Effect Of Heat Conduction Research Articles

Studies on the formation mechanism of incomplete fusion defects in ultra-narrow gap laser wire filling welding

Austenitic stainless steel has many excellent properties and it has been extensively used in rail transit trains. Ultra-narrow gap laser wire filling welding is the preferred welding method to connect austenitic stainless steel. However, with this method, it is easy to form defects such as lack of fusion in the welding process. In order to clarify the mechanism of incomplete fusion defects, this article analyzed incomplete fusion defects from the groove design and the welding wire transfer behavior, integrating with high-speed photography and finite element simulation. It is found that the form of narrow gap groove has a close relationship with the formation of fusible defects. Besides, under the condition of globular transfer, all the complicated recoil force, blocking effect of the large droplet on laser energy, the eruption of metal vapor and turbulence of the molten pool caused by large droplet transfer will cause violent disturbance of the molten pool and seriously affect its heat conduction effect. The base metal incompletely melting, the lack of transfer driving force on the droplet and the failure of droplet wetting and spreading on the sidewalls of the groove are the main causes of welding non-fusion defects. Designing reasonable groove, ensuring the stability of wire feeding, adopting liquid bridge transfer mode and appropriately increasing heat input are conducive to reduce the occurrence of incomplete fusion, which is essential to improve the welding quality and enhance the safety of rail vehicles.

Preparation and performance of conductive tack coat on asphalt pavement

The purpose is to solve the problems of inadequate snow melting uniformity, difficult construction, and large heat storage in existing energy conversion pavements for deicing and snow melting. The conductive layers of different adhesive materials were prepared with emulsified asphalt modified by waterborne epoxy resin (MEAWER) and modified waterborne epoxy resin (MWER) as matrix. According to the surface resistivity and temperature change, the conductivities and heat conduction effects of the two types of conductive adhesive layers were compared and evaluated, respectively, and the road performance is determined based on multiple conditions. The results showed that when the amount of carbon fiber was 120 g/m2, compared to MEAWER, the surface resistivity of MWER was 18.7% lower, the surface temperature was 3.6 ℃ higher, and the bonding strength was 46.1% higher. The thermogenesis and heat dissipation efficiency of MWER could reach 6.9 and 2.76 ℃/h at −5 ℃, respectively. Moreover, its minimum shear and drawing strengths exceeded 1.3 and 0.5 MPa, respectively. Based on the temperature change and road performance of the conductive adhesive layer, it was recommended that 120 and 100 g/m2 of carbon fiber be distributed in the MEAWER and MWER conductive tack coats, respectively.

Influence of spherical inclusions on effective thermoelectric properties of thermoelectric composite materials**Project supported by the Ningbo Natural Science Foundation, China (Grant Nos. 2019A610151 and 2018A610081), the Natural Science Foundation of Zhejiang Province, China (Grant Nos. LY17A020001 and LY20A020002), the National Natural Science Foundation of China (Grant No. 11402063), and the K C Wong Magna Fund in Ningbo University, China.

A homogenization theory is developed to predict the influence of spherical inclusions on the effective thermoelectric properties of thermoelectric composite materials based on the general principles of thermodynamics and Mori–Tanaka method. The closed-form solutions of effective Seebeck coefficient, electric conductivity, heat conductivity, and figure of merit for such thermoelectric materials are obtained by solving the nonlinear coupled transport equations of electricity and heat. It is found that the effective figure of merit of thermoelectric material containing spherical inclusions can be higher than that of each constituent in the absence of size effect and interface effect. Some interesting examples of actual thermoelectric composites with spherical inclusions, such as insulated cavities, inclusions subjected to conductive electric and heat exchange and thermoelectric inclusions, are considered, and the numerical results lead to the conclusion that considerable enhancement of the effective figure of merit is achievable by introducing inclusions. In this paper, we provide a theoretical foundation for analytically and computationally treating the thermoelectric composites with more complicated inclusion structures, and thus pointing out a new route to their design and optimization.

Ballistic-Diffusive Model for Heat Transport in Superlattices and the Minimum Effective Heat Conductivity.

