Complex Internal Flow Research Articles

Derivation of analytic formulae for several resonance frequencies of the SparkJet actuator

Building on previous research that emphasized the importance of focusing on resonance frequencies for a fundamental understanding of the thrust and complex internal flow phenomena of the SparkJet actuators, this study theoretically derives analytic formulae for several important resonance frequencies. The research addresses the typical configuration of the SparkJet actuators, which consists of a cylindrical cavity and orifice connected by a conical converging nozzle. While the resonance frequencies of the SparkJet actuators can be obtained by solving the eigenvalue problem presented in previous studies, this eigenvalue problem, despite being a typical boundary value problem in the form of an elliptic partial differential equation, is challenging to solve using conventional numerical methods such as iterative methods, because the eigenvalue is included as an unknown in the operator. Consequently, Boundary Element Method (BEM) or methods using the Green functions have been proposed to obtain numerical solutions, but these still require handling large matrix data, resulting in significant computational costs and memory consumption. To overcome this, the current study first simplifies the eigenvalue problem using the conformal mapping presented in previous research. Then, referencing prior studies that claim that three types of resonance frequencies (Helmholtz resonance frequency and resonance frequencies representing streamwise or radial directional oscillation) significantly affect the thrust of the SparkJet actuators, characteristics of these frequencies are mathematically defined. Using these mathematical characteristics, the study derives the asymptotic and approximate solutions of the simplified eigenvalue problem, from which the resonance frequencies are obtained. The analytic formulae proposed in this study theoretically explain the already known geometric tendencies of the resonance frequencies and reveal new geometric factors influencing resonance frequencies, which were previously unknown. This approach is expected to facilitate obtaining several important resonance frequencies of the SparkJet actuator more promptly and accurately and to provide a deeper understanding of the nature of the complex oscillation phenomena inside the actuator.

A new reactor with porous baffle for thermochemical heat storage: Design and performance analysis

The Ca(OH)2/CaO reaction system presents broad development prospects for thermochemical heat storage. However, the poor thermal conductivity of heat storage material and the complex internal flow limit the performance of the reactor. To improve the performance of the reactor, this work designs a new improved fixed bed reactor with baffle. The comparison with and without baffle, as well as the effects of operation conditions and structural parameters of the improved reactor on heat storage and release performance, are numerically investigated. The results show that the improved reactor can enhance heat transfer and flow inside the reactor, resulting in performance improvement. After baffle installation, there is a 64% decrease in the pressure drop, a 29.14% reduction in the heat storage time, and a 35.45% reduction in the heat release time. Besides that, the improved reactor can maintain the high temperature for longer at the outlet during heat release. The orthogonal test design analysis reveals that the inlet velocity of direct heat transfer fluid has the greatest impact on heat storage, while the inlet velocity of indirect heat transfer fluid has the smallest impact, and the same applies to heat release. The thermal conductivity and permeability of the porous baffle have minor impacts, but the air distribution chamber has a noticeable influence. This work can guide the optimization design and operation adjustment of the reactor.

Experimental and simulation analyses of the hydraulic complex internal flow characteristics in an axial pump based on varying frequency vibration ranges technique

Experimental and simulation analyses of the hydraulic complex internal flow characteristics in an axial pump based on varying frequency vibration ranges technique

Interactions of row blades of a marine compressor based on POD analysis

The complex internal flow of the marine low-pressure compressor (LPC) is characterized by a series of unsteady flow structures, presenting extensive temporal and spatial features that pose challenges to direct data analysis. This paper employs the Unsteady Reynolds-averaged Navier Stokes (URANS) method to simulate a marine 1.5-stage LPC with full-channel configuration. Validation of the compressor’s overall characteristics and the unsteady pressure is achieved through comparison with experimental data. Additionally, the Proper Orthogonal Decomposition (POD) method is applied to decompose the velocity and pressure fields in various computational regions. The results demonstrate that the combined use of URANS and POD facilitates detailed insights into blade interactions. The consistency between time-averaged variables and POD modes underscores the practical physical significance of the POD modes. Furthermore, the study reveals that the Rotor-Stator interaction significantly outweighs the Inlet Guide Vane (IGV)-Rotor interaction. The coherent modal pairs generated by different interferences exhibit diverse characteristics within the computational domain, with distinct frequencies observed for the same interference reaction in the upstream and downstream regions. Notably, the POD modes of the rotor pressure field unveil a separation bubble structure.

