Choking Condition Research Articles

Universal benchmark data of the three-dimensional boundary layer blockage and average friction coefficient for in silico code verification

The first law of thermodynamics reveals that all fluids are compressible, and the second law of thermodynamics entails all fluids to have positive viscosity. These established laws reaffirm the possibilities of the occurrence of Sanal flow choking in yocto to yotta scale systems and beyond [Kumar et al., “Discovery of nanoscale Sanal flow choking in cardiovascular system: Exact prediction of the 3D boundary-layer-blockage factor in nanotubes,” Sci. Rep. 11, 15429 (2021); “Sanal flow choking: A paradigm shift in computational fluid dynamics code verification and diagnosing detonation and hemorrhage in real-world fluid-flow systems,” Global Challenges 4, 2000012 (2020)]. The Sanal flow choking occurs in the real-world flows at a critical total-to-static pressure ratio [Kumar et al., “Abstract P422: Sanal flow choking leads to hemorrhagic stroke and other neurological disorders in earth and human spaceflight,” Circul. Res. 129(1), AP422 (2021)]. At the Sanal flow choking condition, the Rayleigh-flow-effect (thermal choking) and the Fanno-flow-effect (choking due to frictional effects) unite at a unique site of the sonic-fluid-throat. In this article, the two-dimensional (2D) and the three-dimensional (3D) boundary-layer-blockage factors and average friction coefficient are generated for different working fluids passing through a cylindrical port, at the Sanal flow choking condition, as universal benchmark data for a credible verification of in silico codes for both adiabatic and diabatic flows. The outlook, advancement, and significance of the analytical methodology, invoked for developing Sanal flow choking model using well-posed initial conditions, for generating the universal benchmark data for computational fluid dynamics code verification are critically reviewed herein. The closed-form analytical models presented herein for predicting the 2D and the 3D boundary-layer-blockage factors at the sonic-fluid-throat of adiabatic and diabatic flows and average friction coefficient in a circular duct at the Sanal flow choking condition are fabulously unaffected with any errors due to discretization and fully freed from empiricism for a credible decision making on various high fidelity numerical simulations. The Sanal flow choking model offers the luxury to the scientific community for solving numerous unresolved problems in boundary layer theory. It provides universal benchmark data for various applications irrespective of the laminar/turbulence flow features in wall-bounded compressible viscous flow systems. The 2D and 3D in silico simulation results are presented for demonstrating conclusively the possibilities of the occurrence of the Sanal flow choking and streamtube flow choking [Kumar et al., “The theoretical prediction of the boundary layer blockage and external flow choking at moving aircraft in ground effects,” Phys. Fluids 33(3), 036108 (2021).] in internal and external flows. The phenomenological manifestation of the flow choking phenomenon reported herein extends disruptive technologies at the cutting-edge to solve century-long unresolved scientific problems in physics of fluids with credibility.

Particle flow regime in a swirling pneumatic conveying system

Particle fluidization is crucial for pneumatic conveying, combustion, and material drying. In this study, a swirling generator was designed to investigate the effect of the swirling flow on the flow regimes of pebble particles in a vertical pipe, and four different sizes of pebble particles were experimentally evaluated. Experiments were performed to investigate the effect of the axial flow and swirling flow on the flow regime, pressure drop, and suspension height of particles in a vertical pipe. The results indicated that the swirling flow has a significant effect on the particle flow regimes in a vertical pipe when the solid mass flow rate and inlet airflow velocity are constant. The swirling flow reduces the pressure drop and increases the particle fluidization in a vertical pipe, and it lifts the particles only when the inlet airflow velocity is higher than the particle velocity for the choking condition.

Effect of Cavitation Formation on Velocity and Vorticity

In this study, flow in a rectangular shape nozzle has been simulated. Fully hexahedral mesh elements are utilized with having boundary layer mesh in order to avoid excessive mesh destiny in the orifice area. It was found that velocity magnitude, axial velocity and vorticity magnitude increases intensely at the beginning of the orifice area which makes it possible for the cavitation phenomena to occur rapidly. The cavitation that occurs inside diesel fuel injector nozzles has a significant impact on atomization, and modelling this phenomenon will be useful in future injector development. The Schnerr cavitation model was chosen among three distinct cavitation models available in Ansys Fluent v21 because it is capable of properly forecasting choke conditions in cavitating flow. The two-phase flow inside a nozzle is assumed to be a homogenous combination of vapor, liquid, and noncondensable gas in this model. The pressure and velocity profiles derived from the simulations were compared to planar throttle flow experimental findings.