There has been much interest in semiconductor superlattices because of their low thermal conductivities. This makes them especially suitable for applications in a variety of devices for the thermoelectric generation of energy, heat control at the nanometric length scale, etc. Recent experiments have confirmed that the effective thermal conductivity of superlattices at room temperature have a minimum for very short periods (in the order of nanometers) as some kinetic calculations had anticipated previously. This work will show advances on a thermodynamic theory of heat transport in nanometric 1D multilayer systems by considering the separation of ballistic and diffusive heat fluxes, which are both described by Guyer-Krumhansl constitutive equations. The dispersion relations, as derived from the ballistic and diffusive heat transport equations, are used to derive an effective heat conductivity of the superlattice and to explain the minimum of the effective thermal conductivity.

Effects of heat conduction of implant surface at thermal stimulation on implant placement.

The purpose of this study is to investigate, in vitro, how two different implant placement methods (one and two-stage implant placement) affect implant surface temperature after thermal stimulation. Two titanium screw implants were used and three thermocouples were attached to the implant surface at 0.5mm (ch1), 5.5mm (ch2) and 9.0mm (ch3) under each platform. Experimental devices were fabricated pouring polymerization resin under a condition that imitated the two embedded technique with the one-stage implant placement model (1-stage) and the two-stage implant placement model (2-stage). A hot water storage device was installed in each model and hot water at three temperatures (60°C, 70°C and 100°C) was flowed. The temperature change over time at the implant surface by the thermocouple was recorded. From the measurement data, the maximum temperature (Max-temp) at the implant surface, the time to reach 47°C (47°C r-time), and the duration of 47°C or more (47°C c-time) were calculated, and the test was repeated 26 times using the same method. The mean of repeated measurements was determined and statistical analysis was performed. Max-temp showed significant differences between each implant placement method, each channel and each thermal stimulation (p < 0.01). In this study suggested that the implant surface could reach 47°C with 60°C thermal stimulation in a 1-stage. In addition, it rose over 47°C at 70°C. The 2-stage implant surface did not rise to 47°C.

Effectiveness of pre-scanning on zinc evaporation in laser spot welding of zinc-coated steels

To prevent zinc vapor–induced spatter, pre-scanning was proposed as a means to evaporate the zinc coating prior to the welding process. This paper studied the effectiveness of various pre-scanning parameters such as beam defocusing distance, beam power, scanning velocity, and geometric scanning path. The gap conditions during pre-scanning were also studied in regard to outgassing of zinc vapor during pre-scanning. To complement the experimental testing, a thermomechanical numerical model was developed and validated by comparing the size of the predicted versus the physical fusion zone. The effectiveness of the pre-scanning process was evaluated by examining the temperature distribution at both the top and faying surfaces of the stack-up under the condition that the temperature at the faying interface is greater than the zinc boiling point through the entire pre-scanning process to prevent condensation of the zinc vapor back onto the faying interface. The study found that no gap at the faying interface should be allowed with the use of pre-scanning to ensure an effective heat conduction across the faying interface. When the pre-scanning path is spiral or circular, the beam translation should initiate at the center and work outward towards the periphery. The line energy is suggested to decrease as the radius increases to avoid excessive accumulation of heat in the path center.

Flash temperature and anti-wear tribofilm growth mechanisms by asperity contact in top-ring/liner conjunction of IC engines

Understanding the growth mechanism of tribofilms at the top of the liner in IC engines is of great importance to protecting the sealing function of the top ring. In present study, the transient heat transfer is investigated between the ring and the liner in the top dead center (TDC) area with considerations of asperity contact pressure, boundary friction coefficient and reciprocating piston motion. A 3D transient model is developed to predict the evolutions of effective heat partition and conduction between the contact surfaces. It is revealed that the flash temperature in the asperity contact pairs of the top-ring/liner conjunction is remarkably higher than the steady-state liner temperature, therefore significantly enhancing the growth rate of anti-wear film formation based on a stress-activated Arrhenius model. The results show that the asperity contact model and the flash temperature model can be employed as an engineering tool for anti-wear tribofilm prediction in top-ring/liner conjunction of IC engines.

