Global rainbow thermometry for droplet-temperature measurement

Abstract

Standard rainbow thermometry connects the scattering angle of the main rainbow maximum, generated by a single droplet, to the droplet's refractive index and thus to its temperature. Droplet nonsphericity influences the rainbow position and therefore degrades the quality of the droplet-temperature measurement. We propose global rainbow thermometry, which measures the average rainbow position that is created by multiple droplets and from which a mean temperature can be derived. The new technique aims at eliminating the nonsphericity effect. The principle of this method is presented, and a typical recorded image is discussed.

Temperature and chemical composition of droplets by optical measurement techniques: a state-of-the-art review

“…Spatial filtering may be required to simultaneously select single droplets when the method is applied to a spray. The CW laser may be replaced advantageously by a pulsed laser which allows the rainbow pattern to be frozen to eliminate perturbations from transient events such as oscillations of the droplet shape (van Beeck and Riethmuller 1998). A CCD camera imaging the focal plane of a lens can be used to observe the angular distribution of the scattered light in the vicinity of the rainbow angle.…”

“…The GRR technique was initiated by van Beeck et al (1999) and can be seen as an extension of SRR to sprays. Potentially, GRR is capable of providing valuable information about the size distribution and the temperature associated with an ensemble of droplets.…”

“…Furthermore, the aim of this method is to eliminate some of the biases of SRR, especially those related to the presence of deformed droplets because non-spherical droplets are a major source of errors. A sphericity default of only 1 % may lead to errors in temperature measurements as high as ±40°C (Moebius 1910;van Beeck et al 1999). van Beeck et al (1999) attempted to select spherical droplets for processing according to the comparison of droplet size measurements from the Airy fringes and from the ripple.…”

“…The inversion procedure using the angular position of the inflexion points was successfully applied to the characterization of an isothermal water spray (van Beeck et al 2003). In the case of water sprays, errors do not exceed 4°C according to different studies (van Beeck et al 1999(van Beeck et al , 2001. In general, the mean droplet diameter derived from the angular distance of the inflection points around the first rainbow maxima greatly differs from the Sauter mean diameter, but an estimate of size dispersion can still be obtained (van Beeck et al 2001).…”

Extended glare point velocimetry and sizing for bubbly flows

“…However, there is a possibility to relax this restriction of sphericity and perform global measurements in flows with non-spherical bubbles. If the average distance ratio over a large number of randomly oriented bubbles is taken, the influence of the non-spherical bubbles is eliminated completely through the superposition of many different tilt angles (this principle is also used in global rainbow thermometry (van Beeck et al (1999))). The proof for this statement is also shown in Figure 6(c), where the solid curve represents the distance ratio as a function of the aspect ratio averaged over all possible tilt angles (but still restricted to the planar case).…”