Thermal Insulation Conditions Research Articles

Fracture Mechanics Analysis of Thermoelectric Materials With Equilateral Triangle Holes

The fracture mechanics for thermoelectric materials with equilateral triangle holes subjected to uniform electric current densities and uniform energy fluxes at infinity was studied by means of the complex variable method. The analytic expressions of temperature fields and stress fields were obtained under the boundary conditions of electric insulation and thermal insulation. Effects of the triangle size, the applied electric current density and the energy flux on the thermoelectric material were analyzed. The results show that, the variations of the current density and the triangle size have obvious influences on the annular energy flux, the annular stress and the annular heat flux.

Simulation study of local thermal runaway of 18650 lithium battery module under multi-point thermal trigger

The thermal runaway of lithium power battery is the key problem of battery safety, according to the standard SAE J2464–2009 single point heating key position, the proposed multi-point trigger based thermal runaway of lithium power battery module simulation method and battery module thermal runaway battery monomer ratio PN, thermal runaway trigger time tn two indicators, the thermal runaway rule of 18650 lithium power battery module under different trigger position, trigger points. The results show that considering the external thermal insulation conditions of the power lithium battery module, the geometric angular position in the module is the most dangerous position, and the number of thermal trigger points is positively correlated with PN.

Contact resistance analysis applied to simultaneous estimation of thermal properties of metals

This work aims to present a method for simultaneously estimating the thermal conductivity, k, and specific heat, cp, in samples of stainless steel 304 and cemented carbide by accounting the imperfect thermal contact at the heater–sample interface. A transient one-dimensional heat conduction model with constant thermal properties is used. The metallic sample is placed in the middle of a polyimide heater and a thermal insulator. A constant heat flux is applied on the specimen upside surface and a thermal insulation condition is maintained on the opposing surface, where a type T thermocouple measures the temperature response. Contact resistance is determined and accounted for as a reducing factor on heat flux. Instead of considering a lumped model, a microscopic approach of the contacting regions is used to describe both the surface roughness and the fluid gap. With the intention of guaranteeing simultaneous and reliable estimates for both thermal properties, sensitivity analysis is used to establish heat flux intensity, experiment duration, time step for data acquisition, and other characteristics of the experiments. To simultaneously achieve both thermal properties, the optimization method BFGS (Broyden–Fletcher–Goldfarb–Shanno) is employed to minimize a least-squares objective function comparing experientially measured and numerically calculated temperatures. The numerical solution for the transient conduction problem is determined with COMSOL, which solves the transient heat diffusion problem by discretizing it applying the finite element method (FEM). The linear system of equations is solved using the PARDISO solver and backward differentiation formula (BDF) method. Heat flux is estimated employing the sequential function specification method (SFSM) as a means of verifying the robustness of parameter estimation and experimental procedure. Moreover, uncertainty analysis is performed to confirm the quality of the results obtained. Finally, the uncertainty analysis outcomes and thermal properties estimated are compared with literature.

Thermodynamic modelling of buried transformer substations for dynamic loading capability assessment considering underground heat accumulative effect

The lifetime cycle and secured service of buried transformers are constrained by their thermal insulation and loading conditions. This paper proposes an extended thermal circuit model for direct-buried transformer substations to dynamically evaluate the transformer loading capability. In the proposed model, the underground thermal interactions and energy balances among heat generation, transfer and storage in the transformer substation are represented with nonlinear thermal resistances and capacitances based on thermal-electrical analogies, and then hot-spot temperature (HST) dynamics can be captured from the nodal analysis on this R-C thermal equivalent circuit. Furthermore, the underground thermal accumulative effect is investigated for dynamic loading capability assessment considering the combined impact of heat accumulation in the surrounding soil caused by fluctuating transformer loads during prior operating periods. Finally, the finite element analysis with measured data is implemented for parameter tuning and model verification of the proposed thermodynamic model, and numerical simulations confirm the improvements of the proposed model for the transformer life extension and load management.

