A theoretical analysis of the systematic errors in determining the thermal characteristics of structural materials by a pulse method in a sample of finite dimensions

The problem of the three-dimensional nonstationary temperature field of a sample of material when a thermal pulse of short duration acts on its surface is solved numerically. The scales of the errors in determining the thermal characteristics of the material by a pulse method due to the assumption that heat transfer in the sample is a one-dimensional process, is estimated. The limits of the ranges of possible variation of the ratio of the area of the heated surface of the sample and the zone of thermal heating, which ensure minimum errors in determining the thermal characteristics, are established. Translated from Izmeritel’naya Tekhnika, No. 4, pp. 34–36, April, 2009.     

Optimizing dimensional accuracy of titanium alloy features produced by wire electrical discharge machining

This study investigates geometrical errors such as cylindricity, circularity and diametral errors of a feature (a hole) produced from wire electrical discharge machining of Ti6Al4V alloy where tension in wire, pulse on time, and flushing pressure are varied. Pareto analysis of variance (ANOVA), Taguchi design of experiment (DoE), and traditional analysis estimate the influence of variables on errors of holes. It was noted that flushing pressure is the most significant factor with individual contributions of 31.02%, 49.5% and 37.84% to circularity, cylindricity, and diametral errors, respectively. The circularity error of holes decreases as the flushing pressure and tension in wire rise, but decreases with the rise of pulse on time. The cylindricity error decreased with the increase of wire tension, flushing pressure and pulse on time. The absolute diametral error reduced as the pulse on time and tension in wire raised, but it raised with the rise of pulse on time. All these trends are associated with the influence of tension in wire on the flexibility of wire, the dependence of heat generation and dissipation on pulse on time, and ability of the flushing pressure to control the cooling, as well as debris removal from the machining zone.     

On the relation between rigid-plastic and elastic-plastic predictions of response to pulse loading

Using a simple model, comparisons are shown between the predictions of elastic-plastic and rigid-plastic analyses as to the permanent displacements due to pulse loading. Several pulse shapes are considered, and the comparisons are made over a large range of the ratio τ/T (pulse duration time/natural period of the structure). When the rise time of the pulse is not zero, these show a wavy character that was not seen in earlier comparisons of this type, where smaller ranges were used. Thus the two predictions are found to be very close at regular intervals, while the rigid-plastic prediction has negative (unconservative) errors in between, relative to the elastic-plastic one. These errors may be large even though the plastic deformation greatly exceeds the elastic displacement at yield. The ‘energy ratio’ criterion for validity of rigid-plastic analysis must be supplemented, for ‘long’ pulses, by consideration of the effect of pulse duration. The waviness of the elastic-plastic spectral plots is explained in terms of the similar shapes of the ‘dynamic amplification’ spectra of wholly elastic shock analysis.     

Application of the transient hot-wire method to gases at low pressures

The need to determine the thermal conductivity of non-ozone-depleting refrigerants implies measurements at pressures below 10 bar in the gaseous phase. In order to apply the transient hot-wire method with proven accuracy to this state, possible sources of systematic errors in the measurements have been carefully assessed theoretically and experimentally. The influence of the finite heat capacity of the hot wire and of the isothermal outer wall of the cell have been identified to affect the measurements substantially. An improved correction method to account for the wire heat capacity is presented, as well as criteria to choose the parameters in the experiments in order to avoid errors due to the outer boundary and due to the finite wire length. The results are presented in dimensionless quantities, and as an example, they are discussed for argon.     

Computational fluid dynamics simulations of an orifice type pulse tube refrigerator: Effects of operating frequency

A numerical study is reported here for the investigation of the fundamental flow and heat transfer processes found in an orifice type pulse tube refrigerator (OPTR). The OPTR is driven by a cyclically moving piston at one end of the system with helium as the working fluid. The regenerator and the various heat exchangers are modeled as porous media and a thermal non-equilibrium model is applied in these regions. The system is studied for different operating frequencies of the driver piston. The simulations reveal interesting steady-periodic flow patterns that develop in the pulse tube due to the fluctuations caused by the piston and the presence of the inertance tube. The predicted secondary-flow recirculation patterns in the pulse tube are found to affect the OPTR performance. When the secondary-flow patterns are well-developed, they help isolate the cold and hot ends of the pulse tube and create a thermal buffer zone at the center of the pulse tube, enhancing the performance of the OPTR.     

