米尔恩

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米尔恩范文第1篇

In a sense,the life of each of us is made up of family, friends and career. If any of them is lacking,our life is not intact and we may feel embarrassed, regretful, or even painful.

One's family is his harhour where he can rest and relax himself when he sails home through winds and waves of the outside world. One's friends are his most trustworthy and most valuable companions. He and his friends will help and encourage each other on the long journey of life. One's career is the reflection of his talent and value which he offers society.

Anyone who longs for a delightful life should cherish his family, take sincere care of fi'iendship and devote himself to his career。

米尔恩范文第2篇

Abstract

This paper deals with new design of low head turbines, as feasible solutions to solve the lack of energy in rural and remote areas, or to provide energy from urban water pipe systems. Propeller turbines are then the subject of this research because they are suitable for small heads, discharges with little variability, easy to manufacture and with low costs associated. Hence, the aims are the design of quite simple tubular propeller turbines and the analysis of hydrodynamic behaviour for different number and configuration of blades, based on CFD analyses and experimental tests development. An advanced hydrodynamic code based on the finite volume method, as well as blades configuration and mesh specific models are used for the impeller and the turbine design. The blade geometry is optimized using mathematical formulations and experimental results, concerning the possible range of operation under best efficiency conditions. Performance curves are obtained for typical characteristic parameters allowing comparisons between CFD and experimental results. Based on the similarity theory applied to turbomachines it is possible to evaluate the hydrodynamic behaviour through a tubular propeller for different sizes, in a scale model application.

Key words: Low head turbines; Fluid dynamics; Tubular propeller; CDF analysis; Performance curves

Ramos, H. M., Sim?o, M., & Borga, A. (2012). CFD and Experimental Study in the Optimization of an Energy Converter for Low Heads. Energy Science and Technology, 4(2), -0. Available from: URL: /index.php/est/article/view/10.3968/j.est.1923847920120402.142

DOI: /10.3968/j.est.1923847920120402.142

INTRODUCTION

Hydrodynamic models of fluid mechanics, also known by computational fluid dynamics (CFD), allow the evaluation of the flow behaviour, for a specific system configuration with associated boundary conditions. These models need the development of theoretical analysis on the physical behaviour of the flow based on mathematical formulations in three-dimensional analyses, with enough accuracy not only for laminar and turbulent flows, but also for the various forms of energy transfer, changing phases of the flow, vorticity occurrence, levels of turbulence and shear stress between interfaces. Hence, this study deals with a new propeller solution based on a facility implementation in order to predict the real evaluation of the pathlines, turbulence and losses effects, for different operating conditions.

Pico turbines are cost effective means of producing electricity of low power being under analysis for new improvements, despite the less attention that researchers and manufactures have been paid to those engines’ technology. Thus, the challenge of this work is therefore to provide new engineering designs and implementation methods that can be effectively customized and applied for possible energy recovery projects in water systems of low head and relative flow rates, such as from natural small rivers or streams, water supply, irrigation and drainage systems, treatment plants or aquaculture factories.

Definitions for pico-hydropower vary, but the term generally refers to power systems below 5 kW. At isolated regions, such systems are suitable for individual households and powering data loggers or management control systems in water companies.

A fixed geometry propeller turbine was built and tested for a runner speed of 1000 rpm, suited to 35 m head and about 5 l/s flow rate, reaching good efficiencies (Howey, 2009). It was also designed and installed in field a fixed geometry propeller turbine with a spiral casing showing an overall mechanical efficiency of 65% (Simpson & Williams, 2006). Recent optimizations of low-head axial-flow hydro turbines have enabled to reach interesting operating efficiencies. Several researchers (e.g. Demetriades, 1997; Upadhyay, 2004; Alexander et al., 2009) have developed models of medium sizes for propellers turbines, but until now there are no any relevant expression studies for propellers working as micro turbines, allowing new developments of improvements in its design, efficiency and versatility of operation based on both computational modelling and experimental analysis. A new fixed blade runner called “mixer” suitable for upgrading old units of Francis turbines installed in low head hydropower plants was recently developed by (Skotak, 2009). This new turbine suitable for a head of 5 m, with a runner diameter of 2250 mm and a discharge of 23 m3/s allows obtaining higher efficiency by optimizing the shape of the runner blades.

These new design solutions are usually appropriate to hydropower schemes with large discharge values, leaving an open field for developing new geometries optimization models with smaller sizes, in order to cover a large range of applications, where low power are available, especially in water pipe systems.

Through turbo machine similarity it is possible to estimate different operating conditions from an equivalent turbine, even though the scale effects associated. The behaviour of the system as a whole can differ, depending on the scale adopted, and the configuration of the runner, in particular the blades shape (Ramos et al., 2009). Consequently, the design and the development of micro turbines cannot be only based on the methodology of exactly scaling down from large turbines. Economic, hydrodynamic and manufacturing constraints give opportunities to create new designs adequate to each type of water system or infrastructure, depending on its main characteristics.

This study provides analysis based on a new blade model configuration and CFD analysis, as well as new hydraulic energy converters suitable for applications of micro-scale for low head and flow rate conditions, which can be easily implemented both in remote areas, as well as in water pipe systems in urban environment, with non-negligible flow energy available that would be wasted or dissipated. The proposed new tubular propeller turbines (with 4 and 5 blades) represent a cheap and easy installation solution to cover a range of low power, head and discharge values which are not available in the market.

