Tornado Flow Paper

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Abraham Kogane-mail: [email protected]

Meir Kogan

Solar Research Facilities Unit,Weizmann Institute of Science,

P.O. Box 26, Rehovot 76100, Israel

The Tornado Flow Configuration—An Effective Method for Screeningof a Solar Reactor WindowThe working fluid in solar receivers, utilized for effecting chemical reactions, is usuflown through a sealed enclosure provided with a quartz window. When one of thetants or products of reaction is a powder, care must be taken to prevent contact oincandescent powder particles with the window, in order to obviate its destructiooverheating. Attempts made in the past to screen the window against particle depoby a ‘‘curtain’’ of an auxiliary gas stream showed that very substantial flow ratesauxiliary gas (30—80% of the main stream flow rate) were necessary for perfect winscreening. The heat absorbed by the auxiliary gas stream represented a major loenergy. In an effort to reduce the auxiliary stream flow rate to a minimum, a certainpattern akin to the natural tornado phenomenon has recently been developed inlaboratory. It enabled effective reactor window screening by an auxiliary gas flowless than 5% of the main gas flow rate. The tornado effect is discussed and demonsby a smoke flow visualization technique.@DOI: 10.1115/1.1487882#

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1 IntroductionIn many volumetric solar receivers used for effecting chemi

reactions, the working fluid must be contained within a seaenclosure provided with a window transparent to incident soradiation. This situation arises whenever the reactor is designeoperate at a pressure different from atmospheric pressure. Mover, even with solar reactors operating at atmospheric presthe reactor enclosure must be sealed by a transparent winwhenever it is necessary to prevent air from interfering withchemical process or to prevent hazardous chemicals from esing into the atmosphere.

In some reactions that make use of solar energy, the reactanthe products of reaction contain a solid phase component inform of a powder suspension. The irradiated powder particlesbe incandescent. If allowed to come into direct contact withtransparent quartz window, they could blend with it, forming hspots at its surface. To prevent the destruction of the windowlocal overheating, it is necessary to prevent the solid particfrom contacting the window surface.

The use of acurtain of an auxiliary gas flow in the vicinity ofthe inner surface of the reactor window has been suggested ipast as a means for screening the window against contact withmain fluid flow inside the reactor. Two typical designs basedthis notion will be mentioned here by way of illustration.

Litterst @1# performed experimental work with a circulating fluidized bed equipped with a transparent window for admissionconcentrated solar radiation. To protect the window from conwith hot powder particles, the window was removed from tmain stream in the fluidized bed by mounting it on a T-branchthe fluidized bed column, and compressed air was injecthrough radially positioned tubes near the inner surface ofwindow. It was estimated that input of screening air necessaryadequate protection of the window should be 30–50% of the mstream.

More recently, Steinfeld et al.@2# developed a solar reactor iwhich ZnO reacts with natural gas, producing Zn and synthgas. It consists of an insulated cylindrical cavity that containcircular, windowed, aperture to let in concentrated solar ene

Contributed by the Solar Energy Division of the THE AMERICAN SOCIETY OFMECHANICAL ENGINEERS for publication in the ASME JOURNAL OF SOLAR EN-ERGY ENGINEERING. Manuscript received by the ASME Solar Energy Division Otober 2000; final revision, October 2001. Associate Editor: R. Hogan.

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Particles of ZnO, conveyed in a flow of natural gas, are contiously injected into the reactor cavity via a tangential inlet plocated at the back of the cavity. Inside the reactor’s cavity,gas particle stream forms a vortex flow that progresses towthe front following a helical path. The chemical products, Zn vpor and syngas, continuously exit the cavity via a tangential ouport located at the front of the cavity, behind the aperture. In frof the 6-cm aperture, the cavity receiver was equipped witdiverging conical aluminum funnel, so that the window couldmounted 7 cm in front of the focal plane, where radiation intensis about 10 times smaller and dust deposition is unlikely to occThe window is actively cooled and kept clear of particles

-Fig. 1 Reactor model M1; flow visualization test

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means of an auxiliary flow of gas, which is injected tangentiaand radially at the window and aperture planes, respectively.

Using computational fluid dynamics to calculate particle trajtories for different geometries and flow boundary conditions,timization was accomplished for minimizing the auxiliary flo

Fig. 2 Reactor model M2b-CPC assembly

Fig. 3 Impeller-like disk

Journal of Solar Energy Engineering


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while keeping the window clear of particles. In the optimizeresult, the auxiliary gas flow rate amounted to 83% of the mgas flow rate.

