Choking Phenomenon Research Articles

Choking phenomenon and pressure drop mechanism in electronic expansion valves

A quasi one-dimensional model in which oblique evaporation wave theory and shock wave theory are employed is developed to investigate the choking phenomenon and the pressure drop mechanism in electronic expansion valves (EEV). The data predicted by the model are validated by the experimental data presented by Zhang. The results show that the choking mass flow rate of EEVs calculated by Abuaf’s model, which was developed with water, agrees well with the tested data when R22 is used as the working fluid. The relative deviation between the predicted data and Zhang’s test results is in the range from −5% to 6%. Compared with the test data, the choking mass flow rate of EEVs with the mixture R407c is over predicted by Abuaf’s model. The back pressure at the EEV exit when choking just occurs is decreased as the flow area (opening pulse number) is decreased. The trend is consistent with Zhang’s test results.

Eulerian simulation of heterogeneous gas–solid flows in CFB risers: EMMS-based sub-grid scale model with a revised cluster description

Gas–solid two-phase flow in CFB risers is characterized by the clustering of solid particles producing dynamical multi-scale structures, and how to quantify such heterogeneity is a critical yet unsolved issue. Recently, incorporating the energy minimization multi-scale (EMMS) model with Eulerian approach has obtained encouraging results for simulating the hydrodynamics in CFB risers. However, owing to the cluster diameter correlation used, the present model is still limited to the simulation of Geldart A particles. In this study, a stochastic geometry approach named doubly stochastic Poisson processes is used to analyze the fluctuation characteristics of solid concentration in CFB risers, which provides a mean to define the solid concentration inside clusters. The predicted results are validated by experimental data available in literature, and a revised cluster diameter correlation is then proposed for EMMS model previously developed for cocurrent-up gas–solid flow. Following our previous studies, the EMMS model thus improved is incorporated into an Eulerian–Eulerian description of gas–solid flow as a sub-grid scale model for inter-phase drag force, with which the hydrodynamics of both Geldart A and Geldart B particles in CFB risers are simulated. It is shown that the experimentally found S-shaped axial voidage profiles and the choking phenomenon can be well predicted. The computed one-dimensional slip velocities decrease toward the top of the risers and increase with decreasing cross-sectional averaged voidages. The experimentally found dependence of the root mean square of the solid concentration on its mean value at a given position is also well predicted.

Choking Phenomena of Gas‐Powder Flows at Sonic Velocity

Choking phenomena of gas‐powder flows in a horizontal constant cross‐section duct are experimentally studied using three kinds of powders: Al2O3, hollow Al2O3 (H‐alumina) and Al, with different physical properties such as densities, specific heat capacities and thermal conductivities. The relationship between the mass flowrate and inlet stagnant pressures is analysed and the apparent sonic velocity (ASV) of gas‐powder flows is defined. The influences of physical properties of powders on the ASV are obtained by the comparison between the experimental results of different powders. Two characteristic parameters, the effective specific heat capacity (ESHC) of solid phase and the effective isentropic exponent (EIE) of the mixture, are defined to describe the choking phenomena. The ESHC is not only measured through experiment but also calculated by numerical method. The calculated results are well consistent with the measured ones. As a preliminary application test for choking phenomena ‐ in order to uniform the pulverized coal injection (PCI) into a blast furnace ‐, a model PCI system with two branch pipes was set up and tests were carried out to verify the choking effect. The test results show that, during choking, the PC flow rates in the branch pipes can be well equalized no matter how much the downstream conditions differ from each other.

