不同温度下泡沫混凝土热物理性能的研究

为研究不同温度下聚苯乙烯(EPS)泡沫混凝土热物理性能参数的变化规律,使用ISOMET2114便携式传热分析仪测试了不同温度下(-10～40℃)泡沫混凝土的导热系数、比热容和热扩散系数;同时,通过改变掺入EPS的体积分数,研究EPS泡沫混凝土热物理性能的变化规律,探究EPS对泡沫混凝土保温性能的影响机理。结果表明:温度对于泡沫混凝土的热物理性能有着显著影响,随温度的升高,泡沫混凝土的导热系数及比热容随之增大,而热扩散系数呈下降趋势;而随掺加的EPS颗粒体积分数的增大,泡沫混凝土的导热系数和热扩散系数先减小再增大,比热容先增大后减小;同时,泡沫混凝土的热物理特性是其干密度的函数,其导热系数随干密度的降低而降低。     

准稳态法测量纳米颗粒悬浮液的热物性

通过添加亲水性分散剂,经超声振动制备了粒径为50 nm CuO纳米颗粒悬浮液。理论和实验分析了运用准稳态法测量纳米颗粒悬浮液有效导热系数和比热容等热物性的可行性。加热密度为100～500 W/m~2时,热物性测量结果的合理性和重复性都较好;而加热密度较小时离散性较大。实验表明,常温下2~(wt)％的CuO纳米颗粒悬浮液的有效导热系数约为纯水的1.08倍,比热容约为纯水的1.02倍。     

SiNx薄膜热物性参数实验测量与分析研究

采用一种新的实验测量方案，将金属加热单元与温度探测单元合二为一，间接获得了在半导体和微电子学MEMS领域内有重要用途的SiN_x薄膜的导热系数、发射率、比热容和热扩散系数，并对实验结果进行了不确定度分析，为微电子电路设计和掩模成型工艺等提供了可靠的热物性数据. 实验结果表明，薄膜的导热系数、发射率、热扩散系数远比相应体材质低，而且还与温度、厚度有关，尺寸效应显著，而比热容则与体材质相差不大. 关键词： 微尺度传热 热物性参数 x薄膜')" href="#">SiN_x薄膜 测量技术     

离子液体[EMIm]Ac比热容、导热系数及黏度的研究

本文通过实验研究,测定在一定温度范围内,离子液体[EMIm]Ac及摩尔分数分别为0.8、0.6、0.4和0.2的[EMIm]Ac水溶液的比热容、导热系数及黏度。实验结果表明:比热容和黏度受温度影响较大,比热容随温度升高而增大;黏度随温度升高而急剧减小;导热系数受温度影响较小;添加水后,随着水含量的增加即水溶液摩尔分数的减小,比热容增大,相比于纯离子液体,比热容最高增加70%;导热系数最高增加35%;其黏度急剧减小,最高减小82%。本文的研究结果为[EMIm]Ac及其水溶液在吸收式制冷方面的应用研究提供了可靠的数据。     

SiNx薄膜热物性参数实验测量与分析研究

采用一种新的实验测量方案,将金属加热单元与温度探测单元合二为一,间接获得了在半导体和微电子学MEMS领域内有重要用途的SiNx薄膜的导热系数、发射率、比热容和热扩散系数,并对实验结果进行了不确定度分析,为微电子电路设计和掩模成型工艺等提供了可靠的热物性数据. 实验结果表明,薄膜的导热系数、发射率、热扩散系数远比相应体材质低,而且还与温度、厚度有关,尺寸效应显著,而比热容则与体材质相差不大.     

正二十四烷烃石蜡/膨胀石墨（EG）复合相变材料分子动力学模拟

相变材料(PCMs)在相变时的恒温、高能量密度等特性,经常应用于设备的热管理,但是PCMs导热系数低的缺点影响了其使用范围.本文采用分子动力学方法,模拟了在正二十四烷烃石蜡PCMs中添加不同结构(层状、交叉状)的膨胀石墨(EG)之后构成的复合PCMs的物性.文章通过径向分布函数(RDF)、声子态密度(PDOS)、比热容和导热系数这四个指标,分析了夹角为0°的层状结构,夹角为45°、90°的交叉状EG添加物对于石蜡热物性的影响. EG(0°、45°、90°)添加使得石蜡的原子分布在不同程度上变得更加均匀、紧密,使得石蜡的比热容有所增加.同时,两种类型的添加物提高了石蜡的PDOS,提高了导热系数.其中,EG(90°)添加物对于石蜡导热系数的提升最为明显,石蜡/EG(0°、45°、90°)模型中EG的含量分别为33.63 wt%、30.86 wt%和23.20 wt%,相比于的石蜡的导热系数分别提升了417.1%、345.7%和522.9%. EG的添加能够提高石蜡的导热系数,不同结构的EG对石蜡导热系数的影响有着较大的区别.     

