Estimation de la cartographie du coefficient d'échange convectif par thermographie infrarouge

Le coefficient d'échange convectif est un paramètre pertinent lorsqu'il s'agit de modéliser le comportement thermique d'un système physique. Dans ce texte, à partir de deux modèles thermiques écrits et discrétisés à l'échelle du pixel nous avons construit deux fonctionnelles qui relient variables observables en occurrence les champs de températures mesurés par une caméra infrarouge et les variables à estimer (coefficient d'échange convectif et/ou diffusivité thermique). Les cartographies du coefficient d'échange convectif sont obtenus en minimisant ces deux fonctionnelles. Ici, les résultats que donnent ces deux modèles sont présentés et confrontés. Nous montrons que lorsque les effets de refroidissement sont prépondérants, la diffusion de la chaleur dans le système physique peut être négligée dans l'estimation du coefficient d'échange convectif. Les valeurs de corrélations linéaires entre le champ de température et sa dérivée temporelle montrent que ces deux grandeurs sont relativement bien corrélées

Fast sizing of the width of infinite vertical cracks using constant velocity Flying-Spot thermography

Constant Velocity Flying-Spot thermography consists in scanning the sample surface by a focused CW-laser spot moving at constant speed. This technique was designed to study large surfaces in short times. In this work, we propose a method, based on a Flying-Spot thermography setup, to size the width of vertical cracks by fitting the temperature profile along the line that contains the center of the laser spot and is perpendicular to the crack to its analytical expression. This method is also valid in the opposite configuration, where the laser spot remains at rest and the sample is moving at constant velocity. This configuration is useful for in-line inspection in factories, for detecting and sizing cracks in real time, without stopping the production chain. Experimental measurements on stainless steel samples containing calibrated vertical cracks confirm the validity of the method to measure the crack width with high accuracy, even for submicronic wide cracks.This work has been supported by Ministerio de Economía y Competitividad (DPI2016-77719-R, AEI/FEDER, UE), by Gobierno Vasco (PIBA 2018–15), by Universidad del País Vasco UPV/EHU (GIU16/33) and by Conacyt (Beca Mixta 2017 Movilidad en el extranjero)

Quantitative thermal analysis of heat transfer in liquid–liquid biphasic millifluidic droplet flows

In this paper, infrared thermography is used to propose a simple quantitative approach toward understanding the thermal behaviour of a liquid–liquid biphasic millifluidic droplet flow under isoperibolic conditions. It is shown that due to the isoperibolic boundary condition, the thermal behaviour at the established periodic state can be managed according to different orders, i.e. either a continuous or fluctuating contribution. A complete analytical solution is proposed for the complex problem model, then a simplified model is proposed. Finally, a simple homogeneous equivalent thin body model approximation with a characteristic coefficient function of a biphasic flow mixing law is sufficient for describing the thermal behaviour of the media under isoperibolic conditions. From this theoretical validation, the experimental results concerning the behaviour of a biphasic oil and droplet flow are presented. An analytical representation law is proposed to quantitatively estimate and predict the thermal behaviour of the flow. Moreover, it is demonstrated that with this new method, the thermophysical properties of the phase can be estimated with a deviation less than 5% from that reported by the suppliers.The authors gratefully acknowledge Pierre Guillot and the microchemistry team at LOF for useful discussions, this paper is humbly dedicated to mCp

Infrared thermospectroscopic imaging of heat and mass transfers in laminar microfluidic reactive flows

In this work, a novel image-based method is presented to characterize the heat and mass transfer rates in a Hele- Shaw microfluidic reactor. A Fourier transform infrared (FTIR) spectrometer is used in transmission mode in combination with an infrared (IR) camera to simultaneously measure the molar concentration and the thermal fields in the microfluidic chip within few seconds. A classical exothermic NaOH + HCl → NaCl + H2O chemical reaction is used to produce a multiphase flow and a heat source in the reactor. The molar concentration fields of all the species are measured using the IR spectrum in the mid-IR region, and the heat fields are obtained simultaneously from the proper emission. The quantitative aspect of the method is illustrated by comparing the molar concentration profiles to a reactor model, based on the advection-diffusion-reaction equations. The good agreement between the model and experimental data validates the method, and the expected strong diffusion- limited reaction regime in laminar microfluidic reactor is achieved. Thus, the results of this work provide a new and efficient thermospectroscopic imaging method to perform rapid, contactless and in operando heat and mass transfer characterizations in laminar microfluidic reactive flow

Measurement of in-plane thermal diffusivity of solids moving at constant velocity using laser spot infrared thermography

In this work, an infrared thermography setup is proposed to measure the in-plane thermal diffusivity of (an)isotropic samples that are moving at constant velocity, as it is the case of in-line production or in-line quality control processes in factories. The experiment consists in heating the moving sample with a focused laser spot, which remains at rest, and recording the surface temperature by an infrared camera. An analytical expression for the surface temperature of the moving sample has been obtained. By analyzing the surface temperature in logarithmic scale, three simple linear relations are obtained, whose slopes give the thermal diffusivity in the direction of the sample movement and in the perpendicular direction. These three linear methods, which are not disturbed by heat losses by convection and radiation, are valid for both opaque and semitransparent samples. Measurements performed on calibrated samples confirm the validity of the methods, which are also valid when the sample is at rest and the laser spot scans its surface at constant velocity, the so-called ‘‘flying spot” technique.This work has been supported by Ministerio de Economía y Competitividad (DPI2016-77719-R, AEI/FEDER, UE), by Universidad del País Vasco UPV/EHU (GIU16/33) and by Conacyt (Beca Mixta 2017 Movilidad en el extranjero)

