Showing papers in "Quantitative InfraRed Thermography in 1996"

In Situ Calibration for Quantitative Infrared Thermography

Abstract: This paper presents a method to correct temperatures obtained by infrared thermography using thermo­ couples as reference temperatures. The raw signal values of the infrared detector are compared to the temperatures measured with thermocouples. The relation between raw thermal value and body tempera­ ture includes three parameters which allow to determine a best-fit approximation between detector signal and body temperature. The resulting non-linear system of equations is solved numerically using the Levenberg-Marquardt procedure. These parameters are then used to calculate the corrected tempera­ tures of the body. 1 Introduction Infrared thermography is a powerful method for temperature measurement in turbomachinery research. At the 'Institut fOr Thermische Stromungsmaschinen' at the 'Universitat Karlsruhe' the infrared camera has been used in numerous investigations on heat transfer phenomena. So far, the results achieved were mainly used for qualitative reviews of complex surface temperature distributions. To allow for any quantitative analyses the data have to undergo some further cor­ rection or calibration procedure. This procedure has to account for many different effects (e.g. reflected radiation, gas radiation, translucency of the window) which have an impact on the thermography results. In most experimental set-ups matching real engine conditions, these effects are difficult to quantify. Therefore, any correction procedure on a pure theoretical basis is not very promising. Better accuracy can be achieved with a case specific calibration of the thermography system which is obtained experimentally. Various authors have proposed calibration procedures for thermography systems which rely on calibration experiments. Among them Koschel et al. [1] used an external system to calibrate a pyrometer which was used to measure the temperatures on the surface of turbine blades. In this way the influence of the observation angle and thE: distance between blade surface and detector on the IR-signal could be studied precisely. Nevertheless, the surface emissivity could not be determined accurately and reflection effects could not be simulated in the calibration setup. Carlomagno et al. [2] used a little black body to calibrate an IR-camera under real test conditions. The black body was placed in the test rig at the location at which the surface tem­ perature had to be determined later on. The calibration parameters obtained with the black body were then used in the surface temperature measurements with an additional correction for gray body. This method can only be applied if the test surface can be dealt as a gray body, i.e. its emissivity is independent of temperature, and if the test section allows for the insertion of a black body. Reflected radiation, which occurs in the case of any gray body, cannot be taken into account. Meyers at al. [3] performed temperature measurements on film cooled combustor walls. During the heating-up of the combustor, temperature data were recorded from both the infrared scanner and from the embedded thermocouples. These data were then paired and used for a best fit approximation for the calibration parameters of the camera. Thus, all tem­ perature dependent properties of the system are taken into account. Nevertheless thermal im­ age recording during the heating-up period leads to slightly shifted operating conditions be­ tween calibrating and testing. The calibration method presented in this paper uses a similar approach but thermocouple temperature and thermal image recording takes place at steady state and the 'best-fit' approximation includes three parameters instead of two for higher accu­ racy.

Inspection of aircraft structural components using lock-in thermography

Abstract: 'Institut fOr KunststoffprOfung und Kunststoffkunde, Universitat Stuttgart, 0-70569 Stuttgart, Germany; 2oepartmento di Energetica, Politecnico di Milano, Piazza L. da Vinci 32, /-20133 Milano, Italy; 3K. Malter Air Service, Flugplatz, 0-91074 Herzogen-aurach, Germany; 41nstitut fOr Bauweisenund Konstruktionsforschung, oLR, Pfaffen-waldring 38-40, 0-70569 Stuttgart, Germany; 5oaimler-Benz Aerospace, oomier Luftfahrt GmbH, 0-82230 Wessling, Germany; 6IABG, 0-85521 Ottobrunn, Germany

Lock-in thermography with mechanical loss angle heating at ultrasonic frequencies

Abstract: In this paper the mechanical loss angle heating as an internal thermal wave source is described. Acoustic energy at high frequency (ultrasound) can be coupled into the sample. The high hysteresis loss in the defect region results in increased heating of the defect. When the intensity of the ultrasound is modulated at a low frequency the defect itself emits a thermal wave at this low frequency. The range of this thermal wave is large enough for detection at the surface with lock-in thermography tuned to the intensity modulation. This technique combining ultrasonic waves and lock-in thermography provides a fast imaging of imaginary part of Young's modulus in materials. Results obtained with this selective heating on various real components are presented.