Impact of convection and radiation on direct/hybrid heating stability of field assisted sintering

Field assisted sintering (FAST) processes allow a direct transmission of the heating energy to the specimen (through the electric, magnetic fields or the electrical current). FAST allows higher heating rates, faster sintering response and a better control of the sintered microstructures. However, FAST suffers high heating instability in direct heating configurations which generally takes the form of a hot spot. The origin of these hot spots is well known and is correlated to the convective/radiative cooling at the specimen surfaces and the thermal dissipation in the specimen. Nevertheless, the impact of these cooling fluxes evolves with the sample dimensions, thermal insulation, heating rate and hybrid heating conditions and there is not clear quantification of the relative importance of these fluxes in regards to the previous cited heating conditions. In this work we develop a finite element (FE) tool which can easily explore the heating stability of an “Equivalent Thermal Cavity” (ETC). We illustrate the ETC concept by the case study of the microwave sintering of zirconia. We show that the dominant heat transfer is radiative, but the convective fluxes have a high importance for the temperatures homogenization, in particular in the case of a hybrid heating configuration.

The Spectral Properties of a Pre-Stressed Thermoelastic Layer Coupled to a Half-Space

In the framework of the linearized theory of the propagation of thermoelastic waves, a dynamic coupled problem of oscillations of an inhomogeneous medium under the influence of a thermal load oscillating on its surface simulating the action of a frequency-modulated laser beam is considered. The medium is a thermoelastic prestressed layer rigidly adhered to a thermoelastic prestressed half-space. The surface of the layer is free from mechanical stress and insulated outside the area of thermal load acting. At the media boundary thermal insulation condition is considered: ideal thermal contact and. The initial stress state is assumed to be uniform and is created due to preheating and the action of mechanical forces. As an example, the distribution of the phase velocities of the body with initial heating and without it differences are obtained and shown graphically.

Heating and hot water with wood chips. Is it convenient to replace firewood boilers in southern Chile?

This study estimates the cost of heating and domestic hot water (DHW) with wood chips, under climatic conditions and forest biomass availability in Valdivia, a middle size city in central-southern Chile. The demand for heating and DHW is established for a typical residential building with different thermal insulation conditions. Results of wood chip technology were compared with conventional firewood, liquefied petrol gas (LPG) and diesel technology. Likewise, it is analyzed the profitability of replacing firewood boilers in use by wood chip and LPG boilers. Under almost all scenarios, the higher investment costs of the wood chip solution are more than offset by lower fuel costs, resulting in lower heating and DHW costs than LPG and diesel. The higher automation and combustion efficiency of the wood chip system also results in a lower heating and DHW cost than a firewood system, even if this latter system is already in operation. The thermal insulation significantly reduced the investment and fuel supply costs, financially justifying its inclusion as part of the project to replace the heating and DHW system with wood chips. The alternative with LPG appears to be more attractive only for low levels of heating and DHW demand. The connection of four identical buildings (7680 square meters) in a heating network allows to reach the lowest cost of heating and DHW per dwelling, balancing economies of scale in the boiler investment with the cost of expanding the distribution network.

SELECTED PROBLEMS INVOLVING THE PRESERVATION OF HISTORICAL FRAME BUILDINGS IN SILESIA IN THE CONTEXT OF THEIR FUTURE THERMO-RENEWAL

Abstract The paper discusses the conservation state of selected examples of historical objects preserved in the Silesia Province, having a complex structure with the elements of Prussian wall. They can be found mainly in the so-called company towns created in the late 19th century. The work presents the structural and architectural details characteristic for such objects, with the emphasis on different technical condition of the structural and finishing elements. External envelopes were evaluated in terms of their thermal insulation and humidity condition, depending on their structure, type of material and location, using theoretical and calculation methods. Due to historical value of the buildings and special character of the façade, the paper suggests that the material values, thermal conductivity and humidity level of the envelopes should be closely identified to provide a basis for further analyses.

