Thermal Diffusivity Of Materials Research Articles

Laser-spot lock-in thermography with a non-research-grade thermal camera: From thermal diffusivity measurements to visualizing free convection

The widespread use of Lock-in Thermography (LIT) has been hindered by the high cost of the thermographic cameras available for such applications. We show that a low-cost thermographic camera (we used the QIANLI MEGA-IDEA Super iR Cam 2S) can be straightforwardly used in advanced applications, particularly for measuring the thermal diffusivity of solids using the Laser Spot LIT Technique and the Slope Method. The results of measurements performed in test samples were compared with those obtained using the FLIR SC5000 research-grade camera, whose usefulness for this kind of application has been demonstrated before. The Super iR Cam 2S is almost two orders of magnitude cheaper than the FLIR SC5000 but has an inferior performance. However, a good agreement between the thermal diffusivities of materials, with values between 10 and 100 mm2/s, obtained using both cameras and those reported in the literature demonstrates inexpensive instrumentation usefulness for conducting high-quality research work. The visualization of cracks between samples with different diffusivities was demonstrated too. For the first time, we also show the possibility of using Laser Spot LIT to directly observe a convective effect in a fluid layer closest to the sample’s surface.

Measurement of the thermal properties of materials by the hot plate method considering the convection coefficient around the device

Measurement methods based on the hot plate did not consider the exchange coefficients around the device. These measurements were based on analytical solutions of the unidirectional (1D) heat conduction equation. This paper describes an experimental method to simultaneously estimate the thermal conductivity and thermal diffusivity of materials, taking into account the exchange coefficient around the device. This method is based on the measurement of the temperature at the center of a heating element inserted between two samples, with the unheated surface of the samples being kept constant. As a first step, a two-dimensional (2D) heat transfer model was solved using the finite volume method (FVM) and implemented in MATLAB®. To validate the computational code developed with the 2D model, a comparative study was performed with the full 3D model simulated in COMSOL Multiphysics. This 2D model was then used to perform a sensitivity analysis on the different thermophysical parameters and the convection coefficient. The Levenberg–Marquardt algorithm was used to calculate the estimates. An experimental study was performed on several material samples, leading to an estimation of their thermal properties. In addition, a comparative study based on the asymmetric hot plate (AHP) method was performed, and the results obtained showed a relative error of less than 4%. Therefore, it can be concluded that the proposed model is validated to provide reliable thermophysical properties of materials.

Heat Transfer Augmentation Using Duplex and Triplex Tube Phase Change Material (PCM) Heat Exchanger Configurations

The significance of latent heat thermal energy storage is more substantial when compared to sensible energy storage due to its higher energy storage capability. In this paper, heat transfer enhancement techniques for melting (charging) and solidification (discharging) by using external fins and internal–external fins for a phase change material (PCM) in duplex and triplex tube heat exchangers (DTHX and TTHX) are investigated numerically. A two-dimensional analysis is carried out using ANSYS Fluent for various configurations. Moreover, the effect of different critical parameters, number of fins, fin length, fin thickness, and the heat exchanger tube material are evaluated in terms of the total time of complete phase change of the PCM. Four cases are investigated; cases 1 and 2 are based upon a DTHX while cases 3 and 4 are TTHXs. By considering case 1 as a reference case, it is found that case 2 and case 3 reduce the total melting time by 48.76% and 90.12%, respectively. Case 4 achieves the shortest time for complete melting of the PCM, and the total melting time is decreased by 92%. Solidification behaviour for all four cases is also investigated. The novel configurations increase (doubled) the supply of heat transfer fluid (HTF) while at the same time significantly enhance the melting/solidification characteristics for all the cases without disrupting the convectional currents during phase change of the PCM. Tube materials with different thermophysical properties are also investigated with the heat transfer rate and melting time significantly improved with a high thermal diffusivity material. Moreover, the heat transfer is found to increase with fin length and fin thickness.