Polymer crystallization and optical birefringence modulated by solvent evaporation in sessile droplets

Solvent evaporation is a dynamic process associated with mass transfer, heat exchange and complex internal flow. Their complicated effects on crystallization remain unclear in evaporating sessile droplets. Here, we study crystallization in evaporating poly(ethylene oxide) (PEO) sessile droplets and obtain three kinds of spherulites with monochromatic, anisotropic or negative birefringence. Monochromatic spherulites are generated at very slow solvent evaporation and low molecular weight, while high anisotropic spherulites are created under flow field at rapid solvent evaporation and large molecular weight. Ordinary negative spherulites appear at intermediate conditions. The temperature variation is verified during solvent evaporation, but plays an insignificant thermodynamic effect on crystallization. The experimental findings are further validated in different solvents and polymer systems. This work establishes the relationship between solvent evaporation and polymer crystallization in sessile droplets and also provides a guideline for modulating optical properties of deposited films using droplet processing techniques.

Numerical investigation on energy change field in a centrifugal pump as turbine under different flow rates

The pump as turbine (PAT) is an economical and essential device that is widely used in micro hydropower and residual energy recovery. The energy change field, characterized by first and second energy change rate (LK1 and LK2), is investigated in main flow passage components of PAT under five flow rates ranging from 0.5QBEP to QBEP. The relationship between mean partial derivations of mechanical energy and flow rate is non-monotonic in the impeller, and it shows increasing and decreasing trends in the volute and draft tube, respectively. The large-LK2 regions exhibit positive correlation to complex internal flow structures. The mean LK1 (MLK1) in the impeller initially decreases and then increases slightly as flow rate increases, the opposite trends are observed in the volute and draft tube. There is a positive slope segment between the mean LK2 (MLK2) and flow rate in the impeller, which corresponds to the internal flow instability. The maximum values of MLK2 in the impeller and volute are 2.54 and 2.81, respectively, corresponding flow rates are 0.75QBEP and 0.875QBEP. As the flow rate increases, the standard deviations of MLK1 and MLK2 decreases significantly, and then reaches a steady value in impeller and volute region.

Study on the flow characteristic of pump turbine at partial load

Abstract Pump turbine adopts the same runner to realize forward pumping for pumping condition and reverse power generation for turbine condition, which is a widely used power equipment in pumped storage power station. Compared with conventional pump units or turbine units, the pump turbine has a complex internal flow, especially under part load conditions, and the vibration and noise problems of the structure are prominent. In this paper, the internal flow characteristics of the pump turbine at 60% load are studied by numerical simulation. The low-frequency pressure pulsation 3.5fn and 5.5fn that occur under this operating condition are analyzed. The results show that under partial load conditions, the opening of the guide vane decreases, the angle of attack of the water entering the runner increases, and the rotating stall phenomenon occurs in the pump turbine. The stall vortices show periodic variations on the blade inlet pressure surface and undergo the processes of vortex reduction, fusion, splitting and increase, respectively. The frequency of change is 0.5 times the runner rotation frequency (0.5fn ). The frequency of the corresponding monitoring points in the 7 blade rotor is 7 times (1fn -0.5fn ), which is 3.5fn .