1. FLUID DYNAMICS

1.1 Fundamentals

In computational fluid dynamics, the CFDs are important tools to estimate real results from the calibration based on experimental tests, which allow for better understanding the phenomenon associated with the flow behaviour in turbines for different flow conditions (Ramos et al., 2010). In fact, these CFD are advanced models of fluid mechanics widely used in the analysis of complex in setting hydraulic systems, leading to optimal design solutions. FLUENT is a hydrodynamic model that applies the technique of finite volumes to solve the equations that describe the flow, as the continuity equation and the Euler or Navier-Stokes’ equations also known as Reynolds equations. This model features two types of calculation algorithms that can be solved by a system of equations. Regarding the latter option the algorithm SIMPLE is a way to resolve the coupling between velocity and pressure. In the case of Reynolds stress it is used the k-ε model since it is a robust model with proven results on the turbulence analyses. The model includes two equations regarding the properties of turbulence flow, which allows accounting for all purposes of the convection and diffusion of the turbulence intensity. One of the variables is the turbulent kinetic energy, k, while the other represents the rate of dissipation, ε. In summary the dissipation variable determines the scale of turbulence, while the kinetic energy the turbulence intensity.

1.2 k-ε Model

The effect of turbulence normally occurs for high values of Reynolds, and is the cause of production some eddies within the fluid. Associated with turbulent flow it can be identified zones with rotation, diffusion intermittence, highly disordered and dissipative effects. Regions with greater turbulence, which are normally associated to fluctuations of low frequency, can be considered as a boundary condition of the flow and its size can reach the same order of magnitude of the flow itself. As a result, turbulent flow characteristics require specific models to determine the correlation between velocity and pressure. According to the simplifications in the fluid transport equation (Equation (1)), it is possible to make a parallel between these equations and those used by the turbulence k-ε model.

(1)

This k-ε model is a semi-empirical based on transport equations of kinetic energy turbulent (k) and its dissipation rate (ε). The flow transport equations for the k model, are derived from the exact equation, while the transport equation for the ε model is obtained through physical relations (Fluent, 2006). In the derivation of the k-ε model it is assumed the flow is turbulent, and the effects of molecular viscosity are negligible. Thus, the turbulent kinetic energy and its dissipation rate are obtained, respectively, by Equations (2) and (3):

(2)

(3)

where C1ε, C2ε are constants, and σk, σε correspond to the variables turbulent Prandtl k and ε, respectively, determined experimentally with air and water affected by friction in flows with homogeneous and isotropic turbulence (Scott-Pomerantz, 2004). The experience shows that these values provide good results for a wide range of defined border and free of friction. Once, the following constants were adopted: C1ε = 1.44, C2ε = 1.92, σk = 1.0, σε = 1.3. Pk is the product of turbulence due to viscous forces and fluctuations,

(4)

The turbulent dynamic viscosity, μt, is calculated by combining k and ε as follows:

(5)

where, μt is defined as the turbulent dynamic viscosity and is an empirical constant specified in the turbulence model (approximately 0.09).

For high Reynolds numbers, the rate of kinetic energy dissipation is obtained by multiplying the viscosity with the fluctuating vorticity. An exact equation for the transport of vorticity floating is the rate of dissipation, which can be derived from the Navier-Stokes equations transforming the turbulent kinetic energy and the dissipation rate in Equations (6) and (7).

(6)

(7)

where G and are given by

(8)

(9)

To establish a first image of the turbulent regime, it is consider that the flow rate increases and the impeller rotational speed also rises induced by the gradient along the solid walls and the amounts up the viscous stresses. However, the occurrence of different viscous tensions from point to point, determines the curved trajectories of the flow particles, a phenomenon which increases as they approach to the solid boundaries, given the increased role of concentrated stress gradients.

1.3 Mesh Specification

The success of 3D computational modelling in fluid mechanics requires a special attention during the mesh generation. When a flow passes through a turbine, the turbulence (from the effective viscosity variable in space) plays an important role in the dynamic convection, requiring that in complex flows, the amount of turbulence are duly solved with high precision. Due to the strong interaction between flow and turbulence, numerical results tend to be more susceptible to the grid dependency than for a laminar flow. Thus, it is recommended that the study considers sufficiently thin meshes in regions where occur rapid flow changes and concentrated large tangential tensions. In this way, the use of a mesh generation model (workbench-mesh creation) to describe the volumes, allows the calculation space and the appropriate definition of the boundary conditions (Ansys CFX, 2006).

For the mesh occupied by the flow, it is defined a physical preference in the CFD model and an initial control method, setting mesh defaults changing only one parameter “growth rate” to 1.5 since this value is crucial in the choice of the mesh size, concerning the number of elements and nodes. This model use an advanced size function where all the faces are identified, since the entire turbine, until the ones that correspond to more restrictions in places where the mesh is difficult to create, which usually coincides with rather small volumes. Thus, the mesh created on all sides surrounding the body of the impeller, comprised of the blades, interior bulb, and the shaft to connect the generator, corresponds to 333793 elements and 65103 nodes (Figure 1).

Figure 1

Schematic of the Mesh Created for a Tubular Propeller

The boundary conditions specify the values of characteristic variables in the physical limits of the device. As part of the simulations for each case study, there are four types of boundary conditions: inlet and outlet pressure, rotor or impeller and the tubular wall. Areas designated by impeller are defined as moveable walls, with rotational speed around the rotation shaft, which corresponds to the centre of the runner. In other areas of the field corresponding to solid surfaces is imposed the condition of impermeability and uses the standard wall law for turbulent flow simulations. The faces of the elements belonging to periodic surfaces (the area occupied by the fluid) are treated as inner faces of the domain. All simulations were carried out with the fluid corresponding to water density and constant viscosity, with values of ρ = 998.2 kg/m3 and n = 1.01×10-6 m2/s.

