Measurement Of Liquid Film Thickness Research Articles

Measurement of liquid film thickness in micro square channel

In micro channels, slug flow becomes one of the main flow regimes due to strong surface tension. In micro channel slug flow, elongated bubble flows with the thin liquid film confined between the bubble and the channel wall. Liquid film thickness is an important parameter in many applications, e.g., micro heat exchanger, micro reactor, coating process etc. In the present study, liquid film thickness in micro square channels is measured locally and instantaneously with the confocal method. Square channels with hydraulic diameter of D h = 0.3, 0.5 and 1.0 mm are used. In order to investigate the effect of inertial force on the liquid film thickness, three working fluids, ethanol, water and FC-40 are used. At small capillary numbers, liquid film at the channel center becomes very thin and the bubble interface is not axisymmetric. However, as capillary number increases, bubble interface becomes axisymmetric. Transition from non-axisymmetric to axisymmetric flow pattern starts from lower capillary number as Reynolds number increases. An empirical correlation for predicting axisymmetric bubble radius based on capillary number and Weber number is proposed from the present experimental data.

Measurement of Liquid Film Thickness by a Fringe Method

A technique was developed and tested to measure the local thickness of a droplet or a liquid film on a surface of an opaque or thick single transparent plate by an interference fringe pattern that was easily formed by reflecting laser lights. Monochromatic epi-illumination through an objective lens of a conventional microscope was provided by a 5 mW or 300 mW laser and a filter to remove the noise caused by laser speckle. The incremental height difference of the liquid layer between neighboring maxima or minima of fringes was evaluated from the wavelength of the laser light and the refractive index of the liquid. Estimation error of a local inclination angle was discussed using ray tracing under parallel illumination approximation. Droplet profiles evaluated from the interferogram that were obtained by the present fringe method agreed well with those by Laser Focus Displacement Meter. Measurement was made to ensure the usefulness of the present technique. It was made clear that the contact angle of a liquid droplet could be obtained precisely and swiftly, even in small size or small contact angle, and that instantaneous three-dimensional profile of a liquid film on a bubble moving in a microchannel could be measured. The fringe method had sufficient potential to obtain more detailed information about three-dimensional characteristics of liquid film in flows such as the generation, breakdown, and growth of waves and the liquid film on a bubble at the beginning of movement.

Effect of geometric parameters on the liquid film thickness and air core formation in a swirl injector

Many theoretical and experimental studies have been conducted to investigate elements of swirl injector hydrodynamics, such as variations in liquid film thickness or air core diameter. From these studies, some theoretical relationships have been established through an approximate analytical solution of flow hydrodynamics in a swirl nozzle. However, experimental studies on elements such as the measurement of liquid film thickness have not produced conclusive results. In a swirl injector, the atomization process is significantly influenced by the liquid film thickness. Thus, it is possible to investigate the effects of various geometric parameters on spray characteristics through the measurement of liquid film thickness. We used a specially designed injector based on the electrical conductance method to measure the liquid film thickness accurately. The liquid film thickness was measured through precise calibration, and the accuracy of this measurement was demonstrated in comparison with previous theories and experiments. From these results, we present an empirical relation for the liquid film thickness by adding orifice length to an existing analytical equation. The variations and stability of the air core were also examined by visualizing the formation of the air core in the swirl chamber with a high-speed camera system. This study confirms that air core shape and liquid film thickness are directly related. Thus, study of the fluctuations of liquid film thickness under various geometric conditions can be applied to the analysis of internal flow.

Properties of disturbance waves in vertical annular two-phase flow

Disturbance waves play an important role in interfacial transfer of mass, momentum and energy in annular two-phase flow. In spite of their importance, majority of the experimental data available in literature on disturbance wave properties such as velocity, frequency, wavelength and amplitude are limited to near atmospheric conditions (Azzopardi, B.J., 1997. Drops in annular two-phase flow. International Journal of Multiphase Flow, 23, 1–53). In view of this, air–water annular flow experiments have been conducted at three pressure conditions (1.2, 4.0 and 5.8 bar) in a tubular test section having an inside diameter 9.4 mm. At each pressure condition liquid and gas phase flow rates are varied over a large range so that the effects of density ratio, liquid flow rate and gas flow rate on disturbance wave properties can be studied systematically. A liquid film thickness is measured by two flush mounted ring shaped conductance probes located 38.1 mm apart. Disturbance wave velocity, frequency, amplitude and wavelength are estimated from the liquid film thickness measurements by following the statistical analysis methods. Parametric trends in variations of disturbance wave properties are analyzed using the non-dimensional numbers; liquid phase Reynolds number ( Re f), gas phase Reynolds number ( Re g), Weber number ( We) and Strouhal number ( Sr). Finally, the existing correlations available for the prediction of disturbance wave velocity and frequency are analyzed and a new, improved correlation is proposed for the prediction of disturbance wave frequency. The new correlation satisfactorily predicted the current data and the data available in literature.

