Measurement of thermal parameters of a heat insulating material using infrared thermography

The article presents results of research developing methods for determining thermal parameters of a thermal insulating material. This method applies periodic heating as an excitation and an infrared camera is used to measure the temperature distribution on the surface of the tested material. The usefulness of known analytical solution of the inverse problem was examined in simulation study, using a three-dimensional model of the heat diffusion phenomenon in the sample of the material under test. To solve the coefficient inverse problem an approach using an artificial neural network is proposed. The measurements were performed on an experimental setup equipped with a ThermaCAM PM 595 infrared camera and a frame grabber. The experiment allowed verification of the chosen 3-D model of the heat diffusion phenomenon and proved suitability of the proposed test method.     

Nondestructive testing of objects of complex shape using infrared thermography: Determination of the spatiotemporal distribution of the irradiation heat flux

The existing inverse methods used to determine the heat flux density require that the forward problem and the problem domain (geometry) be known. In this paper, in order to determine the spatiotemporal heat flux density without knowing the real problem domain, we propose an approach based on temporal tracking of the thermal front. The proposed approach is particularly relevant when a three-dimensional formulation is adopted for nondestructive testing using infrared thermography. For such a formulation, heat flux density resulting from the external thermal stimulus is needed and must be determined to accurately characterize the defects and reconstruct the internal geometry of the inspected objects. The proposed approach uses only two inputs: the time-dependent temperature of the frontal surface recorded by an infrared camera and the 3D point cloud of the frontal surface collected by a 3D scanner. The method is evaluated numerically on an object of complex shape. We consider the case of pulsed thermal stimulus as well as the cases of unit step and modulated thermal stimuli. An experimental validation is performed on a cylindrical object submitted to a pulsed thermal stimulus and a modulated thermal stimulus. The results show the accuracy of the method which can easily be implemented as the initial step of the three-dimensional quantitative nondestructive testing of objects using infrared thermography.     

Estimation of thermophysical properties by an inverse method with experimentally determined heating region of a thin-layer heater

This paper deals with the problem of estimation of thermophysical parameters by an inverse method. The thermal conductivity in radial and axial direction of a cylindrical sample and the heat capacity were simultaneously estimated using the Levenberg–Marquardt method of minimizing a mean square functional. As heat sources the thin-layer heater KHR 2/10 of diameter  = 50 mm and thickness 0.20 mm made by OMEGA as well as the Kanthal resistance wire of diameter  = 0.1 mm in a form of semicircle were simultaneously used. The main aim of using these two heaters simultaneously, both placed at one of the sample interfaces, was to generate heat fluxes in axial and additionally in radial direction. However, measurements of temperature distribution on the main surface of the thin-layer heater by using the FLIR Systems (ThermaCAM SC 3000 infrared camera) revealed a spatial heterogeneity of its temperature field, and therefore it was necessary to determine the effective heating region of that heater indispensable for solving of the coefficient inverse problem of heat conduction.     

Filtered thermal contrast based technique for testing of material by infrared thermography

A new kind of thermal contrast, called “filtered contrast” is presented, which allows detecting and characterizing material defects using active thermography under some assumptions on physical and thermal parameters of materials. In opposition to known definitions of the thermal contrast, knowledge about defect-free area is not necessary and this contrast is less sensitive to nonuniformity of heat disposal to the material surface. The measurements were performed on an experimental setup equipped with a ThermaCAM PM 595 infrared camera and frame grabber. The step heating was chosen as heat excitation. The results demonstrate usefulness of the 1D model of heat transfer used for determination of depth of subsurface defects. The influence of the parameter of the smoothing filter, required for filtered contrast implementation, thermal parameters of the tested material and defect on expanded uncertainty of determination of defect depth is also presented. Due to significant complexity of the model of heat transfer, the conditions for the “law of propagation of uncertainty” were not fulfilled and a numerical method, i.e., Monte Carlo simulation is applied for the propagation of distributions.     

Infrared thermography applied to the resolution of inverse heat conduction problems: recovery of heat line sources and boundary conditions

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.     

亚表面异质缺陷对功能梯度材料表面温度场的影响

基于双曲型热传导方程,采用镜像法和波函数展开法,求解了含亚表面异质圆柱缺陷的半无限功能梯度材料的表面温度场,给出了功能梯度材料中热波散射的一般解.分析了亚表面异质圆柱缺陷的几何参数(如埋藏深度)和热物理参数(如导热系数、热扩散长度、热扩散率及热弛豫时间等)对功能梯度材料表面温度场的影响.温度波由调制的超短脉冲激光在功能梯度材料表面激发,异质圆柱缺陷表面的边界条件为导热边界.研究结果可望为功能梯度材料的红外热波无损检测、导热反问题提供计算方法和参考数据.     

