Effect of diffuser angle on the discharge coefficient of miniature critical nozzles

Many studies on critical nozzles have been made to accurately measure the mass flow rate of gas and standardize its performance as a flow meter. Recently, much interest has been given to measuring very small mass flow rates in industrial fields, such as MEMS applications. However, the design and performance data of the critical nozzles obtained thus far have been applied mainly to critical nozzles with comparatively large diameters, and available studies on miniature critical nozzles are lacking. In this study, computational fluid dynamics (CFD) method was applied to investigate the influence of the diffuser angle on the discharge coefficient of miniature critical nozzles. In computations, the throat diameter of a critical nozzle varied from 0.2 to 5.0 mm, and the diffuser angle changed from 2° to 8°. The computational results were validated with some available experimental data. The present computational results accurately predicted the discharge coefficient of gas flows through miniature critical nozzles. The discharge coefficient is considerably influenced by the diffuser angle as the throat diameter of the nozzle becomes smaller below a certain value. This implies that miniature critical nozzles should be designed with careful consideration of its effects.     

The humidity effect on air flow rates in a critical flow venturi nozzle

A Critical Flow Venturi Nozzle (CFVN) is usually calibrated using dry air. Yet CFVNs in industrial and calibration service centers are often used to measure flow rates of humid air. Therefore, ISO 9300 provides the calculation method for the humidity effect on discharge coefficients of CFVN. However, since this method is only due to a theoretical analysis, it is important to confirm and check the ISO calculation method for the humidity effect on CFVN with its isentropic analysis by means of an experimental method.In this experiment, three CFVNs with diameters of 0.4, 0.8 and 1.6 mm were calibrated with dry air (with the dew point −40 °C), in a primary air flow standard system with a mercury sealed piston prover, installed at the Korea Research Institute of Standards and Science (KRISS). Another piston prover, a portable dry piston prover, was used as a reference meter and was also calibrated in the primary standard system using dry air. The repeatability of this dry piston prover was confirmed with the deviation being less than 0.05%. The CFVNs were tested with this dry piston prover, using humid air. For air types with high humidity, the humidity effect on flow rates through the CFVNs showed quite significant difference between the experimental results and those from the ISO method with isentropic analysis. But for air types with low humidity, its effect was relatively insignificant.     

The calculation of the uncertainty of the orifice-plate discharge coefficient

The uncertainty of the orifice-plate discharge coefficient given by the Reader-Harris/Gallagher (1998) Equation has been calculated taking account of the uncertainty of the data on which it is based and of the variability in manufacture permitted by ISO 5167–2. This work has shown that using the correct method to determine the uncertainty in ISO 5167-2 has made an insignificant difference to the value given in the standard. However, in other similar cases where the uncertainty for an artefact is based on data from other similar artefacts the uncertainty values obtained by the correct method may be significantly different from those by the incorrect method.     

A review of studies on estimating the discharge coefficient of flow control structures based on the soft computing models

The discharge coefficient (Cd) plays a vital role in the accurate design and safety of weirs, spillways, and dams. In the last decade, Soft Computing(SC) models, which showed excellent capabilities for non-linear mapping between parameters, were widely used to estimate the discharge coefficient of flow control structures. This study provides a comprehensive review of the application of SC models for estimating Cd of different flow control structures such as ogee spillways, orifices, side weirs, etc. In addition, the most common empirical relations which are obtained from laboratory experiments are discussed briefly. The findings revealed that weir length/flow depth ratio, weir length/channel width ratio, weir height/flow depth, and Froude number are widely used to estimate Cd in the side weirs. Besides, the ratio of orifice crest height to height of side orifice, the ratio of main channel width to length of side orifice, ratio of main channel width to height of side orifice, and ratio of the height of side orifice to upstream flow depth were extensively employed to calculate Cd of orifice structures. The common parameters for measuring Cd of labyrinth weirs are, discharge over a labyrinth cycle, weir height, channel width, apex constant, upstream head, discharge over the weir, effective length, convergence constant, sidewall angle, and Froude number. In the weir-gate structure, some factors such as contraction coefficient of the gate, head loss, and weir height are key parameters for the accurate evaluation of Cd. The trends of SC models, features of popular models, and the background of models are discussed briefly in this paper. Also, research gaps and possible directions for new studies are suggested.     

Flow structure in critical flow Venturi nozzle and its effect on the flow rate

The flow fields in toroidal Venturi-nozzles, shaped according to the ISO-9300 Standard, have been investigated using numerical flow simulation. The present study was aimed at clarifying some of the phenomena associated with unchoking the flow in the throat. To this end, the shock structure has been studied for different Reynolds numbers and exit pressure ratios. The flow simulations were carried out in two and three dimensions. The flow fields were always unsteady, displaying a complex shock–boundary layer interaction.     

