Vortex formation


Vortex Shedding. When a streamlined body is placed in a flowing stream, the fluid follows the contours of the body without separating from its surface. If, however, the body is bluff or non streamlined, the fluid separates at some point from the surface and roUs into a vortex. For two-dimensional symmetrical bodies, the changes in local velocity and pressure associated with the separation on one side interact with vortex formation on the opposite side. This feedback quickly causes a stable pattern of alternate vortex shedding so that the downstream wake becomes a staggered pattern of vortices commonly referred to as a Karmen-vortex street. This pattern is shown for a cylindrical obstmction in Figure 15. It is this phenomena that causes the flapping of flags and the clear turbulence behind jet aircraft.  [c.64]

Internal Flow. Depending on the atomizer type and operating conditions, the internal fluid flow can involve compHcated phenomena such as flow separation, boundary layer growth, cavitation, turbulence, vortex formation, and two-phase flow. The internal flow regime is often considered one of the most important stages of Hquid a tomiza tion because it determines the initial Hquid disturbances and conditions that affect the subsequent Hquid breakup and droplet dispersion.  [c.328]

Fermentation. The volume of industrial fermentors range from 20 m to several hundred m, in a few cases exceeding 1000 m. A conventional fermentor (14) is shown in Eigure 4. In most cases, oxygen is requited by the microorganism, and air is suppHed through a bottom sparger at a rate of 0.5 to 2 tank volumes per minute. The Hquid is often agitated to improve gas transfer and mixing. Heat caused by microbial metaboHsm and agitation is removed through a cooling jacket or coH. Baffles are placed near the waH to increase mixing efficiency and prevent vortex formation. Eor microorganisms that are very sensitive to shear stress, air lift fermentors (15) can be used. In the air lift fermentor, the rising air bubbles provide the mixing and ckculation of the culture broth.  [c.289]

Vertical pumps experience a vortex formation due to loss of submergence required by the pump. Observe the suction surface while the pump is in operation, if possible.  [c.916]

Draft tubes are employed to improve the mixing of large quantities of liquids by directing the motion of the liquid. Figure 1 shows such an arrangement, favorable for large ratios of liquid depth to mixer diameter. In such applications a high pumping capacity of the mixer is utilized, especially where mixtures of low viscosity are concerned. The draft tube directs the flow to the regions of the vessel that otherwise would not be agitated by the liquid stream. In the absence of draft tubes and at high rotational velocities of the propeller, baffles generally are located at various points in the vessel. Baffles minimize vortex formation and divide it into a number of local eddies, increasing the total turbulence of the tank. Depending on the application, multiple impellers may be mounted on a single revolving shaft and more than one shaft may be employed in a given tank. In some applications it is desirable to have two adjacent impellers rotating in opposite directions, forming a beater. Sometimes the impellers actually touch the walls of the tank, giving a positive scraping action, which is desirable when thick layers of material tend to stick to the wall.  [c.437]

The predominantly radial flow from the impeller impinges onto the vessel walls, where it splits into two streams. These streams cause mixing by their energy. When turbine mixers are operated at sufficiently high rotational speeds, both radial and tangential flows become pronounced, along with vortex formation. This flow situation warrants the installation of baffles to ensure a more uniform flow distribution throughout the mixing vessel.  [c.438]

Leaf-shaped (broad blade) paddle mixers provide a predominant tangential flow of liquid, but there is also turbulence at the upper and lower edges of the blade. Leaf-type blades are employed for mixing low-viscosity liquids, intensifying heat transfer processes, promoting chemical reactions in a reactor vessel and for dissolving materials. For dissolving applications, leaf blades usually are perforated. During the mixer s rotation, jets are formed at the exits from the holes which promotes the dissolution of materials. The rotational velocity of paddle mixers is in the range of 15 to 45 rpm. Under these conditions, the pumping action is small and there generally is no danger of vortex formation. As such, paddle mixers are most often used in vessels without baffles. However, for broad-blade paddles, which operate at speeds up to 120 rpm, baffles are incorporated into the design to minimize vortex formation.  [c.440]

Based on the pitch of the impeller with regard to the direction of rotation, there are two possible axial flow patterns that in which the impeller pumps the liquid from the bottom to the surface and that in which the impeller pumps liquid from the surface to the bottom. A combination of the three principal types of flow normally is encountered in mixing tanks. The tangential flow following a circular path around the shaft forms a vortex at the surface of the liquid. The vortex formation results from the influence of gravity forces, quantitatively determined by means of the Froude number, which increases at higher speeds, promoting vortex formation. Figure 10 presents a three-dimensional flow pattern, affording a clear image of the liquid flow in the tank obtained by projecting the path of a liquid particle in two planes. Part (A) shows the path that the particle takes at a given impeller speed.  [c.447]

