高温熔融高炉渣颗粒相变冷却特性分析

采用温度法模型对高温熔融高炉渣颗粒的相变冷却特性进行了分析,考虑颗粒固液相热导率随温度的变化及颗粒与环境的辐射换热,获得了高温熔渣颗粒内的温度分布以及相界面位置随时间的推移过程。讨论了变热导率、换热条件、颗粒尺寸,冷却流体速度和温度对相变冷却过程的影响,结果表明:热导率的变化使得颗粒冷却凝固时间延长,高温辐射换热极大加快了冷却速率;颗粒直径增加,相界面移动速度降低,凝固时间增加;冷却流体速度增加,温度降低,相界面移动速度增加,凝固时间缩短。     

非均质移动颗粒床的渗流换热模型与数值模拟

针对非均质移动颗粒床的换热过程,建立了其渗流换热的数学模型,提出了COADI-GS算法,采用此算法对移动颗粒床的气固换热过程进行流热耦合数值模拟。通过对均质与非均质这两种不同移动颗粒床换热模型的仿真结果进行比较,发现两者的床内温度分布有较大差异,非均质移动颗粒床模型能更为准确地描述床层内的渗流换热过程。计算结果表明,供风压力的增加虽有利于熟料颗粒的冷却,但会降低回收风的温度;颗粒温度随床层移动速度的增加而升高,近似为线性关系。     

石墨烯纳米流体相变材料蓄冷特性的数值模拟

鉴于石墨烯高导热性能的特点,将石墨烯纳米流体作为相变材料有望提高蓄冷效率。本文对水基石墨烯纳米流体相变材料的凝固特性进行了数值研究,采用焓-多孔度法追踪固液相界面,分析了石墨烯纳米片质量分数、蓄冷腔体尺寸和几何形状对凝固时间和相界面演化的影响。结果表明,相变材料凝固所需时间随着石墨烯纳米片质量分数的增大显著降低,对于直径为72mm的圆形蓄冷腔体,质量分数为1.2%的石墨烯纳米流体相变材料与去离子水相比凝固时间降低了30.1%,与已有的实验结果相符;随着圆形蓄冷腔体直径减小,石墨烯纳米流体凝固所需时间显著降低,但石墨烯纳米片对凝固的强化作用减弱;在腔体等截面积的情况下,三角形腔体内凝固过程的相界面移动速率明显大于圆形和方形腔体、更有利于促进凝固过程,3种形状腔体内初期凝固都发生在腔体底部、凝固中期相界面形状与腔体本身形状相似、凝固后期相界面趋近于圆形。     

硬脂酸/十八醇/乙酸钠复合相变材料蓄/放热性能

首次将无机相变材料乙酸钠和混合有机相变材料（硬脂酸和十八醇）复合，获得三元复合相变材料。有机和无机相变材料复合可有效解决无机相变材料在相变过程中存在相分离、过冷度大和有机相变材料热导率低的缺点。利用同心套管蓄/放热实验台测试了乙酸钠/硬脂酸/十八醇三元复合相变材料的蓄/放热性能，分析了流体流量及温度对相变材料蓄/放热过程的影响，并结合Fluent数值模拟和实验结果分析了相变过程中相界面的移动规律。研究结果表明，三元复合相变材料在蓄热过程中自然对流起主导作用，放热过程中导热起主导作用，蓄热所需时间小于放热所需时间。蓄热过程中，相变材料的上部相界面横向移动明显快于下部；放热过程中，相变材料的上、下部相界面均匀地径向移动。     

熔渣颗粒空冷相变换热的三维数值模拟

利用VOF方法结合凝固和熔化模型对熔渣颗粒在空气流中的冷却相变过程进行了三维数值模拟,讨论了熔渣颗粒直径和空气速度对冷却凝固过程演变的影响。结果表明:空冷方法能够实现熔渣颗粒表面的快速凝固成型,但同时也造成了颗粒内部的非均匀凝固。熔渣直径越小,完全凝固时间越短;空气流速越大时,其表面换热越强,完全冷却时间越短。颗粒初温为1673.15K、直径为0.5~2mm,风速为1~5m·s-1条件下熔渣颗粒在2s内释放出全部凝固热,后续空气最高温度能达到900K以上。     

