多孔材料导热系数影响因素分析

导热系数是多孔材料的一个重要参数,影响多孔材料导热系数的因素很多,不同的材料有不同的导热系数。在多孔材料的组成固定之后,其导热系数主要随着温度、容重和含水率的变化而变化,其中对其影响最大的是含水率。     

含盐多孔材料传热传质研究

对含盐多孔材料内部温度、质度分布作了现场测定和数值模拟。建立了含盐多孔材料在一维温度质度场的耦合方程组,获得了其温度、质度动态分布规律, 数值模拟结果与实测结果较为吻合, 验证了数值模型的正确性, 对促进含盐多孔材料热质迁移的研究,具有重要的意义     

若干因素对多孔透水混凝土性能的影响

研究了若干因素如骨料级配与粒径、骨灰比、水灰比、外加剂及搅拌工艺等对多孔透水混凝土的空隙率、透水系数与抗压强度等性能的影响.结果表明：骨料粒径与级配、骨灰比是影响多孔透水混凝土空隙率、透水系数与抗压强度的关键因素;水灰比对多孔透水混凝土的性能影响较小;减水剂、硅灰及聚合物乳液等外加剂可改善多孔透水混凝土的性能;水泥裹石法搅拌工艺对多孔透水混凝土的透水系数影响不大,但能使其抗压强度提高、空隙率下降.     

多孔材料湿缓冲循环的节能效果

将多孔材料用于房间内壁面可起到缓冲室内湿度波动的作用.基于理想等温工况,解析出2种稳态湿缓冲日循环机制,其中昼吸夜放循环能有效降低空调湿负荷.通过分析不同城市气候湿度特征和湿度波动潜在峰谷关系,预测了不同季节湿缓冲循环的发生几率.借助EnergyPlus模拟分析了BESTEST标准办公建筑算例应用火山石基复合相变调湿材...     

建筑装修材料可挥发性有机化合物散发特性分析

建筑装修材料是新装修建筑室内可挥发性有机化合物(volatile organic compounds,简称VOCs)的主要散发源.选取南京市某户住宅装修过程中的8种主要建筑装修材料进行环境舱测试,获得材料甲醛与VOCs的浓度变化趋势;结合模型拟合关键散发参数,按照计算出的168 h污染物释放率对所测材料进行散发分级评价...     

温度变化对板材内VOC散发特性影响的研究

扩散系数及分配系数受温度的影响,为系统地研究温度对VOC散发的影响,本文建立了基于传热传质理论下板材内VOC散发的连续模型,并用该模型对常用板材中VOC的散发进行分析.模拟结果同计算的理论值以及实验数据均能较好的吻合.当温度从18℃升高到50℃时,不论是初始时刻的VOC散发速率还是散发稳定时刻的浓度值,都有明显的升高,散发24小时后,板材在50℃下的散发量约为30℃下散发量的2.5倍;同时本文对环境温度tf波动情况下的VOC散发结果模拟表明:空气中污染物在材料表面表现出沉降与二次挥发的现象.对密闭小室中板材内部VOC分布的模拟结果表明,随着散发时间的推移,材料内部的污染物浓度将逐渐趋于均匀.     

室内装修材料中甲醛污染物的检测与防治策略

室内环境污染问题是城市化进程中居民关注的焦点问题,其中装修材料发挥的甲醛污染物很容易对人体健康造成威胁.人民生活水平的不断提升,使房屋装修的样式逐渐多样化,各种个性化的家具和装饰材料均含有危害人体健康的化学物质,在长期的释放过程中会增加室内空气污染的严重程度.本文简要分析室内装修中主要污染物甲醛的来源、特点,提出降低室...     

掺陈化粮对烧结多孔材料性能的影响

利用陈化粮作为成孔剂制备轻质高强高孔洞率的烧结墙体材料,系统研究了成孔剂掺量和粒径对烧结制品性能的影响。结果表明：在相同烧成制度下,掺加试样质量比15%的0.15 mm~0.3 mm陈化米粒径的试件体积密度降低为1.35 g/cm3,气孔率达到45.89%,制品气孔率提高了83%,抗压强度为18.56 MPa;随着气孔率的增大,抗压强度降低,当气孔率大于35%时,成孔剂的粒径对抗压强度影响明显,当气孔率小于35%时,烧结制品性能相对稳定。     

谈影响钢铁企业总图布置的外部因素

对钢铁企业厂址选择和总图布置时需要注意的外部因素进行了分析总结,指出了运输条件、水文气象因素、地形、地貌和地质因素、相关规划因素、建设场地的面积和形状等主要外部因素对钢铁企业总图布置的影响,以引起相关工作者对这些因素的重视。     

