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Research Papers: Flows in Complex Systems

Experimental Investigation of Vortex Flow in a Two-Chamber Solar Thermochemical Reactor

[+] Author and Article Information
Ajay K. Prasad

e-mail: prasad@udel.edu
Center for Fuel Cell Research,
Department of Mechanical Engineering,
University of Delaware, Newark, DE 19716

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the Journal of Fluids Engineering. Manuscript received April 8, 2013; final manuscript received July 3, 2013; published online August 8, 2013. Assoc. Editor: Mark R. Duignan.

J. Fluids Eng 135(11), 111103 (Aug 08, 2013) (12 pages) Paper No: FE-13-1226; doi: 10.1115/1.4024965 History: Received April 08, 2013; Revised July 03, 2013

Recent advances in the field of large-scale solar thermochemical processing have given rise to substantial research efforts and demonstration projects. Many applications of high-temperature solar-thermal technology employ an enclosed cavity environment, thus requiring a transparent window through which concentrated solar energy can enter. One configuration employed is a two-cavity reactor connected by a narrow aperture, where solar flux entering through the window is focused at the aperture plane before diverging into the lower chamber, where the chemical reaction occurs. For the Zn/ZnO thermochemical cycle where Zn is solar-thermally reduced from ZnO in a high-temperature cavity environment, effective removal of the product gas stream containing zinc vapor is of paramount importance to prevent fouling by condensation on the reactor window. Two argon-jet configurations, tangential and radial, located around the circumference of the upper chamber are used to control the gas flow within the reactor cavity. First, the tangential jets drive a vortex flow, and second, the radial wall jet travels across the window before converging at the reactor center line and turning downward to create a downward jet. The tangential jet-induced flow creates a rotating vortex, contributing to overall flow stability, and the radial jet-induced downward flow counters the updraft created by the vortex while actively cooling and sweeping clear the inner surface of the window. Flow visualization in a full-scale transparent model of the reactor using smoke and laser illumination is employed to characterize the effectiveness of aerodynamic window clearing and to characterize the processes by which a vortex flow develops and breaks down in a two-chamber solar reactor geometry. Based on a large dataset of flow visualization images, a metric is developed to define vortex stability over a wide range of flow conditions and identify an ideal operating range for which a vortex formation path is established that maintains stable flow patterns and removes product gases while minimizing the use of argon gas. The predominant influence of vortex instability and breakdown is identified and examined for the case of a beam-down, two-chamber solar reactor geometry.

Copyright © 2013 by ASME
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Figures

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Fig. 1

Cross section of GRAFSTRR, a cylindrical, bichambered, solar-thermochemical reactor. Relevant dimensions of the upper (H1) and lower (H2) chambers are shown. Concentrated solar energy enters through the upper chamber as a converging cone of light and then diverges into the lower chamber, where the thermochemical reaction takes place. H1 = 85 mm, H2 = 265 mm, D1 = 180 mm, Da = 40 mm, D2 = 20 mm, D3 = 660 mm, and β = 40 deg.

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Fig. 2

Schematic representing a single tile section of the reactor, of which there are 15 in total. The vortex flow is generated between the window plane and aperture plane. Product vapor evolving off the reaction surface in the lower chamber is swept into the vortex flow and removed through the outlet port.

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Fig. 3

A fouled quartz window after vortex breakdown during high-temperature experiments. Zn and ZnO can be seen deposited on the window surface, in addition to the swirling pattern by which they were deposited.

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Fig. 4

Section View A (shown in Fig. 6), depicting the vortex generation mechanism. The radial gas feed enters as a circular wall jet across the surface of the quartz window from its circumference. The tangential feed is directed by vanes oriented tangentially to the upper chamber.

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Fig. 5

Tangential jet vane configuration for a generic vortex ring swirler. Wi and We are the vane entrance and exit widths, respectively. θ denotes the angle of the entering jet to the tangent at the point of entry. Flow QT delivers gas equally to the tangential vanes.

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Fig. 6

Experimental setup for flow visualization within the transparent mock reactor. Lens positions 1 and 2 were used to image the upper and lower chambers, respectively. View A is shown in Fig. 4, giving a more detailed description of the flow generation mechanism.

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Fig. 7

Flow visualization reactor. Large hoses deliver smoke to the buffer cavity from the smoke plenum below. Smoke diffuses through holes in the reaction tiles and enters the lower chamber of the reactor at nearly zero velocity.

