Giovanni Maria Carlomagno

Recent advances in the use of infrared thermography

Infrared thermography transforms the thermal energy, emitted by objects in the infrared band of the electromagnetic spectrum, into a visible image. This feature represents a great potentiality to be exploited in many fields, but this technique is still not adequately enclosed in industrial instrumentation because of a lack of adequate knowledge; at first sight, it seems too expensive and difficult to use. The aim of the present paper is to shortly overview existing work and to describe the most relevant experiences devoted to the use of infrared thermography in three main fields, i.e. thermo-fluid dynamics, technology and cultural heritage, which have been performed in the department the authors belong to. Results may be regarded from two points of view, either as validating infrared thermography as a full measurement instrument, or as presenting infrared thermography as a novel technique able to deal with several requirements, which are difficult to perform with other techniques. This study is also an attempt to give indications for a synergic use of the different thermographic methods and sharing experiences in the different fields.

A survey on infrared thermography for convective heat transfer measurements

During the past several years infrared thermography has evolved into a powerful investigative means of thermo-fluid-dynamic analysis to measure convective heat fluxes as well as to investigate the surface flow field behaviour over complicated body shapes. The basic concepts that govern this innovative measurement technique together with some particular aspects linked to its use are herein reviewed. Different operating methods together with their implementations are also discussed. Finally, the capability of infrared thermography to deal with several simple, or complex, fluid flow configurations is analysed.

Non-destructive control of industrial materials by means of lock-in thermography

Lock-in thermography is employed for non-destructive control to evaluate several aspects of industrial interest: inclusions of spurious materials in both carbon-epoxy and glass-epoxy, impact damage and delaminations occurring around holes during drilling in carbon-epoxy, bonding improvements in Certran® after plasma treatments and steel modifications after welding. Phase images are analysed to find quantitative information for industrial characterization.

Comparison between thermographic techniques for frescoes NDT

The use of infrared thermography in the architectural restoration field is examined. Three samples, made of a support of marble, brick, or tuff, covered with a layer of plaster with inclusions to simulate detachments or cracks in frescoes, are considered. Different techniques: pulse thermography, lateral heating thermography, lock-in or modulated thermography and pulse phase thermography are employed to detect the flaws artificially created; advantages and disadvantages of each technique are discussed. It is found that pulse thermography is easy and fast to use for information about the state of the art treasures, but data may be affected by non-uniform heating and local variation of thermal emission; the lateral heating can help to overcome interference effects due to non-uniform heating but it is more troublesome to use. When the evaluation regards rare art treasures the lock-in technique seems to be the only response since it is able to operate within very low increase of surface temperature; this technique is also able to give information about the material composition. The pulse phase thermography may be used to detect more in depth flaws but it needs higher temperature increase with respect to the ambient temperature and so it is recommended to control, before testing, the temperature sensitivity of the artefact.

Infrared thermography for thermo-fluid-dynamics

Introduction and historical groundings - Physical background - Infrared scanner - Heat flux sensors - Restoration of thermal images - Some practical considerations - Applications.

Heat Transfer Measurements on a Rotating Disk

Heat transfer to a rotating disk is measured for a wide range of Reynolds number values in the laminar, transitional and turbulent flow regimes. Measurements are performed by making use of the heated-thin-foil technique and by gauging temperature maps with an infrared scanning radiometer. The use of the IR radiometer is advantageous on account of its relatively good spatial resolution and thermal sensitivity and because it allows one to perform measurements down to very low local Reynolds numbers. Data is obtained on three disks, having an external diameter varying from 150mm to 450mm; the smallest disk is used only to measure the adiabatic wall temperature and can rotate up to 21,O00rpm. Heat transfer results are presented in terms of Nusselt and Reynolds numbers based on the local radius and show a substantial agreement with previous experimental and theoretical analyses. Transition to turbulent flow is found at about Re=250,000. A discussion about the role played by the adiabatic wall temperature is also included.

Influence of Shear Layer Dynamics on Impingement Heat Transfer

Measurements of convective heat transfer coefficients on a flat plate with an air jet impinging on it perpendicularly are made to investigate the influence of some governing parameters. Particular attention is focused on the effects of the shear layer dynamics. For certain flow conditions and/or test arrangements, coherent structures and/or recirculation currents are observed affecting the distribution of the heat transfer coefficients. Measurements of wall temperature as well as of adiabatic wall temperature of the stream are made by means of an infrared scanning radiometer, and the heat transfer coefficients are calculated by means of the so-called heated thin foil technique. The data are reduced in dimensionless form as Nusselt numbers and compared with data from the available literature. Both spatial distributions and averaged values of the Nusselt number are discussed. A new explanation for the second peak in the local Nusselt number is proposed.

Application of infrared thermography to adhesion science

Infrared thermography (IRT) is a non-contact, non-intrusive technique which converts the invisible thermal energy, radiated from the surface of an object in the infrared band of the electromagnetic spectrum, into a video signal, each energy level being generally represented by a color or a gray level. IRT has been considered an exciting scientific breakthrough since its introduction in the early 1960s. Indeed, the new generation of fully-computerized infrared imaging systems can provide both qualitative and quantitative measurements which are useful in many industrial and research fields. Nevertheless, infrared thermography is still not completely exploited. The reason for this lies, in part, in the lack of knowledge, since at first sight IRT seems to be too expensive and difficult to use and, in part, in the industrial inertia to change the routine procedures. The aim of this review article is to provide the reader with a background to infrared theory and with an overview of the most relevant applications of IRT to the adhesion field. The use of IRT as a non-destructive evaluation technique with the two different approaches of pulse thermography (PT) and lock-in thermography (LT) is discussed. Many applications are described which involve several different materials (metals, plastics, plaster, composites, hybrid composites and sandwiches) and different types of bonds (coatings, sandwiches and joints). The results show that both PT and LT are able to detect material modifications caused by surface treatments, presence of inhomogeneities in bulk materials, as well as disbonding, delamination, and cracks and slag inclusions in bonded structures. The LT is also capable of evaluating materials characteristics (e.g., variations in density, porosity, hardness, etc., which induce variations in the phase angle), the dimension of the heat affected zone in welded joints, coating thickness, bondline thickness, the effects of adhesive thickness, the effects induced in bonded structures by substrate surface treatments, and the effects of crosslinking in polymers. The LT technique is particularly advantageous in the evaluation of frescoes, mosaics and antique artworks. The reported applications provide also information which is useful for decision making about the use of IRT alone, or combined with other techniques.

Boundary layer diagnostics by means of an infrared scanning radiometer

A computerized infrared (IR) scanning radiometer is employed to characterize the boundary layer development over a model wing, having a Göttingen 797 cross-section, by measuring the temperature distribution over its heated surface. The Reynolds analogy is used to relate heat transfer measurements to skin friction. The results show that IR thermography is capable of rapidly detecting location and extent of transition and separation regions of the boundary layer over the whole surface of the tested model wing. Thus, the IR technique appears to be a suitable and effective diagnostic tool for aerodynamic research in wind tunnels.

Azimuthal instability in an impinging jet: adiabatic wall temperature distribution

The present work has been aimed at gaining some new insights into instability phenomena arising when an air jet impinges on a flat plate under certain conditions. At a critical Mach number, depending on the impingement distance, the jet loses its circumferential appearance with the formation of evenly equidistant azimuthal structures, whose number and location depend on the nozzle geometry and on the flow conditions. The instability is investigated in terms of pressure and adiabatic wall temperature; the latter is measured by means of an infrared scanning radiometer. Entrainment effects are found to play a key role in the priming and evolution of the instability.