Heat and mass transfer analysis of a suspension of reacting particles subjected to concentrated solar radiation – Application to the steam-gasification of carbonaceous materials

A chemical reactor for the steam-gasification of carbonaceous materials (e.g. coal, coke, biomass) using high-temperature solar process heat is modeled by means of a two-phase formulation that couples radiative, convective, and conductive heat transfer to the chemical kinetics for polydisperse suspensions of reacting particles. The governing mass and energy conservation equations are solved by applying advanced Monte–Carlo and finite-volume techniques with smoothing and underrelaxation. Validation is accomplished by comparing the numerically calculated temperatures, product compositions, and chemical conversions with the experimentally measured values obtained from testing a 5 kW solar reactor prototype in a high-flux solar furnace. A unique feature of the reactor concept is that the gas-particle flow is directly exposed to concentrated solar radiation, providing efficient radiative heat transfer to the reaction site for driving the high-temperature highly endothermic process.     

An experimental study of the statistical parameters of gas–liquid two-phase slug flow in horizontal pipeline

Experiments were performed in a horizontal test loop with inner diameter 50 mm to study the gas–liquid slug flow. The translational velocities of elongated bubbles, lengths of liquid slugs and elongated bubbles, and slug frequencies were measured using two pairs of conductivity probes. Correlations are presented for elongated bubble translational velocity, length of elongated bubble and slug frequency, respectively. It was found that the translational velocity of elongated bubble is not only dependent on Froude number, but also is significantly affected by the distance from the entrance of pipeline in the higher mixture velocity range. Mean liquid slug length is relatively insensitive to the gas and liquid flow rates in the higher mixture velocity range, however in the lower mixture velocity range, the mean liquid slug length is affected by the mixture velocity. Mean slug frequency clearly increases as the liquid superficial velocity increases but it weakly depends on the gas superficial velocity.     

Heat transfer and exergy analysis of cascaded latent heat storage with gravity-assisted heat pipes for concentrating solar power applications

A thermal network model is developed to predict the performance of latent heat thermal energy storage (LHTES) systems including cascaded phase change materials (PCMs) and embedded heat pipes/thermosyphons. Because the design of LHTES systems involves a compromise between the amount of energy stored, the heat transfer rate, and the quality of the released thermal energy, an exergy analysis is also carried out to identify the preferred LHTES design. It is found that the LHTES with the lowest melting temperature PCM yields the highest exergy efficiency. However, a cascaded LHTES recovers the largest amount of exergy during a 24 h charging–discharging cycle. Quantitatively, the cascaded LHTES recovers about 10% more exergy during a 24 h charging–discharging cycle compared to the best non-cascaded LHTES considered in this work.     

Surface and nonlocal effects on the nonlinear free vibration of non-uniform nanobeams

The surface and nonlocal effects on the nonlinear flexural free vibrations of elastically supported non-uniform cross section nanobeams are studied simultaneously. The formulations are derived based on both Euler–Bernoulli beam theory (EBT) and Timoshenko beam theory (TBT) independently using Hamilton’s principle in conjunction with Eringen’s nonlocal elasticity theory. Green’s strain tensor together with von Kármán assumptions are employed to model the geometrical nonlinearity. The differential quadrature method (DQM) as an efficient and accurate numerical tool in conjunction with a direct iterative method is adopted to obtain the nonlinear vibration frequencies of nanobeams subjected to different boundary conditions. After demonstrating the fast rate of convergence of the method, it is shown that the results are in excellent agreement with the previous studies in the limit cases. The influences of surface free energy, nonlocal parameter, length of non-uniform nanobeams, variation of nanobeam width and elastic medium parameters on the nonlinear free vibrations are investigated.     

High-calcium fly ash as the fourth constituent in concrete: problems,solutions and perspectives

Even though Hellenic high-calcium fly ashes of different origin are widely used by the cement industry for the production of several CEM II types of cements according to EN 197-1, their systematic use in concrete still presents some difficulties. This inhibits the establishment of specifications for their addition. Main problems concerning the quality, are focused on variations in chemical and mineralogical composition, necessity for supplementary grinding, high proportion of free-CaO and periodically high proportion of SO₃ content.These problems as well as the solutions, for every day use by the concrete industry, applied during the construction of a dam, are discussed, in this paper. To overcome these problems, untreated fly ash was cheaply upgraded by grinding at a specially designed ball mill, with simultaneously hydration, for the reduction of free-CaO.Details also (i) for fly ash variations in relation to their origin, (ii) the grinding plant and (iii) the industrial production of fly ash, are given. Finally, in a separate chapter of this paper, aiming to explain the treatment of fly ashes followed during their industrial production, data of the mechanical strength of mixtures of cements incorporating fly ashes with different treatment, concerning their free-CaO and their fineness, are given.     

