Aerodynamic and Structural Characteristics of a Centrifugal Compressor Impeller

In this study, we determined an aerodynamic configuration to design structures applying composites for large-scale horizontal-axis wind turbine blades. A new aerodynamic and structural design method for large wind turbine blades is presented. The rated power of the wind turbine blade is 25 MW class. The tip speed ratio is 7. The diameter of the designed blade is 260 m. Therefore, thick airfoils were selected to design large-scale wind turbine blades considering structural stiffness and maximum lift coefficients. For the aerodynamic design method, it was designed with the optimal angle of attack having the maximum lift-to-drag ratio. The blade element theory and vortex theory were applied to aerodynamic design. For the aerodynamic design results, its validity was investigated via aerodynamic performance analysis. As a result of analyzing aerodynamic performance, it was confirmed that higher power was generated. At 12.5 m/s of rated wind speed, electrical power was 28.32 MW. The structural design considering the aerodynamic design results was carried out. The composite laminate theory was adopted. Structural safety was evaluated for the designed blades. Finally, the structural design results were analyzed as sufficiently valid.

Aerodynamic Performance of a centrifugal fan with additionally installed splitter blades in the impeller has been investigated numerically using three-dimensional Reynolds-averaged Navier-Stokes equations. The shear stress transport turbulence model and hexahedral grids system were used to analyze the flow in the centrifugal fan. From results of the flow analysis, considerable energy loss by flow separation was observed in the blade passages. Splitter blades were applied between the main blades to reduce the loss and enhance fan performance. The chord length ratio of splitter to main blade, the angle between splitter and main blade, and the height ratio of outlet and inlet of impeller were selected as the geometric parameters, and their effects on the aerodynamic performance of the centrifugal fan have been investigated. The performance of the centrifugal fan with added splitter blades was improved conspicuously compared to the centrifugal fan without splitter blades. It was found that the installation of splitter blades in the impeller is effective to improve the aerodynamic performance of a centrifugal fan by reducing the flow separation generated between main blades in the impeller.

The vibration performance of centrifugal impellers is important for pumps and hydraulic excitation is a key source of vibration. The complex internal secondary flow in the centrifugal impeller brings degradation on vibration performances. An attempt of optimization by controlling the thickness distribution of centrifugal impeller blade is given to repress the internal secondary flow and alleviating vibration. The usual method of modifying an impeller on vibration performance is applying splitter blades. In this study, an ordinarily designed impeller is improved by the optimization attempt and the optimized impeller (OPT) is compared with the prototype impeller (PRT) with traditional splitter blades. The vibration performances of the impellers, the PRT, the ordinary impeller (ODN), and the OPT, are investigated numerically and experimentally. Meanwhile, further study on the influence of the thickness distribution optimization on vibration is conducted. There is a relative velocity gradient from suction side (SS) to pressure side (PS) in impeller ODN, causing nonuniformity of energy distribution. By means of thickness distribution optimization, the impeller blade angle on the PS and SS along the blade-aligned streamwise location is, respectively, modified and therefore the flow field can be reordered. The energy transfer in impeller is also redistributed after the modification of blade thickness distribution. What is more, experimental research upon impeller PRT and impeller OPT is also complemented to support the computational fluid dynamics (CFD) results. The experimental results show that the hydraulic performance of the impellers basically agree with the CFD results and the vibration data also proves a better vibration performance of the OPT.

This paper introduces two simplified equations, "Nori formulae," for calculating maximum allowable stress (MAS) in mechanical design engineering. These formulae eliminate the need for traditional safety margins or factors of safety along with factored values across applications ranging from safety pins to spacecraft components. The first formula determines the maximum allowable stress in tension, while the second addresses the maximum allowable stress in shear. The Nori formulae were developed by incorporating fundamental mechanical properties of steel: ultimate tensile strength, yield strength, elongation, and reduction of area—all commonly derived from engineering stress-strain curves in standard tensile testing. This approach effectively utilizes existing global databases of elongation and reduction of area measurements, requiring no additional data generation that would otherwise demand significant time and financial investment. This work addresses a critical gap in the field, as various international design codes and standards currently employ inconsistent design philosophies and safety margins when determining the maximum allowable stress for steel materials.

Transition of amplitude–frequency characteristic in rotor–stator interaction of a pump-turbine with splitter blades

Aerodynamic performance of wind turbine governs the overall energy efficiency, which has been an ever-lasting research focus in the field of wind power technology. Due to the coupling effect among the highly complex environmental and structural uncertainties, the practical aerodynamic performance may not be reliably predicted. To aggravate, this performance declines with time in service. It is of great significance to efficiently and reliably assess the impact of uncertain factors and reduce these influences on wind turbine aerodynamic performance. This paper establishes an uncertainty analysis and robustness optimization model of wind turbine aerodynamic performance considering wind speed and pitch angle error uncertainties. An approach combined the no-instrusive probabilistic collocation method is used, and the blade element momentum theory is applied to quantify influences of variable uncertainties on NREL 5 MW wind turbine aerodynamic performance. The optimization target is to reduce the sensitivity of wind turbine aerodynamic performance to uncertainties, as well as maintain capture power. The results show that the wind turbine aerodynamic and mechanical performance will be greatly affected with uncertain factors. By optimizing and adjusting wind turbine rotor speed and blade pitch angle, the wind turbine rotor power and thrust load variation can be reduced to 9.14% and 9.36%, respectively, which indeed reduces the uncertainty effects.