Thermal diffusivity estimation in metallic alloys using a one-dimensional flux-based thermography

This work proposes a simple one-dimensional (1D) thermography technique to estimate a metallic alloy thermal diffusivity by employing a uniform flux-based heating source. A theoretical model is developed and validated to account for the sample dimensions, material thermal properties and the sample initial and boundary conditions. These conditions include the sample initial temperature, the effective convection heat transfer coefficient with the surrounding environment and the uniform heat flux supplied to the sample. The adopted theoretical model is tested against simulated thermal measurements to retrieve a sample unknown boundary conditions along with the material thermal diffusivity of interest following the Nelder-Mead optimization approach, which offers a high flexibility to the proposed technique. This technique is experimentally validated over a tempered aluminum alloy (Al-2024 T4) of relatively high thermal diffusivity and a one-order of magnitude lower thermal diffusivity annealed stainless-steel alloy (SS-304) delivering an uncertainty lower than 2 % in material thermal diffusivity estimation.

Defect measurement limits using flash thermography with application to additive manufacturing

This paper assesses the possibility of measuring small, shallow defects in low thermal diffusivity materials with existing pulse thermography techniques. Defects like these are commonly introduced in additive manufacturing (AM), and their presence can cause inconsistencies in the mechanical properties of the final part. An axisymmetric, numerical model was created to simulate the heat transfer within these low thermal diffusivity structures during flash thermography. Deviations from the ideal conditions commonly used in flash thermography models such as conduction across the flaw and in-depth absorption of the incident pulse were included in the model and their effects on defect measurability were investigated. These nonideal conditions (in addition to free convection) introduce depth measurement errors of >10% even for large aspect ratio defects (such as delaminations). The figures provided quantify the amount of error associated with these individual parameters and can be used to determine if a given thermography system is ideal. The simulation results also demonstrate that traditional 1D thermography models may be used to approximate the depth of a defect with negligible error if the defect has an aspect ratio greater than 6. Smaller defects may also be measured with minimal error. If flash thermography were performed during AM to identify the layers where defects most likely reside, the measurement limits would be even less restrictive.

Simplification and Improvement of the Efficiency of Testing Thermophysical Properties of Facade Composite Materials

The choice of highly efficient materials for the opaque parts of the building facades is the most effective factor in increasing its thermal protection. A decrease in the coefficient of U-value of opaque parts of a building directly affects the consumption of both thermal energy and the energy demand for cooling. Two-component or multi-component composite materials today occupy a large place in modern construction. This article analyzes the methodology for testing the thermophysical properties of these materials, reveals a new approach to determine to it, taking into account the links between the thermal conductivity and the thermal diffusivity of materials. The article analyzes the relationship between buildings and the surfaces of the outer envelope and the dependence of the energy efficiency index of the building.

HIGHLY RESPONSIVE AND ACCURATE TEMPERATURE MEASUREMENTS IN ORTHOGONAL CUTTING THROUGH INNOVATIVE SINGLE LEG THERMOCOUPLE

During machining, most of the thermal sources are resulting from the conversion of mechanical energy during the chip shearing mechanism and the intense frictions between the tool, the chip, and the machined surface. Thermal gradients are high and localized, especially for low thermal diffusivity material like titanium alloys. Moreover, heat rates come close to few million degrees per second in the shearing zones. Even though several authors performed thermography techniques to determine the temperature distribution into the machined material, their results remain underestimated due to experimental limitations. In this research, a new temperature measurement technique has been developed based on a single wire thermocouple to measure at one exact location both the temperature and the heating rate during orthogonal cutting test. The tests are performed with uncut chip thicknesses of 0.020 mm and 0.100 mm and with a cutting speed of 120 m/min with two different cutting angles. Results are discussed and compared to prior studies.