Laser triangulation for liquid film thickness measurements through multiple interfaces

Laser triangulation is used to measure the thickness of a liquid film in a test section consisting of a quartz viewing window, a water layer, and a hydrophobic membrane. The triangulation sensor acquires measurements to the bounding surfaces of the film while peering through multiple interfaces. This allows the difference between the two measurements to constitute the local film thickness. A refraction model is developed and applied to the analysis of data collected from the experiment. For verification, an empirical method is also developed and compared to the analytical approach. The measurement technique is intended to assess the stability of liquid films for use as gas-liquid contactors.

Heat transfer and crisis phenomena with intense boiling in the falling wave liquid films

Experimental data on heat transfer with intense evaporation in the falling films of liquid nitrogen were analysed. According to data generalization, heat transfer at evaporation becomes more intense under the precrisis modes at high heat fluxes for two studied boundary conditions on the heat-releasing surface: Tw ≈ const and qw ≈ const. The relative contributions of conductive and convective components of heat transfer for different heat fluxes were estimated due to statistical treatment of the wave characteristics carried out by the capacitance probes for measurement of the local liquid film thickness. It was found out that heat transfer intensification is mainly caused by a drastic decrease in thermal resistance of the local zones with intensely evaporating residual layer between large waves. At that, the convective component of heat transfer related to wave perturbations on a free surface of a liquid film decreases significantly with a rise of heat fluxes. New data on pulsations of the local temperature of the heat-releasing surface were obtained at different points along the flow with the modes of “dry spot” formation.

Characteristics of the Liquid Film and Pressure Drop in Horizontal, Annular, Two-phase Flow Through Round, Square and Triangular Tubes

A unique set of liquid film thickness and pressure drop data has been obtained for horizontal, annular flow of air and water through round, square and triangular tube using a noninvasive, optical liquid film thickness measurement system. In the square and triangular tubes, the liquid film was thinned in the corners, indicating an effect of turbulent secondary flows in these geometries. In addition, when the film thickness is nondimensionalized using a friction velocity based on the total pressure drop, it is found that virtually all the data lie below h+=35 and that dryout occurs at approximately h+=5. The two-phase friction factor for the annular flows examined in all the tubes can be correlated using the Lockhart–Martinelli parameter, Xtt, when adjustments are made for the hydraulic diameters of the nonround tubes.

単チャンネル流路内スペーサによる液膜厚さ分布への影響評価に関する研究(一般セッション 液膜挙動)

The unsteady measurement of liquid film thickness is essential to investigate liquid film structure in the two phase annular-mist flow and especially to establish the dry out mechanism of liquid film on fuel rod around spacer in the new fuel assembly of BWR. The basic experiment is conducted to multi-points resistance probe to measure liquid film thickness profile in flow single channel with same cross-sectional configurations. Influences of spacer in rod gap on the liquid film thickness were clarified experimentally.

Measurement of liquid film thickness and the determination of spin-up radius on a rotating disc using an electrical resistance technique

A method of rapid measurement of liquid film thickness on a spinning disc surface is presented. The concepts used are based on the analysis of the electrical resistance of the liquid film and its relationship to film thickness when high frequency voltages are applied. A comparison with the Nusselt model is made for a wide range of operating conditions. Experimental data are presented and calculations of mean radial flow velocity based on film thickness measurements are made and shown to compare well with a simplified two-dimensional model. An empirical model for the extent of the spin-up radius is developed to provide a guide to the zone at which the Nusselt model breaks down.

2130 多点埋め込み式抵抗プローブによる液膜厚さ分布の計測に関する研究

The measurement of liquid film thickness is essential to investigate liquid film structure in the two phase annular-mist flow and especially to establish the dry out mechanism of liquid film on fuel rod around spacer in the fuel assembly of BWR. The basic experiment is conducted to multi-points resistance probe to measure liquid film thickness profile in a rectangle tube or a triangle tube.

