Parallel-wire Probes Research Articles

Hydrodynamic model of interfacial area for trickling flow in a packed bed

To predict local interfacial area in a packed bed under the trickling flow, a wavy annular flow model was developed by introducing the shape of waves in a thin liquid film. The trickling flowin a packed bed was approximated by an annular flow through a number of equivalent channels in which the continuous gas and liquid were completely separated by a wavy interface. Film thicknesses were measured by parallel wire probes to estimate the wave structure on the interface. By integrating the interfacial areas over a certain time period, the time-averaged local interfacial area was evaluated from a low to high interaction flow regime: 1) from trickling to bubbly flow and 2) from trickling to pulsing flow. The interfacial area predicted by the wavy annular flow model in the trickling flow was moderately higher than those predicted with the empirical correlations developed by others. The results show that the interfacial area increases more significantly as the flow regime changes from trickling to pulsing flow than that increases as the flow regime changes from trickling to bubbly flow. As a result, the wave structure on the interface should be considered to predict more accurate interfacial area in a packed bed.

Liquid film thickness behaviour within a large diameter vertical 180° return bend

Experimental results of liquid film thickness distribution of an air–water mixture flowing through a vertical 180° return bend are reported. Measurements of liquid film thickness were achieved using flush mounted pin and parallel wire probes. The bend has a diameter of 127mm and a curvature ratio (R/D) of 3. The superficial velocities of air ranged from 3.5 to 16.1m/s and those for water from 0.02 to 0.2m/s. At these superficial velocity ranges, the flow pattern investigated in this work focused on churn and annular flows. It was found that at liquid and gas superficial velocities of 0.02m/s and 6.2m/s, respectively, the averaged liquid film thickness peak at 90°. At gas superficial velocity of 16.1m/s, the relationship between them is linear due to the shear forces overcoming gravity. Additionally, it was found that deposition of entrained droplets keeps the liquid film on the outside of the bend. The results of polar plots of average liquid film thickness in the bend showed that the distribution of the liquid film is not symmetrical with thicker films on the inside of the bend due to the action of gravity. Experimental results on average liquid film thickness showed good agreement with the simulation data reported in the literature.

A study on the effect of gas flow rate on the wave characteristics in two-phase gas–liquid annular flow

Interfacial waves play a very important role in the mass, momentum and energy transport phenomena in annular flow. In this paper, film thickness time–trace measurements for air–water annular flow were collected in a small vertical tube using a parallel wire probe. Using the data, a typical disturbance wave shape was obtained and wave properties (e.g., width, height, speed and roughness) were presented. The liquid mass flux ranged from 100 to 200 kg/m 2 s and the gas mass flux ranged from 18 to 47 kg/m 2 s. Disturbance wave characteristics were defined and the effects of changing the gas flow rate on the wave spacing, wave width, wave peak height and wave base height were studied. An average velocity model for the wave and base regions has been developed to determine the wave velocity. The investigation method could be further extended to annular-mist flow which frequently occurs in boiling water reactors.

A preliminary study of two-phase annular flow at microgravity: experimental data of film thickness

Film thickness time trace measurements were taken in normal (earth gravity) and microgravity (μ-g) conditions during parabolic flights operated by NASA Johnson Space Center in Houston, Texas. The measurements were taken using a parallel wire probe. Using the data, average film thickness, film minimum thickness, wave peak height, wave frequency and wave velocity were calculated over a wide range of liquid and gas mass velocities. The liquid mass velocity ranged from 49 to 318 kg/m 2 s, and the gas mass velocity ranged from 2.3 to 25 kg/m 2 s. While, based on average values of the film thickness, there were small differences between the 1-g and μ-g results, the wave base height and frequency were strongly affected by the reduction in gravity.

The effect of cross-flow velocity on mean-square derivatives measured using hot wires

It is well known that significant errors occur in the velocity derivative moments measured in turbulent flows when the measuring transducer is too large or Taylor's hypothesis is used in high-turbulence-intensity flows. An additional error occurs when velocity derivative moments are measured with hot wires in high-turbulence-intensity flows, because the wires cannot resolve the individual velocity components in these flows. Estimates of the error this causes in the derivative moments measured with single-, cross-, and parallel-wire probes are developed herein. The errors are significant in the derivative moments measured with cross-wire probes, but are smaller in derivative moments measured with single- and parallel-wire probes. For example, the relative errors in \(\) measured in the far field of the round jet are 30–50% smaller than predicted in previous analyses.

The pulsed wire anemometer: review and further developments

The purpose of the present paper is to review work that has been done on the pulsed wire anemometer technique and also suggest further developments that could be made in its range of application. The aper discusses the three types of probes that have been used in pulsed wire anemometry: the crossed wire velocity probe, the parallel wire wall shear stress probe and the parallel wire velocity probe. The work shows that the crossed wire and the parallel wire techniques can be used to make velocity, turbulence and wall shear stress measurements in highly turbulent flows without any upper restriction on turbulence level. Comments are also made on the potential of a parallel wire probe for use in highly turbulent flows that would enable higher order velocity cross-product terms to be measured.

