Superficial Vapor Velocity Research Articles

Experimental investigation of hydrodynamics and suspension characteristics in a boiling stirred tank reactor with helical coils

The boiling stirred tank reactor (BSTR) is widely used in the chemical industry, while the relevant basic data of hydrodynamics and suspension characteristics for reactor design are quite limited. In this work, the hydrodynamics and suspension characteristics in a BSTR with helical coils (BSTR@HC) were studied using three types of impellers. Particle image velocimetry and visualization experiments reveal different flow fields and nucleation processes of three impellers. Suspension experiments show that superficial vapor velocity, solid loading, and particle size play an important role for just-suspension impeller speed Njsv and power consumption Pjsv, while cloud height is mainly determined by superficial vapor velocity. Among the three impellers, radial impeller shows the best suspension performance in BSTR@HC. Correlations of Njsv and Pjsv were developed with relative mean deviations of 2.7% and 4.6%, respectively. Furthermore, a model for the estimation of energy efficiency was developed by evaluate specific power consumption εjsv.

Influence of gas density on void fraction and pressure drop in upward vapor–liquid flow

The influence of gas density on the void fraction, flow map, and pressure drop was examined for an upward vapor–liquid flow. The knowledge of the flow map is necessary to ensure a safe operational condition in a nuclear zone at high pressure. Moreover, the pressure gradient and its volumetric flow rate are related to the profitability of petroleum wells. Considering these scenarios, a modified cascade refrigeration cycle was constructed to generate and measure single-phase streams with different liquid and vapor density ratios. These single-phase liquid and vapor streams were mixed and injected into an upward vertical test section with an inner diameter of 26.64 mm and length of 5.8 m. The pressure ranged from 1.7 to 2.3 MPa, liquid–vapor density ratio ranged from 10.2 to 15.2, and total mass flux ranged from 733.1 to 882.9 kg.s−1.m−2. The void fraction, pressure gradient, and frictional pressure component were measured using quick-closing valves, temperature probes, and absolute and differential pressure sensors. The experimental data and transition line behavior of the flow map at high pressures in an upward two-phase flow as well as the proposed correction factors for the pressure gradient and its frictional component reflect the novelty of this study. Most existing studies have focused only on the influence of the gas density on the horizontal flow. A key observation of our study is that the liquid–vapor density ratio significantly changes the churn and annular transition lines. According to a vertical flow map found in the literature, the disappearance of the churn region can be foreseen and an annular transition occurs at lower gas velocities. The liquid–vapor density ratio does not influence the void fraction or slip velocity ratio; both increase with the superficial vapor velocity. Correction factors for the pressure gradient and its frictional component were proposed. The mean deviation for the pressure gradient was reduced from 27 % to 16 % after application of the correction factor. Half of the experimental points presented absolute relative errors lower than 30 % for the frictional pressure gradient. A cross-validation between the experimental data reported by other authors and the proposed correction factors was conducted to assess their response. In this cross-validation, 126 additional experimental points were used for the pressure gradient whereas 1270 points were used for its frictional component. Overall, the mean deviation of the pressure gradient was reduced from 92.7 % to 31.9 % after the application of the correction factor. Similarly, the mean deviation of the frictional gradient was reduced from 70.7 % to 31.3 % after the application of the correction factor. There was no evident influence of the liquid–vapor density ratio on the total pressure gradient and drift relationships for Froude numbers spanning from 2.6 to 7.4.