Equivalent model for an axial compressor used for aero engines based on 1D and 3D analytical models and performance data

This paper proposes a method to generate an equivalent model for predicting compressor performance by correcting a one-dimensional (1D) meanline analysis model based on the results of three-dimensional computational fluid dynamics (3D CFD) and compressor performance at an engine operating point. By comparing the results predicted by the 1D and 3D analysis methods, the compressor operating region and empirical model that needs improvement in the 1D meanline model are identified. As a result, the off-design profile loss model and the model for the incidence angle in the choking condition are selected as models that need improvement; the models are adjusted based on the 3D CFD analysis results. Correction factors for deviation angle, minimum loss incidence angle, and total loss coefficient are proposed. The correction factors are calculated using an iterative method to match the result for the 1D meanline model and the compressor performance at an engine operating point. The method proposed is applied to a three-stage axial compressor used for an aero engine. This confirms that not only compressor performance near the operating point, but also performance near surge and choking, are well predicted.

The Polytropic Approach in Modeling Compressible Flows Through Constant Cross-Section Pipes

Abstract A compressible flow with wall friction has been predicted in a constant cross section duct by means of a barotropic modeling approach, and new analytical formulas have been proposed that also allow any possible heat transfer to the walls to be taken into account. A comparison between the distributions of the steady-state flow properties, pertaining to the new formulas, and those of a classic Fanno analysis has been performed. In order to better understand the limits of the polytropic approach in nearly chocked flow applications, a numerical code, which adopts a variable polytropic coefficient along the duct, has been developed. The steady-state numerical distributions along the pipe, obtained for either a viscous adiabatic or an inviscid diabatic flow by means of this approach, coincide with those of the Fanno and Rayleigh models for Mach numbers up to 1. A constant polytropic exponent can be adopted for a Fanno flow that is far from choking conditions, while it cannot be adopted for the simulation of a Rayleigh flow, even when the flow is not close to choking conditions. Finally, under the assumption of diabatic flows with wall friction, the polytropic approach, with a constant polytropic exponent, is shown to be able to accurately approximate cases in which no local maximum is present for the temperature along the duct. The Mach number value at the location where the local maximum temperature possibly occurs has been obtained by means of a new analytical formula.

مدل سازی عددی و بهینه سازی هندسی اجکتور تک‌فاز مافوق صوت

In the present study, the performance of the 2D single-phase supersonic ejector with working fluid of air is simulated in ANSYS CFX. The aim is to investigate the velocity field, pressure distribution, primary nozzle flow regime, and entertainment ratio in different operational conditions. The primary pressure inlet with bar and the secondary inlet as an opening with bar at different outlet pressures are simulated. The turbulence model is used. The sidewalls are considered symmetry boundary conditions and the no-slip condition is applied to the ejector walls. Then, the ejector geometry is optimized using Multi-Objective Genetic Algorithm (MOGA) in order to reach greater efficiency. Optimization is performed considering geometric parameters including primary nozzle exit diameter, nozzle exit position, diameter, and length of the constant area section. Sensitivity analysis results show that the diameter of constant area section has major effect on the entertainment and pressure ratios as two objective functions. The nozzle exit position and external diameter of primary nozzle are the second and third dominant parameters that respectively influence the performance of the ejector, while the effect of constant area section length is negligible. Results indicate that by increasing the pressure ratio, the shock train moves upstream and at the design point, the last oblique shock is located in the exit of the constant area section, letting the remaining pressure recovery be done in the subsonic diffuser which reduces the pressure losses and increases the efficiency. Above the critical pressure ratio, due to the movement of the shock train to upstream weakening the shock strength, the suction pressure increases. Then, the pressure difference is reduced, leading to the lower secondary mass flow suction. The optimized ejector in the double choking condition has a % higher entertainment ratio and its operational range is enhanced by percent in comparison to the original geometry.

