Diffraction patterns of laser beams with thermal self-phase modulation by optically thin films

Summary A mathematical model of the far-field diffraction pattern of a laser beam with laser heating self-phase modulation through an optically thin film is presented. We deal with the steady-state temperature field generated by the weak absorption of a laser beam in an optically and thermally thin film bounded by two transparent plates, the cell walls, whose thermal exchange to the surrounding ambient has been assumed to be linear in the temperature difference to the ambient. These hypotheses describe the application of the steady-state ?thermal lens? spectroscopy to the detection of the optical or thermal properties of liquid samples. We give a very simple expression for the diffracted intensity that appears suitable for fast computer calculations. The good accuracy of the technique is shown by comparison with the results given by more intricate approaches. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Graded finite element simulation of thermal stress in inhomogeneous high-Tc superconductor

YBa₂Cu₃O_y is an orthotropic material with different material properties in a, b and c directions, such as Young’s modulus, coefficient of thermal expansion (CTE), and thermal conductivity. It is assumed that the material properties of inhomogeneous high temperature superconductor (HTS) vary with different height coordinate and temperature. A model is presented in this paper to calculate the thermal stress of inhomogeneous HTS when temperature decreases from ambient to operating conditions (cryogenic temperatures). By fitting a second order polynomial to the experimental data, value of the material properties of inhomogeneous HTS can be obtained. Then, through the proposed graded finite element method, the coupled thermo-mechanical equations were solved numerically. The numerical results show that the temperature profiles distribute the function of time after soaking. It is notable that the temperature profile reaches steady in a very short period of time, so the thermal stress suddenly increases to a very high level for a bulk superconductor. It is also shown that the closer to the sample internal region it is, the larger the heat fluxes are. Besides, the maximum tensile stresses, i.e. the peeling stresses, occur near bottom corner of inhomogeneous HTS. It is intended that the model presented in this paper could be useful to researchers who are interested in mechanical properties of inhomogeneous HTS.     

Auto-wave regime of heating of dielectric media by electromagnetic radiation

The process of heating a moving medium by an electromagnetic radiation in high-frequency range is considered in the presence of heat exchange with the ambient medium in the approximation of a thermally thin layer. The existence of temperature profiles in the form of auto-waves is established. The analytic and numerical solutions are compared.     

INVESTIGATION OF A QUASI-STEADY LIQUID CRYSTAL TECHNIQUE FOR FILM COOLING HEAT TRANSFER MEASUREMENTS

A quasi-steady technique to simultaneously measure the local heat transfer coefficient and cooling effectiveness on surfaces involving film cooling situations is investigated. The method employs a composite slab consisting of a very thin laminate layer of low-thermal-conductivity material superposed upon a highly conductive metal substrate. The resulting heat transfer in the thin laminate is described by one-dimensional conduction. A very thin coating of thermochromic liquid crystals sprayed onto the surface of the laminate is used in conjunction with a computer image processing procedure to provide local surface temperature data. This information, combined with the substrate and mainstream gas temperatures, provides highly detailed (90 video pixels/cm²) local convection heat transfer distributions. The method is used to conduct flat-plate film cooling experiments consisting of a single row of discrete holes inclined at 35 to the mainstream flow. The local surface temperature is influenced by the combination of two interacting fluid streams at different temperatures. A numerical analysis was performed to assess the assumptions underlying the data reduction procedure. The experimental uncertainty of 7% in the heat transfer coefficient is comparable to prior studies. Furthermore, the uncertainty of 5% in the film cooling effectiveness, coupled with the negligible lateral conduction errors, indicates the present technique offers a unique capability for accurate measurement of the local film cooling effectiveness.     

