基于小波变换的核磁共振FID信号的去噪方法研究

以医用核磁共振自由感应衰减(FID)信号的去噪处理为研究目标,基于小波变换技术,分析了有效信号和噪声的小波变换特性,针对三种去噪方法--模极大值、阈值收缩和平移不变,提出了改进方案,并以阈值收缩去噪方法为例,对临床获取的神经胶质瘤病例的大量数据样本进行了去噪、分析实验,探讨了适用于本类FID信号去噪处理的小波基函数、分解参数、分解层次和阈值策略的选取方法,通过实验建立了最优选取方案.实验结果有效地验证了新方案良好的去噪性能.本研究对于医用磁共振数据的高效处理具有很好的参考价值.     

基于核磁共振的癌症非侵入式诊断理论与技术研究

分析了核磁共振技术在癌症临床诊断领域的应用现状、相关理论与技术的发展趋势,概述了对核磁共振设备产生的FID信号、MRI和MRS的数据分析技术,并以MRS为重点,研究了FID信号和波谱数据的特点,提出了利用小波变换和模式识别技术对FID信号和波谱数据进行分析的技术方案,并对信号的小波阈值去噪、小波基函数的选择、MRS特征识别与提取等问题进行了探索性研究,对核磁共振为基础的相关技术在癌症临床诊断领域内应用的进一步发展提示了方向。     

基于灰色关联度的超临界水自然循环换热系数影响因素分析

在严重事故评价中,超临界水冷反应堆将充分利用自然循环的驱动力在循环中带走堆芯余热。影响超临界水换热系数的影响因素有很多,而定量地确定各个因素之间对换热系数的影响的大小,对于改善超临界水冷反应堆的设计,具有一定的现实参考意义。通过提取超临界水自然循环的实验数据点,选取加热段功率、管径、系统压力、入口温度、出口温度、出口壁温和系统循环流量等7个因素,应用灰色关联度分析方法,分析这些因素对换热系数的关联程度的大小。不同的分辨率对计算关联度的大小有较大影响。选取的分辨率越小时,各因素之间的关联度区分的越大。通过超临界水自然循环换热系数的灰色关联度大小的分析,可以为换热系数的预测提供理论依据。     

超临界水自然循环特性影响参数研究分析

运用ANSYS平台下的CFX软件,对超临界水自然循环回路进行了数值模拟计算,得出超临界水自然循环流量和换热系数随功率的变化趋势。在研究范围内,随着功率的增加,自然循环流量和换热系数先逐渐增大,然后达到一个最大循环流量和换热系数。此后随着功率的继续增加,自然循环流量和换热系数将逐渐减小。流动换热进入密度波不稳定状态。在最大循环流量和换热系数附近时,加热器的出口温度接近拟临界温度点附近。通过数值模拟计算结果和国内已有的实验结果的比较,两者对比符合良好。     

核磁共振癌症诊断数据处理技术的研究与实现

分析了癌症临床诊断领域中MRI、MRS、PACS和小波变换技术的应用研究现状,针对目前主流核磁共振设备的特点,利用医疗图像存档与通信系统(PACS),提出了基于FTP/Telnet网络通讯技术从不同核磁共振设备获取NMR数据的方法.研究了FID信号的存储格式,以Philips Intera1.5T系统产生的FID信号向Windows标准格式的转换为例,给出了适用于目前主流核磁共振设备的核磁共振底层数据格式转换的方法,并给出了具体的程序实现.基于小波变换阈值去噪技术进行了FID信号的去噪研究,利用自行构造的小波滤波器组取得了较好的去噪实验效果.     

小波变换及其在γ能谱去噪处理中的应用

为了利用小波变换对HPGeγ谱仪测量的γ能谱进行消噪处理,首先论述了小波变换基本原理以及一维信号消噪过程中小波函数、分解层数、阈值函数以及阈值规则选取方法,然后结合已测的137Cs、140Laγ能谱所固有的噪声特性,采用小波工具箱对其进行了消噪处理.消噪结果与MCNP仿真计算结果进行了比对,计算了两者之间绝对误差的2范数,对比曲线图表明两者相当一致.     

矩形回路内超临界二氧化碳自然循环实验研究

为认识超临界二氧化碳自然循环基本特性,开展超临界二氧化碳在简单矩形回路内自然循环特性的实验研究,研究系统压力和冷热段流体温差对自然循环流量的影响,分析回路结构对自然循环特性的影响。结果表明:循环流量存在峰值;峰值点前,随加热功率增加流量快速上升,峰值点后流量变化平缓;在本试验参数条件下未观测到流动不稳定现象;压力对循环流量影响与亚临界自然循环类似,压力越高循环流量峰值越大,回路冷热段温差对循环流量影响较大;加热段出口流体温度接近拟临界温度时,很小的回路温差变化即可引起循环流量较大变化;加热段布置方式对超临界二氧化碳自然循环流量变化特性影响较大,对回路稳定性的影响需要进一步进行实验验证。     