Modelowanie procesów zachodzących w zbiorniku ciśnieniowym reaktora wodnego wrzącego podczas spadku ciśnienia w warunkach pracy awaryjnej

Evaluation of the two-phase water mixture level in the case of the sudden depressurization of the Reactor Pressure Vessel resulting from an accident scenario is an important aspect in the reactor safety analysis. This paper discusses results of simulations of the water dynamics and heat transfer during the process of an abrupt depressurization of a vessel filled up to a certain level with saturated liquid water and with the rest of the vessel occupied by steam under saturation conditions. During the pressure decrease e.g. due to a break in the steam pipeline, the liquid water evaporates abruptly leading to strong transients in the vessel. These transients and the sudden emergence of void in the area occupied by liquid at the beginning, result in the elevation of the two-phase mixture. This work presents several approaches for modelling of the void fraction, the level swell and the collapse level. The first approach was based on the churn turbulent drift-flux correlation and an explicit analytic equation for the averge void fraction as a function of dimendsionless superficial vapor velocity. The second and the third aproaches were based on dimensionless analysis and purely empirical corelations. The models were verified against independent experimental data. The models represent the Reactor Pressure Vessel of the Integral Test Facility Karlstein (INKA) – a dedicated test facility for experimental investigation of KERENA – a new medium size Boiling Water Reactor design of Framatome. The comparison of the simulations results against the reference data shows a good agreement.

Perilaku Antarmuka Berdasarkan Data Beda Tekanan pada Peristiwa Kondensasi Aliran Uap Dengan Pendinginan dari Luar Searah pada Pipa Horisontal Berbasis Domain Waktu

A two-phase flow pattern experiment on the condensation event of steam flow with external cooling based on the measurement of pressure difference within a horizontal pipe is carried out by varying the superficial velocity. Annulus pipes with inner pipe material made of copper and outer pipe made of galvanized iron (GIP) within an insulation of 10 mm thick were used in this experiment. The length of the pipe is 1.6 meters, outer diameter of 4 inch and inner diameter of 17 mm. The two-phase flow pattern was investigated based on differential pressure fluctuations between the inlet and outlet. To support the observation, flow pattern visualization was performed using a transparent pipe with a diameter of ¾ inch and a length of 1.3 meters connected to the test pipe section. The superficial vapor velocity was carried out from JG = 0.0689 m/s to JG = 1,9117 m/s. The results showed stratified flow patterns for the lowest superficial velocity and also obtained wavy, wavy-slug, and slug. Annular flow patterns can not be observed in this experiment. In general, increasing superficial velocity of steam will cause a significant increase in pressure fluctuations.

Forced convection boiling in a stator–rotor–stator spinning disc reactor

Boiling of a pure fluid inside the rotor–stator cavities of a stator–rotor–stator spinning disc reactor (srs‐SDR) is studied, as a function of rotational velocity ω, average temperature driving force and mass flow rate . The average boiling heat transfer coefficient hb increases a factor 3 by increasing ω up to 105 rad s−1, independently of and . The performance of the srs‐SDR, in terms of hb vs. specific energy input ϵ, is similar to tubular boiling, where pressure drop provides the energy input. The srs‐SDR enables operation at Wm , yielding values of hb not practically obtainable in passive evaporators, due to prohibitively high pressure drops required. Since hb is increased independently of the superficial vapor velocity, hb is not a function of and the local vapor fraction. Therefore, the srs‐SDR enables a higher degree of control and flexibility of the boiling process, compared to passive flow boiling. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3763–3773, 2016

Two-Phase Flow and Heat Transfer during Steam Condensation in a Converging Microchannel with Different Convergence Angles

The present study experimentally investigates the effect of convergence angle of microchannel on two-phase flow and heat transfer during steam condensation. Three condensation regimes, from the inlet to the outlet, are identified: mist/annular flow, injection flow, and slug-bubbly flow. Flow pattern maps are constructed using superficial vapor and liquid velocities as the coordinates, wherein relatively distinct boundaries between the flow patterns can be identified. The experimental results show that the condensation heat flux increases with an increase in the convergence angle and/or the steam mass flux at a given coolant flow ratebut decreases with an increase in the coolant flow rate at a given steam mass flux. The results further demonstrate that the local condensation heat transfer coefficient in the mist/annular flow region is much higher than that in other condensation regimes. Moreover, the local condensation heat transfer coefficient in the mist/annular flow and injection flow region decreases with an increase in the convergence angle.

