ROTATING DRUM HEAT EXCHANGER

A Sparcely Filled Liquid/Liquid Rotating Drum Heat Exchanger (RDHE) was investigated for its heat transfer characteristics. Drums upto 10cm in diameter and 1.0m long were rotated upto 350 rpm. Liquids investigated varied in kinematic viscosity from water at 2.2 × 10^-3m²/hr to 32wt% LiCl solution at 6.3× 10^-3m²/hr. In this range data showed that the film heat transfer coefficient did not depend on kinematic viscosity. This resulted in a “best fit” correlation of Nusselt Number (100 to 800) with Peclet Number (200,000 to 2,000,000) as oppossed to Nusselt Number with Reynolds Number (2000 to 20,000). Analysis suggested that the heat transfer between two liquid films adhering to opposite surfaces of a rotating drum was controlled by conduction not a convection mechanism. The RDHE has found application in the heat transfer between two liquids of similar pressure where little to no pressure drops are allowed.     

Effects of internals on phase holdup and backmixing in a slightlyexpanded-bed reactor with gas-liquid concurrent upflow

Five different internals were designed, and their effects on phase holdup and backmixing were investigated in a gas-liquid concurrent upflow reactor where the spherical alumina packing particles of three diameters (3.0, 4.5 and 6.0 mm) were slightly expanded under the conditions of varied superficial gas velocities (6.77×10^-2-3.61×10^-1 m·s^-1) and superficial liquid velocities (9.47×10^-4-2.17×10^-3 m·s^-1). The experimental results show that the gas holdup increases with the superficial gas velocity and particle size, opposite to the variational trend of liquid holdup. When an internal component is installed amid the upflow reactor, a higher gas holdup, a less liquid holdup and a larger Peclet number characterizing the weaker backmixing are obtained compared to those in the bed without internals under the same operating conditions. Additionally, the minimal backmixing is observed in the reactor equipped with the internals with a novel multi-step design. Finally, empirical correlations were proposed for estimating gas holdup, liquid holdup and Peclet number with the relative deviations within 11%, 12% and 25%, respectively.     

以最佳温度均匀度和最小熵产为目标的航天器热循环试验系统运行参数优化

以数值模拟的方法研究了不同运行参数下航天器热循环试验箱内温度均匀度与熵产的变化规律。结果表明,在4.3×10³≤Re≤8.6×10⁵、4.62×10¹³≤Gr≤1.38×10¹⁴范围内,由于强浮升力的作用,壁面附近出现回流区,温度由上往下降低,中轴线附近气体加速下沉,温度由上往下升高。箱内量纲1温度标准偏差随Reynolds数增大而增大,随Grashof数变化不明显;混合对流过程中流动熵产远小于传热熵产,熵产数值随Reynolds数、Grashof数的增大而增大。提出了壁面Nusselt数、试验箱内量纲1平均温度、量纲1温度标准偏差及量纲1传热熵产随Reynolds数、Grashof数变化的关联式。     

TRANSIENT BEHAVIOR OF NATURAL CONVECTION INSIDE A VERTICAL CYLINDER

The transient behavior for natural convection heat transfer within a vertical cylindrical enclosure was studied using the SIMPLER numerical algorithm. Initially, the temperature of the internal fluid was uniform. The unsteady state heat transfer was initiated by a temperature step change applied to all boundaries. The boundary temperature was then increased at the same rate as the center temperature until pseudosteady state was achieved. (Pseudosteady state is comparable to steady state convection for a fluid with uniform heat generation.) The transient heat transfer coefficient and the intensity of flow, along with the magnitude and the location of the minimum temperature, mean temperature, and maximum stream function are presented. Temperature and stream function contours are used to show the penetration of heat from the boundary and the development of the flow patterns. The local Nusselt number is also provided as supplementary information. A cylinder with HID = 1 was considered, with the Prandtl number of the fluid equal to 7 and a Rayleigh number of 10⁵. A characteristic length defined as 6 x (volume)/(surface area), which was found to produce the best regression results in our previous studies, was used in defining the Nusselt number and the Rayleigh number. The initial heat transfer rate was found to be correlated by

Nu =0.938 (Dimensionless Time) ^-0567

Oscillatory phenomena were found in support of the prediction made by Patterson and Imberger.     