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Fig. 8

Schematic of flow patterns developed in the upper and lower chambers of the reactor. Sequential cross sections of velocity vectors are shown for increasing swirl on the left. First, low swirl initiates B-type breakdown of the vortex, where recirculation of the auxiliary radial flow is present (a). With increasing swirl and Reynolds numbers, B-type bubble is elongated (b). With sufficient elongation, the vortex can attach to the pressure outlet and form a single-cell laminar vortex flow (c). Here, radial flow makes a clean attachment to the quartz window before turning downward. Lastly, with further increased swirl and insufficient Reynolds number, S-type vortex breakdown and recirculation of the lower-chamber bulk fluid (red, underneath the dotted line) is initiated (d). On the right, these flow patterns are shown in the context of the rector geometry.

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Fig. 9

A stabilized vortex inside the lower chamber of the reactor, wherein the vortex core maintains full attachment with the outlet tube at the bottom. Boundaries of the laser plane and image are shown. Bulk fluid (smoke-laden) inside the reactor is completely prevented from entering the upper chamber, and auxiliary flows from the upper chamber impart stability and rotation to the lower chamber as they proceed towards the outlet.

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Fig. 10

Upper chamber of the reactor, showing conditions of stagnant and diffuse smoke (a), the beginning of smoke rotation (b), and the absence of smoke once the vortex flow has attached to the outlet (c). Image (a) also depicts the stagnation height Hs, defined as the distance between the aperture plane and the uppermost point of the smoke formation in the upper chamber.

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Fig. 11

Vortex attachment progression under low swirl conditions. Vortex attachment proceeds from frame (a)–(d) over a time period of approximately 10 seconds, at which point its base matches the outlet diameter exactly. Once the vortex is attached, its axis does precess about the reactor's center line. In addition, occasional shedding of small eddies was observed from the lower end of the vortex flow.

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Fig. 12

Flow schematic of recirculation in the lower chamber. Blue (circular loops above the dotted line) flow lines indicate recirculation zones of auxiliary gas, red (wide spiraling flow line originating at the stagnation point denoted with an “x”) represents circulating bulk fluid, and purple (central tight spiraling flow line) represents auxiliary fluid exiting the recirculation zones. The auxiliary gas recirculation zones are contained within a circulating shroud of auxiliary gas and can be visualized as clear pocket.

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Fig. 13

Flow schematic of an attached vortex. Blue (central) flow line indicates auxiliary gas, red (annular) flow line represents circulating bulk fluid, and the flow line in-between the two (purple) represents rotating fluid in transition between the vortex core and bulk-fluid rotation. The dotted line represents the vortex-core boundary, which is clearly visible in the images of Figs. 9 and 11.

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Fig. 14

A plume of swirling smoke drawn up from the lower chamber through the aperture plane due to lower pressure at the vortex core. The rotation of the plume around its axis is clearly visible during visualization. This type of vortex breakdown (S) is detrimental, as it can deposit significant amounts of product onto the reactor window and shut down the reaction.

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Fig. 15

Flow pattern of recirculation developed in the upper chamber of the reactor during S-type vortex breakdown. Blue flow lines (originating at the window plane) indicate auxiliary gas, red (underneath the shroud depicted as a dotted line) represents a plume of recirculating fluid penetrating into the upper chamber from the lower chamber, and purple (surrounding the shroud and connecting to the outlet) represents fluid exiting the recirculation. A shroud of auxiliary gas circulates and translates around the plume and passes into the lower chamber. The plume boundary is indicated by the dashed red line.

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Fig. 16

Plot of nondimensional stagnation height versus tangential flow rate for various radial flow rates. Stagnation height is highly sensitive to tangential flow rates on both the lower and higher ends of the tested range. With radial flows above 5 L/min (shown as dashed lines), extreme turbulence was initiated, and although the stagnation metric was extracted, the flow configuration was unsteady and intermittent.

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Fig. 17

Contour plot showing the effect of Reynolds and swirl numbers on nondimensional stagnation height HS. Plume formation is sufficiently suppressed for Re < 400 and the full range of Sa. Increasing Re beyond 1000 initiates turbulent vortex breakdown. For Re > 400 and Sa > 0.12, the plume easily penetrates into the upper chamber. Significant swirl-induced vortex breakdown was observed for Sa≈ 0.2.

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Fig. 18

Precession of the plume in the upper chamber of the reactor. Arrows indicate the precession orientation. The plume makes one complete clockwise precession about the reactor axis from (a)–(d) over a period of 0.67 seconds.

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Fig. 19

Plume-precession frequency versus Reynolds number. Frequency increases linearly with Re.

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Fig. 20

Plume-precession frequency versus radial flow rate (QR) for three values of tangential flow rate (QT). Frequency increases with both QR and QT at roughly equal rates (average slope = 8.6/L for fixed QT and 11.5/L for fixed QR).

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