2 Summary of Preliminary Work at the WeizmannInstitute of Science

A preliminary reactor model M1 was tested at the Solar Rsearch Facilities Unit of the Weizmann Institute of Science.consists of a cylindrical Pyrex vessel divided by an annular pation into a reaction zone and a narrow buffer zone near the tothe vessel~Fig. 1!. A main stream of gas flows into the reactiozone, while an auxiliary gas stream is directed into the buzone. The auxiliary stream discharges from the buffer zothrough the annular partition into the reaction zone, wheremixes with the main stream. The gas mixture leaves the reathrough a port at the bottom of the reactor.

A series of flow visualization tests was performed with reacM1. One of the two gas streams was made visible by charginwith a heavy smoke, while the other gas stream was left innatural transparent condition.

In all the flow visualization tests performed with reactor Mthe main gas stream entered into the cylindrical reaction ztangentially, generating in it a whirling gas flow. In some of ttests, the auxiliary stream was introduced into the buffer zonetangentially, in order to induce a whirling motion concurrent withe motion of the main stream and to facilitate their smooth meing. In other tests, the auxiliary stream entered the buffer zradially.

These tests demonstrated that it is possible to completely elnate the main gas from the buffer zone by using an auxiliarystream, flowing at a rate of about 30% of the main gas flow ra

Surprisingly, the introduction of the auxiliary stream radialinto the buffer zone gave somewhat better results than those

Fig. 4 Reactor model M2a

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Fig. 5 M2b flow pattern; smoke-charged radial auxiliary flowÀ2 LÕM

Fig. 6 Planar laser beam optical configuration

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tained in tests with a whirling auxiliary flow. This unexpecteresult became clear only during our recent work, as it willexplained below.

3 Design of Reactor Model M2Following the preliminary flow visualization tests performe

with reactor model M1, we proceeded with the design of reacmodel M2, which was intended to address simultaneously a nber of separate technical issues and to solve them in a coheway:

• The piping for admission of the main stream~methane! andof the auxiliary gas stream should be designed in a compway, in order to avoid obstacles in the path of propagationthe concentrated radiation that fans out from the CompoParabolic Concentrator~CPC!, through the quartz windowinto the reactor chamber.

• For the same reason, the quartz window must be installethe immediate vicinity of the CPC. It should be clamped asealed between the CPC structure and the reactor metaling.

• The quartz window should be cooled in order to remove hof absorption.

• The main gas stream should be introduced preferably intoreaction chamber in a direction away from the window,

Fig. 7 Flow pattern of Fig. 5 visualized by laser cross-sectionillumination; smoke-charged radial auxiliary flow À2 LÕM

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order to cooperate with the auxiliary stream in the taskpreventing direct contact between solid particles and the wdow.

• The radiation emerging out of the CPC diverges througvery wide angle. This dictates a thin, poor insulation ceilito the reactor. Introducing the main~methane! flow throughthe ceiling should help in recycling heat from the ceiling bato the reactor chamber.

These goals were met by replacing the discrete entry tuwhich were used in reactor M1 for introduction of the main aauxiliary streams into the reactor, by two narrow annular passa~Fig. 2!.

The annular passage defined by spacers placed between thflange and a second metal flange enables the introduction oauxiliary stream into the reactor cavity in the form of a planar thfilm of gas adjacent to the inner surface of the window. Tstream flows radially from the periphery to the axis of symme

A second annular passage, defined by spacers between theond metal flange and the top-end ceramic annular structure, sefor the delivery of the main gas stream into the reactor cavitythe shape of a hollow conic film, which flows in directions awfrom the window.

With this design, we achieve an unobstructed passage forcentrated solar radiation from the CPC—through the quawindow—into the reactor cavity. The annular partition usedreactor M1 for the definition of a buffer zone does not appea

Fig. 8 M2b flow pattern; smoke-charged radial auxiliary flowÀ2 LÕM; tangential main flow À10 LÕM



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the reactor M2 design. The two gas streams are delivered toreactor cavity in the form of continuous thin films.

Moreover, in the reactor M2 design, the quartz window andmetal flanges are cooled effectively by the two high-velocity gstreams.

To enable generation of a whirling main gas stream insidereactor cavity, an impeller-like disk~Fig. 3! was inserted into acircular groove cut in the top-end ceramic structure. The mainstream flowing through the annular passage into the reactor cais deflected by the slant channels of the impeller-like disk, thacquiring an angular momentum.

4 Flow Visualization Test Series with Reactor ModelM2: The Tornado Effect

Different versions of reactor M2 were built around a commcylindrical metal casing. The only practical way to scrutinize tflow inside the reactor cavity appeared to be by charging onethe two gas streams with smoke, by illuminating the cavity andobserving it through the quartz window.

