利用CFD分析催化转化器不同设计因素的影响

用于降低排放的催化转化器已经成为汽车不可或缺的重要装置,为进一步优化设计催化转化器,必须对催化器的各方向影响因素进行全面分析,以得到最佳匹配,使新型催化器的使用环境达到最佳,为此通过ANSYS－CFD（计算流体力学）对催化器内部温度场和流场进行数据值模拟,结合比较不同设计因素对催化器内部温度场和流场的影响。     

Investigation of the flow field inside flat-plate collector tube using PIV technique

The thermofluid process inside the tube of flat-plate collectors is complex because the non-uniform heating of the tube results in the formation of stably and unstably stratified layers of fluid that interact with each other. The measurement and investigation of the flow behaviour inside the collector tube is very challenging. We report on a novel application of the particle image velocimetry (PIV) technique to remotely measure the velocity field inside the collector tube. The two-dimensional velocity fields were measured in the midplane of a collector tube for the Reynolds number range of 150-900 at unheated and four different heating conditions. We have presented and discussed in detail the technique implementation and the associated challenges. The results have shown that the collector heating significantly alters the structure and magnitude of the mean velocity field and influences the heat transfer to the fluid. It is observed that the collector heating causes a significant asymmetry in the mean velocity profiles over the given range of Reynolds numbers and heating conditions.     

Comparative study on heat transfer enhancement of nanofluids flow in ribs tube using CFD simulation

This study presents, a numerical investigation of two‐dimensional turbulent nanofluids flow in different ribs tube configurations on heat transfer, friction, and thermal performance coefficients using ANSYS‐FLUENT software version‐16. Governing equations of mass, momentum, and energy have been solved by means of a finite volume method (FVM). Four types of nanoparticles namely; Al₂O₃, CuO, SiO₂, and ZnO with volume fraction range (1%‐4%) and different size of nanoparticles (dp = 30 nm, 40 nm, 50 nm, and 60 nm) with various Reynolds number (10 000‐30 000) in a constant heat flux tube with rectangular, triangular, and trapezoidal ribs were conducted for simulation. The results exhibit that Nusselt number for all cases enhanced with Reynolds number and nanofluid volume fraction increases. Likewise, the results also reveal that SiO₂ with volume fractions of 4% and diameters of nanoparticles of 30 nm in triangular ribs offered the highest Nusselt number at Reynolds number of Re = 30 000. In addition, the higher value of thermal performance factor was obtained at Reynolds number of Re = 10 000.     

CFD modeling of flow and heat transfer in a thermosyphon

In the present study a gas/liquid two-phase flow and the simultaneous evaporation and condensation phenomena in a thermosyphon was modeled. The volume of fluid (VOF) technique was used to model the interaction between these phases. Experiments in a thermosyphon were carried out at different operating conditions. The CFD predicted temperature profile in the thermosyphon was compared with experimental measurements and a good agreement was observed. It was concluded that CFD is a useful tool to model and explain the complex flow and heat transfer in a thermosyphon.     

依托高楼的太阳能热气流电站系统的CFD模拟

设计了一种依托高楼的太阳能热气流电站,建立了相应的数学模型。利用Fluent软件对该系统的流场及温度场进行了数值模拟,并对电站结构进行了优化设计。模拟结果表明:随着烟囱高度的增加,烟囱内气流的温度不断上升,在出口处由于回流的影响温度稍有下降,气流速度不断增大,气流的压力分布先减小后增加。由于平板集热器的加热作用使烟囱内气流的温度分布不均匀,设计中可将平板型储热器换为带有肋片扩展表面的储热器,并对烟囱与扩压管联接处采用流线形联接进行过渡,以减少此处的流动阻力。同时,设计一个收缩型的烟囱出口,以保证电站系统产生较大的抽力,从而提高电站系统的能量转换率。     

CFD modeling of heat transfer and fluid flow inside a pent-roof type combustion chamber using dynamic model

A numerical work has been performed to analyze the heat transfer and fluid flow in a pent-roof type combustion chamber. Dynamic mesh model was used to simulation piston intake stroke. Revolution of piston (1000 ≤ n ≤ 5000) is the main governing parameter on heat and fluid flow. k–ε turbulence model was used to predict the flow in the cylinder of a non-compressing fluid. They were solved with finite volume method and FLUENT 12.0 commercial code. Velocity profiles, temperature distribution, pressure distribution and velocity vectors are presented. It is found that the inclined surface of pent-roof type of combustion chamber reduces the swirl effect and it can be a control parameter for heat and fluid flow.     