Measurement of liquid film thickness using a laser light absorption method

A photometric technique for film thickness measurements is described in this paper. It is based on the absorption of light passing through a layer of dyed liquid and takes advantage of small sized diode laser sources and sensitive light sensors. The method is non-intrusive, easy to use and to calibrate and can be implemented at relatively low cost. Laboratory tests of the technique have yielded satisfactory accuracy and repeatability. Moreover, the technique allows measurements with a good spatial resolution. Flowing film thickness measurements made photometrically are compared directly to measurements taken simultaneously with a “parallel wire conductance probe”. Time-averaged film heights, RMS values and other statistical information have been obtained by analyzing these instantaneous film thickness records. With regard to the time-averaged values of liquid film thickness, there is a satisfactory agreement between the two measuring techniques.

Falling liquid film thickness measurement by an optical-electronic method

An optical-electronic method was developed for measurement of falling liquid film thicknesses. The method is based on the postulate that a sheet light beam passing tangentially through a vertical liquid film on a cylinder will be blocked by the falling liquid film. Hence, when the beam is much wider than the film, the output of a photodiode probe which is located on the opposite side of the cylinder from the light source will be reduced by an amount proportional to the film thickness. The shadow of the transient film shape will also appear on a screen behind the falling film. Therefore, the variation of the amplitude of the film waves can be measured from the output of the photodiode and the average film thickness can be measured from pictures of the shadow using computer aided image analysis. The average film thickness measured agrees well with commonly accepted empirical equations. The maximum relative deviation between the experimental and calculated results is 18.5% with a standard deviation of 4.34×10−2 mm. Compared with popular conductance parallel-wire probes, this method has the advantages of convenience, noninvasive, higher spatial and temporal resolution, smaller hysteresis and it does not disturb the model’s surface.

Automated optical liquid film thickness measurement method

A nonintrusive, automated, optical film thickness measurement technique has been developed to be used with a wide range of fluids and flow configurations. In this method, light is reflected from the surface of a liquid film flowing over a transparent wall. This reflected light generates an image on the outside of the wall which is captured and digitized using a charge coupled device camera and framegrabber card in a desktop computer. The image is processed to determine the positions of the reflected light rays, with which the film thickness is calculated. The entire process is automated, allowing for the collection of 600 data points in about 4 min using a personal computer with a 486 microprocessor. Film thicknesses on the order of 0.01 mm may be determined using inexpensive components, with the possibility of greater precision using more advanced imaging equipment. An automated calibration procedure allows for the determination of the necessary physical parameters, so the index of refraction of the test fluid or the test section wall need not be known a priori. Static liquid measurements agree to within 2.2% of measurements made using the needle-contact method. Film thickness data are presented for both round and square test sections operating under annular, two-phase flow conditions with air and water.

Measurement of 2-dimensional local instantaneous liquid film thickness around simulated nuclear fuel rod by ultrasonic transmission technique

An accurate knowledge of hydro-dynamic behavior of a liquid film flow on nuclear fuel rods is indispensable for analysis of the CHF under postulated loss-of-coolant-accidents in boiling water reactors. This work is concerned with a new development of ultrasonic transmission technique for film flow measurements. The technique adopted a rotating reflector, capable of measuring time-dependent spatial distribution of liquid film thickness around a simulated nuclear fuel rod. The scanning time is currently 4 ms for reconstruction of one image of the circumferential film thickness distribution.

Liquid Film Thickness Measurement by the Fiber-Optical Probe

Liquid Film Thickness Measurement by the Fiber-Optical Probe

混相流先端技術 １１ 超音波エコー法による模擬燃料棒周りに形成される液膜流の瞬時・二次元液膜厚さ分布の測定

混相流先端技術 １１ 超音波エコー法による模擬燃料棒周りに形成される液膜流の瞬時・二次元液膜厚さ分布の測定

Fluorescent imaging system for global measurement of liquid film thickness and dynamic contact angle in free surface flows