Hypersonic inlet flow field reconstruction dominated by shock wave and boundary layer based on small sample physics-informed neural networks

High Mach number inlets face complex challenges such as shock wave/boundary layer interference, adversely impacting the aerodynamic characteristics and stable operating boundaries. Traditional computational fluid dynamics (CFD) simulations for performance and flow field analysis are slow, and data-driven deep learning technologies struggle with predicting flow fields in complex aerodynamic phenomena like shock wave/boundary layer interactions, boundary layer separation, and Mach disks. This study introduces a rapid, robust method for reconstructing hypersonic viscous inlet flow fields using inlet geometry design parameters. This method is called shock wave and boundary layer dominated two-dimensional multiphysical field reconstruction model based on multi-scale sensory field fusion residual physical information neural network (PIMSRF_ResNet), ensuring high-precision training data. Using the optimal Pareto solution set from multi-objective optimization with a small sample intelligence method, 150 sets of multi-physics fields under Ma10 conditions with varying geometric design parameters are calculated. Compared to traditional pure data-driven models, PIMSRF_ResNet's structural similarity in the test set reaches 0.9773, with a peak signal-to-noise ratio close to 30 dB and a correlation coefficient exceeding 98%. These experimental results demonstrated that the proposed method is a promising tool for real-time prediction of complex internal flow fields dominated by high Mach number shock waves and boundary layers.

Investigation of the Internal Flow Characteristics of a Tiltrotor Aircraft Engine Inlet in a Gust Environment

In the vertical take-off and landing (VTOL) state of tiltrotor aircraft, the inlet entrance encounters the incoming airflow at a 90° attack angle, resulting in highly complex internal flow characteristics that are extremely susceptible to gusts. Meanwhile, the flow quality at the inlet exit directly affects the performance of the aircraft’s engine. This work made use of an unsteady numerical simulation method based on sliding meshes to investigate the internal flow characteristics of the inlet during the hover state of a typical tiltrotor aircraft and the effects of head-on gusts on the inlet’s aerodynamic characteristics. The results show that during the hover state, the tiltrotor aircraft inlet features three pairs of transverse vortices and one streamwise vortex at the aerodynamic interface plane (AIP). The transverse vortices generated due to the rotational motion of the air have the largest scale and exert the strongest influence on the inlet’s performance, which is characterized by pronounced unsteady features. Additionally, strong unsteady characteristics are present within the inlet. Head-on gusts mainly affect the mechanical energy and non-uniformity of the air sucked into the inlet by influencing the direction of the rotor’s induced slipstream, thereby impacting the performance of the inlet. The larger head-on gusts have beneficial effects on the performance of the inlet. When the gust velocity reaches 12 m/s, there is a 1.01% increase in the total pressure recovery (σ) of the inlet, a 25.72% decrease in the circumferential distortion index (DC60), and a reduction of 62.84% in the area where the swirl angle |α| exceeds 15°. Conversely, when the gust velocity of head-on gusts reaches 12 m/s in the opposite direction, the inlet’s total pressure recovery decreases by 1.13%, the circumferential distortion index increases by 14.57%, and the area where the swirl angle exceeds 15° increases by 69.59%, adversely affecting the performance of the inlet. Additionally, the presence of gusts alters the unsteady characteristics within the inlet.

Design and construction of a helium cooling experimental loop

The helium cooling tritium breeding blanket and helium cooling divertor, which produce tritium or heat, are important components in Chinese Fusion Engineering Test Reactor (CFETR) design. The helium cooling system, which function is similar to the primary loop of Pressurized Water Reactor (PWR), is designed to extract energy from blankets and divertors. These helium cooling components have many complex internal flow channels to extract high thermal load. They also have to withstand electromagnetic force, thermal stress and coolant pressure, etc. Therefore, out-of-pile experiments must be carried out as far as possible to verify the design. Under CFETR project support, a helium cooling experiment loop is built to carry out the thermo-hydraulic experiments for these components and to accumulate experience of helium cooling system. According to the cooling requirements of blanket, the loop test section operating parameters are flow rate 2.5kg/s, temperature 550°C and pressure 12MPa. During preliminary commissioning, these parameters have been reached. A high heat flux testing facility will connect to this loop for future experiments.

