Fluidic lens by using thermal lens effect

This research presents a novel fluidic lens based on thermal lens effect. Effects of the pump power and the pump beam intensity distribution to the probe beam profile in the dual thermal lens system are investigated experimentally and theoretically. A model, which accounts for heat conduction, natural heat convection and ray tracing in inhomogeneous medium is developed to predict the characteristics of the thermal lens system. Numerical results show the advantage of the uniform pump beam in reducing the spherical aberration compared with the Gaussian pump beam. An experiment with the uniform pump beam is carried out to confirm the numerical prediction. Experimental results show a good agreement with the calculated results. Finally, the pump power is varied to adjust the focal length of the system.     

Theoretical and experimental studies on laser transformation hardening of steel by customized beam

An exact solution was developed for a quasi-steady-moving-interface heat transfer problem in laser transformation hardening by a beam customized to a flat-top rectangular shape. The absorption and release of the latent heat were taken into account. The solution provided the body temperature, heating rate, and subsequent cooling rate. Using the IT and CCT diagrams, the rapid austenitization and pearlitic/martensitic phase transformation were determined for prediction of the hardening effect. In the experimental study, a diffractive optical kinoform (DOK) was used to customize a conventional Gaussian laser spot beam to a flat-top rectangular beam. A reasonable agreement between the theoretical predictions and the experimental measurements was obtained.     

Thermal fatigue on pistons induced by shaped high power laser. Part I: Experimental study of transient temperature field and temperature oscillation

Thermal fatigue behavior is one of the foremost considerations in the design and operation of diesel engines. It is found that thermal fatigue is closely related to the temperature field and temperature fluctuation in the structure. In this paper, spatially shaped high power laser was introduced to simulate thermal loadings on the piston. The incident Gaussian beam was transformed into concentric multi-circular beam of specific intensity distribution with the help of diffractive optical element (DOE), and the transient temperature fields in the piston similar to those under working conditions could be achieved by setting up appropriate loading cycles. Simulation tests for typical thermal loading conditions, i.e., thermal high cycle fatigue (HCF) and thermal shock (or thermal low cycle fatigue, LCF) were carried out. Several important parameters that affect the transient temperature fields and/or temperature oscillations, including controlling mode, intensity distribution of shaped laser, laser power, temporal profile of laser pulse, heating time and cooling time in one thermal cycle, etc., were investigated and discussed. The results show that as a novel method, the shaped high power laser can simulate thermal loadings on pistons efficiently, and it is helpful in the study of thermal fatigue behavior in pistons.     

Spatially Resolved Measurements of Thermal Stresses by Picosecond Time-Domain Probe Beam Deflection

Measurements of thermally induced strains with micron-scale lateral spatial resolution, picosecond time resolution, and sub-picometer vertical sensitivity are achieved using a newly developed experimental method, time-domain probe beam defection (TD-PBD). TD-PBD is a pump-probe optical technique that combines an ultrafast laser oscillator as the light source, high frequency (10 MHz) modulation of the pump beam, and a wide range of time delays (0–4 ns) between the pump and probe. Deflections of the probe beam are measured by a position sensitive detector and an rf lockin amplifier. The beam-deflection data are analyzed using a detailed model of heat transport and thermally generated stresses and strains. Comparisons between the model and the data enable quantitative measurements of the coefficient of thermal expansion with a spatial resolution of 4 µm.     

Visualization of hydrogen jet using deformation of the laser beam profile

Hydrogen has the widest flammable range, the fastest flame propagation speed, and the lowest ignition energy, so its safety needs special attention before the wide application of hydrogen energy. The main objective of this work is to propose a new method to evaluate hydrogen jet pressure by using a Helium–Neon laser through the jet. A mathematical model was proposed, which describes the deformation of the laser beam profile passing through an axisymmetric circular hydrogen jet pressure flow field in detail. This research attempts to apply the expression of density Gaussian distribution, ideal gas equation, and Gladstone Dale equation to disclose the deformation of laser beam profile under different outlet conditions. The experimental uncertainty is about 3 × 10^?3. A non-contact optical experimental system is established to visually measure the density gradient distribution of the gas jet. Our findings show that the hydrogen jet can be regarded as a gas-phase lens, and the deformation of the laser beam profile in the horizontal direction increases linearly with jet pressure. Finally, the preliminary results of calculations of the spot area with the theoretical model were presented and compared with the images of the laser beam profile passing through the jet in the experiment. The theoretical model gave similar results and the overall agreement with the experiment was satisfactory. Our technology exhibits high sensitivity in the measurement of hydrogen leakage pressure, providing a theoretical basis for non-contact, non-damaged, high-response, and high-sensitivity detection of hydrogen leakage.     