纤维长径比对纤维多孔材料性能的影响

以氧化铝短切纤维为基体,分别经20目、40目、60目筛超洗,以聚丙烯酰胺为分散剂、硅溶胶为粘结剂、叔丁醇/水二元混合溶液为料浆液相溶剂,采用冷冻浇注工艺制备氧化铝纤维多孔陶瓷。在1500℃对样品进行烧结,测试烧结后样品的压缩强度;用扫描电镜观察样品表面形貌,研究纤维长径比对材料力学性能和隔热性能的影响。结果表明,随过筛目数增大,氧化铝多孔纤维材料密度减小,孔隙率增加,抗压强度下降。纤维过40目筛样品抗压强度略低于过20目筛样品,过60目筛样品的抗压强度最差。在隔热性能测试中,纤维过40目筛样品冷面温度略低于过60目筛样品,过20目筛样品的隔热性能最差。过40目筛纤维长径比为20～35,其所制备样品的隔热性能和力学性能俱佳。     

多孔建筑材料热湿传递过程的研究

从理论计算和实验研究等方面对国内外多孔建筑材料的传热、传湿及热湿耦合传递研究方面的进展进行了综合分析。指出了目前多孔建筑材料热湿传递过程研究中存在的问题,并对未来的研究方向进行了展望。     

墙体特性与自然通风耦合作用的墙体传热模型

对单面墙体自然通风房间的传热过程进行数值模拟。主要讨论了不同自然对流强度对墙内侧对流换热系数的影响,并分析了不同墙体结构的室内空气温度变化情况。结果表明：墙内侧值的变化会随瑞利数（Ra）的增加而增加,室内平均温度随房间通风量增加振幅不断减小,房间热环境随通风量增加而改善。绝热层总厚度不变时,绝热层位置离热源越近隔热效果越好。     

墙体特性与自然通风耦合作用的墙体传热模型

对单面墙体自然通风房间的传热过程进行数值模拟.主要讨论了不同自然对流强度时墙内侧对流换热系数的影响,并分析了不同墙体结构的室内空气温度变化情况.结果表明:墙内侧值的变化会随瑞利数(Ra)的增加而增加,室内平均温度随房间通风量增加振幅不断减小,房间热环境随通风量增加而改善.绝热层总厚度不变时,绝热层位置离热源越近隔热效果越好.     

Modelling of volatile organic compounds emission from dry building materials

A numerical and an analytical model were developed to predict the volatile organic compound (VOC) emission rate from dry building materials. Both models consider the mass diffusion process within the material and the mass convection and diffusion processes in the boundary layer. All the parameters, the mass diffusion coefficient of the material, the material/air partition coefficient, and the mass transfer coefficient of the air can be either found in the literature or calculated using known principles.

The predictions of the models were validated at two levels: with experimental results from the specially designed test and with predictions made by a CFD model. The results indicated that there was generally good agreement between the model predictions, the experimental results, and the CFD results. The analytical and numerical models then were used to investigate the impact of air velocity on emission rates from dry building materials. Results showed that the impact of air velocity on the VOC emission rate increased as the VOC diffusion coefficient of the material increased. For the material with a diffusion coefficient >10⁻¹⁰ m²/s, the VOC emission rate increased as the velocity increased; air velocity had significant effect on the VOC emission. For the material with a VOC diffusion coefficient <10⁻¹⁰ m²/s, the VOC emission rate increased as the velocity increased only in the short-term; <24 h. In the medium to long-term time range, the VOC emission rate decreased slightly as the air velocity increased; velocity did not have much impact on these materials. Furthermore, the study also found that the VOC concentration distribution within the material; the VOC emission rate and the VOC concentration in the air were linearly proportional to the initial concentration. However, the normalized emitted mass was not a function of the initial concentration: it was a function of the properties of the VOC and the material.     

3D-CFD analysis of diffusion and emission of VOCs in a FLEC cavity

This study is performed as a part of research that examines the emission and diffusion characteristics of volatile organic compounds (VOCs) from indoor building materials. In this paper, the flow field and the emission field of VOCs from the surface of building materials in a Field and Laboratory Emission Cell (FLEC) cavity are examined by 3D Computational Fluid Dynamics (CFD) analysis. The flow field within the FLEC cavity is laminar. With a total flow of 250 ml/min, the air velocity near the test material surface ranges from 0.1 to 4.5 cm/s. Three types of emission from building materials are studied here: (i) emission phenomena controlled by internal diffusion, (ii) emission phenomena controlled by external diffusion, and (iii) emission phenomena controlled by mixed diffusion (internal + external diffusion). In the case of internal diffusion material, with respect to the concentration distribution in the cavity, the local VOC emission rate becomes uniform and the FLEC works well. However, in the case of evaporation type (external diffusion) material, or mixed type materials (internal + external diffusion) when the resistance to transporting VOCs in the material is small, the FLEC is not suitable for emission testing because of the thin FLEC cavity. In this case, the mean emission rate is restricted to a small value, since the VOC concentration in the cavity rises to the same value as the surface concentration through molecular diffusion within the thin cavity, and the concentration gradient normal to the surface becomes small. The diffusion field and emission rate depend on the cavity concentration and on the Loading Factor. That is, when the testing material surface in the cavity is partially sealed to decrease the Loading Factor, the emission rate become higher with the decrease in the exposed area of the testing material. PRACTICAL IMPLICATIONS: The flow field and diffusion field within the FLEC cavity are investigated by CFD method. After presenting a summary of the velocity distributed over the surface of test material and the emission properties of different type materials in FLEC, the paper pointed out that there is a bias in the airflow inside the FLEC cavity but do not influence the result of test emission rate, and the FLEC method is unsuitable for evaporation type materials in which the mass transfer of the surface controls the emission rate.     

