Orifice plate cavitation mechanism and its influencing factors

The orifice plate energy dissipater is an economic and highly efficient dissipater. However, there is a risk of cavitaion around the orifice plate flow： In order to provide references for engineering practice, we examined the cavitation mechanism around the orifice plate and its influencing factors by utilizing mathematical analysis methods to analyze the flow conditions around the orifice plate in view of gas bubble dynamics. Through the research presented in this paper, the following can be observed： The critical radius and the critical pressure of the gas nucleus in orifice plate flow increase with its initial state parameter r0 ; the development speed of bubbles stabilizes at a certain value after experiencing a peak value and a small valley value; and the orifice plate cavitation is closely related to the distribution of the gas nucleus in flow. For computing the orifice plate cavitation number, we ought to take into account the effects of pressure fluctuation. The development time of the gas nucleus from the initial radius to the critical radius is about 107-10-5 s; therefore, the gas nucleus has sufficient time to develop into bubbles in the negative half-cycle of flow fluctuation. The orifice critical cavitation number is closely related to the orifice plate size, and especially closely related with the ratio of the orifice plate radius to the tunnel radius. The approximate formula for the critical cavitation number of the square orifice plate that only considers the main influencing factor was obtained by model experiments.

Experimental Study on the Flow Characteristics of a Plate with a Mechanically Choked Orifice

The mechanically choked orifice plate (MCOP) is a new type of device for flow control by which choking conditionsfor incompressible fluids can be obtained with relatively small pressure losses. Given the lack of relevant results anddata in the literature, in the present study, we concentrate on the experimental determination of the flow coefficientfor the annular orifice, the pressure distribution in the MCOP, and the characteristics of the choked flow itself. Asconfirmed by the experimental results, the Reynolds number, the orifice plate thickness, the plug taper, and theeccentricity have an obvious influence on the aforementioned flow coefficient. The pressure drop in the MCOPis mainly generated near the orifice plate, and the pressure upstream of the orifice plate is slightly reduced in theflow direction, while the pressure downstream of the orifice plate displays a recovery trend. The choked flow rateof the MCOP can be adjusted by replacing the spring with a maximum flow control deviation of 4.91%.

A criterion for flow mechanisms through vertical sharp-edged orifice and model for the orifice discharge coefficient

According to the experimental data of the orifice discharge coefficient for the flow through a vertical sharp-edged orifice obtained in the previous study of this work,a theoretical criterion for flow mechanisms of small orifice（viz.thick-walled orifice and nozzle） and large orifice（viz.thin-walled orifice） was proposed based on the ratio of orifice diameter to plate thickness.It can help explain the dissipation of the mechanical energy loss in the flow process for the two flow mechanisms under different operating regimes.The main parameters such as orifice diameter,plate thickness and liquid head were correlated,and a semi-empirical model for orifice coefficient and an empirical model with high precision at the stable region were developed.

Experimental study on prediction model of wet gas pressure drop across single-orifice plate in horizontal pipes in the low gas phase Froude number region

The pressure drop prediction of wet gas across single-orifice plate in horizontal pipes had been solved satisfactorily under an annular-mist flow in the upstream of orifice plates.However,this pressure drop prediction is still not clearly determined when the upstream is in an intermittent flow or stratified flow,which is corresponding to a region of low FrG(gas phase Froude number)in the flow pattern map of wet gases.In this study,the wet gas pressure drop across a single-orifice plate was experimentally investigated in the low FrG region.By the experiment,the flow pattern transition in the downstream of single-orifice plates,as well as the effects of FrG and FrL(liquid phase Froude number)on UG(gas phase multiplier),were determined and compared when the upstream is in the flow pattern transition and the stratified flow region,respectively.Prediction performances were examined on the available pressure drop models.It was found that no model could be capable of jointly predicting the wet gas pressure drop in the low FrG region with an acceptable accuracy.With a new method of correlating FrG and FrL simultaneously,new correlations were proposed for the low FrG region.Among which the modified Chisholm model shows the best prediction accuracies,with the prediction deviations of UG being within 7%and 3%when the upstream is in flow pattern transition and stratified flow region,respectively.