Lock-in thermography as a measurement technique in heat transfer

Abstract: An experimental temperature oscillation technique is described for determining local distributions of the heat transfer coefficient or local distributions of the thermal diffusivity of heat transferring walls. By heating uniformly one surface of the wall with sinusoidally modulated energy a temperature oscillation is generated which results in a wavelike propagation behavior of heat flow and temperature within the wall. The characteristic of the temperature oscillations at both faces of the wall depends directly on the local heat transfer conditions and the thermal diffusivity of the wall material. So the local values of the heat transfer coefficient or the thermal diffusivity can be calculated from the measured amplitudes or from the phases of the temperature waves at the surfaces. To demonstrate the applicability of the method first experiments were performed. The measured results agree reasonably well with data obtained from literature. 1. Introduction Because of their simplicity and accuracy temperature oscillation techniques obtained an increasing importance for measuring purposes in heat transfer. The basic idea of these techniques is to supply modulated energy to the testing object which results in a wavelike propagation behavior of heat flow and temperature within the object material. By measuring and analyzing the temporal and spatial propagation behavior of the thermal waves, numerous thermophysical parameters of the testing object can be determined. With an earlier developed method [1] for measuring the local heat transfer coefficient or the local thermal diffusivity periodic temperature oscillations are optically generated at one spot of the object surface by periodic heating with a focused laser beam. This method was then refined by Wandelt [2] to achieve a much higher accuracy. Measurements which cover the whole object surface have to be performed point by point in a rasterlike fashion. At each pOint one has to wait a minimum time, until the local stationary state is achieved. So measuring times can be long which might be not acceptable for practical applications. The technique presented overcomes the restrictions of the method described above. Local distributions of the heat transfer coefficient as well as local distributions of the thermal diffusivity can be determined for the whole testing object within a short time. 2. Measurement principle The measurement principle is illustrated in figure 1. Sinusoidally modulated thermal energy is supplied uniformly to the whole surface of the object under consideration. From the surface a nearly plane thermal wave propagates into the material after the initial transient behavior. The local amplitudes and phases of the temperature waves at both sides of the wall depend on the properties of the wall, the frequency of the oscillation and the heat flux to the surrounding. By measuring the local temperature oscillations at each surface point and by evaluating the local phases and amplitudes of these oscillations, a phase angle image and an amplitude image can be extracted for the whole surface. In principle, a map of the local heat transfer coefficient or a map of the locally varying thermal diffusivity can now be calculated from the data of both the images. For monitoring and recording thermal waves a rapid infrared scanning device (AGEMA THV900LW/ST) is used, which allows a non-contacting measurement of the small wall temperature changes with high thermal, spatial and time resolution. The thermal information from all scanned pOints are analyzed with a PC.

Thermal ellipsometry in steady-state and by lock-in thermography. Application to anisotropic materials characterization

Abstract: A technique for lock-in thermography that we recently developed was applied for the measurement of the in-plane principal diffusivities of orthotropic materials. The surface is point heated with a modulated laser beam and a 2-D synchronous detection is performed of the elliptical thermal field which develops around it. Diffusivities are inferred from the slope of the phase lag profiles. The approach was first validated with a duralumin (isotropic) sample. It was then applied on a unidirectional C/epoxy sample. The aspect ratio of the surface isotherms is imposed by the anisotropy of the considered material. Therefore, possible in-depth variations of the anisotropy unavoidably modify the aspect ratio distribution at the surface. Based on this observation, an inversion method was recently proposed to monitor the internal fibre orientation changes in composites. Results obtained with a continuous fibres C/epoxy sample and with an injection moulded short fibres sample are reported.

Experimental investigations of defect sizing by transient thermography

Abstract: Details are presented of an experimental and theoretical investigation of the defect sizing capabilities of the transient thermography technique. Back-drilled hole artificial defects, of diameters from 2 to 10 mm and at depths between 0.5 and 1.7 mm, in bakelite plates, were thermographically imaged. Full-width at half maximum (FWHM) of a defect image is shown to be accurately related to defect diameter. These measurements are also shown to be in excellent agreement with finite-difference simulations of the response of this type of defect. The temporal dependences of the images are shown to provide an indication of defect depth.

