Local Liquid Film Research Articles

Liquid film circumferential asymmetry prediction in horizontal annular two-phase flow

This study considers the prediction of the degree of asymmetry in the circumferential distribution of the liquid film in the tube cross section of horizontal annular gas–liquid two-phase flow, endemic of the lower region of this flow regime near the stratified-wavy flow transition boundary. Focusing on disturbance waves as the predominant mechanism for transporting the liquid in the annular film from the bottom to the top of the tube to counterbalance the draining effect of gravity, a new prediction method for the degree of asymmetry in the annular liquid film is proposed that outperforms existing correlations. Flow pattern maps for horizontal gas–liquid two-phase flow of frequent use in the design of evaporators and condensers can thus be explicitly updated to account for both symmetric and asymmetric annular flows. The underlying experimental database contains 184 measured liquid film circumferential profiles, corresponding to 1276 local liquid film thickness measurements collected from 15 different literature studies for tube diameters from 8.15mm to 95.3mm.

ESTIMATION OF LOCAL LIQUID FILM THICKNESS IN TWO-PHASE ANNULAR FLOW

In many semi-empirical analyses of flow boiling heat transfer, an annular flow is often assumed as a model flow and the local liquid film thickness is a key parameter in the analysis. This work considers a simple electrical conductance technique to estimate the local liquid film thickness in two-phase annular flows. In this approach, many electrodes are mounted flush with the inner wall of the pipe. Voltage differences between two neighboring electrodes for concentric annular flows with various liquid film thicknesses are obtained before the main experiments and logged in a look-up table. For an actual application in the annual flow, voltage differences of neighboring electrodes are measured and then corresponding local film thicknesses are determined by the interpolation of the look-up table. Even though the proposed technique is quite simple and straightforward, the numerical and static phantom experiments support its usefulness.

Liquid film behavior in annular two-phase flow under flow oscillation conditions

Abstract Experiments were carried out to investigate the effects of sinusoidal forced oscillation of the inlet flow rate on the time variations of local liquid film thickness and the frequencies of large wave’s passing in steam–water annular two-phase flows. The liquid film thickness oscillated with the same period as the inlet flow rate. The mean film thickness in the thin film regions decreased and approached to an asymptotic value with an increase in the oscillation period of the inlet flow rate. This result was consistent with the experimental results of the occurrence of liquid film dryout under flow oscillation conditions reported in the literature. It was hence considered that the axial liquid transport from the thick to thin film regions mitigates the reduction of the critical heat flux caused by the flow oscillation. It was also found that the wave frequency in the thin film region increased with a decrease in the oscillation period. This observation suggested that the disturbance waves contribute to the enhancements of the liquid transport and consequently the critical heat flux associated with the liquid film dryout under flow oscillation conditions.

Laminar film condensation from a downward-flowing steam-air mixture onto a horizontal circular tube

This investigation develops a numerical model of laminar film condensation from a downward-flowing steam-air mixture onto a horizontal circular tube. The significant non-similarity of the coupled two-phase flow laminar film condensation is such that the boundary layer governing conservations of momentum, species and energy in the mixture and liquid phases are solved by finite-volume methods. Numerical analysis of both the local condensate film thickness and heat transfer characteristics elucidated the simultaneous effects of inlet-to-wall temperature difference and inlet air concentration, the Reynolds number of the mixture, and the dimensionless parameter, F, by adopting a unified condensate parameter. The local Nusselt number and liquid film thickness increase as both the non-condensable air mass fraction and the tube temperature decreases. The numerical results on local heat transfer and film thickness for low/no non-condensable gas (air) agree closely with the theoretical results of Yang [Sheng-An Yang, Superheated laminar film condensation on a nonisothermal horizontal tube, J. Thermophys. Heat Transfer 11(4) (1997) 526–532], Fuji [T. Fujii, H. Uehare, C. Kurata, Laminar filmwise condensation of flowing vapor on a horizontal tube, Int. J. Heat Mass Transfer 15 (1972) 235–246] and Homescu and Panday [D. Homescu, P.K. Panday, Forced convection condensation on a horizontal tube: Influence of turbulence in the vapor and liquid phases, J. Heat Transfer Trans. ASME 121(4) (1999) 874–885]. Meanwhile, the comparison of average heat transfer coefficient with the experimental data from Rose and Lee [W.C. Lee, J.W. Rose, Forced convection film condensation on a horizontal tube with and without non-condensing gases, Int. J. Heat Mass Transfer 27(4) (1984) 519–528] demonstrates reasonably good agreement with the parameter F ( = gdh fg μ L / ⌊ k L U 0 2 ( T 0 - T W ) ⌋ , the ratio of gravity to mixture velocity) and G (= ( T 0 - T W ) ( k L / h fg μ L ) ( ρ L μ L / ρ m μ m ) , the suction effect).