A Concise Model and Analysis for Heat-Induced Withdrawal Reflex Caused by Millimeter Wave Radiation

In this study, we consider the heat-induced withdrawal reflex caused by exposure to an electromagnetic beam. We propose a concise dose-response relation for predicting the occurrence of withdrawal reflex from a given spatial temperature profile. Our model is distilled from sub-step components in the ADT CHEETEH-E model developed at the Institute for Defense Analyses. Our model has only two parameters: the activation temperature of nociceptors and the critical threshold on the activated volume. When the spatial temperature profile is measurable, the two parameters can be determined from test data. We connect this dose-response relation to a temperature evolution model for electromagnetic heating. The resulting composite model governs the process from the electromagnetic beam deposited on the skin to the binary outcome of subject’s reflex response. We carry out non-dimensionalization in the time evolution model. The temperature solution of the non-dimensional system is the product of the applied power density and a parameter-free function. The effects of physical parameters are contained in non-dimensional time and depth. Scaling the physical temperature distribution into a parameter-free function greatly simplifies the analytical solution, and helps to pinpoint the effects of beam spot area and applied power density. With this formulation, we study the theoretical behaviors of the system, including the time of reflex, effect of heat conduction, biological latency in observed reflex, energy consumption by the time of reflex, and the strategy of selecting test conditions in experiments for the purpose of inferring model parameters from test data.

Numerical Study of Transient Bio-Heat Transfer Model With Heat Transfer Coefficient and Conduction Effect in Cylindrical Living Tissue

The human thermal comfort is affected by the body’s heat exchange mechanism conduction, convection, radiation, and evaporation. The mode of heat transfer between the body and environment depends upon the human internal physiological phenomena, together with the boundary conditions. The present paper provides the comprehensive overview of the thermoregulatory system of human body and studies the numerical solution of unsteady-state one dimensional Pennes bio-heat equation with appropriate boundary conditions. The solution is used to observe the temperature profiles at different thermal conductivities, and different heat transfer coefficients in the living tissue at the various time steps. Various physical and physiological factors across the cylindrical living tissue have been incorporated in the model.

Highly thermally conductive polystyrene/polypropylene/boron nitride composites with 3D segregated structure prepared by solution-mixing and hot-pressing method

Continuous heat conduction networks are very significant for filler-filled polymer composites to enhance their thermal conductivity (TC). 3D segregated structure has been considered as an effective approach to construct continuous heat conduction networks. Herein, we prepared the polystyrene/polypropylene/boron nitride (PS/PP/BN) ternary composites with 3D segregated filler networks by solution-mixing and hot-pressing method. The ternary PS/PP/BN composite containing 50 wt% BN achieves TC of 5.57 W/m·K, which is 29 times than that of PS resin (0.19 W/m·K) and 2.76 folds of the PS/BN binary counterpart (2.02 W/m·K). More importantly, the ternary PS/PP/BN composite exhibits favorable thermal stability after 50 hot-cold cycles or thermal ageing test for 50 h, whose TC drops slightly by 3.2% and 3.4%, respectively. The typical 3D segregated structure has been constructed in our PS/PP/BN ternary composites, which can provide effective heat conduction pathways. Moreover, BN platelets align along the PP microspheres in the PS matrix during the hot-pressing process. These two factors make the TC of the PS/PP/BN ternary composites increase sharply. Our strategy can potentially pave the way for the easy, low-cost and large-scale preparation of highly thermally conductive polymer-based composites as thermal management materials.