2. BLADE MODEL CONFIGURATION (BMC)

In the design of the impeller blades it is important to analyse different slopes (i.e. angle variation) in order to determine the best results that lead to a best efficiency point (BEP). In the blades design it was adopted a minimal thickness as possible in order to avoid disturbances in the flow, causing additional losses that might constrain its effectiveness. For the flow rates considered in micro turbines, the maximum thickness of 1mm was taken into account for the blades due to limitations of the mesh generation. Figure 2 shows the velocities triangles to take in the optimization of the blade configuration. It shows the parameters are associated to each other from the direction of the blades, as the angle variation by the vectors indicated. Hence, a blade model configuration (BMC) was developed to estimate the best blade orientation to get the best efficiency operating conditions. It is a lengthy process that requires special care and sensitivity analyses to various characteristic parameters associated to the inlet and outlet velocity triangles.

Figure 2

Velocity Vectors in a Blade of a Turbine Propeller

In Figure 2, velocity vectors are identified by the vectors of absolute (v), periphery (c) and relative runner blade velocity (u). From them and according to the shown detail in Figure 2, it is established some essential relationships to calculate the turbine discharge for a given configuration. Knowing the periphery velocity (c) at the inlet and outlet of a blade, which depends on the impeller rotational speed (ω) and the blade radius (r),

(10)

and the absolute velocity (ν) depends on the discharge (Q) that pass through the impeller (Ramos et al., 2009),

(11)

being S the tubular cross section area, re and ri the tip and hub blade radius between the runner periphery and internal bulb, respectively.

According to the angles of a blade on the periphery of the inlet and outlet (subscript 1 and 2, respectively) of the impeller yields the following Equations (12) and (13),

(12)

(13)

To reduce the losses in the turbine, it is assumed the flow at downstream of the impeller is irrotacional, influenced by a vortex formation, depending on the radius of the blade, the flow cross-section and the discharge value as presented in Equation (14),

(14)

In fact as the angle of the blade changes from the inlet to the outlet, between the upstream section, where the flow impulse the blade, to downstream section, the efficiency changes and may lead to better or worse values depending on the angle variation along the blade profile.

Based on the Equations (12) to (14), the blade model configuration determines the angles for a given rotational speed, leading to an optimum performance. The input values known as the head, discharge, rotational speed for the rated conditions, runner diameter, relation between the runner bulb and periphery diameter, the open blade angle (ap) which depends on the number of blades, the angles of the inlet and outlet from the axis to the periphery in each blade are calculated, as well as the power, the specific speed and the constant vortex velocity by Equation (14). Knowing all the data provided by the input conditions, the correct blade configuration from the bulb section (ri) to the periphery (re) (Figure 3) is then determined.

(a)

(b)

Figure 3

Scheme of a Propeller: (a) Plan View with Five Blades and Five Profiles in a Blade; (b) Parameters Associated to the Tracing Profiles in Each Blade

The five profiles (i.e. P1 to P5) in each blade are then drawn from two fundamental equations (Equations (15) and (16)), where, j, represents the number of chosen points required to perform the tracing blade profile; yc and xc are the centre of the blade; x1 and x2 the inlet and outlet coordinates of the blade represented in Figure 3 (b); and rh the radius of the blade curvature.

(15)

(16)

The representation of the profiles on each blade configuration (Figure 3 (a)) corresponds to each line between (ri) and (re), the bulb radius and the runner periphery, respectively.

As the upstream of momentum per unit time is given by Equation (17),

(17)

deriving it, the Equation (18) is then obtained,

(18)

which by algebraic manipulation yields Equation (19),

(19)

After its integration the binary is obtained by Equation (20),

(20)

As the motor or mechanical power is given by Equation (21),

(21)

and the hydraulic power by Equation (22),

(22)

the efficiency is given by Equation (23),

(23)

In order to design the tubular propeller to the available lab conditions, it is considered the outer impeller diameter of 100 mm, with a bulb diameter of 50 mm. According to Table 1 the blades design is built to operate with a discharge of 4 l/s at 300 rpm for the rotational speed.

Table 1

Values for the Blade Profiles Design

(a) Propeller with Five Blades

(b) Propeller with Four Blades

Figure 4

Design of Different Profiles for Each Blade (Impellers Configuration)

To design the blades profile presented in Figure 4 (for five (a) and four (b) blades) the variables presented in Table 1 are used in Equations (15) and (16).

3. CFD analyses

3.1 Performance Curves

For the two designed impellers (with five and four blades) which are characterised by its specific speed Nsqt as stated in Equation (24):

(24)

were developed comparisons between the blade model configuration (BMC) and CFD analyses. Table 2 confirms a good agreement between BMC (for a maximum theoretical efficiency of 100 %) and the CFD simulations, even the existing losses, turbulence effects, anisotropy in zones of high flow circulation, scale effects, which are not considered in the theoretical methodology of the BMC. CFD simulations use, in a first stage, the angles obtained by BMC, but sensitivity investigation regarding the best efficiency operating conditions induces small corrections to those angles as shown in Table 2.