Visualization of emergency viscous two-phase venting behaviors

Safety design of emergency relief system must consider adequate vent sizing whether the flow is a single phase or two-phase vapor–liquid flow and two-phase flow generally requires a larger relief area. It is an even more complex situation due to the effect of friction loss in a relief line with high viscous fluids. Generally, viscous fluids are polymer solutions or melts. The purpose of the present work is to study the effects of the viscosity of the liquid on the venting behavior and ascertain how well SAFIRE program can model the emergency venting of a non-reacting fluid. The fluids used in this study were water solutions made by adding a neutral polymer of PVP (polyvinyl-pyrrolidone) to water with 10 weight percent of the concentrations of the additive in the water. The viscosity was adjusted by selecting the different molecular weight with 40,000 g/mol and 3,60,000 g/mol of PVP in simulating the conversion condition of polymerization. At ambient temperature, the viscosity was estimated to be 0.007 and 0.113 Ns/m 2, respectively. The blow-down was performed by a purposed built bench scale reactor with volume of 1600 cc and a 0.1 m 3 pilot reactor at 150 °C where the relief pressure was about 5 bar. Venting experiment in the pilot reactor is important in simulating the large vessel in industry and avoiding the small pipe effect in bench scale. Simulation of the characteristics of two-phase flow was conducted by SAFIRE program. Experimental data of pressure and mass flow rate were compared with the results of simulation by using bubbly, churn-turbulent, and homogeneous flow model. The microscopic dynamic behaviors were studied by the fast photography technique in a 1600 cc reactor with two windows to allow the flow visualization. Experimental data of superficial vapor velocity and bubble rise velocity were directly measured in the fast photography in the blow-down. It is expected that the present study may contribute a better understanding of the dynamic behavior and the mechanism of flashing flow.

Two-phase flow characteristics of liquid nitrogen in vertically upward 0.5 and 1.0 mm micro-tubes: Visualization studies

Application of liquid nitrogen to cooling is widely employed in many fields, such as cooling of the high temperature superconducting devices, cryosurgery and so on, in which liquid nitrogen is generally forced to flow inside very small passages to maintain good thermal performance and stability. In order to have a full understanding of the flow and heat transfer characteristics of liquid nitrogen in micro-tube, high-speed digital photography was employed to acquire the typical two-phase flow patterns of liquid nitrogen in vertically upward micro-tubes of 0.531 and 1.042 mm inner diameters. It was found from the experimental results that the flow patterns were mainly bubbly flow, slug flow, churn flow and annular flow. And the confined bubble flow, mist flow, bubble condensation and flow oscillation were also observed. These flow patterns were characterized in different types of flow regime maps. The surface tension force and the size of the diameter were revealed to be the major factors affecting the flow pattern transitions. It was found that the transition boundaries of the slug/churn flow and churn/annular flow of the present experiment shifted to lower superficial vapor velocity; while the transition boundary of the bubbly/slug flow shifted to higher superficial vapor velocity compared to the results of the room-temperature fluids in the tubes with the similar hydraulic diameters. The corresponding transition boundaries moved to lower superficial velocity when reducing the inner diameter of the micro-tubes. Time-averaged void fraction and heat transfer characteristics for individual flow patterns were presented and special attention was paid to the effect of the diameter on the variation of void fraction.

Two-phase flow pattern and frictional performance across small rectangular channels