Measurement Method of Thermal Diffusivity of the Building Wall for Summer and Winter Seasons in Poland

The thermal diffusivity of building materials is an extremely important parameter influencing the subsequent thermal comfort of building users. By definition, thermal diffusivity describes how quickly heat from a hot source can flow through a material. Therefore, this parameter includes both the thermal conductivity and the heat capacity of the material. This parameter is often neglected in heat-related calculations which, in the case of dynamic problems, leads to unreliable results. It should be taken into account that heat flows through all materials at a finite speed. On the other hand, knowing the correct thermal diffusivity value of building materials, it is possible to accurately determine the internal parameters in rooms over time. There are several methods for determining thermal diffusivity, most of which are destined to determine this property in laboratories. The aim of the present research is to show how the thermal diffusivity of materials can be determined in existing buildings. The presented method can be used to determine more real thermal parameters used for thermal calculations in buildings, for example, during energy audits or when calculating the demand for cooling for air conditioning or heat for space heating. This research presents the results for a 60 cm brick wall. Thermal diffusivity was determined for specific summer and winter days—most representative of the whole year. This research has shown that the applied method should be used in the summer period, due to the fact that the wall has greater temperature fluctuations. The obtained results are comparable with the previously mentioned laboratory methods. However, due to the fact that the materials analyzed on the spot, the results are more reliable, and also take into account changes in the value of thermal diffusivity resulting from the use of binders, inaccuracies in joining and external layers made of other materials.

Measurement of thermal properties of bulk materials and thin films by modulated thermoreflectance (MTR)

Thermal conductivity and thermal diffusivity of materials must be known at high accuracy for thermal engineering applications, in order to understand energy dissipation in devices and engines. Thermal conductivity libraries can be unreliable since their reported values may not reflect the complexities of the samples under investigation, and new materials may not yet be listed. Over the past three decades, modulated thermoreflectance (MTR) has emerged and evolved as a reliable, non-contact, and noninvasive technique to measure the thermal properties of a wide range of bulk and thin film materials and their interfaces. This Tutorial discusses the basis of, and recent advances in, the MTR technique, and its applications in the thermal characterization of a variety of materials. MTR experiments use two micrometer-wide laser beams. Experimental data (amplitude and phase vs the offset between the pump and the probe) give information about heat diffusion around the heat source along several tens of micrometers. Heat diffusion equations are used to fit the experimental data and extract the required thermal properties. Importantly, best fit procedures are not always needed because some analytical approximations provide evidence of the required thermal properties. We present many examples (bulk materials, layered sample, etc.) which illustrate this.

Analytical solutions of heat storage and heat transfer performance of parallel‐plate regenerators in Stirling cycle

Parallel-plate regenerators (PPR), with flow resistance lower than traditional wire-mesh regenerators, can improve the thermal efficiency of Stirling engines (SEs). However, as working frequency or plate thickness increase, the heat cannot penetrate into the plate effectively, resulting only the surface part of the plates to have substantial temperature variation, while the internal part fails to store and exchange heat energy. In order to obtain high performance of PPR, the heat storage efficiency and the heat transfer coefficient, as well as their influential factors, are theoretically studied. Three parameters are found to play an important role, which are working frequency, plate thickness, and thermal diffusivity of materials. Their roles can be represented by a dimensionless parameter as a whole, which is the relative thickness, e. By the critical value of relative thickness, ecr = 2.4, two distinct working conditions can be divided, thermally penetrated condition as e < ecr and thermally non-penetrated condition as e > ecr. Under thermally penetrated condition, the heat storage efficiency is high, and the heat transfer coefficient is high enough when e > 1.6, while under thermally non-penetrated condition, the heat storage efficiency is low. In conclusion, by comprehensively considering the heat storage efficiency and heat transfer coefficient, it is recommended that the relative thickness e should be chosen within the range [1.6, 2.4]. And the optimal working frequency, plate thickness, and suitable material can be determined accordingly.