Figure 4 is a cross section of reactor model M2a, with the Creplaced by a flange for ease of visual observation.

Flow patterns inside reactor M2a, obtained by introducingdially a smoke-charged auxiliary gas stream into the reactor caat a rate of 2 L/M are illustrated in Fig. 5. No main gas stream wadmitted to the reactor in the test shown in Fig. 5a. It is observedthat the auxiliary flow progresses adjacent to the reactor windfor some distance, but it detaches from the window surface bereaching the window center. During the test shown in Fig. 5b, amain gas stream of 10 L/M was also introduced radially intoreactor cavity. In this case, the annular area of the window, whis swept by the auxiliary stream, is smaller than the correspondarea in Fig. 5a, since the auxiliary stream is sucked away frothe window by the main gas stream.

While it is possible to get from these pictures some geneimpression about the flow characteristics, there is no way totermine from them what is the thickness distribution of the auxiary flow layer, nor can we ascertain from them whether the mgas stream achieved direct contact with the window.

The following optical arrangement was used in order to enavisual inspection of a cross section of the flow inside the reac~Fig. 6!. A laser beam directed towards the reactor windowdiffracted by passage through a transverse cylindrical glassThe laser radiation emerges from the glass cylinder as a plsheet of light that illuminates a cross section of flow insidereactor cavity.

Figure 7 is a visualization of the flow patterns of Fig. 5, otained by the method of laser cross-section illumination. Maflow details that could not be discerned in Fig. 5 are visibleFig. 7. We found out soon that this simple illumination techniqcan be a powerful tool in the investigation of complex floconfigurations.

The flow configuration visualized in Fig. 8 was also generaby introducing a main stream of 10 L/M and an auxiliary smokcharged stream of 2 L/M into the reactor cavity. The auxiliastream entered the cavity radially, as in the case of Fig. 5.main stream, however, was introduced into the cavity tangentiaThe gas exits the reactor cavity through a port at the end ofcavity opposite the window, centered on the reactor axis of symetry, as in the case depicted in Fig. 5. Under these boundconditions, the main gas stream approximates a free vortex fl

Neglecting compressibility effects, the tangential velocityu andthe pressurep in a free vortex flow are given by:

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Fig. 9 M2a flow pattern; smoke-charged radial auxiliary flow À1 LÕM; illumination by planar laser beam

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Fig. 10 Tornado at Manhattan, Kansas, USA, May 31, 1949
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respectively, wherer is the radial distance andp0 is the limitingpressure, corresponding tor 5`.

Thus, the whirling motion of the main gas stream produceradial pressure variation in the reactor cavity, the pressure bhighest on the periphery and lowest at the reactor axis of symtry and the pressure gradient increasing steeply towards the axsymmetry. The auxiliary stream does not participate in the whing motion of the main stream, since it is introduced into treactor cavity in an essentially radial direction and becauseflowing as a thin layer in direct contact with the non-rotatinwindow. Yet, the strong negative radial pressure gradient pduced by the whirling motion of the main stream is imposedthe boundary layer flow of the non-rotating auxiliary streamfact well known in fluid mechanics@3#. In the absence of a centrifugal force to balance the pressure gradient, the auxiliary streis accelerated vigorously towards the center of the window, floing as a thin boundary layer on the surface of the window.

It must be emphasized that by extracting the outflowing strefrom the reactor cavity through the central port, the axis ofwhirling motion inside the cavity is determined and it coincidwith the axis of symmetry of the reactor. This is an example ofstrong tendency towards two-dimensionality, which is observedthe flow of contained rotating fluids@4#.

The nature of the flow configuration generated by a mainstream entering the reactor cavity tangentially and an auxiliarystream flowing into it radially, in close proximity to the windowsurface, differs basically from the flow configuration obtainedintroducing both the main and auxiliary streams radially into treactor cavity. In the last case, the auxiliary stream can succeedisplace the main stream from the vicinity of the window onlysheerbrute force, when its flow rate is comparable in size to thmain gas flow rate. In the former case, by contrast, an increasflow rate of the main whirling stream enhances the flow stabi



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of the auxiliary stream in the form of a boundary layer attachedthe window surface. The window screening capacity of the auiary stream does not diminish when the main stream flow rgoes up. It continues to flow radially on the surface of the windall the way to its center. There it turns abruptly by 90 deg aproceeds as a tubular flow along the axis of symmetry towardsexit port.