This study presents flow visualizations and two-phase frictional pressure drop data for three rectangular channels with channel height of 3, 6 and 9 mm, and a fixed width of 3 mm. It is found that the stratified flow pattern still exists for an aspect ratio of unity at a low mass flux of 100 kg/m 2 s but it completely vanishes when G > 200 kg/m 2s. For the same plug flow of intermittent flow pattern, the number of plug increases whereas its length decreases when the aspect ratio is increased. This is especially pronounced when the mass flux is further increased over 500 kg/m 2 s. The major departure of the observed flow pattern relative to the conventional Mandhane flow map is the transition boundary for slug/annular had been moved to a much lower superficial vapor velocity. The two-phase frictional pressure drop data are compared to homogeneous and Chisholm method, Wambsganss and Ide-Fukano correlations. It is found that none of the existing methods or correlations can satisfactorily predict the two-phase pressure gradient in rectangular channels. A modified C factor of Chisholm method considering the effect of aspect ratio was proposed from the empirical fit with the data sets of Wambsganss et al., Ide-Fukano, and this study. The corresponding mean deviations of the proposed correlation against the datasets are 24.99%, 10.83% and 10.73%, respectively. This correlation is applicable in wide rages of mass flux (50 < G < 700 kg/m 2 s), vapor quality (0.001 < x < 0.95), Martinelli parameter (0.05 < X < 20) and aspect ratio (0.1 < A < 1.0).

System Limit: The Ultimate Capacity of Fractionators

The system limit is reached when the superficial vapor velocity in the tower exceeds the settling velocity of large liquid droplets. At higher vapor velocities, ascending vapor lifts and carries over much of the tray liquid, causing the tower to flood. This flood cannot be debottlenecked by improving the internals or by increasing tray spacing. The system limit represents the ultimate capacity limit of the vast majority of the existing trays and of all the existing packings. In some applications, where very open packings or trays are used, such as refinery vacuum towers, the system limit is the actual capacity limit. Until recently no published methods were available to predict it. The factors that affect it are poorly understood. In the late 1950s and early 1960s, Fractionation Research Inc. (FRI) systematically studied this limit in a 1.22m diameter column and developed several different correlations. Publication of this first commercial scale ‘system limit’ data by FRI last year permitted us to explore this system limit, shedding much-needed new light on this phenomenon. The Stupin correlation, based on a dispersed liquid phase falling through an ascending vapor phase, was shown to give excellent prediction of experimental data on the ultimate capacity in the liquid rate range of 40–140m3 h–1 m-2 tower cross section area. However, the correlation tends to predict high at lower liquid rates. The current paper extends Stupin's previous model into the low liquid rate region. This correlation provides a useful means to check the limiting capacity that could be expected for a given column diameter and set targets for the capacities of new devices. The challenge is to develop devices that exceed this capacity limit.

Measurements of solid particle suspension concentrations in boiling pools

An experimental study was performed to measure vertical particle suspension concentrations in contaminated boiling pools. The study was conducted in a nucleate boiling regime. Tests were conducted with distilled water and solid nickel particles ranging in size from 5 to 40 μm. The test apparatus was 30 cm wide and 120 cm high. Particle concentrations were on the order of milligrams per cubic centimeter of slurry. Heat fluxes on the order of 100 kW m −2 were attained by electrical bottom heating. Corresponding superficial vapor velocities were on the order of several centimeters per second. Measurements were aimed to reveal the dependence of dilute particle suspension distributions on the pool depth, heat flux and particle loading. The results indicate that only a fraction of the particle loading gets suspended. This fraction turns out to be largely dependent on the pool depth and heat flux, but is insensitive to particle loading variations. The particle distribution within the suspension was analyzed with the dispersion–sedimentation model. The work demonstrates the applicability of this model for the conditions of boiling pools and partial particle suspensions. Furthermore, the results indicate that for shallow pools and small size particles the distributions are fairly uniform. Consequently, the suspension concentration is substantially a function of the fraction suspended. The work provides experimental data to evaluate this fraction for a range of heat fluxes and pool depths. The knowledge of particle suspension concentrations has important industrial and environmental applications in the power generation, nuclear and chemical engineering industries. For example, the results could be used to evaluate the amount of particle released from a contaminated boiling-pool spill, or could be used to reduce conservatism applied in analyses of pools containing larger particles.

