Numerical investigation on influence of diffuser vane height of centrifugal pump

Vaned diffusers are extensively used in centrifugal pumps, but the influence of vane height on internal flow field and overall performance is not explicit. This paper mainly presents numerical investigation on influence mechanism of diffuser vane height in a single-stage centrifugal pump. The head values were carried out on a low specific speed centrifugal pump equipped with different diffuser vane height by numerical simulation and experimental method. And the deviation between numerical results and experimental results were < 5%. The diffuser vane height h/b ratio is changed as 0, 0.3, 0.4, 0.5, 0.6, 0.8, and 1 in this study. The numerical analysis shows that reducing diffuser vane height could eliminate the vortex which appears at tongue region. Meanwhile, the influence of rotor-stator interaction was reduced by reducing the vane height. Consequently, the energy loss in the volute and the diffuser could both be decreased at design flow point and over flow point. In the other hand, the circumferential velocity at partial flow point gets larger which could lead to large frictional loss. In general, reducing the diffuser vane height at design and over flow point could improve the output work of impeller.     

Numerical simulations of the performance of solar fired flash pyrolysis reactors

Numerical simulations of free falling and entrained flow, solar fired, biomass flash pyrolysis reactors are described. Good agreement with experimental results was obtained. Magnification factors exceeding 5 were predicted for some experimental conditions.     

Numerical and experimental investigations on effect of fan height on the performance of piezoelectric fan in microelectronic cooling

A piezoelectric fan is an attractive device to remove heat from microelectronic systems due to its low power consumption, minimal noise and compactness. In the present study, a piezoelectric fan is investigated to analyze the cooling capability for possible use in electronic devices. Both numerical and experimental analyses are carried out on the piezoelectric fan which was oriented horizontally. The FLUENT 6.3 software is used in the 2D simulation to predict the heat transfer coefficient and the flow fields using a dynamic mesh option to observe the fan swinging phenomena. Two heat sources in in-line arrangement are used in the experiment. The flow measurements are carried out at different piezoelectric fan heights by using a particle image velocimetry (PIV) system. The result shows that the piezofan height of h_p/l_p = 0.23 can reduce the temperature of the heat source surface as much as 68.9 °C. The numerical and experimental values of heat transfer coefficients are plotted and found in good agreement.     

Numerical study of the aerodynamic performance of blunt trailing-edge airfoil considering the sensitive roughness height

For rough wind turbine airfoil and its blunt trailing-edge modification, the aerodynamic performance has been numerically investigated to facilitate a greater understanding of the effects of the blunt trailing-edge modification on the aerodynamic performance enhancement of airfoil with sensitive roughness height. The S834 airfoil from National Renewable Energy Laboratory is used for the simulation. The lift and drag coefficients of S834 airfoil with smooth or rough surface are calculated by the k-ω SST turbulence model, and are compared with wind tunnel test results. The aerodynamic performance of airfoils with different roughness heights is studied to obtain the sensitive roughness heights of suction and pressure surfaces. The mathematical expression of the blunt trailing-edge airfoil profile is established using the coordinate's rotation combined with the zoom coefficient of coordinate. Then, the S834 airfoil with sensitive roughness height is modified to be symmetrical blunt trailing-edge modification, and the lift and drag coefficients and the lift-drag ratio are also calculated and analyzed. Results indicate that the sensitive roughness height of suction surface is 0.5 mm, and the pressure surface is insensitive to the roughness height. Through the blunt trailing-edge modification, the lift coefficient and the maximum lift-drag ratio obviously increase for rough airfoil, and the sensitivity of airfoil to roughness height is reduced. The research provides significant guidance for designing the wind turbine airfoil under conditions of rough blade.     

On the dependence of thermosyphonic solar system performance on collectors' azimuthal orientation

Thermosyphonic solar hot water systems with off-south facing collectors are investigated. Each of the systems has two solar collectors which are either facing the same or opposite direction. In the second configuration, a different solar radiation distribution is seen by each of the collectors.The performance of each of the systems was numerically simulated and mixed tank temperature as well as bulk efficiencies are reported.The conventional south facing system is used as a reference by which all other systems are measured, thus, reported results are expected not to be too sensitive to the latitude of the system's location.Simulations indicate that system performance is more dependent on azimuthal orientation during the winter than the summer. Tools are provided to enable the designer of an off-south system to evaluate necessary collectors area if the equivalent south system is known.     

