Search Results (25)

The article analyses the use of servomotors in the control systems of industrial equipment, focusing on the alternative offered by position and speed synchronization in relation to classical mechanical mechanisms. A complete methodology is presented to determine the dynamic parameters of the adopted kinematic system using electronic motion profiles. The results obtained constitute a mathematical model of the execution chain and an analysis of the basic quantities for linear motion, supported by actual measurements of the drive parameters. The merit of the article is to show that the servomotors can significantly simplify the design of the device, make it more flexible in adaptation to different assortments, and allow integration with systems predicting the technical condition of the device. The analysis of the results revealed significant differences in the constant rotational speed of the servomotor, which do not align with previous findings. The results suggest that changing the angular working range of the assembly to the range (205°;270°) could significantly affect the generated linear acceleration, reducing the risk of stalling. The calculations and graphs conducted allowed for the accurate representation of the actual mechanical system, considering its dynamic characteristics. The key conclusion is that precise mathematical modelling is essential to ensure the stability and durability of engineering components. Full article

This study investigates a dual-stage axial-flow fan within a specific power plant context. Numerical simulations encompassing both steady-state and stall conditions were conducted utilizing the Reynolds-averaged Navier–Stokes (RANS) equations coupled with the Realizable k–ε turbulence model. The findings reveal that, under normal operating conditions, there exists a positive correlation between the mass flow rate and outlet pressure with gas density while displaying a negative correlation with dynamic viscosity. Regardless of the changes in air density, the volumetric flow rate at the maximum outlet pressure of the fan remains essentially the same. When a stall occurs, the volumetric flow rate rapidly decreases to a specific value and then decreases slowly. The analysis of the three-dimensional flow field within the first-stage rotor was performed before and after the rotational stall occurrence. Notably, stall inception predominantly manifests at the blade tip. As the flow rate diminishes, the leakage area at the blade tip within a passage expands, directing the trajectory of the leakage vortex toward the leading edge of the blade. Upon reaching a critical flow rate, the backflow induced by the blade tip leakage vortex obstructs the entire passage at the blade tip, progressively evolving into a stall cell, thereby affecting flow within both passages concurrently. Full article

Understanding the flow instabilities encountered by the turbocharger compressor is an important step toward improving its overall design for performance and efficiency. While an experimental study using Particle Image Velocimetry was previously conducted to examine the flow field at the inlet of the turbocharger compressor, the present work complements that effort by analyzing the flow structures leading to stall instability within the same impeller. Experimentally validated three-dimensional computational fluid dynamics predictions are carried out at three discrete mass flow rates, including 77 g/s (stable, maximum flow condition), 57 g/s (near peak efficiency), and 30 g/s (with strong reverse flow from the impeller) at a fixed rotational speed of 80,000 rpm. Large stationary stall cells were observed deep within the impeller at 30 g/s, occupying a significant portion of the blade passage near the shroud between the suction surface of the main blades and the pressure surface of the splitter blades. These stall cells are mainly created when a substantial portion of the inlet core flow is unable to follow the impeller’s axial to radial bend against the adverse pressure gradient and becomes entrained by the reverse flow and the tip leakage flow, giving rise to a region of low-momentum fluid in its wake. This phenomenon was observed to a lesser extent at 57 g/s and was completely absent at 77 g/s. On the other hand, the inducer rotating stall was found to be most dominant at 57 g/s. The entrainment of the tip leakage flow by the core flow moving into the impeller, leading to the generation of an unstable, wavy shear layer at the inducer plane, was instrumental in the generation of rotating stall. The present analyses provide a detailed characterization of both stationary and rotating stall cells and demonstrate the physics behind their formation, as well as their effect on compressor efficiency. The study also characterizes the entropy generation within the impeller under different operating conditions. While at 77 g/s, the entropy generation is mostly concentrated near the shroud of the impeller with the core flow being almost isentropic, at 30 g/s, there is a significant increase in the area within the blade passage that shows elevated entropy production. The tip leakage flow, its interaction with the blades and the core forward flow, and the reverse flow within the impeller are found to be the major sources of irreversibilities. Full article