Determination of thermal diffusivity of the material by numerical-analytical model of a semi-bounded body

A mathematical description of the material thermal diffusivity aт in a semi-bounded body is proposed with a relatively simple algorithm for its numerical and analytical by solving the inverse problem of thermal conductivity. To solve the problem, it is necessary to obtain the temperature values of the unbounded plate as a result of a thermophysical experiment. A plate can be conditionally considered as a semi-bounded body as long as the Fourier number Fo ≤ Foк (Foк ≈ 0.04–0.06). It is assumed that the temperature distribution over cross-section of the heated layer of the plate R is sufficiently described by a power function whose exponent depends linearly on the Fourier number. A simple algebraic expression is obtained for calculating aт in the time interval ∆τ from the dynamics of temperature change T(Rп , τ) of a plate surface with thickness Rп heated under boundary conditions of the second kind. Temperature of the second surface of the plate T(0, τ) is used only to determine the time of the end of experiment τк. The moment of time τк, in which the temperature perturbation reaches the adiabatic surface x = 0, can be set by the condition T(Rп , τк) – T(0, τ = 0) = 0,1 K. The method of approximate calculation of dynamics of changes in depth of the heated layer R by the values of Rп , τк , and τ is proposed. Calculation of a т for the time interval ∆τ is reduced to an iterative solution of a system of three algebraic equations by matching the Fourier number, for example, using a standard Microsoft Excel procedure. Estimation of the accuracy of a т calculation was made by the test (initial) temperature field of the refractory plate with the thickness Rп = 0.05 m, calculated by the finite difference method under the initial condition T(x, τ = 0) = 300 (0 ≤ x ≤ Rп) at radiation-convective heating. The heating time was 260 s. Calculation of aт, i was performed for 10 time moments τi + 1 = τi + Δτ, τ = 26 s. Average mass temperature of the heated layer for the whole time was T = 302 K. The arithmetic-mean absolute deviation of aт(T = 302 K) from the initial value at the same temperature was 2.8 %. Application of the method will simplify the conduct and processing of experiments to determine the thermal diffusivity of materials.

Thermal accumulation at kilohertz repetition rates inside fused silica under ultrafast laser irradiation.

Thermal accumulation effect has proved to reduce ablation threshold and improve the ablation rate during multi-pulse ultrafast laser ablation. It was widely believed that this effect cannot be triggered until the laser repetition rate is raised to the megahertz range. In this Letter, we experimentally discover strong thermal accumulation in fused silica at kilohertz repetition rates and its significant contribution to enhance ablation rate. It is found that the threshold repetition rates to trigger thermal accumulation are intrinsically determined by material thermal diffusivity and insensitive to ambient conditions. We observe two-fold enhancement of the ablation rate and clearly discriminate the contribution from thermal and non-thermal accumulation effects by 35% and 50%-70%, respectively. A multi-physics model is developed to assist the understanding of the process. This Letter promotes the fundamental understanding of thermal/non-thermal accumulation effects and opens the door to low-repetition-rate thermal accumulation for low thermal diffusivity materials.

Determination of Thermal Diffusivity of Material by the Numerical-Analytical Model of a Semi-Bounded Body

A mathematical description of a material’s thermal diffusivity ae in a semi-bounded body is proposed with a relatively simple algorithm for its numerical and analytical calculation by solving the inverse problem of thermal conductivity. To solve the problem, it is necessary to obtain the temperature values of the unbounded plate as a result of a thermophysical experiment. A plate can be conditionally considered as a semi-bounded body as long as Fourier number Fo ≤ Foe (Foe = 0.04–0.06). It is assumed that the temperature distribution over a cross-section of the heated layer of the plate with thickness R is sufficiently described by a power-like function whose exponent depends linearly on the Fourier number. A simple algebraic expression is obtained for calculating ahc in time interval Δτ from the dynamics of temperature change T(Rp, τ) of a plate surface with thickness Rp heated at boundary conditions of the second kind. Temperature T(0, τ) of the second surface of the plate is used only to determine end time τe of the experiment. Moment of time τe, at which the temperature perturbation reaches adiabatic surface x = 0, can be set by the condition T(Rp, τe) – T(0, τ = 0) = 0.1 K. An approximate method of calculating the dynamics of changes in depth of heated layer R by values of Rp, τe, and τ is proposed. The calculation of ahc for time interval Δτ is reduced to an iterative solution of a system of three algebraic equations by matching the Fourier number, for example, using a standard Microsoft Excel procedure. Estimation of the accuracy of calculation of ahc at radiation-convective heating was performed using the initial temperature field of the refractory plate with thickness Rp = 0.05 m, calculated by the finite difference method under initial condition T(x, τ = 0) = 300 (0 ≤ x ≤ Rp). The heating time was 260 s. Calculation of ahc, i has been performed for ten time moments τi + 1 = τi + Δτ, Δτ = 26 s. Average mass temperature of the heated layer for the entire time was τe T = 302 K. The arithmetic-mean absolute deviation of ae (T = 302 K) from the initial value at the same temperature was 2.8%. Application of the method will simplify conducting and processing experiments to determine thermal diffusivity of materials.