Design and validation of a reconfigurable polysonic wind tunnel

A new polysonic wind tunnel has been designed and constructed at Saint Louis University. The flow paths and flow conditioning needs were designed by PhD students and the structural design and manufacturing were accomplished by local pressure vessel and manufacturing firms. Starting with a set of two 25 hp air compressors, an air filter and dryer system with an operating pressure range between 100–200 psia, and a 14 m3 storage tank, the total cost for additional components and labor was on the order of $750,000, where design and installation was completed in approximately 18 months. The facility is uniquely reconfigurable to accommodate straight and curved flow paths, with existing configurations supporting a supersonic shock wave-boundary layer interaction study in a rectangular channel and another study of a transonic linear cascade of high-pressure turbine airfoils. The design process and resulting components are described herein, and include how to improve supersonic nozzle design by adjusting for displacement thickness growth as well as how to diagnose and correct choked flow conditions which existed in the first iteration of the flow path circuit. Results also include a sample schlieren image of a normal shock wave-boundary layer interaction and airfoil static pressure distributions and total pressure ratio across the turbine showing successful configuration implementations.

Peningkatan Peran Guru Sekolah Dasar Dalam Pengobatan Darurat untuk Menghilangkan Rasa Sakit Gigi dan Mulut

Abstract - Oral and dental health is a condition that needs to be considered and maintained to improve the quality of life. One of the efforts made to maintain dental and oral health in the school environment is by conducting UKGS (Usaha Kesehatan Gigi Sekolah). In UKGS activities, teachers have a role in providing emergency treatment to relieve pain. The emergency treatment administered by teachers in schools can perform in cases such as avulsions, choking, and abscesses. This community service aims to increase the role of elementary school teachers through UKGS to maintain, improve the oral health of all students at SDN in the Karangtengah area, Baturaden District, Purwokerto. The method of implementation was basely on optimizing the potential of UKGS teacher cadres, including identification of target schools and participants, pre-test implementation, maturation, coaching, enhancement of UKGS teacher cadres. Coaching and training have been carried out for UKGS supervisors for abscess, choking and avelse conditions. Post-test activities to gain insight into emergency medicine and relieve tooth and mouth pain. Results: p data analysis using the Wilcoxon asymp test.sig.(2-tailed) of 0.001 (<0.05), this value indicates that there is an influence on the level of knowledge about emergency medicine. The analysis performed with the Wilcoxon Test; there was a significant difference (sig = 0.001, p <0.05). There was an increase in the average pre-test value of 76.92 and post-test of 97.85.Conclusion: There is increasing knowledge about emergency medicine to relieve tooth and mouth pain in elementary school teachers.

Compound-choking theory for supersonic ejectors working with real gas

The performance of supersonic ejectors is mainly limited by the choking of the flow. Below a critical pressure, the entrainment capability of the ejector remains indeed constant. As the coefficient of performance of ejector-based heat driven refrigeration cycles (HDRC) is directly linked to the ejector entrainment ratio, it constitutes a real limitation towards better overall cycle performance. The compound-choking criterion has recently demonstrated to better explain this limitation compared to the Fabri-choking theory for supersonic ejectors working with air. However, as all ejector-driven HDRCs work with real gas refrigerants, this criterion needs to be extended to real gases. In the present work, the compound-choking criterion is first extended analytically to real gases. Wall-resolved turbulence modelings for supersonic ejectors working with R134a and blends of R134a and HydroFluoroOlefeins (HFO) are then performed. An improved thermodynamic model is finally used to demonstrate the superiority of the compound-choking criterion over the Fabri-choking one for supersonic ejectors. Results show that the β parameter constitutes an unambiguous indicator of the ejector choking condition, much better suited to determine the operating regime than the sonic line criterion. Moreover, the mean error in entrainment ratio predictions is reduced from 17.54% to 5.28% when using the compound-choking theory instead of the Fabri-choking one, with virtually no difference in the complexity or computational cost of the model.

Performance analysis of ERS using R134a - An experimental investigation (Part 1) – Ejector design and performance analysis

The present study proposed a mathematical model for calculating the entrainment ratio and area-ratio for the single-phase ejector working double choking conditions. The model contains gas dynamic equations and is used to calculate performance of ejector for R134a and R141b refrigerants. The model is also validated with the experimental results of the previous researcher and found results are in good coherence with average deviation of -0.287% and 2.48% for area-ratio and entrainment ratio respectively.

