Time-dependent Temperature Distribution Research Articles

Application of pulsed laser annealing to ferromagnetic GaMnAs

In this experimental and theoretical work we focus on the technique of pulsed laser annealing applied to the metastable ferromagnetic semiconductor GaMnAs. Analytical heat-flow calculations are used to illustrate the position and time-dependent temperature distribution during the whole laser annealing process. Such heat-flow calculations will also play an indispensable role for the preparation of other new metastable diluted ferromagnetic semiconductors by ion implantation and subsequent laser annealing. The structural, magnetic, and magnetotransport properties of ferromagnetic GaMnAs have been probed in dependence on the annealing parameters, e.g., the number of laser pulses and the pulse length. Annealing with a single KrF laser pulse of 30 ns and $0.26\text{ }\text{J}/{\text{cm}}^{2}$ with the photon energy above the GaAs band gap energy leads to similar magnetic properties like annealing with a single 3 ns Nd:YAG laser pulse with the photon energy below the GaAs band gap energy. We observed that possibly due to Mn diffusion and decreasing hole concentration, several laser pulses degrade the structural and magnetic properties of GaMnAs. Our results reveal the largest saturation magnetization in Mn-implanted GaAs annealed with a single KrF laser pulse.

Enhancement of Heat Transfer Behind Sliding Bubbles

The time-dependent temperature distribution on an inclined, thin-foil uniform-heat-generation heater was used to infer the heat transfer enhancement caused by the passage of an FC-87 bubble sliding beneath the lower surface of the heater. A two-camera system was used: One camera recorded color images of a liquid crystal layer applied to the upper (dry) side of the heater while a second camera simultaneously recorded the position, size, and shape of the bubble from below. The temperature response of the heater could then be correlated directly to the bubble characteristics at any given time during its passage. The data along the line bisecting the bubble wake from the nine bubbles comprising 54 bubble images were analyzed. The heat transfer in the wake behind the sliding cap-shaped bubbles is very effective compared with the natural convection that occurs before the passage of the bubble. The maximum values of heat transfer coefficient in the range of 2500 W/m2 K were produced in very sharply peaked curves. The point of maximum cooling measured as a fraction of the local driving temperature difference before the bubble passage was identified and correlated with some success to the streamwise length of the bubble. The location of the maximum heat transfer coefficient was reasonably correlated with bubble width. The level of the maximum heat transfer coefficient when cast as a Nusselt number based on bubble width grew to a saturation value as the bubble moved across the plate. A constant value of Nusselt number requires that the heat transfer coefficient falls as the bubble grows past some critical bubble size. This behavior was observed for the larger cap-shaped bubbles.

Coupled electromagnetic thermal and kinetic modeling for microwave processing of polymers with temperature- and cure-dependent permittivity using 3D FEM

This paper presents a self-consistent 3D marching-in-time multiphysics model, which includes electromagnetic field distribution, microwave power absorption, heat transfer, and polymer curing kinetics. Temperature- and cure-dependent permittivity and curing kinetics for DGEBA/DDS based on experimental data are explicitly included in the model. An edge- based finite element method (FEM) is implemented for the electromagnetic model, whereas node-based FEM is used in the heat transfer model. The numerical results can be used to determine the time-dependent temperature distribution and curing profile across the polymer sample, as well as the electromagnetic field distribution within the cavity applicator. The numerical results are compared with the measured data and a good agreement is achieved.

Numerical modelling of temperature variations in a Chinese solar greenhouse

The time-dependent temperature distributions inside a Chinese solar greenhouse are numerically predicted from external climatic conditions using a computational fluid dynamics (CFD) analysis. The boundary conditions are based on hourly measured data for the solar insolation and the sky, soil (1 m below the soil surface) and outside air temperatures, plus other parameters describing the external convection and radiation. The numerical model takes into account all of the heat transfer mechanisms including the variable solar insolation, the air infiltration, the heat capacities of the thick walls and the ground and the natural convection inside the greenhouse. The temperatures were measured experimentally in an enclosed solar greenhouse with a 12 m span and 5.5 m ridge height during the winter in northern China with the south roof covered with a thin plastic film during the daytime and with a thermal blanket added at night to reduce heat losses. The large temperature variations in the greenhouse were measured and predicted for the climatic conditions in northern China during three clear days followed by a cloudy day during the winter. The simulated air and soil temperatures have the same profile as the measured temperatures with the average temperature differences between the simulated and measured air temperatures during the nighttime less than 1.0 °C on the clear days and no more than 1.5 °C during the entire cloudy day.

