Investigation of the pressure response of different Pitot tubes

Pitot tubes are commonly used to measure gas flow in ducts. The integration of the velocity profile which allows the calculation of the gas flow is described in several international standards such as ISO 3966 or ISO 10780.The common working principle of Pitot tubes is based on the measurement of the differential pressure between the two different pressure taps. The gas velocity is related to this differential pressure through a flow coefficient depending on the Pitot tube type.In case of stable flow, in a pressurized duct, fluctuations of the in-line pressure, even low, can occur. If the response times of the two pressure lines (static and total) between the Pitot tube head and the differential pressure sensor are not equal, these fluctuations can be seen as fluctuations of the measured differential pressure and then of the calculated velocity.This phenomenon is investigated for different design of Pitot tubes and the difference in behaviour of the two pressure lines is highlighted.     

The impact of geometric parameters of a S-type Pitot tube on the flow velocity measurements for greenhouse gas emission monitoring

In the monitoring of greenhouse gas emission from industrial smoke-stacks, the most common device used to measure the stack gas velocity is the S-type Pitot tube in South Korea, which is used to estimate the volumetric flow rate by what is termed the Continuous Emission Monitoring System (CEMS). The S-type Pitot tube installed in the stack is inevitably affected during velocity measurements by velocity changes, yaw and pitch angle misalignments due to the harsh environments. Various geometries of the S-type Pitot tube can affect the characteristics of the S-type Pitot tube coefficients, including the degree of sensitivity to velocity changes and yaw and pitch yaw angle misalignments. Nevertheless, there are no detailed guidelines pertaining to the S-type Pitot tube geometry considering accurate and reliable measurements in the ISO, EPA and ASTM international standards. In the present study, S-type Pitot tubes with various geometric parameters, in this case the distance between the impact and wake orifices and the bending angle of the orifices, were manufactured by a 3D printer. Wind tunnel experiments were conducted in the Korea Research Institute of Standards and Science (KRISS) air speed standard system to determine the optimal geometry of an S-type Pitot tube for the accuracy velocity measurements in actual smokestacks which undergo velocity changes and yaw and pitch angle misalignments. Particle image velocimetry was also used to understand the flow phenomena around an S-type Pitot tube under various geometric and misalignment conditions by means of qualitative visualization. The results indicate that S-type Pitot tubes with a long effective length have more constant distributions of the S-type Pitot tube coefficients when the velocity changes from 2 m/s to 15 m/s. The error indexes for yaw angle misalignments show that S-type Pitot tube models with large effective lengths are less affected by yaw angle misalignments. The S-type Pitot tube coefficients were mostly insensitive to the both positive and negative pitch angle misalignments regardless of the velocity and geometry of the various models tested.     

Calculation of the Pitot tube correction factor for Newtonian and non-Newtonian fluids

This paper presents the numerical investigation performed to calculate the correction factor for Pitot tubes. The purely viscous non-Newtonian fluids with the power-law model constitutive equation were considered. It was shown that the power-law index, the Reynolds number, and the distance between the impact and static tubes have a major influence on the Pitot tube correction factor. The problem was solved for a wide range of these parameters. It was shown that employing Bernoulli's equation could lead to large errors, which depend on the magnitude of the kinetic energy and energy friction loss terms. A neural network model was used to correlate the correction factor of a Pitot tube as a function of these three parameters. This correlation is valid for most Newtonian, pseudoplastic, and dilatant fluids at low Reynolds number.     

The effect of turbulence on a multi-hole Pitot calibration

Accurate calibrations of multi-hole Pitot tubes require thousands of measurements spanning ranges of the fluid's velocity, and the pitch and yaw angles. When calibrating a commercially-manufactured multi-hole Pitot tube in NIST's low-turbulence wind tunnel, we found hysteresis in certain ranges of airspeed, pitch angle, and yaw angle. In the worst case, the hysteresis caused a calibration error of 30%. We demonstrate that the hysteresis was caused by a flow instability associated with flow separation. A turbulence intensity of only 1% removes the hysteresis; however, the calibration depends on the turbulence intensity over the entire range of our measurements (0.25–2%). Therefore, multi-hole Pitot tubes should be calibrated and used at the same turbulence levels.     