Measurements of solid contaminant emission rates from boiling pools

An experimental study was performed to measure release rates of solid particles from boiling pools. Sensitivity studies were conducted to reveal the dependence of the particle carry-over rate on the pool depth, the boiling rate and the vertical distance from the pool surface. Experimental data on particle concentrations as a function of size at the pool surface are also provided. The study was conducted in a nucleate boiling regime. Tests were performed with distilled water and solid nickel particles ranging in size from 5 to 40 μm. The test apparatus was made of a cylindrical vessel, 30 cm in diameter and 120 cm in height. Slurry particle concentrations were on the order of milligrams per cubic centimeter of slurry. Heat fluxes on the order of 100 kW/m 2 were attained by electrical bottom heating. Corresponding superficial vapor velocities were on the order of several centimeters per second. The phenomenon of particle emission from the surface of boiling pools pauses major environmental and industrial problems. Such particles are often undesirable contaminants, especially if toxic or radioactive. Exploration of this subject is of considerable importance to the chemical, nuclear, and power generation industries. The current study provides important data to support safety analyses and development of analytical tools. Surface contamination concentrations and particle emission rates are essential data for environmental contamination evaluations of spills and boiling, respectively. The study indicates, among other things, that emission rates intensify as pools get shallower. A released particle flux is strongest near the surface; and at a decreasing pace, diminishes to a constant rate as the distance from the surface is increased. Decontamination factors of the boiling process were found to be in a range of 10 3–10 4.

The effects of inclination angle on flooding in a helically fluted tube with a twisted insert

This paper develops an experimental flooding correlation as a function of the inclination angle for a fluted tube with a twisted insert for application to absorption heat pump systems. Fluted tubes have been used to enhance both heat and mass transfer during absorption and desorption processes. In the present tests, a plastic twisted tape was inserted to increase the absorption rate in counter current absorption. The effects of the twisted insert and the angle of inclination on flooding were examined. Water-ethyl alcohol solution flows downward in the fluted tube while air flows counter current in an upward direction. The flooding mechanism in a fluted tube with a twisted insert was analyzed by visual observation, and experimental correlations for flooding for both vertical and nearly horizontal tubes were developed. The experimental data from this paper were compared with those for smooth tubes from the literature. The results show that flooding was initiated from the top for inclination angles less than 60° while it started from the bottom of the tube for the angles larger than 60°. The fluctuation in pressure drop was very strong when flooding started from the bottom while it was not as strong when it started from the top. The effect of the position of the insert was not significant for the nearly horizontal tube while it had some effect for the vertical tube. For the fluted tube with an insert, the flooding vapor velocity was the highest for the inclination angle between 40° and 60° for superficial liquid velocities less than 1.178 × l0 −2 m/s. When the superficial velocity of liquid flow is higher than 1.178 × 10 −2 m/s, the superficial flooding vapor velocity increases with an increase of the inclination angle.

Phase Splitting Of Wet Steam in Annular Flow Through a Horizontal Branching Tee

Summary A phase-splitting equation for flow of a two-phase fluid through a tee junction was derived. It shows that the fluid quality at an outlet of the tee is determined by the relationship between the liquid and vapor extraction ratios of the fluid through that outlet. This equation applies to any tee junction regardless of its geometry, orientation, and inclination. This paper presents its application to the flow of wet steam through a horizontal branching tee. Experimental data were for wet steam flowing through a standard branching tee of 2- and 4-in. sizes in a horizontal plane, inlet pressures from 400 to 800 psig, inlet qualities from 0.2 to 0.8, inlet vapor velocities from 40 to 120 ft/sec, and for vapor-extraction ratios in the 0.2 to 0.8 range. These data were analyzed focusing on the effect of the vapor-extraction ratio of the run stream on the liquid-extraction ratio of the same stream. A correlation between these two extraction ratios has been established with steam quality, superficial vapor velocity, and critical velocity at the inlet to the tee as controlling parameters. Using this correlation and the phase-split equation, we are able to predict steam qualities exiting from horizontal branching tees to within ± 15% of the experimental values, which is more accurate than other phase-splitting models.