The above-described behavior of the whirling main flow aradial boundary layer auxiliary flow configuration was demostrated in the test series illustrated in Fig. 9, during whichradial auxiliary stream was maintained constant, at a flow rate1 L/M, while the main whirling stream flow rate was increasgradually from 3 L/M to 20 L/M.

In the case of a main stream flow rate of 3 L/M~Fig. 9a!, thesmoke-charged auxiliary stream becomes detached from thedow surface almost immediately upon its entry into the reaccavity. Nevertheless, it seems to perform as a window screendisplacing the relatively weak main stream by the bulky volumoccupies. By increasing the main stream flow rate to 5 L/M,stabilizing boundary layer influence upon the auxiliary streamcomes fully effective~Fig. 9b!. This influence becomes increaingly pronounced, up to the maximum main stream flow ratetained in this test series—20 L/M~Fig. 9d!.

It is observed that even in the extreme case illustrated in9d, in which the rate of flow of the auxiliary stream equals bare5% of the main stream flow rate, the auxiliary stream provideseffective gas dynamic window screen by adhering to the windsurface over its whole extent as a fast sweeping thin boundlayer. A patent application has been filed with respect to thescribed process.

The interaction of a bulk whirling gas motion and a radboundary layer flow described above occurs in nature in anado. The free vortex-like whirling motion of a tornado generaat high altitude is propagated down to the earth surface.

Fig. 11 Reactor model M2c



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strong whirling motion is arrested by friction within a boundalayer adjacent to the ground. In the absence of a centrifugal foin the ground boundary layer, the air in it is accelerated towathe tornado core by the pressure gradient induced in the bounlayer from the main tornado flow above ground. Soil and deb~or water, in the case of a tornado over the ocean! are swept by theviolent wind in the boundary layer towards the tornado a~Fig. 10!.

The physical circumstances that govern the tornado andwhirling flow generated in our laboratory are quite similar. Yethese two phenomena differ in a number of respects:

• Compared to the laboratory experiment, the tornado isevent of a huge scale. North American tornadoes aver275—375 m in width. Their funnels may extend down froas high as 7600 m. The wind speed in the vortex has nebeen measured, but judging from the effects and from thretical considerations, it is of the order of 160—635 km/h@5#.

• Tornadoes are transient phenomena, sometimes lastingabout 20 min. By contrast, the whirling motion describabove can be maintained in the laboratory under steady~orquasi-steady! conditions indefinitely.

• Tornadoes occur in the atmosphere uncontained by rboundaries, except for the ground or the ocean surface.laboratory tornadois a contained gas flow phenomenon, ocurring under axially symmetric boundary conditions.

A rigorous mathematical treatment of the problem of a viscofluid rotating around an axis perpendicular to an impervious stionary boundary may be found in Ref.@4#. The important dimen-sionless parameter by which such flows are characterized isEkman number

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Fig. 12 Reactor model M2d

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It is a gross measure of how the typical viscous force compato the centrifugal force and it is, in essence, the inverse of Rnolds number for the rotating flow. A very small value ofE im-plies the existence of an Ekman boundary layer of thicknE1/2 L.

In our application, with typical valuesV572 rad/sec, L56 cm, n50.14 cm2/sec, the value ofE is 5.431025 and theexpected boundary layer thickness at the reactor window is 0cm.

During operation of a solar reactor under real conditions,temperatures around 1500°C, strong destabilizing gas dynaeffects can be expected in connection with buoyancy due to inmogeneous heating of gas inside the reactor cavity. Thereforeinteresting to explore to what extent do the dimensions ofreactor cavity influence stability of the contained tornado confiration.

All the experiments described above were performed withactor M2a. Its small cavity has a cone-shaped frustum offollowing dimensions: D1580 mm, D2510 mm, L5100 mm~Fig. 4!.

A well-developed tornado flow configuration was obtainedan auxiliary gas flow rate of 2 L/M and a main gas flow ra

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greater than 10 L/M. The tornado funnel extended from the eport to the center of the window, within61 mm.

Smoke flow visualization tests were also performed withenlarged cavity reactor model M2b~Fig. 2!. The maximum cavitycross-section diameter in this reactor model is 150 mm, anddistance from the exit port to the window is 160 mm. A fuldeveloped stable tornado flow was obtained for an auxiliaryflow rate of 2 L/M and a main gas flow rate greater than 14 L/

A reactor model with a very large cavity was then preparedremoving all the internal parts from the M2 model casing. Tresulting configuration~Fig. 11! has a cavity diameter of 250 mmand the distance from exit port to window is 380 mm. Forauxiliary gas flow rate of 2 L/M and a main gas flow rate exceeing 10 L/M, a tornado funnel with one end anchored at the eport and the second end reaching up to the window planeobtained even with this extreme configuration. However, the fnel end was not stationary. It was rather meandering onaround the window surface, while the auxiliary flow covered twindow completely.