Air velocity and flow measurement using a Pitot tube

The accurate measurement of both air velocity and volumetric airflow can be accomplished using a Pitot tube, a differential pressure transducer, and a computer system which includes the necessary hardware and software to convert the raw transducer signals into the proper engineering units. The incorporation of sensors to measure the air temperature, barometric pressure, and relative humidity can further increase the accuracy of the velocity and flow measurements. The Pitot tube measures air velocity directly by means of a pressure transducer which generates an electrical signal which is proportional to the difference between the pressure generated by the total pressure and the still air (static pressure). The volumetric flow is then calculated by measuring the average velocity of an air stream passing through a passage of a known diameter. When measuring volumetric flow, the ‘passage of a known diameter’ must be designed to reduce air turbulence as the air mass flows over the Pitot tube. Also, the placement of the pitot tube in the passage will influence how accurately the measured flow tracks the actual flow through the passage. Calibrating the measurement system in a wind tunnel can further increase the accuracy of the velocity and the flow measurements. This objective of this paper is to provide the field engineer with single, concise source of information on flow measurement using a Pitot tube.     

Smokestack gas velocity measurements using 3D pitot tubes in a coal-fired power plant

On the path to carbon neutrality to reduce greenhouse gas (GHG) emissions, the Korean government has mandated legislation for controlling and monitoring GHG emissions emitted from smokestacks. A continuous emission measurement (CEM) method is considered to be the most reliable for determining CO2 emissions from stationary sources. In Korea, an S-type Pitot tube is the most popular technique to measure the gas velocity in a smokestack, but it will result in a certain error when the non-axial velocity components exist. To vanquish this limitation, Korea Research Institute of Standards and Science (KRISS) developed a nulling smokestack flow measurement (NSFM) instrument equipped with 3D Pitot tubes for taking on-site stack gas velocity measurements. 3D Pitot tubes used in this research, such as prism Pitot tube and sphere Pitot tube, are calibrated in the KRISS airspeed system. The instrument using 3D Pitot tubes with the nulling technique is expected to diminish the restriction on S-type Pitot tubes, and to enhance the quality of the GHG emission measurements in the smokestack. The 3D Pitot tubes can measure both axial and non-axial velocity components of a flow, whereas the S-type Pitot tubes can measure only the axial velocity component. The averaged axial velocity of the stack gas as measured by this instrument has expanded uncertainty of 3.3% (P = 95%, k = 2) for both prism and sphere Pitot tubes.     

Experimental and numerical investigations of the factors affecting the S-type Pitot tube coefficients

In greenhouse gas emission monitoring from industrial stacks, the most common device used to measure stack gas velocity is the S-type Pitot tube. Various factors such as the Reynolds number and misalignment of the installation angle can be additional error sources for the S-type Pitot tube coefficients due to harsh environments. Manufacturing quality of the S-type Pitot tube is also a factor affecting on the measurement uncertainty of stack gas velocity. In the present study, wind tunnel experiments were conducted in Korea Research Institute of Standards and Science (KRISS) standard air speed system to examine the effects of various factors on the S-type Pitot tube coefficients. Numerical simulations were also used to understand flow phenomena around the S-type Pitot tube in the presence of misalignment and distortion of the geometry. The results indicate that misalignment of the pitch and yaw angle change within ±10° changes the S-type Pitot tube coefficients by approximately 2% compared with normal values. The manufacturing quality resulted in unstable values of the coefficients within 2%. However, variations of the Reynolds number (Re_D=3.0×10³–2.2×10⁴) had no significant effect on the S-type Pitot tube coefficients.     

皮托静压管的流量测量及不确定度评估

本文介绍了皮托静压管的结构原理及使用场合,讨论了使用皮托静压管进行流量测量时的测量计算方法,分析了实际测量条件下不同因素对流量测量结果的影响,并对测量结果的不确定度进行评估。     

A Pitot tube system for obtaining water velocity profiles with millimeter resolution in devices with limited optical access

An automated, miniature, S-type Pitot tube system was created to obtain fluid velocity profiles at low flows in equipment having limited optical access, which prevents the use of standard imaging techniques. Calibration of this non-standard Pitot tube at small differential pressures with a custom, low-pressure system is also described. Application of this system to a vertical, high-pressure, water tunnel facility (HWTF) is presented. The HWTF uses static flow conditioning elements to stabilize individual gaseous, liquid, or solid particles with water for optical viewing. Stabilization of these particles in the viewing section of the HWTF requires a specific flow field, created by a combination of a radially expanding test section and a special flow conditioner located upstream of the test section. Analysis of the conditioned flow field in the viewing section of the HWTF required measurements across its diameter at three locations at 1 mm spatial resolution. The custom S-type Pitot tube system resolved pressure differences of <100 Pa created by water flowing at 5–30 cm/s while providing a relatively low response time of ~300 s despite the small diameter (<1 mm) and long length (340 mm) of the Pitot tube needed to fit the HWTF geometry. Particle imaging velocimetry measurements in the central, viewable part of the HWTF confirmed the Pitot tube measurements in this region.     

