Flow Choking Research Articles

Experimental and numerical studies on self-excited periodic oscillation of vapor condensation in a sonic nozzle

The unsteady nozzle flow induced by vapor condensation has many different self-excited periodic oscillation modes and affects flow field and mass flow-rate of sonic nozzle. A detailed discussion of thermal choking and oscillation frequencies of this unsteady flow was carried out. To investigate these different oscillation modes, an experimental sensor for measuring inlet humidity/temperature, and monitoring pressure oscillation was built. A numerical model for condensation process in sonic nozzle was established and validated by experimental data of moist nitrogen reported by Wyslouzil and Lamanna. The output of measurement sensor for nozzle flow of moist air agreed with the results of this numerical model. At last, four different oscillation modes caused by vapor condensation were explored. The rules of frequencies and amplitudes of pressure signals for these oscillation modes were discussed in detail. All results can promote the study on the effect of vapor condensation on accurate metering of sonic nozzle.

Comparison of Correlations for Predicting Oil Flow Rate Passing Through Chokes

Multiphase flow occurs in almost all producing oil wells and nearly every flowing well has some sort of choke to regulate the flowing rate, for the following reasons: (1) to maintain sufficient back pressure to prevent sand entry; (2) to protect surface equipment from high pressure; (3) to prevent gas or water coning; and (4) to produce the reservoir at the optimum flow rate. One of the most important problems in the petroleum industry is measuring fluid flow rate. For calculation of fluid flow, various methods are used. In this article, a comparison based on data obtained from an Asmari reservoir located in southwest Iran was performed. The following procedures were used to choose the best correlation: (1) evaluation of oil flow rate in production unit by separator test; (2) evaluation of oil flow rate by using the choke flow rate equations; and (3) comparison of these correlations and choosing the best fitted correlation for these data. Results showed that Baxendall (1958) correlation has the least errors and can be used to predict flow rate in this reservoir.

A Performance Prediction Model for Low-Speed Centrifugal Fans

A generalized model for mapping the trend of the performance characteristics of a double-discharge centrifugal fan is developed based on the work by Casey and Robinson (C&R), which formulated compressor performance maps for tip-speed Mach numbers ranging from 0.4 to 2 using test data obtained from turbochargers with vaneless diffusers. The current paper focuses on low-speed applications for Mach number below 0.4. The C&R model uses four nondimensional parameters at the design condition including the flow coefficient, the work input coefficient, the tip-speed Mach number, and the polytropic efficiency, in developing a prediction model that requires limited geometrical knowledge of the centrifugal turbomachine. For the low-speed fan case, the C&R formulas are further extended to a low-speed, incompressible analysis. The effort described in this paper begins by comparing generalized results using efficiency data obtained from a series of fan measurements to that using the C&R model. For the efficiency map, the C&R model is found to heavily depend on the ratio of the flow coefficient at peak efficiency to that at the choke flow condition. Since choke flow is generally not applicable in the low-speed centrifugal fan operational environment, an alternate, but accurate estimation method based on fan free delivery derived from the fan test data is presented. Using this new estimation procedure, the modified C&R model predicts reasonably well using the double-discharge centrifugal fan data for high-flow coefficients, but fails to correlate with the data for low-flow coefficients. To address this undesirable characteristic, additional modifications to the C&R model are also presented for the fan application at low flow conditions. A Reynolds number correction is implemented in the work input prediction of the C&R model to account for low-speed test conditions. The new model provides reasonable prediction with the current fan data in both work input and pressure rise coefficients. Along with the developments for the efficiency and work input coefficient maps, the use of fan shut-off and free delivery conditions are also discussed for low-speed applications.

Hydraulics of crest spillway with large unit discharge and low Froude number

With respect to the crest spillway with large unit discharge and low Froude number, the hydraulics of the slit-type energy dissipater at the outlet should be noticed due to the complicated flow regimes. In the present paper, some issues about hydraulic characteristics were experimentally investigated by means of five slit-type outlets and four tetrahedrons, including the flow choking, impact to river banks and jet trajectory. The main findings are as follows. The critical Froude number for the flow choking decreases with increasing outlet width of the slit-type energy dissipater. If the flow Froude number is expressed by the parameters just before this energy dissipater, the tetrahedron placed inside the side wall of the outlet could efficiently avoid the flow impact to the river bank of same side, and compared with the jet trajectory of the slit-type energy dissipater, the outlet with tetrahedron has different trajectory trend, i.e., the distance of the jet trajectory decreases with the increase of the water head due to special form of the outlet tetrahedron.