Nonsynchronous Vibration (NSV) due to a Flow-Induced Aerodynamic Instability in a Composite Fan Stator

Abstract This paper describes the identification and prediction of a new class of nonsynchronous vibration (NSV) problem encountered during the development of an advanced design composite fan stator for an aircraft engine application. Variable exhaust nozzle testing on an instrumented engine is used to map out the NSV boundary, with both choke- and stall-side instability zones present that converge toward the nominal fan operating line and place a limit on the high-speed operating range. Time-accurate three-dimensional viscous CFD analyses are used to demonstrate that the NSV instability is being driven by dynamic stalling of the fan stator due to unsteady shock-boundary layer interaction. The effects of downstream struts in the front frame of the engine are found to exasperate the problem, with the two fat service struts in the bypass duct generating significant spatial variations in the stator flow field. Strain gage measurements indicate that the stator vanes experiencing the highest vibratory strains correspond to the low static pressure regions of the fan stator assembly located approximately 90 degrees away from the two fat struts. The CFD analyses confirm the low static pressure sectors of the stator assembly are the passages in which the flow-induced NSV instability is initiated due to localized choking phenomena. The CFD predictions of the instability frequency are in reasonable agreement with the strain gage data, with the strain gage data indicating that the NSV response occurs at a frequency approximately 25% below the frequency of the fundamental bending mode. The flow patterns predicted by the CFD analyses are also correlated with the results of an engine flow visualization test to demonstrate the complex nature of the flow field.

Theoretical Analysis on Choking of Convergent Nozzle Flows with Boundary Layers

To understand choking phenomena of real nozzle flows, an analysis of weak shock waves in duct flows with boundary layers has been carried out by the control volume method where an appropriate mean flow is assumed over a given cross section of the flows. Relations for plane sound waves as the limiting case of the weak shock waves are presented. The criteria for choking and discharge coefficients of convergent nozzles taking account of boundary layers are shown and discussed. As a result, it is found that the ratio of the boundary layer thickness to the duct half-height (or radius in axisymmetric geometries) at the nozzle exit of a convergent nozzle significantly affects choking conditions as well as discharge coefficients.

Visualization of secondary flow choking phenomena in a supersonic air ejector

The present study deals with the visualization of the air flow inside a supersonic ejector. Our attention is more precisely focused on the choked flow phenomenon which occurs along the mixing chamber of the secondary nozzle and which can be visualized by CFD. Laser tomography visualizations are used to validate the CFD model. The evolution of flow configuration in the ejector with the primary stagnation pressure is examined both in the case of zero secondary flow and in the case of free entrainment of induced air.

Analysis of the Magnetohydrodynamic Energy Bypass Engine for High-Speed Airbreathing Propulsion

The performance of the MHD energy bypass air-breathing engine for high-speed propulsion is analyzed in this investigation. This engine is a specific type of the general class of inverse cycle engines. In this paper, the general relationship between engine performance (specific impulse and specific thrust) and the overall total pressure ratio through an engine (from inlet plane to exit plane) is first developed and illustrated. Engines with large total pressure decreases, regardless of cause or source, are seen to have exponentially decreasing performance. The ideal inverse cycle engine (of which the MHD engine is a sub-set) is then demonstrated to have a significant total pressure decrease across the engine; this total pressure decrease is cycle-driven, degrades rapidly with energy bypass ratio, and is independent of any irreversibility. The ideal MHD engine (inverse cycle engine with no irreversibility other than that inherent in the MHD work interaction processes) is next examined and is seen to have an additional large total pressure decrease due to MHD-generated irreversibility in the decelerator and the accelerator. This irreversibility mainly occurs in the deceleration process. Both inherent total pressure losses (inverse cycle and MHD irreversibility) result in a significant narrowing of the performance capability of the MHD bypass engine. The fundamental characteristics of MHD flow acceleration and flow deceleration from the standpoint of irreversibility and second-law constraints are next examined in order to clarify issues regarding flow losses and parameter selection in the MM modules. Severe constraints are seen to exist in the decelerator in terms of allowable deceleration Mach numbers and volumetric (length) required for meaningful energy bypass (work interaction). Considerable difficulties are also encountered and discussed due to thermal/work choking phenomena associated with the deceleration process. Lastly, full engine simulations utilizing inlet shock systems, finite-rate chemistry, wall cooling with thermally balanced engine (fuel heat sink), fuel injection and mixing, friction, etc. are shown and discussed for both the MHD engine and the conventional scramjet. The MHD bypass engine has significantly lower performance in all categories across the Mach number range (8 to 12.2). The lower performance is attributed to the combined effects of 1) additional irreversibility and cooling requirements associated with the MHD components and 2) the total pressure decrease associated with the inverse cycle itself.

