共查询到18条相似文献，搜索用时 0 毫秒
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Microwave heating technology is a powerful mean to ensure successful sintering of ceramic materials. In sintering experiments, low loss insulators, conductors and high‐loss ceramics are microwaved so as to get optimal mechanical and structural properties. It is known that low‐loss ceramic materials such alumina and zirconia exhibit long waiting time before reaching a critical coupling temperature at which microwaves can be readily absorbed. On the other hand, some ceramics such as silicon carbid have a high loss factor and therefore can be used as a process stimulus for microwave sintering of microwave transparent ceramics. Furthermore, successful sintering experiments often require the use of carefully designed insulating structure in order to minimize thermal gradients caused by heat loss from surfaces. All these problems have led to the introduction of microwave hybrid heating (MHH) schemes using higher dielectric loss susceptors, insulation or coating. Since MHH depend mainly on human expertise, the optimization of sintering experiments will certainly benefit from numerical simulations. The transmission line matrix (TLM) is used to study two MHH schemes where both a susceptor and an insulating matrix were, respectively, used as process stimulus for microwave heating of multiple alumina samples within a three‐dimensional multimode cavity. The effects of such MHH schemes and target settings on electric field distribution and power absorption rates are reported in this paper. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
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Mohamed H. Bakr Natalia K. Nikolova Peter A. W. Basl 《International Journal of Numerical Modelling》2005,18(6):441-455
We present a novel efficient algorithm for the estimation of S‐parameter sensitivities in homogeneous and lossless transmission line modelling (TLM) problems. Our approach estimates S‐parameter adjoint‐based sensitivities without actually carrying out any adjoint simulation. By applying a transformation to the original TLM simulation we establish an isomorphism between the original and the adjoint problem. The unique properties of the TLM node in a lossless and homogeneous problem are also exploited in establishing the isomorphism. For an electromagnetic structure with Np ports, only the Np original simulations utilized in evaluating the S‐parameters are required to estimate their sensitivities as well. Our novel approach is illustrated through estimating S‐parameter sensitivities with respect to waveguide discontinuities. Good match is obtained between our sensitivity estimates and those calculated using finite differences at the response level. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
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We present a novel algorithm for efficient estimation of S‐parameter sensitivities with the time‐domain transmission line modelling (TLM) method. The original electromagnetic structure is simulated using TLM to obtain the S‐parameters in the desired frequency band. For each port, an adjoint TLM simulation that runs backward in time is derived and solved. The sensitivities of the S‐parameters in the desired frequency band are estimated using only the original and adjoint simulations. For a structure with Np ports and n designable parameters, our approach requires only Np additional simulations regardless of n. This can be easily contrasted with the 2nNp additional simulations required by the central difference approximation. Our algorithm is illustrated through the estimation of S‐parameter sensitivities with respect to the dimensions of waveguide discontinuities. Very good match is obtained between our sensitivity estimates and those obtained using central difference approximation. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
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Marcello Pesare Agostino Giorgio Anna Gina Perri 《International Journal of Numerical Modelling》2001,14(5):395-409
In this paper, an analytical model for the electrothermal solution to the non‐linear 3‐D heat flow equation for multilayer structure electronic devices is proposed. Compared with previous models presented in literature, it is general and can be easily applied to a large variety of integrated devices, provided that their structure can be represented as an arbitrary number of superimposed layers with a 2‐D embedded thermal source, so as to include the effect of the package. The proposed method is independent of the specific physical properties of the layers, hence GaAs MESFETs and HEMTs as well as silicon and silicon‐on‐insulator MOSFETs and heterostructure LASERs can be analysed. Moreover, it takes into account the dependence of the thermal conductivity of all the layers on the temperature; the heat equation is solved coupled with the device current–voltage relation in order to give physical consistence to the experimental evidence that a temperature increase causes a degradation of the electrical performances and that the electrical power is not uniformly distributed. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