Modelling of thermal conductance during microthermal machining with scanning thermal microscope using an inverse methodology

In this study, a general methodology for determining the thermal conductance between the probe tip and the workpiece during microthermal machining using Scanning Thermal Microscopy (SThM) has been proposed. The processing system was considered as an inverse heat conduction problem with an unknown thermal conductance. Temperature dependence for the material properties and thermal conductance in the analysis of heat conduction is taken into account. The conjugate gradient method is used to solve the inverse problem. Furthermore, this methodology can also be applied to estimate the thermal contact conductance in other transient heat conduction problems, like metal casting process, injection molding process, and electronic circuit systems.     

含双圆柱亚表面缺陷板条材料的表面温度分布

基于非傅里叶热传导方程,采用复变函数法和镜像法,研究了含双圆柱亚表面缺陷板条材料热波散射的温度场,并给出了热波散射温度场的解析解。分析了入射波波数、热扩散长度、缺陷的埋藏深度以及板条材料的厚度等对板条表面温度分布的影响。温度波由调制光束在材料表面激发,缺陷表面的边界条件为绝热。该分析方法和数值结果可为工程材料结构的传热分析、热波成像和材料内部缺陷评估,以及热物理反问题研究提供参考。     

Determination of thermophysical characteristics of materials at thermal effect of constant power

On the basis of the solution to the coefficient inverse problem of heat conductivity we have proposed a method for determination of thermophysical characteristics of material according to temperature measurement in its depth as an approximation of semi-infinite solid, plate of finite thickness, and plate with a layer of ideal conductor at thermal effect of constant power on their surface. The method does not require experimental data smoothing, serves to remove restrictions for heating mode selection, and increases accuracy of thermophysical parameters determination.     

Optical method for determining the hardness of metals and alloys

We present an optical method for determining the hardness of metals, based on the correlation between the thermal diffusion coefficient and the hardness of the material. Spatial and temporal modulation localized in the nonstationary thermal wave profiles is transferred to the surface of the metal, where the time evolution of the speckle images produced by a probing laser beam is recorded by a CCD camera. A special algorithm for processing the speckle images allows us to determine the thermal diffusion coefficient. Experimental studies on steel samples confirmed the workability of the proposed method. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 460–465, May–June, 2009.     

The optimization method in design problems of spherical layered thermal shells

The inverse problem of designing multilayered spherical shells, intended for thermal cloaking a spherical body or concentrating heat in it, has been analyzed. The stationary heat conduction equation for an anisotropic medium is applied as an original mathematical model. The optimization method is used to reduce this inverse problem to an extreme problem, where the role of controls is played by the thermal conductivities of shell layers. A numerical algorithm for solving the problem is proposed, and the results of computational experiments are discussed.     

Thermal wave scattering by two overlapping and parallel cylinders

In this paper an analytical solution of the temperature of an opaque material containing two overlapping and parallel subsurface cylinders, illuminated by a modulated light beam, is presented. The method is based on the expansion of plane and cylindrical thermal waves in series of Bessel and Hankel functions. This model is addressed to the study of heat propagation in composite materials with interconnection between inclusions, as is the case of inverse opals and fiber reinforced composites. Measurements on calibrated samples using lock-in infrared thermography confirm the validity of the model.     

ESTIMATION OF THERMAL CONTACT CONDUCTANCE DURING RESISTANCE SPOT WELDING

This article describes an experimental procedure conducted to estimate and investigate the transient thermal contact conductance (or thermal contact resistance) between the electrodes and workpieces during resistance spot welding. A fine thermal metrology was developed to collect thermal histories near the welding region. Indeed, the electrode tip was instrumented with several interior microthermocouples for measuring the transient temperature response during the welding process. A simple mathematical model, using an inverse heat transfer method, was built for the estimation of the transient heat transfer coefficient from interior transient temperature measurements. A simple resistance welding case of two steel sheets was investigated. The initial transient values of thermal contact conductance were found to be in agreement with those observed in the dry copper–steel solid contact case. At the end of the process, the transient heat transfer coefficient reaches a high value corresponding to the best heat transfer phenomenon at the interface during the welding process. When the metal is melted, the contact quality increases due to the high-applied electrode force. Higher electrode force and heating temperatures produce lower thermal contact resistance. The results obtained show the capabilities and the power of the coupled thermal metrology and transient inverse technique developed to investigate thermal history of resistance spot welding.     

Study of Phase Change and Supercooling in Micro-Channels by Infrared Thermography

The present work presents a fast and simple new experimental method, designed to enhance the observation and characterization of thermal phenomena at microscale. Supercooling of water was carried out in micro-channels and recorded with a high-frequency infrared camera. The method is based on the coupling of microfluidics, infrared thermography, and inverse techniques. The objective is to extract a maximum of information from the experiment to perform advanced characterization of the system. First, a thermal modeling of such a system was written, then the image processing from infrared recordings allowed estimating the thermal properties (diffusivity) and the source term (energy released by the phase change). The novelty of the approach is the ability of measuring the heat released by the phase change and using this displacement to calculate the ice front propagation velocity and thermal properties. This method is appropriate for many other applications and is mainly devoted to the characterization of fluids during phase change at microscale.     