Double S-type Pitot Tube for velocity field study of fire whirls

In this paper we present a pressure measurement instrument based on the S-type Pitot tube modified to measure the two velocity components of a high temperature flow assuming that the flow is locally two-dimensional. The development of this new device, which we designate as the Double S-type Pitot Tube, is related to the difficulty and the lack of precision of measurements with a standard S-type Pitot tube in flows with unknown directions like the case of fire whirls in laboratory experiments. The design construction and calibration method of the device are described. The pitch angle of the flow and the velocity coefficients of the probe are analysed experimentally in a wind tunnel calibration as well as the associated errors. The use of this sensor in a fire whirl laboratory test is shown and the results are compared with those of simple S-type Pitot tubes in the same test. The obtained results show the applicability and better performance of the novel device.     

A mathematical model of the self-averaging Pitot tube: A mathematical model of a flow sensor

Flowmeters with self-averaging Pitot tubes are more and more often applied in practice. Their advantages are practically no additional flow losses, usability in the case of high temperature of fluids and simplicity of fitting. A mathematical model of a self-averaging Pitot tube including the influence of the probe shape, selected constructional features and flow conditions on the quantity of differential pressure gained has been given in this paper. The values and ranges of variations of the coefficients established for the model have been assessed on the basis of the numerically computed velocity and pressure fields around and inside the probe. Velocity and pressure fields were calculated by means of solving conservation equation and turbulence models. The characteristics linking values of the flow coefficient with values of the Reynolds number have been presented. The conclusions have been formulated taking flow metrology needs into account.     

Pitometry as a validation tool for water flow measurement in large diameter pipelines

Accurate measurement of water flow rates in large diameter pipelines is a challenge for water companies that need to produce, transport and distribute increasing quantities of water. To a large extent, this challenge results from the impossibility of recalibration of the flow meters within the periodicity established in the metrological regulations since the removal of a large size flow meter from its site of operation in the field and its transport to a calibration laboratory is in most cases technically and economically impracticable. Because of this scenario, this paper presents the pitometry technique as an interesting alternative to solve problems related to the validation of water flow measurements performed by flow measurement systems installed in large diameter conduits. The technique is based on the determination of the water flow rate by mapping the velocity profile of the water flow inside the pipe by means of Cole type Pitot tubes. The water flow rate is determined in a cross section of the pipe located near and in series to the flowmeter to be evaluated. Based on the results obtained in a great number of water flow measurements already performed by applying the pitometry technique in large diameter pipelines in the field, it is possible to conclude that this methodology is perfectly applicable in the validation of the performance of flow meters installed in these conduits solving satisfactorily the issues related to its operation.     

管束效应对HFC245fa与HCFC123膜状凝结换热影响

建立试验系统、改进试验方法,试验研究HFC245fa与HCFC123在光管与3种强化换热管(2D-A,3D-A与3D-B)管束外冷凝换热特性。试验管束由4列排深为5排的列管构成,换热管公称外径为19.05 mm、有效换热长度为500 mm。试验中,利用改进的Wilson图解法获得水侧对流传热系数,通过轮转试验方法消除管束试验中各试验管换热本构差异等因素对管束效应测试分析的影响。试验结果表明,Kern模型预测值与HFC245fa与HCFC123光管管束外凝结换热结果偏差随试验热通量升高而增大;管束效应对光管与三维表面强化管(3D-B)凝结换热影响比其对二维表面低肋管(2D-A)影响显著;管束效应对HFC245fa在3D-B管外凝结换热影响在n>3后超过Nusselt管束模型预测值;HFC245fa在3D-B管束外凝结换热性能随管排深度的变化规律与其在光管管束外的变化规律及Nusselt模型显示规律明显不同。     

Effect of the Pitot microtube diameter on pressure measurement in plane supersonic microjets

The article reports on results of estimating the Pitot tube diameter effect on the streamwise and lateral pressure distributions in plane supersonic air microjets. The study is aimed at obtaining reliable information on the microjet structure. The nozzles with the size of 22.3 × 2593 μm and 83.3 × 3823 μm are used in experiments. The supersonic flow from that nozzles is investigated by glass Pitot tubes in inner/outer diameters of 24/70 and 16/42 μm. Additional measurements are performed by a Pitot microtube 8 μm in outer diameter and ≈0.1 μm wall thickness. The value ranges for accuracy determination of supersonic jets main characteristics (shock cell sizes and supersonic core length) have been found.     

