冷冻外科连续冻融循环理论研究

冷冻外科实践证明，组织经历连续冻-融循环后治疗效果确切，并且比单一冻融过程疗效有所提高。在焓法理论基础上，通过应用有限元数值分析方法对模拟生物组织中连续两次冻融循环过程进行理论分析。研究了连续冻融循环过程中，组织中温度场、冰球生长和温度梯度场的变化规律。结果表明，实施连续冻融循环，组织中冰球反复冻融、温度和温度梯度波动明显，这些因素均为疗效提高的热力学机理。这些分析可以为从组织学、生物力学和生化理论角度进一步分析连续冻融损伤机理提供热力学依据。     

利用熵产最小法分析R290/CO2复叠式制冷循环

介绍了低温环境下采用自然工质R290和CO2的复叠式制冷循环，用熵产最小法对R290／CO2复叠式低温制冷循环进行了分析，利用熵产最小法确定R290/CO2复叠式低温制冷循环的最佳中间温度。为提高R290／CO2复叠式低温制冷循环的效率，应减少蒸发器、冷凝器和冷凝蒸发器的传热温差，可以看出R290／CO2的复叠式制冷循环在低温制冷条件下有很好的发展前景。     

低温换热器的效率分析

分析了低温换热器中传热过程的热力学特性，定义了低温换热器传热效率，得到了效率的计算式。讨论了传热单元数、介质热容量比等参数对低温换热器效率的影响，比较了各种流型低温换热器效率的相对大小，给出了使效率达最大值的最佳热容量比的计算方法。     

冻融循环作用下玄武岩纤维-矿渣粉-粉煤灰混凝土压拉强度试验与细观结构

对玄武岩纤维-矿渣粉-粉煤灰混凝土(BF-SP-FAC)进行了单轴抗压试验、劈裂抗拉试验、冻融循环试验、气孔结构测试试验和SEM分析。研究了不同冻融次数下BF-SP-FAC冻融损伤量、抗压强度、抗拉强度的变化,分析了气孔结构参数(含气量、气孔比表面积、气泡间距系数和气泡平均弦长)与BF-SP-FAC抗压强度、抗拉强度、冻融损伤量的关系,运用灰关联熵分析法讨论了BF-SP-FAC气孔结构参数对抗压强度、抗拉强度、冻融损伤量影响的主次关系。结果表明:相同冻融次数下,与其他纤维掺量相比,玄武岩纤维掺量为0.18vol%时,BF-SP-FAC抗冻性能较好,抗压强度和抗拉强度最高;在相同玄武岩纤维掺量下,随含气量、气泡间距系数、气泡平均弦长的增大,BF-SP-FAC抗压强度和抗拉强度减小,而冻融损伤量增大;随气孔比表面积的增加,BF-SP-FAC抗压强度和抗拉强度增大,而冻融损伤量减小。气孔比表面积是影响BF-SP-FAC强度的最主要因素,而气泡平均弦长是影响BF-SP-FAC冻融损伤量的主要因素,最小灰熵关联度分别为0.998和0.993。气孔结构参数与强度、冻融损伤关系的建立,可预估混凝土的强度与冻融损伤。      

低温换热器的Yong效率分析

分析了低温换热器中传热过程的热力学特性，定义了低温换器传热Ｙｏｎｇ效率，得到了Ｙｏｎｇ效率的计算式，讨论了传热单元数，介质热容量比等参数对低温换热器Ｙｏｎｇ效率的影响，比较了各种流型低温热器Ｙｏｎｇ效率的相对大小，给出了使Ｙｏｎｇ效率达最大值的最佳热容量比的计算方法。     

低温微创手术液氮传输过程的压力和传热分析

在低温微创手术中,采用液氮作低温工质,对肿瘤组织进行低温冷冻,为了能精确控制低温治疗探针的温度,需要了解冷疗手术过程中液氮传输的工况.对液氮在输送管中压力和传热进行分析计算,结果表明:低温流体在传输中气、液两相成分比决定于换热和流动阻力,通过对影响换热和压降的主要因素进行试验,从而获得合理的传输模式.     

涡流管热力学方法的研究

依据热力学第一、第二定律，采用熵产分析方法，建立了适合的涡流管热力学模型，得出产生温差的最小压比，并在h-s图上表示出涡流管的能量分离过程。为涡流管的理论研究提供一种新的方法。     

低温辐射传热的特点及其对低温绝热的影响

本文从简述辐射传热的一般理论着手，提出了低温下辐射传热的几个问题，分析了材料的辐射性质及对低温绝热的影响，描述了低温辐射传热的计算，最后提出了对减少低温辐射传热的一些看法。     