A diffusion-based approximate model for radiation heat transfer in a solar thermochemical reactor

An approximate numerical method for fast calculations of the radiation heat transfer in a solar thermochemical reactor cavity is formulated based on the separate treatment of the solar and thermal radiative exchange by the diffusion approach. The usual P₁ approximation is generalized by applying an equivalent radiation diffusion coefficient for the optically thin central part of the cavity. The resulting boundary-value problems are solved using the finite element algorithm. The accuracy of the model is assessed by comparing the results to those obtained by a pathlength-based Monte Carlo simulation. The applicability of the proposed model is demonstrated by performing calculations for an example problem, which incorporates a range of parameters typical for a solar chemical reactor and the spectral radiative properties of polydisperse zinc oxide particles.     

Optimization analysis of the thermal cloaking problem for a cylindrical body

The inverse heat transfer problem associated with constructing multilayer material shells cloaking the presence of a cylindrical body in an externally applied temperature field is studied. As the original mathematical model, the steady-state heat equation for an anisotropic shell is used. With the help of the optimization method, this inverse problem is reduced to the corresponding control problem. A numerical algorithm of its solution based on the particle swarm optimization is proposed, and the results of numerical experiments are discussed.     

Measuring flow resistivity of porous materials at low frequencies range via acoustic transmitted waves

An acoustic transmissivity method is proposed for measuring flow resistivity of porous materials having rigid frame. Flow resistivity of porous material is defined as the ratio between the pressure difference across a sample and the velocity of flow of air through that sample per unit cube. The proposed method is based on a temporal model of the direct and inverse scattering problem for the diffusion of transient low-frequency waves in a homogeneous isotropic slab of porous material having a rigid frame. The transmission scattering operator for a slab of porous material is derived from the response of the medium to an incident acoustic pulse. The flow resistivity is determined from the solution of the inverse problem. The minimization between experiment and theory is made in the time domain. Tests are performed using industrial plastic foams. Experimental and numerical results, and prospects are discussed.     

加载功率与壳温对AlGaN/GaN高速电子迁移率晶体管器件热阻的影响

结温是制约器件性能和可靠性的关键因素, 通常利用热阻计算器件的工作结温. 然而, 器件的热阻并不是固定值, 它随器件的施加功率、温度环境等工作条件的改变而变化. 针对该问题, 本文以CREE公司生产的高速电子迁移率晶体管(HEMT)器件为研究对象, 利用红外热像测温法与Sentaurus TCAD模拟法相结合, 测量研究了AlGaN/GaN HEMT器件在不同加载功率以及管壳温度下热阻的变化规律. 研究发现: 当器件壳温由80 ℃升高至130 ℃时, 其热阻由5.9 ℃/W变化为6.8 ℃/W, 增大15%, 其热阻与结温呈正反馈效应; 当器件的加载功率从2.8 W增加至14 W时, 其热阻从5.3 ℃/W变化为6.5 ℃/W, 增大22%. 对其热阻变化机理的研究发现: 在不同的管壳温度以及不同的加载功率条件下, 由于材料导热系数的变化导致其热阻随温度与加载功率的变化而变化.     

Estimating thermal transport in deep X-ray lithography with an inversion method

This study proposes a general methodology for estimating the depth profile of the heat source of the thermal transport system during deep X-ray lithography. The exposure process in a lithography system is considered as an inverse heat conduction problem with an unknown heat source. The conjugate gradient method is used to solve the inverse problem. Numerical results confirm that the method proposed herein can accurately estimate the heat source even involving the inevitable measurement errors. Furthermore, this methodology can also be applied to estimate the local distribution of temperatures when using scanning thermal microscopy (SThM) to microthermally machine materials and will contribute to increase the quality of microthermally machined products. In addition, a thermomechanical data-storage system, which utilizes a resistively heated atomic-force-microscopy (AFM) cantilever tip to read and write data bits, can also adopt this inverse methodology to control the temperature of a polymer substrate.     

Thermal diffusivity measurements of spherical samples using active infrared thermography

This work presents a method for measuring the thermal diffusivity of spherical samples using active infrared thermography. The principal novelty of this method lies in the deduction of an analytical model to obtain the spatial and temporal distribution of temperature in spherical samples. The model is obtained from the classical theory of heat conduction or the 3D heat diffusion equation. In order to analyze the behavior of the model, an active infrared thermography is used in order to monitor the spatial and temporal temperature distribution. Three different materials are used as spherical samples and they are heated by radiation increasing this way its temperature. The recorded data is fitted to the model by adjusting the diffusivity parameter. The results of the diffusivity values obtained using this model are consistent with those obtained from a standard thermal properties analyzer.