Temperature amplification during laser heating of polycrystalline silicon microcantilevers due to temperature-dependent optical properties

The absorption of optical energy and subsequent thermal transport are investigated experimentally and numerically for 500 μm long, 250 μm wide, and 2.25 μm thick polycrystalline silicon microcantilevers irradiated by an 808 nm continuous-wave laser. Temperature profiles were measured using Raman thermometry at 314 and 532 mW of laser power, and the microcantilever peak temperature was measured for laser powers up to 719 mW. A modular technique for multilayered structures is used to calculate the optical absorption in polysilicon microcantilevers, and the thermal response is then calculated with a two-dimensional, finite difference model. Very good agreement is obtained between the measured and calculated temperature profiles and peak temperatures versus laser power. The abrupt increase or amplification of peak temperature for laser powers between 415 and 440 mW is shown to be a result of peaks in the temperature-dependent optical absorptance.     

Thermal fatigue on pistons induced by shaped high power laser. Part II: Design of spatial intensity distribution via numerical simulation

In the laser induced thermal fatigue simulation test on pistons, the high power laser was transformed from the incident Gaussian beam into a concentric multi-circular pattern with specific intensity ratio. The spatial intensity distribution of the shaped beam, which determines the temperature field in the piston, must be designed before a diffractive optical element (DOE) can be manufactured. In this paper, a reverse method based on finite element model (FEM) was proposed to design the intensity distribution in order to simulate the thermal loadings on pistons. Temperature fields were obtained by solving a transient three-dimensional heat conduction equation with convective boundary conditions at the surfaces of the piston workpiece. The numerical model then was validated by approaching the computational results to the experimental data. During the process, some important parameters including laser absorptivity, convective heat transfer coefficient, thermal conductivity and Biot number were also validated. Then, optimization procedure was processed to find favorable spatial intensity distribution for the shaped beam, with the aid of the validated FEM. The analysis shows that the reverse method incorporated with numerical simulation can reduce design cycle and design expense efficiently. This method can serve as a kind of virtual experimental vehicle as well, which makes the thermal fatigue simulation test more controllable and predictable.     

Influence of interface conditions on laser diode ignition of pyrotechnic mixtures: application to the design of an ignition device

This paper treats of numerical modelling which simulates the laser ignition of pyrotechnic mixtures. The computation zone is divided into two fields. The first is used to take account of the heat loss with the outside. It can represent an optical fibre or a sapphire protective porthole. The second field represents the reactive tablet which absorbs the laser diode's beam. A specific feature of the model is that it incorporates a thermal contact resistance R_c between the two computation fields. Through knowledge of the thermal, optical and kinetic properties, this code makes it possible to compute the ignition conditions. The latter are defined by the energy E₅₀ and the time t_i of ignition of any pyrotechnic mixture and for various ignition systems.This work was validated in the case of an ignition system consisting of a laser diode with an optical lens re-focussing system. The reactive tablet contains 62% by mass of iron and 38% by mass of KClO₄. Its porosity is 25.8%. After an evaluation of the laser's coefficient of absorption, the variations of the ignition parameters E₅₀ and t_i are studied as a function of the thermal contact resistance R_c. Temperature profiles are obtained as a function of time and for various values of the thermal contact resistance R_c. More fundamental observations are made concerning the position of the hot spot corresponding to priming. From this study, which concerns the heat exchange between the two media, several practical conclusions are given concerning the design of an ignition device. By evaluation of the thermal contact resistance R_c, comparison with test results becomes possible and the results of the computations are in reasonable agreement with the test measurements.     

基于吸收式热泵的循环水余热利用技术在大型抽凝机组热电联产应用的安全性研究

基于吸收式热泵的循环水余热利用技术应用于大型抽凝机组热电联产,可以使低品质的热量得到应用,增加供热能力,但由于此项技术刚刚应用于大型抽凝机组,尚未十分成熟。从安全性角度出发,通过现场试验来验证吸收式热泵在大型抽凝机组应用的安全性是有保证的。     

Numerical simulation of thermal loading produced by shaped high power laser onto engine parts

Recently a new method for simulating the thermal loading on pistons of diesel engines was reported. The spatially shaped high power laser is employed as the heat source, and some preliminary experimental and numerical work was carried out. In this paper, a further effort was made to extend this simulation method to some other important engine parts such as cylinder heads. The incident Gaussian beam was transformed into concentric multi-circular patterns of specific intensity distributions, with the aid of diffractive optical elements (DOEs). By incorporating the appropriate repetitive laser pulses, the designed transient temperature fields and thermal loadings in the engine parts could be simulated. Thermal–structural numerical models for pistons and cylinder heads were built to predict the transient temperature and thermal stress. The models were also employed to find the optimal intensity distributions of the transformed laser beam that could produce the target transient temperature fields. Comparison of experimental and numerical results demonstrated that this systematic approach is effective in simulating the thermal loading on the engine parts.     

