共查询到20条相似文献,搜索用时 9 毫秒
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Néstor VásquezLuis Sánchez George E Klinzing Shrikant Dhodapkar 《Powder Technology》2003,137(3):167-183
Pneumatic conveying, employing the dense phase plug flow regimen, is largely used to transport bulk solids. This process permits the conveying of large amounts of material in economical manner with less particle and pipe degradation compared to dilute phase conveying. By using an experimental system with special measurement devices and different materials of construction and transport, the friction between the material being transported and the pipe wall, the actual motion of the particles was determined, and the degree of fluidization were estimated. This information permits more accurate modeling of dense phase plug flow providing basic parameters to insert in existing and developing models. 相似文献
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Pandaba Patro 《Drying Technology》2016,34(6):703-712
In this article, turbulent gas-solid flow in a vertical pipe is investigated for predicting the heat transfer from the heated wall to the suspension. The Eulerian-Eulerian model is used, incorporating a four-way coupling; i.e., considering inter-particle collisions as well as particle-wall collisions. Both the phases are simulated based on Reynolds averaged Navier-Stokes equations (RANS) with a two-equation k ? ? turbulence model for the gas phase and a granular temperature equation for the solid phase. The closure of the granular temperature (kinetic energy associated with the random motion of the particles) equation is done by the use of kinetic theory of granular flows. The main objective of the study is to investigate the variations of two-phase heat-transfer coefficient and Nusselt number with flow parameters like flow Reynolds number, particulate loading, and particle size. In comparison to single-phase flow, heat transfer is found to be significantly increased with the increase in Reynolds number and particulate loading. This happens because of the presence of the solid particles in a gas flow, which bring changes to the heat-transfer characteristics of the gas phase. Heat transfer increased by adding solid particles for particulate loading in the range of 1 to 20 and particle size in the range of 30 to 50 µm. 相似文献
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Haifeng LuXiaolei Guo Wanjie HuangKai Liu Xin Gong 《Chemical Engineering and Processing: Process Intensification》2011,50(7):702-708
Experimental studies were performed to describe the physical phenomena occurring in dense phase pneumatic conveying of the pulverize coal with a Laval nozzle installed in the pipeline. The maximal coal mass flow rate decreased from 0.87 kg/s to 0.35 kg/s and an obvious decrease in the solids loading ratio was revealed after the Laval nozzle was installed. In addition, the Laval nozzle showed a better capacity of resisting disturbance, which made it easier to control the coal mass flow rate precisely and promoted the stable conveying process. These specific physical phenomena were proved to result from the high pressure drop of the Laval nozzle. Thereby, a mathematic model was developed to predict the two-phase pressure drop across the Laval nozzle. The pressure drop model described the experimental data within the 15% deviation. The main influence factors contributing to the pressure drop of the Laval nozzle were discussed using the model. Then the effects of gas mass flow rate, solids loading ratio, convergence angle, throat diameter and throat length were revealed. 相似文献
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A particle-to-particle collisional heat transfer model in the frame of Eulerian-Eulerian approach was proposed in this paper. By incorporating it into the multi-fluid model to close the enthalpy equations, the heat transfer between different particle classes in a gas bubbling fluidized bed of binary mixture was investigated, based on the CFD simulations of particle mixing in literature (Cooper and Coronella, 2005). The results showed that the particle-to-particle heat exchange coefficient between different particle classes increases with increasing the size of large particle class and the superficial gas velocity. The ratios of the particle-to-particle heat transfer to the gas-to-particle heat transfer range from 8.04% to 15.0% for various calculating conditions. In order to better understand the heat transfer behavior in a dense gas-solid fluidized of binary mixture, it is important to take the particle-to-particle heat transfer into account. 相似文献
