Meeting Washington 2003

7/30/2019 Meeting Washington 2003

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IMPROVED SOLAR REACTOR DESIGN

A. Kogan, M. Kogan, S. Barak

Presented at the 14th

Annual US Hydrogen Meeting

Washington, DC, 46 March 2003

____________________

Solar Research Facilities Unit, Weizmann Institute of Science, Rehovot, Israel.


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Abstract

The paper describes the application of the tornado flow configuration [1] to the

protection of a reactor window from solid particle deposition at room temperature. Asecondary effect of the tornado flow pattern was identified. Friction between the swift jet of

gas leaving the reactor enclosure in the axial direction and the surrounding relatively

quiescent gas engenders a weak stream of gas along torroidal streamlines. Solid particles

entrained by this secondary gas flow are carried towards the reactor window. An efficientgasdynamic method is discussed, by which this negative effect is counteracted and

eliminated.

1. Introduction

Solar receivers utilized for effecting chemical reactions are usually provided with an

aperture for admission of concentrated radiation, which is sealed by a transparent window.

When one of the reactants or products of reaction is a powder, it is essential to preventcontact of irradiated incandescent powder particles with the window, in order to obviate its

destruction by overheating. The usual method of screening the window by flooding its

surface with a curtain of a secondary gas stream requires very substantial secondary gas

flowrates and the heat absorbed by the gas represents a major loss of energy [2,3,4].

2. Screening of a Solar Reactor Window by the Tornado Flow Pattern

In an effort to reduce the secondary gas flowrate to a minimum, a certain flow pattern

akin to the natural tornado phenomenon has recently been developed in our laboratory,which enables effective reactor window screening by a secondary gas flowrate less than 5%of the main gas flowrate. Details of the tornado effect are discussed elsewhere [1].

Here, we shall limit ourselves to a brief exposition of the physical background and to an

illustration of this phenomenon.When a fluid flows along a solid stationary boundary, its motion is retarded in a thin

boundary layer by friction. The retarded fluid boundary layer may thicken progressively in

the direction of flow and ultimately it may detach from the solid boundary and mix with themain flow. Boundary layer detachment can be averted if care is taken to maintain a

uniformly decreasing pressure in the direction of flow. The accelerated main flow entrains

then the fluid in the boundary layer strongly enough to counteract the flow retardation

caused by friction with the stationary boundary.Turning to our particular application, the axisymmetric chamber of the solar reactor is

provided with a transparent window located at one end of the chamber, transversally to the

longitudinal axis. The main gas flow is introduced into the chamber in a manner whirling

around the axis, while the reaction products are withdrawn at the opposite end of thechamber through a narrow central tube oriented along the longitudinal axis. The gas flow

inside the chamber approximates then a free vortex flow, characterized by a drop of

pressure from the periphery of the chamber to its axis.

A secondary flow of protecting gas introduced at the periphery of the window isdirected towards the window central area. It is accelerated by the negative pressure gradient

generated by the free vortex flow. The secondary gas boundary layer flow at the window


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surface is stabilized thereby and it remains attached to the surface all the way to the center

of the window. There, the radially converging streamlines turn abruptly by 90 into the

axial direction, forming a typical tornado-like funnel along the reactor axis.

Synergy between the free vortex flow of the main gas and the boundary layer flow ofthe secondary gas is here exploited in order to protect effectively the reactor window. The

synergy is expressed by the fact that the secondary flow, which is desired to form a stable,

continuous and non-separated protective layer on the window surface is not disturbed by

the whirling main stream. It is rather stabilized by it. Consequently, the secondary flowdoes not need to be injected with high velocity or with a great flowrate in order to adhere to

the surface to be protected, because it uses the energy of the whirling main stream againstwhich protection is sought.

The tornado effect has been demonstrated in a series of simulation tests at room

temperature with the reactor model shown in Fig. 1 [1]. The main gas stream was flownfrom an annular plenum chamber through a narrow annular gap towards the upper part of

the reaction chamber. An impeller-like ring was implanted in the annular gap. The main gasstream acquired an angular momentum during its passage through slanted grooves in the

impeller ring and it entered the reactor cavity in a whirling motion. The secondary gasstream was flown radially from a second annular plenum chamber through a second narrow

annular gap towards the periphery of the inner surface of the window. Both streams

consisted of nitrogen gas. The secondary stream was made visible by charging it with

smoke, while the gas in the main stream was left in its natural transparent condition. In

order to enable visual inspection of a cross section of the flow inside the reaction chamber,a laser beam directed towards the reactor window was diffracted by passage through a

transverse cylindrical glass rod. The monochromatic laser beam emerged from the glass rod

as a planar sheet of light that illuminated a cross section of flow inside the reaction

chamber.The four-tornado configuration tests illustrated in Fig. 2 were performed with a

secondary smoke-charged gas maintained at a constant flowrate of 1 L/M. With a whirlingmain gas stream at a 3-L/M flowrate (Fig. 2a), the secondary flow separated from the

window surface immediately upon its entry into the reaction chamber. When the whirling

main stream was introduced into the reactor cavity at successively higher flowrates (Fig.2bd), the secondary stream became progressively stabilized as a thin boundary layer. For a

main gas flowrate of 20 L/M, the secondary gas moved at high speed in the thin boundary

layer near the window surface. It covered the entire window surface area and it left finally

the reaction chamber through a narrow axially oriented funnel.

