Sizing the Depth and Width of Narrow Cracks in Real Parts by Laser-Spot Lock-In Thermography

We present a complete characterization of the width and depth of a very narrow fatigue crack developed in an Al-alloy dog bone plate using laser-spot lock-in thermography. Unlike visible micrographs, which show many surface scratches, the thermographic image clearly identifies the presence of a single crack about 1.5 mm long. Once detected, we focus a modulated laser beam close to the crack and we record the temperature amplitude. By fitting the numerical model to the temperature profile across the crack, we obtain both the width and depth simultaneously, at the location of the laser spot. Repeating the process for different positions of the laser spot along the crack length, we obtain the distribution of the crack width and depth. We show that the crack has an almost constant depth (0.7 mm) and width (1.5 µm) along 0.7 mm and features a fast reduction in both quantities until the crack vanishes. The results prove the ability of laser-spot lock-in thermography to fully characterize quantitatively narrow cracks, even below 1 µm.This work has been supported by Ministerio de Ciencia e Innovación (PID2019-104347RB-I00, AEI/FEDER, UE), by Gobierno Vasco (PIBA 2018-15), and by Universidad del País Vasco UPV/EHU (GIU19/058)

Cloud morphology and dynamics in Saturn’s northern polar region

We present a study of the cloud morphology and motions in the north polar region of Saturn, from latitude ∼ 70°N to the pole based on Cassini ISS images obtained between January 2009 and November 2014. This region shows a variety of dynamical structures: the permanent hexagon wave and its intense eastward jet, a large field of permanent “puffy” clouds with scales from 10 – 500 km, probably of convective origin, local cyclone and anticyclones vortices with sizes of ∼1,000 km embedded in this field, and finally the intense cyclonic polar vortex. We report changes in the albedo of the clouds that delineate rings of circulation around the polar vortex and the presence of “plume-like” activity in the hexagon jet, in both cases not accompanied with significant variations in the corresponding jets. No meridional migration is observed in the clouds forming and merging in the field of puffy clouds, suggesting that their mergers do not contribute to the maintenance of the polar vortex. Finally, we analyze the dominant growing modes for barotropic and baroclinic instabilities in the hexagon jet, showing that a mode 6 barotropic instability is dominant at the latitude of the hexagon.This work was supported by the Spanish MICIIN projects AYA2015-65041 with FEDER support, Grupos Gobierno Vasco IT -765-13, and UFI11/55 from UPV/EHU

Quantifying the width and angle of inclined cracks using laser-spot lock-in thermography

In this work we introduce a method to determine the width and the orientation of tilted cracks. This method combines laser-spot lock-in thermography and calculations of the sample temperature by means of finite elements modelling. The fitting of the surface temperature, calculated from the numerical model, to experimental lock-in thermography data obtained by focusing the laser spot close to an artificial calibrated inclined crack delivers its width and angle. The agreement between nominal and retrieved values proves the ability of the method to size inclined cracks, even those of micrometric width.This work has been supported by Ministerio de Ciencia e Innovación (PID2019-104347RB-I00, AEI/FEDER, UE), by Gobierno Vasco (PIBA 2018–15) and by Universidad del País Vasco UPV/EHU (GIU19/058)

Experiments on a lab scale TES unit using eutectic metal alloy as PCM

The behavior of a magnesium and zinc eutectic metal alloy used as thermal energy storage (TES) material is tested in a laboratory scale TES unit. The TES unit consists of two concentric tubes with the central tube surrounded by 67 kg of the metal alloy and two caps at both ends of the tube through which the heat transfer fluid (HTF) flows. Charging (melting) and discharging (solidification) processes of the eutectic metal alloy are performed using synthetic oil as the HTF. The experimental results are used to test the validity of the model via simulations performed with a computational fluid dynamics tool. The results corroborate that phase change materials with high thermal conductivity, such as eutectic metal alloys, are ideal for the evaporation process of water in direct steam generation applications due to the quasi-constant melting and solidification temperatures and to its high heat transfer capacityThe authors would like to thank the Department of Industry, Innovation, Commerce and Tourism of the Basque Government for funding Etortek 2011 Energigune’11 grant (IE11-303). The authors would like to express their gratitude to Felix Mendia for his valuable and constructive suggestions and very much appreciate the collaboration of Julian Izaga from IK4-Azterlan and Rafael de Diego from Melfun in the synthesis of the eutectic allo

Operational strategies guideline for packed bed thermal energy storage systems

Thermal energy storage (TES) is presented as a proven element to reach a sustainable and efficient management of any thermally driven process. The inherent thermodynamic limitations associated to a thermal system, such as the unavailability of an appropriate heat source, thermal losses or improvable cycle efficiencies, justify the implementation of a TES. In practice, numerous industrial processes present noticeable enhancement opportunities according to the mentioned gaps. In these terms, solar-thermal power production, intensive heat demanding industries (steelmaking, glass, cement production etc.), or compressed air energy storage are representative examples with such optimization potential

Thermal emissivity spectra and structural phase transitions of the eutectic Mg-51%Zn alloy: A candidate for thermal energy storage

The thermal emissivity spectrum in the mid infrared range (3 to 21 $\mu$m) as well as its dependence on temperature between 225 and 320 $^{\circ}$C has been obtained for the Mg-51%Zn (weight %) eutectic alloy, a candidate for thermal storage. The spectral curves show the typical behaviour of metals and alloys, with emissivity values between 0.05 and 0.2. It was also found that the emissivity spectrum shows variations in each heating cycle during the first few cycles. These changes are associated with the presence of metastable phases in the solid solid phase transition, present in the alloy below the melting point. The absence of signs of oxidation in air is very favourable for the use of this alloy in thermal energy storage systems. Moreover, the total normal emissivity curves obtained from dynamic spectral measurements have allowed analysing the behaviour phase transition sequence present in this alloy. These experimental results indicate that accurate emissivity measurements can be sensitive enough to account for the structural phase transitions in metals and alloys

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)