Table 2

Comparison Between Tip and Hub Angles for the Tubular Propeller with Five Blades

Methodology

It is notorious the difference in power values of power between impellers diameter of 100 and 200 mm. This difference allows concluding that for the propeller with five-blades higher discharge values are need.

Two geometrically similar turbines operating at rotational speeds that satisfy the condition presented in Equation (25), have usually different values of efficiency in particular when the relationship between homologous lengths is high.

(25)

This is due to scale effects noticed between the two machines, driven by the effect of viscosity which causes loss of pressure, preventing thus a quadratic variation to the flow velocity. For different rotational speed values and flow conditions, the efficiency are obtained (Figure 5), for the tubular propellers with five and four blades.

Figure 5

Performance Curves of Efficiency and Head Versus Rotational Speed, for a Runner Diameter of 200 mm: (a) with Four Blades; (b) with Five Blades

Figure 6

Performance Curves of Head and Mechanical Power Versus Discharge for a Runner Diameter of 200 mm: (a) with Four Blades; (b) with Five Blades

Figure 7

Performance Curves of Efficiency and Head Versus Rotational Speed, for a Tubular Propeller, with a Diameter of 100 mm: (a) with Four Blades; (b) with Five Blades

Figure 6 shows the curves of head and mechanical power versus discharge for the two impellers analyzed. In Figure 7, the turbine with a smaller runner diameter (D =100 mm) is performed and it is most suitable for small discharge values as happen with small drinking systems as well as in water distribution lab conditions in which the maximum discharge value is around 5 l/s.

Table 3

Reference Values for Tubular Propeller (D = 200 mm)

Table 3 presents some reference values obtained by CFD modeling for an impeller diameter of 200 mm, with 4 and 5 blades. These values represent a wide range of operation for different rotational speed, discharge and head values.

Based on CFD 3D hydrodynamic simulations for small discharge range values, performance curves are obtained based on the following dimensionless parameters:

Discharge number: (26)

Head number: (27)

Power number: (28)

Figure 8 shows the performance curves for head and power number and efficiency versus discharge number variation for the tubular propeller of D =100 mm with four and five blades, respectively.

Figure 8

Comparison of Power and Head Numbers and Efficiency vs Discharge Number for 4 and 5 Blades Propellers of D = 100mm

For the impeller with 4 blades the BEP is obtained for a rotational speed of 300 rpm (Nsqt = 91 rpm (m, m3/s)), a discharge of 4 l/s. The BEP for the propeller with five blades is obtained for a discharge of 3.4 l/s, a rotational speed of 300 rpm (Nsqt = 80 rpm (m, m3/s)).

3.2 Hydrodynamic Behaviour

Established the BEP for the tubular propellers (with four and five blades) based on CFD simulations, detailed analyses are developed in order to better understand the 3D hydrodynamic behaviour of the flow throughout each impeller. For the tubular propeller (D = 100 mm) with five blades and according to a discharge, rotational speed and head, the flow velocities, total pressure, turbulence intensity, wall shear stress and pathlines are presented in Figure 9.

Figure 9

Fluid Performance Inside Tubular Propeller with Five Blades

Figure 10

Fluid Performance Inside Tubular Propeller with Four Blades

This 3D fluid computational analysis considers steady pressurized flow conditions, keeping a constant rotational speed where the singularities reflect an increasing of turbulence. Analysing Figure 9, there are some instabilities in the flow inside the turbine. This is not only due to the rotation of the impeller as it is associated to the circulation flow, making an anisotropic behaviour in different turbine zones, but also the way of the flow enters into the turbine section, through the propeller and leaves with a rotational movement (in vortex configuration) towards the draft tube or downstream pipe.

Given the characteristic curves of the tubular propeller with four blades, and after established the BEP, it is observed a similar behaviour for the velocity, pressure, turbulence, shear stress, and pathlines as showed in Figure 10.

At upstream of the turbine, the flow has a low velocity, with higher pressure values in this region, presenting irrotacional behaviour. However, when it enters in the field of the impeller rotation, the flow becomes a rotational behaviour. At turbine section, the flow goes through the impeller being influenced by the impeller contour inducing the effect of flow separation with significant effects on the turbulence intensity and wall shear stress. It is also noticed the shear stress is higher near the periphery of the blades conferring some significant flow resistance in this zone.

For these tubular turbines are specified four sectioning plans (Figure 11) to analyse the behaviour of the flow in zones where the flow range can vary and where it is needed a better comprehension about the variation of the flow velocity.

Figure 11

Schematic Representations of the Sectioning Plans for Instantaneous Velocity Analysis

In Figure 12 the fluid enters the turbine with an average speed of 0.32 m/s, decreasing as it approaches the tubular walls due to the well known effect of wall friction effect. As it approaches the curve and the impeller, the flow presents asymmetry behaviour in the velocity distribution.

In Figure 13 the velocity distribution shows a similar behaviour. Along the axis the flow tends to be influenced by the shaft rotation inducing the formation of separation zones.

Figure 12

CFD Simulations for the Variation of the Flow Velocity Across Turbine with Five Blades

Figure 13

CFD Simulations for the Variation of the Flow Velocity Across Turbine with Four Blades

Althougth the number of blades are different, in general way the hydrodynamic behaviour is similar. Comparing Figure 12 and Figure 13, there is an agreement associated with the effects of the flow rotation, the friction and the existence of seperation zones, which induce a variation behaviour along the turbine, which is the base of the efficiency variation for different operational conditions.

