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The generation of ultrasound by pulsed lasers is a thermoelastic process, and in the literature on general thermoelasticity there are presently three different forms of the heat equation in popular use, the classical, Lord-Schulman (LS), and Green-Lindsay (GL) heat equations. The question may thus arise as to which heat equation should be used to model laser generated ultrasound. The purpose of this work is to summarize the current rationale for using these different heat equations, in order to provide a basis for choosing one of the forms. A review of the classical, LS and GL theories is given, and the potential advantages of hyperbolic heat flow theories in laser ultrasonics are discussed. A numerical example is given that clearly shows the small time differences these theories predict, and also points out potential problems with using hyperbolic heat equations on small time scales. 相似文献
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本文通过n_e=1×10~(19)cm~(-3)的热等离子体,在纯热传导冷却机制下的计算,得到了等离子体温度的时空变化,并运用碰撞-辐射模型计算了在这种条件下,类锂铝离子4f-3d,5f-3d跃迁的增益系数,计算表明:只要有合适的初始条件,用纯热传导冷却机制,也能产生软X射线激光增益. 相似文献
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根据爱因斯坦的质能等效关系式,热能具有的等效质量称为热质,从而在固态和气态介质中分别建立了声子气质量和热子气质量的概念.应用牛顿定律建立了含有驱动力、阻力和惯性力的热质(声子气或热子气)运动的动量守恒方程.由于热量在介质中的传递本质上就是热质(声子气和热子气)在介质中的运动,所以热质动量守恒方程就是普适的导热定律,能够统一描述各种条件下的导热规律.当热流密度不是很大从而热质惯性力可以忽略时,热质动量守恒方程就退化为傅里叶导热定律,这表明傅里叶导热定律是特殊条件下的导热定律,对于微纳尺度条件下的导热,热流密度可以极高,由速度空间变化引起的惯性力不能忽略,在稳态导热情况下也将出现非傅里叶导热,此时在计算或者实验中不能用热流密度除温度梯度求导热系数.在超快速加热条件下,必需考虑惯性力,与基于CV导热模型的波动方程相比,普适的导热定律增加了因速度空间变化引起的惯性力项,所以在介质中热波叠加时不会出现产生负温度的非物理现象,表明基于热质运动概念的普适导热定律更为合理.
关键词:
傅里叶导热定律
普适导热定律
热质运动
非傅里叶导热 相似文献
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E. B. Davies 《Journal of statistical physics》1978,18(2):161-170
We construct a model of a chain of atoms coupled at its ends to two reservoirs at different temperatures. In a weak coupling limit the atoms obey a stochastic evolution law and have an equilibrium state with a uniform temperature gradient along the chain. 相似文献
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为提高传统光滑粒子动力学(smoothed particle hydrodynamics, SPH)方法模拟瞬态热传导问题的精度和稳定性,本文提出了一种一阶对称光滑粒子动力学(first order symmetric SPH, FO-SSPH)方法.该方法将具有二阶热传导方程分解成两个一阶偏微分方程,然后基于梯度离散和Taylor级数展开思想,对一阶核梯度形式进行修正,并将得到的局部矩阵对称化.数值结果表明:与传统SPH方法相比,FO-SSPH方法精度高、数值稳定性好; 该方法能较准确地直接施加混合边值
关键词:
瞬态热传导
光滑粒子动力学
非线性 相似文献
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We propose a new concept, the centre of energy, to study
energy diffusion and heat conduction in one-dimensional hard-point
model. For diatom model, we find an anomalous energy diffusion as
$\langle x^2 \rangle\sim t^\beta$ with $\beta=1.33$, which is
independent of initial condition and mass rate. The present model
can be viewed as the model composed by independent quasi-particles,
the centre of energy. In this way, heat current can be calculated.