Quantitative experimental determination of evaporation influencing factors in single droplet levitation

Acoustic levitation has been used successfully as a model system for spray polymerization processes and the determination of particle morphology. The experimental extension for the investigation of heat and mass transfer in levitated liquid droplets requires much more effort. In order to enable the levitation method as a universal tool for the tracking of heat and mass transfer, influencing experimental parameters and their effect on relevant characteristics like droplet deformation, temperature and surrounding flow field were analyzed. These parameters were elucidated experimentally to reveal their mutual dependencies. It was shown that the evaporation rate is affected by the aspect ratio of a droplet, which is directly correlated with its wet-bulb temperature. This was primarily explained by varying acoustic flow field patterns based on classical fluid mechanics, gas velocities and acoustic boundary layer thicknesses, which were visualized and quantified by particle image velocimetry. In order to study the wet-bulb temperature, thermocouple measurements on the levitated droplet were performed. It turned out, that the wet-bulb temperature is dependent on the power input and the resulting sound pressure level as it changes the acoustic field around the droplet. Moreover, the temperature measurement revealed the necessity to establish a temperature correction to account for heat conduction effects along the thermocouple shaft into the droplet. The illustrated experimental setup and measurement procedure is supposed to provide guidelines for the experimental determination of heat and mass transfer in evaporating droplets.

Influence of flow non-uniformity on the dynamic behaviour of three-fluid cross-flow compact heat exchanger

The flow uniformity at any cross-section along the flow is highly influenced due to different practical limitations such as improper header/distributor design, manufacturing defect etc. Hence, the compact three-fluid heat exchanger with cross-flow arrangement is numerically investigated with non-uniform flow at the entrance of central fluid for four feasible flow arrangements (C1,2,3,4) for turbulent flow regime. The experiments are conducted to validate the numerical solutions obtained from Implicit Finite Difference scheme. The dynamic response of the cross-flow three-fluid heat exchanger is analysed with step and sinusoidal perturbation at the entry and both the cooling and heating efficacy of the heat exchanger is discussed. The model is made more realistic by incorporating the dispersive axial heat conduction effect within the fluids as well as at the inlet section of heat exchanger (Dankwert's boundary condition) and wall longitudinal heat conduction in the conducting walls. Four cases of flow non-uniformity (P, Q, R and S), which resemble the most realistic and possible case of non-uniform flow in the heat exchangers are selected for present study. It is learnt that case P and case Q offers the maximum and the minimum augmentation or deterioration in the efficacy of heat exchanger when non-uniform flow exists only in central fluid. The impact of flow non-uniformity on the performance of heat exchanger is more significant in turbulent flow regime than laminar. The analysis is further extended for the flow non-uniformity in all the three fluid streams simultaneously and for certain combinational case of U (uniform flow), P and Q. It is found that flow non-uniformity drastically affects the heat exchanger's efficacy and the upshot of axial dispersion causes more deterioration in the thermal performance than that by longitudinal heat conduction.

Numerical simulation of non-Fourier heat conduction in fins by lattice Boltzmann method

The addition of fin to the surface of microelectronic elements is a very effective way to enhance their heat transfer rate. Since the heat generation of the microelectronic elements is very fast, the Fourier law is no longer valid and the non-Fourier effect must be considered when designing a fin. Cattaneo – Vernotte model (CV model) is the most common model that is used to describe the non-Fourier effect in heat conduction. The mathematical expression of the CV model is essentially a wave equation. The lattice Boltzmann method (LBM) can be used to solve the wave equation in the CV heat conduction problem. In this paper, LBM is used to solve the non-Fourier heat conduction in a fin under periodic boundary conditions. Effects of relaxation time, shape and the frequency of the base temperature oscillations on heat transfer efficiency are analyzed.

Size Effect on Onset and Subsequent Evolution of Adiabatic Shear Band: Theoretical and Numerical Analysis