Lockin thermography for imaging of modulated flow in blood vessels

Abstract: While conventional thermographic applications to the human body aim at revealing local temperature changes normally related with the beginning of a pathologic process, this paper shows how lockin thermography can be modified in order to meet medical needs in the visualization of functionality of blood vessels. A compression cuff is used in connection with the lockin system to create a modulation in the blood flow. The amplitude and phase images thus obtained can provide infonmation about the blood flow in the vessels.

Investigation of resolution in lock-in thermography: theory and experiment

Abstract: The general formalism of wave propagation and scattering effects is applied to optically generated thermal waves in order to describe imaging of near-surface features. The contrast functions of amplitude and phase calculated for flat bottom holes show good agreement with experimental results obtained with lockin-thermography.

Transient thermographic detection of buried defects: attempting to develop the prototype basic inspection procedure

Abstract: The transient thermal NOT basic inspection procedure is proposed. The procedure is based on the statistical analysis of NOT results presented as a single IR image or image sequence. A defect map of the sample under a test is created according to the accepted values of correct detection and false alarm. The proposed methodology could serve for comparing different thermal NOT procedures and data processing algorithms.

Thermal imaging of the effects of beta-irradiation on human body surface

Abstract: It is an experience that ionising radiations produce thermal effects in human organisms. Nevertheless, very few data are available on the quantitative relationship of the dose of ionising radiation and the changes of temperature. The studies were planned, therefore, to reveal the thermal detectability of tissue reactions following radiotherapy treatments. By this way during the radiation treatments the thermal measurements made possible to detect the sequential alterations in the thermal map of the involved body surfaces. The thermal studies were performed in collaboration with the National Institute for Oncology by measurements of patients irradiated with beta rays. In the paper the results of infrared thermogrammetry investigations are given by histographic method.

Experimental determination of the local heat transfer coefficient on a thermally thick wall downstream of a backward-facing step

Abstract: A heat transfer measurement technique based on the combination of infrared thermography and numerical computation is presented in the case of a turbulent reattachment downstream of a backward­ facing step. The presence of a CaF2 window and low surface temperatures has led to develop a specific infrared system calibration. Heat transfer measurement technique and infrared system calibration are both presented in this paper.

Factors affecting the detectability of voids by infrared thermography

Abstract: During the manufacturing of laminar plastics, debondings between layers can occur due to bad curing. Two inspection techniques by infrared thermography have to be considered: Impulse or transient thermography and lock-in thermograpy. For both techniques, the detectability of a defect and the factors affecting it are major issues. The first investigated material is isotropic polyvinyl chloride, the second is anisotropic unidirectional carbon fiber­ reinforced plastic. This article will discuss the above mentioned factors and will derive some practical rules about the detectability of a defect. For the two base materials analysed, the complex contrast is studied as a function of the defect diameter to depth ratio and the modulation frequency of the heating source. Practical rules will be derived and a comparison between the two techniques will be made. Laminar plastics such as woven fiber-reinforced plastics are becoming more and more widely used due to their good mechanical properties and their ability to be tailored to the intended application. During their manufacturing, debondings between layers can occur due to bad curing. Fiber-reinforced plastics in form of sheets are also used to reinforce civil engineering structures like bridges [1]. The presence of air inclusions or delaminations in the bonding layer between the sheet and the structure can drastically reduce the mechanical strength of the reinforcement. Therefore, a fast testing method is needed in order to detect such flaws. Until now, only the time consuming ultrasonic inspection method came into consideration. Infrared thermography is a particularly well suited inspection method, because it is fast and gives an image of the location of the defects. Two inspection techniques by infrared thermography have to be considered: the impulse or transient thermography [2] and the lock-in thermography [3].