新能源汽车固态电池中的锂枝晶抑制研究

相比于商业化的锂离子电池,固态电池具有更高的能量密度和更好的安全性。然而,固体电解质依旧面临锂枝晶生长的问题。以目前已大规模工业化的聚氧化乙烯(PEO)基电解质作为研究对象,通过将PEO与高杨氏模量的石榴石型电解质复合,抑制了锂枝晶在PEO基复合电解质中的生长,不仅使电解质膜的离子电导率从9.8×10-6S/cm增加到了3.8×10-4S/cm,还使锂/锂对称电池的临界电流密度从0.4 m A/cm²提高到1.6 mA/cm²。与此同时,组装的基于金属锂负极与传统石墨负极的软包电池可分别获得334.5 W·h/kg与218.2 W·h/kg的能量密度。其中,钴酸锂/复合电解质/石墨软包全电池循环1 000次后的容量保持率可达92.3%,能够满足新能源汽车的需求。     

多乙烯多胺桥联受阻酚类抗氧剂的合成及其清除DPPH·性能

以二乙烯三胺和三乙烯四胺为桥联基,β-(3,5-二叔丁基-4-羟基苯基)丙酰氯为抗氧化功能基团,通过酰胺化缩合反应合成了两类具有不同对位桥联基团的受阻酚类抗氧剂。采用傅里叶红外光谱和核磁共振氢谱证实了合成的多乙烯多胺桥联受阻酚类抗氧剂的化学结构。DPPH法研究了多乙烯多胺桥联受阻酚类抗氧剂清除自由基的性能,并探索了酚羟基个数和对位桥联基结构对受阻酚类抗氧剂清除自由基性能的影响。结果表明,多乙烯多胺桥联受阻酚类抗氧剂具有良好的清除DPPH·能力,且随着抗氧剂分子中酚羟基个数的增加,清除DPPH·的活性增加,分子中含有4个酚羟基的三乙烯四胺受阻酚类抗氧剂的抗氧化效率（AE）达到2.65×10^-2 L/(mol·s)。对位桥联基结构对受阻酚类抗氧剂清除DPPH·能力有较大影响,季戊四醇为桥联基的受阻酚类抗氧剂1010清除DPPH·能力最强,其抗氧化效率（AE）为3.08×10^-2L/(mol·s);乙二胺为核的1.0代树枝状受阻酚类抗氧剂清除DPPH·能力最弱,其抗氧化效率（AE）为2.60×10^-2 L/(mol·s)。     

在有方形障碍物的方腔中对双扩散自然对流的玻尔兹曼晶格模拟(英文)

Double diffusion convection in a cavity with a hot square obstacle inside is simulated using the lattice Boltzmann method. The results are presented for the Rayleigh numbers 104,105 and 106, the Lewis numbers 0.1, 2 and 10 and aspect ratio A (obstacle height/cavity height) of 0.2, 0.4 and 0.6 for a range of buoyancy number N=0 to?4 with the effect of opposing flow. The results indicate that for|N|b 1, the Nusselt and Sherwood numbers decrease as buoyancy ratio increases, while for|N|N 1, they increase with|N|. As the Lewis number increases, higher buoyan-cy ratio is required to overcome the thermal effects and the minimum value of the Nusselt and Sherwood num-bers occur at higher buoyancy ratios. The increase in the Rayleigh or Lewis number results in the formation of the multi-cell flow in the enclosure and the vortices wil vanish as|N|increases.     