Axial Compressor Mean-Line Analysis: Choking Modelling and Fully-Coupled Integration in Engine Performance Simulations

A mean-line compressor performance calculation method is presented that covers the entire operating range, including the choked region of the map. It can be directly integrated into overall engine performance models, as it is developed in the same simulation environment. The code materializing the model can inherit the same interfaces, fluid models, and solvers, as the engine cycle model, allowing consistent, transparent, and robust simulations. In order to deal with convergence problems when the compressor operates close to or within the choked operation region, an approach to model choking conditions at blade row and overall compressor level is proposed. The choked portion of the compressor characteristics map is thus numerically established, allowing full knowledge and handling of inter-stage flow conditions. Such choking modelling capabilities are illustrated, for the first time in the open literature, for the case of multi-stage compressors. Integration capabilities of the 1D code within an overall engine model are demonstrated through steady state and transient simulations of a contemporary turbofan layout. Advantages offered by this approach are discussed, while comparison of using alternative approaches for representing compressor performance in overall engine models is discussed.

Numerical Investigation of the Centrifugal Compressor Stability Improvement by Half Vaned Low Solidity Diffusers

Centrifugal compressors for the fuel cell vehicles often operate near the surge line compared with the turbocharger compressors. Low solidity and half vaned diffusers are recognized as good ways to improve the stability of the centrifugal compressor. The presented work investigated four diffuser configurations (i.e., the vaneless diffuser (VLD), full-height low solidity vaned diffuser (LSVD), hub-side half vaned diffuser (HVD) and shroud-side half vaned diffuser (SVD)) through steady-state and unsteady numerical simulations. The results show that the best performance is achieved by the LSVD, HVD and SVD at the design, surge and choke conditions. The flow rate at the surge operating point of the HVD has decreased by 15.53% compared with the LSVD, and 9.21% compared with the VLD. At near surge operating point, a longitudinal suction side passage vortex is formed on the hub of the LSVD and rotates as circumferential stall cells. A hairpin vortex is formed along the leading edge and is dragged by the main flow along the suction side as a local vortex shedding. The mechanism of the stability improvement by half vaned diffusers is that the tip leakage vortex migrates from the clearance side to the vane mounting side and replenishes the low-momentum zone on the mounting side. The best position where the half vaned diffuser should be mounted is based on the impeller outlet flow conditions, namely, the location of the wake region, where the meridional velocity and relative stagnation pressure is low.

Choked Flow Behavior of CO2 Refrigerant Flowing Through the Spiral Capillary Tube

The flow characteristics of CO2 refrigerant are numerically studied for an adiabatic spirally coiled capillary tube employing choked flow conditions. The mass, momentum and energy conservation equations are used to develop a numerical model. The existing model is verified with the published results. The choked flow behavior at various geometric parameters viz. tube diameter and spiral pitch is studied. Similarly, the influence of these parameters on the mass flow rate through the tube is observed. A significant change in mass flow rate is due to a change in tube diameter, whereas a minimal variation is observed with the change in surface roughness and spiral pitch. Moreover, it is observed that the coiling effect has a significant influence on the flow behavior of the spiral capillary tube. As the pressure decreases, from unchoked to the choked pressure in the evaporator by 63.46%, the mass flow rate increases by 9.46% only. The capillary tube choking is circumvented by increasing spiral pitch, tube diameter and decreasing the length of the tube. A unique nomogram is developed that gives the best understanding of choked and unchoked flow conditions, that graphical representation is useful to design the spirally coiled capillary tube. By using that, the choked length is identified for the known mass flow rate, even more, the choked mass flow rate is known for a given tube length. Moreover, for the given tube length and evaporator temperature, a nomogram is useful to the known choked values of mass flow rate and exit values of the evaporator pressure and quality of refrigerant.