3. Transfer of Radiation Energy to Reactant Gas

Radiation propagating into the solar reactor is not absorbed directly by the transparent

gas. It heats the reactor wall and part of the heat is transferred to the gas by conduction and

convection (surface heating).

Following a method proposed by Hunt [5] and Yuen et al. [6], a gas may be heated by

concentrated radiation throughout the volume of the reaction chamber by dispersing smallparticles in the gas, to form an opaque cloud. Radiation is absorbed by the particles in

suspension, which in turn exchange heat with the surrounding gas very effectively, in view

of the very large surface area per unit mass of particles (volumetric gas heating).


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Fig. 1. Cross section of reactor D1.

Fig. 2. Evolution of tornado flow configuration. Smoke-charged secondary flow enters the

reactor cavity at the window periphery in the radial direction at flowrate of 1 L/M.

Illumination by planar laser beam.


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4. Tornado Flow Configuration Testing with Powder Seeding

The fast axial flow of gas in the tornado funnel entrains by friction the relatively

quiescent gas around it. Fig. 3 is a qualitative picture of the streamlines in the perturbedquiescent region. These are helical streamlines wrapped around torroidal surfaces.

At close proximity to the entraining jet, solid particles suspended in the perturbed gas

are expected to move downward, concurrent with the entraining funnel jet. Upon

approaching the reactor cavity bottom, they diverge towards the periphery, turning insequence upwards towards the reactor window, then inwards along the window.

Some exploratory room temperature tests, in which activated carbon black (CB) powder

was seeded into the reactor cavity, were performed in order to gain experimental

information about the paths followed by solid particles entrained by gas in a tornado flowpattern. In these tests, the powder was mixed with the main gas stream before its entry into

the reactor.

Fig. 3. Schematic representation of torroidal flow perturbation induced by friction between

the gas moving at high speed in the tornado funnel and the surrounding gas.

Fig. 4 is a picture of the reactor window taken at the end of one such test. It shows two

concentric rings of powder sediment. The outside diameter of the diffuse outer ring equalsthe diameter of the circle along which the powder is admitted into the reactor cavity (6 cm).

The more concentrated inner powder ring is confined between two concentric circles with

diameters of about 2.5 and 0.5 cm, respectively.

The test was repeated with the inner surface of the window wetted by a thin layer oflubrication oil. At the end of the test, a narrow, high density powder ring was observed at


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the periphery of the window and a circular spot of CB of about 0.3 cm in diameter wasspread around the window center, with vestiges of streak lines connecting the outer ring to

the central spot (Fig. 5). By the time the picture shown in Fig. 5 was taken, about 30 min

after the end of the test, the central spot of CB-stained oil spread over the area of a circleabout 1.3 cm in diameter.

Fig. 4. Reactor window stained by powder deposition after a 10-min CB seeding tornado

flow test.

Fig. 5. CB deposit on reactor window at the end of a test similar to the one illustrated in

Fig. 4, when the window surface was wetted by lubrication oil, before start of test.


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Figs. 4 and 5 suggest a flow of CB particles entrained by an upwelling stream of gasalong the periphery of the reactor cavity. Upon reaching the top of the cavity, the gas

stream turned apparently abruptly from the longitudinal into the radial direction, shedding

CB particles onto the window surface. Some of the particles formed the outer black ring,while most of them were swept along the lubricated window surface towards the center ofthe window. This particle flow pattern is compatible with the tornado flow configuration, as

described above. The ascending gas current along the reactor cavity wall is part of a weak

perturbation flow generated by friction between the energetic gas stream in the tornado

funnel and the gas surrounding the funnel.

5. Improvement of the Tornado Flow Window Screening Capacity by the

Introduction of a Weak Tertiary Flow

The negative influence of the induced perturbation flow on the tornado effect could be

eliminated by the introduction of a weak flow of gas along the reactor cavity wall in a

direction countercurrent to the torroidal flow streamlines.Reactor D2 was designed to enable the introduction of a tertiary gas stream F 3 along the

bottom of the reactor cavity, flowing from the periphery of the bottom plane to the exit port

at the reactor axis of symmetry. The F3 stream is supposed to interact by friction with the

weak torroidal flow occurring around the tornado funnel and to diminish its capability toentrain solid particles towards the window. The F3 stream should also prevent

agglomeration of powder at the cavity bottom by entraining it towards the exit port.