Measurement of local thermal contact resistance with a periodic heating method using microscale lock-in thermography

This study proposes a new method for measuring thermal contact resistance using lock-in thermography. By using lock-in thermography with an infrared microscope, the dynamic and spatially resolved temperature behavior of the contact interface was visualized on a microscale with one measurement. In addition, a new thermal contact resistance measurement principle was constructed after solving the three-dimensional thermal conduction equation in the cylindrical coordinates by considering a periodic heat source with a Gaussian intensity distribution and the relative position of the heating point to the sample edge, in the presence of thermal resistance at the contact interface. Consequently, the discontinuous behaviors of the temperature wave, amplitude, and phase lag at the contact interface were observed on a microscale. From that discontinuity, the local thermal contact resistance was analyzed as a fitting parameter by matching the theoretical curve to the measured amplitude and phase lag. Furthermore, the simultaneous analysis of the material thermal diffusivity was demonstrated and the validity of the measurements was confirmed.

Simulation of the thermophysical experiment with remote control to determine the quality properties of the object

Simulation of the thermophysical experiment for determination of thermophysical properties of insulating materials using the impulse method in the influence of a linear heat source on the test material and mathematical model of the process of thermophysical measurement is fulfilled in MathCad software. As a result of the simulation the optimum operating parameters for thermophysical measurements are determined: optimum operating parameters and thermal pulse repetition time interval in minimum measurement error of coefficients of thermal conductivity and thermal diffusivity of materials. The structure of an intelligent information-measuring system of remote control is offered that realizes the structure of an intelligent information-measuring system of remote control using methods of artificial intelligence that let us increase the efficiency and accuracy of determining the thermophysical properties of insulating materials.

Термический коэффициент полезного действия при электронно-лучевой обработке конструкционных материалов

The results of studying the interrelation that links the electron beam welding (EBW) thermal efficiency, thermophysical properties of the material, and welding speed are presented. Thermal efficiency was determined by a combined experimental and computational method based on measuring the penetration areas and evaluating the energy expenditure for melting with the known welding mode parameters. The study was carried out on a series of microsections prepared from welded joints of 5V titanium alloy, grade 40Kh13 steel, D16 duralumin, BrKh1Tsr bronze, and TsM2A molybdenum. It is shown that in EBW with deep penetration, its thermal efficiency increases with increasing the welding speed and decreasing the welded material thermal diffusivity. The experimental dependence of the thermal efficiency on the welding speed is approximated with sufficient accuracy by a logarithmic curve. Thus, in increasing the EBW speed from 20 to 120 m/h, the thermal efficiency increases from 54% to 67% for 5V titanium alloy, from 46% to 63% for 40Kh13 steel, and from 18% to 40% for D16 duralumin. The thermal efficiency in welding bronze (at an EBW speed of 30 m/h) and in welding TsMA molybdenum (at a speed of 20 m/h) was 15% on the average. An analysis of the thermal efficiency curve versus the dimensionless parameter vd/2a has shown that the thermal efficiency for all materials can be approximated by a single logarithmic dependence. An empirical relationship is proposed, using which the welding thermal efficiency can be determined as a function of welding speed and material thermal diffusivity.