Minimum wall pressure coefficient of orifice plate energy dissipater

Orifice plate energy dissipaters have been successfully used in large-scale hydropower projects due to their simple structure, convenient construction procedure, and high energy dissipation ratio. The minimum wall pressure coefficient of an orifice plate can indirectly reflect its cavitation characteristics： the lower the minimum wall pressure coefficient is, the better the ability of the orifice plate to resist cavitation damage is. Thus, it is important to study the minimum wall pressure coefficient of the orifice plate. In this study, this coefficient and related parameters, such as the contraction ratio, defined as the ratio of the orifice plate diameter to the flood-discharging tunnel diameter; the relative thickness, defined as the ratio of the orifice plate thickness to the tunnel diameter; and the Reynolds number of the flow through the orifice plate, were theoretically analyzed, and their relationships were obtained through physical model experiments. It can be concluded that the minimum wall pressure coefficient is mainly dominated by the contraction ratio and relative thickness. The lower the contraction ratio and relative thickness are, the larger the minimum wall pressure coefficient is. The effects of the Reynolds number on the minimum wall pressure coefficient can be neglected when it is larger than 10^5. An emoirical expression was presented to calculate the minimum wall oressure coefficient in this study.

Manufacture of Precious Metal Products： Advancement and Prospect

A survey about OJSC "SIC Supermetal'" as a processor of secondary precious metal raw materials and a manufacturer of precious metal products for technical purposes,has been presented.Brief information has been given about the basic technologies and materials used in production,including dispersion strengthened materials on the basis of platinum alloys and laminar composites.

Experimental study on the degradation of Rhodamine B by hydrodynamic cavitation technique

Hydrodynamic cavitation is a new technique in wastewater treatment processes. The degradation of Rbodamine B was studied on a 220 liters hydrodynamic cavitation setup using multiple hole orifice plates in this paper. The experimental results showed that Rhodamine B was really decomposed by hydrodynamic cavitation. Some factors influencing degradation effect i.e. geometric parameters and operation conditions also were discussed. It was concluded there was the optimal ratio of total area of holes to crosssectional area of the pipe and the rate constant increased with a reduction in the value of the modified cavitional number.

Characteristics of the recirculation zone in a cylindrical confined space and its suppression by an annular orifice plate

The accumulation of pollutants in the recirculation zone can worsen ventilation.It is critical to reduce recirculation zones to improve the ventilation efficiency of buildings.However,the variation rule of the recirculation zone in a cylindrical confined space(CCS)is unclear,and there are few solutions to suppress or eliminate the recirculation zone at present.In this paper,an annular deflector orifice plate for suppressing the recirculation zone was developed based on the structural characteristics of the CCS.This device is simple in structure and can be used flexibly.Through experiments and numerical simulations,the variation rule of the recirculation zone length and the influence of structural parameters of the device on the vortex suppression were explored.Firstly,empirical formulas for calculating the length of the recirculation zone in the CCS were obtained.In addition,it was proved that placing the annular orifice plate inside the CCS effectively reduced the recirculation zone and improved the ventilation efficiency.Compared to the system without the annular orifice plate,the dimensionless length of the recirculation zone was decreased by 76.3%,and the time to completely discharge the pollutants from the CCS was decreased by 16.7%.Finally,parameters of the annular orifice plate that form the best vortex suppression effect were proposed:the porosity range was 40%–50%,uniform in shape with equal ring spacing,and placed more than one inlet diameter away from the inlet.The results help guide the ventilation design of CCS.

Flow field characteristics,mixing and emissions performance of a lab-scale rich-quench-lean trapped-vortex combustor utilizing a quench orifice plate combined with a bluff-body

In this study,the low emission combustion technology of Rich-Quench-Lean(RQL)has been applied in Trapped-Vortex Combustor(TVC),and the combinative RQL-TVC shows a promising low emissions performance.By utilizing a quench orifice plate combined with a bluffbody,a lab-scale RQL-TVC was designed.The flow fields of RQL-TVC were measured by 2-D PIV and predicted by 3-D numerical simulation.Flow structures,radial profiles of normalized mean axial velocity,turbulence intensity and mixing level of the quench zone were analyzed.Results reveal that the dual-vortex and the single-vortex flow patterns both exist in cavities and quench zone of RQL-TVC,and the turbulence intensity is strong in the quench zone with some reverse flows.The spiral vortex was discussed by 3-D streamlines and the detail flow structures of the quench zone were analyzed based on the numerical results.The mixing level of the quench zone was determined,and results show that the quench device enhances the mixing level compared with TVC.Combustion efficiency and emissions performance were investigated experimentally,and results demon-strate that RQL-TVC has relatively higher combustion efficiency and lower emission index of CO,UHC and NO_xthan the same size lab-scale TVC in present work.

Hydraulic Characteristics of Multi-Stage Orifice Plate

Energy dissipater of multi-stage orifice plate, as a kind of effective energy dissipater with characteristics of high energy dissipation ratio and low cavitations risk, has become welcomed more and more by hydraulics researchers. The relationship between the contraction ratio of upper stage orifice plate and the lower one's under the principle of equal-cavitation characteristics, and the reasonable distance between upper stage orifice plate and the lower one under the condition of complying with this principle, are two important factors to be considered for multi-stage energy dissipater design. In the present paper, these two factors were analyzed by theoretical consideration and numerical simulations, and solving methods were put forward. The conclusion in this paper was proved to be reasonable by model experiment.

