1,720,970 research outputs found
Thermal diffusivity measurement in slabs using harmonic and one-dimensional propagation of thermal waves
No full textThe development of a novel approach to the well-known Angstrom ́s method for the measurement of the thermal diffusivity is reported. In this method, the diffusivity is determined from the damping and the phase shift of a periodic thermal signal during its propagation along the specimen. The propagation can be easily monitored by infrared thermography. In general, a non-contact source is used to apply the signal. In the present work, however, a direct-contact source is employed, with a temperature-oscillation signal supplied on a portion of one of the two main surfaces of the specimen, where a homogeneous contact can be yielded by using a proper contact pressure. Such practice implies that the measures of surface temperature can be used to estimate the diffusivity only beyond a certain distance from the source, where the wave-front of the temperature oscillation within the specimen becomes plane and perpendicular to the main surfaces. This distance is investigated here, to establish a general rule for the performance of the experiments. A thermoelectric device based on the Peltier effect is employed as the thermal source. The main difficulty about its use is to obtain a perfectly harmonic and well-balanced thermal signal. This is necessary to avoid a complex processing of the experimental data, and it can be achieved by supplying a current with a properly-chosen time-evolution pattern. Such an approach, which is built upon previous work, is here enhanced by improving the underlying analytical model
Measurement of thermal diffusivity by propagation of thermal waves: a new modification of the Ångström method
No full textA new technique for measurement of the thermal diffusivity is presented here. The technique is a modification of the well-known Ångström’s method, which relies on the steady-periodic propagation of thermal waves along a specimen to estimate the diffusivity. The novelty of the proposed modification is that harmonic thermal waves with mean value equal to the ambient temperature are induced in the specimen by means of a purposely developed source, based on the Peltier effect. This allows estimating the diffusivity by relatively simple processing of surface-temperature data, acquired by infrared thermography.The mathematical formulation of the thermal problem is described here, together with the experimental procedure and the apparatus by which the most proper thermal signal can be achieved. A cross comparison with other measurement techniques is finally reported
Thermal diffusivity measurement of stainless steel by periodic heating technique
No full textThe use of a device based on the Peltier effect can provide heat source and sink suitable for diffusivity measurements. By performing a one-side test it is possible to estimate the diffusivity value, provided that some conditions are met. In a previous work the basic idea and the experimental set up of this method were described. A crucial point is the inversion formula that allows recovering the diffusivity value from the temperature data. The proposed analytical formula works only if the experimental parameters are suitably chosen for the particular sample under test. This paper is focused on the conditions to be met in order to correctly apply that formula. To this end, tests have been performed on a material with known thermal properties
Valutazione delle aree delaminate in strutture edili fibrorinforzate, con il metodo termico
No full textLa recente normativa internazionale ed italiana relativa al consolidamento statico di opere
civili con materiali compositi fibrorinfozati fornisce dati quantitativi precisi sul numero ed
estensione massima ammissibile delle aree distaccate. Purtroppo, la tipologia delle prove
previste contempla quelle non distrutive solo in modo complementare, finalizzate
essenzialmente alla verifica dell'uniformità dell'applicazione. L’assenza di un'affidabile
procedura di controllo in grado di quantificare la posizione e l'estensione di eventuali
difetti di adesione tra il rinforzo ed il substrato riduce la diffusione di questo tipo di
intervento, potenzialmente molto adatto ad esempio al ripristino di viadotti.
