Laser consecutive pulse heating in relation to melting: influence of duty cycle on melting

Laser consecutive pulse heating of steel and titanium is considered and the influence of consecutive pulse duty cycle on the melting process is examined. A model study is introduced to accommodate the phase change process while an experiment is carried out to measure the size of the melting region along the irradiated surface. The simulations are repeated for three duty cycles. It is found that the sizes of melt and mushy zones are influenced by the duty cycle; in which case, radial distribution of temperature is modified significantly for low duty cycle as compared to for high duty cycle.     

Analytical solution for temperature field in thin film initially heated by a short-pulse laser source

Analytical solution for electron and lattice temperature distribution in the solid initially heated by a laser short-pulse is presented. Strained parameters method is introduced when formulating electron and lattice temperature distributions. Laser short pulse heating of gold film is simulated numerically and temperature data at the end of the heating pulse are adopted as initial condition to the governing equations of energy transport for analytical solutions. This enables to solve the governing equations of energy analytically in the cooling period. It is found that electron temperature decays sharply while lattice site temperature increases gradually in the surface regions during the cooling cycle. As the depth from the surface increases change in both temperatures become gradual.     

Pulsed laser heating – Fourier and electron kinetic theory approaches

Laser surface pulse heating of engineering metals is in demand in the metal industry and investigation into laser pulse heating becomes fruitful in this regard. Application of Fourier theory to heat conduction due to high power laser irradiation may give closed form solution to the problem. On the other hand, the heat flux through a given plane depends on the electron energy distribution through the material and at the scale of distance required to examine the problem, the material can no longer be considered as being homogeneous continuum, therefore, errors may occur when considering the Fourier theory in laser heating process. The problem requires to be examined in the quantum field. The present study examines the pulse laser heating process when considering both Fourier conduction and electron-kinetic theory approaches. Analytical solution to Fourier conduction equation is obtained for intensity exponential pulses while numerical scheme is introduced to solve the heat transfer equation resulted from kinetic theory approach. It is found that both Fourier and electron kinetic theory approaches result in similar temperature profiles for the pulses having the same energy content. In the case of electron kinetic theory approach the rise time for surface temperature to reach the melting point is shorter than that obtained from the analytical solution. Received on 23 February 1998     

Laser heating of multilayer assembly and stress levels: elasto-plastic consideration

Laser heating of multilayer assembly results in temperature profiles, which differ in each layer. Depending upon the material and laser pulse properties, high temperature gradients can occur. This, in turn, results in excessive stress levels in the region irradiated by a laser beam. When the magnitude of stress level exceeds the yield stress of the material, plastic deformation is resulted. In this case, elasto-plastic analysis should be applied when modeling the thermal stresses due to a laser heating pulse. In the present study, time exponentially varying pulse laser heating of four and two layer assemblies is considered. The four layer assembly composes of gold, chromium, gold and silicon while two layer assembly is formed from gold and silicon. The resulting temperature field is obtained numerically using a control volume approach. The elasto-plastic analysis employed to compute the stress levels in the substrate material. It is found that stress levels higher than the yield stress of the substrate material occurs in the surface region. This, in turn, results in plastic zone in this region. The size of the plastic region extends towards the solid bulk as the heating progresses.     

Feasibility study on strengthening heating effect of high power short pulse laser on biological tissue by micro/nano metal particles

A new method for enhancing the heating effect of high power short pulse laser on biological tissue by micro/nano metal particles was proposed. Theoretical analysis of the influences of the micro/nano particle kind, the concentration and the microcosmic distribution of micro/nano particles on the temperature response was carried out with a multi-layer hyperbolic heat conduction model with volumetric heat generation. The results indicate that embedding micro/nano particles could improve the surface temperature increase of biological tissue with short duration and reduce the deeper material temperature under the same heating condition, which would help strengthen the heating effects of high power short pulse laser on biological tissue. This study may open a new technical approach for improving laser applications.     

Applications of Crank-Nicolson method with ADI in laser transformation hardening

A two-dimensional numerical solution for pulsed laser transformation hardening is developed using the finite difference method (FDM). The FDM has been developed using Crank-Nicolson scheme which solved by using alternating-direction implicit method. If this present model was compared to the analytical solution, then the Crank-Nicolson scheme showed better results in terms of accuracy, consistency, stability, convergence, and performance than to the explicit scheme. The longer heating duration, higher laser beam intensity, and greater number of pulse had influences on increasing the maximum temperature. The repetitive heating had influences on extending the heat duration and increasing the initial temperature of domain. The shorter cooling duration in repetitive pulse produced higher maximum temperature. The thinner material’s thickness increased the cooling rate, which finally increased the possibility of austenite to transform into martensite phase. In addition, it was also found that the higher maximum temperature always reduced the cooling rate value when temperature cools down toward to the starting temperature of martensite formation. It reduced the possibility of martensite formation. It was also seen that the heat was conducted more effective to the axial direction than to the radial direction.     