An easy method to design gas/vapor relief system with rupture disk

Tank discharge gas/vapor flow problems are frequently encountered in both practice and design. To perform this type of design calculation, the first step is to identify whether the flow is choked or not through a trial-and-error solution of an equation for adiabatic flow with friction from a reservoir through a pipe. Developing a direct method without any trial-and-error to identify a choking condition would be helpful for expediting the flow calculations. This paper presents an easy and quick method to identify the choking of gas flow for an emergency relief system consisting of a rupture disk and vent piping. This greatly simplifies the design calculations. The proposed method for validating the venting adequacy of existing ERS circumvents the iteration calculation and the use of Lapple charts. Three case studies for the design of vent piping for rupture disks support the proposed method.

Potential of the range extension of compressors with a variable inlet prewhirl for automotive turbocharged engines with an ultra-high-power density

Downsizing is an effective way to improve the fuel economy of an engine and to reduce the emissions. For turbocharged engines, further downsizing or increasing the power density requires air to be compressed to a higher pressure. However, conventional compressors have a narrow stable operating range at high pressure ratios and cannot provide such highly pressurized air for ultra-high-power-density engines. Thus, a variable inlet prewhirl is employed to extend the stable operating range of the compressor. This paper investigates the performance of a centrifugal compressor with different prewhirls and speeds, quantitatively estimates the potential of range extension with a variable inlet prewhirl and discusses the mechanisms thought to be responsible for the range extension. The approach combines steady three-dimensional Reynolds-averaged Navier–Stokes simulations with theoretical analysis. In order to make the study universal to various applications with a inlet prewhirl, the inlet prewhirl was generated by giving a velocity direction at the inlet boundary of the numerical method. A variable inlet prewhirl from 0° to 60° improves the stable operating range from 23.5% to 49.0% at a pressure ratio of 4.8. The extended range is based on the shifts in the choke line and the surge line with a positive inlet prewhirl. The decreased choking flow with a positive inlet prewhirl is mainly caused by the drop in the work input from the impeller. The dominant factor in reducing the surge flow of compressor is the highly increased stability of the impeller. It is also found that the impeller tends to choke and the diffuser tends to surge with increasing prewhirl. By extending the stable operating range of the compressor with a variable inlet prewhirl from 0° to 60°, the improvement in the maximum pressure for turbocharged engine is about 64% and the increase in the brake torque is estimated to be up to 42%.

Preliminary design and off-design performance analysis of an Organic Rankine Cycle for geothermal sources

Geothermal fluid of 90°C and 10kg/s can be exploited together with oil in Huabei Oilfield of China. Organic Rankine Cycle is regarded as a reasonable method to utilize these geothermal sources. This study conducts a detailed design and off-design performance analysis based on the preliminary design of turbines and heat exchangers. The radial inflow turbine and plate heat exchanger are selected in this paper. Sliding pressure operation is applied in the simulation and three parameters are considered: geothermal fluid mass flow rate, geothermal fluid temperature and condensing pressure. The results indicate that in all considered conditions the designed radial inflow turbine has smooth off-design performance and no choke or supersonic flow are found at the nozzle and rotor exit. The lager geothermal fluid mass flow rate, the higher geothermal fluid temperature and the lower condensing pressure contribute to the increase of cycle efficiency and net power. Performance maps are illustrated to make system meet different load requirements especially when the geothermal fluid temperature and condensing pressure deviate from the design condition. This model can be used to provide basic data for future detailed design, and predict off-design performance in the initial design phase.

The nature of very faint X-ray binaries: hints from light curves

Very faint X-ray binaries (VFXBs), defined as having peak luminosities LX of 1034–1036 erg s−1, have been uncovered in significant numbers, but remain poorly understood. We analyse three published outburst light curves of two transient VFXBs using the exponential and linear decay formalism of King & Ritter. The decay time-scales and brink luminosities suggest orbital periods of order 1 h. We review various estimates of VFXB properties, and compare these with suggested explanations of the nature of VFXBs. We suggest that: (1) VFXB outbursts showing linear decays might be explained as partial drainings of the disc of ‘normal’ X-ray transients, and many VFXB outbursts may belong to this category; (2) VFXB outbursts showing exponential decays are best explained by old, short-period systems involving mass transfer from a low-mass white dwarf or brown dwarf; (3) persistent (or quasi-persistent) VFXBs, which maintain an LX of 1034–1035 erg s−1 for years, may be explained by magnetospheric choking of the accretion flow in a propeller effect, permitting a small portion of the flow to accrete on to the neutron star's surface. We thus predict that (quasi-) persistent VFXBs may also be transitional millisecond pulsars, turning on as millisecond radio pulsars when their LX drops below 1032 erg s−1.

