Erosion of the target surface due to vertical impingement of a gas-solid suspension jet

Fusion Engineering and Design 19 (1992) 299-306 North-Holland

299

Erosion of the target surface due to vertical impingement of a gas-solid suspension jet A k i h i k o Shimizu a a n d Shu H a s e g a w a b

a Department of Energy Conversion, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga.koen, Kasuga 816, Japan b Kururne College of Technology, 1232 Komorino-machi, Kurume 830, Japan Accepted 26 June 1992 Handling Editor: K. Miya

The possibility of a fusion reactor blanket concept in which gas-solid suspension flows are used as coolant, depends crucially on the degree of erosion of the walls of the coolant channels. This paper presents an experimental study on the erosion of a stainless steel target surface on which a gas-solid suspension flow impinges vertically. Vertical impingement of the suspension flow is considered to be a promising flow configuration for first-wall cooling. Three kinds of material (glass beads, alumina and graphite) were used as the suspended particles and the effects of each of the flow parameters are examined separately. Based on the experimental results, the erosion depth per year is estimated for several conditions and discussions are presented on the selection of the advisable flow conditions that lead to minimize the erosion.

1. Introduction There exists an idea of using a gas-solid suspension medium as coolant of the fusion reactor blanket [1,2]. Although gases have some advantages over liquid metals, particularly in that they do not suffer from MHD flow resistance, considerable pressurization is necessary to ensure the required cooling ability because of their inherently poor heat transfer characteristics. This pressurization as well as the expected high temperature of the blanket impose severe restrictions on the selection of the structural materials. Therefore, heat transfer improvement is significant in order to make the gas-cooled blanket concepts more realistic. In fact, the main motive of adopting a suspension medium is to improve the heat transfer of single-phase gas and, at the same time, to preserve the inherent advantages of a gas-cooled blanket. The second motive is to let the suspended particles serve as a tritium breeder as well by selecting an appropriate lithium compound, which will substantially simplify the blanket structure.

Correspondence to: Dr. A. Shimizu, Department of Energy Conversion, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen, Kasuga 816, Japan.

Meanwhile, some technical problems must be examined and solved prior to detailed design work. Among others, the erosion of the coolant channels and heattransfer surfaces due to the continuous impact of the suspended particles, will be crucial, and practically determines the feasibility of such systems. This paper presents experimental results on the erosion of flat plates which are exposed to vertical impingement of a gas-solid suspension jet. The vertical impinging jet of suspension flow is considered by the authors as a possible cooling method of the first wall [3]. Three kinds of solid particles were selected as test materials, and the erosion rate of the target stainless steel was measured systematically. Such parameters as the gaseous phase velocity, the solid-load ratio and the surface temperature of the target plate are examined. Based on the experimental results, the erosion depth per year is estimated for several operating conditions.

2. Experimental facilities and procedures 2.1. Gas-solid flow system A schematic diagram of the experimental apparatus is shown in fig. 1. Air is introduced into the flow

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A. Shimizu, S. Hasegawa / Erosion due to a gas-solid suspension jet

(1) Blower (2) Flow Regulator (3) Rotameter air (4) Feed Mixing Chamber (5) Sight Glass (6) Thermocouple (7) S e cCyclone Test tion(8)

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2.2. Test section and target plates

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Figure 2 shows details of the test section. The gas-solid suspension jet emerges from the nozzle and impinges vertically from below on the target plate, which prevents particles from accumulating in the chamber. The target plates were of stainless steel (SUS304) and before the tests their surfaces were polished with emery paper. The diameter exposed to the jet impingement was 108 mm. The nozzle diameter D is 16.5 mm, and the nozzle-to-plate distance H was fixed at twice the nozzle diameter. Due to this relatively short nozzle-to-plate distance, substantial erosion occurred in a circular region around the stagnation point with almost the same diameter as that of the nozzle. In the following, the stagnation point is taken as the origin of the coordinate system, the distances Y

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air flow .... particle flow Fig. 1. Schematic diagram of the experimental apparatus.