ECT studies of the choking phenomenon in a gas–solid circulating fluidized bed

AbstractChoking is commonly defined as a phenomenon where a sudden change in the solids holdup occurs in a gas–solid fluidization system. In this study, the electrical capacitance tomography (ECT) based on the neural network multicriteria optimization image reconstruction technique (NNMOIRT), developed by this research group, is used to probe the mechanism of choking formation by examining a real‐time, quasi‐3D cross‐sectional flow structure of a circulating fluidized bed riser. The particles used are FCC catalysts. The ECT with NN‐MOIRT reveals a double solids‐ring flow structure and the presence of particle blobs at the center of the bed in the circulating fluidized bed (CFB) riser. This flow structure, when it is present at the entrance region of the riser, undergoes a distinct variation during the choking transition when the gas velocity is below the transport velocity Utr, but does not undergo a distinct variation when the gas velocity is above Utr. This flow structure, when it is present at the upper region of the riser, however, does not vary distinctly at any gas velocity. Such flow structure variations are not appreciably affected by the solids feeding patterns with or without a gas or solids distributor, or by the gas humidity. For the low gas velocities (<Utr), the borescope measurement indicates that the solids concentration in the particle blob and the blob size all increase as the solids circulation rate increases. The disintegration of the enlarged blobs or blob jets and the collapse of the solids suspension characterize the mechanics of the initiation of the choking transition to the dense‐phase fluidization regime attributed to solids suspension instability. © 2004 American Institute of Chemical Engineers AIChE J, 50: 1386–1406, 2004

Characteristics of Choking Behavior in Circulating Fluidized Beds for Group B Particles

The electrical capacitance tomography (ECT) based on the neural network multi-criteria optimization image reconstruction technique (NNMOIRT) developed by this research group is used to study the choking phenomenon for Geldart Group B particles in a circulating fluidized bed (CFB). The Group B particles employed are sand particles with a mean diameter of 240 μm and a particle density of 2200 kg/m 2 . The ECT examines the real-time, quasi-3D cross-sectional flow structure of a gas-solid circulating fluidized bed. The ECT with NNMOIRT reveals a double solids-ring flow structure at a low solids circulation rate and the presence of solids blobs at the center of the bed at a high solids circulation rate before choking in a CFB. This flow structure undergoes a distinct variation during the choking transition forming wall slugs/gas intervals when the gas velocity is below the transport velocity, U tr , or forming open slugs with blobs/ particles at the center when the gas velocity is above U tr . The standard deviation of the solids concentration fluctuation and the cross-sectional time-averaged solids concentration variation with increasing solids circulation rates exhibit a sharp change during the choking transition for a low gas velocity (<U tr ), but this is not the case when the gas velocity is above U tr . The choking transition initiates at the bottom of the bed; as the solids circulation rate increases, choking occurs at the surface of the dense bed leading to an increase in the bed height, eventually reaching the top of the column.

Computational study of the gas flow through a critical nozzle

The critical nozzle is defined as a device to measure the mass flow with only the nozzle supply conditions making use of the flow choking phenomenon at the nozzle throat. The discharge coefficient and critical pressure ratio of the gas flow through the critical nozzle are strongly dependent on the Reynolds number, based on the diameter of the nozzle throat and nozzle supply conditions. Recently a critical nozzle with a small diameter has been extensively used to measure mass flow in a variety of industrial fields. For low Reynolds numbers, prediction of the discharge coefficient and critical pressure is very important since the viscous effects near walls significantly affect the mass flow through the critical nozzle, which is associated with working gas consumption and operation conditions of the critical nozzle. In the present study, computational work using the axisymmetric, compressible, Navier-Stokes equations is carried out to predict the discharge coefficient and critical pressure ratio of gas flow through the critical nozzle. In order to investigate the effect of the working gas and turbulence model on the discharge coefficient, several kinds of gases and several turbulence models are employed. The Reynolds number effects are investigated with several nozzles with different throat diameters. The diffuser angle is varied in order to investigate the effects on the discharge coefficient and critical pressure ratio. The computational results are compared with the previous experimental ones. It is known that the standard k-ε turbulence model with the standard wall function gives the best prediction of the discharge coefficient. The discharge coefficient and critical pressure ratio are given by functions of the Reynolds number and boundary layer integral properties. It is also found that the diffuser angle affects the critical pressure ratio.