Numerical Study of Single Flow Element in a Nuclear Thermal Thrust Chamber

The objective of this study was to develop an efficient and accurate computational methodology to predict detailed thermo-fluid environments of a single flow element in a hypothetical solid-core nuclear thermal thrust chamber assembly. Several numerical and multi-physics thermo-fluid models, such as chemical reactions, turbulence, conjugate heat transfer, porosity, and power generation, were incorporated into an unstructured-grid, pressure-based computational fluid dynamics solver used in this investigation. A secondary objective was to develop a porosity model for simulation of the whole solid-core nuclear thermal engine without resolving thousands of flow channels inside the solid core. Detailed numerical simulations of a single flow element with different power generation profiles were conducted to investigate the root cause of a phenomenon called mid-section corrosion that severely damaged the flow element assembly of early solid-core reactors. Under the assumptions employed in this effort and for the first time, the result demonstrated flow choking in the flow element. The possibility of flow choking in part of the flow element indicated a potential coolant mass flow imbalance, which could lead to a high local thermal gradient in coolant-starved flow elements and possibly the eventual mid-section corrosion.

Measurement, analysis and modeling of centrifugal compressor flow for low pressure ratios

Increasingly stringent emission legislation combined with consumer performance demands has driven the development of downsized engines with complex turbocharger arrangements. To handle the complexity, model-based methods have become a standard tool, and these methods need models that are capable of describing all operating modes of the systems. The models should also be easily parametrized and enable extrapolation. Both single- and multi-stage turbo systems can operate with a pressure drop over their compressors, both stationary and transient. The focus here is to develop models that can describe centrifugal compressors that operate in both normal region and restriction region from standstill to maximum speed. The modeling results rely on an analysis of 305 automotive compressor maps, whereof five contain measured restriction operation and two contain measured standstill characteristic. A standstill compressor is shown to choke at a pressure ratio of approximately 0.5, and the corresponding choking corrected mass flow being approximately 50% of the compressor maximum flow capacity. Both choking pressure ratio and flow are then shown to increase with corrected speed, and the choking pressure ratio is shown to occur at pressure ratios larger than unity for higher speeds. Simple empirical models are proposed and shown to be able to describe high flow and pressure ratios down to choking conditions well. A novel compressor flow model is proposed and validated to capture the high flow asymptote well, for speeds from standstill up to maximum.

Numerical investigation & comparison of a tandem-bladed turbocharger centrifugal compressor stage with conventional design

Extensive numerical investigations of the performance and flow structure in an unshrouded tandem-bladed centrifugal compressor are presented in comparison to a conventional compressor. Stage characteristics are explored for various tip clearance levels, axial spacings and circumferential clockings. Conventional impeller was modified to tandem-bladed design with no modifications in backsweep angle, meridional gas passage and camber distributions in order to have a true comparison with conventional design. Performance degradation is observed for both the conventional and tandem designs with increase in tip clearance. Linear-equation models for correlating stage characteristics with tip clearance are proposed. Comparing two designs, it is clearly evident that the conventional design shows better performance at moderate flow rates. However; near choke flow, tandem design gives better results primarily because of the increase in throat area. Surge point flow rate also seems to drop for tandem compressor resulting in increased range of operation.

A lumped element method for modeling the two-phase choking flow through hydraulic orifices

A novel lumped element methodology for modeling the oil–gas two-phase flow through the hydraulic orifice has been presented, focus on the choking condition. The proposed approach consists of a gas cavitation model developed based on “Full Cavitation Model” and a formulation considering the homogenous fluid assumption for evaluating the mass flow rate. The two-phase choking flow occurring at extreme condition is characterized in detail by using the present method, in terms of the critical pressure, sound speed, filling ratio and the critical speed of pump operation in the hydraulic circuit. Mach number is found to be 1 and the critical speed is estimated by the inlet volumetric flow rate at choking condition of the two-phase flow. The effects of different parameters including the orifice diameter, the upstream pressure and the cavitation model coefficient are investigated to find possible ways of improving the operation of the pump circuit. Reducing the cavitation model coefficient is identified to be both advantageous for reducing the critical pressure and increasing the critical speed.

