(3-17) A Correction Method for Laser Absorption in LIF Measurements inside Engine Cylinders((D-1)Diagnostics 1-LIF) (2001)

A new correction method for laser absorption in LIF measurements inside engine cylinders is proposed. Frequently we have a problem of absorption of excitation laser by high-density molecules in LIF measurements at high concentration media such as in-cylinder gas. We developed a method to correct the attenuation effect by use of two counter propagating laser beams. Corrected information on molecule distribution is obtained by taking square root of data after multiplying two data which were from measurements by each excitation beam. A couple of laser light pulse A and B propagating in counter directions is considered. Pulse A is propagating in ^+x direction and pulse B is going in -x direction. Pulse A induces a fluorescence light [numerical formula] (1) where we considered laser light absorption using Lambert-Beer's law. Here, φ_A, n, η, B, I_A and α(x,y) are detection efficiency, density of the molecule, quantum yield which includes ro-vibronic state population, Einstein's B coefficient, excitation laser light intensity, and absorption coefficient (=σ×n(x, y), σ is absorption cross section). Similarly, fluorescence signal induced by pulse B is described as [numerical formula] (2) Multiplying and taking square root of these two equations, we obtain [numerical formula] (3) This Eq. (3) does not include α. This means that effect from laser absorption was canceled out. It is emphasized that α does not have to be constant, but can be a function of space. That is, this method is applicable to any fields including heterogeneous fields in principle. Eq. (3) means that we need intensity profiles of I_A and I_B at x=0. The origin for x-axis can be determined freely, but I_A and I_B must be known at a same position. Namely, if I_A was measured at the location before incidence to the measurement field (as described in Fig. 1), I_B has to be measured after it propagated through the measurement field ( after the attenuation). In order to conduct absolute measurements, one will need η(x, y) which includes quantum yield and statistical ro-vibronic population determined by temperature. We should note that re-absorption of fluorescence light is not counted into this method, and generally we should be careful on this issue. However, in case of LIF of ketone, which we will demonstrate later in this paper, the fluorescence spectrum is shifted to region from 350 to 450 nm because of vibrational relaxation inside S_1 state and it does not overlap with absorption spectrum which located from 250 to 310 nm. This means that re-absorption of fluorescence light does not occur in case of ketone LIF. Another method using two counter propagating laser beams had been presented by Versluis et al [1], in 1997. Our method is totally new one and is physically different from the method proposed by Versluis et al.. In this paper, differences between Versluis' method and our method are discussed, and it is clarified that our method has a considerable advantage in case of noisy measurement fields. We also demonstrate a fuel distribution measurement by our new method.