ML20138C397

Premixing of Steam Explosions:PM-ALPHA Verification Studies

ML20138C397
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Issue date:	11/29/1996
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12/04/96 CED 10 : 48 FM' 1- 030 ' 252 4730 ,o t. . RE 4. 3. 3v,m g Additional Comments on DOE /ID-10504.

Pre =Wg Of Steam Explosions: PM ALPHAVerification Studice e

' by T G Theofanous. W W Yuen and S Angelini Reviewer: B D Turland, AEA Technology plc .

Date of review: 29 November 1996 INTRODUCTION This document represents the ctilmination of a substantial piece of work to develop a mixing code for steam explosion studies and to validate it against the experimental data. The report makes good use of the (still rather limited)

< experimental data available for this purpose. 'Ihe report concentrates on the presentation of results rather than their evaluation. It would beneat from a i leading chapter on the philosophy of the verification / validation process, accompanied by a matrix indicating which of the code's models are tested, and to '

what extent, by the comparisorts reported. It would further bene 8t from a longer concluding chapter that draws together the results in the context of this matrix.

It is noticeable that efforts are made to compare isothermal particle-water predictions with accepted correlations. There ought to be scope to include similar material on two-phase flow in the absence of particles; this is probably more important in establishing the reliability of the code to predict voiding behaviour. ,

While there are many detailed comments below, these should not detract from the achievement of the authors. The comparisons performed indicate that the code has the ability to make reasonable predictions for reactor conditions.

However, the results should still be used cautiously, as the data currently do not

. exist to provide fullvalidation of the model.

SPECIFIC COMMENTS

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Chapter 2 Single carticle settling While tracking a representative particle in a La6rangian

. fashion gives the expected analytic result, melt mess is usually tracked through the volume fraction. 'Ihis can be much more diffusive.

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2. ama o m 10 04 98 CED 10:49 FAI 1 630 232 4760 Settlinc of particle clouds I have thed to check the consistency between the drag law for particles given by equations 3.14,3,21 and 3.22 of Appendix A with the drift flux formulation, but have been unsuccessful. There appear to be inconsistencies between equation 2.4, and Figures 2 and 3. Taking v., = 0.487 m/s, gives the hquid superficial velocity for a = 0.5 as 0.093 m/s. Figure 2 shows this as 0.12 m/s, while Figure 3 indicates 0.19 m/s. This suggests that it l

is not the superficial velocity that is being plotted in Figure 3 but the flow  !

velocity, which would be 0.186 m/s from equation 2.3. My evaluations of the l

drag coefficient given in Appendix A for this case give a relative velocity of 0.286 m/s, or a superficial velocity of 0.143 m/s. However, PM-ALPHA has produced, according to Figure 3 a value close to 0.2 m/s. My hand calculations indicate that the PM-ALPHA model is not as close to the ddft flux model as implied by Figure 3.

Setthna of narticle clouds The comparison with the drift flux modelis clearly important as it goes some way to establishing the reliability of the drag coefficient modelling in PM-ALPHA (although it should be noted that the particle volume fracuon is unljkely to exceed 20%, where the enhanced drag due to particle-particle effects is not that significant). It is less clear what one is expected to leam from the material presented on transient analysis regarding the validity of the code's models. It would have been usefulinstead/in addition to perform the same compadson with the ddft flux model for gas-water interactions where the fonn of the drag coefficient is rather different.

Section 2.2.2: MAGICO exoeriments It would have been useful to have a short synopsis of the conclusions drawn about the model from the analysis of the MAGICO tests. Besides the qualitative agreement (and general quantitative agreement) on the nature of the interaction. I think the most significant finding is the prediction and measurement oflarge void fractions (greater than 70%)

illustrated in Figure B23). It would be useful to provide a statement on the specific code models that these observations are believed to validate (eg water-steam drag film boiling, radiative heat transfer??).

This looks a very interest.ing analysis of these tests.

The QUEOS Exnedments:

The presentation of results in Figure 4 etc gives an excellent way of quahtatively comparing code results and experimental observations. Perhaps some comment should be made about the apparently coherent release oflarge gas / steam volumes, seen eg at 0.41 s in Figure 4; also on the water spout effect predicted at this time (this seems to provide the mass difference between Meyer's interpretation of the water fraction in the mixing region and the PM-ALPHA values). The acceptability, or otherwise of numerical diffusion, is a complicated matter, because of non-linear feedbacks through the drag laws; it is very easy to underpredict the peak particle volume fraction. Figure 5 does not give units for the liquid flux. Condensation in PM-ALPHA looks too effective at later times in Figure 6 compared with the experimental image.