The accurate prediction of rotating stall inception is critical for determining the stable operating regime of a compressor. Among the two widely accepted pathways to stall, namely, modal and spike, the former is plausibly believed to originate from a global linear instability, and experiments have partially confirmed it. As for the latter, recent computational and experimental findings have shown it to exhibit itself as a rapidly amplified flow perturbation. However, rigorous analysis has yet to be performed to prove that this is due to global linear instability. In this work, an eigenanalysis approach is used to investigate the rotating stall inception of a transonic annular cascade. Steady analyses were performed to compute the performance characteristics at a given rotational speed. A numerical stall boundary was first estimated based on the residual convergence behavior of the steady solver. Eigenanalyses were then performed for flow solutions at a few near-stall points to determine their global linear stability. Once the relevant unstable modes were identified according to the signs of real parts of eigenvalues, they were examined in detail to understand the flow destabilizing mechanism. Furthermore, time-accurate unsteady simulations were performed to verify the obtained eigenvalues and eigenvectors. The eigenanalysis results reveal that at the rotating stall inception condition, multiple unstable modes appear almost simultaneously with a leading mode that grows most rapidly. In addition, it was found that the unstable modes are continuous in their nodal diameters, and are members of a particular family of modes typical of a dynamic system with cyclic symmetries. This is the first time such an interesting structure of the unstable modes is found numerically, which to some extent explains the rich and complex results constantly observed from experiments but have never been consistently explained. The verified eigenanalysis method can be used to predict the onset of a rotating stall with a CPU time cost orders of magnitude lower than time-accurate simulations, thus making compressor stall onset prediction based on the global linear instability approach feasible in engineering practice. Full article

Compressors are an essential component of aircraft engines. Their design and operation must be extremely reliable as engine safety and performance depend greatly on these elements. Axial compressors exhibit instabilities, such as surge or rotating stall, in a region close to the peak of their performance curves. These fluid dynamic instabilities can cause drops in efficiency, stress on the blades, fatigue, and even failures. Compressors are handled therefore by operating with a safety margin far from the surge line. Moreover, models able to predict onset instabilities and to reproduce them are of great interest. A dynamic system able to describe successfully both surge and rotating stall is the model presented by Moore and Greitzer That model has also been used for developing control laws of the compressor dynamics. The present work aims at developing an artificial neural network (ANN) approach able to predict either the permanence of the system in stable working condition or the onset instabilities from a time sequence of the compressor dynamics. Different solutions were tried to find the most suitable model for identifying the system, as well as the effects of the duration of the time sequence on the accuracy of the predicted compressor working conditions. The network was further tried for sequences with different initial values in order to perform a system analysis that included multiple variations from the initial database. The results show how it is possible to identify with high accuracy both rotating stall and surge with the ANN approach. Moreover, the presence of an underlying fluid dynamic model shares some similarities with physically informed AI procedures. Full article

Rotational stall is an unstable flow phenomenon that reduces the performance of centrifugal pumps, usually occurring under partial load conditions. It causes instability in the flow resulting in intense vibrations and noise under certain flow conditions. In this study, the one-equation Wray–Agarwal (WA) turbulence model, which was recently developed, is employed to numerically simulate the internal flow field of a centrifugal pump under the deep stall condition. The aim of this study is to examine the prediction accuracy for stall by using the WA turbulence model. The method based on computational fluid dynamics (CFD) has been widely applied for investigation of complex flow patterns in pumps by solving Reynolds-averaged Navier-Stokes (RANS) equations. Particle image velocimetry (PIV) experimental results were compared with simulations predicted using the WA, renormalization group (RNG) k−ε, shear stress transport (SST) k−ω, and realizable k−ε turbulence models and large eddy simulations (LES). The comparisons indicated that the WA turbulence model can accurately predict the flow separation and has a good agreement with the PIV data. The WA model adds a cross-diffusion term and a blending function to the eddy viscosity R equation, so that this model could be expressed as a one-equation k−ω model or one-equation k−ε model as needed by using the switching function. The results show the strong potential of the WA model for accurately computing the stall in rotating fluid machinery. The outcomes of the study are useful in development and optimization of fluid machinery with a low calculation cost and a high accuracy. Full article

This paper proposes a stage flow parameter analytical model for rapid evaluation of a counter-rotating compressor’s performance for design optimization and its application in the design of a transonic counter-rotating fan. In the first part, the velocity diagram method, considering the influence of flow-path geometry variation for enhancing the accuracy, is used to correlate the aerodynamic parameter between the inlet guide vane (IGV), the upstream rotor (R1), and the downstream CR rotor (R2). A profile loss correlation based on Lieblein’s diffusion factor and a shock loss model from a high-speed fan database are incorporated for predicting the rotor efficiency. In the second part, to verify its effectiveness, the analytical model is used for aiding in the aerodynamic design of a transonic CR fan for indicating the optimized combination of design parameters for good efficiency and a high pressure ratio. According to the analytical model and the simulation results, the final selected samples have higher efficiency, with a moderate pressure ratio (0.949/2.67, 0.890/2.99, and 0.841/2.99 for R = 0.1, 0.5, and 0.9, respectively). Finally, the aerodynamic characteristics of the designed transonic CR fan at a relative rotating speed of N = 1.05~0.8 are calculated by using the CFD software Numeca. Simulations indicate that the designed transonic CR fan has a pressure ratio of 2.76, with an efficiency of 0.8405 at the design point, and the efficiency is maintained above 0.821 with a stall margin of 13.3% for N=1.0. The maximum pressure ratio of this CR fan reaches 3.08 and 3.36 for N = 1.0 and 1.05, respectively. If used to provide thrust, calculations indicate that the thrust of this transonic CR fan is 71.8, 65.9, and 35.8 kN for N = 1.05, 1.0, and 0.8 at the near-choke point for the sea-level condition. Full article