Estimation of heat flux and thermal stresses in multilayer gun barrel with thermal contact resistance

In this study, a conjugate gradient method based on an inverse algorithm is applied to estimate the unknown time-dependent heat flux at the inner surface of gun barrel, in which the interlayer thermal contact resistance between the steel cylinder and the chrome coating is taken into account in the boundary conditions. While knowing the temperature history at the measuring position, no prior information is needed on the functional form of the unknown heat flux. The temperature data calculated from the direct problem are used to simulate the temperature measurement. The influence of measurement errors and initial guess values upon the precision of the estimated results is also investigated. Results show that an excellent estimation on the time-dependent heat flux, temperature distributions, and thermal stresses can be obtained for the case considered in this study.

Phase Change Heat Transfer Enhancement Using Copper Porous Foam

A detailed experimental and analytical study has been performed to evaluate how copper porous foam (CPF) enhances the heat transfer performance in a cylindrical solid/liquid phase change thermal energy storage system. The CPF used in this study had a 95% porosity and the phase change material (PCM) was 99% pure eicosane. The PCM and CPF were contained in a vertical cylinder where the temperature at its radial boundary was held constant, allowing both inward freezing and melting of the PCM. Detailed quantitative time-dependent volumetric temperature distributions and melt/freeze front motion and shape data were obtained. As the material changed phase, a thermal resistance layer built up, resulting in a reduced heat transfer rate between the surface of the container and the phase change front. In the freezing analysis, we analytically determined the effective thermal conductivity of the combined PCM/CPF system and the results compared well to the experimental values. The CPF increased the effective thermal conductivity from 0.423W∕mKto3.06W∕mK. For the melting studies, we employed a heat transfer scaling analysis to model the system and develop heat transfer correlations. The scaling analysis predictions closely matched the experimental data of the solid/liquid interface position and Nusselt number.

Experimental and numerical investigation of heat and mass transfer during drying of Hayward kiwi fruits ( Actinidia Deliciosa Planch)

In this paper we undertake an experimental and numerical study on heat and mass transfer analysis during drying of kiwi fruits. In the experimental part, the effects of various drying conditions in terms of air velocity, temperature and relative humidity on drying characteristics of kiwi fruits are investigated. In the numerical part, the external flow and temperature fields are studied using a commercial CFD package. From these fields, the local distributions of the surface convective heat transfer coefficients for the fruits are determined to predict the local convective mass transfer coefficients through the analogy between the thermal and concentration boundary layers (known as the Chilton–Colburn analogy). In addition, the time-dependent temperature and moisture distributions for different cases are obtained using the code developed to investigate heat and mass transfer aspects inside the fruits. Numerical results are then compared with experimental data and a considerably high agreement is obtained.

Design by Analysis: Direct Route for Cases With Pressure and Thermal Action

This paper focuses on the usage of direct route for design by analysis, which is included in the new European standard for unfired pressure vessels (CEN, 2002, “Unfired Pressure Vessels-Part 3: Design,” European Committee for Standardization No. EN 13445-3, Current Issue 14-2005). The direct route addresses failure modes directly, having, therefore, advantages in comparison to the traditional method, which is stress categorization. Special attention is given to the progressive plastic deformation design check (PD-DC) with space and time-dependent temperature distribution and the fatigue design check (F-DC) with stress components, which do not vary simultaneously. As a simple demonstration example, a nozzle with cold media injection is used to show how the direct route can be applied in such cases. This example is analyzed with abruptly changing injection temperature, which makes a transient thermal analysis necessary. In the case of the PD-DC, Melan’s shakedown theorem and cycling of a finite element model with a linear-elastic ideal-plastic material model are used. In the case of the F-DC, some problems are discussed: One of them is cycle counting in the case of nonproportional loading, and the other one is the use of structural stresses and stress concentration factors.

