Observation of second sound in graphite over 200 K

Second sound refers to the phenomenon of heat propagation as temperature waves in the phonon hydrodynamic transport regime. We directly observe second sound in graphite at temperatures of over 200 K using a sub-picosecond transient grating technique. The experimentally determined dispersion relation of the thermal-wave velocity increases with decreasing grating period, consistent with first-principles-based solution of the Peierls-Boltzmann transport equation. Through simulation, we reveal this increase as a result of thermal zero sound—the thermal waves due to ballistic phonons. Our experimental findings are well explained with the interplay among three groups of phonons: ballistic, diffusive, and hydrodynamic phonons. Our ab initio calculations further predict a large isotope effect on the properties of thermal waves and the existence of second sound at room temperature in isotopically pure graphite.

Thermal Effect on Formation Stability Due to Heterogeneity

This study evaluates physical and chemical changes induced by high thermal gradients on the formation and their impact to the stability. The heat sources that effect the formation’s stability are varied, including drilling (due to drilling bit friction), perforation, electromagnetic heating (laser or microwave), and thermal recovery or stimulation (steam, resistive heating, combustion, microwave, etc.). This study uses an integrated approach to characterize rock heterogeneity and mapping heat propagation from different heat sources. The information obtained from the study is vital to accurately design and enhance well completion and stimulation This is an integrated analysis approach combining different advanced characterization and visualization techniques to map heat propagation in the formation. Advanced statistical analysis is also used to determine the key parameters and build fundamental prediction algorithms. Characterization on the samples was performed before, during, and after the exposure to thermal sources; it comprised thin-section, high speed infrared thermography (IR), differential thermal analysis and thermogravimetric analyzer (DTA/TGA), scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), uniaxial stress, and autoscan (provide hardness, composition, velocity, and spectral absorption). The results are integrated, and machine learning is used to derive a predictive algorithm of heat propagation and mapping in the formation with reference to the key formation variables and heterogeneity distribution. Rock heterogeneity affects the rate and patterns of heat propagation into the formation. Within the rock sample, minerals, laminations, and cementations lead to a heterogeneous, and sometimes anisotropic, distribution of thermal properties (thermal conductivity, heat capacity, diffusivity, etc.). These properties are also affected by the rock structure (porosity, micro-cracks, and fractures) and saturation distribution. The results showed the impact of heat on the mechanical properties of the rocks are due to clays dehydration, mineral dissociations, and micro cracks. High speed thermal imaging provides a unique visualization of heat propagation in heterogeneous rocks. Statistical analysis identified key parameters and their impact on thermal propagation; the output was used to build a machine learning algorithm to predict heat distributions in core samples and near-wellbore. Characterizing rock properties and understanding how heterogeneity modifies heat propagation in rocks enables the design of optimal completion and stimulation strategies. This paper discusses how advanced characterization and analysis, combined with novel algorithms, can improve this understanding, and unleash innovation and optimization. The data and information gathered are critical to develop numerical models for field-scale applications.

Formation Heterogeneity Effect on Steam Injection and Propagation

The objective of this work is to evaluate and understand steam injection in heterogonous formation utilizing a state-of-the-art experimental apparatus. Heat transfer and efficiency for steam injection are evaluated in heterogeneous formation and compared with homogenous formation. The information obtained from the apparatus provided the key in designing effective steam injection for optimized recovery in heterogeneous formations. This paper presents several successful experimental works and proposes solutions to overcome the challenges produced from heavy oil reservoirs. The technology utilizes advanced thermal apparatus to improve heat penetration depth into the formation and efficiency of the thermal heating. Steam is the most used technology due to its high latent heat capacity, cost and maturity. Steam injection should be carefully planned to ensure the injectivity to the target. Heterogeneity adds to the complexity of the operation, as the steam will propagate in different orientations. This study provides the key element to understand steam propagation to maximize the recovery efficiency. The experiments were carried out using heavy oil apparatus, which is designed to accurately simulate reservoir conditions. It measures one meter in length by one meter in width by one and a half meter in height. It has 65 thermocouples, 24 acoustic transducers, 9 vertical wellbores, 9 horizontal wellbores; these data are used for modeling and simulation. The apparatus can use sand or blocks. Thermal technology is very effective to mobilize heavy and viscous oil; steam injection has been successfully and widely deployed due to its reasonable cost, maturity, and efficient thermal transfer to reservoir fluids. Understanding the formation is vital to ensure successful steam-based stimulation, especially in heterogonous reservoirs. To this end, an apparatus was designed to evaluate steam injection in heterogonous formations. This is one-of-a kind studies that evaluates heterogeneity effect at a large scale and provides detailed analysis. First, steam is injected in homogenous formation to establish a baseline of heat propagation in formation. Second, the apparatus is filled in layers resembling a heterogonous formation, and steam is injected at same conditions (i.e., wellbore depth and injection rate and pressure). The device collected a real-time temperature map using 65 thermocouples. 3D graph and animations are plotted to visualize and evaluate the pattern and trend of steam propagation in both homogenous and heterogeneous formations. The apparatus is uniquely deigned to evaluate different scenarios that simulate the field and wellbores more accurately. Due to its volume (one cubic meter), the device is the largest apparatus in literature, and flexibility, the device enables the replication of a heterogeneous formation. The amount of data and information gathered, make the apparatus unique and provide key elements to drive successful steam injection operations.