Effect of mounting geometry on convection occurring under a photovoltaic panel and the corresponding efficiency using CFD

Computational fluid dynamics (CFD) is used to model experimental data corresponding to convection occurring under a photovoltaic (PV) panel. Further experimental data is used to validate the model where the satisfactory agreement is received. A standardised condition is set up to allow the effect of varying three geometric parameters to be examined. These are the air gap height (10–500 mm), air gap orientation angle (0–90° from the horizontal) and fluid velocity magnitude (0–3 m/s). The optimum mounting conditions for the PV panel is obtained and maximised electrical efficiency found to favour angles greater than 50° and air gap heights that give an aspect ratio of 60. Mixed convection opposed to natural convection is found to be more effective, with greater efficiencies obtained for larger fluid velocities.     

Investigating the effect of crevice flow on internal combustion engines using a new simple crevice model implemented in a CFD code

A theoretical investigation is conducted to examine the way the crevice regions affect the mean cylinder pressure, the in-cylinder temperature, and the velocity field of internal combustion engines running at motoring conditions. For the calculation of the wall heat flux, a wall heat transfer formulation developed by the authors is used, while for the simulation of the crevices and the blow-by a newly developed simplified simulation model is presented herein. These sub-models are incorporated into an in-house Computational Fluid Dynamics (CFD) code. The main advantage of the new crevice model is that it can be applied in cases where no detailed information of the ring-pack configuration is available, which is important as this information is rarely known or may have been altered during the engine’s life. Thus, an adequate estimation of the blow-by effect on the cylinder pressure can be drawn. To validate the new model, the measured in-cylinder pressure traces of a diesel engine, located at the authors’ laboratory, running under motoring conditions at four engine speeds were used as reference, together with measured velocity profiles and turbulence data of a motored spark-ignition engine. Comparing the predicted and measured cylinder pressure traces of the diesel engine for all cases examined, it is observed that by incorporating the new crevice sub-model into the in-house CFD code, significant improvements on the predictive accuracy of the model is obtained. The calculated cylinder pressure traces almost coincide with the measured ones, thus avoiding the use of any calibration constants as would have been the case with the crevice effect omitted. Concerning the radial and swirl velocity profiles and the turbulent kinetic energy measured in the spark-ignition engine, the validation process revealed that the developed crevice model has a minor influence on the aforementioned parameters. The theoretical study has been extended by investigating in the same spark-ignition engine, during the induction and compression strokes, the way crevice flow affects the thermodynamic properties of the air trapped in the cylinder.     

水能回收装置引水部件内部流场的CFD分析

根据某石化公司的节能需要,采用水力机械工作原理研究开发出一种卧式轴流式水能回收装置。 为了优化结构、保证性能及提高水能回收效率,在研究开发工程中,对其引水部件的内部复杂的紊流流 场采用CFD技术进行粘性流动的数值模拟,以取代模型试验。通过对流场的可视化分析,表明圆形断 面的蜗壳进水的均匀性、形成的速度环量比矩形断面的更好。为水能回收装置引水部件优化设计提供 了依据。     

CFD investigation of the flow and heat transfer characteristics in a tangential inlet cyclone

This work presents a computational fluid dynamics (CFD) calculation to investigate the flow field and the heat transfer characteristics in a tangential inlet cyclone which is mainly used for the separation of the dens phase of a two phase flow. Governing equations for the steady turbulent 3D flow were solved numerically under certain boundary conditions covering an inlet velocity range of 3 to 30 m/s. Finite volume based Fluent software was used and the RNG k −  turbulence model was adopted for the modeling highly swirling turbulent flow. Good agreement was found between computed pressure drop and experimental data available in the literature. The structure of the vortices and variation of local heat transfer were studied under the effects of inlet velocity.     

Energy efficiency improvement of water pumping system using synchronous reluctance motor fed by perovskite solar cells

Perovskite solar cells (PSCs) are in the forefront of third-generation of photovoltaics and gained a lot of attention as a very promising green technology toward direct solar energy conversion to electricity. PSCs are fabricated following solution-processed techniques at low temperature and they present high power conversion efficiency exceeding 25%, enabling them to be attractive alternative to the silicon-based devices. This research work proposes an efficient and cost-effective photovoltaic (PV) pumping system based on PSCs. For this purpose, lab-scale PSCs were fabricated and their characteristics were determined. In parallel, the geometry of a synchronous reluctance motor (SynRM) driving a 350 m³/day water pump was optimized for maximizing the output power, while minimizing the torque ripple simultaneously. In addition, a perovskite solar array feeding the SynRM via an inverter was designed and implemented. The inverter was properly regulated by a control system which optimized the maximum available power of the PSCs solar array and the SynRM characteristics. Finally, laboratory measurements were performed, including a power generator simulating the behavior of the PSCs array feeding the SynRM. The obtained results confirmed the experimental validation of the proposed approach.     