4. EXPERIMENTAL TESTS

Figure 14 shows the schematic facility for the analysis of the propeller turbines with five and four blades and an impeller diameter of D =100 mm placed in a loop pipe in order to maintain a steady state flow conditions. This setup comprises a pipe system with a pump, for the recirculation, an air vessel to control the pressure at upstream, an electromagnetic flow meter and a downstream reservoir provided with a triangular (90?) weir. There is a valve for the flow control at downstream the air vessel and when it is fully open the maximum possible turbine flow is 5.2 l/s.

Through the turbine upstream curve, the shaft transmits the momentum to a torque balance or a generator.

During the tests it was observed an isotropic behaviour of the flow at upstream of the turbine and an anisotropy through the impeller influenced by the flow rotation and separation of the boundary layer that exists at downstream of the internal impeller bulb. The BEP for the tubular propeller (D =100 mm) under lab conditions is for a rotation speed of 200 rpm (Nsqt = 84 rpm (m, m3/s)), as shown in Figure 15, with dimensionless curves based on head number and efficiency versus discharge number for the impeller with four and five blades, respectively.

Figure 14

Tubular Propeller Installation

Figure 15

Characteristics Curves of Tubular Propeller: (a) with Four Blades; (b) with Five Blades

The behaviour of tubular turbine with five blades (Figure 15 (b)) shows that this turbine is most adequate to operate with higher discharge values that there are not available in the facility.

According to the lab conditions, the experiments are obtained by regulating the discharge control valve, measuring the runner speed in a tachometer Hibok-24 for different flow values measured in an electromagnetic flow meter, and pressure head in transducers at upstream and downstream of the turbine, in undisturbed flow zones. These measurements are then compared with the CFD-3D model simulations. Using an Ultrasonic Doppler Velocimetry (UDV) in the zone of the turbine (Figure 16), the velocity profiles throughout the system are analyzed. With the UDV sample placed on vertical-sloped position of 25?, this device measures the flow velocities allowing the evaluation of the flow behaviour in real time.

Figure 16

Experimental Facility of the Tubular Propeller: UDV (Left); Balance Torque (Center); Rotational Speed Measurement (Right)

Figure 17

Separation of the Boundary Layer and Velocity Profiles

Figure 17 shows different velocity profiles along a runner boundary layer, where they represent the behaviour of the flow separation zone in which the velocity profile inversion tendency is visible.

The most important features to retain in the identification of a turbulent flow are essentially through (i) the flow irregularity by the occurrence of three-dimensional vorticity fluctuations, i.e. the turbulent movements are rotational, (ii) the continuity valid for the turbulent movements, since the smallest scales of these vortices are generally superior to the molecular fluid scale, (iii) the energy dissipation, i.e. the turbulent phenomenon is associated to a significant energy loss, where the turbulence is damped quickly by giving a greater homogeneity and isotropy to the flow motion, (iv) the diffusivity corresponding to a rapid mixing within the fluid domain, followed by transfer of momentum, heat and mass in rapid variations or fluctuations in the flow.

Based on these premises, Figure 18 shows the mean velocity profiles along the turbine for the plans referenced in Figure 11.

Figure 18

Flow Velocity Profiles Obtained by UDV in Sections Represented in Figure 12 (a) to (d)

From these profiles and comparing Figure 18 with Figure 13 a similar behaviour of the fluid is visible, as well as the identification of the section where the separation effect is notorious. When the fluid comes closer to the curve there is certain anisotropy with velocity retardation induced by the shaft rotation, and as soon it passes through the bulb the pressure and velocity decreases induced by the depression existed at downstream the impeller, leading to a separate zone (Ramos et al., 2012). When a fluid moves in the turbulent regime, its domain can be subdivided into two regions, where the movement has its own characteristics: a thin layer near the solid walls in which the tangential stress play an important role (the boundary layer); and the remaining part occupied by the fluid field, where the shear stress is presented with less significance.

5. COMPARISON OF PERFORMANCE CURVES

Dimensionless characteristic parameters of CFD simulations and lab tests were selected and compared as shown in Figure 19, in which H0 and Q0 are the rated values of head and discharge. The comparison of CFD simulations for the two impellers (i.e. with five and four blades) with the lab tests shows typical trends and a reasonable fit in the head performance behaviour.

Figure 19

Comparison Between CFD Simulations and Experimental Results of Tubular Propellers

Figure 20

Performance Curves Between CFD Simulations and Experimental Results: (a) Turbine Tubular Propeller with Five Blades; (b) Turbine Tubular Propeller with Four Blades

Regarding the efficiency values it is noticed a discrepancy justified by scale and losses effects that the CFD does not take into account in the simulations. Figure 20 presents comparisons between efficiency vs specific speed (m, m3/s) curves two rotational speed values (i.e. 70 and 200 rpm). The efficiency values by CFD analyses are higher than the experimental ones, essentially due to negligible factor owing to the friction losses in the mechanical system, such as bearings and seals that CFD codes cannot perform.

In Figure 20 (a), as increasing the rotational speed, there is a higher difference between simulations and tests, due to lab discharge limitations, forcing the propeller with five blades to run out of the optimal operation point. For the propeller with four blades, Figure 20 (b), the lab conditions are much closer to the rated operating point and consequently the results fit better.