The effect of low Reynolds number flows on pitot tube measurements

An investigation on the low Reynolds number effect on hemispherical-tipped Pitot tube measurements was performed by measuring the center-line velocity during the laminar flow of a Newtonian fluid in a 25 mm (1 in.) diameter vertical recirculating pipe loop. The primary objective of the study was to reconsider the available low Reynolds number Pitot tube data in the literature with modern instrumentation.Using the results of this experimental study, a correlation that accurately predicts the low Reynolds number Pitot tube behavior has been developed. The correlation accounts for an additional viscous term in the relationship for the pressure coefficient (C_p) which is not accounted for in Bernoulli's Equation. The correlation is semi-empirical and accurately fits experimental data gathered in this study, as well as a significant body of experimental data available in the literature. The correlation, which is based on a Pitot tube Reynolds number calculated using the opening diameter (d), has been shown to be provide more accurate predictions of C_p for a wide range of opening diameter to outer diameter ratios (0.22≤d/D≤0.6) than available correlations based on outer diameter.The transition Pitot tube Reynolds number, below which Bernoulli's Equation is no longer appropriate, was predicted to be approximately 35, compared to a value of 79 obtained from fitting data collected by Barker. The correlation developed in this study provides smoother transitions at both ends of the low Reynolds range. At the low end (Re<10) it converges with a Stokes Law’ analogy, while at the critical transition (Re~35) it converges asymptotically with Bernoulli's Equation. The correlation also accurately predicts the behavior of the pressure coefficient with Reynolds numbers between these ranges.     

Flow measurement of CO2 in a binary gaseous mixture using an averaging Pitot Tube and Coriolis mass flowmeters

To combat the growing emissions of CO₂ from industrial processes, Carbon Capture and Storage (CCS) and Carbon Capture and Utilization technologies (CCU) have been accepted worldwide to address these pressing concerns. So as to efficiently manage material and financial losses across the entire stream, accurate accounting and monitoring through fiscal metering of CO₂ in CCS transportation pipelines are core and required features for the CCS technologies. Moreover, these technical requirements are part of the legal compliance schemes and guidelines from various regulatory bodies. The CO₂ transportation pipelines will likely have multiple inputs from different capture plants, each with varying composition of CO₂ and thus introducing impurities into the CO₂ stream. The presence of other ordinary or hydrocarbon gases in the CO₂ gas stream could affect the functionality of metering instruments by introducing additional errors, particularly in the case of volumetric flowmeters. In this study, volumetric and direct mass measurement methods for the flow measurement of CO₂ mixtures using two totally different metering principles are experimentally evaluated. An Averaging Pitot Tube with Flow Conditioning Wing (APT-FCW) and Coriolis mass flowmeters (CMF) are used to assess the flow metering of CO₂ in a binary gaseous mixture. Different gases (nitrogen, air, oxygen, argon and propane) are diluted as contaminants into the pure CO₂ gas flow for various mass fractions to produce an adulterated mixture of the CO₂ gas. Comparative analysis of the measurement results under these flow conditions relative to that of pure CO₂ gas show that the measurement error of the APT-FCW sensor increases with the mass fraction of the diluent component, and gases with density closer to that of CO₂ have a much lesser effect on the performance of the APT-FCW flow sensor for smaller mass fractions. The CMF proved to be very reliable in the gas combination processes and as a reference meter for the APT-FCW sensor. Further analytical observations are discussed in detail.     