Unsteady pressure-pulsation-induced severe vibration and high alternating stress can cause some unexpected results, including impeller crack and structural damage of the entire pumping system. In the present paper, a review on pressure pulsations in pumps is carried out based on the published studies. A comprehensive view on pressure pulsations from several aspects is discussed in detail. The contents of the studies include the mechanism of rotor–stator interaction; a prediction model and experimental and numerical investigations of pressure pulsations; unsteady rotating-stall-induced pressure pulsations at off-design flow rates; the relationship between pressure pulsation and the internal flow structure and cavitation; and the reduction in pressure pulsation caused by some effective approaches. It is concluded that unsteady pressure pulsation in pumps is closely associated with complex flow structures, for instance flow separation, cavitation, and rotating stall. The rotor–stator interaction mainly dominates pressure pulsation characterized by the discrete components in pressure spectrum. To reduce pressure pulsation, some effective approaches are proposed, such as increasing the rotor–stator gap, staggered blade, and blade modification. Finally, several suggestions for future works are given and discussed considering the current research. The review contributes to better understanding of pressure pulsations in centrifugal pumps, and may also benefit engineers in controlling pressure pulsations in some fields, such as pumps in nuclear reactor. Full article

The accumulation of ice on aircraft is a typical meteorological issue. The ice accretion on the wing’s leading edge can cause an earlier stall and significantly increase the safety risks. Because the equivalent shape of the wing will change based on the ice pattern on the leading edge, it is crucial to predict the ice pattern of the aircraft and design the anti-icing device. The ice accretion is predicted in the present work through a multi-shot approach. In the current study, a bio-inspired leading edge that can generate multiple pairs of counter-rotating vortices is used to alter the trajectory of the water droplets. This results in a lowering of the ratio of droplet attachment on the leading edge, hence and the ice accretion time, which is an indication of hazardous flight conditions, can be delayed. As a result, the spanwise continuous ice transforms into the discontinuous ice. Meanwhile, the Procrustes analysis provides a result for the thickness of the ice pattern on the wing model based on a variety of parameters for the leading edge. Full article

Wind turbine blades experience excessive load due to inaccuracies in the prediction of aerodynamic loads by conventional methods during design, leading to structural failure. The blade element momentum (BEM) method is possibly the oldest and best-known design tool for evaluating the aerodynamic performance of wind turbine blades due to its simplicity and short processing time. As the turbine rotates, the aerofoil lift coefficient enhances, notably in the rotor’s inboard section, relative to the value predicted by 2D experimentation or computational fluid dynamics (CFD) for the identical angle of attack; this is induced by centrifugal pumping action and the Coriolis force, thus delaying the occurrence of stall. This rotational effect is regarded as having a significant influence on the rotor blade’s aerodynamic performance, which the BEM method does not capture, as it depends on 2D aerofoil characteristics. Correction models derived from the traditional hard computing mathematical method are used in the BEM predictions to take into account stall delay. Unfortunately, it has been observed from the earlier literature that these models either utterly fail or inaccurately predict the enhancement in lift coefficient due to stall delay. Consequently, this paper proposes a novel stall delay correction model based on the soft computing technique known as symbolic regression for high-level precise aerodynamic performance prediction by the BEM process. In complement to the correction model for the lift coefficient, a preliminary correction model for the drag coefficient is also suggested. The model is engendered from the disparity in 3D and 2D aerofoil coefficients over the blade length for different wind speeds for the NREL Phase VI turbine. The proposed model’s accuracy is evaluated by validating the 3D aerofoil coefficients computed from the experimental results of a second wind turbine known as the MEXICO rotor. Full article

