Rainbow Thermometry Research Articles

One-dimensional rainbow thermometry system by using slit apertures

A new rainbow thermometry system by using slit apertures and a laser light sheet, called a one-dimensional rainbow thermometry (ORT) system, has been developed as an extension of global rainbow thermometry (GRT). This system is capable of one-dimensional or line measurements of the size and refractive index of droplets in the spray space, while the conventional GRT system is normally considered a typical "single-point" or "small volume" measurement method. The performance of this new system was tested and verified with both water and ethanol spray. The results show the feasibility and potential of ORT in simultaneous and one-dimensional measurement of the size and refractive index of liquid droplets, especially in the research field of spray evaporation and combustion.

Differential infrared thermography (DIT) in a flashing jet: a feasibility study

This paper discusses the feasibility of differential infrared thermography (DIT) for performing quantitative temperature measurements in a flashing jet. In the literature, the technique has been previously applied to map temperature distributions in diesel sprays. Thanks to the differential operation, DIT enables the local determination of the spray emissivity, independently from a detailed knowledge of the drop size distribution. Temperatures can then be determined through application of the Planck’s radiation law. As a first step, we revised DIT constitutive equations in the framework of the theory of radiative heat transfer in particulate media. The objective is twofold: First, to identify the quantity actually measured by DIT and the limits of applicability of the method; Second, all factors, which may hamper quantitative data interpretation, are carefully reviewed. The analysis includes the choice of the background temperature, multiple scattering effects and the spatial resolution of the optical system; and finally, the experimental temperature data are validated through comparison with theoretical predictions and experimental measurements with thermocouples and global rainbow thermometry. In all cases, DIT shows coherent results with the physics of the problem and a good agreement with experimental data.

Non-Intrusive Measurement of the Temperature and Size Distribution of Water Droplets in a Flat Fan Spray

The measurement of spray characteristics especially the droplets thermal and hydrodynamic properties is very important for many industrial applications. In this paper, a developed global rainbow thermometry method is applied to measure droplets temperature and size distribution in a flat fan air-blast atomizer spray. The spray droplets is illuminated by a continuum laser with the wavelength of 532nm and power of 14mW, and the rainbow light formed by the droplets is received by a linear CCD. To get accurate result, a calibration method for the relation of scattering angle and CCD pixels is designed, which can effectively reduce the error of the measured temperature. Experiment is taken under different temperature at a fixed gas-liquid ratio conditions, and results shows the method is feasible and has a relatively good precision.

Rainbow Thermometry Based on EMD De-Noising and Signal Analysis

A novel Rainbow thermometry method is presented which is based on Empirical Mode Decomposition (EMD) denoising and signal analysis, and permits one to get simultaneously the temperature and size of droplet under low Signal-to-Noise Ratio (SNR) condition. The noise is removed based on EMD and wavelet threshold method, and the inverse parameters are calculated according to the signal decomposition analysis based on EMD. A experiment is taken, and the measurement result shows the validity of the presented method.

Dynamics and thermal behaviour of water sprays

A detailed study (dynamics, thermal and radiative transfer) concerning a single water spray is presented. It is based on comparisons between experimental and numerical results. The experimental part of this work uses both the Global Rainbow Thermometry (GRT) technique, providing informations about the size distribution and the mean temperature of a set of droplets, and a Fourier Transform InfraRed (FTIR) spectrometer yielding the radiative attenuation through the spray. Besides a numerical simulation of the spray is done using an Eulerian–Lagrangian approach. The radiative transfer problem is treated by a Monte Carlo Method using a Ck model and the Mie theory in order to define the spectral radiative properties of the medium. Evaluation of the numerical reliability of the code regarding temperature level predictions and attenuation ability of the spray is allowed by the experimental results. Furthermore, the numerical simulation can explain some experimental observations: the Sauter mean diameter evolution inside the spray is for example clearly related to the polydisperse feature of the spray.

Assessment of refractive index gradients by standard rainbow thermometry

Up to now the application of rainbow thermometry has been limited to particle systems possessing a uniform refractive index. This is mostly due to the absence of an appropriate data inversion algorithm that takes into account the presence of a refractive index gradient. In this paper, for the first time to our knowledge, exploiting a generalization of the Airy theory, a data inversion algorithm for a single droplet, presenting a parabolic refractive index gradient, is proposed. This data inversion algorithm is used to compute the diameter and the refractive index at the core and at the surface of a simulated burning droplet. The results are compared to the analytical solutions showing a satisfactory agreement.

Characterization of a non-isothermal water spray by global rainbow thermometry

The paper shows that the global rainbow thermometry (GRT) technique is a non-intrusive measurement technique, which provides the entire size distribution and the mean temperature of an ensemble of liquid droplets suspended in a gaseous phase. In this aim a non-isothermal flat-fan water spray is characterized by means of the GRT and the results obtained, both for temperature and for size, are in agreement with predictions already done by other authors. Moreover, it is shown that the main source of uncertainty for the GRT measurements is due to the systematic error committed in the determination of the GRT reference angle.