Numerical analysis of an OPTR: Optimization for space applications

A numerical study is reported here for the investigation of the flow and heat transfer processes in a co-axial type single stage orifice type pulse tube refrigerator (OPTR). The OPTR is driven by a cyclically moving piston at one end of the system with helium as the working fluid. The regenerator and the various heat exchangers are modeled as porous media and a thermal non-equilibrium model is applied in these regions. The simulations reveal interesting steady-periodic flow patterns that develop in the pulse tube due to the fluctuations caused by the piston and the presence of the inertance tube. When the secondary flow patterns are well-developed, they help isolate the cold and hot ends of the pulse tube and create a thermal buffer zone at the center of the pulse tube, enhancing the performance of the OPTR.     

Aerodynamic heating of ballistic missile including the effects of gravity

The aerodynamic heating of a ballistic missile due to only convection is analysed taking into consideration the effects of gravity. The amount of heat transferred to the wetted area and to the nose region has been separately determined, unlike A Miele’s treatise without consideration of gravity. The peak heating rates to the wetted area and to the nose of the missile are also investigated. Finally four numerical examples are cited to estimate the errors, in heat transfers and heating rates to both wetted area and nose region of the missile, arising out of neglecting the gravitational forces.     

Pulsed laser coating of hydroxyapatite/titanium nanoparticles on Ti-6Al-4V substrates: multiphysics simulation and experiments

Pulsed laser coating (PLC) of bioceramics/metal nanomaterials on metal substrates was investigated in this research. It is found that due to the nature of the nanosized particles and pulse laser beam, PLC processed hydroxyapatite (HAp) coatings possess strong coating/substrate interfacial bonding strength, and minimum thermal decomposition. Feasibility analysis of PLC is conducted using both simulation and experiments. In the multiphysics simulation, laser interacting with metal nanoparticles and heat conduction is simulated by coupling the electromagnetic (EM) module and heat transfer (HT) module. In experiments, HAp and titanium nanoparticle mixture are coated on Ti-6Al-4V substrate using nanosecond pulsed Nd:YAG laser with wavelength of 1064 nm. Resulting temperature is measured by calibrated infrared (IR) camera and compared with simulation results. Experimental results agree well with simulation which serves as a guidance to find appropriate processing parameters. It is found that resulting temperature increases with increasing of pulse energy linearly and decreasing of pulse duration following the power law. It is recommended that shorter pulses to be used in PLC due to its better sinterability. Microstructure and chemical characterizations confirmed that HAp was physically and chemically maintained due to pulse laser caused rapid heating and cooling processes.     

Evaluation of laser drilling of holes in thermal barrier coated superalloys

Abstract

Laser drilling of precise holes in thermal barrier coated Ni based superalloys has been studied. The interplay between various hole geometrical features such as hole shape, taper, barrelling, undercut, etc. and laser parameters such as pulse energy, pulse width and pulse repetition rate have been examined. The hole diameters are seen to follow a linear dependence on the incoming laser power densities for pulse width up to 2·0 ms. However, such a linear dependence was not observed for a pulse width of 3·0 ms. It was found that high pulse energy and short pulse width (high power density) gave crack free recast layer, whereas low pulse energy and longer pulse width (low power density) gave microcracks in the heat affected layer of superalloy. The significant barrelling observed in IN718 material at low power density values is due to multiple reflection of the incident beam from the cavity in combination with plasma formation at the evaporation front and trapping of the incident radiation causing excessive heating in that region.     

脉冲电流对冷坩埚定向凝固TiAl基合金微观组织和性能的影响

施加脉冲电流进行Ti-45.5Al-4Cr-2.5Nb(原子分数,%)合金的定向凝固并结合仿真实验,研究了脉冲电流对其凝固组织和性能的影响。结果表明：随着频率为200 Hz的脉冲电流密度的增大电流集肤效应增强、熔体内电流集肤效应加重。在集肤层电流焦耳热效应的作用下糊状区侧向散热的热流密度降低,使枝晶的平均偏离角减小,从而使合金的断后延伸率增大。脉冲电流密度较小时,在焦耳热效应和电磁搅拌的作用下熔体中枝晶的重熔和破碎使柱状晶晶粒的径向尺寸减小。但是,过量的焦耳热使熔区的长度增大、温度梯度减小,反而使晶粒的径向尺寸增大。因此随着电流密度的增大TiAl合金的抗拉强度先提高后降低,施加频率为200 Hz、电流密度为35.3 m A/mm的脉冲电流Ti Al合金的抗拉强度最大,比母合金铸锭的抗拉强度提高了70.7%。电流密度为52.9 m A/mm时延伸率达到最大值,比母合金锭的延伸率提高了129.5%。     