Numerical Investigation of Inner Flow Characteristics of a Prototype Pump Turbine with a Single Pier in Draft Tube at Part Load Conditions

Pump turbines operate under various off-design conditions, resulting in complex internal flow patterns. This study employs Reynolds-averaged Navier–Stokes (RANS) numerical methods to investigate the flow characteristics of a prototype pump turbine with a single draft tube pier in turbine mode, and then, the flow characteristics inside the draft tube are discussed with emphasis. Asymmetry between the pier-divided draft tube passage flows is inevitable due to the elbow section’s curvature. Most of the fluid flows out of one passage, while vortex motion dominates the interior of the other one, resulting in completely different pressure fluctuation characteristics for the two flow passages. The large-flow passage is mainly characterized by the wide band in the frequency domain, corresponding to the recirculation zone, while some of the measured points in the low-discharge passage exhibit frequency splitting under kinematic progression. Further analysis demonstrates a low-frequency peak corresponding to the complementary shape between the vortex rope and the recirculation zone. This work elucidates the effects of the pier on the flow behavior and pressure fluctuation characteristics inside the draft tube and fills the research gap on piers in the field of pump turbines.

Frictional boundary condition for lattice Boltzmann modelling of dense granular flows

Hydrodynamic approaches that treat granular materials as a continuum via the homogenization of discrete flow properties have become viable options for efficient predictions of bulk flow behaviours. However, simplified boundary conditions in computational fluid dynamics are often adopted, which have difficulty in describing the complex stick–slip phenomenon at the boundaries. This paper extends the lattice Boltzmann method for granular flow simulations by incorporating a novel frictional boundary condition. The wall slip velocity is first calculated based on the shear rate limited by the Coulomb friction, followed by the reconstruction of unknown density distribution functions through a modified bounce-back scheme. Validation is performed against a unique plane Couette flow configuration, and the analytical solutions for the flow velocity profile and the wall slip velocity, as functions of the friction coefficient, are reproduced by the numerical model. The transition between no-slip and partial-slip regimes is captured well, but the convergence rate drops from second order to first order when slip occurs. The rheological parameters and the basal friction coefficient are calibrated further against the discrete element simulation of a square granular column collapsing over a horizontal bottom plane. It is found that the calibrated continuum model can predict other granular column collapses with different initial aspect ratios and slope inclination angles, including the basal slip and the complex internal flow structures, without any further adjustments to the model parameters. This highlights the generalization ability of the numerical model, which has a wide range of application in granular flow predictions and controls.

Experimental investigation on aerodynamic and noise characteristics of Fenestron

It is difficult to simulate the strong interference and serious flow separation of Fenestron by the CFD method based on the widely used RANS equation, and the detailed experimental data, which could be used to validate the aerodynamic and noise numerical methods, is unavailable. The experimental investigation on the aerodynamic and noise characteristics of Fenestron is carried out. In view of the complex internal flow field in duct and the relative motion between the stationary duct and the rotating rotor, a comprehensive aerodynamics and pressure measurement scheme is designed based on the bottom support rig. In this measurement scheme, the thrust generated by the rotating rotor can be measured by the rotating shaft balance, the thrusts from Fenestron are measured by the external balance and the pressures on duct inner wall are monitored by a pressure measuring system. To fully capture the noise directionality of Fenestron, a series of noise observers located at an arc array are arranged. In terms of the Fenestron test models, the baseline model, the performance improvement model based on the high-performance tail rotor and the noise reduction model based on the non-uniform blade distribution are designed respectively. By the designed measurement scheme, aerodynamic forces and pressure distributions and noise were measured for the three different Fenestron models. The results show that the aerodynamic thrusts of the tail rotor and duct increase greatly and the noise increases slightly for the performance improvement model because of the larger aerodynamics. The rotor aerodynamic performance of the noise reduction model is reduced, but the modulation effect of the tail rotor improves the forces of the duct. The noise radiated by the noise reduction model is reduced and a good noise reduction result is obtained in frequency domain.