Comparison of formaldehyde emission from building finishing materials at various temperatures in under heating system; ONDOL

The objective of this research was to investigate the effect of various temperatures, room, 37 and 50 degrees C, on formaldehyde emission from floor materials, such as laminate and plywood floorings, and furniture materials, such as MDF and particleboard veneered with decorative paper foil, by desiccator's method. The temperature conditions were set up by, measuring the temperature in a Korean under heating system. To maintain an indoor air temperature of 20 degrees C, the temperature of the flooring surface was about 37 degrees C and the temperature of the cement mortar was 50 degrees C. The initial formaldehyde emission of the laminate flooring and plywood flooring was 1.44 and 0.63 mg/l, and for MDF and particleboard it was 4.73 and 4.95 mg/l, respectively. Floor materials were under E1 grade while furniture materials were under E2 grade in terms of formaldehyde emission. Because of the under heating system, the flooring materials were exposed to 37 and 50 degrees C, while the furniture materials mostly existed at room temperature. At 37 and 50 degrees C, the formaldehyde emission level of the flooring materials was already under 0.3 ppm (F level by JIS A 1460, application possibility without area limit) after 10 days and the emission had decreased further (0.03-0.10 mg/l) after 28 days. These levels are not injurious to the human body and will not cause sick house syndrome (SHS). The problem, however, is the furniture materials such as MDF and particleboard. As these materials are not exposed to high temperature (50 degrees C in this experiment) in living condition, it was still E2 grade of formaldehyde emission level at room temperature remained even after 28 days. Although there will be variations with the volume of furniture materials and the indoor conditions, furniture materials are the principal cause of indoor air quality pollution in Korean with the under heating system. PRACTICAL IMPLICATIONS: Koreans spend most of their time sitting on ONDOL (heated) floors, with their buttocks always in contact with the floor surface. The flooring materials are exposed to high temperatures (37-50 degrees C) why the effect of bake-out is rapid. The emission of formaldehyde from furniture materials are more important for the IAQ because usually MDF and particleboard of E2 grade are being used as furniture materials in Korea.     

Simulation of VOC emissions from building materials by using the state-space method

There are many mass-transfer models for predicting VOC emissions from building materials described in the literature. In these models, the volatile organic compound (VOC) emission rate and its concentration in a chamber or a room are usually obtained by analytical method or numerical method. Although these methods demonstrate some salient features, they also have some flaws, e.g., for analytical method the solutions of both room or chamber VOC concentration and building material VOC emission rate are constituted of the sum of an infinite series, in which additional computation for finding roots to a transcendental function is necessary, but sometimes quite complicated. Besides, when it is applied in complex cases such as multilayer emission with internal reaction, the solution is very difficult to get; for conventional numerical methods such as finite difference method, discrete treatment of both time and space may cause calculation errors. Considering that, the state-space method widely used in modern automation control field and the heat transfer field is applied to simulate VOC emissions from building materials. It assumes that a slab of building material is composed of a number of finite layers, in each of which the instantaneous VOC concentration is homogenous during the entire process of emission, while the time is kept continuous. Based on this assumption we can predict both the VOC emissions rate and the concentrations of VOCs in the air of a chamber or room. The method is generally applied to simulate VOC emissions from arbitrary layers of building materials, and the solution is explicit and simple. What's more, the method can be applied to the cases where a reaction producing/removing VOC in building materials exists. For some specific cases the method is validated using the experimental data and the analytical solutions in the literature. The method provides a simple but powerful tool for simulating VOC emissions from building materials, which is especially useful in developing indoor air quality (IAQ) simulation software.     

Priorities in environmental health Part 2

Continuing the format of the inventory, as introduced and outlined in Part 1 (of Priorities in Environmental Health) Part 2 lists the two remaining categories of environmental health hazards. The first category Involves man's indirect rote as the transmitter of the impediment. The second category concerns hazards which occur when man is neither the aetiological agent or the transmitter.