A New Rapid Method of Determining the Thermal Diffusivity of Materials and Finished Articles

A new nonstationary nondestructive rapid method of determining the thermal diffusivity of materials and finished articles has been suggested and tested experimentally without cutting out samples from them and preparation of the latter. The method is based on using an infrared imager to video record the infrared radiation of the surface originating as a result of its local stepwise heating by, say, a focused laser beam. Subsequent computer processing of the patterns of the nonstationary thermal field by the developed models and algorithms makes it possible to determine the thermal diffusivity of a material or an article in the case of one-sided access to them.

Heat engineering properties of energy-efficient material based on plant additives (dried hogweed)

Introduction. The articlecompares the heat engineering properties of construction materials: high-strength concrete, ordinary concrete, and two concrete samples containing a vegetable additive, dried stem fragments of hogweed (Heracleum sosnowskyi). The first sample contains the additive in the crushed form, while the second contains the additive in the form of tubes. The research to estimate heat conduction in the four samples under unsteady conditions.&#x0D; Materials and methods. The thermal diffusivity of two samples with plant additive is determined using a comparative physical experiment. When determining the thermal properties, the accumulating component of the enclosure is taken into account what leads to a long delay in the temperature variation over time. DS18B20 digital temperature sensors connected to an Arduino Uno board were used to measure the temperature. The experiment was carried out in the Challenge 250 chamber at an initial temperature of 20 ºС. Then the chamber temperature was lowered to 0 ºС within 30 minutes. In the next period, the temperature dropped from 0 ºС to –10 ºС. The data obtained are of interest for improving the energy efficiency of buildings.&#x0D; Results. The experiments showed that the tubular-hogweed sample is close to high-strength concrete by its heat engineering properties. The material thermal diffusivity affects the amplitude of temperature fluctuations and the accumulating capacity of the wall.&#x0D; Conclusions. It is established experimentally that at lower thermal diffusivity of a hogweed-containing sample, the amplitude of temperature fluctuations increases. This indicates a greater time lag, just as in the case of a high-strength concrete sample. When using the hogweed as an additive, we obtain a heat-resistant minimum-cost material for enclosures. Considering an issue of the energy efficiency in construction and existing additives to concrete, the researchers have reached the conclusion that it is efficient to use the dried hogweed as the plant additive.

Determination of the Thermal Diffusivity of Materials by a Nondestructive Express Method with the Use of Step-By-Step Local Heating of the Surface and High-Speed Thermography

We propose an express method for measuring the thermal diffusivity of the materials of products that does not require cutting out of the samples of special shapes from the homogeneous mass of the product. The method is based on the local heating of the surface by step-function pulses, e.g., by a focused laser beam with simultaneous high-speed thermography performed with the help of a thermal imager. The subsequent processing of a radially symmetric picture of isotherms carried out by using an original software program makes it possible to find the thermal diffusivity of macrohomogeneous materials with an error lower than 10%. As an example, we present the results of measurements of the thermal diffusivity of the ZrO2:MgO ceramics, PhenoCure Powder composite based on phenol formaldehyde resin, and TPK-5PD polymeric-rubber material.

Impact of evaporative cooling due to wetting of urban materials on local thermal comfort in a street canyon

The choice of urban materials strongly influences the outdoor thermal comfort through their role in the absorption, transport and storage of heat and moisture. The present study investigates the level of evaporative cooling reached after rain events of varying intensities using urban materials with distinct thermal and moisture transport properties. Further, the resulting impact on thermal comfort is studied. The local climate in a street canyon is modeled using a fully-integrated model that couples computational fluid dynamics (CFD) simulations of wind flow with the three-dimensional heat and moisture transport (HAM) in porous urban materials. Wetting due to rain deposition on each surface is calculated using an Eulerian multiphase wind-driven rain model. The results show that an increase in the rainfall intensity extends the effective duration of evaporative cooling, but does not change the reduction in maximum surface temperatures. The improvement in the thermal comfort in the street canyon depends on a combination of factors such as moisture permeability, moisture capacity and thermal diffusivity of materials.