Evaluating the phenomenon of choking in professional golfers.

A study involving 775 professional golfers investigated whether choking occurs in the PGA Tour's Qualifying Tournaments, known among golfers for its high pressure. It was hypothesized that players who were near or at the cutoff for earning a tour card would have higher final round scores than players whose scores entering the final round were either four or five strokes better or worse. However, the data did not support a choking hypothesis. There were no significant differences in final round scores for the conditions.

先細ダクトにおける複合圧縮流れの可視化

The compound choking phenomena by confluence of two subsonic streams flowing in the same converging duct were experimentally investigated by the optical observations as well as the pressure measurements. A criterion for the compound choking is shown and discussed. Also, the experimental data are compared with the results calculated by using an inviscid one-dimensional model.

Choking phenomena of sonic nozzles at low Reynolds numbers

The choking phenomena of sonic nozzles were investigated for the Reynolds number range from 40 to 30 000 for nitrogen gas. The results showed that the critical back pressure ratio is a function of the Reynolds number only, with different characteristics for different nozzle shapes. In ISO-type toroidal-throat Venturi nozzles, the minimum Reynolds number satisfying the choking condition is about 40 and the critical back pressure ratio is only about 0.05 at this minimum Reynolds number. It was also found that the critical back pressure ratio has a local minimum value around Re th=4000 and that the local maximum value is around Re th=10 000 due to the change in characteristics of the boundary layer in the diffuser. On the other hand, the critical back pressure ratio in quadrant nozzles decreases monotonically with decreasing Reynolds number, unlike the former nozzle, and the minimum Reynolds number necessary for choking is estimated to be approximately the same as that in the Venturi nozzle.

One-dimensional analysis of thermal choking in case of heat addition in ducts

The thermal choking phenomenon is of great importance in an inlet isolator in dual-mode ram jet/scramjet combustor. In some cases the choked flow creates a pseudo-shock wave including a shock train in it at the engine inlet and causes large amounts of drag and radically reduces the performance of the engine at high flight Mach numbers. The present paper describes a one-dimensional flow model taking account of the upstream boundary-layer as well as heat addition by using a mass-weighted averaging technique. The simple relationships for the flow field in a constant area duct in which the effect of the upstream boundary-layer is considered but the effect of the wall friction in the duct can be neglected are presented. The results of the calculation such as the maximum heat addition when the thermal choking occurs, the downstream Mach number and the static pressure ratio are presented and examined in detail.

Model for dilute gas–particle flow in constant-area lance with heating and friction

Considering the influence of the heat transfer and friction between the dilute gas–particle flow and the inner wall of lance, a mathematical model describing the behavior of the dilute gas–particle flow passing through constant-area lance has been developed. According to the Mach number checkout method, the choking phenomena for heat and friction of dilute gas–particle flow with the lance wall were determined. At the same time, the critical duct length of gas–particle flow was acquired by the model.

Ecoulement critique d’un liquide en vaporisation à travers une ligne de décharge comportant un orifice

