Critical Flow Venturi Research Articles

Relaxation Effects in Small Critical Nozzles

We computed the flow of four gases (He, N2, CO2, and SF6) through a critical flow venturi (CFV) by augmenting traditional computational fluid dynamics (CFD) with a rate equation that accounts for τrelax, a species-dependent relaxation time that characterizes the equilibration of the vibrational degrees of freedom with the translational and rotational degrees of freedom. Conventional CFD (τrelax=0) underpredicts the flow through small CFVs (throat diameter d=0.593mm) by up to 2.3% for CO2 and by up to 1.2% for SF6. When we used values of τrelax from the acoustics literature, the augmented CFD underpredicted the flow for SF6 by only 0.3%, in the worst case. The augmented predictions for CO2 were within the scatter of previously published experimental data (±0.1%). As expected, both conventional and augmented CFD agree with experiments for He and N2. Thus, augmented CFD enables one to calibrate a small CFV with one gas (e.g., N2) and to use these results as a flow standard with other gases (e.g., CO2) for which reliable values of τrelax and the relaxing heat capacity are available.

The effect of vibrational relaxation on the discharge coefficient of critical flow venturis

This paper identifies a new mechanism that can affect the discharge coefficient of critical nozzle flows. Specifically, vibrational relaxation effects are demonstrated to significantly influence the discharge coefficient of selected gases in critical venturi flows. This phenomenon explains why certain gases — like carbon dioxide — exhibit calibration characteristics that differ significantly from other gases (e.g., N 2, O 2, Ar, He, and H 2). A mathematical model is developed which predicts this behavior, and vibrational non-equilibrium effects are further substantiated by two independent experiments.

The use of critical flow venturi nozzles with saturated wet steam

This paper describes the results of an investigation into the use of critical flow venturi nozzle flowmeters with wet saturated steam. The real-gas flow coefficient has been calculated using the equations of state for steam, which show the steam behaviour to be far from ideal along the saturation line. The nozzle was calibrated using superheated steam and the mass flow rate measured using a condensate weightank was seen to agree with the flow rate measured by the nozzle to within ±2%, although up to half of this uncertainty may be due to the uncertainty in the calibration rig and in the measured parameters used in the nozzle equation. The nozzle was then calibrated in saturated steam with dryness fractions down to 84%. The nozzle can still be used by including a wet steam correction factor in the flow rate calculation, or a more practical approach might be to precede the nozzle by an efficient steam/water separator, assume the steam then to be dry saturated and accept an uncertainty in the mass flow of 3%.

The application of sonic (critical flow) nozzles in the gas industry

Sonic (critical flow venturi) nozzles have been used by some organizations as calibration standards for the accuracy testing of gas meters and have sometimes been used as flow transfer standards in inter-laboratory comparisons, but their routine application has been limited, possibly due to the lack of practical guidelines. The implementation of ISO 9300: 1990 ‘Measurement of gas flow by means of critical flow venturi nozzles’ is discussed in relation to the practical application of flow-calibrated nozzles. Working equations applicable to meter accuracy testing are presented. The high level of repeatability of sonic nozzles has been demonstrated by nozzle inter-comparison work using a ‘Wheastone bridge’ technique in this laboratory and at the Australian National Measurement Laboratory. Considerations in the practical application of the bridge technique are presented in relation to nozzles used for gas meter testing.

A Comparison of Three Critical Flow Venturi Designs

Three critical flow venturi designs have been examined from the standpoint of their sensitivity to initial boundary-layer thickness, inlet flow nonuniformity, separation, and transition point location. These sensitivity studies were carried out with a finite-difference computer code which represents a coupled solution of the inviscid potential core and the viscous boundary layer. Since it makes good sense to select a design that is least sensitive to these effects, it is expected that this information will be of interest to potential users of these devices. Differences between the sensitivities of the various designs were found to be small; however, a slight advantage is seen in designs which employ a circular arc entrance section and a circular arc throat.

The Choking Pressure Ratio of a Critical Flow Venturi

The critical flow venturi has many important applications in the measurement and control of gas flow. In many of these applications, it is desirable to minimize the pressure loss required to maintain critical flow conditions. The performance of the venturi may be characterized by the ratio of outlet static pressure to inlet total pressure just sufficiently small to produce critical flow. This ratio is called choking pressure ratio (CPR). The optimization of diffusers for critical flow Venturis is discussed and suggestions for designs practice are presented. Test results are given for six different diffuser configurations, and a comparison is made with data on 11 configurations from other investigators. This work was done under contract to the National Aeronautics and Space Administration—Marshall Space Flight Center. It is shown that, for the small divergence angles considered, a simply defined diffuser effectiveness parameter is approximately independent of flow conditions and may be used to predict choking pressure ratio. Even very short diffusers greatly improve performance, and, for longer diffusers, critical flow can be maintained at total pressure losses of 5 percent.

A Multiple Critical Flow Venturi Airflow Metering System for Gas Turbine Engines

An air metering system has been constructed using an array of critical flow venturis. By plugging individual venturis, the total metering area is varied to provide an airflow range from 25 lb per sec to 1200 lb per sec. A unique flow control valve and air straightening section provides low air velocity distortion at the entrance to the meter.