考虑粘结滑移的冻融损伤纤维梁柱模型研究

伸入梁柱节点以及柱与基础交界处纵向受力钢筋的粘结滑移效应会显著影响构件的侧向变形。为准确评估冻融损伤后钢筋混凝土(Reinforced Concrete, RC)柱的抗震性能，在考虑冻融损伤不均匀分布的纤维模型基础上，以锚固区的粘结滑移效应为研究对象，首先基于拉拔试验建立可考虑冻融损伤分布的粘结强度退化规律，进而根据简化粘结应力分布假设，通过建立控制方程进行理论推导得到冻融损伤粘结滑移计算方法，并与冻融钢筋混凝土拉拔试验数据进行了对比验证。进而基于有限元分析软件OpenSEES，将该文模型嵌套于零长度截面单元中，提出可综合考虑冻融不均匀损伤与粘结滑移效应的纤维梁柱模型，根据6榀冻融RC柱拟静力加载试验数据进行了验证，并与仅考虑冻融损伤的纤维模型进行了对比。结果表明:与纤维模型计算结果相比，采用该文模型计算所得滞回曲线与试验结果吻合更好，在承载力、极限位移和累积耗能等方面的计算误差较小，表明所建模型可更为准确地反映冻融损伤后RC柱的地震响应。     

混凝土结构冻融损伤理论及冻融可靠度分析

该文基于疲劳累积损伤理论,对混凝土的冻融损伤特性进行了可靠性分析。把混凝土结构冻融破坏近似看作由不同正负峰值温度差顺序作用产生的疲劳累积损伤破坏,给出了常幅温差及变幅温差作用下基于疲劳累积损伤的冻融可靠度计算分析模型,并对某一混凝土构件的冻融损伤可靠度进行了计算分析。     

Entropy generation theory for characterizing the freezing and thawing injury of biological materials

As an important approach for long-term preservation of biological material, cryopreservation has been widely studied and applied in clinics. However, one of the most critical issues involved–the injury of biology material induced during freezing and thawing process still remains to be incompletely answered due to complexity of the problem itself. In this paper, we proposed for the first time to interpret the freezing or thermal injury by using the irreversible thermodynamics theory which is generalized in concept and has wide applicability. Comprehensive entropy generation analysis was performed on several typical freezing/thawing processes of biological tissues subject to cryopreservation. Particularly, variation of entropy generation rate due to injury induced change of thermal properties of the biological materials was investigated. Several useful indexes were suggested to quantify the freezing injury or viability of the biological material, through a combination with certain specific measurements. This study may possibly open a new theoretical strategy for evaluating the final output of either cryobiology or hyperthermia practices.     

载冷气泡直接接触式对流换热过程熵产分析

研究了载冷气泡和制冰溶液的直接接触换热过程,建立了对流换热微分方程,得到了努塞尔数和雷诺数、普朗特数的传热关联式,通过实验数据验证了模型的准确性。利用传热关联式,针对换热过程的熵产作了分析,在以气泡直径作为换热特征尺寸下,换热过程中熵产数随着雷诺数的变化存在最小值。研究为直接接触式对流换热过程的优化提供了理论基础。     

Preliminary survey on the mechanisms of the wave-like behaviors of heat transfer in living tissues

The mechanism of the wave like behaviors of heat transfer in living tissues were studied through introducing a new concept of multi-mode energy coupling. A phenomenological thermal wave model of bioheat transfer was obtained. A new conceptual equation was proposed to correlate the heat flux with the temperature gradient, thus the intriguing high magnitude of the characteristic time in living tissues was better understood. A hypothesis was proposed for further testing the current theory. Numerical calculations were performed to study the temperature transients in the skin stratified as three layers with different thermal parameters. Deviations between the thermal wave model of bio-heat transfer and the Pennes' equation were studied for several typical transient bio-heat processes and the possible practical implications were discussed. A simple temperature criterion has been established to determine when the thermal wave propagation dominates the principal heat transfer process.     

Dynamic Low-Frequency Electrical Impedance of Biological Materials Subject to Freezing and Its Implementation in Cryosurgical Monitoring

A precise monitoring of the extent of freezing during a cryosurgical process has been an important problem in health-care clinics. Among various existing techniques, the dynamic electrical impedance utilizing the impedance jump to detect the ice moving front, is a suitable way because the impedance of frozen tissue is much higher (3 to 4 orders of magnitude or even larger) than that of unfrozen tissue. Based on two experimental setups, the dynamic low-frequency impedance (DLFI) and impedance changing rate (ICR) for selected biological materials (fresh pork and rabbit tissues) subject to freezing were systematically measured. Their transient behaviors were investigated, and implementations in a practical cryosurgery to detect ice front propagation were analyzed. Furthermore, in vitro experiments were performed on distilled water and phantom gel. The experimental results, obtained with a two-electrode measuring technique, are as follows: (1) The impedance of all samples has a rapid response to the external freezing. (2) The impedance will not jump into an insulation region when the cooling temperature is not low enough. (3) As an alternative to DLFI, ICR imaging can also give important information for the phase-change process, which may lead to an efficient method to detect the ice-ball growth. (4) There is an evident variation in DLFI for different biological tissues when subjected to the same cooling temperatures; this value also differs for the same tissue under different cooling conditions.     