一种利用太阳能的激光供能方法

设计了一种太阳能光伏发电-激光供能系统,讨论了系统中能量转换效率和供能损耗问题。实验采用人工太阳模拟发射器,通过光伏发电给半导体激光器提供能量发射红外激光,采用透镜准直后照射到光电池上转换成电能。测量了各装置的输入输出功率,计算了能量转换装置的转换效率和传输过程中的能量损耗,提出了改进系统的有效途径。     

Solar cells and batteries with unusual selective coatings for combined supply of heat and electricity by solar power stations

Optical coatings are used to increase the efficiency, extend the life, and to improve the electrophysical characteristics and stability of solar energy converters based on various physical principles, including semiconductor solar cells. When solar cells are placed on the exterior of collectors in photothermal systems, and generate both electric and thermal power, the optical coating applied to their surfaces gives them highly unusual selective properties, namely, reduced reflection of solar radiation (and high transparency in this part of the spectrum), which leads to higher integrated solar absorption coefficient, and enhanced infrared reflection which ensures that the thermal emission coefficient is as low as possible. Solar cells then not only generate electric power but, at the same time, covered by these coatings, act as selective optical surfaces for solar collectors.     

Dual-pump CARS temperature and major species concentration measurements in counter-flow methane flames using narrowband pump and broadband Stokes lasers

Dual-pump coherent anti-Stokes Raman scattering (CARS) is used to measure temperature and species profiles in representative non-premixed and partially-premixed CH₄/O₂/N₂ flames. A new laser system has been developed to generate a tunable single-frequency beam for the second pump beam in the dual-pump N₂-CO₂ CARS process. The second harmonic output (∼532 nm) from an injection-seeded Nd:YAG laser is used as one of the narrowband pump beams. The second single-longitudinal-mode pump beam centered near 561 nm is generated using an injection-seeded optical parametric oscillator, consisting of two non-linear β-BBO crystals, pumped using the third harmonic output (∼355 nm) of the same Nd:YAG laser. A broadband dye laser (BBDL), pumped using the second harmonic output of an unseeded Nd:YAG laser, is employed to produce the Stokes beam centered near 607 nm with full-width-at-half-maximum of ∼250 cm⁻¹. The three beams are focused between two opposing nozzles of a counter-flow burner facility to measure temperature and major species concentrations in a variety of CH₄/O₂/N₂ non-premixed and partially-premixed flames stabilized at a global strain rate of 20 s⁻¹ at atmospheric-pressure. For the non-premixed flames, excellent agreement is observed between the measured profiles of temperature and CO₂/N₂ concentration ratios with those calculated using an opposed-flow flame code with detailed chemistry and molecular transport submodels. For partially-premixed flames, with the rich side premixing level beyond the stable premixed flame limit, the calculations overestimate the distance between the premixed and the non-premixed flamefronts. Consequently, the calculated temperatures near the rich, premixed flame are higher than those measured. Accurate prediction of the distance between the premixed and the non-premixed flames provides an interesting challenge for future computations.     

Transient conductive–radiative numerical analysis of multilayer thin films heated by different laser pulses

A thermal and optical one-dimensional numerical analysis of semi-transparent single and multilayer thin films on a transparent thermically semi-infinite glass substrate, irradiated by a laser source, is presented, in classical conductive Fourier hypothesis. The absorption is evaluated by means of the classical optical matrix method. Both in thermal and optical models the effects of temperature on the properties are taken into account. A pulsed laser source impinges on the glass side of the structure. Four different typical pulse shapes are compared at the same energy amount: a rectangular on–off shape, a symmetric triangular shape, a Gaussian shape, an asymmetric triangular Weibull profile. Numerical calculations are performed, with reference to a Nd-YAG pulsed laser and to the three structures by means of which an amorphous silicon photovoltaic cell is progressively manufactured: a single transparent conductive oxide SnO₂ (TCO) layer, a double layer of amorphous silicon (a-Si) and TCO, an Al/a-Si/TCO multilayer thin film. In each case the single or composite thin films are on a glass substrate. Results are reported in terms of depth and time profiles of radiative coefficients and temperature.     