In spite of the tremendous advances in computing power and continuous improvements in simulation software made in recent decades, the accurate estimation of wind turbine performance using numerical methods remains challenging. Wind turbine aerodynamics, especially when operating outside of the design envelope, is highly complex: blade stall, laminar-to-turbulent boundary layer transition, rotational effects (lift augmentation near blade root), and tip losses are present. The scope of this research is to show that the classic Reynolds-Averaged Navier–Stokes (RANS) modeling approach, although extensively tried and tested, is not yet exhausted. The NREL Phase VI rotor was used as a basis for numerical methodology development, verification and validation. The numerical model results are compared in detail with the available measured data, both globally (turbine torque and thrust, and blade bending moment) and locally (pressure coefficient distributions and aerodynamic force coefficients at several locations on the blade) over the entire experimental wind speed range. Stall initiation and spread over the blade span are well captured by the model, and rotor performance is predicted with good accuracy. RANS still presents significant value for wind turbine engineering, with a great balance between accuracy and computational cost. The present work brings potential impact on all applications of wind turbines, especially targeting offshore wind energy extraction for which great development is expected in the near future. Full article

The predictions of the onset of rotating stall and surge are very important in the preliminary design stage of a compressor. Rotating stall and surge are complex instabilities that cause efficiency loss and reduced pressure rise, and, therefore, compressor designers attempt to avoid them in the design stage. There are many criteria for predicting stability limits, including empirical, theoretical, and numerical investigations in the literature. However, these investigations have important limitations. The present study establishes a new method in which the stall and post-stall behavior of a compressor is estimated by an equivalent reconstructed compressor using special combinations of single-passage flow behavior in different mass flow rates. The combinations are generated such that pre-stall, in-stall, and surge flow regimes and between one and eight stall cells are reproduced in the full-annulus compressor. The method requires the least computational requirements and is time efficient. The results indicate that secondary flow total energy and spectral entropy are indeed correlated with compressor operating conditions. The predictions of the onset of stall and surge for the investigated compressor show good agreement with the experimental data. Full article

The onset of rotating stall and surge in compressors limits the operating range of aero-engines. Accurately predicting the key features during these events is critical in the engine design process. In this paper, a three-dimensional computational model for transient simulation of multi-stage axial compressors during stall is proposed. The kinetic equations describing the dynamic process of the compression system are constructed, with a 3D through-flow model for the compression part and a 1D gas collector model for the outlet part. The calculation of the source term is performed using the developed body-force model, which realizes the correlation between the deviation angle and the loss coefficient with the inlet parameters in various flow regions. Validated on a single-stage compressor and a single-rotor fan, the results show that the method is capable of capturing the stall and surge features correctly and that the three-dimensional structure of the stall cell can be captured. In addition, this model could be used for the analysis of the surge load, which is significant for the structural integrity of the compressor. Full article

An aero-structural algorithm to reduce the energy consumption of a propeller-driven aircraft is developed through a propeller design method coupled with a Particle Swarm Optimization (PSO). A wide range of propeller parameters is considered in the optimization, including the geometry of the airfoil at each propeller section. The propeller performance prediction tool employs a convergence improved Blade Element Momentum Theory fed by airfoil aerodynamic characteristics obtained from XFOIL and a validated OpenFOAM. A stall angle correction is estimated from experimental NACA 4-digits data and employed where convergence issues emerge. The aerodynamic data are corrected to account for compressibility, three-dimensional, viscous, and Reynolds number effects. The coefficients for the rotational corrections are proposed from experimental data fitting. A structural model based on Euler-Bernoulli beam theory is employed and validated against Finite Element Analysis, while the impact of centrifugal forces is discussed. A case of study is carried out where the chord and pitch distributions are compared to minimal losses distribution from vortex theory. Wind tunnel tests were performed with printed propellers to conclude the feasibility of the entire routine and the differences between XFOIL and CFD optimal propellers. Finally, the optimal CFD propeller is compared against a commercial propeller with the same diameter, pitch, and operational conditions, showing higher thrust and efficiency. Full article

Focusing on the identification of dynamic system stability, a hybrid neural network model is proposed in this research for the rotating stall phenomenon in an axial compressor. Based on the data fusion of the amplitude of the spatial mode, the nonlinear property is well characterized in the feature extraction of the rotating stall. This method of data processing can effectively avoid the inaccurate recognition of single or multiple measuring sensors only depending on pressure. With the analysis on the spatial mode, a chaotic characteristic was shown in the development of the amplitude with the first-order spatial mode. With the prerequisite of revealing the essence of this dynamic system, a hybrid radial basis function (RBF) neural network was adopted to represent the properties of the system by artificial intelligence learning. Combining the advantages of the methods of K-means and Gradient Descent (GD), the Chaos–K-means–GD–RBF fusion model was established based on the phase space reconstruction of the chaotic sequence. Compared with the two methods mentioned above, the calculation accuracy was significantly improved in the hybrid neural network model. By taking the strategy of global sample entropy and difference quotient criterion identification, a warning of inception can be suggested in advance of 12.3 revolutions (296 ms) with a multi-step prediction before the stall arrival. Full article