Effect of variable duty cycle flow pulsations on heat transfer enhancement for an impinging air jet

A series of experiments has been conducted in which a pulsed air jet is impinged upon a heated surface for the purpose of enhancing heat transfer relative to the corresponding steady air jet. Traditional variables such as jet to plate spacing, Reynolds number, and pulse frequency have been investigated. One additional flow variable – the duty cycle – representing the ratio of pulse cycle on-time to total cycle time is introduced and shown to be significant in determining the level of heat transfer enhancement. Specifically, heat transfer enhancement exceeding 50% is shown for a variety of operating conditions. In each case, the duty cycle producing the best heat transfer is shown to depend upon each of the other flow parameters. Recommendations are made for further experimentation into optimizing the duty cycle parameter for any particular application.     

Mechanical response and heat buildup in repetitively impacted elastomers

Experimental and analytical investigations were made to find the rebound characteristics and temperature rise in elastomer slabs subjected to repetitive impact. For its mechanical response, the elastomer was modeled as a linearly viscoelastic material, and its frequency and temperature dependent moduli were determined by Rheo-vibron tests. The experimental traces of impact force vs. time were studied in relation to various input parameters (e.g., impact velocity and slab thickness). An iterative method was devised to calculate the pulse from the input parameters enabling the analyst to estimate the peak force or duration of impacts occurring in a specific damper. As opposed to single (isolated) impacts, the repetitive impact process is conducive to temperature rise in the elastomer slabs resulting in a steady state temperature distribution. Under various input parameters, the temperature on the surface and inside the rubber was measured in the steady state. Both a simplified and a pulsed-heat-generative mathematical model was used to analytically estimate these temperatures.     

Numerical approach to pulsed laser heating of semi-infinite aluminum substance

Processing of the reflective materials, such as aluminum, with a pulsed CO₂ laser beam depends largely on laser output power and pulse form. To enhance the understanding of the effect of pulse parameters on laser machining a modeling of laser induced heating is essential. The present study develops the heat transfer model allowing temporal variation of CO₂ laser output pulse, phase change process and temperature dependent thermal properties. A numerical technique is introduced to solve the resulting heat transfer equation. Aluminum is selected as workpiece and its surface reflectivity is taken into account in the computation. Thermal integration due to repetitive pulsing is also discussed. It is found that time corresponding to maximum temperature can be predicted by proper selection of pulsed parameters and the ability of the material to follow the laser pulse profile depends upon the pulse shape.     

Dynamic yield-strength determination at elevated temperatures after nanosecond pulse heating

An experimental method is described that has been used to determine the yield strength of 6061-T6 aluminum after extremely short times at elevated temperature. The method combines electron-beam pulse heating and onedimensional stress-wave loading. A 3.5-MeV pulsed electron-beam source (pulse width of 70 nanoseconds) is used to deposit energy uniformly through the thickness and along a limited region of a slender aluminum rod. An axial compressive stress wave, produced by projectile impact on one end of the rod, propagates into the heated region a few microseconds after energy deposition. The nanosecond electron-beam pulse increases the internal energy of the material before it can expand to equilibrium dimensions at the elevated temperature. Additional time is therefore required for the specimen to equilibrate mechanically through the propagation of radial release waves which originate at the stress-free boundary of the sample. The deformation produced by these radial relief waves is coupled with microstructural changes that also contribute to a reduction in the yield strength of the material at elevated temperature, as well as at room temperature following electron-beam heating.     

Tensile and mixed-mode strength of a thin film-substrate interface under laser induced pulse loading

Laser induced stress waves are used to characterize intrinsic interfacial strength of thin films under both tensile and mixed-mode conditions. A short-duration compressive pulse induced by pulsed-laser ablation of a sacrificial layer on one side of a substrate is allowed to impinge upon a thin test film on the opposite surface. Laser-interferometric measurements of test film displacement enable calculation of the stresses generated at the interface. The tensile stress at the onset of failure is taken to be the intrinsic tensile strength of the interface. Fused-silica substrates, with their negative nonlinear elasticity, cause the compressive stress wave generated by the pulse laser to evolve a decompression shock, critical for generation of the fast fall times needed for significant loading of surface film interfaces. By allowing the stress pulse to mode convert as it reflects from an oblique surface, a high amplitude shear wave with rapid fall time is generated and used to realize mixed-mode loading of thin film interfaces. We report intrinsic strengths of an aluminum/fused silica interface under both tensile and mixed-mode conditions. The failure mechanism under mixed-mode loading differs significantly from that observed under pure tensile loading, resulting in a higher interfacial strength for the mixed-mode case. Inferred strengths are found to be independent, as they should be, of experimental parameters.     