IR heat transfer measurements in a rotating channel

Abstract: The main aim of the present study is to develop a new experimental methodology that allows accurate measurements of the local heat transfer distribution nearby a 180deg sharp turn in a rotating square channel to be performed by means of infrared thermography. Another objective is to prove that the use of infrared thermography may be appropriate to experimentally study this type of problems. To perform heat transfer measurements, the heated-thin-foil technique is used and the channel is put in rotation in a vacuum tank so as to minimise the convective heat transfer losses at the surface of the foil on the channel outside. Some preliminary results in terms of temperature distributions and averaged Nusselt number Nu profiles are presented. 1. Introduction To increase the thermodynamic efficiency of gas turbine engines is necessary to increase the gas entry temperature. Present advanced gas turbines operate at gas entry temperatures much higher than metal creeping temperatures and therefore require intensive cooling of their blades especially in the early stages. A classical way to cool turbine blades is by internal forced convection: generally, the cooling air from the compressor is supplied through the hub section into the blade interior and, after flowing through a serpentine passage, is discharged at the blade trailing edge. The serpentine passage is mostly made of several adjacent straight ducts, spanwise aligned, which are connected by 180deg turns. The presence of these turns causes separation of the flow with consequent high variations of the convective heat transfer coeffi­ cients. Furthermore the rotation of the turbine blade gives rise to Corio lis and much stronger buoyancy forces that may completely change the distribution of the local heat transfer coeffi­ cient. To increase the blade life, which depends also on thermal stresses, it is necessary to know the distribution of the local convective heat transfer coefficient. In the case of radially outward flow, the Coriolis force produces a secondary flow (in the form of a symmetric pair of secondary vortices), in the plane perpendicular to the direction of the moving fluid, which pushes the particles in the center of the channel towards the trailing sur­ face, then along the latter in the direction of the side walls and finally back to the leading sur­ face. The presence of these two secondary cells enhances the heat transfer in the vicinity of the trailing wall and reduces it at the leading surface with respect to the non-rotating case. When the flow is reversed, i.e. radially inward flow, one has only to change the role played by the leading surface with that of the trailing one and vice versa. Furthermore, the heating at the walls causes a temperature difference between the core and the wall regions, so that the in­ duced density difference and the strong centripetal acceleration due to rotation give rise to a buoyant effect. This effect magnifies the influence of the Coriolis force in the radially outward

Infrared imaging techniques for the measurement of complex near-field antenna patterns

Abstract: This paper describes the application of "plane-to-plane" (PTP) iterative Fourier processing to infrared thermographic images of microwave fields to calculate the near-field and far-field patterns of radiating antennas. The PTP technique allows recovery of the phase by combining intensity (magnitude) measurements made on two planes, both in the radiating near field of the antenna under test. Starting with an estimate of the phase and the measured magnitudes, Fourier processing techniques are used to iteratively "propagate" between the planes to determine the correct phase distribution at each plane. We describe the technique and show excellent comparisons made between predicted and measured results.

Heat transfer identification induced by multihole cooling in combustion chambers

Abstract: This paper deals with the identification of the heat transfer occurring in the combustion chambers of turbomachines. Several test plates, reproducing at different scales the walls of combustion chambers have been used to quantify the effectiveness of a 30· staggered multihole cooling. Using a physical model and temperature measurements made by infrared thermography, the behavior of the convection heat transfer coefficient and the adiabatic wall temperature downstream the hole region have been determined. A modification of the model gave their behavior in the perforated area, then correlations of the effectiveness versus the streamwise distance and the blowing ratio were performed.

Investigation of temperature distribution during plastic deformation of stainless steel