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s ~ g a s channel by use of a blower. The air flow rate was measured by a rotameter, which was positioned just after the blower. The air humidity in the laboratory was always kept less than 50% in order to prevent agglomeration of particles. The particle feed system consists of a feed hopper, a ball valve, a slide-plate-type solid feed regulator and a vibrator, the latter as well as the pressure-equating path assuring smooth feeding of particles. Particles are continuously dropped from the feed hopper to a feed chamber through the feed regulator with which the solid flow rate was fixed at a certain value by selecting an appropriate slide plate, with several holes of a certain diameter. The feeding rate of each slide plate was calibrated in advance. After being fed with particles in the feed mixing chamber, the flow enters a straight acceleration part of 4 m length, through which a fully developed turbulent flow profile is formed at the nozzle exit. The temperature of the flow was measured at the entrance of the test section by use of a thermocouple. After leaving the test section, the flow enters a series of cyclones, where particles are separated from the flow and recovered into a receiving hopper.

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301

302

A. Shimizu, S. Hasegawa / Erosion due to a gas-solid suspension jet

Table 1 Properties of the particles Glass beads Graphite Average diameter (/zm) 52.7 Standard deviation (/zm) 7.46 Density (kg/m 3) 2520 Knoop hardness (g/cm 2) 373-550

22.9 5.31 2260 35

AI203 30.3 12.91 3970 1692-2470

and Z being measured from there, parallel with and normal to the surface, respectively. In order to monitor the surface temperature of the target plate, a thermocouple was positioned flush with the impingement surface at a location 15 mm from the stagnation point. In order to examine the effects of the target-plate temperature, the plate was heated from outside by use of a gas burner, and the temperature was tentatively fixed at 350°C by valve operation of the burner. Meanwhile, the impinging flows were at the room temperature throughout the experiment. 2.3. Particles

As the dispersed solid particles, three powders were tested: alumina (A1203), glass beads and graphite, which were chosen as examples of hard, medium and soft particles, respectively. In view of its softness and stable physical properties, graphite is a promising candidate for the suspended phase of the blanket coolant, if the role of tritium breeding is not left to the particles. The physical properties of the particles are summarized in table 1, while histograms of their size distributions and photomicrographs are shown in fig. 3. Glass beads and graphite particles are almost spherical, while alumina particles are of irregular shape, so that the so-called Martin diameter was adopted for the size of alumina.

a certain time interval of exposure was estimated by subtracting the resultant contour from the original, undamaged contour. In view of the difficulty in finding the real impingement velocities of the particles onto the surface, the average gas velocity U at the nozzle exit was, instead, used in processing the experimental data. It ranged from 23.4 to 84.4 m/s, which corresponds to a range of nozzle Reynolds numbers, Re = U D / v , from 25 000 to 92 500, while the range of the examined solid-load ratio F was from 0 to 0.917. In describing the erosion rate, it has been customary to make use of the weight of the material which has been lost from the target plate, divided by the integrated weight of the particles which have impinged on it [4-6]. However, the main motive of the present study is to examine the feasibility of a suspension-cooled blanket, and, in particular, that of first-wall cooling by use of a suspension jet. In the following, therefore, the erosion rate will be discussed mainly in terms of the maximum erosion depth of the target plate. The maximum erosion does not necessarily occur at the stagnation point. When the gas burner is used to raise the plate temperature, the resultant contours inevitably include a deformation due to the heating, which must be

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2. 4. Measurement o f erosion

The erosion rate was obtained by measuring the surface contour of the test plate along a certain line through the stagnation point. For this purpose, a noncontact-type depth gauge (Union Optical Co., LTD.), with 1/zm resolution in the contour measurement, was used. Since the test plates were polished using emery paper prior to each measurement, their initial surface contour was not necessarily flat but had a somewhat uneven waving shape. Therefore, the erosion rate after

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A. Shimizu, S. Hasegawa / Erosion due to a gas-solid suspension jet

distinguished from the deformation due to erosion, Therefore, the original non-eroded contour of the plate was postulated by fourth-order polynomial curves ob-

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tained from several points in the undamaged region. An example is illustrated in fig. 4, in which the measured profile and its modified shape are drawn.

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A. Shimizu, S. Hasegawa / Erosion due to a gas-solid suspension jet

304 3. Results and discussion

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3.1. Characteristics o f the erosion for each o f the particles

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operation, are drawn under the abscissa, which may be useful for intuitive understanding of the erosion speed. As is seen from the figure, erosion of 1 mm depth can occur only with about 10 hr impingement of alumina particles, while graphite particles cause no such severe erosion. In the following, the discussion will be restricted to the results of glass beads.