CONCLUSIONS

Optimizing analyses for new tubular propellers (with five and four blades) adequate for low-head pipe systems and small discharge values are key solutions of the utmost interest to water companies to supply energy to data acquisition systems, control of the operational management in rural and isolated areas or even supply renewable energy to small regions, where it is very expensive to extend the energy line to these locations. These solutions are also adequate for small pumping systems and water treatment plants. The proposed new tubular propeller turbines (with 4 and 5 blades) represent a cheap, easy installation, good performances and competitive solutions to cover a power, head and discharge values lower than 8 kW, 20 m and 200 l/s respectively, corresponding a range of application not obtainable for existent commercial turbines available in the market.

These devices can be installed at the entrance and the exit of reservoirs or tanks or in some off-grid treatment plants, where are located the most electromechanical equipment which needs energy, or even in pipe systems for water drinking or drainage, where is necessary to provide power to supply control systems or for collect data.

Table 4

Main Characteristics of New Tubular Propeller for Low-Head Solutions

1) CFD analysis for lab conditions; 2) Experimental tests; 3) CFD turbomachine similarity from a propeller with 100 mm

Table 4 shows a summary of the main characteristics of the converters developed in this study which aim at providing small power outputs, usually available in most of the pressurized pipe systems.

A significant range of possible applications is presented in which traditional turbines cannot still cover in a cost-effective manner. These machines are economic solutions, because they are quite simple, normally composed by a runner installed in a pipe-curve, without volute, neither a guide vane. They are appropriate for operation under almost constant-flow conditions, as for water pipe systems equipped with a discharge control valve.

Fluid computational 3D analysis together with a blade model configuration (BMC) and experimental tests help to better understand the phenomenon associated with the hydrodynamic and turbine behaviour, leading a greater knowledge of interaction between the machine geometry, the hydraulic flow conditions and the turbine performance. These developments allow finding the best solution in terms of design, behaviour and configuration, whereby a good basis of calculations has become a point of promising research. This work provides also a good guideline for possible new design of low power turbines in order to highlight the continuity development of new energy converters to support cost-effective micro-hydro solutions.

ACKNOWLEDGMENTS

To projects HYLOW from 7th Framework Programme (grant No. 212423) and FCT (PTDC/ECM/65731/2006) which contributed to the development of this research work, allowing components of computational dynamic and laboratorial investigation associated to the performance of tubular turbines.

REFERENCES

[1] Howey, D. A. (2009). Axial Flux Permanent Magnet Generators for Pico-Hydropower. EWB-UK Research Conference 2009, Hosted by the Royal Academy of Engineering, February 20th.

[2] Simpson, R. G., & Williams, A. A. (2006). Application of Computational Fluid Dynamics to the Design of Pico Propeller Turbines. ICREDC-06, School of Engineering and Applied Sciences, University of the District of Columbia, Washington DC, USA.

[3] Demetriades, G. M. (1997). Design of Low-Cost Propeller Turbines for Standalone Micro-hydroelectric Generation Units (Doctoral Dissertation). University of Nottingham, UK.

[4] Upadhyay, D. (2004). Low Head Turbine Development using Computational Fluid Dynamics (Doctoral Dissertation). University of Nottingham, UK.

[5] Alexander, K. V., Giddens, E. P., & Fuller, A. M. (2009). Axial-Flow Turbines for Microhydro Systems. Elsevier Journal of Renewable Energy, 34, 35-47.

[6] Skotak, A., Mikulasek, J., & Obrovsky, J. (2009). Development of the New High Specific Speed Fixed Blade Turbine Runner. International Journal of Fluid Machinery and Systems, 2(4).

[7] Ramos, H. M., Borga, A., & Sim?o, M. (2009). New Design Solutions for Low-Power Energy Production in Water Pipe Systems. Water Science and Engineering, 2(4), 69-84. doi:10.3882/j.issn.1674-2370.2009.04.007

[8] Ramos, H. M., Borga, A., & Sim?o, M. (2010). Energy Production in Water Supply Systems: Computational Analysis for New Design Solutions. London, Taylor & Francis Group.

[9] Fluent 6.3. (2006). User’s Guide. USA.

[10] Scott-Pomerantz, C. D. (2004). The k-Epsilon Model in the Theory of Turbulence (Doctoral Dissertation). University of Pittsburg, USA.

[11] Ansys CFX 11.0. (2006). Tutorials. PA 15317, South Pointe 275 Technology Drive, Canonsburg.

米尔恩范文第3篇

ISSN Print: 2161-4954

ISSN Online: 2161-4962

Aims & Scope

The World Journal of Nano Science and Engineering contains original and innovative research pertaining to the applications of the physical, chemical and biological sciences to engineering at nano scale. The highest priority is given to scientific research that transcends the classical boundaries and introduces cutting-edge frontiers. Fields of interest include, but are not limited to

· Computational Nanotechnology· Energy at the Nanoscale· Molecular nanotechnology· Molecular Self-Assembly· Nanobiotechnology· Nanodevices· Nanoelectronics· Nanoengineering· Nanolithography· Nanomachining· Nanomaterials· Nanomedicine· Nano-Optics· Nanophysics

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米尔恩范文第4篇

抢劫的前一天晚上，他心满意足地躺在床上，把自己的那把六发毒气枪又仔细检查了一遍。埃米尔自然不打算开枪。如果迫不得已，那么一把毒气枪也足够应付了。“美梦很快就要成真了！”