Energy saving has become a trend as well as a countermeasure for global environmental issues. It can be one of the efficient solutions for reducing energy consumptions by adopting integrally geared compressors instead of inline compressors for industrial plants. The development of key technologies for the design centrifugal compressor such as CFD, FEM, Rotor dynamic Analysis has enabled integrally geared centrifugal compressors for higher pressure applications than ever. A commercial size, 5 stages integrally geared CO2 centrifugal compressor of 20MPaG discharge pressure level was designed, manufactured and evaluated the fluid dynamic performances and rotor stability. It was installed in OEM’s factory and the full load and full pressure test was carried out using real CO2 gas. The actual compressor fluid dynamic performance has shown superior results even at high pressure supercritical gas conditions at the high pressure stages of the compressor. The mechanical performances such as rotor vibration and bearing temperature were also measured and observed that the vibration was substantially low and within safe limits even at high pressure operating conditions above 20MPaG. Also the authors established a novel on Direct Rotor Excitation Method to experimentally investigate the rotor stability even at a high operating pressure level of 20MPaG. With this technique the measurements of the actual rotor natural frequencies and the damping ratios (log decrement) during high pressure operating conditions provided more precise results rather than conventional casing excitation method. The open impeller blade vibration was measured during running condition by using a non-contact blade vibration measurement technique. This measurement results revealed that impeller blades stress against the centrifugal force and the external excitation force was very low compared to the allowable stress value of the impeller material. The fluid dynamic characteristics and mechanical reliability are assessed as acceptable including super critical gas condition of CO2 gas.

The regenerative liquid ring (RLR) pump is a type of rotodynamic pump which combines the mechanical impulses of the impeller with centrifugal force. The head is increased through recirculation, or regeneration, of the fluid in the impeller blades. The design of the regenerative machine has evolved over many decades and improvements have been made in the design, particularly in the shape and number of impeller blades. However, few publications exist on other design aspects such as the radial and axial clearances between the impeller and the casing, and more crucially most researchers tend to investigate these elements in isolation. This highlights the key driver and novelty of this current research, which applies modern Computational Fluid Dynamics (CFD) tools to systematically assess the effects on pump performance over a large range of impeller-casing clearance parameters and understand their operational behaviour. The sizes of the clearances are dictated by the manufacturer’s drawing tolerances on critical pump components. The results of the parametric clearance analysis showed that there could be a potential variation in head performance by as much as 25%. Furthermore, it highlighted the need to experimentally test a number of pumps within a representative tolerance range. Three test pumps were therefore manufactured and tested. A discrepancy between the CFD and experimental results was observed but the experimental results confirmed that variations in clearances have an impact on the performance, particularly on the developed head. This was more noticeable at higher operational speeds where the pressure differential varied by as much as 19%. A one-way Fluid-Structure Interaction analysis of the three test impellers was carried out to shed some light on the operational behaviour of the pumps and the effects on the clearances due to non-uniform loading by importing pressure profiles from the corresponding CFD simulations. Assessing the deformation against the computational and experimental pressure, the results indicate that the maximum allowable radial design clearance should be reduced by 35%. Furthermore, with this radial design tolerance the magnitudes of the drive end and non-drive end clearances have a lower impact on the performance. The RLR pump is unique and requires further comparison studies between experimental testing and computational analysis.

In nuclear power plant, anchor bolts for pressurizer supports are sufficiently used in terms of safety reason, but field inspections have reported that some bolts exceed the limit of their allowable hardness. Because the high level of hardness may lead to failures due to the stress corrosion or fracture toughness, a regular inspection is required for the bolts in nuclear power plant. Thus, this research measures the hardness of bolts currently used in pressurizer supports and then estimates maximum allowable stresses preventing failures by stress corrosion and fracture toughness. Using the ANSYS program, the stresses of the bolts in the regular condition and accidental condition have been calculated, and the possible maximum stress has been compared with the estimated allowable stresses. From the results, the stresses of bolts in the accidental condition satisfy the allowable safety stress from the stress corrosion failure. However, in the future, it shall be needed to consider the reflection of the structure assembling method on the assembling procedure to ensure the pressurizer integrity during maintenance period time.