Vortex formation is a condition that arises from centrifugal acceleration acting on gravitational acceleration. The circular motion of the entire contents of the tank predominates over the flow of the liquid from the impeller. Flow orientation thus is important not only in cases of noticeable vortex formation, but  [c.448]

Vortex formation leads to a considerable drop in mixing efficiency and should be suppressed as much as possible in practical applications to increase the homogenizing effects of mixers. The preferable method of vortex suppression is to install vertical baffles at the walls of the mixing tank. These impede rotational flow without interfering with the radial or longitudinal flow. Figure 11 illustrates such a system.  [c.449]

The distribution of velocity components (radial, tangential and axial) under conditions of mixing with baffles in comparison with the conditions of vortex formation is presented in Figure 12. The dashed lines in Figure 12 indicate non-baffled conditions. Comparison of the non-baffled and fully baffled velocity curves (solid line) leads to the following set of conclusions on vortex suppression when dealing with perfectly miscible liquids  [c.449]

The circulation increases, and the difference between the circulation rate under fully baffled conditions and at the vortex formation rises to two to four times the original value. This means that the power input increases considerably in the range of two to ten times the input without baffles.  [c.449]

For turbine mixers that the width of a baffle should not exceed more than one-twelfth of the tank diameter and, for propeller mixers, no more than one-eighteenth the tank diameter. With side-entering, inclined or off-center propellers, as shown in Figure 13, baffles are not required. Instead, shrouded impellers and diffuser rings may be used to suppress vortex formation. These devices contribute to flow resistance and reduce circulation by creating intense shear and abnormal turbulence  [c.450]

Because the liquid layers move at different velocities relative to each other, planes of slip exist between them and, from Newton s law, the forces of viscous friction arise. Consequently, the resistance force depends on the viscosity of the medium as is determined by the viscosity coefficient, fi. At higher particle velocities (or higher medium velocities relative to the particle) the flow around the object is broken, forming swirling fluid patches (Figure 14(B)). The formation of vortices is influenced by the relative flow velocity, the shape of the particle and the smoothness of the object s surface. The higher the velocity, the more complicated the particle shape and/or the greater the roughness, the more intense the vortex formation. Eventually, this leads to the generation of eddies along the downstream surface of the particle (Figure 14(C)).  [c.291]

During the mixing of fluids, it is essential to avoid solid body rotation and a large eentral surfaee vortex. When solid body rotation oeeurs, adequate mixing is not aehieved beeause the fluid rotates as if it were a single mass as shown in Figure 7-9a. Centrifugal foree of the fluid eauses a eentral surfaee vortex to be thrown outward by the impeller. Entrainment of air results if the vortex reaehes an impeller, resulting in redueed mixing of the fluids. This situation ean be averted by installing baffles on the vessel walls, whieh impede rotational flow without interfering with radial or longitudinal flow. Effeetive baffling is attained by installing vertieal strips perpendieular to the wall of the tank. With the exeeption of large tanks, four baffles are adequate to prevent swirling and vortex formation. For propellers, the width of the baffle should be less one-eighteenth the diameter of the tank for  [c.563]

Redueing vortex formation may also be aehieved by plaeing an impeller in an off-eenter position. This ereates an unbalaneed flow pattern, redueing or eliminating the swirl and thereby inereasing or maximizing the power eonsumption. The exaet position is eritieal, sinee too far or too little off-eenter in one direetion or the other will eause greater swirling, erratie vortexing, and dangerously high shaft stresses. Changes in viseosity and tank size also affeet the flow pattern in sueh vessels. Off-eenter mounting of radial or axial flow impellers is readily employed as a substitute for baffled tank installations. It is eommon praetiee with propellers, but less with turbine agitators. Off-eenter mounting ean also be useful for a turbine operated in the medium viseosity range and with non-Newtonian fluids where baffles eause stagnation with little swirl of the fluid. Off-eenter mountings have been quite effeetive in the suspension of paper pulp. Figure 7-10 illustrates an angular off-eenter position for propellers, whieh is effeetive without using baffles.  [c.564]

