Dynamic Void Fraction Research Articles

ANALYSIS OF THERMAL PERFORMANCE IN A TWO-PHASE THERMOSYPHON LOOP BASED ON FLOW VISUALIZATION AND AN IMAGE PROCESSING TECHNIQUE

Thermal performance was analyzed based on flow visualization and an image processing technique in a two-phase thermosyphon loop (TPTL) with boiling water as the working fluid at low pressure. The bubble geometries, bubble frequency, and dynamic void fraction were estimated using direct image analysis combined with the power spectrum and statistical analysis. At a heating rate of 400 W and a filling ratio of 0.88, the thermal performance of the TPTL was enhanced with a thermal efficiency of 91% and effective thermal conductivity of 11,336 Wm<sup>-1</sup> °C<sup>-1</sup>. The enhancement was due to the higher frequency cap bubble flow of 11.2 Hz obtained by the direct flow visualization. The bigger Taylor bubbles with slug flow were examined to contribute to the negative effect on the heat transfer rate due to their film boiling regimes. The detailed analysis reveals the mechanism of bubble flow interacting with thermal performance.

Data-assisted CFD modeling of transient blast furnace tapping with a dynamic deadman

The blast furnace (BF) hearth’s condition is crucial for a long and healthy campaign life. Previous numerical investigations on BF drainage commonly treated the deadman as a static, porous medium. In this work, we employ a coupled computational fluid dynamics (CFD) – discrete element method (DEM) to account for its dynamic behavior. The model is utilized in several short-term simulations to generate a database of deadman states based on iron and slag levels, coke properties and the burden weight distribution. In contrast to the previous Eulerian deadman descriptions, we directly obtain an inhomogeneous description of the porosity from a particle size distribution. The database is utilized in the dynamic void fraction model, to describe the transient deadman in a fully Eulerian framework for long-term simulations. In essence, we present a strategy to mitigate the coupling between the dynamics of a packed bed and the fluid flow through it by connecting the bed’s evolution to global state parameters such as the levels of liquids in which the particles float. This methodology is not restricted to blast furnaces, but can also be applied to other problems involving slow granular motion with dynamic interstitial fluids.

Void fraction inference in cavitating fuel injector flows

It has been shown that hydrodynamic cavitation within fuel injectors plays a significant role in their performance, with the desirable effect of broadening the resultant fuel spray. Experiments are challenging owing to the relatively small geometries, high pressure, and high Reynold’s number (Re) associated with such flows. Previous studies have observed cavitation in optically transparent nozzles at slower flows. By utilizing acoustic and vibration measurement techniques cavitation activity may be measured in a steel fuel injector at more practical Re flows used in applications. We report here experimental measurements taken using a laser vibrometer and a commercial fuel injector. Previous studies have demonstrated a resonant frequency shift as a function of injection pressure. Among competing hypotheses, our working hypothesis is that this shift is the result of mass unloading of cantilever mode oscillations of the fuel injector. The dynamic void fraction caused by cavitation activity within the fuel injector can then be inferred from the measured frequency shift. We report measurements of mode shapes and frequencies for static and flowing fuel injectors as functions of the flow rate.It has been shown that hydrodynamic cavitation within fuel injectors plays a significant role in their performance, with the desirable effect of broadening the resultant fuel spray. Experiments are challenging owing to the relatively small geometries, high pressure, and high Reynold’s number (Re) associated with such flows. Previous studies have observed cavitation in optically transparent nozzles at slower flows. By utilizing acoustic and vibration measurement techniques cavitation activity may be measured in a steel fuel injector at more practical Re flows used in applications. We report here experimental measurements taken using a laser vibrometer and a commercial fuel injector. Previous studies have demonstrated a resonant frequency shift as a function of injection pressure. Among competing hypotheses, our working hypothesis is that this shift is the result of mass unloading of cantilever mode oscillations of the fuel injector. The dynamic void fraction caused by cavitation activity within the fuel in...

Investigation of a method for real time quantification of gas bubbles in pipelines.

The need to measure the dynamic void fraction (the proportion of flowing bubbly liquid that is gas) is common across many power, processing and manufacturing industries. Many such pipelines and liquids are optically opaque, and work on margins that require a low cost solution that is not commensurate with the size of the challenge. Such a solution will therefore be a compromise, and in this paper costs are reduced by using a narrowband acoustic solution that cannot, on its own, contain enough information to characterize the void fraction in real time unambiguously. The ambiguity is reduced using likely estimates of the general shape of the bubble size distribution so that, with a single source-receiver pair attached to the outside of the pipe, the absolute gas content can be estimated. While the data that are required a priori (the general shape of the bubble size distribution) are not identical to the output of the inversion (the absolute void fraction of gas entrained as bubbles in the flow), the requirement for such a priori information could limit the usefulness of the technique in industry.

Investigation of flow boiling in horizontal tubes: Part I—A new diabatic two-phase flow pattern map

Several important modifications to the flow pattern map of Kattan–Thome–Favrat [J. Heat Transfer 120(1) (1998) 140–147] made, resulting in a significantly new version of the map. Based on the dynamic void fraction measurements described in [Int. J. Multiphase Flow 30 (2004) 125–137], the stratified–wavy region has been subdivided into three subzones: slug, slug/stratified–wavy and stratified–wavy. Furthermore, annular-to-dryout and dryout-to-mist flow transition curves have been added and integrated into the new flow pattern map, identified by distinct trends of the heat transfer coefficient as a function of vapor quality and by flow pattern observations to determine (and then predict) the inception and completion of dryout in horizontal tubes.