3D infrared thermospectroscopic imaging

AbstractThis work reports a multispectral tomography technique in transmission mode (called 3DITI for 3D Infrared Thermospectroscopic Imaging) based on a middle wavelength infrared (MWIR) focal plane array. This technique relies on an MWIR camera (1.5 to 5.5 μm) used in combination with a multispectral IR monochromator (400 nm to 20 μm), and a sample mounted on a rotary stage for the measurement of its transmittance at several angular positions. Based on the projections expressed in terms of a sinogram, spatial three-dimensional (3D) cubes (proper emission and absorptivity) are reconstructed using a back-projection method based on inverse Radon transform. As a validation case, IR absorptivity tomography of a reflective metallic screw is performed within a very short time, i.e., shorter than 1 min, to monitor 72 angular positions of the sample. Then, the absorptivity and proper emission tomographies of a butane-propane-air burner flame and microfluidic perfluoroalkoxy (PFA) tubing filled with water and ethanol are obtained. These unique data evidence that 3D thermo-chemical information in complex semi-transparent media can be obtained using the proposed 3DITI method. Moreover, this measurement technique presents new problems in the acquisition, storage and processing of big data. In fact, the quantity of reconstructed data can reach several TB (a tomographic sample cube of 1.5 × 1.5 × 3 cm3 is composed of more than 1 million pixels per wavelength)

Bayesian Inference for 3D Volumetric Heat Sources Reconstruction from Surfacic IR Imaging

The domain of non-destructive testing (NDT) or thermal characterization is currently often done by using contactless methods based on the use of an IR camera to monitor the transient temperature response of a system or sample warmed by using any heat source. Though many techniques use optical excitation (flash lamps, lasers, etc.), some techniques use volumetric sources such as acoustic or induction waves. In this paper, we propose a new inverse processing method, which allows for the estimation of 3D fields of heat sources from surface temperature measurements. This method should be associated with volumetric heat source generation. To validate the method, a volumetric source was generated by the Joule effect in a homogeneous PVC sample using an electrical thin cylindrical wire molded in the material. The inverse processing allows us to retrieve the depth of the wire and its geometrical shape and size. This tool could be a new procedure for retrieving 3D defects on NDT

Mid-infrared spectroscopic thermotransmittance measurements in dielectric materials for thermal imaging

Thermal considerations affect the performance of most microsystems. Although surface techniques can give information on the thermal properties within the material or about buried heat sources and defects, mapping temperature and thermal properties in three dimension (3D) is critical and has not been addressed yet. Infrared thermography, commonly used for opaque materials, is not adapted to semi-transparent samples such as microfluidic chips or semiconductor materials in the infrared range. This work aims at answering these needs by using the variations of transmittance with temperature to obtain information on the temperature within the thickness of the sample. We use a tunable mid-infrared light source combined with an infrared camera to measure these variations of transmittance in a glass wafer. We couple this technique with a thermal model to extract the thermotransmittance coefficient—the coefficient of temperature variation of the transmittance. We then introduce a semiempirical model based on Lorentz oscillators to estimate the temperature-dependent optical properties of our sample in the mid-IR spectral range. Combined with the measurement, this paper reports the spectroscopic behavior of the thermotransmittance coefficient in the mid-IR range and a way to predict it

High power density laser estimation using quantitative thermal imaging method

The knowledge of the amplitude and the spatial distribution of an excitation flux is of great interest for the quantification of heat sources. In this work, the development of a non-contact imaging powermeter based on the association of a bolometer with an infrared camera is described. This powermeter allows, thanks to infrared thermographic measurements and image processing methods, the quantitative estimation of the spatial distribution of the power of the flux delivered by a high-power laser. First, the experimental setup used is described. Then, the complete model- ling of the heat transfer within the bolometer using the 3D thermal quadrupole formalism is presented. After that, an inverse method based on the Wiener filter in Fourier-Laplace transform spaces to estimate the spatial distribution of the power flux is described. Finally, power estimation results using two metallic plates as a bolometer are presented and discusse

The periodic pulse photothermal radiometry technique within the front face configuration

The front face photothermal radiometry technique has been improved in order to estimate the thermal conductivity of thin films with better accuracy compared to existing techniques. The experimental pro- cedure is based on the front face response to a nanoseconds laser pulse repeated periodically at high fre- quency, i. e., a Dirac comb waveform. Averaging the thermal response by considering thousands successive pulses allows improving largely the signal noise ratio. The unknown thermal properties and related experimental parameters are identified by minimizing the gap between the measured signal and the theoretical response that accounts with the pulse waveform, the repetition frequency and the detector transfer function. Minimization is first achieved by implementing first a simplex technique that gives an initial set of values to start the Metropolis–Hastings algorithm in a second step. Application of the proposed methodology is done considering amorphous GeTe film deposited on a Si wafer. It is shown that this experimental method as well as the implementation of the Bayes minimization technique allows to identify the thin film intrinsic thermal conductivity with high accuracy considering some uncertainty on the other parameters assumed to be known