Si presentano gli ottimi risultati ottenuti con una specifica tecnica termografica di
controllo non-distruttivo, che ben si adatta alle caratteristiche meccaniche e termiche
altamente anisotrope dei materiali compositi comunemente impiegati
Active and passive thermography to detect Moisture in Building Material
No full textDampness is one of the main problems of preservation of ancient building. Among non-destructive methods for quantitative evaluation of damages of buildings materials, thermographic applications are based on the measure of thermal properties variations. Those variations highly depend on water content inside these materials. Water presence in the superficial layer of the wall produces a cooling effect due the evaporation process, which affects the surface temperature. The evaporation rate can be properly calculated when all the energy exchanges across the wall surface are known The solution of the energy balance equation gives theoretical temperatures that describe the cooling during evaporation and it allows calculation of
evaporation rates. A good agreement with experimental values by weighing test has been found in lab tests. Furthermore the thermographic passive measurements of the surface cooling allow to estimate the water content. The same values of water content can be obtained by an active approach. In the paper the reference method is considered. It describes dependencies of thermal effusivity and diffusivity on water content with respect to dry material. The distribution of moisture in building porous materials is derived from thermal effusivity. In the paper are also reported the experimental results obtained in estimating water content by front-surface temperature. Authors describe the contribution due to the different kinds of heat exchange, and determine reliable values of water content and evaporative flux using both active and passive techniques
Characterization of radiant systems for heating and cooling
No full textWater based surface embedded heating and cooling systems exhibit peculiar features compared with other HVAC systems. Therefore, many studies have been developed defining how to design and test the components of these systems. Nevertheless, advanced technical solutions requires a deeper analysis of the performances both of the components and of the overall assembly. In the following study a low-thickness heating and cooling radiant module with a non-conventional geometry of the embedded piping layer points out some open issues. The thermal performance of this system have been measured inside a climatic chamber; tests are combined with simulations in order to set up numerical models, successively used for the optimization of the system. A test facility has been set up for measuring the performance of a full scale heating and cooling floor designed for industrial applications.Different alternative methods have been applied to measure the heat delivered to the conditioned environment. Among them, the well-known enthalpy balance is used as a reference. A new method, based on quantitative infrared thermography, has been developed in order to determine local heat fluxes, with centimetric spatial resolution on the surfaces. Results collected for two years by the different methods have been compared and analyzed. The comparison, applied both to wide and narrow surfaces, suggests some useful remarks on the characterization procedures derived from current standards
Thermal parameters estimation by heating and cooling and thermographic measurement
No full textThe use of a Peltier device in thermal contact with a specimen can provide heat source and sink. The utilization of this equipment is experimented, applying successive heating and cooling stages on various specimens and measuring the temperature outside of the contact area. Oscillating temperature in time and space can provide indication on the thermal diffusivity.The appealing ability of sourcing and sinking heat, with average null effect on temperature, may represent an innovative approach with respect to traditional stimulation technique.A simple analytical model is exploited for recovering the inversion equations. Numerical simulation allows to better understand the real process on the sample
Tecniche fototermiche e termografiche per la misura della diffusività
No full textLa collaborazione di tre laboratori ha permesso di utilizzare differenti tecniche di misura della diffusività termica, alcune fototermiche, altre termografiche, applicandole ad un campione di acciaio AISI 304. Lo scopo è quello di mettere a confronto le tecniche termografiche con quelle fototermiche, quest'ultime considerate di riferimento, al fine di utilizzare in campo le prime con uno specificato grado di attendibilità. Si descrive brevemente ogni singola tecnica e alla fine viene compilata una tabella di risultati con la stima dell'errore, in modo da dare una valutazione oggettiva dell’affidabilità delle varie misure
Modeling of thermal non-destructive evaluation techniques for composite materials
No full textThe inspection of composite structures by thermal non-destructive evaluation (TNDE) techniques based on Infrared Thermography is relatively common in the aerospace industry. However, state-of-the-art is far to be reached and there is still place for substantial improvement, in terms of development of the inspection procedures, optimization of the experimental set-up and assessment of the inversion algorithms. This requires the availability of large amounts of reference data, which can be provided in a cost-effective way by mathematically modeling the thermal behavior of the inspected structures.The numerical simulation of procedures for thermographic inspection is investigated in this work. The analysis is focused on the strategies that can be implemented to model the heat conduction processes in composite structures typical of TNDE procedures by means of commercial software packages for coupled mechanical-thermal analyses. The results of a pilot application of a commercial software packages to a real case are finally presented
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