Design and Thermal Simulation of Induction Machines for Traction in Electric and Hybrid Electric Vehicles

An electric traction machine for an electric or a hybrid electric vehicle is usually designed for a specific operating point or cycle. For such an operating point or cycle, the masses and the cooling circuit of the electric machine determine the time dependent temperature distribution within the machine. For a specific load cycle, the thermal simulation of the machine can reveal possible mass and size reductions for a given insulation class of the machine. In addition, such simulations allow the comparison of various cooling concepts. In the machine design process, the first step is a conventional electromagnetic machine design. From the geometric data of this design and the material properties, the parameters of a thermal equivalent circuit can be derived. The differential and algebraic equations of the thermal equivalent circuit are solved by a simulation tool to predict the temperatures of the critical parts in the electric machine. A thermal equivalent circuit is accurate enough to predict the thermal behavior of the critical parts in the electric machine, and yet not too complex, to obtain simulation results with moderate numerical effort. This enables an iterative design process to optimize the drive.

Fabrication of periodic structures in thin metal films by pulsed laser irradiation

We developed a technique for the fabrication of 1D and 2D periodic structures in thin gold and silver films by irradiation with two and four intersecting beams of a nanosecond pulsed laser. The periodically alternating intensity distribution caused by interference induces mass transfer in thin metal films. The mechanisms and kinetics of mass transfer depend on the film thickness, morphology, and laser intensity. In the films thinner than some transitional value, h*, redistribution of material between hot and cold regions on the substrate occurs as a result of melting and beading of the film in hot regions and subsequent motion of liquid drops towards cold regions in the optically induced temperature gradient. In the films thicker h* redistribution of the film material occurs by hydrodynamic flow of the molten film in the temperature gradient, with subsequent crystallization in the cold regions and formation of tall and narrow ridges or rims. Theoretical models were developed that allowed us to calculate the time dependent temperature distributions around irradiated regions of the film. From the temperature distributions we calculated the expected speed of liquid beads, as well the distances between the ridges, the rim diameters and found good agreement with the measured values.

Self-organization of a 2D lattice on a surface of Ge single crystal after irradiation with Nd:YAG laser

Experimentally observed self-organization of a 2D lattice on the surface of Ge single crystal after irradiation by pulsed Nd:YAG laser is reported. The 2D lattice consists of nano-size hills arranged in a pattern of C6i point group symmetry and is characterized by translational symmetry with the period of 1μm. Calculations of time-dependent distribution of temperature in the bulk of the Ge sample are presented to explain the phenomenon. The calculations show that overheating of the crystal lattice occurs at laser radiation intensities exceeding 30MW/cm2. According to synergetic ideas, the presence of the non-equilibrium liquid phase of Ge and a huge gradient of temperature (∼3×108K/m) can lead to self-organization of the 2D lattice similar to Benard cells.

Numerical Modeling of the Long-Term Behavior of Passive Fire Protection Mortars Applied on Concrete Tunnels

Experiences show that the design of overlay mortars is a complicated matter as far as long-term durability and dimensional stability are concerned. In many cases, applied mortars crack or delaminate a few years after application. The same is true for passive fire protection mortars (PFP mortars). The mortar must remain crack free and must not delaminate during the planned service life, that means the fire protection requirements must be combined with the durability requirements. In this paper, an approach based on a numerical model is used to forecast the long-term behavior of PFP mortars. By means of this model, the time-dependent humidity, temperature, stress, and strain distributions, as well as the crack formation can be calculated in a realistic way. The minimal adhesive strength between overlay and substrate which is necessary to prevent delamination during the planned service life has been calculated. Material properties and climatic boundary conditions have been taken into account. The results allow the tailored optimization of PFP mortars.