Computation of forced convection in slow flow through ducts and packed beds—I extensions of the graetz problem

A computational study of forced convection processes in ducts and packed beds at low Reynolds numbers has been made. The results give a better understanding of these processes, especially for small Peclet numbers. It is demonstrated that two distinct forms of Nusselt numbers are relevant for low Peclet numbeRs. One, used mostly by theorists, is related to the local driving force; the other, used mostly by experimentalists, is related to easily measurable temperatures and concentrations. As an example the Graetz problem has been solved numerically over a wide range of Peclet numbers, and new asymptotes have been obtained for the region of small Peclet numbers.     

CHEMICALLY INDUCED MIGRATION OF PARTICLES ACROSS FLUID STREAMLINES

The velocity of a colloidal particle that moves because of a gradient of concentration of a molecular solute depends on the concentration field at the surface of the particle. Effects of macroscopic convection of the suspending fluid on two such transport phenomena, capillary-driven movement of fluid particles and diffusiophoresis of rigid particles, are considered here. In the case of fluid particles our results also apply to motion caused by a temperature gradient. If the particles are in a laminar flow with the solute gradient directed perpendicular to the direction of flow, as might arise in the boundary layer near a surface to which the particles are being deposited, the local solute concentration field around each particle is disturbed from that of pure diffusion of the solute. Using published results for these concentration disturbances in a simple-shear flow, we determine the effect of the imposed velocity gradient on the speed of the particles in the direction of the solute gradient. For both fluid and rigid particles, the correction due to macroscopic shear is 0(Pe^3/2:) where Pe is the Peclet number based on particle radius and fluid shear rate; this effect opposes the zero-shear particle velocity. A possible consequence of this result is that by increasing the shear rate in a laminar boundary layer in the hope of enhancing the rate of particle adsorption, one may actually be decreasing the rate.     

NUSSELT NUMBER FOR DIRECT CONTACT EVAPORATION OF A MOVING ELLIPSOIDAL DROP IN AN IMMISCIBLE LIQUID

Since Sideman et al.^[1] derived an analytical solution of the Nusselt number for a spherical drop in the direct contact evaporation in an immiscible liquid, many researchers have performed theoretical research on this field under different assumptions [2]-[6]. However drops moving in another immiscible liquid do not take a spherical form, but oblate form, which may be well approximated as an ellipsoid. This paper establishes the mathematical model of heat transfer for a moving ellipsoidal drop in an immiscible liquid, and presents results from numerical calculation. The relationship of the Nusselt number with the Weber number and the Peclet number is given, which is suitable for a relatively large range of We and Pe. The theoretical results for the Nusselt number show good agreement with experimental data. Some important conclusions are obtained.     

Effects of stagnant and dispersion conductivities on non-darcian forced convection in square Packed-Sphere channels

This paper presents a numerical and experimental investigation of the effects of stagnant and dispersion conductivities on non-Darcian forced convection in square packed-sphere channels. The theoretical prediction of Nusselt number are found to be in good agreement with the experimental data. A larger near-wall damping of stagnant conductivity is found for the present water-steel medium than for that described by the mixing rule based on the volume fraction. A nonlinear Peclet number dependence, Pe^λ and λ = 0,88, for dispersion conductivity is found to induce better agreement between the theoretical and experimental results especially for the cases of high Peclet number and high D_c/d.     

Dielectric behaviour of betaine arsenate

Dielectric constant measurements of the ferroelectric material Betaine Arsenate showed a maximum value of 1.057*10⁶ (tan δ = 1.73*10^-2). The dielectric constant is compared with hysteresis loops. It is shown that close below T_c, a small signal of 1.3 V/m is sufficient to switch the spontaneous polarization.     