Use of the Laplace transformation for data reduction in the flash method of measuring thermal diffusivity

This paper presents a new way to reduce data in the laser flash method of measuring thermal diffusivity. Experimental temperature vs time data are first transformed by using the Laplace transformation, and then they are fitted with an appropriate theoretical formula. The data reduction procedure is more efficient and enables the use of more realistic models of heat conduction in the sample, because the theoretical formulae for transformed temperatures have a simpler form than those for nontransformed ones. Some examples of the theoretical formulae of transformed temperatures are included here for one- and two-dimensional heat transfer, respectively. The models described take into account a finite pulse time and heat losses from the sample. Two fitting algorithms are proposed. Experimentally, the data reduction procedure has been tested for a correction of the finite pulse time effect in the flash method. The results show that the accuracy of our procedure is comparable with other data reduction methods. Provided that the shape and duration of the pulse are known, this procedure allows elimination of the finite pulse time effect on calculation of the thermal diffusivity for any transformable heat pulse time function, even in cases where the other specialized data reduction procedures have failed.     

Robust dynamical decoupling

Quantum computers, which process information encoded in quantum mechanical systems, hold the potential to solve some of the hardest computational problems. A substantial obstacle for the further development of quantum computers is the fact that the lifetime of quantum information is usually too short to allow practical computation. A promising method for increasing the lifetime, known as dynamical decoupling (DD), consists of applying a periodic series of inversion pulses to the quantum bits. In the present review, we give an overview of this technique and compare different pulse sequences proposed earlier. We show that pulse imperfections, which are always present in experimental implementations, limit the performance of DD. The loss of coherence due to the accumulation of pulse errors can even exceed the perturbation from the environment. This effect can be largely eliminated by a judicious design of pulses and sequences. The corresponding sequences are largely immune to pulse imperfections and provide an increase of the coherence time of the system by several orders of magnitude.     

Numerical Analysis of a Radiant Heat Flux Calibration System

Heat flux gauges are one of the devices that are used to determine the heat loads to which high-speed aerospace structures are subjected during flight. Prior to installation, these gauges are calibrated. The calibration system must be well understood if the heat flux gauges are to provide useful data during flight tests. A pseudo three-dimensional model of the radiant heat flux gauge calibration system was developed. The radiant heat flux gauge calibration system consists of a graphite plate heater and a circular foil heat flux gauge. The numerical model simulates the combined convection, radiation, and mass loss by chemical reaction on the graphite plate surface. It can be used to identify errors due to heater element erosion, and the deviations in the predicted heat fluxes due to uncertainties in various physical parameters of the system. A fourth-order finite difference scheme is used to solve the steady-state governing equations and to determine the temperature distribution in the gauge and the graphite plate, the incident heat flux on the gauge face, and the flat plate erosion. Initial gauge heat flux predictions from the model are found to be within ±5% of experimental results.     

Effect of pulse voltage and aluminum purity on the characteristics of anodic aluminum oxide using hybrid pulse anodization at room temperature

Most of the anodic aluminum oxide (AAO) templates are performed by potentiostatical method at 0-10 °C to inhibit the Joule's heat enhanced dissolution in aluminum oxide for well-ordered cell configuration. In this article, we propose the hybrid pulse anodization (HPA) method with effective suppression of Joule's heat generation to fabricate AAO at room temperature. Effects of purity of aluminum (Al) foils and pulse voltage on the evolution of pore characteristics have been investigated. The AAO morphology is captured by scanning electron microscope and analyzed via gray-scale imaging in order to identify the pore size distribution. The increased applied potential results in the widened pores and non-uniform cell arrangement due to the increased current density and variation. Moreover, low-purity Al foils lead to the reduced AAO distribution uniformity owing to the uneven electric field induced pits on the Al surface for inferior pore arrangement. Extending both the positive and negative pulse period from 1 s to 5 s during HPA can enhance the distribution uniformity of AAO from high-purity Al by up to about 95%. In addition, the relationship between AAO configuration and Al purity and pulse voltage is further discussed and established.     