Three-dimensional periodic thermoelastichydrodynamic modeling of hydrodynamic processes of a thrust bearing

The article presents the basic principles of three-dimensional mathematical modeling of the operation of a thrust plain bearing with fixed pads of the compressor. The model is based on the periodic thermoelastichydrodynamic (PTEHD) theory which allows calculating the temperature at the inlet to the pad and considering the complete thermal pattern. A description of the main provisions of the numerical implementation is given. In the stationary mode of the bearings operation, using the Sm2Px3Tx program, numerical experiments were carried out aimed at studying different boundary conditions to the Reynolds equation, the physics of the hydrodynamic process in the lubricating and boundary films of the bearing and the heat propagation in the body of the pad and thrust collar.

Calculation of aerial contact wire volume heated by a moving electric ARC

The article is devoted to the study of the problem of electrified railways electric locomotive current collectors’ interaction violation with an aerial contact wire, accompanied by the occurrence of an electric arc. Current collection disorders, accompanied by arcing, have a destructive effect on the contacting elements, causing their thermal erosion. Places where current collection violations occur should be registered and diagnosed in a timely manner in order to prevent an emergency situation associated with burnout or breakage of the aerial contact wire. In order to further develop the system of technical diagnostics for current collection disorders accompanied by arcing, it is necessary to study the nature and parameters of the processes occurring during these violations. The main part of the article is devoted to the study of the area of heating the aerial contact wire by a moving electric arc. The characteristics describing the heated area are geometric parameters, including the area in the cross section of the wire and the volume of the area. To obtain the necessary data, the method of heat sources was applied, which is a mathematical model that describes the process of heat propagation in an aerial contact wire. The electric arc arising when the current collector breaks off is considered as a mobile heat source distributed over the aerial contact wire surface in the heating zone and having a limited radius of the heating spot. Based on the heat propagation process peculiarities inside the aerial contact wire, a method for calculating the volume of the heated area bounded by an isothermal surface of a certain temperature is presented. The heated area was visualized using the PTC MathCAD.

Through-Plane and In-Plane Thermal Diffusivity Determination of Graphene Nanoplatelets by Photothermal Beam Deflection Spectrometry

In this work, in-plane and through-plane thermal diffusivities and conductivities of a freestanding sheet of graphene nanoplatelets are determined using photothermal beam deflection spectrometry. Two experimental methods were employed in order to observe the effect of load pressures on the thermal diffusivity and conductivity of the materials. The in-plane thermal diffusivity was determined by the use of a slope method supported by a new theoretical model, whereas the through-plane thermal diffusivity was determined by a frequency scan method in which the obtained data were processed with a specifically developed least-squares data processing algorithm. On the basis of the determined values, the in-plane and through-plane thermal conductivities and their dependences on the values of thermal diffusivity were found. The results show a significant difference in the character of thermal parameter dependence between the two methods. In the case of the in-plane configuration of the experimental setup, the thermal conductivity decreases with the increase in thermal diffusivity, whereas with the through-plane variant, the thermal conductivity increases with an increase in thermal diffusivity for the whole range of the loading pressure used. This behavior is due to the dependence of heat propagation on changes introduced in the graphene nano-platelets structure by compression.