3 ω线法测量液体热参数影响因素分析

首先给出基于谐波探测技术测量液体导热系数和热扩散系数的3ω线法的基本原理.采用实验和理论分析相结合的方法,分析了加热丝结构参数和自身热参数对温度波动和测量结果的影响.给出了加热丝直径和长度的合理范围.液体导热系数比较小时可以利用三次谐波的实部和虚部的交点测量液体的比热容.     

含水层储能影响因素分析

本文建立了热泵耦合含水层储能系统的地下水流动及热质迁移的耦合数学模型,模拟了不同井距和在不同水文地质条件下对井抽灌储能区域的温度场变化.通过分析得出:增大井距可延长发生热突破的时间、提高储能效果,但储能对地下环境的热影响也相应增大;含水层介质的渗透系数(或渗透率)、弥散度以及导热系数越大则储能效果越差,体积比热容越大储能效果越好,且体积比热容对储能效果的影响较显著.     

杏鲍菇热风干燥中热质传递的实验及模拟研究

杏鲍菇作为一种营养价值高、市场需求量大的典型食用菌,热风干燥是其主要加工方法。本文以干燥条件和切片厚度为实验变量,探究热风干燥中杏鲍菇切片的干燥特性(干基含水率、干燥速率)随时间的变化规律。考虑切片的收缩变形和热物性(导热系数、比热容)随含水率的变化关系,建立了热风干燥条件下杏鲍菇切片的热质耦合数值模型。结果表明：杏鲍菇切片的干燥速率随干燥温度的升高而增大,随切片厚度的增加而减小;杏鲍菇切片在对流干燥过程的收缩变形存在明显的非线性,尤其是在降速干燥阶段。并且,数值模型的预测结果与实验测试数据吻合良好,为准确预测杏鲍菇切片的热风干燥特性提供了依据,对菌类干燥工艺优化有重要的指导意义。     

不同材料薄壁微球结构低温冷却特性数值模拟

为了研究重力效应以及材料物性对球壳内工质冷却过程的影响,获得球壳内均匀的温度场,对球壳冷却过程进行传热分析,并建立了球壳冷却过程的二维旋转轴对称模型,对该非稳态的冷却过程进行数值仿真。研究表明,在工质没有发生相变的冷却过程中,自然对流速度非常小,重力效应对球壳内温度场的影响非常微弱,可以忽略不计;而对于材料的物性影响,根据引入的定义,比热容和密度对冷却时间的影响分别是导热系数影响程度的25.2倍和66.5倍,因此比热容和密度是影响冷却特性的主导因素,而导热系数的影响较小可以忽略;对于不同材料的球壳,单位体积热容小的材料会得到更好的冷却效果。     

Solar contribution to space heating

Summary The solar contribution to space heating and cooling is evaluated through an analytical procedure. The method used simulates the building as a thermal network in which each node corresponds to the centre of a room and each mesh arm to a wall; building materials and frames are explicitly taken into account. The network is excited by a set of generators corresponding to external excitations (e.g. environmental temperature, solar flux, etc.) which are described by a truncated Fourier series. The network is resolved by an analytical method through the matrix formalism and gives either the time diagrams of indoor temperatures in each room, or the diagrams of the auxiliary energy required to maintain a set room temperature. The application to a residential environment is presented as an example and is discussed in relation to a passive retrofitting design. The results are briefly compared with those obtained through established procedures, to give evidence of the occurrence of strong fluctuations on the average values which significantly influence the conditions of comfort. The method can be implemented at the level of microcomputers.     

Energy analysis of a passive solar system

This work aims at presenting the numerical solution to a natural convection problem concerning the use of a passive solar system for building heating purpose. The system consists of a modification of the well-known Trombe-Michel passive system. The main differences consist of thermal insulation on the southern wall surface, the presence of two solar ducts separated by a thin metallic plate with collector function, and a thermal storage over the ceiling of the heated rooms. The numerical solution to the simple mathematical model — based on energy and mass conservation equations — is achieved by a finite difference method, which allows to determine both the time-dependent temperature profile on each component of the system and the air flow pattern in the solar ducts. A comparison between the numerical results and some experimental data is reported: it shows a very satisfactory agreement. At last, the hour by hour energy fluxes are shown in some graphs.     

Thermal evaluation of concentrator-assisted solar distillation system

A simple analysis of a concentrator-assisted solar distillation unit has been presented in terms of concentrator and solar still parameters. An analytical expression for instantaneous thermal efficiency of the system is derived using the energy balance equation for each component of the system. It is observed that the instantaneous and overall efficiency of the concentrator-assisted solar still is significantly improved compared to a collector-cum-distillation unit due to reduced heat loss in the concentrator. The derived expression for instantaneous thermal efficiency can be used for testing of any solar distillation unit working under either a passive or active mode of operation.     