The influence of thermal contact between medium on the surface waves propagation in a prestressed thermoelastic layered half-space

In the framework of the linearized theory of the propagation of thermoelastic waves, a dynamic mixed coupled problem on the oscillations of an inhomogeneous medium under the action of a thermal load oscillating on its surface is considered. The medium is a thermoelastic prestressed layer rigidly coupled with a thermoelastic prestressed half-space. At the boundary of the medium, two types of thermal conditions are considered: ideal thermal contact and heat insulation. Using operational calculus methods, the boundary value problem with mixed thermal boundary conditions is reduced to an integral equation of the first kind with respect to the unknown distribution function of the heat flux in the contact zone. We studied the distribution of the poles of the Green’s function of a medium, taking into account the effect of initial strains and preheating. It is shown that thermal conditions at the interface between the medium have little effect on the distribution of the vertical displacement of the layer surface in the natural state. At the same time, the condition of thermal insulation between the layer and half-space allows to compensate for the effect of body pre-heating on the formation of the surface wave field.

Conjugate thermal-hydrodynamic model for the study of heavy oil transport

In the present work, the thermal impact induced by the environment on the hydrodynamics of heavy oil transport in pipelines is analyzed. Here, the thermal dependence of the dynamic viscosity of heavy oils, in addition to the mechanical heating caused by viscous dissipation are taken into account; therefore, the mathematical models that represent the flow and temperature fields of the heavy oil have to be solved in a coupled manner. Moreover, given that the pipeline is protected by a thermal insulation (soil), the domain of study defines a conjugate system between the heavy oil, the pipeline and the soil, which indicates that before knowing the temperature in the fluid region, temperatures in the other two zones must be calculated. To define properly how thermal energy is transferred and generated during the crude oil transport, the analysis is carried out in dimensionless form, solving the conjugate mathematical model by using numerical approaches and simplified asymptotic approximations. The main results reveal that the transport of heavy oils is seriously affected by a small reduction in temperature, increasing substantially the dynamic viscosity and the flow rate is reduced significantly in comparison with the case of a full thermal insulation condition. Then, we can infer that when the losses of thermal energy and its effects on the hydrodynamics are controlled, an efficient transport process is obtained. Employing the asymptotic analysis, results show that such a condition can be achieved by two ways: the first one by improving the thermal insulation, and the second one by optimizing the ratio of the internal radius to the length of the pipeline or increasing the volumetric flow rate, which indicates that the conjugate phenomenon is a modified version of the Graetz-Nusselt problem.

The effects of electrical and thermal boundary condition on the simulation of radiofrequency ablation of liver cancer for tumours located near to the liver boundary

Effects of different boundary conditions prescribed across the boundaries of radiofrequency ablation (RFA) models of liver cancer are investigated for the case where the tumour is at the liver boundary. Ground and Robin-type conditions (electrical field) and body temperature and thermal insulation (thermal field) conditions are examined. 3D models of the human liver based on publicly-available CT images of the liver are developed. An artificial tumour is placed inside the liver at the boundary. Simulations are carried out using the finite element method. The numerical results indicated that different electrical and thermal boundary conditions led to different predictions of the electrical potential, temperature and thermal coagulation distributions. Ground and body temperature conditions presented an unnatural physical conditions around the ablation site, which results in more intense Joule heating and excessive heat loss from the tissue. This led to thermal damage volumes that are smaller than the cases when the Robin type or the thermal insulation conditions are prescribed. The present study suggests that RFA simulations in the future must take into consideration the choice of the type of electrical and thermal boundary conditions to be prescribed in the case where the tumour is located near to the liver boundary.

Evaluation of heat conduction in a laser irradiated tooth with the three-phase-lag bio-heat transfer model

In this study, a dental short pulse laser with a Gaussian beam profile was applied normally to the top surface of a mineral organ i.e. the human tooth for a root canal therapy. A numerical method of finite difference is adopted to solve the time-dependent heat transfer equation. The real boundary conditions of thermal insulation on the sharp segment of the root canal and periodic heat flux on the top boundary of the tooth were applied. The comparison of a three-phase-lag (TPL) bio-heat transfer model with other heat transfer studies has shown that this new bio-heat model (TPL) could accurately predict the thermal behaviour of a non-homogeneous structure such as the human tooth. It was observed that decreasing of the pulse time, increasing of the pulse number and laser spot radius are the logical approaches for dental pulp ablation in comparison with increasing of laser intensity or increment of continuous irradiation time. Based on the results of this investigation, an optimum dental laser with heat flux of 5 W/cm2 that could make adequate temperature increments in the range of 5 °C to 8 °C inside the dental pulp after 30 s is suggested. The time of each pulse was 0.1 s, and eight times were iterated in the first ten seconds of treatment time while the laser radius was 3 mm.