Numerical simulations on performance enhancement of a cross-flow latent thermal energy storage heat exchanger

基于列管式换热器具有传热面积大、结构紧凑、操作弹性大等优点,使其在相变储能领域具有广阔的应用前景。本文建立一种新型列管式相变蓄热器模型,在不考虑自然对流的情况下,利用Fluent软件对相变蓄热器进行二维储热过程的数值模拟。本文主要研究斯蒂芬数、雷诺数、列管排列方式、肋片数以及相变材料的导热系数对熔化过程的影响,并对熔化过程中固液分界面的移动规律进行了分析。模拟结果表明,内肋片强化换热效果明显,特别是对应用低导热系数相变材料[导热系数小于1 W/(m·K)]的列管式蓄热器,相对于无肋片结构,加入肋片（N_fn=2）可缩短熔化时间52.6%。     

Effect of diffuser vane shape on the performance of a centrifugal compressor stage

The present paper reports the results of experimental investigations on the effect of diffuser vane shape on the performance of a centrifugal compressor stage.These studies were conducted on the chosen stage having a backward curved impeller of 500 mm tip diameter and 24.5 mm width and its design flow coefficient is Фd=0.0535.Three different low solidity diffuser vane shapes namely uncambered aerofoil,constant thickness flat plate and circular arc cambered constant thickness plate were chosen as the variants for diffuser vane shape and all the three shapes have the same thickness to chord ratio(t/c=0.1).Flow coefficient,polytropic efficiency,total head coefficient,power coefficient and static pressure recovery coefficient were chosen as the parameters for evaluating the effect of diffuser vane shape on the stage performance.The results show that there is reasonable improvement in stage efficiency and total head coefficient with the use of the chosen diffuser vane shapes as compared to conventional vaneless diffuser.It is also noticed that the aero foil shaped LSD has shown better performance when compared to flat plate and circular arc profiles.The aerofoil vane shape of the diffuser blade is seen to be tolerant over a considerable range of incidence.     

Numerical simulations on the stability of spherical flame structures

The paper presents investigations concerning the stability of spherical flames in a premixed lean hydrogen-air atmosphere and their evolution in case of instabilities. This is done by means of numerical simulations using the thermo-diffusive model with one-step finite rate chemical reaction and radiative heat loss under optically thin conditions. In the first part spherical symmetry is imposed leading to a one-dimensional problem. The results obtained in this way are compared with the asymptotic analysis and the numerical simulations from the literature. In the second part these solutions are employed as initial conditions for fully three-dimensional simulations using a high-resolution pseudo-spectral method. It allows the investigation of the nonlinear transient behavior of spherical flames with respect to three-dimensional perturbations. Different scenarios of their evolution are observed: extinction, spherical growth, and splitting. Also, for the first time, a steady flame ball is computed in a three-dimensional simulation. The different numerical and physical issues are discussed in detail and are related to available experimental observations as well as to theoretical analyses.     

Influence of the wakes of rotating spokes on the performance of a turbine exhaust diffuser

The design of turbine exhaust diffusers is still commonly based upon conservative design charts which do not consider the influence of the components installed upstream such as the last turbine stage.Based on investigations with different spoke wheels which approximate the blade wakes of the last turbine stage,the influence of a rotating bladed disc on the flow in a exhaust diffuser is presented.The pressure recovery for an annual diffuser with 15° half cone angle and the 5 mm spoke wheel used in the model test is very high and nearly independent on the rotational speed and the corresponding Strouhal number.For a 20° annual diffuser,the pressure recovery increases with Strouhal number     

Numerical simulations of the energy performance of a PEM water electrolysis based high-pressure hydrogen refueling station

The present paper proposes an energy analysis on a hydrogen production and storage system. The dynamic and multi-physical investigation is inherent to an energy system composed of a PEM electrolyzer, a diaphragm compressor, a gaseous storage system, ancillaries, and control procedures. To perform this investigation, a model previously developed by the authors has been used. The case-study simulation shows how the electrolyzer has the predominant rate, accounting for 88.5% of the daily 24-h energy demand (13 MWh). The electrolyzer specific energy consumption for 205 kg of hydrogen generated resulted to be 56.3 kWh/kg. The other components have required 1.5 MWh, with a specific energy of about 7.5 kWh/kg. The overall system efficiency resulted to be 52.9%, including all the components and their energy consumption and guaranteeing 14.9 metric tons of avoided carbon dioxide.     