CFD simulations on small hydrogen releases inside a ventilated facility and assessment of ventilation efficiency

The use of stationary H₂ and fuel cell systems is expected to increase rapidly in the future. In order to facilitate the safe introduction of this new technology, the HyPer project, funded by the EC, developed a public harmonized Installation Permitting Guidance (IPG) document for the installation of small stationary H₂ and fuel cell systems for use in various environments. The present contribution focuses on the safety assessment of a facility, inside which a small H₂ fuel cell system (4.8 kW_e) is installed and operated. Dispersion experiments were designed and performed by partner UNIPI. The scenarios considered cover releases occurring inside the fuel cell at the valve of the inlet gas pipeline just before the pressure regulator, which controls the H₂ flow to the fuel cell system. H₂ was expected to leak out of the fuel cell into the facility and then outdoors through the ventilation system. The initial leakage diameter was chosen based on the Italian technical guidelines for the enforcement of the ATEX European directive. Several natural ventilation configurations were examined. The performed tests were simulated by NCSRD using the ADREA-HF code. The numerical analysis took into account the full interior of the fuel cell, in order to investigate for any potential accumulation effects. Comparisons between predicted and experimental H₂ concentrations at 4 sensor locations inside the facility are reported. Finally, an overall assessment of the ventilation efficiency was made based on the simulations and experiments.     

CFD analysis on the influence of helical carving in a vortex flow solar reactor

Solar thermal cracking of natural gas is a promising technology, which has attracted researchers in recent years for its potential to lead to the development of CO₂ free hydrogen production process. However, experimental access to the reaction chamber of solar cracking reactors is a challenge due to the high temperature process as the instruments capable of measuring fluid flow cannot survive the medium inside the reactor. However, computational fluid dynamics (CFD) can provide an insight into the flow, where experimental access is limited or not possible. This paper presents a CFD analysis for directly irradiated solar thermochemical reactor to characterize the influence of flow behavior on the heat transfer and solar cracking process. The heat transfer by radiation from carbon particles is considered by providing global absorption and scattering coefficients in the computational domain obtained from Mie code. The flow field is based on RNG k–? model derived using renormalization group theory. This technique accounts for the effect of swirl on turbulence thereby enhancing accuracy for the swirl flows. Validation of the numerical results is carried out by making a comparison with the experimental results. Highlighting the effects of carving on the solar reactor walls, this study presents numerical analyses of solar reactor geometry for two cases; namely, when there is no vortex forming carving in the cavity, and when there is vortex forming helical carving. The results show that carving has significant influence on the flow behavior, however, it has very little effect on the outlet temperature. The numerical results also show that the radiative heat transfer mechanism is the dominant means of heat transfer compared to the effects of conduction and convection.     

Investigation of the performance of a staggered configuration of tidal turbines using CFD

This paper investigates the influence of wake interaction and blockage on the performance of individual turbines in a staggered configuration in a tidal stream farm using the CFD based Immersed Body Force turbine modelling method. The inflow condition to each turbine is unknown in advance making it difficult to apply the correct loading to individual devices. In such cases, it is necessary to establish an appropriate range of operating points by varying the loading or body forces in order to understand the influence of wake interaction and blockage on the performance of the individual devices. The performance of the downstream turbines was heavily affected by the wake interaction from the upstream turbines, though there were accelerated regions within the farm which could be potentially used to increase the overall power extraction from the farm. Laterally closely packed turbines can improve the performance of those turbines due to the blockage effect, but this could also affect the performance of downstream turbines. Thus balancing both the effect of blockage and wake interaction continues to be a huge challenge for optimising the performance of devices in a tidal stream farm.     

水平换热通道自然循环流动特性分析

通过建立考虑密度变化的三维CFD(计算流体动力学)模型,对单相自然循环条件下水平换热回路的流动特性进行了研究。在稳态流动分析中,首先通过三维CFD模型,获取了加热管道的径向温度和速度分布,发现管道截面温度场和速度场均为不均匀分布,这分别是由径向自然对流和对流换热系数决定的;其次,应用量纲分析的方法,整理归纳系统无量纲参数间的线性关系。在非稳态分析中发现水平换热通道出现不规则复杂流动,证明了混沌现象的存在,产生这种现象的原因是自然循环的特性与回路布置方式双重作用的结果。     

A study on rotational augmentation using CFD analysis of flow in the inboard region of the MEXICO rotor blades