Inverse estimated transient thermal responses of multicoated optical fiber under time-dependent heat transfer environments

A conjugate gradient method based on an inverse algorithm is applied in this study to estimate the unknown time-dependent convection heat transfer coefficient under the varying ambient temperatures in multicoated optical fibers by reading the transient temperature measurement data at the fiber surface, when the optical fibers are subjected to transient thermal loading. No prior information is available on the functional form of the unknown convection heat transfer coefficient in the present study; thus, it is classified as the function estimation in inverse calculation. The accuracy of the inverse analysis is examined by using the simulated exact and inexact temperature measurements. Results show that the estimation of time-dependent convection heat transfer coefficient and transient temperature distributions can be obtained and are even effected by the varying ambient temperatures for all the test cases considered in this study.

Experimental and Computational Studies of Jet Fuel Flow near the Freeze Point

To study the flow behavior of jet fuel at low temperatures, a wing-tank thermal simulator, which represents the fuel tank of a commercial aircraft, was fabricated. The simulator was subjected to cooling in an environmental chamber. Experimental results show that fuel flowability and pumpability decrease substantially as temperature is reduced. Time-dependent temperature and velocity distributions were numerically simulated for static cooling. Viscosities were obtained from viscometer measurements using different jet fuel samples. It was observed that near the freeze-point temperature, low freeze-point fuels tend to have higher viscosities than high freeze-point fuels. Measured viscosities were used in computational-fluid-dynamics simulations of jet fuel that was cooled. The calculations show that stringers, ribs, and other structures strongly promote fuel cooling. Also, the cooler, denser fuel resides near the bottom surface of the fuel tank simulator. The presence of an ullage space within the tank was found to strongly influence the fuel temperature profile by sometimes reducing cooling from the upper surface. In other instances, the ullage space enhanced cooling.

Modeling the thermal conditions around sites of microwave drilling in bone

4704 Citation: Journal of Biomechanics 2006; Vol. 39 Suppl. 1, page S382 Modeling the thermal conditions around sites of microwave drilling in bone R.R. Mann1, O. Aktushev2, A. Gefen1, E. Jerby2 1 Depts. of Biomedical Engineering and 2 Physical Electronics, Tel Aviv University, Israel A new method for orthopaedic drilling was developed, based on microwave energy that is localized at a ``hot spot'' to penetrate bone tissue [1,2]. However, the heat generated during microwave drilling was shown to cause some tissue carbonization and collagen denaturation in vitro [2]. The goal of this study was to develop a model of time-dependent temperature distribution around the drilling site, in order to optimize the drilling process. A finite-difference time-domain (FDTD) model [3] was employed. The model calculates the temperature fields around the drilling site for a given drill bit diameter, microwave drilling power, and bone geometry (e.g. cortical thickness and presence of underlying marrow). The model allows determination of the size of tissue regions exceeding critical temperature and the time exposure to temperature. Generally, we found that the radius and depth of bone that exceeded a 50°C threshold (for osteocyte cell death) increased with the drilling depth, and damage area was wider when bone marrow was considered. Overall, the model emerged as an essential engineering tool for improving the technology and design of orthopaedic microwave drills. Reference: 1. Jerby E., Dikhtyar V., Aktushev O., Groslick U. The microwave drill. Science 2002; 298: 587-589. 2. Eshet Y., Mann R.R., Anaton A., Yacoby T., Gefen A., Jerby E. Microwave drilling of bones. IEEE Transactions on Biomedical Engineering, accepted for publication. 3. Jerby E., Aktushev O., Dikhtyar V. Theoretical analysis of the microwave-drill near-field localized heating effect. Journal of Applied Physics 2004; 97: 034909.

A temperature and mass dependent thermal model for fire response prediction of marine composites

Abstract An accurate assessment of accumulative damage of a composite ship structure subjected to fire is strongly reliant on the accurate characterization of the time dependent temperature distribution within the composite system. Current state-of-the-art analysis tools assume that thermal–mechanical properties of composite structures are independent of temperature change and mass loss. In this paper, a temperature and mass dependent heat diffusion model is developed to characterize the temperature and mass dependent heat conduction, energy consumption resulting from the decomposition, and the energy transfer associated with vaporous migration. The temperature dependent thermal conductivity and specific heat capacity are determined for the composite at a given resin decomposition stage using a recently developed small-scale test apparatus. Given the temperature dependent thermal properties of the fiber and resin materials, the resulting temperature dependent thermal properties of a woven fabric composite are computed from the newly developed thermal–mechanical analysis tool (TMAT). To assess the effect of using single heating rate based mass loss on the composite fire response prediction, the kinetic parameters are obtained from the post-processing of thermogravimetric analysis (TGA) test data conducted at various heating rates. The effects of temperature dependent thermal conductivity, specific heat capacity, and kinetic parameters determined at different heating rates are explored through the application of the temperature and mass dependent fire model to a composite plate subjected to a hydrocarbon fire. The accuracy of the temperature and mass dependent thermal model is demonstrated by comparing its response prediction with available experimental data.

