Добірка наукової літератури з теми "Silos Aerodynamics"

The article presents study of wind effect on silo parks, which was carried out by computer simulation methods. A special modeling technique was created as a software module for the Ansys Workbench platform. A finite element mesh was developed in accordance with two requirements. Through the use of this mesh, which doesn’t contain needless elements which can be used for simplification of calculations and reduction of execution time, it is possible to describe the turbulent airflow in sufficient detail. The dimensions of all mesh elements are determined by special relations as the functions of the silo diameter and the Reynolds number. The major stage in this investigation was modeling of various options for flowing silos and their groups. As a result of the study, we have obtained aerodynamic characteristics of individual silos as part of silo parks and plotted charts of the distribution of pressure coefficients over the cylinder surface, changing the size of the silos, distances between them and local wind regime. Based on these data, we have drawn a conclusion about the optimal space planning locations of silos for different wind directions. Visualizations of turbulent flow around models at different speeds have also been obtained in this study.

Introduction. Preservation of grain crops in flat-bottomed metal silos is not possible without monitoring and control of air flows in the internal volume. In the silos with uncontrolled air flows, there is a redistribution of moisture and additional moistening of the grain surface layer that leads to losses of about 2% of grain. The aim of this work is to develop a technology for preventing the grain surface layer from humidifying during storage in metal silos. Materials and Methods. Under laboratory conditions, aerodynamic parameters of wheat and soybean grains were determined in the range of filtration rates less than 0.15 m/s. In metal silos, with a capacity of 2,000, 3,000 and 10,000 t, the temperature and relative humidity of the air inside at the top of the silos and outside were measured simultaneously. The temperature and relative humidity measurement period was 30 min for two and five months. Autonomous recorders were used for measurement. Results. A new objective standard of grain ventilation is proposed ‒ minimum (critical) filtration rate, which ensures moisture removal outside the silo. The analytical studies produced an equation for calculating the weighted average filtration rate of air leaving the grain mass. The total air flow rate corresponding to the weighted average filtration rate will provide a filtration rate of at least critical over the entire surface and will exclude moisture settling. The periods of air saturation with moisture up to 100% in the supervisory space under the roof of the silo have been determined experimentally. The mechanism of heat emission up to the silo roof space from the depth of grain mass during storage has been clarified. Discussion and Conclusion. The algorithm for safe active ventilation of grain and roof space in metal silos is proposed. Limit values of relative humidity of atmospheric air are recommended, the use of which will exclude humidification of grain mass when ventilating actively in the range of temperature difference between grain and atmosphere up to 30 °С and more. The obtained data can be used by mechanical engineers in the manufacture of metal silos and by grain producers in the operation of the said silos.

The kinetic energy of flying insect prey is a formidable challenge for orb-weaving spiders. These spiders construct two-dimensional, round webs from a combination of stiff, strong radial silk and highly elastic, glue-coated capture spirals. Orb webs must first stop the flight of insect prey and then retain those insects long enough to be subdued by the spiders. Consequently, spider silks rank among the toughest known biomaterials. The large number of silk threads composing a web suggests that aerodynamic dissipation may also play an important role in stopping prey. Here, we quantify energy dissipation in orb webs spun by diverse species of spiders using data derived from high-speed videos of web deformation under prey impact. By integrating video data with material testing of silks, we compare the relative contributions of radial silk, the capture spiral and aerodynamic dissipation. Radial silk dominated energy absorption in all webs, with the potential to account for approximately 100 per cent of the work of stopping prey in larger webs. The most generous estimates for the roles of capture spirals and aerodynamic dissipation show that they rarely contribute more than 30 per cent and 10 per cent of the total work of stopping prey, respectively, and then only for smaller orb webs. The reliance of spider orb webs upon internal energy absorption by radial threads for prey capture suggests that the material properties of the capture spirals are largely unconstrained by the selective pressures of stopping prey and can instead evolve freely in response to alternative functional constraints such as adhering to prey.

Abstract Capture of a prey by spider orb webs is a dynamic process with energy dissipation. The dynamic response of spider orb webs under prey impact requires a multi-scale modeling by considering the material microstructures and the assembly of spider silks in the macro-scale. To better understand the prey capture process, this paper addresses a multi-scale approach to uncover the underlying energy dissipation mechanisms. Simulation results show that the microstructures of spider dragline silk play a significant role on energy absorption during prey capture. The alteration of the microstructures, material internal friction, and plastic deformation lead to energy dissipation, which is called material damping. In addition to the material damping in the micro-scale modeling, the energy dissipation due to drag force on the prey is also taken into consideration in the macro-scale modeling. The results indicate that aerodynamic drag, i.e., aero-damping, plays a significant role when the prey size is larger than a critical size.