Supercritical flow in circular conduit bends

ABSTRACTA complex flow pattern occurring in a circular conduit bend with supercritical flow has been studied. The main objective was to obtain reliable criteria for prediction of the onset of helical and choking flow, in order to facilitate the design of bottom outlets, tunnel spillways and drainage collectors with bends. A series of scale model experiments provided data for developing empirical relationships describing the effects of the bend curvature, bend deflection angle and approach flow conditions (relative flow depth and Froude number) on the inception of helical and choking flow. Expressions for the energy bend loss coefficient were also proposed. The bend curvature and approach flow conditions proved to have a major influence on the onset of helical flow, while the deflection angle becomes an important parameter for angles less than 45°. Bend curvature and deflection angle have commensurable influence on the onset of choking flow.

Choke modeling and flow splitting in a gas-condensate offshore platform

Well production rate is a major factor for simulation of commingled gas wells. It is the base of computations of well flowing pressure, pressure drop, well performance, flow regime, liquid holdup, reservoir performance, material balance and average reservoir pressure. Typically, well flow rate is measured using flow meters like orifice, but in some wells flow rates remain unknown due to shortage of flow meters and commingled production.In this paper, well flow splitting of a gas-condensate field with two platforms is studied. Each platform consists of ten wells and wellhead pressure is controlled by a choke valve. Total flow rate of platforms mixed production is measured after sending their production to refinery. The proposed methodology for dividing total flow between wells consists of choke modeling using test separator data and flow modeling in the wells. Choke models include mechanistic and Ashford and Pierce as theoretical models and Gilbert, Ros and Pilehvari as empirical models. In most investigations choke size is represented with length unit such as inches but in this work, flow is calculated on the base of choke opening percent and choke size as a function of choke opening percent is obtained. Chokes are modeled with upstream temperature, upstream pressure and downstream pressure. Rich gas flow calculations with the proposed method were compared with reported total flow of platforms and showed mean absolute percentage error of 3.2% for mechanistic model as the best of studied models. This error can cause 0.5% error in calculated well flowing pressure.Sensitivity analysis on operation parameters showed that since the gas-condensate field is lean, gas flow rate is the major phase for modeling and uncertainty of gas-condensate ratio and changing specific gravity and molecular weight of condensate with time do not have considerable effect on well flow rate calculations.

Thrust Enhancement Through Bubble Injection Into an Expanding-Contracting Nozzle With a Throat

This paper addresses the concept of thrust augmentation through bubble injection into an expanding-contracting nozzle with a throat. The presence of a throat in an expanding-contracting nozzle can result in flow transition from the subsonic regime to the supersonic regime (choked conditions) for a bubbly mixture flow, which may result in a substantial increase in jet thrust. This increase would primarily arise from the fact that the injected gas bubbles expand drastically in the supersonic region of the flow. In the current work, an analytical 1D model is developed to capture choked bubbly flow in an expanding-contracting nozzle with a throat. The study provides analytical and numerical support to analytical observations and serves as a design tool for nozzle geometries that can achieve efficient choked bubbly flows through nozzles. Starting from the 1D mixture continuity and momentum equations, along with an equation of state for the bubbly mixture, expressions for mixture velocity and gas volume fraction were derived. Starting with a fixed geometry and an imposed upstream pressure for a choked flow in the nozzle, the derived expressions were iteratively solved to obtain the exit pressures and velocities for different injected gas volume fractions. The variation of thrust enhancement with the injected gas volume fraction was also studied. Additionally, the geometric parameters were varied (area of the exit, area of the throat) to understand the influence of the nozzle geometry on the thrust enhancement and on the flow conditions at the inlet. This parametric study provides a performance map that can be used to design a bubble augmented waterjet propulsor, which can achieve and exploit supersonic flow. It was found that the optimum geometry for choked flows, unlike the optimum geometry under purely subsonic flows, had a dependence on the injected gas volume fraction. Furthermore, for the same injected gas volume fraction the optimum geometry for choked flows resulted in greater thrust enhancement compared to the optimum geometry for purely subsonic flows.