翌日早晨，闹钟准时在九点整响起。起床后，埃米尔刮了刮胡子，并给自己做了顿早餐：两只鸡蛋、面包和咖啡。吃完早餐，他又重新核对了一遍自己的计划。

十二点左右，运钞车将抵达中央银行。接着，他们会在十五分钟之内把两家公司的全部薪水装进车里。

十一点，埃米尔・雅恩克出门。他一身笔挺西装，正要去火车总站――他已在前天晚上托运了一只箱子过来，这是为那笔巨额现钞准备的。埃米尔从保管站取出箱子后，便往停车场方向走去。他来来回回反复打量着停放在那儿的每一辆汽车，考虑再三后终于选定了其中一辆深蓝色的“欧宝上将”。

又过了一刻钟，他顺利到达实施大计的目的地。他拿出一条工装裤换在身上，一个时髦小伙子便完全变成一副机械工的模样了。一切就绪，估计不会出什么差错了。

埃米尔看了看表，十二点二十分。运钞车此刻肯定已经在路上了。他好比一支即将离弦的弓箭正蓄势待发，可就在这当口，距离他两个车位的前方，一辆绿色的小型福特货运车突然开动了。真见鬼！但愿它赶紧开走，他心里诅咒着。

埃米尔越来越焦躁不安。从市中心开来的运钞车已经到了，但那辆绿色的货运车正慢腾腾地往停车场的出口方向移动。突然，司机迅速发动汽车，但随即又立刻刹住。这时，运钞车司机只得停下车。埃米尔看到，那司机摇下车窗，冲着绿色货运车的司机破口大骂，随后响起了枪声。福特货运车里跳下两名男子，他们把运钞车司机和副手拖出车外，关进货运车厢，接着，只见两辆车同时开走了。

埃米尔・雅恩克呆呆望着驶离的运钞车，他的百万马克就在刚才、在自己的眼皮底下被几个陌生人轻而易举地夺走了！等着瞧吧，我要给你们点儿颜色看看！埃米尔迅速把车开往大街，尾随两辆车行驶了两百米左右的路程，接着便在最近的一间电话亭旁停了下来。他迅速拨通报警电话，一股脑儿地把亲眼目睹的一切告诉了警方。

半小时后，警方逮捕了那三名男子，当时他们正在停车场，准备将那笔巨额现金搬进一辆红色大众汽车。同时，埃米尔也选择了一个安全的位置，远远地注视着这一幕。这是他们应得的下场。谁都不能这么轻而易举地从埃米尔・雅恩克手里夺走本该属于他的东西。

那场袭击行动后的第六个周二，埃米尔・雅恩克去了趟警局，他受到了表彰，获奖九万马克。

米尔恩范文第5篇

小熊维尼的原型是英国动物园里一只擅长表演的黑熊，米尔恩的儿子克里斯多夫·罗宾每次去动物园都要拥抱它，认为它和自己的玩具熊一样可爱。米尔恩用绵绵的父爱书写了这些故事，这些有趣的故事广受欢迎，很快成为世界经典。这些书在英国重版了七十多次，并被译成二十多种语言。迪士尼接拍这部著名的儿童文学作品后，小熊维尼一下子风靡全球，成为全世界儿童的至爱。

小熊维尼的全部故事都发生在百亩森林里，这是一处“长满绿树野草、开满鲜花的土地”。故事里面的人物——小熊维尼、小猪皮杰、老驴伊尔、野兔瑞比、袋鼠小豆都是克里斯多夫·罗宾的玩具。说他们是“玩具”他们可能不太同意，因为他们会思考“怎样去迷惑蜜蜂”这样难度很大的问题，会制定计划去做“捉拿长鼻怪”这样惊心动魄的事情，还会背诗、唱歌、种菜、探险。他们虽然个头、性格迥然不同，但是都以自己鲜明的个性，参与了罗宾多姿多彩的童年生活，怎么能说他们仅仅是“玩具”呢？

米尔恩毫无疑问是所有孩子的父亲。他把每个孩子的特点都写得很清楚，你只要看上三两行就会了解每个孩子的“个性特征”：小熊维尼善良憨厚，特别贪吃；小猪皮杰胆小怕事；野兔瑞比勤劳谨慎，但是性格里明显有些急躁和尖刻；老驴伊尔是一个很消极的朋友；跳跳虎总是不断地跳动以致于干扰别人的生活，而自己一点意识不到。米尔恩似乎特别擅长写这些孩子的“缺点”，事实上在百亩森林发生的很多故事，就是这些孩子们身上的“缺点”在不断地“碰撞”，从而使他们的历险过程显得跌宕起伏。

在“捉拿长鼻怪”这个故事里，贪吃的维尼和胆小的小猪计划捉拿长鼻怪，商量好用维尼最喜爱的蜂蜜做诱饵，放在小猪皮杰挖好的陷阱里。结果贪吃的维尼晚上睡觉的时候惦记着“长鼻怪会给我剩下一点吗”，他“想象着每只长鼻怪好像都是冲着他的蜂蜜来的，而且还把他的蜂蜜都吃光了”，“就这样在床上痛苦地翻来覆去了好长一段时间”，最后实在受不了了，跑到陷阱里，把只剩薄薄一小层的蜂蜜舔了一干二净。

而他那个胆小的朋友小猪皮杰，一睁开眼睛，就“用勇敢而肯定的语气”鼓励自己，然后“用更响亮的声音使劲地高喊”，终于大着胆子来到陷阱旁边。这时候维尼因为“一滴没剩”吃完罐底的蜂蜜，脑袋不幸被困在蜂蜜罐里，“悲伤而绝望地在陷阱里哭喊”。小猪恰好往陷阱里看了一眼，读者都以为这下维尼终于得救了。可是这个胆小的家伙只瞄了一眼，“扯着嗓子”喊了两声“救命”，就“惊恐万分，连忙掉头用最快的速度飞跑了回去”，“一边跑一边大叫着救命”，以致于“脸色发白，舌头发直，不停地喘着粗气，有些口齿不清了”。读者看到这里，忍不住笑出声来。