Drift-flux correlation disengagement models: Part II — Shape-based correlations for disengagement prediction via churn-turbulent drift-flux correlation

In Part I of this work the theoretical foundation was laid for predicting disengagement via volumetric gas production and an axial void fraction profile. Earlier work indicated for non-foaming systems one of the two drift-flux correlations can be chosen on the basis of viscosity. Herein, for the churn-turbulent drift-flux correlation (low-viscosity system), the dependence average void fraction and dimensionless superficial vapor velocity [1] on vessel shape (e.g., vertical cylinder, horizontal cylinder, or sphere) is explored. There is very little shape dependence and it is quantified below. A simple two-constant correlation is suggested, which satisfies the conditions at low and high dimensionless superficial vapor velocity conditions. Constants are given for vertical cylinders, horizontal cylinders, and spheres for the churnturbulent drift-flux correlation. The disengagement model is based on a constant energy generation per unit mass of liquid. Thus, the application to runaway reaction with vaporization is straightforward. The application of the correlation to reactive systems producing gas (i.e., gassy systems) and to those both producing gas and involving vaporization (i.e., hybrid systems) in non-constant cross-sectional area vessels is clarified here. Rating calculation equations are developed, and the calculations are illustrated. For a rating calculation, the maximum gas rate (based on vent capacity) is known. The calculation of maximum average void fraction (i.e., 1-fill ratio) and gas generation per unit of liquid is straightforward. Thus, the rate of reaction (i.e., gas generation) must be found to assure that the vent capacity is sufficient

Drift-flux correlation disengagement models: Part I — Theory: Analytic and numeric integration details

In the design of pressure relief systems for vessels containing liquid, the phase of the flow through the vent line is very important. Mounting the line on the top of the vessel does not necessarily guarantee all vapor flow. One must calculate whether vapor bubbles formed in the liquid will disengage before they reach the vent entrance. Disengagement can be predicted via an axial void fraction profile that is calculated based upon volumetric gas production. It is assumed that the liquid phase is continuous and that pseudo-steady state is reached. The disengagement model is based on a constant energy generation per unit mass of liquid. For non-foaming systems, one of two drift-flux correlations can be chosen on the basis of viscosity. The churn-turbulent drift-flux correlation is for low-viscosity systems, and the DIERS' viscous-bubbly drift-flux correlation is for high-viscosity system [1, 2]. This model reduces to a single ordinary differential equation (ode). Analytic integration results for this model are possible for constant cross-sectional area vessels (e.g., vertical cylinder) and non-unity distribution parameters Co [3–5]. If this calculation shows the bubbles do not disengage, either a partial differential equation model must be solved or the coupling equation must be used. The coupling equation uses the maximum void fraction (calculated from the ode) and ties together the vessel and vent models. The ode solutions relate the local and average void fractions to the dimensionless superficial vapor velocity. For the churn-turbulent drift-flux correlation, explicit relationships are presented for the first time. They validate the earlier approximation of Fauske et al. [6] (see also Ref. [3]). For the DIERS' viscous-bubbly drift-flux correlation, implicit relationships are presented for the first time in the open literature. The earlier approximation of Fauske et al. [6] (see also Ref. [7]) fit the data, but is different than these integration results. Further work is in progress to refit the data [8] and to clarify the best model to use. For non-constant cross-sectional area vessels (e.g., horizontal cylinders and spheres), the analytic integration is difficult, but numeric results have been presented [3, 7]. The details of the numeric integration are presented and discussed here. As the cross-sectional area converges (toward the top of the vessel), the vapor concentration (i.e., the void fraction) and velocity increase. The maximum local void fraction occurs at the top of the vessel. Numerical difficulties are encountered as a result of the cross-sectional area going to zero at the bottom and top of the vessel. The pseudo-steady-state model imposes void fractions of minimum and maximum at these extremes (i.e., 0 and 1/Co).