Effects of bleed hole size on supersonic boundary layer bleed mass flow rate

The bleed hole diameter, depth, and boundary layer thickness are key design parameters of a supersonic bleed system. The evolution trend of single-hole bleed flow coefficient with the ratio of boundary layer thickness to bleed hole diameter and the ratio of bleed hole depth to diameter is investigated by numerical simulations under choking and non-choking conditions. The results show that the subsonic leading edge of the circular hole and the subsonic part of the boundary layer are the main factors causing lateral flow of the bleed hole. The effect of diameter on bleed mass flow rate is due to the viscous effect which reduces the effective diameter. The larger the ratio of displacement thickness to bleed hole diameter, the more obvious the viscous effect is. The depth affects bleed flow rate by changing the opening and closing states of the separation zone. When a certain depth is reached, the development of the boundary layer reduces the effective captured stream tube and thus reduces the bleed mass flow rate. The main objective of the study is to obtain the physical mechanism of the bleed hole size parameters affecting the bleed mass flow rate, and to provide theoretical guidance for the selection of the size of bleed holes in the design of a porous arrays bleed system in hypersonic inlets.

A 60 GHz PCB Wideband Antenna-in-Package for 5G/6G Applications

A low-cost, wideband printed circuit boards (PCBs) manufactured antenna is proposed in this letter for 5G/6G antenna-in-package (AiP) applications with wideband common-mode current (surface-wave) choking characteristics. The antenna is based on a grounded, wideband, high-efficiency electromagnetic structure (WHEMS) and a backing cavity, in which a radiating choke is introduced. By changing the choking condition of a quarter-wave length, widely applied in traditional low-frequency baluns, wideband common-mode current suppression is achieved. The enclosing wall of the backing cavity is realized by a combination of isolated grounded vias and connected grounded vias based on current polarization on the wall. Finally, a 22% impedance bandwidth (60–75 GHz) and a relative gain of 8–10 dBi within the band is achieved.

Sanal Flow Choking: A Paradigm Shift in Computational Fluid Dynamics Code Verification and Diagnosing Detonation and Hemorrhage in Real-World Fluid-Flow Systems.

The discovery of Sanal flow choking is a scientific breakthrough and a paradigm shift in the diagnostics of the detonation/hemorrhage in real‐world fluid flow systems. The closed‐form analytical models capable of predicting the boundary‐layer blockage factor for both 2D and 3D cases at the Sanal flow choking for adiabatic and diabatic fluid flow conditions are critically reviewed here. The beauty and novelty of these models stem from the veracity that at the Sanal flow choking condition for diabatic flows all the conservation laws of nature are satisfied at a unique location, which allows for computational fluid dynamics (CFD) code verification. At the Sanal flow choking condition both the thermal choking and the wall‐friction‐induced flow choking occur at a single sonic fluid throat location. The blockage factor predicted at the Sanal flow choking condition can be taken as an infallible data for various in silico model verification, validation, and calibration. The 3D blockage factor at the Sanal flow choking is found to be 45.12% lower than the 2D case of a wall‐bounded diabatic fluid flow system with air as the working fluid. The physical insight of Sanal flow choking presented in this review article sheds light on finding solutions, through in silico experiments in base flow and nanoflows, for numerous unresolved problems carried forward over the centuries in physical, chemical, and biological sciences for humankind.

Performance analysis of a rotating detonation combustor based on stagnation pressure measurements

This study explores the effect of differing inlet and outlet boundary conditions on the operation and performance of a rotating detonation combustor (RDC) over an annulus mass flux range of 50 to 210 kgs−1m−2 and equivalence ratios of 0.7, 1.0, and 1.3. The RDC is equipped with either a uniform outlet restriction or with a set of nozzle guide vanes to simulate turbine integration. Stagnation pressure data from Kiel probes placed in the high-enthalpy exhaust flow are presented for the operational envelope. The RDC’s operation is categorized into different modes distinguished by the number of co- and counter-rotating combustion waves in the annulus. With increasing mass flux, a typical progression proceeding from a pair of counter-rotating waves, to a single detonation wave, and then further to multiple co-rotating waves is observed with wave speeds reaching up to 84% of the CJ velocity. It is shown that a choking condition at the outlet throat correlates with the transition from two counter-rotating waves to a single wave detonation regime. The channel Mach number is then calculated from measured pressure ratios and is shown to agree with area ratio-based estimates. The pressure gain of the RDC is expressed as the stagnation pressure change from the air plenum to the outlet throat, and it is shown that the specific operating mode of the device – in conjunction with the chosen injector and outlet area ratios – can significantly decrease the pressure gain performance in some cases, while not significantly affecting it in others. While no positive pressure gain was achieved in the experiments, the presented experimental data compare well with numerical results of similar boundary conditions and underline the importance of minimizing injector pressure loss while applying outlet restrictions to the combustor. The data also suggest that specific geometric combinations may lead to adverse modes such as longitudinally pulsing combustion, resulting in a reduction in the measured pressure gain relative to numerical results. This observation occurs more often for geometric combinations which are the most promising for exhibiting positive pressure gain, and suppressing these modes will be an important topic to achieving this goal. It is further shown that transition regions exist between modes of one and multiple co-rotating waves, and that wave multiplication may be necessary to unlock further increases in the pressure gain.