Measurement of dynamic void fractions in stratified types of flow

A new non-intrusive computerized image analysis and optical observation method for accurately measuring dynamic void fraction has recently been developed by Thome and coworkers [Dynamic void fractions in stratified types of flow, part I: new optical measurement technique, Int. J. Multiphase Flow 31 (2004); Dynamic void fractions in stratified types of flow part II: measurements for R-22 and R-410a, Int. J. Multiphase Flow 31 (2004)]. This technique is applied to circular horizontal sight glass tubes using a monochromatic laser sheet to illuminate the two-phase flow coupled with image processing to measure cross-sectional void fractions and dry angles in stratified types of flow. The refraction effects on the cross-sectional images are overcome by reconstructing the video images by computer. From these, the shape at the vapor–liquid interface is detected and the void fraction is accurately determined (to an estimated accuracy of about ±0.01) over a wide range of void fraction values (from 0.05 to 0.95). In addition, the dry angle around the upper perimeter of the tube is also obtained. The system has been coupled to a flow boiling test facility to obtain dynamic and time-averaged void fractions in a horizontal tube for two refrigerants: R-22 and R-410A. About 227,000 images were analysed so far in this study for a 13.6 mm tube to provide the same number of dynamic void fraction measurements and 238 time-averaged void fraction values. A summary of the technique is described here with additional analysis of the previous results and new results (based on processing 83,000 new images) are presented here for void fractions in an 8.0 mm sightglass tube with R-22 at mass velocities of 100 and 150 kg/(m 2 s). The new method also has the potential to measure interfacial wave contours and other pertinent geometrical characteristics of stratified two-phase flows.

Interfacial measurements in stratified types of flow. Part II: Measurements for R-22 and R-410A

A new optical void fraction measurement system has been coupled to a flow boiling test facility to obtain dynamic and time-averaged void fractions in a horizontal tube. A series of evaporation tests have been run for two refrigerants. R-22 was tested under mass velocity conditions of 70, 100, 150 and 200 kg/m 2 s and R-410A for 70, 150, 200 and 300 kg/m 2 s in a 13.6 mm diameter glass tube. Using our newly developed image processing system, about 227 000 images have been analyzed in this study to provide the same number of dynamic void fraction measurements. From these images, 238 time-averaged void fraction values have been obtained for vapor qualities from 0.01 to 0.95. These experimental points show very good agreement with the horizontal version of the Rouhani–Axelsson drift flux void fraction model.

Gas-Liquid Distribution in the Developing Region of Horizontal Intermittent Flows

Experiments have been carried out to analyze the evolution of the air/water flow structure along a horizontal 60 mm i.d. straight pipe. Plug and slug flow regimes were observed. In order to investigate the local structure of the flow, dynamic void fraction measurements have been performed by means of ring impedance probes. From dynamic void fraction measurements the mean void fraction values, the probability distributions of void, the liquid film height in the gas cavities and the slug frequencies have been evaluated. The analysis of the results showed that the flow structure deeply modifies along the pipe and that minor effects have to be ascribed to gas injection mode. The examination of the probability distribution function enabled the identification of the operating conditions at which the transition occurs, thus making it possible to propose a new objective criterion of identification.

Dynamic void fraction measurements in horizontal ducts with sudden area contraction

Void fraction distributions in horizontal pipes with sudden area contraction have been experimentally investigated. The experiments are aimed at analysing the effect of the singularity characteristics on void fraction profiles and phase distribution. The technique employed to obtain the cross-sectional average void fraction along the test pipes is based on the instantaneous measurement of the electrical impedance of the air–water mixture. The instrumentation set up consists of ring electrode pairs placed on the internal wall of the cylindrical test duct, flush to the pipe surface. Two different test sections, a 70\\60 mm i.d. pipe and a 70\\36 mm i.d. pipe have been employed. Plug and slug flow regimes were investigated in order to obtain the mean void fraction along the channel in five different locations upstream and downstream the singularity. The analysis of the results, in terms of mean values and probability distributions, shows that the characteristics of the flow restriction deeply modify the flow structure upstream and downstream the discontinuity.

Slug flow regime identification from dynamic void fraction measurements in vertical air-water flows

An existing design of conductivity probe for the measurement of void fraction has been developed and tested. Calibration checks showed that the time averaged output of the instrument was close to linear over the whole range of void fraction, with a tendency to overestimate the true value. Computer modelling of the probe's response to different void distributions supported the calibration results. Two meters were used to examine the dynamic variation of void fraction in air water flows in a vertical 32 mm diameter tube. Flow patterns ranging from bubbly to annular were observed. Examination of the void fraction traces and their probability distribution functions enabled the identification of six flow regimes: discrete bubble flow, spherical cap bubble flow, stable slug flow, unstable slug flow, churn flow and annular flow. The locations of flow regime transition boundaries on a flow pattern map, deduced from this objective analysis, are in agreement with the data of other workers. The unstable slug flow regime corresponds to a region some workers have identified as churn flow, it provides explanations for some of the apparent anomalies concerning the location of the slug to churn flow boundary. Cross-correlation of the signals from the two probes has provided statistical data on the lengths of slugs and Taylor bubbles which suggested modifications to the previous theories enabling the entry to and exit from the unstable slug flow regime to be predicted.