Direct Photography

Ignition process and flame development were investigated by direct photography method using a light sensitive high-speed color digital video camera. The sensitive camera could clearly capture spray ignition and flame development with extremely dark flame. Direct imaging can also be used to obtain information of the flame development after ignition and also as evidence in order to understand the burning process diagnostics. Furthermore, direct photograph method provides instantaneous observation of flame development and flame pattern area with real time measurement. In direct photograph technique, the optical equipment can be broadly divided into two optical setups, one is high intensity photograph and the other is high sensitivity photograph, as shown in Fig.3. The details comparisons of the photograph condition are clearly shown in Table 1. To compensate for these differences, the camera lens which comes with the different focal length is arranged in front of high-speed camera. As shown in Fig.3(a), images of the spatial distribution of natural flame luminosity are obtained shortly after commencement of firing operation. The high intensity images is observed with the lens (Nikon 135 mm f/2D) in order to suppress the quantities for high intensity combustion flame.

Fig.3 Schematic diagram of direct photography:

(a) High intensity photography

(b)High sensitivity photography

Generally, diesel combustion has a high flame luminance because of its diffusion flame. However, during ignition process, the blue flame appears first and continues with the luminous flame. In addition, the intensity blue flame or initial flame becomes more obvious and are nearly dependent of combustion parameter such as oxygen concentration and ambient density reflects the relative change in the flame development intensity. The overall flame intensity decreases with decreasing oxygen concentration and increasing ambient density. Due to the initial flame is very low luminosity, the optical set up under high intensity method cannot be performed in order to capture flame under extremely low diffusion flame.

Therefore, the temporal blue flame development visualization or low luminance image is captured by high sensitivity photograph method as presented in Fig.3(b). According to Fig.3(b), the camera lens was used with short telephoto lenses (Nikon 50 mm f/1.4) in order to capture a large amount of light especially the low flame intensity during the initial flame development.

Figure 4 compares the initial flame development images with the different direct photography method but appears in the same time period. As seen in Fig.4, the images on the left, with high intensity photography shows the reddish dark flame. On the contrary, images on the right, with high sensitivity photography show clearly the region of blue flame. Both flames are indicator to the well-mixtured area but the high sensitivity photography providing the clearer of well-mixtured area during ignition.