米尔恩非常擅长描写人物的心理，把他们细微的心理活动描绘得惟妙惟肖。当小熊维尼看到老驴伊尔过生日没有得到礼物而伤心时，准备把自己最心爱的一小罐蜂蜜送给他。小猪皮杰马上接口说：“我能不能也把这个送给他，作为我俩送给他的礼物？”写出了孩子的天真可爱。当伊尔听到小猪皮杰“愧疚而难受”地告诉他准备送给他的生日礼物气球弄破了以后，“两个人都没有说话，很长一段时间的沉默，老驴眼眶里装满了晶莹的泪珠。”过了好长一段时间，老驴终于开口了，他努力挤出了一句话：“唉！我的气球！我的生日气球，没了。”然后他十分怀念地问：“这个气球是什么颜色的——在它还是完整的气球的时候？”“当它还是完整的气球的时候，它有多大？”非常传神地写出伊尔在听说气球破碎后那种失望的心情。在“初涉长鼻怪”这个故事里，维尼和小猪一起谈论陷阱里放什么东西做诱饵，小猪提议放橡子，维尼想到放蜂蜜。这时候，小猪皮杰“脑子里突然闪出一个念头：如果在陷阱里放橡子，那我就得到橡树上去摘橡子；如果放蜂蜜，维尼就得拿出他的蜂蜜，哈哈，那我就省力多了。”于是小猪皮杰说：“那好吧，就放蜂蜜吧。”“其实维尼这时也想到了这个问题，他正准备说‘那好吧，就放橡子吧’，但是他晚了一步，只能无奈地同意了这个决定”。这么细致入微的心理描写，这么传神生动的演绎，让读者都有心头一动的感觉。这些孩童狡黠的“小心眼”，我们恐怕都曾有过吧。从这一点上，这位儿童文学家真是一位心理描写的大师！

我们看到了百亩森林里的孩子们这么明显的缺点，也看到了他们彼此的小心眼，但是没有感觉那是瑕疵或者缺陷，反而感觉他们那么可爱，那么纯真，浑身散发着和我们童年几乎一模一样的气质和情趣。只有米尔恩这样用慈父的柔情和细腻的童心，才能写出这么精彩真实的儿童心灵世界。

在阅读《小熊维尼历险记》系列丛书的时候，读者总是怀疑米尔恩手里有一支点石成金的神笔。因为他讲述的故事情节是那样的简单，那样的生活化，可是他总能将平淡简单的情节变幻得摇曳多姿，妙趣横生。“伊尔的两个生日礼物”里，小熊维尼本来是准备送一小罐蜂蜜的，可是他因为饿了忘记了这是送伊尔的礼物，就吃完了所有的蜂蜜，只拿来一个洗干净的蜂蜜罐子；小猪皮杰送给伊尔的礼物气球在伊尔还没有看到之前就爆炸成碎片了。这样的情节读者都觉得没戏可看了，但是米尔恩天才的点化却有了神来之笔：伊尔觉得这是一个太合适的罐子，“他用牙齿把气球碎片衔了起来，然后小心地把它们放进罐子里”，“然后又把气球碎片从罐子里衔出来，小心地放在地上”。“伊尔带着愉快的心情一次又一次重复着这些动作。”

伊尔的这个动作，使他的两个好朋友都很开心。维尼说：“我真是太高兴了，我能够送你一个这么有用的罐子，可以装东西。”小猪皮杰说：“我也好高兴，我能够送你这么一个东西，你可以把它装进一个有用的罐子里。”“但是伊尔似乎并没有听见他们在说些什么，只是专心地把那些五颜六色的气球碎片衔出来，又放回去……再也没有比这更开心的时候了……”

应该说，再也没有比这更好的结局了！对于孩子来说，快乐是最重要的，就算在你眼中破碎不堪毫无用途的东西，他们同样能从中找到快乐，这就足够了。

米尔恩塑造的“百亩森林世界”不仅环境优美，长满绿树鲜花，而且祥和快乐，自由宽松。米尔恩像一位慈父，精心守护着这片乐园。他对孩子们极为宽容，小熊维尼可以说“亲爱的蜜蜂酿蜜的目的，就是让我去吃它”；小猪皮杰可以说“袋鼠只有在冬天才会变得凶猛”，他们说什么话都是合情合理的，米尔恩默认这些，而且含着微笑点头赞同。他不允许孩子之外的“异声”，不允许有成人的观点去否定孩子们自由发散的思维。在那片冬天里会铺满白雪的百亩森林里，留下了孩子们各式各样的脚印：深的浅的，大的小的，充满疑问的，欢声笑语的，那是他们成长的脚印。

成年人面对大自然的神奇莫测，经历人生的艰苦磨难，会不由自主发出“天问”，问天，问大自然，想穷极宇宙里无尽的奥秘。孩子们在试探着迈出人生第一步的时候，也会发出孩子们的“天问”，他们用的是白雪一样纯净的心灵，发出的是天籁一般稚嫩的疑问。米尔恩给他们尽可能多的自由和关爱，让他们嗅着好奇的空气，踩着想象的空间，做着他们想做的事情。这样，他们的思维才不会被固定，他们的个性才能够充分释放，他们成长的路才会充满主动性和进取心。