Quantitative measurements in thermo-fluid dynamics based on colour processing

Abstract

Colour can be expressed as a weighted combination of three attributes: hue, intensity, and saturation. Non-coherent light reflected by thermo-sensitive liquid crystals holds a variable hue, moving in a generally narrow temperature interval and also depending on its inclination with respect to the plane of the crystals and on the characteristics of the impinging light. In experimental practice it is not feasible to ensure uniform lighting over an extensive area and its entire view under the same angle. Thus, the acquired hue field is non-uniform even if the liquid crystal sheet is isothermal. However, by means of proper filtering and calibration of the colour attribute, this optical technique, besides being non-intrusive and inexpensive, is capable of mapping the temperature with an accuracy better than 5% of its measuring-range amplitude. A similar method can be applied for measuring the thickness of a thin liquid film. In this case, the colour attribute to be processed is its intensity. In fact, the light transmitted through a dyed liquid decreases with an increasing thickness of the layer. Again, a perfectly uniform light source is unattainable and the recorded intensity field is non-homogeneous even if the liquid free surface is flat. Nevertheless, the film thickness can be determined by this colour-processing procedure with an accuracy better than 8% of the measuring-range amplitude, which is dictated by the utilised dyestuff concentration. Further thermo-fluid dynamic measurements performed over extensive areas could be handled with analogous methodologies. Surface temperature by emitted infrared waves and void fraction in ducts by light absorption are particular examples.