Laminar natural convection above a horizontal laser beam

The steady laminar natural convective plume above a horizontal laser beam has been studied. The plume, which is caused by absorption of thermal energy from the beam, is three-dimensional. The three-dimensionality is a consequence of mass continuity and the variation in the thermal energy absorption in the propagation direction. Flow visualizations have verified the three-dimensionality in that a significant velocity component, in a direction opposite to that of laser beam propagation, was observed.The problem is reduced by similarity analysis to a system of ordinary differential equations which are solved numerically for the Prandtl number, Pr = 1.0. Integral approximations are also presented for Pr = 0.7, 1.0, 10.0 and 100.0. The effect of Pr on the velocities, and temperature boundary layer thicknesses is discussed in detail.     

Numerical simulation of the growth characteristics of laser chemical vapor deposition of silicon carbide

A thermal model, which involves heat transfer in substrate and gases, mass transfer in gases, and chemical reaction on the top surface of the substrate, is set up to simulate the Laser Chemical Vapor Deposition (LCVD) process of Silicon Carbide (SiC) by a finite volume method. Methyltrichlorosilane (MTS; SiCl₃CH₃) and hydrogen (H₂) are chosen as precursor and carrier gas, respectively. A designed set of model cases is executed for both stationary and moving laser beams. For the cases of stationary laser beam, the shape of the SiC deposits is higher and wider with increasing laser power. For the cases of moving laser beam, a narrow strip of SiC deposits is formed along the laser scanning path. Due to the low sticking coefficient of SiC deposits at high temperature, the volcano-like defects occur on the top center of the SiC deposits for both stationary and moving laser beams.     

电厂循环水吸收式热泵利用系统分析

热电厂的循环水所具有的热量一般是通过冷却塔释放给大气环境,为对这部分余热回收利用,进行了水源热泵能量系统的分析研究。将热泵系统供给的热量扣除消耗的驱动蒸汽热量,再考虑导致新增的驱动电耗,及以凝汽器真空下降引起发电的热耗增加作为修正,可确定最终节能量。通过对实际热电厂4台200 MW供热机组的循环水源吸收式热泵系统进行计算,可获知年节约标准煤9 985.7 t,该方案实现了比较理想的工程节能效果。     

Ultrafast Electron–Phonon Coupling at Metal-Dielectric Interface

Energy transfer from photo-excited electrons in a metal thin film to the dielectric substrate is important for understanding the ultrafast heat transfer process across the two materials. Substantial research has been conducted to investigate heat transfer in a metal-dielectric structure. In this work, a two-temperature model in metal was used to analyze the interface electron and dielectric substrate coupling. An improved temperature and wavelength-dependent Drude–Lorentz model was implemented to interpret the signals obtained in optical measurements. Ultrafast pump-and-probe measurements on Au-Si samples were carried out, where the probe photon energy was chosen to be close to the interband transition threshold of gold to minimize the influence of non-equilibrium electrons on the optical response and maximize the thermal modulation to the optical reflectance. Electron-substrate interface thermal conductance at different pump laser fluences was obtained, and was found to increase with the interface temperature.     

Effects of high-level energy pumping on radiative thermal loading of phosphate laser glass

Lasers have been employed as an ignition source for explosive detonation and combustion initiation. The radiative heat absorption in the energy pumping process is expected to alter laser optical conditions and thus affects their performances. The objective of the current research is to establish various thermal and performance characteristics of a solid-state laser designed for explosive detonation under high-level energy pumping, which include maximum temperature change, temperature rise rate, lasing delay, pulse period, etc. Some characteristics of high-level pumping are also compared with those of low-level pumping. A modified Michelson interferometer is employed together with a high-speed framing camera for gross-field detection and photo-detectors for pulse observation, respectively. The interferometric technique appears to be useful for evaluating the thermal behaviors and surface irregularities of laser rods.     

Investigations on thermal and optical performances of a glazing roof with PCM layer

The thermal and optical performances of a roof in a building containing phase change material (PCM) were investigated in this paper. The glazing roof model consists of two layers of glass and one layer of PCM. The purpose of filling the roof structure with PCM is to utilize the solar energy efficiently. The effectiveness of thermal and optical performances of the roof PCM system was determined by analyzing the heat flux and temperature at the indoor surface with different absorption coefficients and refractive index of PCM in solid and liquid states. The results show that the absorption coefficients and refractive index of solid and liquid PCMs have both effects on thermal performance in the roof PCM system. Of all the thermal performances, the effect on internal temperature, temperature lag, and total transmitted energy is smaller and the effect on solar transmittance and transmitted solar energy is bigger. The absorption coefficients have the opposite effect with the refractive index on interior temperature lag. Considering the indoor daylight, increasing the refractive index and absorption coefficient of liquid PCM is a better method to better the thermal performance of a roof PCM system. Copyright © 2017 John Wiley & Sons, Ltd.