Original experimental results related to the critical flashing flows through a horizontal pressure relief line involving a circular orifice are presented. On the basis of the evolution of the physical variables measured as a function of the pressure in the downstream vessel, the occurrence of the flow with the simultaneous double locations of the critical section is demonstrated and analysed experimentally. For a high enough pressure drop between the two extreme vessels, the double choking phenomenon (i.e. the simultaneous locations of a critical section at the vicinity of the orifice as well as of one other at the outlet of the line) is observed for the flow across the pressure relief line in steady-state conditions. Depending of the value of the subcooling of the fluid at the inlet of the discharge line, the characteristics of the axial pressure profile downstream from the orifice as well as the flow visualisation support the existence of several two-phase jet structures. Finally, the influence of the orifice geometry on the liquid superheat and on the critical mass flow-rate is discussed. Résumé Des résultats expérimentaux originaux relatifs aux écoulements critiques en vaporisation à travers une ligne de décharge horizontale munie d’un orifice circulaire sont présentés. Sur la base de l’évolution des variables physiques en fonction de la pression dans le réservoir aval, l’occurrence des différents régimes parmi lesquels l’écoulement à double localisations simultanées de la section critique est analysée. Pour une différence de pression entre les deux réservoirs extrêmes suffisamment élevée, la double criticité de l’écoulement (c’est-à-dire les localisations simultanées d’une section critique observée au voisinage de l’orifice et d’une autre à l’extrémité de la ligne) est obtenue dans un état stationnaire. Suivant le degré de sous-refroidissement du fluide à l’entrée de la ligne, la nature du profil axial de pression à l’aval de l’orifice ainsi que la visualisation de l’écoulement supportent l’existence de différentes structures de jet. L’influence de la géométrie de l’orifice sur la métastabilité du liquide et le débit-masse critique est finalement discutée.

Steady-state critical two-phase flashing flow with possible multiple choking phenomenon

In this paper, the modelling of pure fluid steady-state adiabatic flashing flow through a pipe involving an abrupt enlargement is presented. The thermal non-equilibrium two-phase flow model DEM has been adjusted for subcooled inlet conditions close to saturation and/or for inlet two-phase mixture state. The multichoking flow phenomenon occurs when two basic criteria are simultaneously fulfilled. A general procedure taking into account this possible occurrence is developed on the basis of two iterative algorithms. The first algorithm is applied to the mass flow rate upstream from the enlargement and the second algorithm is based on the length of the pipe downstream from the enlargement. The proposed methodology involves the improved physical one-dimensional two-phase flow model DEM and the global non-equilibrium model through the abrupt enlargement. (C) 1999 Elsevier Science Ltd. All rights reserved.

Steady-state critical two-phase flashing flow with possible multiple choking phenomenon

The numerical results from the proposed methodology given in part 1 are presented. The sensitivity of the wall roughness of the pipe is analysed for both cases: subcooled liquid or two-phase mixture at the entrance of the pipe. A systematic comparison between models HEM and DEM and the experimental results shows that the metastability effects of the liquid phase must be taken into account even for long pipes. The predictions of the DEM are compared favourably with the experimental data of the mass flow rate as well as of the pressure profile obtained for simple-choked and double-choked steam-water flashing flows. (C) 1999 Elsevier Science Ltd. All rights reserved.

高温衝撃風洞における駆動気体汚染の測定

Driver gas contamination in high-enthalpy shock tunnels has been detected using a gasdynamical device composed of a duct and a wedge. The detection technique was developed in the T 5 shock tunnel, and a detector has been newly designed for use in the HEK and HIEST shock tunnels with some modifications. Driver gas in low concentration is satisfactorily detected at the test sections of both shock tunnels. The detector allows the flow visualization of duct internal flow, and a sequence of Schlieren pictures during a shot gives a better understanding of unsteady duct choking phenomena. Futhermore, the useful test time is compared among three shock tunnels and the optimum length-to-diameter ratio of shock tube is discussed.

Choking in water supply structures and natural channels

Techniques are presented to predict the choking phenomenon which occurs in natural and man made waterways with constriction features. The techniques use the energy equation, the momentum principle, and the continuity equation in one dimensional flow derivation; both the form and the friction losses are considered in the study. The different loss coefficients assumed and introduced in the governing equations are extracted and grouped in functional relationships to predict the choking occurrences. Governing equations are transformed to relate a threshold value of Froude number, termed as the limiting Froude number, to the geometry of the constriction feature and to the discharge coefficient for different flow regimes including subcritical and supercritical approach flow conditions. Governing flow equations in constricted channels result in a Froude number termed as the working Froude number. By comparing the limiting and the working Froude numbers, occurrences of choking phenomenon can be predicted. Theoretical derivations are used in conjunction with laboratory data for horizontal board constrictions to develop a more general coefficient of discharge applicable to severe contractions and to estimate the subcritical upstream flow conditions.