Energy and water consumption and carbon footprint of school buildings in hot climate conditions. Results from life cycle assessment

Abstract Optimized energy use and water provision in school buildings play an important role in the sustainability performance of municipalities, and are included in the local sustainable energy policies. Hot climate conditions exacerbate the need for the use of cooling devices and are usually associated to water scarcity problems. Additionally, school buildings in these areas are usually lacking good thermal insulation conditions and energy efficiency measures. This work analyses the energy, material and water requirement activities of two schools located in a hot climate area, and evaluates the aggregated energy and water consumption, the water scarcity exacerbation problems, and the associated carbon footprint through Life Cycle Assessment, which allows the quantification of the impacts along the whole value chain of the school activities per student. Additionally, the effects of different improvement measures, such as the implementation of renewable energy sources and the optimization of energy use based on energy efficiency measures, such as changes in the lighting technologies, are quantified. The results show that schools could reduce the fossil energy demand of the building in the operating and maintenance phase per student between 4.89% and 6.03% by means of the implementation of non-renewable heating measures, between 64.06% and 78.98% by means of the implementation of renewable heating solutions, and between 12.05% and 9.54% by means of the implementation of lighting substitution measures.

A Dynamic Method of Nanoindentation

A new dynamic method of measuring the local value of hardness and contact stiffness at the nanoscale range using a probe in self-oscillation mode at resonant eigenfrequency is proposed. The possibility is shown of measuring the force of contact interaction of a probe with a sample in a range from ten to several hundred μN with shift of the probe’s resonant eigenfrequency, at a resolution up to one hundred nN. The advantage of this method is the absence of a drift component of the measuring signal during the time interval of a test cycle. This ensures operability of the method without special conditions of vibration insulation and thermal insulation that are necessary for instrumental indentation.

Heat Loss Analysis of a Steel Piston and a YSZ Coated Piston in a Heavy-Duty Diesel Engine Using Phosphor Thermometry Measurements

Diesel engine manufacturers strive towards further efficiency improvements. Thus, reducing in-cylinder heat losses is becoming increasingly important. Understanding how location, thermal insulation, and engine operating conditions affect the heat transfer to the combustion chamber walls is fundamental for the future reduction of in-cylinder heat losses. This study investigates the effect of a 1mm-thick plasma-sprayed yttria-stabilized zirconia (YSZ) coating on a piston. Such a coated piston and a similar steel piston are compared to each other based on experimental data for the heat release, the heat transfer rate to the oil in the piston cooling gallery, the local instantaneous surface temperature, and the local instantaneous surface heat flux. The surface temperature was measured for different crank angle positions using phosphor thermometry. The fuel was chosen to be n-heptane to facilitate surface temperature measurements during non-skip-fire, thermally stabilized operating conditions. Assuming one-dimensional heat transfer inside each piston, the local instantaneous surface heat flux was calculated using the heat transfer rate to the oil in the piston cooling gallery and the surface temperature measurements. The results from this study show that the surface temperature variation is similar for both pistons. The instantaneous heat flux during combustion is however significantly greater for the steel piston than the coated piston. The heat release analysis also indicates that combustion is slower for the piston with the yttria-stabilized zirconia coating. (Less)

Experimental Study of Single-phase Pressure Drop Characteristics in Horizontal Internal Helical Finned Tubes