圆管束中导流器对其自然对流换热的影响

对无限大空间中5根同规格圆管组成的圆管束在管间放置导流器的自然对流换热进行了数值模拟研究。考虑了Ra在10³～10⁴范围内,导流器偏转角为0°～60°,圆管间距为2～4倍圆管直径的自然对流换热,分析了5根圆管的局部Nusselt数（Nu_loc）和平均Nusselt数（Nu_ave）。研究结果表明,导流器对管束结构的自然对流换热影响体现在两方面：一是对导流器下方圆管而言相当于障碍物削弱其换热,二是对导流器上方圆管而言隔绝了下方羽状流的影响,从而增强系统的整体换热。在圆管间距较大时,与无导流器的圆管束相比,C1～C4各圆管换热下降趋势明显放缓,并且从C4、C5圆管换热趋势上升,系统整体换热增强。圆管间距S=2D,Ra=10³,导流器主要起障碍物的作用,削弱C1～C4各圆管换热,导致系统整体换热下降。当导流器偏转角度为45°时系统换热达到最大值,较无导流器时换热最多有着21%的提升。     

The influence of evaporation on the deposition of particles in impactors

As an ab initio study theoretical work has been carried out to investigate evaporation effects on particle deposition in an impactor. Equations of particle motion and evaporation using some simplified assumptions were solved simultaneously to trace the particle's movement. In the impactor evaporation reduces the particle's mass and decreases the sticking probability. If it happens in the cascade impactor the particle would be deposited in a later stage than that corresponding to the original particle diameter. The impactor efficiency is strongly dependent on Pe^* (pseudo-Peclet number), the ratio of convected amount to diffused amount and pressure field. Other observations and results are discussed.     

基于M-L湍流模型的浮空器强迫对流换热

外表面强迫对流换热是影响临近空间浮空器热控的重要因素，而流体的流动状态对强迫对流换热具有十分重要的影响。目前对浮空器外表面强迫对流换热的仿真研究多采用雷诺时均方程，将流动作为全湍流进行计算，且并未考虑转捩现象的影响。为了研究转捩现象对强迫对流换热的影响，首先通过在Reynolds数为1.14×10⁶情况下采用M-L转捩模型球体浮空器绕流得到的结果与实验结果以及采用Shear Stress Transfer（SST）k-ω、k-ε模型模拟结果进行对比分析，验证了M-L转捩模型在模拟球体浮空器强迫对流换热时的优越性。在验证数值模拟方法的基础上，分析了Reynolds数对球体浮空器强迫对流换热的影响。基于数值模拟得到的结果，在Reynolds数为10⁶～10⁸的范围内，拟合得到了球体浮空器强迫对流换热关系式。     

The Anisotropy of the Surface Tension of Polar Liquids: The Case of Liquid Crystals

The contact angle of the liquid crystal (LC) 4-phentyl, 4'-cyano biphenyl (5CB) on glass or polyethylene does not correspond to the value expected from its surface tension measured by the “du Nouy” method (29 × 10^-3 JM^-2). The value deduced from the Young-Dupre law is 40 × 10^-3 JM^-2.

Both these values have already been reported in the literature. Their apparent discrepancy is explained by the LC surface tension anisotropy. We show that 5CB as many other LCs orient perpendicular to the free surface but parallel to glass or polyethylene Thus both values of the surface tension correspond to two different molecular orientations.

The LC oriented perpendicularly has a surface tension of 29 × 10^-3 JM^-2 and 40 × 10^-3 JM when it lies parallel to the surface. We suggest that the anisotropy between the perpendicular and parallel state of LCs also exists between two perpendicular orientations. Furthermore, such anisotropy associated with any polar molecules explains the difference of the interfacial energy of similar polar or non polar (i.e., octanol-octane) compounds and water.     