Fluorescent dye dissolved in a liquid flow was used to outline liquid-gas free boundaries and, with digital imaging, to observe quantitatively surface wave propagation and pattern formation, as well as contact-line velocity and contact angle in thin film flows on horizontal and inclined substrates. Using the relatively inexpensive system described here, a fluid depth measurement with a precision of ±0.02 mm is obtained routinely in flows of several millimeters depth over an area of approximately one square meter, and essentially unlimited continuous time spans. Dynamic contact angles are measured, for the first time, on liquid fronts with significant three-dimensional curvature such as rivulets draining down an inclined plate at any speed or global location. Procedures to normalize results quantitatively for any nonuniformities of the incident illumination are given. Estimates of the contribution to the experimental error by other effects, such as variations in dye concentration and temperature, and image digital register capacity, are also discussed. Illustrative results for two fluids and several dyes are given. Refinements to decrease the local error further to ±0.005 mm or less are described.

Measurement of liquid-film thickness by laser interferometry

Here the variation of a liquid-film thickness at small Reynolds numbers is discussed. The film thickness measurement by laser interferometry corresponds to the liquid flowing on the inner surface of a small-bore glass tube. An adequate theoretical background for the techniques used in this experiment is discussed to demonstrate the capability of the experimental technique. An advantage of this method is that it shows the shape of the thin film on the inner surface of the vertical tube at a point in a horizontal cross section. The results obtained from this experiment show that the flow of liquid films on vertical surfaces is inherently unstable and three dimensional even at a Reynolds number smaller than 1.

Liquid-to-wall shear stress distribution in stratified/atomization flow

Experiments were conducted in a horizontal flow loop (24 mm i.d.), using an electrochemical technique, for measuring liquid-to-wall shear stress, at various positions around the pipe circumference. Measurements of liquid film thickness, pressure drop as well as visual observations were also made. These data complement similar information obtained by Paras et al. (1994), in a horizontal 50.8 mm i.d. flow system. Mean values and other statistical information are obtained from the analysis of the shear stress and film thickness data. Visual studies of the gas/liquid interface confirm that its profile is concave rather than flat. The data show that the time-averaged shear stress tends to decrease in the lateral direction (i.e. away from the pipe bottom, Θ = 0°) along which the liquid film gradually becomes thinner. Only for relatively low superficial gas velocities ( e. g. U G < 15m/s), the mean shear stress is almost constant up to θ = 45°; i.e. in a flow region where there is a relatively thick liquid layer. An expression is proposed for predicting the shear stress circumferential distribution. By means of momentum balances (utilizing the new data), the average gas/liquid interfacial friction factor is determined with improved accuracy. Additionally an equivalent gas/liquid interface roughness is expressed in terms of wave characteristics, i.e. wave intermittency and amplitude.

A Capacitance Sensor for Two-Phase Liquid Film Thickness Measurements in a Square Duct

The use of capacitance sensors for measuring liquid film thickness or phase concentration in two-phase flow has gained popularity in recent years. In designing such sensors, there are many issues which must be considered in order to optimize performance: desired temporal and spatial resolution, two-phase flow regime, permittivity of the phases, duct geometry, electrical shielding, and temperature variation in the flow field. These issues are discussed, and the design of a 12.7 mm square cross section capacitance sensor which measures liquid film thickness in either stratified are annular two-phase flow is presented. Using a composite material analysis and an effective permittivity ratio, predictive relations for capacitance as a function of liquid film thickness have been derived for stratified and annular film patterns. The analysis eliminates the need for calibrating the sensor for stratified and annular flow regimes. Optical measurements of liquid film thickness using a high resolution CCD camera are compared against those using the capacitance sensor in conjunction with the predictive relations. The sensor was tested on a bench top for a stratified film pattern with no flow and two different electrode configurations (upwards and side configurations) using FC-87, a low-permittivity (εr = 1.72) dielectric fluid. The standard deviations between the film thicknesses measured optically and those predicted using the capacitance sensor and analysis are 0.014 and 0.019 mm for the respective upward and side electrode configurations. The sensor was also implemented in a vertical flow boiling facility, which uses FC-72 (εr = 1.75) as the working fluid. Time-averaged film thicknesses measured using the capacitance sensor are compared against ensemble-averaged measurements using the CCD camera for annular vertical upflow and downflow. The upflow and downflow standard deviations are 0.17 and 0.093 mm, respectively. As expected, the agreement for vertical flow is not as good as that for the horizontal no-flow case, because large fluctuations in film thickness are characteristic of annular two-phase flow, and the uncertainty of the photographic measurement is increased.