应用小波新阈值函数对低放射性活性炭测氡γ能谱的消噪

提出了一种用于低放射性活性炭测氡的γ能谱消噪的小波阈值函数。在Matlab平台上,通过对野外实测γ能谱的消噪处理,对比分析了多点均滑消噪、傅里叶变换消噪、小波软阈值消噪及新阈值函数消噪的优缺点。结果表明:应用阈值函数对低放射性γ能谱的消噪,在反映γ能谱特征的峰本比、峰高、峰面积及本底值参数方面的效果优于传统的方法,且保留了原始信号的细节特征,具有较高的实用价值。     

改进小波阈值方法对γ能谱去噪的研究

在γ能谱测量过程中,如何有效地对谱数据进行平滑处理,关系到峰位道址的确定和峰面积的计算是否准确。本文基于小波分析,针对影响去噪结果的关键要素--阈值选择方法和阈值函数,分析两者的缺陷并对其进行改进。研究结果表明：改进后小波阈值方法的平滑度指标和去噪前后能谱能量比值分别为0.985 3和0.999 8,其去噪效果明显优于其他方法。     

超临界水自然循环回路流动和传热特性数值模拟

正在国际原子能机构的协调研究计划项目的支持下,以超临界水自然循环试验回路为研究对象,采用计算流体软件ANSYS CFX对10个试验工况进行了数值模拟,分析了自然循环情况下超临界水的流动及传热特性。计算发现,整个计算工况范围内采用层流模型可以很好的模拟回路内的自然循环流动,计算得到回路循环流量、试验段内主流温度及内壁温度随加热功率的变化与试验值均符合较好,计算结果如     

一维开式自然循环系统分析程序的实验验证

基于华龙一号非能动安全壳热量导出系统（PCS）综合性能实验装置实验结果,对采用基于漂移流模型开发的华龙一号PCS程序（PCS?NCCP）进行验证,对比分析了设计工况及非设计工况下PCS?NCCP程序计算值与实验值之间的误差。结果显示,所开发的PCS?NCCP程序能模拟PCS的排热能力、稳态运行特性和动态响应特性,程序计算值能很好地跟踪实验的趋势和幅值变化,绝大部分计算误差落在±20%范围内,验证了PCS?NCCP程序的准确性。     

并联通道内超临界水流动不稳定性的数值模拟研究

超临界水的流动不稳定性特征研究是超临界水冷堆热工水力设计的重点,为进一步获得超临界水流动不稳定性发生的内部机理,采用系统分析程序RELAP5对已有实验本体进行建模,并基于已有超临界水不稳定性实验数据开展了计算方法的验证;系统研究了并联通道内超临界水的流动不稳定性规律,并对比研究了超临界水与亚临界水的不稳定边界。结果表明,超临界水的流动不稳定界限功率随入口温度的增加存在变化拐点;相同入口温度下,随压力上升,不稳定界限功率增加,超临界水相比亚临界气-液两相流具有更好的稳定性;无量纲准则数在超临界条件下具有适用性,超临界水不稳定性变化规律与亚临界水具有相似性。      

超临界水拟临界点的热膨胀系数计算研究

在临界点附近时,超临界水的热膨胀系数很大,随着压力的升高,拟临界点的热膨胀系数迅速下降,拟临界点的热膨胀系数是计算临界相变转换数的基础,对于分析超临界压力下的流动不稳定性非常重要。因此,计算超临界水在拟临界点的热膨胀系数,对于了解和掌握超临界水堆中能量的转换或热量传递非常重要。运用MATLAB曲线拟合工具箱,对超临界水的拟临界点的膨胀系数进行了拟合回归分析。拟合得出了超临界水的拟临界点的热膨胀系数的计算公式。该公式具有结构简单易于计算的特点,最大绝对误差为0.20 K-1,最大的相对误差为0.19%,计算精度满足工业研究与分析的要求。     

Analysis of Reverse Flow in Inverted U-Tubes of Steam Generator under Natural Circulation Condition

In this paper, we report on the analysis of reverse flow in inverted U-tubes of a steam generator under natural circulation condition. The mechanism of reverse flow in inverted U-tubes of the steam generator with natural circulation is graphically analyzed by using the full-range characteristic curve of parallel U-tubes. The mathematical model and numerical calculation method for analyzing the reverse flow in inverted U-tubes of the steam generator with natural circulation have been developed. The reverse flow in an inverted U-tube steam generator of a simulated pressurized water reactor with natural circulation is analyzed. Through the calculation, the mass flow rates of normal and reverse flows in individual U-tubes are obtained. The predicted sharp drop of the fluid temperature in the inlet plenum of the steam generator due to reverse flow agrees very well with the experimental data. This indicates that the developed mathematical model and solution method can be used to correctly predict the reverse flow in the inverted U-tubes of the steam generator with natural circulation. The obtained results also show that in the analysis of natural circulation flow in the primary circuit, the reverse flow in the inverted U-tubes of the steam generator must be taken into account.     