Simultaneous estimation of the temperature and heat rate distributions within the combustion region by a new inverse radiation analysis

A new inverse radiation analysis is presented for estimating the heat rate and temperature distributions in the combustion region from the information of the temperature and heat flux profiles of wall elements in the system. The Monte Carlo method is employed to solve the radiative heat transfer equation. The inverse radiation problem is posed as a minimization problem of the least squares criterion, which is solved by the conjugate gradient method. The performance of the present technique of inverse analysis is evaluated and the effects of the errors of the absorption coefficient, emissivity and convective heat transfer coefficient on the inverse analysis are investigated. The results show that the present technique is robust and yields accurate estimation even with noisy measurement.     

水平管降膜蒸发器传热特性研究

建立了水平管降膜蒸发和管内凝结传热实验台,通过对实验结果的归纳,获得了水平管降膜蒸发器总传热系数随喷淋密度、蒸发温度、传热温差和蒸汽入口流速的变化规律,以及管间距对传热特性的影响。结果表明,总传热系数随喷淋密度、蒸发温度的增大而增大,随传热温差的增大而减小,而蒸汽流速对传热系数影响较小;在本文研究的管间距范围内,当管间距为46.7 mm时,总传热系数最高。     

辐冷器视场角对其制冷性能的影响

本文对抛物面G型辐冷器,在水平视场角变化的情况下不同工况下的制冷性能进行了计算。通过数值计算发现,视场角的减小导致辐冷器制冷性能的明显下降,虽然视场角影响辐冷器的因素很复杂,但最后的结果是两者呈现出良好的线性关系。其次,通过对辐冷器降温特性的研究,提出了通过改变辐冷器外壳控温方式来减小视场角变化对辐冷器制冷性能的负面影响,计算结果表明这种解决措施是非常有效的。     

斜抽运无机液体激光器的流场热分布

激光二极管斜抽运的多增益段串接的液体激光器能够明显地提高激光光束质量、获得较高的输出功率.针对斜抽运子增益段工作时所涉及的流动、传热和壁面耦合,建立了计算子增益段流场热分布的流-热-固耦合模型,应用有限单元法完成了其瞬态流场热分布的数值模拟.该方法排除了不精确的换热系数对计算结果的影响,使得换热系数不再是计算的先决条件,而只是计算结果之一,并且为评价流道形状、流速、吸收系数等因素对流场热的影响,以及进一步改进和控制液体激光介质的流场热分布,提供了可靠的分析方法.数值模拟研究表明:换热系数是空间位置的函数,     

Heat transfer in locally heated liquid films moving under the action of gravity and gas flow

Heat transfer in a viscous liquid film moving under the action of gravity and a gas flow on a substrate with a locally heated rectangular area is investigated. The heat exchange coefficient is given on the liquid-gas surface; the heat flux to the liquid is given on the heated area; the substrate surface outside the heated area is heat-insulated. An analytical solution in a form of a convergent series is obtained for the liquid temperature distribution in the film. The influence of the dimensionless criteria on the obtained solution is analyzed. The effect of heat flux inhomogeneity on the temperature distribution is considered.     

Numerical simulation of tissue freezing by liquid nitrogen based cryoprobe

A model is presented to simulate the cooling processes during tumor cryosurgery with different kinds of flows through the cryoprobe. The heat flux between the cryoprobe wall and the tumor, the heat transfer coefficient under different inflow conditions are obtained numerically. The impact of the inlet mass flow rate, gas volume fraction on these parameters is investigated. It is found that the heat transfer coefficient decreased significantly when inflow changed from two-phase annular flow to droplet flow, and to gas flow. The inlet gas volume fraction and flow velocity only significantly affect the freezing ability of the probe when the inflow is gas or in droplet phase. Simulation of the tumor temperature profiles under different flow conditions show that the heat transfer coefficient is a crucial parameter in temperature prediction during cryosurgery. Results indicate that when the cryoprobe wall is assumed at a constant temperature conventionally, the cooling effect could be overestimated. It would be more reasonable to use the constant wall heat transfer coefficient to simulate the cooling progress under a specific flow.     