Due to the intense enhancement of the convective heat transfer, internal helical finned tubes are widely applied into the field of HVAC. An experimental setup was built for investigating single-phase flow characteristics of internal helical-rib roughness. The single-phase flow characteristics for water-ethylene glycol mixture in three internal finned tubes and a smooth tube were obtained. The nominal diameter of the two test tubes was 22.48mm and 16.662mm, the number of the fins was 60 and 38, the helix angle was 45 degrees and 60 degrees and the relative roughness was 0.022 and 0.053, respectively. The Prandtl number varied from 10.0 to 177 and the Reynolds number ranged from 185 to 40000. The friction factors were measured under the condition of thermal insulation. Experimental results showed that the smooth-tube results were compared to the Filonenko equation with satisfactory agreement. Transition for internal helical finned tubes occurred at lower Reynolds numbers than that of their smooth tube counterparts. The trend of the friction factor of the internal helical finned tube is remarkably different from that of plain tube in the transition regime. For Tube 2 and Tube 3, the flow turns into the fully turbulent flow at Re=14500, while Tube-4 at Re=9500, which is obviously different from that of the plain tube.

Optimal Low Temperature Charging of Lithium-ion Batteries

A lithium-plating side reaction at the lithiated graphite (LiC6) anode leads to poor safety of the lithium-ion battery. Faster charging at normal temperature may lead to a plating side reaction during the end of charging at the anode-separator interface. At lower temperature, the lithium-plating side reaction may become thermodynamically favorable during almost the entire charging period, even at low rates. This paper uses an electrochemical engineering model and dynamic optimization framework to derive charging profiles to minimize lithium plating at low temperatures. Transport parameters for lithium-ion battery are very sensitive at low temperatures. This paper shows the derivation of the optimal charging profile considering strict lower bounds on the plating reaction depending on various thermal insulation conditions (adiabatic, isothermal, and normal heat transfer coefficient) surrounding the battery.

The Analysis of Pipeline Transportation Process for CO2 Captured From Reference Coal-Fired 900 MW Power Plant to Sequestration Region

Abstract Three commercially available intercooled compression strategies for compressing CO2 were studied. All of the compression concepts required a final delivery pressure of 153 bar at the inlet to the pipeline. Then, simulations were used to determine the maximum safe pipeline distance to subsequent booster stations as a function of inlet pressure, environmental temperature, thickness of the thermal insulation and ground level heat flux conditions. The results show that subcooled liquid transport increases energy efficiency and minimises the cost of CO2 transport over long distances under heat transfer conditions. The study also found that the thermal insulation layer should not be laid on the external surface of the pipe in atmospheric conditions in Poland. The most important problems from the environmental protection point of view are rigorous and robust hazard identification which indirectly affects CO2 transportation. This paper analyses ways of reducing transport risk by means of safety valves.

Internal heating of lithium-ion batteries using alternating current based on the heat generation model in frequency domain

This study develops a method to internally preheat lithium-ion batteries at low temperatures with sinusoidal alternating current (AC). A heat generation rate model in frequency domain is developed based on the equivalent electrical circuit. Using this model as the source term, a lumped energy conservation model is adopted to predict the temperature rise. These models are validated against the experimental results of preheating an 18650 cell at different thermal insulation conditions. The effects of current amplitude and frequency on the heating rate are illustrated with a series of simulated contours of heating time. These contours indicate that the heating rate increases with higher amplitude, lower frequency and better thermal insulation. The cell subjected to an alternating current with an amplitude of 7 A (2.25 C) and a frequency of 1 Hz, under a calibrated heat transfer coefficient of 15.9 W m−2 K−1, can be heated from −20 °C to 5 °C within 15 min and the temperature distribution remains essentially uniform. No capacity loss is found after repeated AC preheating tests, indicating this method incurs little damage to the battery health. These models are computationally-efficient and can be used in real time to control the preheating devices in electric vehicles.

Temperature distribution in solder joints of high-temperature superconductors

Equations that make it possible to calculate the temperature variations in solder contact joints of current leads with tape high-temperature superconductors are derived. The proposed calculation procedures allow computing for different states of superconductor and its cooling conditions in media of gaseous and liquid cryogenic agents, as well as investigating the thermal state of the joint in thermal insulation conditions.