Evaluation of hot-film,dual optical and Pitot tube probes for liquid–liquid two-phase flow measurements

An experimental study of kerosene–water upward two-phase flow in a vertical pipe was carried out using hot-film, dual optical and Pitot tube probes to measure the water, kerosene drops and mixture velocities. Experiments were conducted in a vertical pipe of 77.8 mm inner diameter at 4.2 m from the inlet (L/D=54). The tests were carried out for constant superficial water velocities of 0.29, 0.59 and 0.77 m/s (flow rates = 83, 167 and 220 l/min) and volume fractions of 4.2%, 9.2%, 18.6% and 28.2%. The Fluent 6.3.26 was used to model the single and two-phase flow and to reproduce the results for the experimental study. Two methods were used to evaluate the accuracy of the probes for the measurement of the velocities of water, drops and mixture for two-phase flow: (i) comparison of measured local velocities with predictions from the CFD simulation; (ii) comparison between the area-averaged velocities calculated from the integration of the local measurements of water, drops and mixture velocities and velocities calculated from flow meters’ measurements.The results for single phase flow measured using Pitot tube and hot-film probe agree well with CFD predictions. In the case of two-phase flow, the water and drops velocities were measured by hot-film and dual optical probes respectively. The latter was also used to measure the volume fraction. These three measured parameters were used to calculate the mixture velocity. The Pitot tube was also used to measure the mixture velocity by applying the same principle used for single phase flow velocity. Overall the mixture local velocity measured by Pitot tube and that calculated from hot-film and dual optical probe measurements agreed well with Fluent predictions. The discrepancy between the mixture area-averaged velocity and velocity calculated from flow meters was less than 10% except for one test case. It is concluded that the combined hot-film and optical approach can be used for water and drop velocity measurements with good accuracy for the flow conditions considered in this study. The Pitot tube can also be used for the measurement of mixture velocities for conditions of mixture velocities greater than 0.4 m/s. The small discrepancy between the predictions and experimental data from the present study and literature demonstrated that both instrumentation and CFD simulations have the potential for two-phase flow investigation and industrial applications.     

Interference effects of three sonic nozzles of different throat diameters in the same meter tube

For calibration of a large capacity gas flow meter, a sonic nozzle bank may be used as a reference system. International standards (ISO9300:1990) allow installation of a single nozzle in a meter tube as a flow transfer standard. For multiple nozzles in a single tube, the effect of interference between sonic nozzles and the chamber wall must be measured to predict the discharge coefficient of a nozzle array from those of single nozzles. The interference effect between neighboring nozzles can be additional error sources in mass flow measurement. Sonic nozzles with three different throat diameters (d=4.3, 8.1, and 13.4 mm) were tested in a single meter tube in three geometrical arrangements. The mass flow rate was measured against a primary gas flow standard system. Three installation plates for sonic nozzles were made to vary the distance between nozzles and distance from the chamber wall. Discharge coefficients of the three individual nozzles were in agreement with the ISO recommended equation within ±0.2%. Discharge coefficients of the nozzle bank calculated from those of the individual sonic nozzle were the same as the direct measurements within ±0.098% at the 95% confidence level for all cases. For these experiments, the results were not influenced by the proximity of the tube wall or the interaction of the nozzles.     

Analysis of the flow rate characteristics of valveless piezoelectric pump with fractal-like Y-shape branching tubes

Microchannel heat sink with high heat transfer coefficients has been extensively investigated due to its wide application prospective in electronic cooling. However, this cooling system requires a separate pump to drive the fluid transfer, which is uneasy to minimize and reduces their reliability and applicability of the whole system. In order to avoid these problems, valveless piezoelectric pump with fractal-like Y-shape branching tubes is proposed. Fractal-like Y-shape branching tube used in microchannel heat sinks is exploited as no-moving-part valve of the valveless piezoelectric pump. In order to obtain flow characteristics of the pump, the relationship between tube structure and flow rate of the pump is studied. Specifically, the flow resistances of fractal-like Y-shape branching tubes and flow rate of the pump are analyzed by using fractal theory. Then, finite element software is employed to simulate the flow field of the tube, and the relationships between pressure drop and flow rate along merging and dividing flows are obtained. Finally, valveless piezoelectric pumps with fractal-like Y-shape branching tubes with different fractal dimensions of diameter distribution are fabricated, and flow rate experiment is conducted. The experimental results show that the flow rate of the pump increases with the rise of fractal dimension of the tube diameter. When fractal dimension is 3, the maximum flow rate of the valveless pump is 29.16 mL/min under 100 V peak to peak （13 Hz） power supply, which reveals the relationship between flow rate and fractal dimensions of tube diameter distribution. This paper investigates the flow characteristics of valveless piezoelectric pump with fractal-like Y-shape branching tubes, which provides certain references for valveless piezoelectric pump with fractal-like Y-shape branching tubes in application on electronic chip cooling.