太阳能吸热器玻璃窗辐射与导热计算

在太阳能塔式热发电空气吸热器工作过程中,吸热器玻璃窗受到聚焦太阳光、环境以及吸热面（体）的复合辐射。本文采用改进蒙特卡罗法、有限容积法,研究了太阳能空气吸热器石英玻璃窗半透明介质内部辐射与导热的耦合换热,研究了不同厚度半透明介质石英玻璃窗在工作面处于不同第三类边界条件下的稳态温度分布。研究表明石英玻璃的厚度和内侧空气温...     

Coupled thermal-optic effects and electrical modulation mechanism of birefringence crystal with Gaussian laser incidence

We study the Gaussian laser transmission in lithium niobate crystal(LiNbO3) by using the finite element method to solve the electromagnetic field's frequency domain equation and energy equation. The heat generated is identified by calculating the transmission loss of the electromagnetic wave in the birefringence crystal, and the calculated value of the heat generated is substituted into the energy equation. The electromagnetic wave's energy losses induced by its multiple refractions and reflections along with the resulting physical property changes of the lithium niobate crystal are considered.Influences of ambient temperature and heat transfer coefficient on refraction and walk-off angles of O-ray and E-ray in the cases of different incident powers and crystal thicknesses are analyzed. The E-ray electrical modulation instances, in which the polarized light waveform is adjusted to the rated condition via an applied electrical field in the cases of different ambient temperatures and heat transfer coefficients, are provided to conclude that there is a correlation between ambient temperature and applied electrical field intensity and a correlation between surface heat transfer coefficient and applied electrical field intensity. The applicable electrical modulation ranges without crystal breakdown are proposed. The study shows that the electrical field-adjustable heat transfer coefficient range becomes narrow as the incident power decreases and wide as the crystal thickness increases. In addition, it is pointed out that controlling the ambient temperature is easier than controlling the heat transfer coefficient. The results of the present study can be used as a quantitative theoretical basis for removing the adverse effects induced by thermal deposition due to linear laser absorption in the crystal, such as depolarization or wave front distortion, and indicate the feasibility of adjusting the refractive index in the window area by changing the heat transfer boundary conditions in a wide-spectrum laser.     

正常和嵌合肿瘤乳腺温度分布特征的有限元分析

基于乳腺解剖学结构和生理学特征,建立了适合应用于正常和嵌合肿瘤乳腺三维热传递的分析模型.该模型综合考虑了乳腺代谢产热、血液灌注和动静脉血管与乳腺组织之间的热交换等因素对温度场的影响.数值研究了正常和嵌合肿瘤乳腺的温度分布特征,着重分析代谢产热、血液灌注率、乳腺肿瘤的大小、位置与数目以及空气对流系数、环境温度等因素对乳腺温度分布的影响.结果表明:血液灌注率、代谢产热、乳腺肿瘤的大小、位置与数目对乳腺温度分布特征的影响明显;环境条件对嵌含肿瘤乳腺的体表温度分布影响较大.研究结果将有助于指导乳腺红外热图像的实验并为其定性分析提供重要的参考.     

脉冲高能量密度等离子体法制备TiN薄膜及其摩擦磨损性能研究

利用脉冲高能量密度等离子体技术在室温条件下在45号钢基材上制备出了超硬耐磨TiN薄膜.利用XRD，XPS，AES，SEM等手段分析了薄膜的成分及显微组织结构，并测试了薄膜的硬度分布及摩擦磨损性能.结果表明：薄膜主要组成相为TiN，薄膜组织致密、均匀，与基材之间存在较宽的混合界面；薄膜硬度高，在干滑动磨损实验条件下具有优异的耐磨性及较低的摩擦系数. 关键词： 脉冲高能量密度等离子体 TiN膜 显微组织 耐磨性     

Theoretical model of ice nucleation induced by acoustic cavitation. Part 1: Pressure and temperature profiles around a single bubble

This paper deals with the inertial cavitation of a single gas bubble in a liquid submitted to an ultrasonic wave. The aim was to calculate accurately the pressure and temperature at the bubble wall and in the liquid adjacent to the wall just before and just after the collapse. Two different approaches were proposed for modeling the heat transfer between the ambient liquid and the gas: the simplified approach (A) with liquid acting as perfect heat sink, the rigorous approach (B) with liquid acting as a normal heat conducting medium. The time profiles of the bubble radius, gas temperature, interface temperature and pressure corresponding to the above models were compared and important differences were observed excepted for the bubble size. The exact pressure and temperature distributions in the liquid corresponding to the second model (B) were also presented. These profiles are necessary for the prediction of any physical phenomena occurring around the cavitation bubble, with possible applications to sono-crystallization.     