Methods for creating the self-regulating mechanisms of passive systems for ensuring thermal regime of devices for space application

The passive systems for ensuring the thermal regime of space objects, which are in current use, can preserve the temperature at the same level only under a certain orientation of spacecraft and at a constant heat release of the equipment. At a variable orientation of the space object, its temperature may vary due to a possible illumination of the radiator-emitter of the system for ensuring the thermal regime by the solar or planet radiation. At a variable heat release the object temperature changes due to an off-design regime of the radiator operation. To compensate for the given effects the active elements are used — the heaters and coolers. This reduces the reliability of thermal regulation system and, consequently, the reliability of the entire spacecraft. The solutions are proposed, which create in the absence of active elements in the thermal regulation system the internal mechanisms enabling a compensation of the variability of heat fluxes both at a reorientation of the spacecraft and at a variation of the internal heat release.     

轻质建筑中定形相变内隔墙板冬季应用效果研究

现代轻质建筑热容较小,窗墙比较大,冬季在利用透射太阳辐射采暖的同时易造成室内温度波动过大。本文提出将定形相变蓄能板安装在建筑内隔墙表面,以增加建筑热容,减小室温波动。采用经实验验证程序,模拟分析了相变板热物性、表面换热系数和用量对冬季室内温度波动的影响,给出了北京地区相变内隔墙房间的设计原则。     

轿车热环境模拟及风口面积对乘客舒适性的影响

通过数值计算的方法模拟了车内的速度场和温度场。计算采用SIMPLE算法,紊流采用k-ε模型,考虑了自然对流换热的影响,应用整体求解法计算气固耦合传热问题。用射线追踪法计算太阳热辐射,S2S模型计算固体壁面间的辐射。利用PMV和EHT指标从总体和局部两个方面分析车内舒适程度,并研究了喷口面积对热环境的影响。研究表明,改变喷口面积可以在一定范围内调节车内热舒适性,空调设计时可以利用这一点来提高冷负荷的利用率。     

The Analytical Approach for the Two-dimensional O(3) Non-linear σ Model

In the lattice gauge theory,by using the variational cumulant expansion,we calculate the internal energy and specific heat for the two-dimensional O(3) non-linear σ model.A comparison with Monte Carlo data is also presented.     

Cell-level hazard evaluation of 18650 form-factor Lithium-ion battery with different cathode materials

The thermal runaway process was studied in a Fire Propagation Apparatus (FPA) for three types of Lithium-ion batteries (LIB) of 18650 form-factor. Cathode materials are lithium cobalt oxide (LiCoO₂, or LCO), lithium nickel manganese cobalt oxide (LiNi_1/3Mn_1/3Co_1/3O₂, or NMC), and lithium iron phosphate (LiFePO₄, or LFP). All batteries have a graphite anode and were at a 100% state-of-charge. Each LIB was externally heated to a thermal runaway event, with the heat input at constant values of 20.4 or 34.1 W, which yielded heating rates on the order of 1 K/s, representative of the thermal runaway propagation process. The mass loss fraction before the thermal runaway events and the maximum values are similar under different heat inputs for a given type of LIB. For different types of LIBs, the maximum mass loss fraction shows the trend of LCO>NMC>LFP. Under the same heating condition, NMC has the highest maximum surface temperature followed by LCO then LFP. A lumped heat transfer thermal runaway model is developed using two decomposition reactions and one internal short circuit reaction to model the internal heat generation. The effective model parameters are optimized using the measured surface temperature and mass loss fraction. The model is able to simulate the thermal runaway behavior of LIB under external heating conditions and reasonably matches the experimental data of LIBs with different cathodes. The model predicts that under the same heat input condition, the thermal runaway time of LCO is shorter than NMC and LFP; the effective average internal heat generations are 22.6, 20.2, and 11.5 kJ for LCO, NMC, and LFP, respectively. The thermal runaway model will be used to predict the thermal runaway propagation in a LIB module.     

聚光太阳能PV/T空气集热器能量和(火用)效率分析

文中建立了带有散热翅片的复合抛物面聚光太阳能PV/T空气集热器内部传热过程的一维稳态数学模型,对传热过程进行了数值模拟,对集热器热、电、(火用)和净电效率进行了计算.分析了空气质量流量、入射光强度、风速对集热器的空气温度及系统各效率的影响.结果表明:随着入射光强度的增加,空气进出口温差、热和(火用)效率是增大的,而电效率则有所降低.随着空气流量的增加,系统的净电效率和进出口温度差是降低的.通过计算可知集热器的(火用)效率在18%～11%,热效率可达65%,净电效率低于2%,并明显受空气质量流速的影响.     

Design,manufacture and testing of a closed cycle thermosyphon rankine engine

The Thermosyphon Rankine Engine (TSR) is a recent concept for power generation using solar or other available low grade heat sources. The basis of the engine is the modification of a heat pipe, with its excellent heat and mass transfer characteristics, to incorporate a turbine, thereby making the system into a Rankine Cycle Engine.The TSR is directed towards power production from solar ponds, geothermal energy and heat produced by solar collectors, as well as for waste heat utilisation for electrical power generation.A theoretical formulation and results from experiments on prototype units are presented. Based on the results, it is concluded that the TSR engine may play an important role for conversion into electrical energy of thermal energy produced by conventional solar collectors, geothermal sources and waste heat.