Phase Distribution of High-Pressure Steam-Water Flow at Large-Diameter Tee Junctions (Data Bank Contribution)

Experimental data are presented for the phase distribution of high pressure (2.75 to 5.5 MPa) steam-water mixtures at horizontal, equal-sided dividing tee junctions of two different sizes (49.3 and 97.3 mm i.d.). These data correspond to wide ranges of inlet qualities, inlet superficial vapor velocities, and extraction rates. Influences of the independent parameters on the phase-distribution phenomenon are presented and discussed. Comparisons between the present data and existing models have shown that the applicability of some models can be extended to the present test conditions.

Heat Transfer to Curved Surfaces from Heat Generating Pools

Experiments were conducted on heat transfer from internally heated ZnSO4-H2O pools to curved surfaces. These experiments extended existing data for nonboiling pools to higher Rayleigh numbers. The data for convective downward heat transfer from nonboiling pools to a curved surface were reasonably close to the Mayinger correlation extrapolated to higher Rayleigh numbers and lower ratios of pool depth to radius of curvature. Sideward heat transfer to a surface could be described by Nu = 0.7 Ra0.2. Insulating the upper pool surface from the atmosphere had no effect on either sideward or downward heat transfer. An investigation was also made on effects of curvature on heat transfer from boiling pools. Nusselt numbers for sideward heat transfer were proportional to a boiling Reynolds number based on superficial vapor velocity to the 0.275 power and quite close to the correlation for a pool with flat vertical walls. Downward boiling heat transfer to a curved surface was proportional to the Reynolds number to the 0.1 power.

Flow Behavior of Volume-Heated Boiling Pools: Implications with Respect to Transition Phase Accident Conditions

Observations of two-phase flow fields in single-component volume-heated boiling pools were made. Photographic observations, together with pool-average void fraction measurements, indicate that the churn-turbulent flow regime is stable for superficial vapor velocities up to nearly five times the Kutateladze dispersal limit. Within this range of conditions, a churn-turbulent drift flux model provides a reasonable prediction of the pool-average void fraction data. An extrapolation of the data to transition phase accident conditions suggests that intense boilup could occur where the pool-average void fraction would be >0.6 for steel vaporization rates equivalent to power levels >1% of nominal liquid-metal fast breeder reactor power density. The extended stability of bubbly flow to unusually large vapor fluxes and void fractions, observed in some experiments, is a major unresolved issue.

A study of sieve‐tray efficiencies

AbstractThe performance of sieve trays in the rectification of the methanol‐water system without entrainment or leakage from the perforations was studied in an 8‐in.–diameter five‐tray column. The trays had a 2‐in. weir height and 4‐in. length of liquid path. Three tray geometries were studied: 1/4‐in. diameter holes on 3/4‐in. triangular spacing, 1/8‐in. holes on 3/8‐in. triangular spacing, and 3/16‐in. on 7/16‐in. triangular spacing. The superficial vapor velocity was varied from 2.2 ft./sec. to the limit of stable operation, which for this apparatus was 4.4 ft./sec. The ratio LM/VM within the column was varied from 1 to 0.5. The Murphree plate efficiency varied greatly from 105% at low concentration to 82% at high concentrations of methanol. Variations of 10 or 12 efficiency % were noted owing to changing velocities and tray geometries. Measurements of concentration gradients, foam heights, and gas pressure drops are also reported. This paper proposes a method of calculating the point efficiency and the number of individual‐phase mass transfer units independent of the actual concentration gradient on the tray.The method is applied to the methanol‐water data, and calculated point efficiencies range from 50 to 65%. The value of 1/NL for the methanol‐water system is found to be small. The values of NG and the effect of the velocity on NG are believed to be the first in the literature for a tray in distillation operation. The effect of velocity is shown to be in agreement with the theory proposed by Gerster and co‐workers. It is shown that kG' aG decreases for increasing free area and increasing hole size. Finally variation in LM/VM is shown to have little effect on EMV.