熔渣颗粒空冷相变换热的三维数值模拟

利用VOF方法结合凝固和熔化模型对熔渣颗粒在空气流中的冷却相变过程进行了三维数值模拟,讨论了熔渣颗粒直径和空气速度对冷却凝固过程演变的影响。结果表明：空冷方法能够实现熔渣颗粒表面的快速凝固成型,但同时也造成了颗粒内部的非均匀凝固。熔渣直径越小,完全凝固时间越短;空气流速越大时, 其表面换热越强, 完全冷却时间越短。颗粒初温为1673.15 K、直径为0.5~2 mm,风速为1~5 m·s^-1条件下熔渣颗粒在2 s内释放出全部凝固热,后续空气最高温度能达到900 K以上。     

冷却方式对Y_2O_3-ZrO_3陶瓷相变及力学性能的影响

研究了常压烧结Y_2O_3-ZrO_2陶瓷不同冷却方式对其力学性能及t—m相变的影响。试验结果表明;冷却方式不同可以使材料中t-ZrO_2相含量不同,这与t—m相变的成核有关,成核过程是温度和时间的函数。随冷却速度增大,相变开始点温度(M_s)降低,相变终了点温度(M_f)则不受冷却速度的影响。     

气淬高温熔渣颗粒运动和换热特性的数值分析

建立了高炉渣颗粒运动与换热过程的数学模型,利用FORTRAN语言编写程序,通过四阶Runge?Kutta方法求解其动力学和传热方程,计算时充分考虑熔渣与冷却空气主要热物性参数随温度的变化,采用温度回升法计算熔渣凝固过程释放的潜热,提出在气淬空气中添加喷雾强化熔渣冷却,考察了渣粒尺寸对换热过程的影响。结果表明,飞行过程中渣粒速度受气淬空气影响先增大后减小;温度降低趋势随运动距离增加而减小,主要受对流换热系数影响,凝固过程持续时间较短;喷雾使渣粒在飞行过程中整体冷却速率明显升高,最终温度明显降低,而对熔渣的运动影响较小;相同初始工况下,熔渣粒径越小,运动越易受流场影响,渣粒整体冷却速率较高,换热效果越好。     

基于二维梯度树状肋相变储热系统强化传热机理

设计了双碟式光热-光电储热发电系统,针对相变储热系统传热特性进行研究,建立了六纵肋、雪花型肋、梯度树状肋相变储热模型,采用Fluent软件对石蜡蓄释热过程进行模拟。通过非稳态传热温度场和速度场的变化分析石蜡熔化和凝固的传热机理。结果表明,石蜡熔化过程伴随着热传导与自然对流的协同作用,凝固过程对流换热微弱以热传导为主。从场协同的角度分析,采用梯度树状肋使空间温度分布更均匀,可提高流体速度场和温度场的协同程度。石蜡熔化温度分别为315、340、360 K,完全熔化时间依次为224、374、703 s;完全凝固时间依次为3439、1089、842 s。可见,随着熔化温度的升高,完全熔化时间增长,完全凝固时间缩短。因此,在选择相变材料时要综合考虑熔化温度、蓄释热初温和终温及储热量的要求。     

基于二维梯度树状肋相变储热系统强化传热机理

设计了双碟式光热-光电储热发电系统,针对相变储热系统传热特性进行研究,建立了六纵肋、雪花型肋、梯度树状肋相变储热模型,采用Fluent软件对石蜡蓄释热过程进行模拟。通过非稳态传热温度场和速度场的变化分析石蜡熔化和凝固的传热机理。结果表明,石蜡熔化过程伴随着热传导与自然对流的协同作用,凝固过程对流换热微弱以热传导为主。从场协同的角度分析,采用梯度树状肋使空间温度分布更均匀,可提高流体速度场和温度场的协同程度。石蜡熔化温度分别为315、340、360 K,完全熔化时间依次为224、374、703 s;完全凝固时间依次为3439、1089、842 s。可见,随着熔化温度的升高,完全熔化时间增长,完全凝固时间缩短。因此,在选择相变材料时要综合考虑熔化温度、蓄释热初温和终温及储热量的要求。     