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Heverhagen JT von Tengg-Kobligk H Baudendistel KT Jia G Polzer H Henry H Levine AL Rosol TJ Knopp MV 《Magma (New York, N.Y.)》2004,17(1):5-11
Benign prostate hyperplasia (BPH) is a major disease and its non-surgical therapy a major area of interest. The purpose of this study was to establish perfusion parameters in beagles with BPH using dynamic contrast-enhanced (DCE) MRI and to investigate changes due to the effects of finasteride treatment. Twelve male beagles (mean age 4.4±0.9,years) were divided into a control and treatment group that received a daily dose of 1 mg/kg finasteride. DCE MRI was carried out in a clinical scanner using a 3D spoiled gradient echo sequence prior to and during treatment. 0.2 mmol/kg contrast agent (gadoteridol) was administered with an injection rate of 0.2 ml/s followed by a 15 ml flush of saline. Contrast enhancement was evaluated by pharmacokinetic mapping of a two-compartment model with colour overlay images in addition to regional ROI analysis. Quantitative parameters were defined by the amplitude of contrast enhancement A, the exchange rate kep and the time to maximum signal enhancement. Dynamic contrast-enhanced MRI investigations of the prostate revealed two distinct zones, an inner, periurethral zone and an outer, parenchymal zone. The periurethral zone is highly vascularized, whereas the parenchymal zone is moderately vascularized when compared to other parenchymal organs. During treatment, in the parenchymal zone the intensity of enhancement (amplitude A) and the time to maximum signal enhancement increased, while the exchange rate kep decreased. Dynamic contrast-enhanced MRI of BPH reveals distinct differences between individual zones within the prostate. Moreover, changes during successful treatment suggest increased blood volume per volume of tissue and decreased vessel leakiness. 相似文献
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S. Aliouat Bellia A. Saidane M. Benzohra J. M. Saiter A. Hamou 《International Journal of Numerical Modelling》2008,21(6):531-549
The skin is sensitive to temperature change and the effect may not be significant while the temperature at the surface is below 44°C. However, higher surface temperatures (above 44°C) will further incur time burning and carbonization so that irreversible damage may happen. An investigation of the heating intensity and the duration of the exposure to the heating source suggested that when the surface temperature is greater than 51°C, the exposure time required to destroy the epidermis is so short that trans‐epidermal necrosis may occur. In this paper, we present one‐ and two‐dimensional numerical models based on transmission line matrix (TLM) method for a quantitative prediction of skin burn injury resulting from the exposure of the skin surface to a high temperature heat source. Transient temperatures are numerically estimated by solving the Pennes' bioheat equation, and the damage function denoting the extent of burn injury is calculated using the Arrhenius assumptions for protein damage rate. The TLM model is used to analyse the effects of exposure time and geometrical dimensions of mutlilayered skin, on the transient temperature distribution and damage extension. TLM results showed good agreement with other numerical sources, suggesting that TLM modelling can be used as a tool for an effective thermal diagnostic of burn injuries. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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Chi Chung Wong 《International Journal of Numerical Modelling》2002,15(3):283-286
The aim of this note is to point out that the boundary condition for the network modelling of thermal problems may have been incorrectly used in some previous studies. It is shown that the accuracy of the network analogue or the equivalent finite‐difference method is on the par with the finite‐element method for very fast transient thermal simulations. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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恐惧情绪是人体应对外界刺激的一种反应,其产生会引起人体面部皮肤温度的变化。根据红外热图反映物体表面温度分布的原理,提出了一种基于面部红外热图对恐惧情绪进行识别的方法。首先采用指数函数拟合方法对现有的传热模型进行简化,将红外热像仪所采集的面部红外热图转化为血液灌注伪彩色图,寻找感兴趣区域(前额区域);然后,提取感兴趣区域的血液灌注变化曲线的相关特征(斜率、置信度、均值、标准偏差),并采用斯皮尔曼相关系数分析其与受试对象恐惧程度自评分之间的相关性;最后采用与恐惧程度自评分相关性最高的标准偏差对受试对象进行恐惧情绪的识别。实验结果表明,当受试对象产生恐惧情绪时,前额区域的血液灌注呈现出明显的下降趋势,这与现有的恐惧情绪研究结论一致;血液灌注值的标准偏差可以作为恐惧情绪识别的主要特征(阈值0.14)。采用本文所提出的方法,对28例样本进行测试,受试对象恐惧情绪的识别准确率达到85.7%,具有较高的可靠性。 相似文献
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In this paper, a new semiempirical DC thermal model of low‐ and high‐power GaAs MESFETs is proposed. The model takes into account the effect of device negative output conductance and simulates external thermal effects modelling the dependence on temperature of the device threshold voltage and the maximum saturation drain–source current. A number of GaAs MESFETs, very different from a geometrical and technological point of view, have been characterized as a function of temperature and modelled by our model with high accuracy. The CPU extraction time results are moderate in any example. Results have been compared with the Rodriguez–Tellez model, showing improvements of accuracy better than 30 per cent. The model can be successfully used n MMIC CAD applications. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