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Multi-scale modeling of dense phase gas-particle flow 总被引:1,自引:0,他引:1
Yutaka Tsuji 《Chemical engineering science》2007,62(13):3410-3418
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对布置有涡发生器小翼对的圆管内部湍流流动和强化传热特性进行了三维数值模拟。研究了涡发生器的形状和对数对流动传热特性的影响并采用综合性能指标进行优化。结果表明:涡发生器后横截面上产生的多纵向涡结构使管内流体混合更加充分,促进了壁面边界层与主流的动量和能量交换,提高了传热强度。每排4对矩形小翼时的换热性能最好,Nusselt数比光管平均提高了27.2%。相同形状下,每排4对涡发生器的综合性能均高于每排3对;相同对数下,梯形小翼的综合性能最好,三角形小翼次之,矩形小翼最差。每排4对梯形小翼时的整体综合性能最优,性能评价标准达到了0.97~1.07。 相似文献
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黄阔 《化学工业与工程技术》2012,33(5):12-16
对半圆形单弓折流板管壳式换热器壳侧局部传热性能、局部流速及阻力进行了实验研究,测量结果表明,其壳侧局部传热膜系数的变化规律与相应点速率变化规律相一致。所取24个测量位置点分布较为合理,基本上反映了换热器局部相应换热及流场分布规律。同时归纳出换热器壳侧的平均努塞尔准数Nu与雷诺准数Re的关联式及流动阻力系数f与雷诺准数Re的关联式,为换热器的实际运用提供了参考依据。 相似文献
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Cai Y. MaChristopher J. Tighe Robert I. GruarTariq Mahmud Jawwad A. DarrXue Z. Wang 《The Journal of Supercritical Fluids》2011,57(3):236-246
Continuous hydrothermal flow synthesis processes are of interest for the manufacture of nanoparticle metal oxides. In such processes, nanoparticle nuclei (in a slurry) which are initially formed, may continue to grow and agglomerate to generate larger particles as they pass through the synthesis apparatus. These processes can widen the size distribution and also affect the ultimate particle shape in the recovered product. Therefore, fast cooling or quenching the initial nanoparticle slurry using a highly efficient heat exchanger may minimise or stop further crystallisation/agglomeration processes. This may be achieved by optimising the design of the heat exchanger based on detailed examination of flow patterns and heat transfer profiles using a computational fluid dynamics (CFD) modelling approach. The predicted flow and heat transfer patterns in the heat exchanger can also provide detailed information for the identification of any heat transfer deterioration or hot spots where further reactions may occur. This paper employs a CFD modelling approach to simulate the heat transfer processes in a tubular heat exchanger of a continuous hydrothermal flow synthesis system and also to examine the effect of various operating conditions, including inlet temperature and flowrate of hot slurry and inlet flowrate of cooling water, on the fluid and thermal features in the heat exchanger. The simulated results show that the predicted temperature and heat transfer coefficient are in good agreement with experimental measurements. 相似文献
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通过对倾斜角35°首尾相连的三分螺旋折流板换热器的数值模拟,展示了其壳侧通道内流体在典型切面上的流场和流线分布,以及典型切面上典型直线的流动和换热参数分布,并与性能测试结果进行了对比。结果表明,数值模拟结果与实验结果是吻合的。螺旋折流板形成的近似螺旋通道,使换热器壳侧流体受离心力和向心力共同作用形成了迪恩涡二次流,且在每个螺旋周期内都存在;二次流增强了主流区域流体与靠近壁面流体的掺混,使得壳侧典型切面上中心线和折流板外缘直线的轴向速度较大;除主流中心区域外,壳侧流体在二次流的作用下具有均匀的湍流动能;二次流所在区域内,壳侧同心柱面内典型直线上换热系数相差不大,但由于二次流能使其附近区域传热面上的流体得到不断卷吸掺混,由此强化传热。 相似文献
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相变墙体可以有效降低建筑能耗并提高热舒适性。相变屋顶多孔砖内部填充相变温度为25~33℃的石蜡,引入考虑辐射与空气温度的等效温度,本文利用高性能计算显卡(GPU)加速基于焓法的多松弛时间格子玻尔兹曼算法(MRT-LBM),分析了2020年8月典型天气下中国7个城市应用不同相变温度的石蜡对屋顶热响应特性的影响。采用相变温度为27℃的石蜡进行研究,结果表明,上海地区相变屋顶中的石蜡没有发生相变,空调运行期间由屋顶进入室内单位面积的热量高达324.3kJ/m2;成都地区相变屋顶中的石蜡液化率变化最大为30%,内表面温度在25.6~27.3℃波动,温差仅为1.7℃,由屋顶进入室内单位面积的热量仅为50.9kJ/m2;武汉和北京地区的液相率变化很接近,其相变时间达到全天的50%,对应的由屋顶进入室内单位面积的热量为上海地区的0.37倍;昆明与哈尔滨地区屋顶内表面温度波动分别达到4.5℃与4.4℃,但对应的温度均在28℃以内;广州地区相变屋顶中的石蜡几乎没有发生相变,室内温度波动幅度为成都的2.6倍。北京、成都、武汉和昆明地区适合采用相变温度为27℃的石蜡,而广州、上海和哈尔滨地区适合采用相变温度为29℃、31℃和25℃的石蜡。本文工作可为相变建筑优化设计提供参考。 相似文献
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Fabrizio Scala 《Chemical engineering science》2007,62(16):4159-4176