Numerical solution of the problem of ignition of a combustible liquid by a single hot particle

The problem of ignition of a typical combustible liquid by a single metal particle heated to high temperatures is solved numerically using a gas-phase model of ignition taking into account thermal conductivity, liquid vaporization, diffusion and convection of fuel vapor in air, crystallization of the particle, formation of a vapor gap between the particle and liquid, temperature dependence of the thermal characteristics of interacting substances, and air humidity. The scales of the effects of the initial temperature and particle size and shape on the delay of the examined process are determined. The limiting values for ignition initiation are found for the characteristic parameters of the ignition source (initial temperature and size) and air humidity.     

Effect of carbon nanofiber additives on thermal behavior of phase change materials

Thermal performance of nanocomposite carbon nanofibers filled paraffin wax was studied experimentally and analytically. The transient temperature response of made nanocomposite was measured during its solidification process and the cooling rate was predicted. It was found that nanocomposite thermal conductivities were enhanced significantly causing the cooling rate to increase. An analytical model was introduced based on one-dimensional heat conduction approach to predict the effective thermal conductivity for the new nanocomposites and its findings showed good agreement with the experimental data. Also, a comparative study was performed to investigate the effect of carbon nanofibers surface characteristics on thermal performance of paraffin wax.     

水冷壁气流床气化炉渣层热应力数值模拟方法的改进

固态渣层能够保护气流床气化炉的水冷壁,防止其受到高温合成气直接辐射以及液态熔渣的侵蚀。本文提出一种数值模拟渣层热应力的改进方法,并应用该改进方法对降温阶段渣层热应力的变化进行模拟研究。在渣层热应力的数值模拟研究中,经常假定水冷壁渣层的热应力变化基于一个固定的参考温度（比如环境温度25℃）。然而对于降温阶段的水冷壁气流床气化炉,一个固定的参考温度值并不能表征渣层"无应力"的初始状态,在此基础上计算将会得到一个不合理的渣层应力分布结果。针对该问题,提出了一种改进方法：将水冷壁渣层分割为多个子计算域,每个子计算域内单独设置参考温度,以此实现在整个水冷壁渣层上施加一个近似为降温初始时刻的参考温度分布,从而使渣层在降温初始时刻处于"无应力"状态。同时,对前人文献中的三维水冷壁渣层结构在降温过程中的热应力变化情况进行计算,以此测试改进方法的准确性,改进方法得到的模拟结果与其他参考文献得到的渣层热应力变化趋势一致。     

水平轴离心铸造套筒凝固过程温度场数值计算

本文通过分析水平轴离心铸造长／径比小于5的圆筒型铸件的凝固过程，建立了表征铸件凝固过程和二维传热模型．并采用交替陷式差分的方法对离心铸造45#碳钢单材质套筒凝固过程温度场进行了数值计算。结果表明：套筒凝固过程表现为沿径向从靠近模具的铸件外表面层向内表面层、轴向从靠近端盖的铸件层向套筒长度中心部位顺序凝固的特点，模具初温对铸件的凝固速度也有重要的影响。提出减少套筒内表面形成的型腔对外界冷空气的卷吸作用。或在铸件金属浇注完毕后浇注一层低熔点保护渣对保持套筒的这种顺序凝固的特点、以加强最后凝固的内层金属液对外层的补缩作用有一定效果。     

高频氢等离子体增强还原制备超细铜粉

利用高频感应热氢等离子体强化还原制备超细铜粉,考察了加料速率、还原氢气流量、氢气分布位置、反应区空间、冷却温度等因素对铜粉颗粒性能的影响,对制备的铜粉颗粒进行氧含量、XRD晶体结构、松装密度、粒度分布和比表面积的表征。结果表明,优化的工艺条件为反应区内径100 mm,加料速率4 g/min,淬火气氩气气量500 L/h,氢气气量500 L/h并通入少量载气,由氢等离子电离产生的氢自由基可强化反应实现瞬时还原,不仅可控制铜粉形貌,还能有效控制铜粉颗粒大小;利用该方法制备出粒径分布100?200 nm、分散性好的超细球形铜粉颗粒。该方法操作简便、产品纯度高、气氛可控、对环境污染小。     

