Revue Générale de Thermique最新文献

This paper presents an experimental study of free and forced convective heat transfer along vertical slender cylinders. The local heat transfer coefficient is determined from the measurement of the surface temperature distribution performed by quantitative infrared thermography. It is found that the convective heat transfer is strongly dependent on the cylinder curvature and misalignment with the flow. The effect of proximity of two cylinders is emphasized in the case of forced convection. Correlations are proposed for the two types of convection. It is worth noting that circumstances exist where the turbulent heat transfer in free convection can be of the same order of magnitude as for laminar forced convection. The outcome of the study demonstrates the suitability of quantitative infrared thermography to solve complex problems and to provide a deeper understanding of the heat transfer on slender cylinders.

In contrast to most infrared radiometry techniques used for nondestructive evaluation which follow the sample cooling after pulsed heating, the technique termed time-resolved infrared radiometry with step heating (TRIR) follows the surface temperature rise as a function of time during the heating pulse. This approach allows identification of subsurface features and determination of thermal properties with the same speed as other thermal techniques, but keeps the required heating power and resulting surface temperature small. This permits the use of heat sources such as microwaves and RF induction heating where high peak power is often not available. One of the most attractive features of the TRIR method is the ability to calibrate the temperature response early, when the sample is thermally-thick. This allows correction for inhomogeneous heat source distributions and differentiation between backing materials. A fast algorithm has been developed to calculate thermal transit times and therefore generate quantitative depth images of subsurface features. This paper will describe the TRIR approach and the analysis of its time response, including the calibration at early times. Examples will be described for laser heating on zirconia coatings, corroded aluminum, and graphite composites, and the use of microwaves and RF induction heating as heating sources.

Quantitative transient IR thermography has been applied to the characterization of hidden corrosion in metals. A dedicated 3D numerical model of heat transfer has been used to solve the direct thermal problem and to simulate the test. Theoretical modelling allows the verification of limits of the ID solution and the derivation of coefficients which take heat diffusion into account. An analysis of inversion accuracy was carried out. A simple algorithm based on a surface temperature time-derivative is proposed for detecting thickness variations. Then, material loss in an area of arbitrary shape is evaluated applying a modified algorithm, originally developed for a ID thermal model. The potential of dedicated image processing in enhancing a signal-to-noise ratio is explored. The feasibility of corrosion quantification by the proposed inversion algorithm is demonstrated with experimental results. Detection and evaluation of hidden material loss within a boiler section, typically used at a power plant station, has been performed. The external surface was heated with flash lamps and temperature response was analyzed both in time and space domains. The masking effect due to the noisy inspected surface (not painted and affected by a long time service) were substantially removed before evaluating corrosion. Obtained results have been compared with measurements produced by the ultrasonic method.

Pulsed phase thermography (PPT) was recently introduced, and up to now analysis of this infrared thermographic approach for non-destructive evaluation has been limited to qualitative aspects. The study presented in this paper is the first attempt to extract quantitative information from PPT results. The approach proposed is based on neural networks well known for their ability to handle complex non-linear problems with access to partial noisy data. In the paper, a thermal model is first presented. This model helps in designing the neural network architecture. PPT fundamentals based on pulsed and lock-in thermography concepts are briefly recalled. Also found in the paper are sections on noise with relations to phase and frequency, neural networks, experimental data on both aluminum and plastic materials. The papers concludes with possible directions of work. The proposed method combining PPT with neural network analysis is shown to be encouraging. The sampling frequency with respect to inspected material thermal conductivity is an experimental limitation.

The heat-photon conversion phenomenon can be used to obtain a thermal image of an electromagnetic field. The electromagnetic field is partially absorbed by a sensitive paint or by a coating deposited on structures or on thin films. A map of the temperature increase of this absorbing medium is an image of the electric or magnetic intensity field distribution, depending on the electric and magnetic properties of the medium. A brief history of the various techniques used to obtain thermal images of electromagnetic fields is first presented. Emphasis is then put on infrared thermography which has been preferentially used in the past 20 years. An analysis of the thermal problems involved is presented. It appears that the solution to these problems is the key for the enhancement of the technique and for really quantitative work. Original solutions have been developed at ONERA, based on the combined use of optimised thin films with controlled electric conductivity, very sensitive infrared cameras, lock-in infrared thermography, and microwave interferometry. In these conditions, quantitative images of both amplitude and phase are obtained. Such an electromagnetic field imaging technique is a powerful tool which has no equivalent and which can be used for several types of applications such as: i) antenna radiation pattern characterization; ii) mode propagation characterization in waveguides; iii) study of absorption phenomena in complex materials; iv) nondestructive evaluation of dielectric structures (electromagnetic windows) or radar absorbing materials; v) knowledge of surface currents distribution on metallic structures.

In this paper we present an application of infrared thermography for inverse heat conduction problems resolution. The approach described in the paper is based on a Boundary Element Method formulation of the transient heat diffusion equation. The inverse problems under investigation concern the time and space reconstruction of unknown boundary conditions or heat line source strength. As there is a lack of information in the system, some additional measurements are necessary to solve the problem. In the examples proposed in this paper the extra information is provided by an infrared scanner. The measurements contained in the infrared pictures are used in the model as a Dirichlet boundary condition or as a special boundary condition prescribing both temperature and heat flux density on the scanned boundary. We present some experimental results concerning line source strength identification and the reconstruction of unknown heat fluxes applied on an out of reach boundary. All the examples presented in this paper are related to 2D transient diffusion. As the inverse problem is ill-posed, time and space regularization techniques are used to stabilize the solution and reduce the sensitivity of the latter to measurement errors.

Very small temperature variations are quantified by a statistical treatment of standard infrared equipment images. This procedure determines the signal amplitude value, comparing the noise and the noisy signal dispersions characterized by their variances. This robust and simple method has the advantage of needing no link to the reference signal and of treating any kind of signal shape. It is applied here to thermoelastic analysis of applied and residual stresses.

Photothermal radiometry allows for remote measurement of local harmonic heat transport where the phase angle (between remote optical energy deposition and resulting temperature modulation) is sensitive to subsurface features or defects. Phase sensitive modulation thermography (or ‘lock-in thermography’) combines the advantages of photothermal radiometry with the fast technique of infrared imaging thereby revealing hidden defects in a short time. In this paper the principle and various applications are described and analyzed. While this lock-in thermography is based on remote optical heating of the whole area of interest, one can heat defects selectively with modulated ultrasound which is converted into heat by the mechanical loss angle effect which is enhanced in defect regions. This ‘ultrasonic lock-in thermography’ provides images showing defects in a way that is similar to dark field imaging in optical microscopy.

The development of turbulence models and wall laws for the numerical simulation of flows in complex geometries requires a detailed experimental analysis of turbulence and of the phenomena that appear in turbulent boundary layers. There is a strong need to develop new measurement systems allowing the determination of unsteady wall heat transfer coefficients. In order to improve the knowledge of the unsteady phenomena occurring in perturbed boundary layers, a fundamental study is conducted on the interaction of a single vortex with a flat plate. An experimental methodology using a specific thermal sensor whose surface temperature is measured by an infrared thermography system is presented. It allows the characterization of the unsteady convective heat transfer coefficient whose evolution is compared with the fluctuations of the wall friction coefficient, calculated from velocity profiles measured by laser Doppler velocimetry.