled by a mass flow meter, as the oxidizer mixes with fuel. There are several modules in the experimental set-up, i.e., the air supply system, the Bunsen burner and the OH-PLIF laser detection system.  
The flame is stabilized on a turbulent Bunsen burner with a weak annulus CH4/air co-flame at the burner rim. The burner was introduced in detail elsewhere [8; 15], and a brief introduction is given here. A sketch of the burner is depicted in Figure 1. To obtain a well-mixed unburned mixture and to prevent flash back, the fuel and air supplied to the burner are firstly impinged to a plate and then pass through two sintered metals, with the characteristic dimension decreasing in order. Turbulence is generated by a perforated plate installed 40 mm upstream of the burner exit, as designated in Figure 1. The burner is water-cooled to avoid water vapor condensation on the burner's surface. Three pressures have been investigated, namely \(P=0.1\), 0.5 and 1.0 MPa, and the equivalence ratio is fixed at \(\phi=1.0\). The bulk inlet velocity is kept constant at 3 m/s and the mass flow increases linearly with pressure.  
The instantaneous OH radical of the flame, which is used to identify the flame front, was measured with OH-PLIF. The OH-PLIF measurement system consists of a Nd:YAG laser (Quanta-Ray Lab-190),a pumped dye laser (Sirah PRSC-G-3000) and an ICCD camera (LaVision Image ProX). The laser source is generated by Nd:YAG laser at 532 nm with the repetition rate of 10 Hz and pulse energy of 500 mW. The dye laser doubles the laser frequency, resulting in a wavelength of 282.769 nm, which corresponds to the peak excitation wavelength of the OH radical in the flame. A laser sheet with a height of 50 mm and a thickness of approximately 0.5 mm is formed at the flame position through the use of optical elements. After the excitation, the flame releases an OH fluorescence signal of 308 nm. The fluorescence signal is captured with a CCD camera, which is equipped with intensified Relay Optics (LaVision VC08-0094) and an OH bandpass filter (LaVision VZ08-0222). Detailed information regarding the OH-PLIF measurements can be found in our previous work [8].  
To obtain reliable turbulence intensity \(u^{\prime}\), the instantaneous flow field was measured at 10 positions, by a single hot-wire anemometer (Dantec 55P11), along the burner's diameter, 10 mm above the nozzle [8; 15]. The plate geometry used in the experiment and the relation of turbulence intensity with bulk inlet velocity are illustrated in Figure 2. The Figure shows that the perforated plate exhibits a turbulence level of about 10%.  
Figure 1: The sketch of the premixed turbulent Bunsen burner.  
Figure 2: The schematic of the perforated plate used and the relationship between turbulence intensity and bulk inlet velocity.