Computational and experimental study of pinch on the performance of a vaneless diffuser in a centrifugal compressor

This study focuses on the vaneless diffuser of a centrifugal compressor.The examined stage consists of an un-shrouded impeller,a parallel wall vaneless diffuser and a volute.The walls of the diffuser were movable allowingdifferent pinch configurations to be investigated.The baseline geometry had no pinch i.e.the height of the dif-fuser was equal to the height of the impeller flow channel plus the axial running clearance.The work consists ofboth numerical and experimental parts.Quasi-steady, turbulent,fully 3D numerical simulations were conducted.The inlet cone,rotor and diffuser were modelled.Six different configurations were studied.The height of thepinch was altered and the pinch made to different walls was tested.Two of the numerically studied cases werealso experimentally investigated.The overall performance of the compressor,the circumferential static and totalpressure and the spanwise total pressure distribution before and after the diffuser were measured.The numericaland experimental studies showed that the pinch improved the efficiency of the compressor.     

Investigation into the influence of diffuser stagger angles on the flow field and performance of a centrifugal compressor

The performance graphs of a centrifugal compressor under different diffuser stagger angles were measured, and the influence of different stagger angles of vanes on the stage performance as well as flow field was investigated numerically. The results show that the performance graph shifts when the diffuser stagger angle is altered; the influence of different stagger angles of vanes on the flow field inside the centrifugal compressor is great. Large scale vortices appear on the diffuser vane under larger incidence. An optimal diffuser stagger angle should exist at a specified flow rate to make the characteristic of the flow optimized. The incidence corresponding to the highest efficiency is not 0° at the design condition. Translated from Journal of Engineering Thermophysics, 2006, 27(1): 61–64 [译自: 工程热物理学报]     

The effect of material difference and flange nominal size on the sealing performance of new gasketless flanges

This paper deals with a new seal system between flange joints without using a gasket. This gasketless flange includes a groove and an annular lip that is machined in one of the flange rings which when removed being in contact with the other flange to form a seal line when the flanges are assembled. In this study, firstly, fundamental dimensions are examined for unplasticized polyvinyl chloride (PVC-U JIS) to obtain the best sealing performance. Then, the effects of material difference and flange nominal size upon the sealing performance of the new gasketless flange are investigated for two types of materials, 0.25% carbon steel (S25C JIS) and PVC-U. It is found that the critical internal pressure at which leakage appears is mainly controlled by the maximum stress at the annular lip for each material even if the flange nominal sizes are different. The gasketless flange made by PVC-U shows the higher critical internal pressure compared with the case of S25C if the same clamping forces are applied. The effect of stress relaxation for PVC-U on the sealing performance is also considered. Then, it may be concluded that this PVC-U gasketless flange as well as S25C has good sealing performance.     

Numerical study on air-side performance of an integrated fin and micro-channel heat exchanger

A new type of aluminum heat exchanger with integrated fin and micro-channel has been proposed. The air-side heat transfer and flow characteristics of the integrated fin and micro-channel heat exchanger are systematically analyzed by a 3D numerical simulation. The effect of flow depth, fin height, fin pitch and fin thickness at different Reynolds number is evaluated by calculating Colburn factor j and Fanning friction factor f. A parametric study method is used to analyze the fin designed parameters affecting the performance of the heat exchanger. The results show that the contribution ratio of the fin geometries in descending order is flow depth, fin pitch, fin height and fin thickness. The air-side performance of the integrated fin and micro-channel heat exchanger is compared with that of the multi-louver fin micro-channel heat exchanger and the wavy fin micro-channel heat exchanger.     