Effect of Vaneless Diffuser Shape on Performance of Centrifugal Compressor

The influence of four different vaneless diffuser shapes on the performance of centrifugal compressors is numerically studied in this paper. One of the studied shapes was a parallel wall diffuser. Two others had the width reduced only from hub and shroud and the rest had the width reduced from hub and shroud divided evenly. Then the numerical simulation was employed and the overall compressor aerodynamic performance was studied. The detailed velocity and pressure distribution and energy loss within the centrifugal compressor with different diffuser geometries and different operating conditions were analyzed. The results revealed that shroud pinch significantly improved the overall compressor aerodynamic performance more than any other pinch types, and the best performance can be achieved by pinched diffusers under the design condition compared with pinched diffusers under the near surge condition or choking condition. The range of energy loss, namely the static entropy area in the compressor, become reduced with the above three pinches diffusers.

Supersonic Bleed Rate Model for a Circular Orifice Based on Geometric Similarity

On the basis of the geometric similarity between polygons and circles, a bleed rate model of a supersonic inviscid circular hole is first established through a gradual approximation by regular polygons. The bleed mass-flow rate depends on the parameters at the entrance. A supersonic inviscid bleed choking model is further established by using the inflow Mach number and the pressure ratio when the expansion wave is parallel to the entrance. For a depthless hole at Mach 2.0 and with a pressure ratio of 0.5, the difference of flow coefficient predicted by the present model to that of computational fluid dynamics is 1.4% under nonchoking conditions. Under choking conditions with a pressure ratio of 0.1, the difference is 3.1%. Furthermore, the influence of the boundary layer on the bleed mass-flow rate is transformed into the influence of the displacement thickness on the effective size of the bleed hole. Based on that, the viscous bleed rate model is established, which shows a maximum error of 7.7% for a depthless bleed hole over laminar flow at Mach 4.0 and Reynolds number . Compared with the reported results of the full bleed hole, the difference of the bleed mass-flow rate predicted by the present model is 3.8%.

Transient modelling and simulation of gas turbine secondary air system

The behaviour of the jet engine during transient operation and specifically its secondary air system (SAS) is the point of this work. This paper presents a methodological approach to develop a fast, one-dimensional transient platform for preliminary analysis of the flow behaviour in gas turbine engines secondary air system. For this purpose, different elements of the system including rotating chamber, pipe, turbine blade cooling, orifice, and labyrinth seal are modelled in a modular form. The validity of the developed models for each component is checked against experimental/publicly available data. Then, using a flow network simulation approach, the secondary air system of a two-spool turbofan engine is modelled and simulated in transient mode. The coupling effect between volume packing and swirl are considered in the simulation, under two pre-defined scenarios for step and scheduled boundary condition variations. In the step-change scenario, the boundary conditions are changed instantly to represent the flow behaviour of the SAS under extreme operating conditions (i.e. shaft fracture, flameout, etc.). In the scheduled scenario, the boundary conditions vary linearly with time to represent the performance of the SAS under normal operating conditions (i.e. acceleration and deceleration). The key findings include the fact that, under normal engine operation, the flow in the SAS varies smoothly and converges much faster than the primary flow by around one magnitude. Thus, it is reasonable to use steady-state SAS model to simulate SAS flow behaviour under these conditions. However, under extreme conditions (e.g. flameout), which could induce an abrupt change in the primary airflow properties (pressure, temperature), reverse airflow or choking conditions in SAS may be observed. This could result in a malfunction of the SAS, inducing further damages to the engine.