Influence of divergent section on flow fields and discharge coefficient of ISO 9300 toroidal-throat sonic nozzle

The effect of divergent section of ISO 9300 toroidal-throat nozzle on discharge coefficient was analyzed based on the inviscid transonic flow model and laminar boundary layer theory. A series of numerical simulations were conducted to verify the results of theory, and investigate the effect of divergent section length L and diffuser angle θ operated at different Reynolds numbers. Combined with the numerical results in this study and the experimental data reported by Nakao, it showed the discharge coefficient increases with the rise of diffuser angle θ or the drop of divergent section length L. A lot of new results about the effect of divergent section were obtained. It indicated that the effect of divergent section on discharge coefficient of ISO 9300 toroidal-throat nozzle should be considered when Re<1.1×10⁴. At last, a concept of effective critical flow was proposed to discuss the effect of divergent section on discharge coefficient.     

Numerical analysis of flow in a Francis turbine on an equal critical cavitation coefficient line

Numerical simulation and model test were applied to study the cavitation flow in Francis turbines. The SST k-ω turbulence model and the mixture model were used to simulate the cavitating flow in the Francis turbine. An equal critical cavitation coefficient line was calculated and the flow in the Francis turbine was analyzed. Simulation results show reasonable agreement with the experimental data. It is confirmed that these cavitation model and numerical method is a useful way to study the two-phase cavitation flow in Francis turbines. On the equal critical cavitation coefficient line, the energy loss in the turbine may be caused by the rotating of vortex rope in the draft tube or flow separation in the runner. The study of equal critical cavitation coefficient line can provide a basic guidance for industry practice.     

The effect of surface curvature on the friction coefficient

A simple idealized model is considered, which predicts an interesting effect that appears not to have been stressed before. If two bodies whose surfaces are curved are in contact and in relative motion, with factional shear between them, then the force resisting the motion is equal to the sum of two terms. The first of these is the frictional shear integrated over the contact region; this is the usual result. The second term is the one of main interest. It depends upon the relative curvature and is negative. Interpreted as a statement about macroscopic contact of surfaces, this means that if curved bodies are in contact and relative motion then the effective coefficient of friction between them is the sum of the basic friction coefficient between the materials and a negative term depending on their relative curvature. Some observable consequences of this effect are discussed. The phenomenon also has relevance to the building of microscopic models of surface contact. This is also examined in some detail.The effect is a consequence of the fact that, even for symmetrically curved surfaces, the contact region is asymmetrical owing to the frictional shear. This gives rise to an extra force in the direction of motion. The argument assumes that the contact is predominantly linear elastic in nature. Viscoelastic effects tend to reduce and finally to cancel the effect described.     

Experimental and numerical investigation on the performance of small-sized cavitating venturis

Cavitating venturis (CVs) are simple devices which can be used in different industrial applications to passively control the flow rate of fluids. In this research the operation of small-sized CVs is characterized and their capabilities in regulating the mass flow rate were experimentally and numerically investigated. The effect of upstream and downstream pressures, as well as geometrical parameters such as the throat diameter, throat length, and diffuser angle on the mass flow rate and critical pressure ratio were studied. For experimental data acquisition, three CVs with throat diameters of 0.7, 1 and 1.5 mm were manufactured and tested. The fabricated CVs were tested at different upstream and downstream pressures in order to measure their output mass flow rate and to obtain their characteristic curves. The flow inside the CVs was also simulated by computational fluid dynamics. The numerical results showed agreement with the experimental data by a maximum deviation of 5–10% and confirmed that the numerical approach can be used to predict the critical pressure ratio and mass flow rate at cavitaing condition. It is found that despite the small size of venturis, they are capable of controlling the mass flow rate and exhibit the normal characteristics. By decreasing the throat diameter, their cavitating mode became more limited. Results also show that increasing the diffuser angle and throat length leads to a decrease in critical pressure ratio.     

振动功率流测量方法的研究

研究梁结构中功率流的测量中分别用加速度传感器和PVDF传感器进行功率流测量.通过压电加速度传感器与PVDF传感器测量进行实验比较,结果表明,设计制作的正弦形状和余弦形状传感器是适用可行的.     

Numerical investigation of the effect of surface roughness on the flow coefficient of an eccentric butterfly valve

Journal of Mechanical Science and Technology - Butterfly valves are used widely as flow controllers in many industrial applications. In order to design and select an appropriate valve for a...     

Influence of wall roughness on discharge coefficient of sonic nozzles

To research the influence of roughness on discharge coefficient of axisymmetric sonic nozzles systematically, a turbulence model was established, and standard k–ε model was used in the turbulent core region while Wall Functions was carried out in the boundary layer region. A series of numerical simulations were conducted to research discharge coefficients of 6 critical flow Venturi nozzles with throat diameter ranging from 0.5 to 100 mm when Reynolds numbers ranges from 10⁴ to 10⁹ and relative roughness from 10⁻² to 10⁻⁶. The validity of the simulation model was confirmed by both the experimental data of Stewart and ISO 9300 empirical equation. According to the simulation results and theoretical analysis, the relations between discharge coefficient and relative roughness were obtained. It is recommended that the dimensionless parameter relative roughness should be used in ISO 9300 rather than absolute roughness. Additionally, when the machining of nozzle cannot satisfy the ISO 9300 requirement or the Reynolds numbers exceed the upper limits of the ISO 9300 equation, the effect of roughness should be considered, and the relative roughness of sonic nozzle should be provided clearly in the further experiment of discharge coefficient.     