A vertical cylindrical, and mechanical agitated pressure vessel, equipped with baffles to prevent vortex formation is the most widely used fermenter configuration. The baffles are typically one-tenth of the fermenter diameter in widtli, and are welded to supports tliat extend from the sidewall. A small space between the sidewall and the baffle enables cleaning. Internal heat transfer tube bundles can also be used as baffles. The vessels must withstand a 45 psig internal pressure and full vacuum of -14.7 psig, and comply with the ASME code.  [c.857]

Solid partieles in liquids generally tend to settle to the bottom of a vessel under gravity due to their exeess density. To maintain a suspension, some form of agitation is normally provided together with wall baffles to prevent vortex formation in the swirling flow (Figure 2.14).  [c.43]

Combustion was modeled as a heat addition within a zone which is propagated at burning velocity relative to expansion flow. The higher rate of side relief, including vortex formation, is a direct consequence of the incorporation of gravity, which makes it possible to simulate the buoyancy of low-density combustion products. Buoyancy generates large, upward velocities at the expense of expansion flow in front of the flame. As a consequence, the flame propagates at a speed which is only about twice its burning velocity.  [c.109]

The vertical tubes serve as baffles to a certain extent, but not enough to prevent some vortex formation. The helical coll installations may have sidewall baffles (usually four Ao or Xj dla.), or baffles assembled with the coil itself (See Figures 5-23H, 5-231, 5-38 and 5-39.)  [c.329]

Poor intake layout can give rise to pump problems, unstable running, losing prime, and cavitation. If the pump is drawing water from a sump, the position of the intake and the shape of the sump must be chosen to avoid vortex formation and resultant air ingestion and pump instability. Figure 32.48 shows the proportions for PD and centrifugal pump suctions and Figure 32.49 gives details of baffles that can be fitted in tanks to reduce vortexing. If large flow rates and numbers of pumps are involved it is advisable to commission model tests to ensure that pump behavior is not affected for all flow rates and pump combinations.  [c.503]

Although furnace sizes (dimensions for a given MW production) do not vary too widely between principal manufacturers, the type of firing employed by each is generally quite distinctive. This indicates that the furnace size is not strongly controlled by the type of firing system, particularly for pulverized-coal firing (10) (70% through 74-p.m (200 mesh) screen for a mean size of ca 40—50 p.m). The furnace needs to be sufficiendy large to permit the oxygen enough time to penetrate (diffuse through) the blanketing CO2 layer evolving from the burning coal particle. The residual ash particles ate, of course, considerably smaller than the parent coal particle, on the order of a mean size of ca 10 p.m before post-combustion agglomeration. Although flame temperatures should be high for combustion efficiency in order to minimize CO formation and combustible carbon loss, it is further requited that the combustion products (gases) are sufficiently cooled to enter the convection banks below the temperature at which slagging occurs. These contradictory conditions (aside from pollution control requirements) influence the furnace size and have led to solutions such as tangential firing. Another popular and widely appHed solution was the introduction of separate combustion chambers auxiUary to the main furnace. This is generally referred to as cyclone firing or the cyclone furnace (9). This furnace (Fig. 6) is a horizontal, refractory-lined, water-cooled cylinder firing cmshed coal, 95% through a 4.76-mm (4-mesh) screen. Fuel and air are mixed by a swirling (centrifugal) motion which promotes turbulence. The fuel is burned at high heat-release rates in the range of 16.7—30.0 UJ/(m-h)((4.5-8.0) x 10 Btu/(ft -h)) at combustion temperatures in excess of 1649°C. At these temperatures the coal ash content forms a hquid slag film on the water-cooled refractory-lined wall because of the centrifugal force imparted to the coal particles. The gaseous products of combustion ate discharged into the main gas-cooling boiler furnace and the film of molten slag on the walls continually drains away from the burner end and discharges through the tap opening into the main boiler furnace from which it drains into a tank for further disposition. In tangential firing systems, a cyclone (ie, vortex flame) is created in the main furnace, ie, the fuel and air admission system is corner installed. For opposed firing, the fuel-air system is generally designed to impart an initial swid to the reactants before furnace entry.  [c.144]

Mixer power under gassed conditions, varies as T/is increased as shown in Figure 21. The initial small drop in power consumption is associated with a steadily increasing but limited reduction in the pumping capacity of the turbine as the gas bubbles in the vortex cavities increase in size. If gas throughput is further increased, at an T/ = 0.025, there is a clear point of inflection in the curve which is associated with formation of alternating large and clinging cavities. FI alone is not sufficient to correlate P because the curves as shown in Figure 21 are different at different mixer speeds (12).  [c.431]