The adiabatic shear instability of ductile materials has attracted more and more attentions of researchers and groups, who have been sparing no effort in further understanding of the underlying mechanism since the first experimental depiction of adiabatic shear instability by Zener and Hollomon. As for the adiabatic shear instability, many factors account for its occurrence, including heat conduction, inertia effect, microstructure effect and so on. However, lots of experimental evidence has shown that metal materials display a strong size effect when the characteristic length scale is in the order of microns. The size effect has also been observed in the analysis of shear band in the ductile materials because the order of the bandwidth stays within the microscale range. However, a comprehensive understanding of the whole process of adiabatic shear banding (ASB), including the early onset and the subsequent evolution, is still lacking. In this work, a gradient plasticity model based on the Taylor-based nonlocal theory feasible for the linear perturbation analysis and convenient for numerical calculation is proposed to investigate the strain gradient on the onset of ASB and the coupling effect of heat conduction, inertia effect and strain gradient at the early stage, as well as on the subsequent evolution process at later stages. As for the onset of ASB, the linear perturbation method is used to consider the effect on the initial formation of ASB. After the investigation of the onset of ASB, the characteristic line method is applied to describe the subsequent nonlinear evolution process of ASB. Three stages of ASB evolution are clearly depicted during the evolution process, and the significance of size effect on the ASB nonlinear evolution process of ASB at different stages is analyzed. With the help of linear perturbation analysis and characteristic line method, a comprehensive description of the role of strain gradient in the ASB from the early onset to the end of the evolution is provided.

Thermoelastic damping in micro and nano-mechanical resonators utilizing entropy generation approach and heat conduction model with a single delay term

Analysis of thermoelastic damping (TED) of micro and nano-beam resonators plays a very important role in designing the resonator with high-quality factors. TED of micro and nano-beam resonators has therefore been an interesting and challenging area of research in recent years. Prediction of TED is more challenging under the effect of non-Fourier heat conduction. In the past, several works have been dedicated to TED modeling with the influence of non-Fourier heat conduction in view of the fact that non-Fourier heat conduction model is more relevant for analysis of TED for small scale devices. The present work aims to investigate TED in micro and nano-mechanical resonators utilizing non-Fourier heat conduction model with a single delay term introduced by Quintanilla in 2011. We derive an explicit formula of the quality factor for TED based on the entropy generation approach. With the help of numerical results, we present the influence of TED in the context of normalized frequency as well as beam thickness. Further, the effects of the time delay parameter and the material constants on TED have been discussed in detail. The results of the present model are compared to those obtained for GN-III model. It has been observed that the current model offers a high-quality factor as compared to GN-III model.

Heat Transfer Model of Multilayer Thermal Protective Clothing for High-Temperature Operation

The thermal protective clothing for high-temperature operation usually consists of three-layer fabrics and a gap called the air layer or Layer IV between Layer III and skin. In order to design more effective thermal protective clothing at less cost, based on the heat transfer principles, we establish heat transfer models of fabrics and air layer, which are one-dimensional nonlinear partial differential equations with constant coefficients. In the three-layer fabrics, we consider the effects of heat conduction and heat radiation in Layer I but only consider heat conduction in Layer II and Layer III. Furthermore, the heat transfer model of Layer IV is decoupled and simplified to steady-state heat conduction in Layer IV and radiation heat transfer on surface of Layer IV. According to the explicit difference schemes for the models, we use the parameters in an experiment which puts a thermal manikin in high-temperature environment for some time and measures the temperature of lateral skin at regular time, to solve the models and calculate the temperature of each layer. With MATLAB, the visual interface of three-dimensional temperature distribution is provided, which is reference for functional design of thermal protective clothing. We also compare the simulation result of skin surface with the experimental data. The results show that at the same position, the temperature rises over time but with decreasing rate and finally reaches the steady state. Moreover, at one moment after reaching the steady state, the temperature has a gradual decrease with the increase of distance from the external environment.

Influence of inhomogeneous porosity on effusion cooling

Effusion cooling is an effective cooling technology which is considered as the next logical step of the cooling technology for gas turbine blades. Additive manufacturing technology makes it possible to fabricate effusion cooling structures in gas turbine blades with the optimal parameters and a low cost. However, the sizes of cooling holes were usually inhomogeneous owing to the limited accuracies of additive manufacturing technology. This study investigated the influence of inhomogeneous porosity on effusion cooling. The results showed that the inhomogeneous porosity dramatically affected the cooling effects of the surface film and the internal holes. However, the inhomogeneous porosity had a very slight effect on both global and local effusion cooling efficiency (less than 1%) even when the non-uniformity reached 20%. The intensive heat conduction inside the solid matrix played an important role for eliminating the adverse influence of the inhomogeneous porosity. Higher thermal conductivity, thicker thickness or narrower holes spacing both resulted in a smaller heat resistance and hence enhanced the heat conduction effect. However, the inhomogeneous porosity had a remarkable effect on effusion cooling efficiency when a low-thermal-conductivity Thermal Barrier Coating (TBC) covered the surface. Decreasing the thermal conductivity of TBC would further aggravate the inhomogeneous problem.