Numerical modeling for non-steady thermal stress analysis of slag layer in a membrane wall entrained-flow gasifier

Membrane wall entrained-flow gasifier (MWEFG) generally has a long service life for the protection of the solid slag layer adhered to the metal wall during operation. However, cracking will generate in the slag layer when the temperature changes greatly in the gasifier, which results from the thermal stress exceeding the strength of slag deposit. In this study, the gasification experiment was performed in a bench-scale MWEFG and a three-dimensional model was developed for non-steady thermal stress analysis of slag layer. Based on experimental data, the thermal stresses of cooling process were numerically simulated using transient thermal analysis. The results indicate that the thermal stresses of the slag layer are tensile during the cooling and the Von Mises stresses (SEQVs) increase with the reduction of temperature. Comparing the results of the selected nodes, it is found that the SEQV always increases from the slag surface to the slag-SiC interface. The contributions of slag porosity and thickness were also investigated. We draw a conclusion that the maximum SEQV decreases with the increasing porosity, while it increases with the increasing slag thickness.     

Transient group combustion of the pulverized coal particles in spherical cloud

A transient group combustion model for the pulverized coal particles in a spherical cloud is developed to predict the transient group combustion characteristics. The submodels, which account for the detailed combustion process of both homogeneous and heterogeneous reactions, are also applied for the pulverized coal particles. The numerical simulation of the collective behaviors of ignition and its subsequent burning were carried out. The ignition mechanism is characterized by the heterogeneous process of the dilute cloud and the homogeneous process of the dense cloud. Two dominant flame structures are observed: one flame penetrating inside the cloud and the other moving outside the cloud. The effects of various parameters (i.e., radiation heat transfer, group combustion number, air temperature, air oxygen concentration, particle size distribution, and particle number density) on the particle mass burning rate and on the overall performance of the group combustion are examined. The results are in good agreement with existing experimental data.     

Enhancements of thermal conductivities with Cu,CuO, and carbon nanotube nanofluids and application of MWNT/water nanofluid on a water chiller system

In this study, enhancements of thermal conductivities of ethylene glycol, water, and synthetic engine oil in the presence of copper (Cu), copper oxide (CuO), and multi-walled carbon nanotube (MWNT) are investigated using both physical mixing method (two-step method) and chemical reduction method (one-step method). The chemical reduction method is, however, used only for nanofluid containing Cu nanoparticle in water. The thermal conductivities of the nanofluids are measured by a modified transient hot wire method. Experimental results show that nanofluids with low concentration of Cu, CuO, or carbon nanotube (CNT) have considerably higher thermal conductivity than identical base liquids. For CuO-ethylene glycol suspensions at 5 vol.%, MWNT-ethylene glycol at 1 vol.%, MWNT-water at 1.5 vol.%, and MWNT-synthetic engine oil at 2 vol.%, thermal conductivity is enhanced by 22.4, 12.4, 17, and 30%, respectively. For Cu-water at 0.1 vol.%, thermal conductivity is increased by 23.8%. The thermal conductivity improvement for CuO and CNT nanofluids is approximately linear with the volume fraction. On the other hand, a strong dependence of thermal conductivity on the measured time is observed for Cu-water nanofluid. The system performance of a 10-RT water chiller (air conditioner) subject to MWNT/water nanofluid is experimentally investigated. The system is tested at the standard water chiller rating condition in the range of the flow rate from 60 to 140 L/min. In spite of the static measurement of thermal conductivity of nanofluid shows only 1.3% increase at room temperature relative to the base fluid at volume fraction of 0.001 (0.1 vol.%), it is observed that a 4.2% increase of cooling capacity and a small decrease of power consumption about 0.8% occur for the nanofluid system at a flow rate of 100 L/min. This result clearly indicates that the enhancement of cooling capacity is not just related to thermal conductivity alone. Dynamic effect, such as nanoparticle dispersion may effectively augment the system performance. It is also found that the dynamic dispersion is comparatively effective at lower flow rate regime, e.g., transition or laminar flow and becomes less effective at higher flow rate regime. Test results show that the coefficient of performance of the water chiller is increased by 5.15% relative to that without nanofluid.