Mass flowrate characteristics of supercritical CO2 flowing through an electronic expansion valve

Electronic expansion valves (EEV) are employed increasingly in transcritical CO2 systems. In order to investigate the mass flowrate characteristics of supercritical CO2 flowing through an EEV, the effects of EEV outlet pressure on the mass flowrate of CO2 is studied under constant EEV inlet pressure and inlet temperature. The two-phase flow patterns of supercritical CO2 flowing through an EEV are revealed, and the conventional choked flow judgment formulas is proved effective for supercritical CO2. Based on Bernoulli Equation with expansion factor, the mass flowrate correlation of supercritical CO2 flowing through an EEV is presented. The influence of the expansion coefficient on the mass flowrate is considered in the correlation in which the correction coefficient is only affected by the EEV opening, the changing rate of the density and the pressure difference in the entrance region. The mass flowrate predicted by the correlation in this paper shows a good agreement with experimental data. The maximum relative errors are within 10%. The average and standard deviations are 0.76% and 5.9%, respectively.

Prediction of two-phase choked-flow through safety valves

Different models of two-phase choked flow through safety valves are applied in order to evaluate their capabilities of prediction in different thermal-hydraulic conditions. Experimental data available in the literature for two-phase fluid and subcooled liquid upstream the safety valve have been compared with the models predictions. Both flashing flows and non-flashing flows of liquid and incondensable gases have been considered. The present paper shows that for flashing flows good predictions are obtained by using the two-phase valve discharge coefficient defined by Lenzing and multiplying it by the critical flow rate in an ideal nozzle evaluated by either Omega Method or the Homogeneous Non-equilibrium Direct Integration. In case of non-flashing flows of water and air, Leung/Darby formulation of the two-phase valve discharge coefficient together with the Omega Method is more suitable to the prediction of flow rate.

Prediction of natural gas flow through chokes using support vector machine algorithm

In oil and gas fields, it is a common practice to flow liquid and gas mixtures through choke valves. In general, different types of primary valves are employed to control pressure and flow rate when the producing well directs the natural gas to the processing equipment. In this case, the valve normally is affected by elevated levels of flow (or velocity) as well as solid materials suspended in the gas phase (e.g., fine sand and other debris). Both surface and subsurface chokes may be installed to regulate flow rates and to protect the porous medium and surface facilities from unusual pressure instabilities.In this study a reliable, novel, computer based predictive model using Least-Squares Support Vector Machine (LSSVM) algorithm is applied to predict choke flow coefficient in both nozzle and orifice type chokes in subsonic natural gas flow conditions. The average absolute relative deviation of the proposed model from reported data for nozzle-type and orifice-type choke are nearly 0.25% and 0.15% and the squared correlation coefficient is around 0.9961 and 0.9982 respectively.

Experimental study of abrupt discharge of water at supercritical conditions

Future SuperCritical Water-cooled nuclear Reactors (SCWRs) will operate at a coolant pressure close to 25MPa and at outlet temperatures ranging from 500°C to 625°C, i.e., above the critical pressure and temperature of the water (22.06MPa and 373.95°C, respectively). Coolant pressures higher than critical values will be used to avoid boiling and eventual critical heat flux that may occur. In addition, the outlet flow enthalpy in future supercritical water-cooled nuclear reactors will be much higher than those of actual ones, which can increase overall nuclear plant efficiencies of up to 48%. However, under such flow conditions, thermal–hydraulic behaviors of supercritical water are not fully known, i.e., pressure drop, the deterioration of forced convection heat transfer, critical (choked) flow, blow-down flow rate, etc. In particular, the knowledge of critical discharge of supercritical fluids is mandatory to perform nuclear-reactor safety analyses and to design key mechanical components. Nevertheless, existing choked-flow data have been collected from experiments at atmospheric discharge pressure conditions, but in most cases using working fluids different than water. Therefore, a supercritical water facility has been built at the École Polytechnique de Montréal. In this paper, a new database containing 524 data points is obtained using this facility and compared with available information from the open literature.

Theoretical Model and Application of Nodal Analysis in Layered Water Flooding Regime - Take Bi-Layer Reservoir as an Example

A bi-layer mathematical model of nodal analysis under the condition of separate layer injection-production has been set up on the study of inflow performance relationship between injectors and producers, horizontal flow, vertical flow, choke flow etc., according to nodal analysis principle. In theory, this method can be used to determine reasonable working system of injectors and producers, and can be optimized the allocation and injection allocation, and can also be made water flooding regime in its optimum condition.