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Z. Zhou M. S. Khanniche P. Igic S. T. Kong M. Towers P. A. Mawby 《International Journal of Numerical Modelling》2006,19(1):33-46
A fast power losses calculation method for long real time thermal simulation of IGBT module for a three‐phase inverter system is presented in this paper. The speed‐up is obtained by simplifying the representation of the three‐phase inverter at the system modelling stage. This allows the inverter system to be simulated predicting the effective voltages and currents whilst using large time‐step. An average power losses is calculated during each clock period, using a pre‐defined look‐up table, which stores the switching and on‐state losses generated by either direct measurement or automatically based upon compact models for the semiconductor devices. This simulation methodology brings together accurate models of the electrical systems performance, state of the art‐device compact models and a realistic simulation of the thermal performance in a usable period of CPU time and is suitable for a long real time thermal simulation of inverter power devices with arbitrary load. Thermal simulation results show that with the same IGBT characteristics applied, the proposed model can give the almost same thermal performance compared to the full physically based device modelling approach. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
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基于最小均方自适应滤波器的热工过程建模方法 总被引：2，自引：0，他引：2
针对热工对象的时变性特点及其在运行过程中易受到不确定性干扰的影响,提出一种基于最小均方(least mean square,LMS)自适应滤波器的热工过程建模方法。LMS滤波器以未知对象的输入和输出作为激励和期望信号,通过最速下降法得到未知对象的有限脉冲响应(finity impluse response,FIR)模型,其与差分方程或传递函数是等价的。实验仿真和某电厂实际运行数据验证了该算法的有效性。这种建模方法避免了复杂的机理分析,其抽头权值的分布可以表征热工对象的动态特性,为分析热工对象提供了一种手段。 相似文献
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新型RBF神经网络及在热工过程建模中的应用 总被引：20，自引：13，他引：20
文中提出了一种基于免疫原理的新型径向基函数（RBF－Radial Basis Function)神经网络模型。该模型利用人工免疫系统的记忆、学习和自组织调节原理，进行RBF神经网络隐层中心数量和位置的选择，并采用递推最小二乘算法确定网络输出层的权值。将这种新型的RBF神经网络应用于建立热工过程的非线性模型。仿真研究表明，这种建模方法不仅计算量较小，而且精度高，并有较好的泛化能力。 相似文献
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We deal with the problem of identifying the parameters of an equivalent lumped RC multiport network from time domain tabulated data of a corresponding distributed real eigenvalues problem, as obtained from either measurements or accurate simulation tools. A novel step response matrix identification procedure is proposed, based on the combination of an outer nonlinear least squares iteration for the relocation of time constants, and an inner convex programming cycle for the identification of the corresponding residue matrix terms, able to guarantee a priori the passivity property of the equivalent RC network. The structure of the algorithm and its principal functions connections are shown, and the mathematical features of the proposed formulation of the identification problem are accurately described and commented. Moreover, the possibility of getting direct synthesis of a Foster generalized concretely passive multiport, as a consequence of the optimal identification of a passive real eigenvalues model from data, is discussed. Since the technique is well suited (although not limited) to the reduced analysis of typical electro‐thermal problems, a set of significant case studies in this area is considered. In this way the algorithm present implementation is validated, also comparing it to previous well assessed methods, evidencing its large potential and value. 相似文献
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MR monitoring of tumour thermal therapy 总被引：2，自引：0，他引：2
Thermal therapy of tumour including hyperthermia and thermal ablation by heat or cold delivery requires on line monitoring. Due to its temperature sensitivity, Magnetic Resonance Imaging (MRI) allows thermal mapping at the time of the treatment. The different techniques of MR temperature monitoring based on water proton resonance frequency (PRF), longitudinal relaxation time Tl, diffusion coefficient and MR Spectroscopic Imaging (MRSI) are reviewed and debated. The PRF method appears the most widely used and the most efficient at high magnetic field in spite of important drawbacks. The Tl method is the easiest method of visualisation of qualitative temperature distribution and quantitative measurement seems possible in the tissue surrounding the tumour up to a temperature of 45–65°C. Despite its high temperature sensitivity, application of the diffusion method in vivo is restricted due to its high motion sensitivity. The recent MRSI technique seems very promising provided acquisition times can be reduced. Results from the literature indicate that MR temperature monitoring in vivo can be achieved in vivo with a precision of about 3°C in 13 s for a voxel of 16 mm3 (1.5 × 1.5 × 7 mm) in 1.5 T scanners. 相似文献