Numerical modelling of a direct current non-transferred thermal plasma torch for optimal performance

A laminar steady-state 2D axisymmetric model of a direct current (DC) thermal plasma torch using a magneto-hydrodynamic approach has been developed. The model takes into account the entire torch system comprising the plasma gas injection, the inner region of the torch, and the jet exiting into the ambient environment. Numerical results are obtained for two different power inputs chosen from published experimental data. The temperature predictions at the torch exit are found in good agreement with experimental results. Velocity analysis of the plasma jet has been presented and the impact of electromagnetic force on jet velocity is analysed. The Lorentz force arising due to the coupling of fluid motion and electromagnetic forces shoots up the jet velocity to significantly high values near the cathode tip. Temperature and velocity profiles are in good agreement with the characteristics of a long laminar plasma jet. An operating value of heat transfer coefficient (h) has been suggested for optimal torch operation, thus ensuring a low anode erosion rate and acceptable thermal efficiency. The argon torch has the maximum temperature and longest jet length among the plasma gases considered.     

激光加热物体的三维模型计算

将物体的热传导系数、比热容、密度、物体对激光的吸收系数考虑成温度的函数,并考虑到物体向周围空间有热辐射和对流换热交换能量等情况下,用控制容积法对激光加热物体作三维真实边界的计算分析.这一结果在激光加热物体温度场的计算上有广泛的实用性.     

An aeroacoustically driven thermoacoustic heat pump

The mean flow of gas in a pipe past a cavity can excite the resonant acoustic modes of the cavity--much like blowing across the top of a bottle. The periodic shedding of vortices from the leading edge of the mouth of the cavity feeds energy into the acoustic modes which, in turn, affect the shedding of the next vortex. This so-called aeroacoustic whistle can excite very high amplitude acoustic standing waves within a cavity defined by coaxial side branches closed at their ends. The amplitude of these standing waves can easily be 20% of the ambient pressure at optimal gas flow rates and ambient pressures within the main pipe. A standing wave thermoacoustic heat pump is a device which utilizes the in-phase pressure and displacement oscillations to pump heat across a porous medium thereby establishing, or maintaining, a temperature gradient. Experimental results of a combined system of aeroacoustic sound source and a simple thermoacoustic stack will be presented.     

强化管管外升膜蒸发换热特性实验

升膜蒸发是利用微细槽道对液体的毛细抽吸作用,在强化管外表面覆盖一层薄液膜,进而以薄膜蒸发的形式实现强化换热.本文针对强化管在水中的浸入深度,蒸发压力,加热壁面过热度等因素对升膜蒸发换热性能的影响展开实验研究.实验结果表明随着管外液位的降低升膜蒸发换热系数明显提高,此外,蒸发压力和加热壁面过热度因素对升膜蒸发换热性能也有着显著的影响.     

样品侧面热交换对调制PTR信号的影响

给出材料在调制光辐照下温度起伏量的理论公式 ,表明侧面热交换系数直接影响贝塞耳函数本征值的确定是一个必须考虑的因素。通过模拟计算讨论分析了侧面热交换对不透明样品光热辐射信号 (PTR)频率特性和径向分布的影响 ,并在两种材料样品的实验结果中得到验证。得出侧面热交换使PTR信号幅度下降 ,相位延迟减少 ,在低调制工作频率区域表现明显的结果 ,为光热辐射测量选择实验条件提供参考依据。