自由电子激光器的平面脉冲微型摇摆器磁场误差分析

建立了平面脉冲微型摇摆器磁场分布误差分析的数理模型，采用数值模拟方法，分析计算了微型摇摆器几何尺寸误差对遥摆器磁场分布轨迹的影响，其结果将为平面脉冲微型摇摆器的工程设计提供可靠的理论依据。     

Ten Years of Parameter Estimation Applied to Dynamic Thermophysical Property Measurements

The parameter estimation theory is considered the best way to estimate thermophysical properties from dynamic experiments. This approach deals with measurement and model errors in a statistical context and provides useful information to optimize the experiment. The experience gained in ten years of implementation of inverse algorithms based on the parameter estimation theory (OLS, MAP, and Kalman filtering) is summarized and presented. Several examples of estimation of thermophysical properties using transient and pulse techniques are reported and discussed. The thermal conductivity, specific heat capacity, and total hemispherical emissivity of different materials (light insulators, Pyrex, and niobium) are presented and compared with data obtained with consolidated techniques and with literature data.     

Stability of compound superconductors under localized heat pulses

Superconducting state stability of commercial Nb-Ti superconductors under heat pulses, generated at different lengths of superconductor, was investigated. The case of a one-dimensional superconductor without heat transfer to liquid helium was examined. It was found that due to dissipation of heat along the superconductor, the energy of irreversible quench may exceed considerably the energy needed for its adiabatic heating to critical temperature T_c (at given magnetic field and current). Such overheating, as it follows from the simplified analytical model and from experiments, depends on current density, magnetic field, the length of the heat generation and on the properties of matrix, but it is in broad limits independent of the form and duration of the pulse. Possible reasons of quantitative disagreement between calculations and experimental results are discussed.     

Numerical simulation and error analysis for thermal diffusivity measurements using laser-induced thermal grating technique

The laser-induced thermal grating technique has been used to determine the thermal diffusivity of liquids and liquid mixtures. But the dynamic behaviour of the transient thermal grating has not yet been thoroughly investigated, and the systematic errors, which result from the departures from one-dimensional heat conduction, have scarcely been studied quantitatively. In this paper. a three-dimensional numerical simulation and results of the transient thermal grating technique are presented, which enable a good understanding of the dynamic behaviour of the transient thermal grating. The results of this simulation are important for the proper design of the experimental setup to keep the systematic errors for the diffusivity measurement small. Based on the simulation method, the systematic errors were analyzed quantitatively. Here, the following effects were studied: (I) sample thickness, (2) intersection angle, (3) absorption, (4) Gaussian beam intensity distribution and focusing of heating laser beam, and (5) heating pulse duration and laser power. This error analysis makes it possible to specify the criteria for optimum measuring conditions, to correct the measured thermal-diffusivity values for systematic errors, and to estimate the accuracy of the measurements.     

Effect of pulse frequency on heat transfer at the stagnation point of an impinging turbulent jet

Heat transfer in an impact pulsed air jet is numerically studied using the Reynolds stress model. It is shown that both enhancement and suppression in the heat transfer are possible in an impinging pulsed jet as compared with a steady flow. The heat transfer intensifies with the pulse frequency at a stagnation point in the region of small distances between a pipe exit cross section and an obstacle (H/D ≥ 6), while an increase in the pulse frequency causes a decrease in the heat transfer for H/D > 8. An increase in the Reynolds number causes a deintensification of the heat transfer, and the data for all frequencies approach the single-phase flow mode. A comparison with available data by other authors is made, and satisfactory agreement is obtained with respect to the pulse frequency effect on the heat transfer between a gas jet and the impact surface.     

Measurement of the Thermal Diffusivity of Solids with an Axisymmetrically Positioned Source of Heat Pulse

The pulse method of measurement of the thermal diffusivity of cylindrical samples is considered: an optimum version of normalization of the geometric parameters of a heat pulse, the thicknesses of a cylinder to the radius, and significance of the length of a heat pulse are discussed. The method is realized on an automated experimental setup with simultaneous recording of a thermal signal and the shape and length of a laser pulse. Nonlinear effects are eliminated by decreasing the energy density on the front surface of the sample. The setup presented allows measurement of the thermal diffusivity within a wide range of its values with an error not exceeding 5%. The obtained results of the determination of the thermal diffusivity of Al, Cu, and Fe are presented in comparison with the literature data.