DRYING OF GELATINIZED WHOLE WHEAT

Whole grains of gelatinized durum and soft wheat were dried by forced and natural convection at 40, 60, 80, and 100°C. Magnetic resonance images taken periodically during drying indicated that Fick's diffusion is not applicable to describe the moisture transfer during drying of the gelatinized wheat grains. A simple mathematical model based on overall moisture balance fitted the experimental data very well. The drying took place in the falling rate period, which was approximated by two regions - first and second falling rate periods (FFRP and SFRP). The internal drying coefficient linearly increased with increasing drying temperature, and was almost an order of magnitude (from 10⁴ to 10⁵ s^-1) higher during FFRP than SFRP. The soft wheat dried faster than the durum wheat. The effect of forced convection was more pronounced during FFRP than SFRP.     

湿筛混凝土动态受压力学性能及破坏形态细观数值模拟

湿筛混凝土通常用来代替由于骨料粒径较大而不便开展物理试验的水工全级配混凝土。为了研究加载速率对湿筛混凝土力学性能及破坏形态的影响,从细观角度出发,运用颗粒流离散元软件PFC^2D建立湿筛二级配混凝土细观数值试件,根据拟静态单轴压缩(应变速率10^-5 s^-1)试验数据标定出数值试件中砂浆颗粒之间、粗骨料颗粒之间及砂浆颗粒与粗骨料颗粒接触面之间的细观参数,进而开展应变速率为10^-4 s^-1、10^-3 s^-1、10^-2s^-1的动态加载并进行动态力学性能及破坏形态的数值模拟和机理分析。结果表明,不同应变速率下试件的应力-应变曲线形状相近,峰值应力随着应变速率的增加而增大,增长率为7.3%～37.9%,峰值应力处应变增大幅度不大。试件破坏形态与物理试验现象吻合较好,随着应变速率的增加,裂缝数量不断增加,裂缝分布趋于均匀,裂缝数量增长率平均为峰值应力增长率的4.2倍。此外,随着应变速率的增加,数值试件内部...     

超临界压力下CO2在螺旋管内的混合对流换热

在恒热流条件下,对超临界压力CO₂在内径为9 mm,绕径为283 mm,节距为32 mm的螺旋管内垂直上升混合对流的传热特性进行了实验研究,实验参数范围为:进口压力8 MPa、质量流速0～650 kg·m^-2·s^-1、内壁热负荷0～50 kW·m^-2。研究发现:受热螺旋管内超临界压力CO₂的壁温及传热特性由变物性、浮升力及离心力的耦合作用共同支配,变物性及浮升力影响的相对大小可用Buoyancy数定性表征,当Bo>8×10^-7时,自然对流占主导作用,浮升力作用引起强烈的二次流效应,显著强化传热;在浮升力和离心力共同作用下,截面周向温度最低点出现在外下侧区域,且当浮升力作用占优时,底部区域的传热系数大于外侧,当离心力作用占优时,底部区域的传热系数小于外侧。基于本实验获取的2346个数据点,得出了计算Nu实验关联式,90%以上的实验值与拟合公式计算值偏差在±20%以内。     

湿化学法制备石榴石型固态电解质Li7La3Zr2O12

锂电池因能量密度高、循环寿命长、绿色清洁等特点被广泛应用,但其液态电解质易泄漏、挥发,且隔膜易被锂枝晶刺穿造成短路,引发危险。固态电解质大多是不具燃烧性的无机材料,室温下离子电导率较高、电化学窗口宽且适用温度范围广。因此,采用固态电解质替代液态电解质具有十分重要的意义。相对于其他类型固态电解质,石榴石型氧化物Li₇La₃Zr₂O₁₂（LLZO）具有离子电导率高、电化学窗口宽（>5V vs. Li/Li⁺）、对锂稳定性好和热稳定性高等特点,是非常具有发展潜力的无机固态电解质。本文采用溶胶-凝胶法和低温燃烧法两种湿化学法合成LLZO粉末,对应的电解质片在40℃时的离子电导率分别为1.22×10^-5S/cm和3.87×10^-6S/cm,活化能分别为0.34eV和0.32eV。从实验结果综合比较,溶胶-凝胶法为最佳制备方法。