3.2. Effects of the flow l~elocity

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In order to clarify the effects of the jet velocity, the relation between the erosion depth, measured at the stagnation point, and the total weight of particles that have impinged on the plate, is shown in fig. 7 for several flow velocities, the solid-load ratio being fixed at 0.3. The target plate temperature was about 25°C. The erosion depth is seen to increase almost linearly with the particle's weight and the slope of this line increases rapidly with the velocity. In order to obtain a closed expression for the erosion rate, these slopes (the erosion depth per unit weight of particles) are replotted in fig. 8 versus the velocities. The figure includes the corresponding results of the measurement in which the plate temperature was raised. Two lines in fig. 8 indicate the least-square approximations for the heated and unheated cases. As is seen in the figure, the exponents of the velocity, which dominate the erosion rate, are 2.52 and 5.10 for room and raised temperature, respectively. It is concluded that even a slight

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increase in the flow velocity accelerates the erosion remarkably, and that the higher the target-plate temperature, the severer the erosion damage. 3.3. Effects o f solid-load ratio Figures 9 and 10 summarize the effects of the solid-load ratio on the erosion, the velocity being fixed at 47 m / s . It can be noticed that the higher the solid-load ratio, the smaller the erosion depth per unit weight of particles, which is observed for both temperature levels. Although there is no experimental evidence, it is postulated that when collisions in the impingement region between oncoming and rebounding particles become frequent due to the increase in the solid-load ratio, the average impact energy of the

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total number of particles is reduced, which may possibly lead to the reduction of the erosion per particle. A second point which deserves attention is the distinct profile observed in fig. 9 for the case F = 0.9 and T = 350°C. This may probably be caused by peculiar velocity and surface-temperature distributions. However, detailed analysis is left for future investigations.

4. Summary The total erosion depths per year corresponding to various operating conditions were estimated from extrapolations of the above experimental results. They are summarized in table 2. Although it is unlikely that a combination of stainless steel target and suspension flows of air and the three kinds of particles examined here would be adopted as it is in the real blanket system, some suggestions on the practical selection may be obtained from it. According to table 2, it is apparent that graphite is a much more realistic choice than the others, in view of erosion. As a candidate for the suspended phase, materials as SiC, Li20, ti4SiO4, t i A l O 2 and graphite have been proposed so far [1-3]. Although no exact data on the hardness of these materials are available at the present stage, it is postulated that SiC has almost the same hardness as alumina, while those of Li 2O, Li 4 SiO4 and LiA10 2 are close to that of the glass beads. Graphite is much softer than the others. Generally speaking, a too large flow velocity should be avoided in any case. If the required heat-transfer

306

A. Shimizu, S. Hasegawa / Erosion due to a gas-solid suspension jet

Table 2 Erosion depth per year estimated for several conditions

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Dispersed particles

Gas velocity (m/s)

Loading ratio

Erosion depth (mm/year)

Glass beads Glass beads Glass beads Glass beads Glass beads Glass beads Graphite A120 3 Glass beads Glass beads Glass beads Glass beads Glass beads Graphite A120 3

10 30 50 100 30 30 30 10 10 50 100 30 30 50 10

0.5 0.5 0.5 0.5 5 10 0.5 20 0.5 0.5 0.5 5 10 0.5 20

0.0055 ~ 4.5 102 7000 8.1 9.0 1.0 1.0 0.095'~ 23 / 250 17 ,~ 25 5.4 53

performance is not attained, it is recommended, from the view point of erosion, to make up the loss by increasing the solid-load ratio instead of increasing the velocity. It is concluded, at least from the results of the present study, that the idea of leaving the role of tritium breeding to the suspended phase is not likely to be realistic, unless some measures are taken to cope with the erosion or unless a certain tritium c o m p o u n d with much smaller hardness is found. Of such measures, coating of the inner walls of the flow channels with other materials of higher hardness, such as SiC, may be recommended. In that case, attrition of the suspended particles and the resultant continuous re-

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duction in the particle size will become another hurdle for the realization of suspension-cooled blanket systems.

References [1] D.K. Sze, ANS Trans. 22 (1975) 21. [2] Y. Gohar et al., Fusion Technol. 15 (1989) 876. [3] A. Shimizu et al., Advances in Enhanced Heat Transfer, Symposium Vol. of 18th ASME and AIChE National Heat Transfer Conf., 1979, p. 155. [4] C.E. Smeltzer, J. Basic Eng. (Sept. 1970) 639. [5] N. Gat and W. Tabakoff, Wear 50 (1978) 85. [6] J.H. Neilson and A. Gilchrist, Wear 11 (1968) 111.