Os silos metálicos cilíndricos de chapa corrugada e cobertura cônica são as unidades mais utilizadas no Brasil para o armazenamento de produtos granulares. As principais ações variáveis que atuam sobre os silos são as pressões devidas aos produtos armazenados e ao vento, sendo esta ação crítica quando o silo se encontra vazio. Devido à grande eficiência estrutural da forma cilíndrica e à resistência elevada do aço, estas estruturas são leves e delgadas e, portanto, suscetíveis a perdas de estabilidade local e global e arrancamento. Com a finalidade de avaliar estes efeitos foram realizados estudos teóricos e experimentais sobre as ações do vento em silos. O trabalho foi desenvolvido com ensaios de modelos aerodinâmicos e aeroelásticos em um túnel de vento na Universidade de Cranfield, Inglaterra, com o objetivo de determinar os coeficientes aerodinâmicos no costado e na cobertura. Os resultados mostram que os valores dos coeficientes recomendados pela Norma Brasileira de vento, NBR 6123 (1990), são adequados para o costado. Para a cobertura cônica, como não são especificados pela NBR, são recomendados valores dos coeficientes aerodinâmicos determinados nos ensaios. Conclui-se também que a colocação externa das colunas é a favor da segurança e que o uso de anéis enrijecedores no costado é indicado e muito importante para a estabilidade local e global da estrutura do silo.
The steel cylindrical silos made of corrugated sheets with conical roofs are the most used units to the storage of granular materials. The main silo loads are the pressures due to the stored material and to the wind, being this action the critical one when the silo is empty. Due to the high efficiency of the cylindrical form and to the high strength of the steel, these structures are thin and light-weight and, as a consequence, susceptible to the loss of local and global stability and to the pull out of the structure. With the aim to assess these effects related to the wind loading in silos, some theoretical and experimental studies were conducted. The work was carried out with aerodynamic and aeroelastic models tested in a boundary layer wind tunnel in the University of Cranfield, England, with the objective to determine the aerodynamic coefficients of the cylinder and the conical roof. The results show that the coefficients of the Brazilian Code of wind loads, NBR 6123 (1990), are adequate to the cylinder. The coefficients to the conical roof are suggested based on our tests, considering that there are no values specified by the NBR. As well it is concluded that the outside columns is on the side of safety and it is indicated the use of wind rings attached to the cylinder, which are very important to the local and global stability of the silo structure.

For heavy-duty gas turbines with silo combustors, the transition section between the combustor and the turbine poses unique aerodynamic design challenges due to the non-axis symmetrical shape of the transition from the combustor (pipe), to hot gas casing (torus), to turbine inlet (annulus). The inlet segment is the transition piece located between the hot gas casing and the outer radius of the turbine. The shape of the inlet segment is designed such that it provides minimum pressure loss of the hot gas flowing from the silo combustor to the turbine inlet. Another requirement of the inlet segment shape is to provide as homogeneous as possible flow conditions at the turbine inlet. This paper shows the results from CFD analysis of an inlet segment. The results agree very well with pressure tap measurements in the engine at several discrete circumferential locations. In addition, thermal crystal measurements mounted at several leading edge locations of the vanes at the inlet of the turbine were able to confirm from the CFD the radial and circumferential temperature distribution typically produced in a silo combustor. A new inlet segment profile shape has been designed to produce an equivalent pressure distribution for the turbine inlet but with a reduced surface area, which helps to reduce the cooling requirements for this part.

Lean blow out (LBO) has a big impact on emission formation at part load of gas turbines, where flame temperature is low and flame stabilization is an issue. With improved combustion behavior at LBO conditions the operation flexibility of a silo gas turbine can be increased within the scope of retrofitting. In multi burner arrangements a part of the preheated air designated for combustion is used for impingement cooling of the burner front panel and subsequently injected into the primary combustion zone. In this region of flame stabilization air and unburned fuel as well as burned products are mixed to sustain stable combustion. The object of this study is to determine the level of dilution of the flow field by the cooling air with the focus on the conditions below LBO that can impair flame stability. The question addressed in this paper is how mixing of the front panel cooling air with the incoming reactants and the combustion products in multi burner arrangements can be computed in a numerically efficient way. As test case for the methodology the local distribution of cooling air in a silo combustor is presented. In this numerical study mixing processes of air-fuel mixture and cooling air as well as aerodynamic interaction of adjacent burners in a multi burner systems are investigated using isothermal Reynolds Averaged Navier Stokes (RANS) simulations. Former published single burner water channel experiments and Large Eddy Simulations (LES) [1] serve as a baseline. Single burner RANS simulations are done and compared to measurement and LES to validate the velocity and scalar fields. A Schmidt number variation is used to modify the mixing process in the RANS single burner calculations. Based on the LES the single burner is modified to address the multi burner conditions and calculated with LES and RANS. Finally the multi burner system is computed with the settings applied in the single burner configuration. Using the symmetry of the investigated burner matrix an efficient methodology is implemented that allows computation of one sixth of a silo combustor. The results expose a strong burner-burner interaction of the recirculation zones and in contrast to the single burner configuration regions of concentrated cooling air.

Based on fundamental research concerning swirling flows, including the vortex breakdown phenomenon, as well as on stability considerations of premixed flames, a second generation of low emission burners has been developed. The lean premixing technique provides NOx-emissions below 25ppmv for natural gas. For liquid fuels the oxides of nitrogen are limited to 42ppmv (oil no. 2). The novel burner technology will be applied to the well-known ABB silo combustor. As a first step the Conical Premix Burner will be used to retrofit the ABB type 11N. For the ABB gas turbine type 8 the design of a novel fully annular combustor is in progress. Most of the conceptual work concerning burner aerodynamics and burner-burner interaction has been carried out on scaled-down burner- and combustor-models. For a second step a sector of the combustor in 1:1 scale has been tested at atmospheric pressure. Additional high pressure tests provide information about the combustor performance at engine conditions. The present paper summarizes the results of the first two steps beginning with the early ideas in the conceptual phase up to the 1:1 tests which prove the low-NOx capability for both gaseous and liquid fuels under atmospheric pressure conditions.