Global rainbow thermometry: improvements in the data inversion algorithm and validation technique in liquid-liquid suspension.

Improvements on the validation of a nonintrusive laser-based measurement technique are presented. This new technique, called global rainbow thermometry (GRT), is capable of determining the temperature and the size distributions of liquid droplets dispersed in a liquid or gaseous bulk. We propose a new data inversion algorithm that takes into account the whole rainbow pattern. Experimental validation of the GRT technique is performed for a liquid-liquid suspension. We performed the validation by comparing the measurements obtained with the GRT technique for the mean droplet temperature and size with the results obtained with alternative techniques.

Global rainbow thermometry assessed by Airy and Lorenz-Mie theories and compared with phase Doppler anemometry.

Global rainbow thermometry (GRT) measures the mean size and temperature of an ensemble of spray droplets. The domain of validity of the Airy theory for this technique is established through comparison with Lorenz-Mie theory. The temperature derivation from the inflection points of the Airy rainbow pattern appears to be independent of the type of spray dispersion. Measurements in a water spray are reported. The mean diameter obtained from the rainbow pattern lies between the arithmetic and the Sauter mean diameters measured by phase Doppler anemometry. The temperature measurement by GRT is shown to be accurate within a few degrees Celsius.

Global Rainbow Thermometry for Mean Temperature and Size Measurement of Spray Droplets

Global rainbow thermometry is a new technique for measuring the average size and temperature of spray droplets. For data inversion a global rainbow pattern is employed, which is formed by constructive interference of laser light scattered by an ensemble of spherical droplets. The non-spherical droplets and liquid ligaments provide a uniform background and hence do not influence the interference pattern from which average size and temperature are derived. This is a large improvement with respect to standard rainbow thermometry, investigated since 1988, which is strongly influenced by particle shape. Moreover, the technique is applicable to smaller droplets than the standard technique because the global pattern is not spoiled by a ripple structure. Data inversion schemes based on inflection points, minima and maxima are discussed with respect to spray dispersion and droplet flux. The temperature derivation from inflection points appears to be independent of spray dispersion. Preliminary measurements in a heated water spray are reported. The mean diameter obtained from the rainbow pattern is smaller than the arithmetic mean diameter measured by phase-Doppler anemometry. The accuracy of the temperature measurement by global rainbow thermometry is shown to be a few degrees Celsius.

Global rainbow thermometry for droplet-temperature measurement

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.

Rainbow refractometry applied to radially inhomogeneous spheres: the critical case of evaporating droplets

A detailed study of rainbow thermometry and its application to droplets in reactive systems is presented. To this end the light-scattering history of a vaporizing droplet under unsteady conditions is discussed. Unlike in previous papers, the reduction of the droplet's diameter is also taken into account in addition to the variation of the refractive-index profile. A finely stratified sphere model with thousands of layers (i.e., 20,000) is used to compute the scattering patterns of a radially inhomogeneous evaporating droplet at different heating times and therefore with different diameters and refractive-index profiles. In the studied case the temperatures inferred from rainbow thermometry do not represent the actual temperatures inside the droplet. They do not represent an average internal temperature or even the surface or the core temperature. For droplets with a temperature that increases from the core to the surface, the inferred values are always lower than the minimum temperature inside the droplet. Therefore the rainbow technique should be applied with caution in all cases in which droplet inhomogeneities are suspected. In addition, a careful analysis of the scattering in the rainbow region is presented. Because of the physical structure of the rainbow, a marked uncertainty in the inferred temperatures also has to be considered in the case of homogenous droplets. For inhomogeneous spheres this intrinsic uncertainty has to be added to the effects caused by the internal profiles of the refractive index.

Application of Rainbow Thermometry to the Study of Fuel Droplet Heat-Up and Evaporation Characteristics

Advanced, nonintrusive, laser-based diagnostics are being developed for simultaneously measuring the size, velocity, temperature, and instantaneous regression rates of vaporizing/burning fuel droplets in polydisperse flow environments. The size and velocity of the droplets are measured using a conventional phase Doppler particle analyzer (PDPA), and the droplet temperatures are simultaneously measured with a rainbow thermometer. This integrated diagnostic has been applied to the study of fuel droplet heat-up characteristics in a swirl-stabilized kerosene spray flame. It has also been shown that a novel extension of rainbow thermometry can be used additionally to extract the instantaneous droplet vaporization rate. The feasibility of measuring the instantaneous regression rate has also been demonstrated using controlled experiments with a vaporizing/burning stream of ethanol droplets.