Based on theory of dynamic billiard, the divergent exponent of heat
conductivity is estimated to be $\alpha=0.33$, which is confirmed by
a simple numerical calculation. 相似文献
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Effective heat dissipation from nano-fluidic devices is sometimes necessary to ensure their performance and lifespan. In the molecular dynamics simulation of nanoscale convective heat transfer, thermostats cannot be directly applied to the fluid because of the non-uniform temperature distribution. Periodic boundary is typically utilised, but unrealistic axial heat conduction exists when there is a temperature difference between the outlet and images of inlet atoms. In this paper, the effect of axial conduction caused by periodic boundary is investigated through the Péclet number (Pe). Taking viscous dissipation into consideration, the magnitude of outlet thermal diffusion is observed to decrease with increasing Pe. The local average temperature of fluid changes in an exponential form except in the region close to the outlet. Results show that the contribution of outlet axial conduction to the local average temperature is less than 2.0% when Pe > 10. The main reason is that the magnitude of fluid velocity and viscous heat dissipation in nanochannels is much larger than that in macro-channels at the same Péclet number. 相似文献
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Steady-state heat conduction problems arisen in connection with various physical and engineering problems where the functions satisfy a given partial differential equation and particular boundary conditions, have attracted much attention and research recently. These problems are independent of time and involve only space coordinates, as in Poisson’s equation or the Laplace equation with Dirichlet, Neuman, or mixed conditions. When the problems are too complex, it is difficult to find an analytical solution, the only choice left is an approximate numerical solution. This paper deals with the numerical solution of three-dimensional steady-state heat conduction problems using the meshless reproducing kernel particle method (RKPM). A variational method is used to obtain the discrete equations. The essential boundary conditions are enforced by the penalty method. The effectiveness of RKPM for three-dimensional steady-state heat conduction problems is investigated by two numerical examples. 相似文献
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Zheng Kuang Walter Perrie Dun Liu Stuart Edwardson Jian Cheng Geoff Dearden Ken Watkins 《Applied Surface Science》2009,255(22):9040-9044
A high repetition rate picosecond laser system is combined with a spatial light modulator (SLM) for diffractive multiple beam processing. The effect of the zero order beam is eliminated by adding a Fresnel zone lens (FZL) to defocus the un-diffracted beam at the processing plane. Chromatic dispersion, which is evident with a large bandwidth femtosecond pulses leading to the problem of distorted hole shape is eliminated due to the much narrower spectral bandwidth, 0.1 nm at 10 ps pulselength, resulting in highly uniform intensity spots, independent of diffraction angle. In addition, high-throughput processing is demonstrated by combining the high power laser output, 2.5 W at λ ≈ 1064 nm and fast repetition rate, f ≈ 20 kHz with P > 1.2 W diffracted into 25 parallel beams. This has the effect of creating an “effective” repetition rate of 500 kHz without restrictive scan speeds. 相似文献
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The determination of the thermal histories in a tube-plate combination when welded with a high intensity heat source is discussed in this paper. The influence of the geometry of the plate and the use of nitrogen gas during the welding process on the thermal histories is shown. With the thermal histories known, the distortion due to thermal stresses could be determined. 相似文献
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An experimental setup was built to study the influence of conduction heat loss on the convective heat transfer performance enhanced by an impinging jet in cross-flows. Results revealed that the conduction heat loss ratio (Ec/E) is between 12.0% and 40.1%, and it decreases nonlinearly with the ratio of jet-to-cross-flow velocity. The relative Nusselt number increases with the ratio of jet-to-cross-flow velocity. The maximum peak value and the average are 8.1 and 6.4, respectively. The distribution of the relative Nusselt number seems to be flattened by assuming a constant conduction heat loss ratio. 相似文献
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Robert Alicki 《Journal of statistical physics》1979,20(6):671-677
The formula for the entropy production in open quantum systems is examined for the Davies model of heat conduction.This work is supported by Polish Ministry of Higher Education Science and Technology, project MRI 7. 相似文献
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Asma Hayat Anwar Latif M. Khaleeq-ur-Rahman K. A. Bhatti Arslan Usman 《辐射效应与固体损伤》2013,168(6):403-409
Laser-induced modifications on platinum (Pt) and silicon (Si) are compared by considering the development of various features on the irradiated surface. The experiments were carried out both in air and under vacuum. The interaction of 50 pulses of 1064 nm Nd:YAG laser with both targets in air resulted in non-linear phenomena. The periphery of the irradiated spot on the Pt surface exhibits wave-like patterns with a featureless central portion. A non-uniform distribution of cones of different sizes is also observed on the irradiated surface. In the case of silicon, the laser-induced periodic surface structures along with the formation of micro-column, rectangular blocks and grid are prominently observed features. However, when both the targets were irradiated with the same number of shots under vacuum (~10?3 Torr), the surface morphologies of both the targets exhibited the hydrodynamic sputtering but with more explosive expulsion in Pt when compared with silicon. In platinum, there is a periodic variation in the distance between adjacent cones formed in various ablated zones. The Gaussian beam mode TEM00 provided the evidences for melt pool formation in silicon when irradiated under vacuum. Additionally, we observed other mechanisms including splashing, sputtering, burning, re-solidification and redeposition on the surface of irradiated targets. 相似文献
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The physical process of forming a modified region in soda-lime glass was investigated using 1 kHz intense femtosecond laser pulses from a Ti: sapphire laser at 775 nm. Through the modifications induced by the femtosecond laser radiation using selective chemical etching techniques, we fabricated reproducible and defined microstructures and further studied their morphologies and etching properties. Moreover, a possible physical mechanism for the femtosecond laser modification in soda-lime glass was proposed. 相似文献
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通过建立声子散射概率函数描述声子在输运过程中的散射,提出了一种模拟声子弹道扩散导热的蒙特卡罗方法,并将其应用于硅纳米薄膜中的稳态和瞬态弹道扩散导热过程的研究. 提出的蒙特卡罗方法对边界发射的声子束进行跟踪,根据散射概率函数模拟声子束在传播区域内经历的散射过程,并通过统计声子束的分布得到温度分布. 稳态导热过程的模拟发现,尺寸效应会引起边界温度跳跃,其值随着Knudsen数的增大而增大;计算的硅纳米薄膜的热导率随着厚度的增大而增大,与文献中的实验数据和理论模型相符. 通过瞬态导热过程的模拟得到了纳米薄膜内的温度分布随时间的变化,发现瞬态导热过程中的热波现象与空间尺度相关,材料尺寸越小,弹道输运越强,薄膜中的热波现象也越显著.
关键词:
纳米薄膜
弹道扩散导热
蒙特卡罗模拟
尺寸效应 相似文献