The effect of surface strain relaxation on HAADF imaging

In this work the effects of strain on high-angle annular dark field (HAADF) images taken in zone axis conditions have been quantitatively studied. In particular, the presence of dark contrast zones in experimental HAADF images of InGaAs–GaAs interfaces is here interpreted in terms of strain relaxation at the surface. The consistence of this assumption is demonstrated by means of experiments and simulations performed for different In compositions, specimen tilt and thickness conditions. It is shown how the HAADF contrast mechanism is related to the bending of the lattice planes in the first surface region. Finally, a generalization of the 1s approximation that is able to qualitatively describe the effect of strain on HAADF images is presented.     

The effect of deionisation time on the electrical discharge machining performance

An experimental and numerical study to evaluate the time needed for dielectric deionisation between consecutive discharges as well as its effect on performance of electrical discharge machining was done. In the numerical study, the open voltage (gap width definition), current intensity, discharge duration and discharge interval were used among the large number of input parameters used experimentally in the process of electrical discharge machining. The performance parameters values, as well as the time needed for a complete deionisation of the dielectric liquid using a finite element model were calculated. In the numerical study, the time needed for the complete deionisation of the dielectric liquid was calculated using the condition that the temperature reached in the boundaries of the plasma channel with the electrodes has a value between 6,000 and 4,500 °C. In the experimental study, the researchers acquired the time needed for the complete deionisation of the dielectric liquid using small discharge duration and a discharge interval variable between the minimum acceptable by the control of the machine and the one that causes a significant decrease in the material removal rate. The numerical results are in agreement with the experimental data.     

Influence of crest geometric on discharge coefficient efficiency of labyrinth weirs

Labyrinth type weirs are structures that, due to their geometry, allow the discharge capacity to be increased compared to linear weirs. They are a favorable option for dam rehabilitation and upstream level control. There are various geometries of labyrinth type weirs such as trapezoidal, triangular or piano key as well as different types of crest profiles. Geometric changes are directly related to hydraulic efficiency. The objective of this work was to analyze the hydraulic performance of a labyrinth type weir, by simulating several geometries of the apex and of the crest using Computational Fluid Dynamics (CFD). For model validation, experimental studies reported in the literature were used. Tests were carried out with trapezoidal and circular apexes and four types of crest profiles: sharp-crest, half-round, quarter-round and Waterways Experiment Station (WES). The results revealed a determination coefficient of R² = 0.984 between experimental and simulated data with CFD, which provides statistical agreement. Simulations showed that circular-apex weirs are more efficient than those with trapezoidal apex, because they have a higher discharge coefficient (4.7% higher). Of the four types of crest profiles analyzed, the half-round and the WES crest profiles had similar discharge coefficients and were generally greater than those of the sharp-crest and the quarter-round (5.26% y 8.5% higher) profiles. Nevertheless, to facilitate a practical construction process, it is recommended to use a half-round profile. For hydraulic heads with H_T/P > 0.5 ratio, all profiles generated sub-atmospheric pressures on the side walls of the weir. However, when H_T/P ≈ 0.8 ratio the half-round crest generated a higher negative pressure (−1500 Pa), while the sharp-crest profile managed to increase the pressure by 76% (−350 Pa), but with a greater area of negative pressure. On the other hand, the WES profile reduced the negative-pressure area by 50%.     

Theoretical discharge coefficient of a critical circular-arc nozzle with laminar boundary layer and its verification by measurements using super-accurate nozzles

The discharge coefficient of a circular-arc critical Venturi nozzle is derived theoretically by combining theories to calculate mass flow defects caused by the core flow distribution and the laminar boundary layer. The equation obtained is verified by measurements using a constant volume tank system and nozzles of similar shape machined by super accurate lathes, which achieved mirror finish without polishing thus resulting in a machining error of less than 1 μm. The agreement of the theoretical and measured discharge coefficients is better than 0.04% at Reynolds numbers larger than 8×10⁴, the minimum used. The equation derived is in an analytical form, which enables an estimation of the effects of specific heat ratio as well as nozzle shape.     

振动摆辗的体积效应实验研究

根据振动摆辗机的工作原理,设计了一套基于AT89C51单片机的振动摆辗力检测系统.系统选择了合适的测力传感器,设计了信号调理电路,利用单片机采集摆辗力数据,并通过串口将数据传送到PC右口连行分析处理,为振动摆动辗压技术下金属成形的体积效应研究提供了一种简单可行的实验方法.