Thermal Analysis of Electric Machines for Combined Stirling Engine-Generator Performance

The performance of an electric machine depends on its ability to resist rising internal temperature and ambient temperature. In particular when it is a combination with a heat engine, it is essential to know the thermal characteristics of the electric machine in connection with its operating environment to decide which type of machine for a better result. This work will make a comparative thermal study of three types of generators namely: the permanent magnet generator (PMSG), the squirrel cage asynchronous generator (SCIG) and the switched reluctance generator (SRG), all driven by Stirling engine. The method involves solving the heat propagation equation to determine the thermal resistance network for each machine. The resolution of the network combined with the finite element method will allow a comparison of the temperature rise and its effect on the performance of each machine. The simulation results show that the temperature of the PMSG windings stabilizes at 430 K while that of the others stabilizes at 373 K and 346 K respectively. However, when comparing the performances for the specifications of this work (i.e., produce minimum electric power of 2kW at low speed generated by the Stirling engine), PMSG is the one that fulfil all the requirements. For the use of this machine for the generator set, it will be necessary to use magnets of types GNS-39EH whose operating temperature is approximately 473K (200 ° C) with magnetic induction of 1.22 T. Keywords: choice of machines, thermal network, Finite Element Method, machine’s performances, Stirling engine.

Thermal Analysis of Electric Machines for Combined Stirling Engine-Generator Performance

The performance of an electric machine depends on its ability to resist rising internal temperature and ambient temperature. In particular when it is a combination with a heat engine, it is essential to know the thermal characteristics of the electric machine in connection with its operating environment to decide which type of machine for a better result. This work will make a comparative thermal study of three types of generators namely: the permanent magnet generator (PMSG), the squirrel cage asynchronous generator (SCIG) and the switched reluctance generator (SRG), all driven by Stirling engine. The method involves solving the heat propagation equation to determine the thermal resistance network for each machine. The resolution of the network combined with the finite element method will allow a comparison of the temperature rise and its effect on the performance of each machine. The simulation results show that the temperature of the PMSG windings stabilizes at 430 K while that of the others stabilizes at 373 K and 346 K respectively. However, when comparing the performances for the specifications of this work (i.e., produce minimum electric power of 2kW at low speed generated by the Stirling engine), PMSG is the one that fulfil all the requirements. For the use of this machine for the generator set, it will be necessary to use magnets of types GNS-39EH whose operating temperature is approximately 473K (200 ° C) with magnetic induction of 1.22 T. Keywords: choice of machines, thermal network, Finite Element Method, machine’s performances, Stirling engine.

Comparison of temperature mapping methods for experimental validation of numerical heat transfer analysis of biomaterials and medical devices

Titanium represents an important biomaterial for implantable medical devices. During medical device manufacturing by means of welding, implant structures are partially exposed to high temperatures. Additionally, active implants such as pacemakers can heat up during operation. Therefore, numerical studies of heat propagation within titanium structures represent an essential tool to assess functionality and safety of medical devices. The current study focusses on the development of a method for experimental validation of numerical heat transfer analysis of biomaterials such as titanium. Numerical heat transfer analysis was performed using the software Abaqus. A finite-element model was established including material properties such as density, thermal conductivity und specific heat capacity. Temperature distribution among a locally applied thermal load was calculated. Furthermore, effects such as convection were considered. For validation, an experimental setup was implemented according to the numerical calculation using a local heating tool. Heat propagation in the sample was determined, respectively. Radiation-based heat determination was performed using an infrared thermographic camera aligned parallel to the sample surface. Contact-based heat determination was performed using thermocouples fixed to the surface at defined distances from the point of local heat input. For evaluation of numerical and experimental results, temperature- time curves were compared for five distinct measuring points, respectively. While infrared thermography offers the advantage of non-contact measurements, difficulties may arise from the definition of correct emissivity and challenging sample surface characteristics, such as metallic reflectance and surface texture. The thermocouple-based temperature measurement shows a high sensitivity to local temperature changes, but it is not always suitable due to the influence on the sample by thermocouple fixation. Infrared thermography and thermocouple based temperature measurements represent suitable procedures for experimental validation of numerical heat transfer analysis of titanium. An individual decision for the most suitable method must be made considering the specific sample and its further application.

On Solving Problems of Thermal Conductivity on an Anisotropic Plane with a Weakly Permeable Film

The problem of thermal conductivity on an anisotropic plane (x; y) divided into two halfplanes D1(1 < x < 0; y 2 R) and D2(0 < x < 1; y 2 R) by a weakly permeable film x = 0 is considered at given heat sources and a given initial temperature. The anisotropy ellipses are arbitrary (in magnitude and direction) and are the same at all points of the plane. Using the method of convolution of Fourier expansions, the solution of the problem is expressed in single quadratures through the well-known solution of the classical Cauchy problem on an isotropic plane without a film. The results obtained are of practical interest in the problems of heat propagation and conservation in materials with anisotropic properties (crystalline, fibrous materials), in the presence of a thermal insulation film.