Calibration of an RGB, CCD Camera and Interpretation of its Two-Color Images for KL and Temperature 2005-01-0648
The two-color method for measuring temperature and optical thickness of soot (KL) has become a standard diagnostic tool for the evaluation of engine designs and technologies relative to soot formation and flame temperature. Implementation of the two-color technique typically requires two cameras or a set of half-pass mirrors and optical narrow band-pass filters. In this paper, a technique for collecting and interpreting two-color images with a single calibrated camera without image splitting and filtering hardware is demonstrated and discussed. This method uses a relatively inexpensive commercial, 10-bit, RGB color, CCD camera capable of 16 μs exposure times. The CCD has published spectral response curves in the visible range, but a method for obtaining the spectral response for the optical system using a monochromator is discussed. Two-dimensional images of transient spray flames of different soot concentrations are shown along with an uncertainty analysis and a comparison of the method with previous 2-D, narrow band two-color methods.
The two-color images show uneven temperature and KL distributions for flames of various fuels. Analysis shows that the temperature and KL values given by the images of heavily sooting fuels are primarily a result of conditions (temperature and soot concentration) within a 1-2 mm region on the surface of the jet, where a turbulent diffusion flame is present. As soot concentration decreases, the region of influence affecting the two-color image thickens, allowing more influence from within the jet, lowering the measured temperature. Therefore, a low sooting jet appears to have a lower temperature than a high sooting jet. Within a given flame however, regions of high soot and high temperature do not always correlate, indicating that actual variations in temperature and soot concentration are real even if not as dramatic as indicated by the images.