The Effect of Axial Conduction of the Wall on Temperature Conditions and Effectiveness of Heat Exchangers with Parallel Flow of Heat Carriers

The effect of heat conduction in the wall on temperature distribution along a heat exchanger with parallel flow of heat carriers and on heat exchanger effectiveness has been investigated. The problem was solved based on a system of one-dimensional (cross-section averaged) energy equations for two heat carriers and the heat conduction equation for the wall. The wall ends were assumed to be heat insulated. The system was solved using the finite-difference technique. For the special cases, the system was solved analytically. The parameter determining the effect of the wall axial conduction on the heat exchanger effectiveness and the value of this parameter at which this effect can be neglected have been found. This dimensionless parameter is proportional to the heat transfer coefficient, thermal resistance of the wall, and square of the heat-exchanger length to the tube wall thickness ratio. Besides the parameter describing the effect of axial heat conduction, the solution depends on the number of heat transfer units and the ratios of thermal equivalents of the heat carriers and heat transfer coefficients on the hot and cold sides. For a cocurrent flow of the heat carriers, the best result is attained when the value of these two ratios is 1. In this case, the effectiveness of the heat exchanger does not change as compared to that with no effect of the axial heat conduction of the wall. For a countercurrent heat exchanger, the effect of axial heat conduction on the heat exchanger effectiveness is at a minimum when the ratios of thermal equivalents of the heat carriers and heat transfer coefficients on the cold and hot sides are equal. The estimations based on the results of calculations demonstrate that, in case of microchannel heat removal, the effect of axial heat conduction in the heat exchanger wall can considerably reduce the heat exchanger effectiveness.

Piecewise compensation of thermal errors of a ball screw driven CNC axis

The article presents compensation of thermal errors of a computerized numerical control (CNC) axis. A partial differential equation (PDE) model of ball screw temperature distribution was utilized. The model takes into account non-steady-state localized heat sources and the effect of heat conduction between the screw and the machine body. To tune and validate the model, temperature distribution of the axis was measured on-line during its operation. The change of the model parameters related to the change of the axis movement velocity was presented. The article shows, that utilization of the thermal PDE model to compensate thermal errors, though effortful, is beneficial, but noticeably less accurate than the utilization of compensation based on the measured temperature values.

Thermodynamic analysis and prediction of reservoir temperature distribution for steam stimulation

During steam stimulation of horizontal wells in heavy oil reservoir, it is necessary to estimate the reservoir temperature distribution, which has a huge impact on oil production and ultimate oil recovery. Most of previous models were derived based on infinite boundary condition and constant fluid velocity, which were not consistent with real situation of reservoir. In this paper, new energy balance equations combined with mobile boundary conditions were developed for increasing the accuracy of predicted results. Based on dimensionless conversion and numerical analysis, the optimum solution of equation system could be easily obtained, and its good performance in predicting temperature distribution was also validated. Moreover, thermodynamic mechanism of hot fluid, latent heat and sensible heat of steam, and their contributions to heating reservoir were also discussed to investigate the heat transfer pattern in reservoir. Finally, factors that could influence the reservoir temperature distribution were analyzed. It is found that: (a) both the heat conduction and the heat convection contribute to the heat transfer in hot liquid zone, and the heat convection predominates; (b) both the condensation front and heat front move outward with the injection of steam, but their expanding rates gradually drop with time; (c) as the radius increases, the radial fluid velocity will go down, leading the heat convection effect to be weakened and the heat conduction effect to become obvious; (d) there are some factors that can influence the reservoir temperature distribution including steam injection duration, fluid velocity, volumetric heat capacity of fluid, steam quality and thermal conductivity of reservoir.