On the robustness and accuracy of large-eddy simulation in predicting complex internal flow of a gas-turbine combustor

The objective of this study is to evaluate the effects of numerical and model setups on the large-eddy simulation (LES) predictive capability for the internal flow of a propulsion-relevant configuration. The specific focus is placed on assessing the LES technique with lower mesh resolutions, which is of technological relevance to practical industrial design. A set of Riemann flux formulations and commonly used subgrid-scale models are considered in this work to produce a hierarchy of LES setups with different dissipation effects (both numerically and physically). The LES results obtained from different setups are compared qualitatively in terms of the key flow characteristics and evaluated quantitatively against the experimental measurements. The error landscape is generated to reveal the predictive qualities of different LES setups. The study shows that the choice of numerical flux formulation plays a prominent role in governing the general flow patterns, while the effect of subgrid-scale model is mainly manifested in transient flow characteristics, such as vortex breakdown and swirl-induced vortical structures. Based on the error analysis, it is found that lower dissipative LES setup is not always beneficial to the LES accuracy. This is in contrast to the commonly accepted understanding in literature for the LES, which was established solely with canonical flow configurations.

Marangoni instability of an evaporating binary mixture droplet

Evaporation of a binary mixture droplet (BMD) is a common natural phenomenon and widely applied in many industrial fields. For the case of a sessile BMD being the only contact-line pinning throughout an entire evaporation, a theoretical model describing the evaporating dynamics is established when considering the comprehensive effect of evaporative cooling, the thermal Marangoni effect, the solutal Marangoni effect, the convection effect, and the Stefan flow. The dynamics of a binary ethanol–water droplet on a heated substrate is simulated using a cylindrical coordinate system. The reasons for Marangoni instability-driven flow (MIF) are discussed, and the influence of initial ethanol concentration and substrate heating temperature are examined. An evaporating BMD first forms a MIF at the contact line and quickly affects the whole droplet. Under the influence of the Marangoni instability, the BMD presents a complex internal flow structure with multiple-vortex and nonlinear temperature and ethanol concentration distributions. The positive feedback induced by vortices and the nonlinear distribution of concentration and temperature promotes the development of a MIF. At low initial ethanol concentrations, the MIF loses its driving force and turns into a stable counterclockwise single-vortex flow as ethanol evaporates completely. However, at high initial ethanol concentrations, the MIF exists in the entire evaporation. Increasing ethanol concentration and substrate heating temperature can delay the appearance of the MIF; ethanol concentration affects the MIF duration time, and heating temperature alters the MIF intensity. To enhance flow intensity and mass transfer of BMDs, the temperature difference should first be increased, followed by increased ethanol concentration.

Investigation of the complex 3D flow structure within a selective catalytic reduction (SCR) reactor of a coal-fired power plant

A selective catalytic reduction (SCR) reactor is commonly used to remove nitrogen oxides (NOx) from coal-fired boilers. Uniformity of the flow passing through the catalyst layer is important for increasing denitrification (de-NOx) efficiency. In order to examine flow uniformity, this study conducted an experimental and numerical analysis of the complex internal flow within a realistic SCR model. Magnetic resonance velocimetry (MRV) was utilized to obtain non-invasive measurements of three-dimensional three-component average velocity and validate Reynolds-averaged Navier-Stokes (RANS) numerical simulations. The computational results showed similar overall flow structure compared with the MRV results. Parameters representing flow quality such as relative standard deviation (RSD) and recirculation zone strength (RZS) were calculated by integrating the flow field. These parameters have the largest value after the straightener area and decrease towards the catalyst reactor, and are not significantly affected by Reynolds number upstream of the catalyst layer. The recirculation zone size was analyzed using spanwise uniformity and skewness indicators. As the recirculation zone induces biased flow, the non-reacted NOx concentration was more prominent in the outlet zone opposite of the recirculating area in the corresponding actual on-site SCR reactor. Based on this finding, a meaningful correlation between flow maldistribution and de-NOx reaction could be deduced.

Blade redesign based on secondary flow suppression to improve the dynamic performance of a centrifugal pump