This work presents the influence of splitters on the performance of small-scale turbopumps for organic Rankine cycle applications numerically. Different design parameters, such as impeller diameter at the outlet, number of blades, blade angle at the trailing edge, and the length and location of the splitter blades, are studied for both shrouded and unshrouded impellers at tip clearance ratios of 0.05 and 0.10 (relative to blade height). The results suggest that splitter blades can increase impeller head coefficient and slip factors up to 10%–24% depending on the blade outlet angle. At the same time, the total efficiency is not influenced significantly. The CFD calculations predict that the placement of splitters in the middle of the blade channel leads to the maximum head coefficient for shrouded impellers. In contrast, for the tip clearance ratio of 0.10, a higher head coefficient is observed for placement closer to the suction side of the main blade. After that, different meridional profiles are studied to investigate their influence on the performance characteristics and mitigation of tip clearance effects. Profiles with a sharper change of cross-section area at the inlet rather than a linear change are found more advantageous, as higher head rise, total efficiency, and static efficiency are observed. In the end, the splitter blade profiles are optimized through techniques of design of experiments, leading to a 2.43% improvement in total efficiency. In contrast, the head rise remains constant, and static efficiency decreases by 0.49% for an impeller with a tip clearance ratio of 0.05.

In order to extend the operating range of a centrifugal compressor, inclined discrete cavities located upstream of the impeller leading edge were optimized in this work. Aerodynamic performance analysis was performed using three-dimensional Reynolds-averaged Navier-Stokes equations with the shear stress transport turbulence model. A parametric study on aerodynamic performances of the centrifugal compressor with the inclined discrete cavities was conducted with six geometrical parameters. Through the parametric study, three geometric parameters were selected as design variables for optimization. Peak adiabatic efficiency and stall margin were selected as objective functions. The Latin hypercube sampling method was used to select the design points, and the radial basis neural network was used to construct surrogate models of the objective functions. A hybrid method combining the particle swarm optimization showed better overall performance in finding global optimum than the genetic algorithm. Pareto-optimal solutions provided the designs which enhance considerably both the performance parameters compared to the reference design.

In order to study the effect of different numbers of impeller blades on the performance of mixed-flow pump “saddle zone”, the external characteristic test and numerical simulation of mixed-flow pumps with three different impeller blade numbers were carried out. Based on high-precision numerical prediction, the internal flow field and tip leakage flow field of mixed flow pump under design conditions and stall conditions are investigated. By studying the vorticity transport in the stall flow field, the specific location of the high loss area inside the mixed flow pump impeller with different numbers of blades is located. The research results show that the increase in the number of impeller blades improve the pump head and efficiency under design conditions. Compared to the 4-blade impeller, the head and efficiency of the 5-blade impeller are increased by 5.4% and 21.9% respectively. However, the increase in the number of blades also leads to the widening of the “saddle area” of the mixed-flow pump, which leads to the early occurrence of stall and increases the instability of the mixed-flow pump. As the mixed-flow pump enters the stall condition, the inlet of the mixed-flow pump has a spiral swirl structure near the end wall for different blade numbers, but the depth and range of the swirling flow are different due to the change in the number of blades. At the same time, the change in the number of blades also makes the flow angle at 75% span change significantly, but the flow angle at 95% span is not much different because the tip leakage flow recirculates at the leading edge. Through the analysis of the vorticity transport results in the impeller with different numbers of blades, it is found that the reasons for the increase in the values of the vorticity transport in the stall condition are mainly impacted by the swirl flow at the impeller inlet, the tip leakage flow at the leading edge and the increased unsteady flow structures.

Generation of allowable stress for indigenously developed 304HCu Stainless steel

Building codes generally specify maximum allowable compressive stresses for basic pile materials that control the structural design of piles. Sometimes these prescriptive type provisions are qualified by performance type provisions that permit such allowable stresses to be exceeded under controlled conditions. Combined prescriptive and performance regulations governing pile foundation design and installation is recommended because totally prescriptive or totally performance type provisions are impractical under the present state of the art. A relatively high structural factor of safety must be reflected in allowable stresses for pile material to provide for the many uncertainties currently existing in pile foundation design and installation and the several conditions under which the actual structural factor of safety can be less than assumed. Until we have more precise answers to all of our problems in designing and installing satisfactory pile foundations, any move to reduce the structural factors of safety below current levels would be ill advised. An examination of the current allowable stresses and proposed increases indicates that under today's state of the art, a practical limit has been reached or in some cases exceeded and proposed increases are generally unwarranted or undesirable.

To more fully explore the effect of blade numbers on aerodynamic performance and noise of small axial flow fan, some solutions are adopted to obtain the parameters’ distribution of the flow field.Firstly, the standard k-ε turbulence model is used to calculate the steady flow field of six different fan blades(such as 5,7,9,11,13,15) , and the SIMPLE algorithm is applied to couple vecolity and pressure. Secondly, the large eddy simulation in conjunction with the FH-W noise model are used to compute the unsteady flow field and noise. Finally, the experimental results verify that the calculation methods of steady flow field and unsteady flow field are correct. The conclusions show: (1)Total pressure and efficiency generally maintain the trend of firstly increasing and then decreasing with increasing the blade numbers, and it is the greatest when fan blade number is 11. The flow rate coupled with the maximum efficiency has never changed with increasing the blade numbers. (2)With the increasing blades, overall sound pressure level of the aerodynamic noise is gradually decreasing near the outlet of fan tip, while it is firstly decreasing and then increasing before decreasing again 1 meter away from the central axis of the impeller along the outlet. When fan blade number is 11, overall sound pressure level of the aerodynamic noise is the greatest.