Inclination effects on circumferential film flow distribution in annular gas/liquid flows

AbstractMeasurements of the circumferentially resolved axial liquid film flow rate have been obtained for a 38 mm internal dia. pipe at inclinations of 0°, 20°, 45°, 70 and 85° from the horizontal. The method employed is based on that used by Butterworth, and consists of a film extraction technique which utilizes a unit with a wall that is porous over a sector of the pipe. Liquid entrained fraction was determined from the total film flow rate. Results show that the local liquid film flow rate is not symmetrical. Flow rates are higher on the lower part of the pipe. As the inclination angle deviates from horizontal, the film flow rate becomes systematically less asymmetric. There is not a strong dependence of the entrained fraction on pipe inclination. For the case when the pipe is horizontal, the circumferential variation of film flow rate is well predicted by a horizontal annular flow model. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Effect of inclination on circumferential film thickness variation in annular gas/liquid flow

Time-varying, circumferentially local liquid film thickness data have been collected on a 38 mm internal diameter pipe at inclinations of 0 ∘ , 30 ∘ , 45 ∘ , 60 ∘ and 85 ∘ from the horizontal using flush-mounted and parallel wire conductance probes. Analysis of these data permits time-averaged thicknesses, probability density functions, and power spectral densities to be determined. Results show that the distribution of the liquid film is not symmetrical with thicker films on the lower part of the pipe, which are dominated by large disturbance waves. Moreover, as the inclination angle deviates from horizontal, the film thickness distribution becomes systematically less asymmetric. The probability density functions show a strong narrow peak where the liquid film is less disturbed by the presence of waves. The power spectra show that a large portion of wave energy at the bottom is carried by waves of frequency ⩽ 12 Hz . There is no influence of liquid velocity on the shape of the spectra. However, the dominant frequency appears to decrease with increasing liquid flow rate. The frequency of the disturbance waves at the bottom decreases with increasing inclination. Moreover, the spectra tend to flatten out with increasing inclination, due to the more uniform distribution of energy among waves of a broad frequency range.

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.

The flow characteristics of an upward gas‐liquid two‐phase flow in a vertical tube with a wire coil: Part 2. Effect on void fraction and liquid film thickness

AbstractAn experimental study has been carried out to clarify the characteristics of the void fraction and the liquid film thickness of the air‐water two‐phase flow in vertical tubes of 25‐mm inside diameter with wire coils of varying wire diameter and pitch. The flow pattern in the experiment on the average void fraction and the local void fraction distribution in cross section was a bubble flow, and the liquid film thickness was in the region of semiannular and annular flows. It is clarified from these experiments that the average void fraction in tubes with wire coils is lower than that in a smooth tube and decreases with the wire diameter owing to the centrifugal force of the swirl flow which concentrates bubbles at the center of the tube, that the local liquid film thickness becomes more uniform with a decrease in the pitch of the wire coil, and that the liquid film becomes thicker after the passage through the wire coil with an increase in the wire diameter. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 652–664, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10067

Experimental study on the characteristics of liquid layer and disturbance waves in horizontal annular flow

The mechanism for transporting liquid from the bottom of the pipe to the top still to be established in the prediction of the film thickness distribution in horizontal annular two-phase flow. To resolve this issue, using five parallel-wire conductance probes, time records of local liquid film thickness at five circumferential positions were collected. The characteristics of circumferential liquid film thickness profiles and its variation with gas and liquid velocities were obtained. The basic features of probability distribution function, probability density function, auto-correlation, cross-correlation and power spectrum density function of the disturbance waves in annular flow were studied respectively. The characteristics of circumferential profiles of disturbance waves and its variation with gas and liquid velocities were presented.