Microscope activation near the wall during explosive boiling

The inception process of nucleation in explosive boiling systems is theoretically investigated. With the effect of pulse heating or sudden cooling, the temperature distribution near the surface during explosive boiling is calculated. The liquid near the wall can maintain a stable layer induced by strong attractive force, and there exists maximum supersaturation beyond this stable layer. As the surface temperature and temperature gradient are high enough, the critical distance of maximum supersaturation can be larger than the radius of critical bubble, and the homogeneous nucleation will dominate the inception boiling process. For explosive boiling induced by pulse heating, homogeneous nucleation forms after a short time; while homogeneous nucleation can dominate the initial explosive boiling induced by sudden cooling.     

3-D analysis of temperature distribution in the material during pulsed laser and material interaction

Recently, lasers are being increasingly used in the industry owing to their precision and low cost. Material is heated and evaporated during laser and material interaction due to the absorption of laser beams by the material. In this study, a 3-D Laser heating model including evaporation has been solved using the electron- kinetic theory approach. The basis in examining the problem using the kinetic theory approach is to describe the heat conduction through electron-phonon and molecule-phonon collisions. The problem is solved by using the electron-kinetic theory approach in such a way that heat conduction is taken into account until the material is heated to its melting temperature and non-conduction limited heat transfer is considered after the melting temperature is reached. Non-conduction limited heat transfer through the phase change process is resulted from vacancy-molecule collisions. A numerical scheme is introduced to solve the governing equation, owing to the fact that the energy equation resulted is in the form of integro-differential equation. Four different materials, namely iron, nickel, tantalum and titanium are chosen in this study determine the material response to laser pulse heating. For each material, time dependent temperature distribution through the depth of the material and on the surface of the material is computed and analyzed for four different materials.     

Seed bubbles stabilize flow and heat transfer in parallel microchannels

Seed bubbles are generated on microheaters located at the microchannel upstream and driven by a pulse voltage signal, to improve flow and heat transfer performance in microchannels. The present study investigates how seed bubbles stabilize flow and heat transfer in micro-boiling systems. For the forced convection flow, when heat flux at the wall surface is continuously increased, flow instability is self-sustained in microchannels with large oscillation amplitudes and long periods. Introduction of seed bubbles in time sequence improves flow and heat transfer performance significantly. Low frequency (∼10 Hz) seed bubbles not only decrease oscillation amplitudes of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures, but also shorten oscillation cycle periods. High frequency (∼100 Hz or high) seed bubbles completely suppress the flow instability and the heat transfer system displays stable parameters of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures. Flow visualizations show that a quasi-stable boundary interface from spheric bubble to elongated bubble is maintained in a very narrow distance range at any time. The seed bubble technique almost does not increase the pressure drop across microsystems, which is thoroughly different from those reported in the literature. The higher the seed bubble frequency, the more decreased heating surface temperatures are. A saturation seed bubble frequency of 1000–2000 Hz can be reached, at which heat transfer enhancement attains the maximum degree, inferring a complete thermal equilibrium of vapor and liquid phases in microchannels. Benefits of the seed bubble technique are the stabilization of flow and heat transfer, decreasing heating surface temperatures and improving temperature uniformity of the heating surface.     

Supersonic aerodynamic performance of truncated cones with repetitive laser pulse energy depositions

We investigate the drag characteristics of truncated cones in Mach 1.94 flow with repetitive laser pulse energy depositions with a frequency of up to 80 kHz. The drag decrement is almost in proportion to the laser pulse repetition frequency, and scales with a greater-than-square power of the truncation diameter. The performance of the latter is associated with the effective area of pressure modulation and the effective residence time of vortices which are baroclinically generated after the interaction between laser-heated gas bubbles and the bow shock wave. With employing a concave head, the drag decrement is enhanced. With increasing the truncation diameter, the efficiency of energy deposition becomes higher; yet, within the operation range of this study the drag coefficient still remains high.     