The mass transfer coefficient around freely moving active particles under bubbling/slugging fluidized bed conditions was measured in a lab-scale reactor. The technique used for the measurements consisted in the oxidation reaction of carbon monoxide at over one or few Pt catalyst spheres immersed in an inert bed of sand. It was shown that this technique is simple and accurate, and allows to overcome most of the difficulties and uncertainties associated with other available techniques. The experimental campaign was carried out by varying the fluidization velocity (0.15-0.90 m/s), the active particle size (1.0-10.0 mm) and the inert particle size (0.1-1.4 mm). Results were analyzed in terms of the particle Sherwood number. Experimental data showed that Sh is not influenced by the fluidization velocity and by a change of regime from bubbling to slugging, whereas it increases with a square root dependence with the minimum fluidization velocity and with the active particle size. These results strongly suggest that the active particles only reside in the dense phase and never enter the bubble/slug phase. Data were excellently fitted by a Frössling-type correlation:
Sh=2.0·εmf+K·(Remf/εmf)1/2·Sc1/3 相似文献
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Direct contact steam condensation used in steam jet injectors, and direct contact feed water heaters, has been studied using CFD simulations. Fairly good agreement has been obtained, with the available experimental data of plume length and the profiles of axial and radial temperature. Further, critical analyses have been carried out for all the published semi-empirical models and correlations. In addition, CFD analysis has been extended to examine the role of nozzle diameter and geometry, on heat transfer phenomenon which governs the direct contact steam condensation phenomenon. A new hydrodynamic model has been formulated which estimates the interfacial area available for condensation. A rational correlation has been developed for the estimation of interfacial area, expressed in terms of Nusselt number (Nu), Reynolds number (Re), Prandtl number (Pr) and ratio of viscosity of steam and water. 相似文献
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Effects of the particle-particle heat transfer in a gas-solid turbulent flow in a riser were evaluated. An Eulerian/Lagrangian four-way interaction formulation including the particle collisions in conjunction with the k − τ and the kθ − τθ model equations were used in the numerical simulation. Inter-particles and particle-wall interactions were accounted for with an inelastic collision model, where the restitution coefficient was evaluated for each collision. The special case when the flow initially contains two groups of hot and cold particles was treated in details. Particular attention was given to the nature of heat transfer to particles due to inter-particle interactions. The results showed that the effect of particle-particle heat transfer was more significant for smaller sizes, lower flow Reynolds numbers, and for higher loading ratios. Solid thermal properties, however, did not have a noticeable effect on the inter-particle heat transfer. The simulation results indicates that although the heat transferred to each group of hot and cold particles was significant, the mean values of gas and particle temperatures and suspension heat transfer was insensitive to the inter-particle heat transfer. 相似文献
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J.P. Solano R. Herrero S. Espín A.N. Phan A.P. Harvey 《Chemical Engineering Research and Design》2012
Oscillatory baffled reactors (OBRs) are a means of process intensification as they allow processes with long residence time to be converted from batch to continuous processing. Helically baffled OBRs have only been developed at “mesoscale” so far, but at this scale have displayed significant advantages in terms of the increased range of conditions over which plug flow is achieved. Scale-up studies are underway to determine whether this is replicated at larger scales. This paper reports fluid mechanical modeling of a helically baffled oscillatory flow for the first time. Time-dependent flow structures induced in tubular reactors have been analyzed on the basis of periodic, laminar flow numerical simulation. A reversing swirled core flow and its interaction with the unsteady mechanism of vortex shedding downstream of the wires has been described. This has allowed greater understanding of the flow structures, which will underpin optimal design and scale-up. The potential for heat transfer enhancement is discussed, considering the compound effect of oscillatory motion and helical coil inserts. The results show that the heat transfer for the helical baffled tube could be enhanced by a factor of 4 compared to a smooth tube in the tested range of oscillation conditions. 相似文献