Abstract: The object of the work was to find the temperature fields and their evolution during tensile test and simple shear test of metal in order to use them in the examination of material's behaviour. In initial stage of metal's elongation, the temperature distribution allows to determine the beginning of plastic deformation. In the homogeneous state of stress temperature measurements enable to investigate a process of energy storage during the plastic deformation. Heterogeneous temperature distribution observed in the subsequent, advanced state of deformation are related to evolution of the localized plastic deformation, leading to necking and damage. Furthermore, examination of the temperature distribution obtained during static simple shear test enables us to confirm the occurrence of macroscopic shear bands, calculated in theoretical approach. As we know from experiments, heating of a body produces changes in stress, strain and temperature. Conversely, deformation of a body is associated with a change of its internal energy, and consequently in its temperature distribution. This mutual interaction between deformation and the temperature fields is called thermomechanical coupling. Effects of thermomechanical coupling, occurring in nature, have a long history both in theoretical and experimental approaches. Considerable increase in capabilities and in accuracy of temperature measurement, related to elastic and plastic deformation of metals, appearing after the non-contact measurement approaches and based on detection of infrared radiation, were introduced [1-3]. This type of measurement system was used in the present investigations. It consists of a thermovision camera, coupled to a computer system of data acquisition and conversion, which enables to store the obtained temperature distributions on the hard disc of a computer and to further process it by dedicated software. The accuracy of temperature measurement is about 0.2 K; in some approaches it can be higher, even 0.01 K. All the tests were performed in a Instron testing machine, most of them during a tensile test, on flat samples of austenitic steel. In order to secure higher and more homogeneous emissivity, the surface of the samples was blackened by a carbon powder. 2. Identification of the beginning of plastic deformation based on temperature measurements. Empirical identification of the boundary between the elastic and the plastic regimes of deformation is complex and ambiguous, since ambiguous is also the definition of the yield point as well as the term used for elastic deformation. The methods utilizing the thermal emission are based on a qualitative change of the temperature behaviour of specimen under mechanical loading. During elastic deformation, these changes are, in general, linearly dependent on stress and are negative in the case of the elongation test. Otherwise, during the plastic deformation, always accompanied by an increase in temperature, the temperature changes are nonlinear. The change of temperature ~T of a specimen, subjected to adiabatic uniaxial elastic deformation, can be described as follows: aTlla

Convective heat transfer along slender cylinders

Abstract: An experimental investigation of the forced convection along vertical. slender cylinders aligned in a uniform air flow is performed using quantitative infrared thermography. In the laminar regime, the heat transfer data are correlated in terms of a local dimensionless curvature parameter and validate the thermal boundary layer model for a single slender cylinder. Turbulent heat transfer occurs when the stand­ off distance between two parallel, slender cylinders is sufficiently small to affect the axisymmetry of the boundary layer. An enhancement factor of three can be achieved at the transition region. In the turbulent regime, the curvature parameter alone does not describe the thermal exchange.

Microwaves holography revealed by photothermal films and lock-in IR thermography: Application to NDE of dielectric and radar absorbing materials

Abstract: The amplitude and phase space distributions of EM (electromagnetic) fields (Xand Ku bands) are imaged and measured using microwaves interferometry revealed by photothermal films and lock-in infrared thermography. Such EM fields imaging is a powerful tool for NDE (non Destructive Evaluation) of dielectric and radar absorbing materials.

Combustion graphology used to improve emulsions of water-in-heavy fuel oil

Abstract: Combustion graphology uses the infrared and luminous radiation of the flame and cenosphere of a burning droplet of heavy fuel oil in order to carry out scientific research with technical industrial applications. The rational combustion of the optimal water-heavy fuel oil emulsions in industrial power fumaces determines several advantages of which the most important is the de-pollution of the environment. Graphological testing for water-heavy fuel oil emulsion droplets is performed on a simulator and the result of the experiments is presented for two situations: when the effect of secondary atomisation is partially present and when the secondary atomisation is total, the whole droplet exploding. Experimental results are presented for samples processed in laboratory and in an industrial emulsifying installation.

Influence of the radiation diffraction in image converter of the thermograph upon its metrological parameters

Abstract: The influence of radiation diffraction upon geometrical resolution of the thermograph with series structure of measuring line has been analysed in the paper. Analysis of thermograph properties in bands of 3 - 5 J.LIT1 and 8 - 1 0 ~m proved that when applying the aperture diaphragms the influence of the radiation diffraction upon geometrical resolution may by significant. This influence is stronger for larger values of the hole numbers in the applied diaphragms. It was shown that the thermal resolution of the thermography system can be improved by reducing the bandnwidth of the electronic system of the camera. The correctness of thermal transformations obtained by means of thermographic equipment is a function of several factors which are dependent or independent on the equipment construction. The dependence on construction factors that are of significant influence upon the quality of thermal transformations, are parameters of the transformation synthesis system. These construction factors/parameters of elements used in the thermograph measuring line determine values of the metrological parameters of the equipment. The most important of them are: thermal resolution (TR) '<\T and geometrical resolution (GR) .<\G. Construction parameters of the image analysis system that influence the TR and GR are different for different measuring conditions (e.g. filters and diaphragms change parameters of the optical line ) [1].