Low-frequency vibration induced by the complex internal flow of centrifugal pumps has been a long-standing challenge in the design of low-vibration pumps. In this paper, a theoretical method of blade thickness calculation is proposed for the passive control of complex flow in centrifugal impellers, and the effects of blade modification based on secondary flow suppression on the dynamic characteristics of model pump are studied by both experimental tests and numerical simulations. The experimental results show that pressure pulsations are affected by flow rate, and their amplitude at the blade passing frequency (fBPF) dominates and increases at partial load conditions. The redesigned impeller improves the dynamic performance of the model pump considering the relatively lower pressure amplitude at fBPF and the vibration level at low-frequency band under almost all concerned flow rates. Based on steady-state numerical results, the visualization and quantitative analysis of impeller outflow by introducing the secondary flow coefficient and the static pressure energy coefficient can reveal the correlation between secondary flow suppression and dynamic performance improvement. The proposed blade modification can effectively inhibit the development of secondary flow in the boundary layers and improve the uniformity of static pressure energy distribution of impeller outflow, which results in the reduction of pressure pulsation amplitude at fBPF and subsequent decrease in the low-frequency band vibration level. Meanwhile, pump operation under partial load conditions will intensify the secondary flow in the boundary layers, which contributes to a more non-uniform energy distribution of impeller outflow and deteriorates the dynamic performance of the model pump.

Blade design and analysis of centrifugal compressors for the transcritical carbon dioxide refrigeration cycle

The compressor is one important component in the transcritical CO2 (carbon dioxide) refrigeration cycle. The CO2 compressor has a small volume at the same refrigeration capacity because its refrigeration capacity per unit volume is large. The radial compressor often used in the CO2 refrigeration cycle has complex internal flow and difficult blade modeling. The paper presented a new centrifugal compressor with radial flow, whose blades are straight and easy to process. Taking a single stage compressor blade for example, the effects of the velocity coefficient, energy head coefficient, rotational speed of the moving blade, blade height, inlet radius and radial distance of the blade on the efficiency and pressure ratio were studied. According to the conclusions, the paper designed and simulated the blades of two compressors of the transcritical CO2 two-stage refrigeration cycle. Simulation result showed that the efficiencies of the two compressors were respectively 84.97% and 87.23%.

Aerodynamic noise due to complex internal flow through cordless vacuum cleaner and suction tower station

Although the demands and sales of cordless vacuum cleaner are on the rise in the global home appliance markets, consumers face the inconvenience of short battery life and frequent emptying of the dust box. To solve this inconvenience, a station tower that can automatically charge the battery and empty the dust box has been developed. However, aerodynamic noise generated when the station tower sucks dust causes another inconvenience. The purpose of this study is to identify the major aerodynamic noise sources that occur when such a station tower is connected to a wireless vacuum cleaner and operate, and to develop a low-noise design based on this. First, the entire flow path of a cordless vacuum cleaner and a station tower was simulated using the unsteady compressible RANS equations. Then, the main aerodynamic noise sources were identified using the vortex sound sources. From these results, the flow region through the head throat of the cordless vacuum cleaner was identified one of the most significant aerodynamic noise sources. Based on the identified noise source mechanism, new throat design was proposed, and its validity was investigated using a high-resolution Large Eddy Simulation technique. It was found that the strength of vortex sound source of pipe flow through the new head throat was much less than the original one. Finally, the new design geometry was applied to create a prototype, and the sound pressure spectrum measured for the prototype was compared to the original, which showed that the radiated noise level of the new one was less than the original one.

Modelling, Optimization, and Experimental Studies of Refrigeration CO2 Ejectors: A Review

CO2 is regarded as an effective and environmentally friendly refrigerant. Using a CO2 ejector is a proven method for enhancing the effectiveness of a transcritical CO2 refrigerant system. However, the complex internal flow of a CO2 ejector, involving supersonic effects, phase change effects, metastable effects, and so on, makes it difficult to understand. In order to summarize the current state of the technology and knowledge gaps, this work provides a comprehensive literature review on CO2 ejectors. In the first part, mathematical modelling and simulation calculations of CO2 ejectors are presented, and an overview and classification of ejector models are given. In the second part, the structural optimization part of the ejector is described in detail, and the nozzle structure, the mixing chamber length, improvements to multi-jet systems, and the impact of these factors on the system performance are analyzed. In the third part, flow visualization is used to study the complex flow phenomenon, and the effect of the shock wave on the entrained rate of the ejector is discussed. Finally, the paper outlines the relationship between all ejector technologies, working fluids, and ejector performance and makes valid recommendations for further research and development of CO2 ejectors.