矩形回路内超临界水稳态自然循环特性数值分析

采用计算流体力学(CFD)方法对简单矩形回路内的稳态自然循环进行数值模拟研究,并对超临界条件下的重力压降计算方法进行评估分析。结果表明,稳态自然循环流量随加热功率的变化,加热段出口流体温度在拟临界点附近时出现最大值,该最大值随加热段入口流体温度的增加而减小;加热段的温度整体上升并向拟临界区移动时,加热段进出口间的密度差、速度差趋于增加,而压降趋于减小。重力压降计算方法评估表明,Ornatskiy与Razumovskiy所推荐的公式在计算较长管道(2 m)内重力压降时结果偏小,最大偏差接近-30%,辛普森公式可以较好地计算较长管道内的重力压降,可用于处理实验数据。     

Experimental study on convective heat transfer of water flow in a heated tube under natural circulation

This paper reports on an experimental study on transitional heat transfer of water flow in a heated vertical tube under natural circulation conditions. In the experiments the local and average heat transfer coefficients were obtained. The experimental data were compared with the predictions by a forced flow correlation available in the literature. The comparisons show that the Nusselt number value in the fully developed region is about 30% lower than the predictions by the forced flow correlation due to flow laminarization in the layer induced by co-current bulk natural circulation and free convection. By using the Rayleigh number Ra to represent the influence of free convection on heat transfer, the empirical correlations for the calculation of local and average heat transfer behavior in the tube at natural circulation have been developed. The empirical correlations are in good agreement with the experimental data. Based on the experimental results, the effect of the thermal entry-length behavior on heat transfer design in the tube under natural circulation was evaluated.     

Experimental study of the coupled thermo-hydraulic–neutronic stability of a natural circulation HPLWR

The HPLWR (high performance light water reactor) is the European concept design for a SCWR (supercritical water reactor). This unique reactor design consists of a three pass core with intermediate mixing plena. As the supercritical water passes through the core, it experiences a significant density reduction. This large change in density could be used as the driving force for natural circulation of the coolant, adding an inherent safety feature to this concept design. The idea of natural circulation has been explored in the past for boiling water reactors (BWR). From those studies, it is known that the different feedback mechanisms can trigger flow instabilities. These can be purely thermo-hydraulic (driven by the friction – mass flow rate or gravity – mass flow rate feedback of the system), or they can be coupled thermo-hydraulic–neutronic (driven by the coupling between friction, mass flow rate and power production). The goal of this study is to explore the stability of a natural circulation HPLWR considering the thermo-hydraulic–neutronic feedback. This was done through a unique experimental facility, DeLight, which is a scaled model of the HPLWR using Freon R23 as a scaling fluid. An artificial neutronic feedback was incorporated into the system based on the average measured density. To model the heat transfer dynamics in the rods, a simple first order model was used with a fixed time constant of 6 s. The results include the measurements of the varying decay ratio (DR) and frequency over a wide range of operating conditions. A clear instability zone was found within the stability plane, which seems to be similar to that of a BWR. Experimental data on the stability of a supercritical loop is rare in open literature, and these data could serve as an important benchmark tool for existing codes and models.     

The development and validation of a thermal hydraulic code in rolling motion

An advanced thermal hydraulic code is established on the basis of RELAP5/MOD3.3 code for the investigation of the thermal hydraulic behavior of nuclear power systems. The RELAP5 code is modified by adding a module calculating the effect of rolling motion and introducing new flow and heat transfer models. The experimental data are used to validate the theoretical models and calculation results. It is shown that the advanced flow and heat transfer models could correctly predict the frictional resistance and heat transfer coefficients in rolling motion. The thermal hydraulic code is used to simulate the operation of a natural circulation system in rolling motion. The calculation results are in good agreement with experimental data. The relative discrepancies between calculation results and experimental data are less than 5%.     

Theoretical investigation on the steady-state natural circulation characteristics of a new type of pressurized water reactor

1 Introduction With respect to the inherent safety of nuclear re- actors, application of passive systems/components including natural circulation phenomena as the main mechanism is intended to simplify the safety-related systems and to improve their reliability, to reduce the effect of human errors and equipment failures, and to provide more time to enable the operators to prevent or mitigate serious accidents. Natural circulation is the main mode of heat removal for removing decay heat from t…     

铅铋自然循环回路热损效应分析

在液态金属自然循环回路的计算分析过程中,已有研究一般忽略散热损失,常导致计算结果与实验结果有较大的区别。为研究散热损失对液态金属自然循环回路稳态特性的影响,利用MATLAB/Simulink编制了含有散热损失模型的铅铋自然循环回路计算程序,并用实验结果进行了验证。利用该程序,分析了不同热功率、中间热交换器二次侧流量和环境温度下散热损失对自然循环回路稳态参数的影响。计算结果表明:通过减小散热损失可提高回路的自然循环流量;当二次侧流量较小时,散热损失对循环流量的影响更为明显;通过增加二次侧流量或适当增加热功率可减小散热量占总热功率的比例,提高热量利用率;当二次侧流量不变时,不同热功率下环境温度对回路的自然循环流量的影响不明显,但热量利用率会随环境温度的升高而增加。