Film-Thickness Distribution for Annular Flow in Directional Wells: Horizontal to Vertical

Summary An experimental and theoretical investigation has been carried out on two-phase annular flow in inclined pipes. The study focused on the effect of the inclination angle on the liquid film thickness distribution. A conductance multiprobe instrument was utilized to measure the local liquid film thickness around half of the pipe periphery for inclination angles of 90°, 75°, 60°, and 45° from the horizontal. A simple analytical model has been developed for the prediction of the liquid film thickness at the top and bottom of the pipe. The model is applicable for the entire range of inclination angles, from horizontal to vertical. Good agreement is observed between the prediction of the model and the experimental data collected in this study and from the literature.

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.

Determination of Heat Transfer Coefficients in Rotating Cone Evaporators: Part I

To gain an understanding of the mechanism of liquid film flow and evaporation on the inner surface of a rotating cone, a theoretical method of calculating the local liquid film thickness and heat transfer coeffcients on a rotating cone was developed by applying a Nusselt-type assumption. Good agreement was found between the predicted and experimental heat transfer coeffcients obtained using a Centritherm evaporator and another rotating surface evaporator having a different cone angle.

121 顔料輝度法による気液二相環状流中の界面波の可視化と液膜厚さの定量測定

In order to make quantitative measurements of local and temporal liquid film thickness in two-phase annular flow by using pigment luminance method (PLM), we made a calibration experiment to determine the relationship between the liquid film thickness and the luminance from pigment dispersed in the liquid film. Furthermore, the relationship between the liquid film thickness and the luminance has been theoretically derived on the basis of the exponential decay law. As a result, we found that the theory generally agrees with a calibration experiment. We investigated the characteristics of the ring wave in detail. As a result, the followings have been elucidated. Firstly, the wave height is decreased as jg is increased, and it jumps only at a particular value of ii, but at any other points of ii it is approximately kept constant. Secondly there are seen two types of the mechanisms for the ring wave generation: appearance through an agglomeration of the ripple wave and through the disintegration of the disturbance wave.

Characteristics of Liquid Film Thickness of Air-Water Annular Two-Phase Flow in a Helically Coiled Tube. Horizontal Coil.

An experimental study has been carried out to clarify the characteristics of the liquid film thickness of air-water two-phase flow in a helically coiled tube placed horizontally. The distribution of the time-averaged local liquid film thickness around a tube periphery varies along the circumference of the coil at low gas velocities. A subsidiary peak of the film thickness forms on the outer side of the coil for large liquid flow rates. The mechanism of peak formation varies with magnitude of the gas velocity. The peripherally averaged liquid film thickness of a tube section in a coiled tube is thicker than that in a straight tube at high gas velocities. When the superficial gas velocity exceeds 20∼30 m/s, the centrifugal force of the gas-phase is dominant on the distribution of the film thickness around the tube periphery at the lower section. At a high gas velocity, it is expected that the characteristics of the liquid film on the downward or upward section of the coil are similar to those in a microgravity environment.

Properties of the liquid layer in horizontal annular flow

Time records have been collected of local liquid film thickness, using parallel-wire conductance probes, in a new 50.8 mm i.d. pipe loop. Statistical analysis of these records allows the study of the circumferential variation of time-averaged thickness, of RMS values and of other quantities. The power spectra show that the dominant frequency associated with large disturbance waves is <10 Hz at low gas velocities, displaying a tendency to increase with gas flow rate. By combining signal cross-correlation data with visual observations, it is shown that the disturbance waves tend to deform rapidly, move on a plane inclined with respect to the vertical and cover an increasingly larger portion of the circumference with increasing gas velocity. The mean large wave height at the pipe bottom is linearly related to the local RMS value. It is also a strong function of gas flow rate, with a moderate dependence on liquid flow rate. The wave steepness shows a similarly strong dependence on gas flow rate. However, the disturbance wave intermittency appears to be almost independent of gas as well as liquid flow rates.

Wavy flow along oscillating wall

A capacitance probe for the measurement of a local liquid film thickness was developed and tested. Characteristics of wavy flows of Newtonian (glycerol) and non-Newtonian liquids (aqueous solution of carboxymethylcellulose and kaolin suspension in water) along a vertical wall were measured by the probe. Attention was also devoted to the influence of wall oscillations upon waves on the film surface. The measured data on wavy flow of glycerine and partially also on carboxymethylcellulose (flow behaviour index n = 0.86) were in a good agreement with Kapitsa theory based, besides others, on the assumption of the sine shape of waves. However the theory cannot be used even as the first approximation for description of wavy flow of kaolin suspension (n = 0.14).