Comparison on Hydraulic Characteristics Between Orifice Plate and Plug

Orifice plate energy dissipater as well as plug energy dissipater, as a kind of effective energy dissipater with characteristics of simple structure, convenient construction and high energy dissipation ratio, has become welcomed more and more by hydraulics researchers. The two kinds of energy dissipaters with sudden reduction and sudden enlargement forms are similar in energy dissipation mechanism, but there are differences in energy dissipation characteristics and cavitation characteristics. In the present paper, the differences between orifice plate and plug in energy loss coefficient, relating to their energy dissipation ratio, in the backflow region length, relating to their energy loss coefficient, and in the lowest wall pressure coefficient, relating to their cavitations risk, were analyzed by numerical simulations and physical experiment, and their features in above three aspects were also revealed. The results of research in the present paper demonstrate that the backflow region length of orifice plate is longer than that of plug at the same contraction ratio, the lowest wall pressure coefficient of plug is smaller than that of orifice plate at the same contraction ratio, and the energy loss coefficient of orifice plate is bigger than that of plug, which illustrates that plug is superior to orifice plate in resistance cavitation damage at the same contraction ratio.

An Analytical Solution of the Potential Velocity Field Induced by a Growing Bubble from a Plate Orifice

An analytical solution is derived with the mirror image method of the velocity field of an inviscid liquid induced by a growing bubble from a plate orifice. The flow is assumed potential, and the bubble shape is idealised as spherical. In deriving the motion equation, the spherical image of a point source, which is a combination of a point source and a line source, is proved approximate to a double source. This approximation enables continuation of the effectiveness of mirror image method to the case studied in this paper. The derived velocity potential equation is verified for the boundary conditions on the bubble surface and the orifice plate. The streamlines of the velocity field are presented and compared with experimental results in the literature.

Vortex Simulation of Axisymmetrical Flows in Cylindrical Geometries．PartII：Application to Pipes Incorporating an Orifice Plate

The Random Vortex Method extended to an azisymmetrical flow is used in the study of the flow field inside pipes incorporating an orifice plate with different contraction ratios and different inlet velocity profiles. Three test-cases, each having experimental measurements available in the literature, are studied. In particular, instantaneous and average velocity fields along with the turbulent statistics for high Reynolds number flows are computed and compared to the corresponding experimental results.These comparisons show the ability and the citations of the method. The results of the numerical simulations are used in the physical analysis of the flow fields and thus allow for a better understanding of the dynamics of the flow in pipes incorporating an orifice plate.

FLOWS THROUGH ENERGY DISSIPATERS WITH SUDDEN REDUCTION AND SUDDEN ENLARGEMENT FORMS

The energy dissipation of flood discharges has been one of important problems that affect directly the safety of hydropower projects. The energy dissipater with sudden reduction and sudden enlargement forms, used widely in large-scale projects has been a kind of effective structure for energy dissipation. The concept of critical thickness was defined, which is related to both the geometric parameters and the hydraulic parameters of the energy dissipater, and the factors affecting the critical thickness, were analzsed by means of dimensional analysis. The empirical expression about the critical thickness was obtained and could be used as the criterion to distinguish the flows through the energy dissipater, i.e., the plug flow and the orifice plate flow. The error analysis showed that the critical thickness calculated by the expression has the errors of smaller than 10% in the estimation of the flows for the energy dissipater mentioned above.

OWC air chamber performance prediction under impulse turbine damping effects

Oscillating water columns(OWCs)are most widely used in coastal wave energy conversion.The air duct opens into the atmosphere through the air turbine,which is the power take-off device,and this results in a pressure drop across the air chamber.However,because of the complex configure of the impulse turbine and its high rotation speed,it is difficult to install it in the experimental simulator and numerical model.Therefore,the turbine damping effects on the operation of the OWC air chamber are induced to predict its performance more accurately.Orifice plates are used as a substitute for the impulse turbine as it generates a similar pressure drop and power output;the experimental and numerical pressure drops and output powers are compared.A 3D numerical wave tank based on the two-phase VOF model is established using the commercial CFD code Fluent,which can predict air flow and pressure variations in the chamber and duct.Water surface elevations,air flow velocity and pressure variation inside the chamber with the orifice plate are studied numerically,and validated by the corresponding experimental data.The air chamber of the Yongsoo OWC pilot plant is used as the engineering project case.The operating performance of the air chamber installed with a 0.428D orifice plate as the substitute for the designed impulse turbine is computed and analyzed.It is found that the turbine damping effects will cause around 30%reduction in the peak values of the pneumatic energy output of the OWC air chamber in the resonant wave domain.