Temperature field at time of pulse current discharge in metal structure with elliptical embedding crack

Theoretical analysis is made on the temperature field at the time of pulse current discharge in a metal structure with an elliptical embedding crack. In finding the temperature field, analogy between the current flow through an elliptical embedding crack and the fluid flow through a barrier is made based on the similarity principle. Boundary conditions derived from this theory are introduced so that the distribution of current density and the temperature field expressions can be obtained. The study provides a theoretic basis to the applications of stopping spatial crack with electromagnetic heating.     

地震动速度脉冲对高温气冷堆核电厂地震反应的影响

为探讨近断层地震动的速度脉冲对结构抗震能力的影响特征,以某高温气冷堆核电厂结构为研究对象,利用有限元软件建立线性三维模型,选择4组具有速度脉冲特性的近断层地震动加速度记录及人工模拟的具有相同加速度反应谱而无速度脉冲的地震动时程分别作为地震动输入,对模型进行动力时程分析,对比在有、无速度脉冲地震动作用下模型的地震反应。研究发现,虽然反应过程中结构仍处于弹性阶段但是地震动的速度脉冲对结构的位移反应具有一定的不利影响,这一点与已有的基本认识不同。因此对于需要安装对位移反应较为敏感设备的高温气冷堆核电厂房,应充分关注地震动速度脉冲对结构反应的影响。     

Aerodynamic thermal environment around transonic tube train in choked/unchoked flow

The aerodynamic thermal environment in an evacuated tube transport (ETT) system is an important factor in ensuring the operational safety of tube trains, where choking can further worsen air flow and aerodynamic heating. A compressible flow solver based on total variation diminishing (TVD) schemes was used to calculate the transonic aerodynamic behaviour of a capsule train in a confined space under low pressure conditions, and the flow fields and aerodynamic heating effect on the train were obtained. The results showed that the flow state around the train could be classified into choked and unchoked flow according to the blockage ratio (BR) and train speed based on the Kantrowitz limit. The wall viscosity caused a difference in the boundary-layer flow and potential flow in the annular space between the train and the tube with an increase in the BR. The choked flow was driven forward by the train, passing through its throat at the speed of sound. Owing to the complicated compressible flow in the tube, the thermal environment around the train gave rise to extreme temperature changes on its surface. In transonic choked flow, the temperature rise of the train head reached a maximum of 525 K, whereas local cooling could occur in the afterbody, causing the surface temperature to fall below the ambient temperature under certain conditions. The findings can be used to guide the design of ETT systems.     

Primary pulse transmission in a strongly nonlinear periodic system

The aim of this work is to study the transmission of stress waves in an impulsively forced semi-infinite repetitive system of linear layers which are coupled by means of strongly nonlinear coupling elements. Only primary pulse transmission and reflection at each nonlinear element is considered. This permits the reduction of the problem to an infinite set of first-order strongly nonlinear ordinary differential equations. A subset of these equations is solved both analytically and numerically. For a system possessing clearance nonlinearities it is found that the primary transmitted pulse propagates to only a finite number of layers, and that further transmission of energy to additional layers can occur only through time-delayed secondary pulses or does not occur at all. Hence, clearance nonlinearities in a periodic layered system can lead to energy entrapment in the leading layers. An alternative continuum approximation methodology is also outlined which reduces the problem of primary pulse transmission to the solution of a single strongly nonlinear partial differential equation. The use of the continuum approximation for studying maximum primary pulse penetration in the system with clearance nonlinearities is discussed.     

Transient behavior of boiling bubbles generated on the small heater of a thermal ink jet printhead

The fundamental behavior of boiling bubbles generated on a small film heater used for thermal ink jet (TIJ) printers is investigated experimentally under the condition of a single pulse heating in a pool of water. The pulse power and the pulse width are varied in wide ranges that include the printing conditions. As the pulse power is increased or the pulse width increased at a fixed high pulse power, numerous fine bubbles appear simultaneously on the heater and then coalesce into a thin vapor film to grow to a vapor bubble, before collapsing at the center of the heater. For a long pulse width sequence, the coalesced bubble repeats the growth and collapse. Bubble behavior is also studied in the same heat flux range using a platinum film heater enabling surface temperature measurement. From a comparison of the two heaters, the dominant mechanism of nucleation on the TIJ heater is believed to be spontaneous nucleation at around the heating rate for printing. The dependence of the size and lifetime of the coalesced bubble on pulse power and pulse width are examined. Based on the analytical model presented by Asai [J. Heat Transfer 113 (1991) 973], the pressure impulse arising during the rapid evaporation of the superheated liquid, presumed to dominate the subsequent growth of the coalesced bubble, is estimated from the measured size of the coalesced bubble. The relationship between the pressure impulse and the superheat energy in the liquid is discussed.