On the temperature dependence of photovoltaic module electrical performance: A review of efficiency/power correlations

A brief discussion is presented regarding the operating temperature of one-sun commercial grade silicon-based solar cells/modules and its effect upon the electrical performance of photovoltaic installations. Suitable tabulations are given for most of the known algebraic forms which express the temperature dependence of solar electrical efficiency and, equivalently, solar power. Finally, the thermal aspects of the major power/energy rating methods are briefly discussed.     

Numerical simulations and physical analysis of an overexpanded reactive gas flow in a planar nozzle

A numerical investigation of the overexpanded reactive gas flow in the ATAC nozzle of rectangular section, equipped with an H₂ injection, is carried out. This study is aiming at reproducing the reacting phenomena occurring when air engulfed in the separated region at the tip of the extension meets the H₂ rich overexpanded flow. Both steady Reynolds Averaged Navier Stokes (RANS) and unsteady Delayed Detached Eddy Simulation (DDES) approaches are used to model turbulence and compared to each other. The effect of the chemical nature of the mixture (either frozen or reacting) on the flow dynamics is discussed. The computations are compared to wall pressure measurements and flowfield visualizations by instantaneous Planar Laser Induced Fluorescence of OH (PLIF-OH) and OH spontaneous emission. A self-sustained flow oscillation at a frequency of around 1 kHz is found numerically, in good agreement with the unsteady wall pressure measurements. An analysis of the space–time characteristics of the propagating disturbances contributes to a better understanding of this phenomenon.     

Numerical Analysis and PIV Measurements of Tip Vortex in an Axial Rotor with Skewed-Swept Blade at Its Leading Edge

Based on the characteristics of axial fans of outdoor units of centralized air-conditioners, using the finite-volume method, applying three-dimensional steady Reynolds-averaged Navier-Stokes equations coupled with Spalart-Allmaras turbulence model equation, and adopting SIMPLE algorithm, numerical analysis is made and applied to analyze the internal flow field of axial rotors with skewed-swept blade at its leading edge. This numerical simulation mainly investigates the formation and development of the tip vortex. Based on the tip vortex characteristics that have been captured, the internal flow numerical results are compared with those obtained by the PIV experiments. This comparison indicates a good agreement between numerical results and PIV results, thus proving the validity of the numerical simulation. In addition, based on the internal flow analyses of the axial rotor with skewed-swept blade at its leading edge, different flow phenomena features are presented. These flow features can be used for further improvements of the present rotor performance characteristics.     

A study on the sealing performance of bolted flange joints with gaskets using finite element analysis

Gaskets play an important role in the sealing performance of bolted flange joints, and their behaviour is complex due to nonlinear material properties combined with permanent deformation. The variation of contact stresses due to the rotation of the flange and the material properties of the gasket play important roles in achieving a leak proof joint. In this paper, a three-dimensional finite element analysis (FEA) of bolted flange joints has been carried out by taking experimentally obtained loading and unloading characteristics of the gaskets. Analysis shows that the distribution of contact stress has a more dominant effect on sealing performance than the limit on flange rotation specified by ASME.     

Numerical simulations of the equilibrium shape of liquid droplets on gradient surfaces

The equilibrium shape of liquid droplets on horizontal and inclined plates that have a surface energy gradient is simulated numerically by applying a finite element method based on the principle of energy minimum in the present study. The numerical results show that the liquid droplet shape changes with locations under the influence of the unbalanced surface tension created by the gradient surface. It is shown that the contact angle reaches the maximum value at the one end of the droplet (2D), but it becomes minimum at the other end; the triple-phase contact line deforms toward the region with a smaller contact angle. It is further shown that the length of the liquid droplet increases with an increase in the surface energy gradient on the surface. More interestingly, an inflexion point appears when the droplet length varies with the center contact angle of the droplet, where the liquid droplet just locates at the transition region from the hydrophilic side to the hydrophobic side. It shifts to the hydrophilic side with the increase in the surface energy gradient. On the inclined gradient surface, the gravity induces a significant deformation of the equilibrium droplet shape towards the bottom of the surface. And the surface energy gradient further enhances the deformation when the unbalanced surface tension is directed to the bottom of the surface. However, the droplet shrinks back when the unbalanced surface tension is opposite to the component of gravity.