For flow past a cyhnder, the vortex street forms at Reynolds numbers above about 40. The vortices initially form in the wake, the point of formation moving closer to the cylinder as Re is increased. At a Reynolds number of 60 to 100, the vortices are formed from eddies attached to the cylinder surface. The vortices move at a velocity slightly less than V. The frequency of vortex shedding/is given in terms of the Strouhal number, which is approximately constant over a wide range of Reynolds numbers.  [c.667]

It is desirable to be able to analyze the controlling features of a stabilizing system so that a good combustion efficiency with respect to pressure loss is attained. Since combustor design involves the formation of turbulent zones with complicated fluid flow and chemical reaction effects, combustor designers must resort to empiricism. A simple bluff body, such as a baffle placed in the flow stream, is the simplest case of flame stabilization. Though the basic flow pattern in each combustor primary zone is similar (fuel and air mixed, ignited by recirculating flame, and burned in a highly turbulent region), there are various ways to create flame stability in the primary zone. However, they are more complicated and difficult to analyze than the simple baffle. Figures 10-3 and 10-4 show two such designs. In one, a strong vortex  [c.376]

Vortex shedding anemometer A device for measuring air velocity by placing an obstruction in a gas flow and measuring the frequency of vortex is formation downstream of the obstruction.  [c.1487]

Related to vortexes formation, depth of vortex related to geometric similarity and equal Froude number.  [c.318]

The frequency of vortex formation is a linear function of the fluid velocity and the width of the obstmction at the point where shedding occurs. Vortex-shedding flow meters use various forms of well-defined symmetrical obstmctions to optimize vortex formation and detect the vortices using sensors which respond to local velocity or pressure changes. Since these meters originally became available in the early 1970s, improved sensors have broadened the appHcation range of the vortex-shedding meter. Vortex-sensing techniques include differential-pressure-sensing diaphragms (having capacitive or inductive pick-off), strain gauges, piezoelectric (see Piezoelectrics) crystals, and thermistors.  [c.64]

Shedding takes place alternately at either side of the object, and the rate of vortex formation and shedding is direc tly proportional to the volumetric flow rate. The vortices are counted and used to develop a signal linearly proportional to the flow rate. The digital signals can easily be totaled over an interval of time to yield the flow rate. Accuracy can be maintained regardless of density, viscosity, temperature, or pressure when the Reynolds number is greater than 10,000. There is usually a low flow cutoff point below which the meter output is clamped at zero. This flowmeter is recommended for use with relatively clean, low viscosity hquids, gases, and vapors, and rangeability of 10 1 to 20 1 is typical. A sufiicieut length of straight-run pipe is necessary to prevent distortion in the fluid velocity profile.  [c.763]

Deflagrative combustion of an extended, flat vapor cloud is very ineffective in producing damaging blast waves because combustion products have a high rate of side relief accompanied by vortex formation.  [c.109]

A much more pronounced vortex formation in expanding combustion products was found by Rosenblatt and Hassig (1986), who employed the DICE code to simulate deflagrative combustion of a large, cylindrical, natural gas-air cloud. DICE is a Eulerian code which solves the dynamic equations of motion using an implicit difference scheme. Its principles are analogous to the ICE code described by Harlow and Amsden (1971).  [c.109]

Figure 5-23A. Ruid flow pattern for propeller mounted center with no baffles. Note vortex formation. By permission, LIghtnin (formerly Mixing Equipment Co.), a unit of General Signal. Figure 5-23A. Ruid flow pattern for propeller mounted center with no baffles. Note vortex formation. By permission, LIghtnin (formerly Mixing Equipment Co.), a unit of General Signal.
Vertica.1 Axia.1 Deposition. The vertical axial deposition (VAD) process (18) was developed by a consortium of Japanese cable manufacturers and Nippon Telephone and Telegraph (NTT). This process also forms a cylindrical soot form. However, deposition is achieved end-on without use of a mandrel and subsequent formation of a central hole. Both the core and cladding are deposited simultaneously using more than one torch (Fig. 12).  [c.256]

N20 is also converted to HNO on PSCs. Since the nitric acid formed remains with the cloud particle, PSC chemistry produces an atmosphere rich in reactive chlorine species but depleted in NO2. These reactions are the key to the formation of the Antarctic hole, and continue until the break-up of the polar vortex and warming of the stratosphere in the spring. PSC particles can grow sufftciendy large to sediment out of the stratosphere. This denitrification process removes HNO as a source of NO2 (83). The volatilized gaseous chlorine and HOCl are photolyzed to chlorine atoms when sunlight returns in the spring. Chlorine atoms re-enter the ozone catalytic destmction cycle (6) and the foUowing (84)  [c.496]