As illustrated in Fig. 6, the main body of reactor D2 is composed of two parts: a quasi-cylindrical shroud (a), superimposed on a cylindrical base block (b). By placing spacers

between the two parts, an annular gap is formed between them. Fig. 7 depicts the assembled

components of reactor D2. Fig. 8 is a picture of reactor D2 test assembly.

Fig. 6. Components of reactor D2.


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Fig. 7. Cross section of reactor D2.

Fig. 8. Picture of reactor D2 assembly.


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6. Prevention of Powder Settling at Reactor Cavity Walls

Much of the seeding powder introduced into reactor D2 in tests without tertiary flow

settled at the horizontal bottom of the reactor cavity. As expected, the activation of thetertiary gas flow F3 alleviated this problem considerably. At the maximum value of F3

(F3=3 L/M), the cavity bottom plane was almost free of powder sediment. Yet, even in this

extreme case, an island of powder agglomerated in an asymmetric shape near the rim of the

exit port (Fig. 9).

Fig. 9. Reduction of powder sedimentation at reactor cavity bottom by activation of tertiary

flow: (a) F3=0, (b) F3= 3 L/M.

In an effort to solve this problem completely, a cylindrical hole of a 5-cm diameter by a

3-cm depth was drilled at the center of the cavity bottom and three interchangeable stainlesssteel inserts were prepared with radii of curvature of 0.5, 1.0 and 2.0 cm, as illustrated in

Fig. 10. By fitting one such insert into the cylindrical hole at the cavity bottom, a rounded

exit port was obtained. The entrainment of powder along the cavity bottom towards the exit

tube was thereby greatly enhanced. With each one of the inserts shown in Fig. 10, a tertiaryflow of 1.8 L/M was sufficient to eliminate completely powder sedimentation. With the

insert with r=2 cm, the cavity bottom remained clear of powder even at the end of tests with

F3=0.


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Fig. 10. Three interchangeable rounded inserts for reactor exit duct.

7. Conclusions

The activation of a tertiary flow, F3, helped only in some marginal cases to overcome

the tendency of the torroidal perturbation flow to entrain powder particles towards the

reactor window. The F3 flow was introduced into reactor D2 at a location too remote from

the window. Its effect of braking the countercurrent torroidal flow by friction wasapparently effective only near the reactor bottom plane.

In order to suppress completely the influence of the torroidal current upon the window

screening capacity of the tornado flow configuration, flow F3 will be introduced into thereactor cavity at a more appropriate location along the reactor wall, not so far removed

from the window as in the design of reactor D2.

The problem of powder settling at the bottom of the reactor cavity has been solved byrounding the rim of the reactor exit port with a generous radius of curvature (2 cm). A

tertiary gas flowing along the bottom of the reactor cavity from the periphery to the axis of

symmetry is also very effective in this regard.


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Acknowledgements

The authors are grateful to the Heineman Foundation for Research, Educational,

Charitable and Scientific Purposes, Rochester, NY, USA, and the Rose Family Foundation,Bedford, NY, USA, which generously supported this work.

References

1. A. Kogan, M. Kogan: The Tornado Flow Configuration An Effective Method for Screening of a Solar

Reactor Window, Journal of Solar Energy Engineering, 124:206214, 2002.

2. T. Litterst: Investigation of Window Damage by Hot Particles in Solar Heated Circulating Fluidized

Beds, Proceedings of the 6th International Symposium on Solar Thermal Concentrating Technologies,

Mojcar, Spain, vol. 1, pp. 359369, 1992.3. A. Steinfeld, M. Brock, A. Meier, A. Weidenkaff, D. Wuillemin: A Solar Chemical Reactor for Co-

production of Zinc and Synthesis Gas, EnergyInternational Journal, 23:803814, 1998.

4. J. Dahl, K. Buechler, R. Finley, T. Stanislaus, A. Weimer, A. Lewandowski, C. Bingham, A. Smeets, A.Schneider:Rapid Solar-Thermal Dissociation of Natural Gas in an Aerosol Flow Reactor, Proceedings

of the 11th SolarPACES International Symposium on Concentrated Solar Power and Chemical Energy

Technologies, Zurich, Switzerland, pp. 223230, 2002.

5. A.J. Hunt:A New Solar Thermal Receiver Utilizing a Small Particle Heat Exchanger, Proceedings of the

14th Intersociety Energy Conversion Engineering Conference, Boston, MA, USA, 1979.

6. W.W. Yuen, F.J. Miller, A.J. Hunt: Heat Transfer Characteristics of a Gas-Particle Mixture underDirect Radiant Heating, Report of the Lawrence Berkeley Lab, LBL-20289 preprint, 1985.

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