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! Chapter 3 Comparison with CHYMES: It is only fair to note that this comparison was only p

• possible by turning off sub-cooling in PM-ALPHA. Much of the detail of the PM-ALPHA predictions depend on the modelling of sub-cooled boiling. The observation that PM-ALPHA often only produces any void somewhat behind the particle front, whereas other codes tend to produce some voiding wherever there are hot particles can have significant implications on the initial flow of water. For instance, we did not reproduce the so-called ETHICCA effect with CHYMES. In j j

addition, CHYMES drag laws were modified for the comparison. However, the

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main result - water depletion is predicted by both codes (at least for low pressure j

l systems close to saturation temperature) - is robust.

The MLYA Exneriments: I will try to clarify the question of time origins for the I

data. The experimental report, which I have, has unequivocal timings, with an i

origin starting at the ignition of the pyrofuse for the thermitic reaction. On this 1

timing the melt first contacted the water at 3140 ms, the peak (measured) l steaming rate was at 3810 ms and the peak pressure occurred at 4215 ms. The j authors have adopted a timescale (their agure 4) where the time of first melt contact is taken to be zero. This is the same timescale used in Figure 1 of j

2 Fletcher and Denham for the measured pressure in the gas space - so the comparison given for pressure in the top frame of Figure 6 (page 3-21) is correct.

1 However, the transient steaming rate figure (flgure 8 of Fletcher and Denham) does not use this time basis - this is because it was dertved from the CHYME calculations with the experimental data over-plotted). There is a significant

1. outflow of gas before the melt reaches the water surface as shown in this figure.

This may be due to (1) preheating and expansion of the gas in the test vessel: (ii) i evaporation of a water film on the test vessel wall (the favoured explanation for similar observations in FARO), and/or (iii) evaporation from the water surface.

' The experimental data on the middle and lower frames of Figure 6 should therefore be shifted to the left by about 0.32 s (error on this is only from my reading of the graph in Fletcher and Denham- it is no more than 0.02 s). The effect of this is to move the measured peak steaming rate ahead of the measured peak pressure. However, I now believe that the measured steaming rates become )

increasingly unreliable (as quoted) due to carry-over of a two-phase mixture:

• similar behaviour has been observed in PREMIX. Unfortunately, while the experimenters noted water carry-over post test, and observed a reduction of l

water height in the vessel post-test of 25 mm (the measured steam would  ;

l produce a reduction of only about 4 mm), there is no infortnation to determine how much of this occurred because of evaporation during the heating of the l i

water. The same comments apply to Figures 7 to 10.

While the PM-ALPHA calculations are as good as or better than any I have seen for MIXA-06, I am not convinced that the real behaviour in the test is being

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A.N L- RE - 5. swuu:a g,gog 12/04/96 WED 10: 50 FAI A 630 353 4730 captured. The most noticeable features are the radial expansion of the melt as it

' enters the water and the apparent lack cf any visual record of droplet break-up.

Both of these effects seem to be connected with sudden expansions of the melt region, due to enhanced steam generation, giving much more coupling between melt and steam than accounted for in CHYMES, and, by the look ofit, in PM-ALPHA. I conjecture that droplet fragmentation is occurring during these rapid events. The formation of smaller particulate then encourages another process of melt spreading. Smaller particles are carried upwards by the central steam flow, move outwards, and fall in the periphery, thus extending the melt envelope l

outwards.

' There is no visual evidence of the predicted extensive voiding atound the melt region - the leading droplets appear to be falling through water - the steam generating region is large because of the spread of the melt droplets.

I agree on the sensitivity of calculations to assumptions on break-up. Has the j l

predicted mean particle size been compared with the experimental value of about l

3 mm? 1 l

This section should contain discussion / conclusions on implications of the comparison for model validation.

The FARO Exteriments: Clearly the initial melt droplet size is very uncertain, as i

is the spread of the melt. L-14 appears to be the test in which the melt stream was best colhmated, but one cannot tell whether the stream contracted as it l

l poured through the gas space, or underwent a mild expansion (in L-11 the melt j

stream appeared to undergo a major expansion). If it is believed that the meltjet contracted (note typo: steam for stream 4 unes from end of page 3-25), then the i

radial meshing with Ar = 5 cm is too small. The choice of break-up parameters appears arbitrary - presumably these were selected to give reasonable agreement with the experimental data. More detailed modelung of the melt release vessel indicates that the melt exit velocity was close to 3 m/s for most of the pour; this will not be replicated by the model shown in Figure 2. I am surprised that a i

Weber number criterion did not limit the droplet size: with the CHYMES implementation of this criterion we almost always get mean particles close to those observed in experiments (typically 3 - 5 mm). The comment on the absence of significantly superheated steam in the experimental data seems to me to be special pleading - it might be right, or the steam flow might be much less concentrated on axis than predicted by PM-ALPHA. giving steam closer to saturation conditions, it is difBcult to relate the scales on the coloured contour plots in Figures 10 and 11 to the colour-scale, particularly because of interpolation effects. Is break-up still occurring after the particles have settled (unless they have solidified)?