Standardization of cryosurgical lesions

In order to establish treatment standards for tumours of various types and sizes, one must have full control over the cryosurgical operation. The requirements for such controlled cryotherapy are discussed, and it is argued that it should be possible to keep the freezing rate between 30–50°C per minute irrespective of tumour size, and that the tissue must be thawed as quickly as possible once the desired minimum temperature has been reached. For controlled cryotherapy one has to rely on freezing damage as damage caused during thawing is difficult to predict.Freezing probes of different designs and sizes must be available to meet the demands mentioned for heat exchange and a pressure gauge must be at hand to record the force with which the freezing probe rests against the tissue. Finally a reliable flowmeter must be found with which to measure the number of calories drained away from the tissue.     

Pilot study on cryogenic heat transfer in biological tissues embedded with large blood vessels

Blood flow through large vessel plays an important role in affecting the temperature profiles of the living tissues under cryosurgery. Besides, arresting of blood vessels due to freezing may possibly cause danger to the patient, which needs to be considered when operating the cryoprobe. However, such important issues received few attentions in the bioheat field even up to date. In this paper, pilot studies were performed to investigate the cryogenic heat transfer behaviors in biological tissues embedded with large blood vessels. First, a simple however intuitive theoretical model was established and then analytically solved. Parametric studies were performed to test the influences of the blood vessel entrance temperature, the vessel diameter, the blood flow velocity and the vessel length etc. to the whole region's temperature distribution. The critical tissue surface temperature to freeze the blood vessel was theoretically predicted. Second, to reveal the role of the countercurrent artery-vein blood flows to the transient phase change in living tissues subject to freezing, qualitative simulating experiments on phantom gel were performed. A 3 cm-diameter, 14 cm-length cylindrical copper block pre-frozen by liquid nitrogen was applied to freeze the gel embedded with two parallel countercurrent 1.1 mm OD/0.8 mm ID Teflon tubes with 30 °C warm water flowing through at the velocity of 0.1 m/s. Temperatures were measured at the selected positions on the tube wall in a step of 2 cm. As a comparison, experiments were also conducted on the same gel without running warm water. It was demonstrated that the countercurrent water flow has significant effect on the freezing progress of the phantom gel in comparison with that of non-flow gel. This study raised an important issue to study the phase change heat transfer of blood vessels to the living tissues subject to cryosurgery, which may have significant clinical applications. The present method can also possibly be extended to wider fields such as heat transfer in buried pipes and collectors.     

生物组织冻结过程中的应力分析

为了预测生物组织低温保存过程中所受到的机械损伤 ,建立了一维球对称模型 ,模拟计算了降温过程相变界面随时间的移动 ,壁面温度随时间的变化 ;组织内温度分布不均引起的热应力随时间变化及其在壁面的分布。主要考虑了降温速率的影响 ,当降温速率低于某一值时 ,降温速率越高 ,组织内部所受热应力越大 ;降温速率超出这个值 ,组织内的热应力不再随降温速率变化     

Extension to Planck''s equation for predicting freezing times of foodstuffs of simple shapes

Planck's equation for predicting freezing times ignores sensible heat and the gradual character of the phase-change process in foodstuffs. By making certain analytical approximations to take these effects into account, a simple freezing-time prediction method is obtained that uses no empirical factor, chart or advanced algebra. The freezing process is divided into cooling and phase-change periods, the latter being calculated with Planck's equation, the former with extensions to Newton's law of cooling. The gradual character of the phase-change process is taken into account by the use of a ‘mean freezing temperature’, while the thermophysical properties of the material are approximated by straight-line segments. The method agrees with published data as well as or better than any previous procedure, including finite-difference computations.     

Thermodynamic analysis of free convection film condensation on an elliptical cylinder

Abstract

This paper aims to perform thermodynamic analysis of saturated vapor flowing slowly onto and condensing on an elliptical cylinder. This is the first approach to investigate how the geometric parameter‐ellipticity and surface tension affect local entropy‐generation rate during film‐wise condensation heat transfer process. The results observe that entropy generation decreases with decreasing ellipticity. It indicates that the entropy generation number is nearly unaffected by surface tension forces at small ellipticity like e ≤ 0.7, but somewhat influenced at large ellipticity for the whole perimeter. From the second law point of view, local entropy generation increases with ellipticity as local heat transfer coefficient does. Furthermore, the entropy‐generation rate due to gravity‐driven film flow friction is proportional to Brinkman group parameter. The irreversibility ratio indicates that film flow friction irreversibility starts to dominate over heat transfer irreversibility in the lower half of streamwise length for higher values of Brinkman group parameter (Br/T = 1).     

A computational study about the types of entropy generation in three different R134a ejector mixing chambers

The purpose of this investigation is to study the efficiency of a R134a ejector operating with three different mixing chambers by means of a CFD based entropy generation analysis. With the aid of the differential equation for entropy, the local entropy generation is pursued. Using this method, the areas where the irreversibilities occur are identified and geometry improvements suggested. The novelty of the numerical procedure presented herein is that the bulk entropy generation is analysed by means of four sources related with viscous dissipation and heat transfer, divided in mean and fluctuating terms. Entropy generation within the boundary layer has also been considered. The latter has proved to be small, as well as the heat transfer contribution. The fluctuating viscous dissipation accounts for more than 75% of the added entropy, being its main sources the shear layer after the nozzle exit and the shock wave trains, independently of their position.