Multiphase Flow in Porous Media with Phase Change. Part II: Analytical Solutions and Experimental Verification for Constant Pressure Steam Injection

An analytical model for the description of a one-dimensional condensing flow of steam in porous media is presented. The model includes an expanded momentum balance and takes into account both compressibility of the steam and the increase of pressure loss due to higher condensation rate or initial water filling of the porous structure. Therefore the model is suited especially for the description of high pressure and high temperature steam injection processes which are often utilized in chemical engineering. The model does not only offer the possibility for quick calculation and optimization of process parameters but also allows the calculation of time dependent temperature and pressure distributions without a numerical simulation. The model is verified by extensive experimental investigations and numerical calculations with a model previously presented in this journal. The comparison of the results shows an excellent agreement with experimental data in a wide range of steam pressures and permeabilities. It is shown that especially for high pressures or permeabilities the inertia term of the momentum balance has to be taken into account for precise calculations.

Three-Dimensional Thermomechanical Buckling of Functionally Graded Materials

Three-dimensional thermomechanical buckling analysis is discussed for functionally graded materials. Material properties are varied continuously in the thickness direction according to a simple power-law distribution. The three-dimensional finite element model is adopted using an 18-node solid element and the assumed-strain mixed formulation is used to prevent locking as well as to maintain kinematic stability for thin structures. The thermal buckling behavior under time-dependent temperature rise is compared with that under time-independent temperature rise. In time-dependent temperature distribution, temperature at each node is obtained by solving the thermomechanical equations and the Crank-Nicolson method is used for a time discretization. In the case of time-independent temperature distribution, thermal buckling is investigated under uniform, linear, and sinusoidal temperature rise. Numerical results are compared with those of previous works. In addition, the changes of critical buckling temperature due to temperature field, volume fraction distributions, and system geometric parameters are studied.

Electron-temperature and energy-flow history in an imploding plasma

The time-dependent radial distribution of the electron temperature in a 0.6 micros, 220-kA gas-puff z-pinch plasma is studied using spatially-resolved observations of line emission from singly to fivefold ionized oxygen ions during the plasma implosion, up to 50 ns before maximum compression. The temperature obtained, together with the previously determined radial distributions of the electron density, plasma radial velocity, and magnetic field, allows for studying the history of the magnetic-field energy coupling to the plasma by comparing the energy deposition and dissipation rates in the plasma. It is found that at this phase of the implosion, approximately 65% of the energy deposited in the plasma is imparted to the plasma radial flow, with the rest of the energy being converted into internal energy and radiation.

Dimensioning components installed in electrical panels with respect to operational temperature

Circuit breakers, disconnectors, fuse disconnectors and fuse holders installed in LV electric panels sometimes suffer overheating, mainly due to inadequate dissipation of the heat generated. A theoretical and experimental investigation is attempted of the mechanisms that can affect heat transfer to a component–conductor system, as well the mass and the external surface area of the component. The resulting system of differential equations is solved using the Laplace transform in the s-plane. The time-dependent temperature distribution in the component–conductor system is provided by an analytical expression, and a simple formula is given for the component's stabilised temperature. Results from the models are compared with experimental measurements performed on a fuse holder and are shown to be consistent. The model accuracy is also investigated. It is convincingly shown that excessive (or additional) external surface area of the component's metallic parts reduces the component temperature. Above a certain level, this can make the component act as a cooler of the conductor. In addition, an increase of the component's mass slows temperature stabilisation. The proposed model could effectively support component design as it involves all the technical characteristics of the system, so that the requirements of international standards are fulfilled in relation to the prevention of overheating.