Again, this section should be supplemented by an evaluation of the implications for the reality of PM-ALPHA predictions. I think a word of caution is necessary, ,

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- as although PM-ALPHA. with the assumptions used, perfonns well against experimental data, it predicts a highly two-dimensional configuration.

Alternatively, good comparisons against the data have also been produced with the one-dimensional code, TEXAS-IV, Untilwe see the nature of the interaction j zone (I expect it to be between these two computational extremes) then it is not j possible to say that one simulation is better than the other.

Chapter 4 I agree with the general comments on break up modelling. As implied in my

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comments above, backing out break up behaviour from the experimental data

! may compensate for other errors in the modelling. As I also noted, it is unclear, even with the visualisation, what break up processes were occurdng in MIXA. I l

j suspect the processes are much more dyriamic than are currently embodied in the models, and coupled strongly with events of enhanced steam generation

. (coolant trapping?). New FARO tests with visualisation should provide

' information on the coherency of the initial pour, besides evidence of any

! subsequent break up.

t Numerical aspects Our expedence is not as comforting as that presented by the authors. I think that numerical diffusion is probably not an important issue for large-scale mixing i

i calculations. However, it becomes important in comparisons with smaller-scale l

experiments, which are often dominated by leading edge effects. Numedcal comparisons that we have perfortned (external to CHYMES) show that upwinding schemes run below the material Courant condition lead to very poor predictions of peak particle fraction, and thus drag. Higher order schemes have to cater for possible discontinuities at the leading edge. Lagrangian approaches, as used by i

the authors for their front tracking, provides much better accuracy, both for i velocity and peak volume fractions. I believe that current schemes in the mixing codes can be improved substantially using physically based Lagrangian limiters, l rather than mathematicallimiters. Fully Lagrangian approaches have the greater l

benefit of handling a spectrum of particle sizes. This may be the best way to treat jet break up and is necessary if one is going to capture the role of the smaller droplets in spreading the melt, as observed in MIXA-06.

l The current presence of numerical diffusion maba the code results difBcult to interpret (eg how far back is the predicted peak concentration from the melt leading edge in the MIXA 06 calculations?). Our experience with more refined i

meshes is that numedcal diffusion is indeed reduced, but the calculations are much more prone to instability of the resulting interface: this numerical instability probably reflects the actual instability of interfaces observed in expedments.

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Concluding Remarks  :

I would expect a more detailed technical evaluation of the calculations presented.

1 am surprised that questions related to the radiation transport modelling, which ,

was clearly important in the FARO simulation, have not been highlighted. I

_would have liked to see more explicit bounds on models emerge form the work.

PM ALPHA Models ,

The details of the correlations embodied in PM-ALPHA will not be reviewed in detail.

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I believe the modelling approach is sound. I note that reactor geometries may impose strongly three-dimensional flow regions, so a 3-D code is riceded for detailed applications (if found to be necessary). I get the impressi4n that the m'odelling philosophy falls between two stools. At places it is admitted that the model necessarily contains many simplifications and constitutive physics that is

. uncertain, but only in the field ofjet break up is a parametric appfoach used. I

would prefer a broader approach to treatment of uncertainties. ,

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With sub-cooling implemented in CHYMES, it is closer in concept to TRIO-MC rather than PM-ALPHA. (EVA should be spelled IVA).

Elsewhere we have queried the use of the drag coefBcients for droflet and bubbly flow. These are derived for bubbles rising at terminal velocity in a: gravitational fleid. It is found that the shape factor for the bubble causes the drag per unit mass of gas to be independent oflength-scale. It is noticeable that no effect of melt droplet shape appears in the con esponding formula for dragicoefBcient for i the melt phase (equation 3.21). A cornpletely different form for the liquid-vapour drag is used for intermediate values of void fraction; this may give! arge changes

' in drag when the transition void fractions are crossed. It is not epdent that there are such sudden changes in flow regime in plenum geometry.

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also the I have not had the time to consider the radiation treatment in det relevant appendices are not included in the excerpt. For dense clbuds of particles, the self-absorption effect will be very important. I woul4 like assurance that this does not allow the region to emit more radiation extemally than that of a l

black-body covering its surface at the same temperature.

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