Rhodacarborane catalysts have been immobilized by attachment to polystyrene beads with appreciable retention of catalytic activity (227). A 13-vertex /oj iJ-hydridorhodacarborane has also been synthesized and demonstrated to possess catalytic activity similar to that of the icosahedral species (228). Ak-oxidation of closo- >(2- P((Z [) 2 - i- > l[l-Bih(Z, results in a brilliant purple dimer. This compound contains two formal Rh " centers linked by a sigma bond and a pak of Rh—H—B bridge bonds. A number of similar dimer complexes have been characterized and the mechanism of dimer formation in these rhodacarborane clusters have been studied in detail (229).  [c.249]

Stirred Tank Agitation Turbine impeller agitators, of a variety of shapes, are used for stirred tanks, predominantly in turbulent flow. Figure 6-39 shows typical stirred tank configurations and time-averaged flow patterns for axial flow and radim flow impellers. In order to prevent formation of a vortex, four vertical baffles are normally installed. These cause top-to-bottom mixing and prevent mixing-ineffective swirhng motion.  [c.660]

In tangential filing (Fig. 27-I6Z ), the burners are arranged in vertical banks at each corner of a square (or nearly square) furnace and directed toward an imaginaiy circle in the center of the furnace. This results in the formation of a large vortex with its axis on the vertical centerline. The burners consist of an arrangement of slots one above the other, admitting, through alternate slots, primary air-fuel mixture and secondary air. It is possible to tilt the burners upward or downward, the maximum inclination to the horizontal being 30°, enabling the operator to selectively utilize in-furnace heat-absorbing surfaces, especially the superheater.  [c.2383]

Toxicity. At low concentrations, inhalation of propylene causes mild intoxication, a tingling sensation, and an inability to concentrate. At higher concentrations, unconsciousness, vomiting, severe vertigo, reduced blood pressure, and disordered heart rhythms may occur. Skin or eye contact with propylene causes freezing burns. Reaction of propylene (see environmental feue) in the atmosphere contributes to the formation of ozone in the lower atmosphere. Ozone can affect the respiratory system, especially in sensitive individuals, such as asthma or allergy sufferers. Ecologically, similar to ethylene, propylene has a stimulating effect on plant growth at low concentrations, but inhibits plant growth at high levels. Carcinogenicity. There is currently no evidence to suggest that this chemical is carcinogenic. Environmental Fate. Propylene is degraded principally by hydroxyl ions in the atmosphere. Propylene released to soil and water is removed primarily through volatilization. Hydrolysis, bioconcentration, and soil adsorption are not expected to be significant fate processes of propylene in soil or aquatic ecosystems. Propylene is readily biodegraded by microorganisms in surface water.  [c.110]

Vaporization Not pertinent Heat of Combustion Not pertinent Heat of Decomposition Not pertinent. H th Hazards formation - Recommended Personal Protective Equipment Approved dust mask protective gloves goggles or face shield Symptoms Following Exposure Highly corrosive to skin and mucous membranes. If ingested, causes violent gastroenteritis, peripheral vascular collapse, vertigo, muscle cramps, coma, and (later) toxic nephritis with glycosuria. Allergic reaction may also occur General Treatment for Exposure INGESTION have victim drink water or milk do NOT induce vomiting. SKIN treat like acid burns external lesions may be scrubbed with 2% solution of sodium thiosulfate Toxicity by Inhalation (Threshold Limit Value) Not pertinent Short-Term Exposure Limits Not pertinent Toxicity by Ingestion Grade 3 50 to 500 mg/kg (human) Late Toxicity Some suggestion of lung cancer Vapor (Gas) Irritant Characteristics Dust or mists may severe uritation of eyes and throat and can cause eye and lung injury. They cannot be tolerated even at low concentrations Liquid or Solid Irritant Characteristics Severe skin irritant causes second- and third-degree bums on short contact and is very injurious to the eyes Odor Threshold Not pertinent.  [c.326]


See pages that mention the term Vortex formation : [c.451]    [c.393]    [c.380]    [c.250]    [c.122]    [c.568]   
Modeling of chemical kinetics and reactor design (2001) -- [ c.563 ]