Performance evaluation of a solar thermal energy storage system using nanoparticle-enhanced phase change material

This paper presents a numerical investigation on the thermal performance of a solar latent heat storage unit composed of rectangular slabs combined with a flat-plate solar collector. The rectangular slabs of the storage unit are vertically arranged and filled with phase change material (PCM: RT50) dispersed with high conductive nanoparticles (Al₂O₃). A heat transfer fluid (HTF: water) goes flow in the solar collector and receives solar thermal energy form the absorber area, then circulates between the slabs to transfer heat by forced convection to nanoparticle-enhanced phase change material (NEPCM). A numerical model based on the finite volume method and the conservation equations was developed to model the heat transfer and flow processes in the storage unit. The developed model was validated by comparing the obtained results with the experimental, numerical and theoretical results published in the literature. The thermal performance of the investigated latent heat storage unit combined with the solar collector was evaluated under the meteorological data of a representative day of the month of July in Marrakesh city, Morocco. The effect of the dispersion of high conductive nanoparticles on the thermal behavior and storage performance was also evaluated and compared with the case of base PCM without additives.     

涡发生器对脱硫系统除雾器性能的影响

通过数值模拟研究了平板、椭圆板、圆管、方管和三角管5种涡发生器分别安装于湿法烟气脱硫系统折流板除雾器中的除雾效率与压降。采用欧拉-拉格朗日方法分别求解气相-液相运动状态,入口流速U _in为3~6m/s,液滴直径范围2~80μm。结果表明,平板的除雾效率与压降增大最多,U _in=3m/s时,除雾效率相比无涡发生器高31.98%,压降高30Pa。平板对2μm液滴具有较高的除雾效率,最大为34.88%,比无涡发生器高20.81%。椭圆板U _in=3m/s时除雾效率比无涡发生器增加29.14%,压降升高20.47Pa。圆管、方管和三角管的总除雾效率分别高达95.05%、97.49%和96.48%。压降方面,方管为无涡发生器的4.18倍,圆管和三角管分别为2.05倍和2.79倍。另外,考察了椭圆板攻角的影响。在相同U _in下,除雾效率随着攻角的增加而增大,50°的椭圆板除雾效率为97.09%~99.15%。攻角对2μm液滴的分级除雾效率影响不大。压降随着攻角的增大而增加,U _in=4m/s时,攻角从0°增加到50°,压降从25.3Pa增加到92.78Pa。椭圆板因其迎流面积与长宽比的匹配以及流线型产生了较多涡流,具有良好的强化除雾效率效果以及较小的压降。     

CO2-C2H6共沸物分离的立方型状态方程选取

汽液平衡热力学模型的准确选取对CO₂-C₂H₆共沸物分离流程的设计和操作分析至关重要。在汽液平衡实验数据的基础上,依据逸度平衡原则,评估vdW、RK、SRK和PR立方型状态方程结合vdW、Margles和CVD混合规则预测CO₂纯物质、CO₂-C₂H₆共沸物和n-C₅H₁₂-CO₂-C₂H₆三元体系汽液平衡的可靠性,采用平均绝对误差的方法进行状态方程的选取。结果表明：SRK状态方程计算CO₂纯物质汽液平衡性质的精度最高;PR状态方程结合Margles混合规则可以准确计算CO₂-C₂H₆共沸体系汽液平衡特性;对于n-C₅H₁₂-CO₂-C₂H₆三元体系,SRK状态方程结合Margles混合规则计算精度明显优于vdW、RK和PR状态方程。通过试差迭代法优化CO₂-C₂H₆共沸体系和n-C₅H₁₂-CO₂-C₂H₆三元体系的二元交互作用参数,状态方程的计算精度得到明显提高。     

Experimental study on a cryogenic loop heat pipe with high heat capacity

Cryogenic loop heat pipes (CLHPs) are efficient heat transfer devices based on two-phase flow. Loop heat pipes for room temperature applications have achieved satisfactory thermal control functions with the benefits of no mechanical moving part, vibration isolation, thermal insulation, long heat transport distance and so on. While there exist many problems for low temperature applications of loop heat pipes, such as limited heat transport capacity, which could not meet the increasing requirement of instrument heat dissipation. This paper presents an advanced CLHP operating at liquid-nitrogen temperature range. An improved condenser structure is introduced to the CLHP, which greatly reduces the flow resistance and increases the cooling capability of the condenser. Many experiments have been carried out on the CLHP prototype for performance test, and one set of the experimental results with a 3.2 MPa fill pressure at room temperature is presented in this paper. It is shown that the advanced CLHP prototype can be operated reliably with a high heat transfer capacity up to 41 W and a limited temperature difference of 6 K across a 0.48 m transport distance.     

A numerical study on airflow and dynamic cross-contamination in the super cleanroom for photolithography process

We performed the numerical study on the characteristics of airflow and cross-contamination in the photolithography process cleanroom. The nonuniformity, the deflection angle and the global cross-contamination were used to analyze the characteristics and performances of cleanroom. From the numerical results, we found that the airflow characteristics of the cleanroom are largely affected by the porosity of the access panel and adjustment of dampers and the cross-contamination varies with the location of source and the passage of time through the concentration ratio.