Systematic error and correction of intensity-based I-PLIF for local pH and concentration measurements in unsteady boundary layers

Planar Laser Induced Fluorescence methods, in particular in their Inhibited version (I-PLIF) are powerful tools to measure local scalar values in various types of flows. The most commonly used LIF methods are intensity-based: Scalar value is extracted from fluoresced light intensity information. However such methods are prone to error when used in challenging configurations (multiphase flows, non-optically-thin systems, etc ). In this short paper, the systematic error on concentration measurement in a dissolved gas boundary layer caused by scalar dependency of the extinction coefficient is discussed, in the context of a non-optically thin system with significant out-of-field absorption. Results of single color I_pH-PLIF measurements are compared to I_pH^r-PLIF (ratiometric) measurements for which the out-of-field absorption is intrinsically accounted for. An empirical correction based on first order concentration statistics derived from ratiometric measurements is proposed. It is used to demonstrate that the oversight of out-of-field absorption in intensity-based methods can lead to significant error on the scalar measurements inside the boundary layer.