IR detection of thermal waves - effect of imaging conditions on the background fluctuation limit

Abstract: Starting from the basic principles of photon detection, a general theoretical description is here given for the incoherent background fluctuation limit of thermal wave detection. Different imaging conditions of IR detection of thermal waves and different detectors are considered. The theoretical limits are compared with measurements obtained for a MeT detector. Good agreement between the observed detection limits and the theoretical prediction is obtained. Thermal waves, excited in solids by intensity-modulated heating, can be used to determine thermo physical parameters, e.g. the thermal diffusivity and effusivity. Since the penetration depth of thermal waves decreases with the modulation frequency of heating, depth-selective information about the thermal properties can be obtained by measuring the amplitude and phase as functions of the modulation frequency. In general, thermal wave measurements are nondestructive and in the case of IR detection of the thermal wave response, thermal waves are most appropriate for non-contact remote measurements of thermal properties and thermal depth profiles. The limit of detection of thermal waves for a given measurement setup, is affected by the total noise of the setup within the measured bandwidth. The total noise is mainly caused by the noise produced in the detector itself, the noise of the electronic system following the detector, and the noise of the incident radiation to which the detector responds. The ultimate limits of detection are set by the fluctuations of the incident radiation, which can be identified, when a cooled detector with low internal noise and a low-noise preamplifier are used. There are two types of fluctuations of the incident radiation: fluctuations of the signal radiation and fluctuations of the background radiation. For the detection of thermal waves, usually the fluctuations in background radiation are dominant and the fluctuations in signal radiation are negligible. 2. Background fluctuations The small variations of the detector signal which correspond to the thermal wave response are distinguished from the background radiation level by filtering the detector signal with the help of a lock-in amplifier at the modulation frequency f of the thermal wave. Nevertheless, the detection is affected by incoherent and coherent fluctuations: - Coherent fluctuations may be due to secondary thermal waves produced in the compo­ nents of the IR optics (lenses and filters) by the modulated laser beam used for the excita­ tion of the thermal wave or by the thermal wave response itself [1]. Since the thermal wave signal depends on the stationary temperature T, coherent noise may also arise from slow changes in the stationary temperature of the sample [2].

Estimation of thermophysical properties of thin plates with averaging techniques and two temperatures model

Abstract: Averaging techniques are well suited for infrared image processing because they consider spatially averaged temperature fields related or not to each constitutive phase of heterogenous media. In the case of simple geometry, such as thin rectangular adjacent plates, one or two dimensional exact analytical relationships can be obtained between averaged temperature related to each plate. These expressions are suitable to estimate thermophysical properties such as thermal diffusivity or thermal resistance at the junction of the plates. The main advantage is to reduce the influence of measurement noise. Experimental results are obtained and some examples of estimation of thermophysical parameters are presented.

Infrared thermography study of a confined impinging circular jet

Abstract: The convective heat transfer at the impingement of a vertical turbulent circular air jet on a horizontal flat plate is inferred from temperature measurements performed by quantitative infrared thermography Steadystate experiments are conducted with the heating-thin-foil method The effects of the jet Reynolds number and the stand-off distance on the thermal exchange coefficient are emphasized The influence of jet confinement is determined The IR results agree with published data and are reproduced by numerical simulations performed with the code FLUENT

Radiative and convective heat transfer in microelectronics

Abstract: This paper presents numerical and experimental results of heat transfer by radiation, convection and conduction in hybrid microelectronic circuits. We chose a heat source with a non-uniform temperature distribution, which agrees with typical cases frequently met in electronics. In this work we evaluate a complex heat transfer coefficient including the non-linear phenomenon for radiative and convective heat dissipation. We apply thermography to confirm the correctness of the simulation.

Photothermal inspections of adhesion strengths and detection of delaminations

Abstract: For more than a decade, photothermal measurement techniques have been used for the nondestructive and contactless evaluation of coating thicknesses and for the investigation of boundaries between a suriace layer and its base material. In the following sections, we aim to describe a few examples whereas the photothermal radiometry has been applied on different interiaces embedded in an optically opaque workpiece. The effect of adhesion defects on photothermal signals will be demonstrated. The obstructed propagation of thermal waves can be explained by thermal contact resistances when delaminations or disbondings are located between a coating and its substrate.