The tornado funnel can be stabilized in a large size reacavity by the use of a tube segment that protrudes into the reacavity through the exit port, thus bringing the actual exit sect

Fig. 13 M2d flow pattern; smoke-charged radial auxiliary flow À2 LÕM



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closer to the window. Tests performed with reactor model M~Fig. 12!, in which the distanced from the window to the exitsection was 45 mm, yielded very stable flow patterns~Fig. 13!.However, for smaller values ofd, there appeared some signsdistortion of the otherwise smooth boundary layer flow atwindow surface.

Finally, reactor model M2e~Fig. 14! was built and tested. Inmodel M2e, the central axial flow exit port is replaced by a serof slant holes on the periphery of the reactor cavity, near itsopposite the window. Here, the main gas stream enters the recavity tangentially and the gas mixture leaves the reactortangentially, like in the reactor described in Ref.@2#. The swirlingflow inside the reactor cavity can be approximated in this casea solid body rotating flow, with:

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The pressure gradient decreases then towards the axis ofmetry, where it becomes zero.

The patterns obtained during a flow visualization test sewith reactor model M2e are illustrated in Fig. 15. An auxiliasmoke-charged gas stream of 2 L/M entered radially into theactor cavity. The tangential main gas flow rate was increasethese tests from 3 L/M to 15 L/M. In all cases, the auxiliastream detached from the window surface immediately upon einto the reactor cavity.

5 ConclusionsThe whirling flow system described above appears to be

ideal solution for the window-screening job. It is a very efficiemethod, requiring an auxiliary gas flow rate of less than 5% ofmain gas flow rate. Its effectiveness is not limited to small swindows. The auxiliary gas sweeps the window surface as aand very fast film that covers the surface completely. It also co

Fig. 14 Reactor model M2e



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effectively the window material, due to the high heat transferefficient of the rapid boundary layer flow. On the other hand, sinthe auxiliary stream does not detach from the window surfacedoes not mix appreciably with the main stream, it does not remmuch process heat from the reacting gas.

Many tests have been performed with five reactor modelsencompass a wide variety of boundary conditions. The three cditions that are required for the generation of a tornado wiscreen have been demonstrated by experiment:

Fig. 15 M2e flow pattern; smoke-charged radial auxiliary flowÀ2LÕM

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• The main gas stream must be introduced tangentially intoreactor cavity.

• The products of reaction should be extracted from the reacavity in the axial direction, preferably through a narrow eport along the axis of symmetry of the reactor.

• The auxiliary gas stream should be introduced essentiallydially into the reactor cavity and adjacent to the window.

AcknowledgmentThe authors are grateful to the Heineman Foundation for

search, Educational, Charitable and Scientific Purposes, Rocter, NY, USA, which generously supported this work. Thanksexpressed to the US National Oceanic and Atmospheric Admitration Library for permission to reproduce the tornado pictureFig. 10, obtained from the NOAA Photo Collection at the Webs

Nomenclature

CPC 5 Compound Parabolic Concentratorc1 , . . . ,c5 5 constants

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d 5 distance from window to exit port~Fig. 12!E 5 Ekman numberL 5 a characteristic lengthp 5 pressure

p0 5 stagnation pressurer 5 radial distancen 5 kinematic viscosityV 5 angular velocity

References@1# Litterst, T., 1992, ‘‘Investigation of Window Damage by Hot Particles in So

Heated Circulating Fluidized Beds,’’Proc. of 6th International Symposium onSolar Thermal Concentrating Technologies, CIEMAT, Mojacar, Spain,1, pp.359–369.

@2# Steinfeld, A., Brack, M., Meier, A., Weidenkaff, A., and Wuillemin, D., 1998‘‘A Solar Chemical Reactor for Co-production of Zinc and Synthesis GaEnergy~Oxford!, 23, pp. 803–814.

@3# Batchelor, G. C., 1967,Introduction to Fluid Flow Mechanics, CambridgeUniversity Press, Cambridge, p. 315.

@4# Greenspan, H. P., 1968,Theory of Rotating Fluids, Cambridge UniversityPress, Cambridge, p. 3.

@5# Encyclopœdia Britannica, 1968, 14th Edition, Vol. 22, pp. 88–89.



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Tornado Flow Paper