Spatial resolution of parallel hot-wire probes for derivative measurements

The spatial resolution of a parallel-wire probe is studied to evaluate the effect of finite wire spacing and wire length on the measurement of turbulence derivative quantities. The study considers the measurement of velocity and temperature derivatives in an isotropoic field. Different spectral models are used for each case to determine if the amount of spatial filtering is sensitive to changes in the shape of the three-dimensional spectrum that is being measured. The data demonstrate that the separation distance is the critical dimension in probe design if the wire length and separation are of comparable magnitude. The amount of spatial filtering predicted is highly sensitive to the spectral model for separation distances greater than 2–3 times the Kolmogorov length scale.

Experimental studies of flooding in nearly horizontal pipes

To investigate the flooding phenomenon in nearly horizontal pipes, experimental studies are performed for air and water countercurrent flow in a test section with a length of 2160 mm, with three different inner diameters of 40, 60 and 70 mm, with different types of end geometry, and with various inclination angles. The effects of the pipe diameter, end geometry and inclination angle on flooding are examined. Two mechanisms governing the transition to flooding are proposed: inner flooding and entrance flooding. The inner flooding is initiated by unstable wave growth, i.e. slugging, at the inner location of the pipe. The entrance flooding is always observed to take place at the entrance of the water flow without slugging. The effect of the inclination angle on flooding is predominant within quite a narrow range of inclination (0° < θ < 1°). A small deviation of the inclination angle from the horizontal plane causes a large deviation of the required air flow rates for flooding. Two local void fractions are measured by parallel wire probes and a scale marked rule in two regions (sub- and super-critical regions). The local effects of the void fraction on slug formation models are investigated. It is found that the differences in the exponents of the void fraction terms of the existing slug formation models mainly result from the definition of the void fraction used. When the void fraction measured in the sub-critical region is used in the slug formation model, Mishima et al.'s model well predicts the trend of the present data. However, if the void fraction measured in the super-critical region is used, Taitel et al.'s model better predicts the present data.

Sensitivity of time domain reflectometry measurements to lateral variations in soil water content

The spatial responses of two types of time domain reflectometry (TDR) probes used for measuring the water content of soils and other porous materials are examined. We investigate the effect of spatial variation in the water content of the sample on the measured value of the water content for both the coaxial cylinders probe and the two parallel wires probe. The response of the instrument to water content distributions that are perturbed slightly from a uniform distribution is calculated, using the conventional electrostatic treatment of electromagnetic wave propagation in the transverse electro‐magnetic (TEM) mode. Under this small perturbation assumption, we show that the spatial weighting function is approximately proportional to the distribution of electromagnetic energy between the electrodes for uniform water content, calculated from a solution of Laplace's equation for each probe type. For coaxial probes, most of the energy (and hence most of the measurement sensitivity) is concentrated around the inner cylinder in a “skin effect” if the ratio of the radii of the inner and outer cylinders is too small. For parallel wire probes, most of the measurement sensitivity is close to the wires if the wire diameter is too small compared to the spacing between them. This can cause significant errors if there is an air gap close to the wires or soil around the wires has been compacted by the process of inserting them into the soil.

The electroacoustic equations for a colloidal suspension

The term ‘electroacoustic’ is used in connection with phenomena that involve the interaction of sound waves and electric fields, such as the voltage difference generated across a transducer by the passage of a sound wave. This paper is concerned with electroacoustic effects in suspensions of electrically charged colloidal particles. The existing methods for calculating electroacoustic effects in suspensions are limited either to the dilute case, or to one particular effect, namely the open-circuit voltage generated between two parallel wire probes by a locally plane sound wave, propagating perpendicular to the wires. In this paper we present a procedure for calculating any electroacoustic effects in suspensions of arbitrary concentration. The only restriction on the method is that the particles must be small compared with the sound wavelength. The procedure involves the solution of a set of differential equations – referred to here as the ‘electroacoustic equations’ – for the macroscopic pressure, velocity and electrical potential in the suspension. The derivation of these equations and the associated boundary conditions form the major part of this paper. General features of the solution are also discussed, and an application is given involving a novel electroacoustic measurement.

Parallel-wire probes for measurement of thick liquid films

A method of calibration for parallel-wire film thickness probes is presented. This method allows calibration of probes made of fine wires with dimensions that are comparable to standard machining tolerances. The resulting probes yield accurate and continuous measurements of film thickness for thick (> 1 mm) films.

An experimental analysis of fluctuating temperature measurements using hot-wires at different overheats

Adjacent parallel hot-wire anemometers at different temperatures have sometimes been used to measure fluctuating temperatures in turbulent flows. This work presents an extensive experimental comparison of temperatures measured with a parallel-wire probe to temperatures simultaneously measured with a standard cold-wire probe. The results show the parallel-wire probe to work well in low intensity flows with temperature signals which are not too small. However, the parallel-wire probe temperature measurements are not accurate for high turbulence intensities or for small temperature signals, and in general the cold-wire system is probably to be preferred.