This work presents an analysis of data from existing as well as new full‐rotor computational fluid dynamics computations on the MEXICO rotor, with focus on the flow around the inboard parts of the blades. The boundary layer separation characteristics on the airfoil sections in the inboard parts of the rotor are analysed using the pressure and the skin friction data at a range of angles of attack. These data are used to gain insight on the relative behaviour of separated boundary layers in 3D flow compared with 2D flow. It has been found that separation on airfoils in rotating flows is different from that in 2D flows in two respects: (i) there is a chord‐wise postponement (or delay) of the separation point, and (ii) the angle of attack at which separation is initiated is higher in 3D compared with 2D. Comments are made on the mechanism of stall delay, and the main differences between the skin friction and pressure distribution behaviours in 2D and 3D rotating flows are highlighted. Copyright © 2014 John Wiley & Sons, Ltd.     

CFD insight of the flow dynamics in a novel swirler for gas turbine combustors

We describe the flow dynamics inside a novel swirler conceptualized for gas turbine combustors. The supreme advantage in this swirler is the ability to vary the swirl number for the same value of Reynolds number. The significance of such advantage against contemporary configurations, which have constant swirl number, is quite evident at low turbine operating loads. The novel geometry and flow pattern are described in details in the present work. The results of four numerical simulations are presented and discussed to study the central recirculation zone, turbulence intensity, and pressure drop at different swirl numbers. The new concept is deemed to enhance the combustion efficiency because of its ability to adjust the swirl number according to the turbine operating load. The current study reports preliminary results which verify the concept behind the proposed swirler. However, intensive numerical and experimental studies are necessary to be carried out in order to characterize the flow dynamics produced by the novel swirler and its impact on the combustion process.     

Evaluation of a combustion model for the simulation of hydrogen spark-ignition engines using a CFD code

The present work deals with the evaluation of a combustion model that has been developed, in order to simulate the power cycle of hydrogen spark-ignition engines. The motivation for the development of such a model is to obtain a simple combustion model with few calibration constants, applicable to a wide range of engine configurations, incorporated in an in-house CFD code using the RNG k–? turbulence model. The calculated cylinder pressure traces, gross heat release rate diagrams and exhaust nitric oxide (NO) emissions are compared with the corresponding measured ones at various engine loads. The engine used is a Cooperative Fuel Research (CFR) engine fueled with hydrogen, operating at a constant engine speed of 600 rpm. This model is composed of various sub-models used for the simulation of combustion of conventional fuels in SI engines; it has been adjusted in the current study specifically for hydrogen combustion. The basic sub-model incorporated for the calculation of the reaction rates is the characteristic conversion time-scale method, meaning that a time-scale is used depending on the laminar conversion time and the turbulent mixing time, which dictates to what extent the combustible gas has reached its chemical equilibrium during a predefined time step. Also, the laminar and turbulent combustion velocity is used to track the flame development within the combustion chamber, using two correlations for the laminar flame speed and the Zimont/Lipatnikov approach for the modeling of the turbulent flame speed, whereas the (NO) emissions are calculated according to the Zeldovich mechanism. From the evaluation conducted, it is revealed that by using the developed hydrogen combustion model and after adjustment of the unique model calibration constant, there is an adequate agreement with measured data (regarding performance and emissions) for the investigated conditions. However, there are a few more issues to be resolved dealing mainly with the ignition process and the applicability of a reliable set of constants for the emission calculations.     

Advancing in the decarbonized future of natural gas transmission networks through a CFD study

The injection of green hydrogen into the natural gas grid is a way to decarbonize the gas sector and build an economic transport route for the large-scale delivery of hydrogen. The suitability of the natural gas infrastructure for this purpose depends on the impact that hydrogen may have on the correct operation of its components and understanding the new flow conditions in the system is essential for this aim. Computational studies can anticipate the expected environment in the pipe system, assessing the readiness of the system. However, the experience on this topic is not extensive enough and deeper understanding is necessary. Here we show a CFD study to simulate the transport of H₂/NG blends in a gas setup with the main characteristics of injection sites and gas pipelines representatives of the transmission gas network. This setup considers a blending station, the pumping and injection procedure, and different pipelines geometries to predict the behavior of various mixtures of H₂/NG. It can be seen how (1) a good mixing is achieved in the blending station after a pipe length equivalent to 20–30 diameters is reached; (2) pumping gas by a piston type compressor shows pulsations in the flow regardless the composition of the blend that can be damped implementing mitigation measurements; and (3) asymmetries in the flow are found when the direction of the fluid changes after section reduction, but 20 diameters downstream of the reduction the flow is fully developed.