Wind Tunnel Modelling of Buoyant Emissions

Atmospheric Pollution 1980, Proceedingsof the 14th InternationalColloquium,Paris,France, May 5-8,1980, M.M.Benarie (Ed.),Studies in EnvironmentalScience,Volume 8 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

117

WIND TUNNEL MODELLING OF BUOYANT EMISSIONS A . G . ROBINS

CEGB, Marchwood Engineering Laboratories, Southampton

SO4 4ZB

ABSTRACT The field study of plume rise and ground level concentration downwind of the Tilbury and Northfleet power stations provides the only adequate data source for use in investigating the accuracy of laboratory scale experiments. The TilburyNorthfleet site was modelled at a scale of 11500 in a 9.1 x 2.7 m wind tunnel and measurements of ground level concentration, plume rise and spread were made in a neutrally stable boundary layer for a range of wind speeds and power station operating conditions.

Diffusion studies for buoyant and passive emissions were a l s o

undertaken in undistrubed boundary layer flows. The applicability of various scaling laws for buoyant emissions was thus determined and the likely accuracy of model studies thereby illustrated.

INTRODUCTION Although it has become relatively common practice to study plume dispersion problems at laboratory scale there have been very few attempts to compare model and full scale observations.

In so far as buoyant emissions are concerned there

are no properly established scaling laws and the applicability of those laws which have been suggested has not been adequately investigated.

It was with these

observations in mind that the present work was undertaken. In order to model a buoyant emission (density, p S ’. diameter D into an external cross-flow (density, p ; conserve the following parameters:

ps/p,

of model and full scale wind speeds is:

2

W/U, gD/U

speed, W) from a source of

speed U) it is necessary to

, and,

R = U(model)/U

=

consequently,the ratio

&, where

E,

the geometric

scale ratio,is defined by the nature of the problem being considered and the dimensions of the available laboratory facility; typically it lies in the range from 1/2000 to 11200.

This comes about because it is necessary to model the lowest

kilometer or so of the atmosphere and to study the concentration field over a sufficient fetch to define the maximum ground level concentration. As all wind tunnels have a minimum operating speed there is thus an implied limit to the lowest wind speed which can be modelled by the above procedure and in order to lower this

118 wind speed it i s necessary to relax the scaling criteria. accomplished by combining the previous

This is usually

three parameters into two and discarding

correct modelling of the emission density and, perhaps, source diameter. However, there is considerable scope for choice in the selection of the combined parameters and it i s essential to define the limitations inherent in these 'relaxed' scaling techniques. The most noteworthy previous investigation is that by Isyumov and Tanaka (ref. 1) who concluded that, of the scaling laws tested, only complete scaling (i.e. conservation of p , / p ,

2

W/U, gD/U ) appeared satisfactory. From a much more limited

study Hoult et a1 (ref. 2) claimed that adequate reproduction of observed full scale plume rise downwind of a short rectangular stack (with strong entrainment effects) could be obtained by modelling the velocity ratio and densimetric Froude number. Not all scaling law forms were examined in these studies and, as a result, no definite recommendations can be made, though Isyumov and Tanaka's conclusion, in

so

far as releases near buildings are concerned, seems intuitively correct. EXPERIMENTAL DETAILS The experiments were carried out in the Marchwood Engineering Laboratories' 24.0 x 9.1 x 2.7 m wind tunnel, in which a 1.2 m deep neutrally stable atmospheric boundary layer (abi) was simulated (ref. 3 ) .

The surface roughness length ( z ) of

the undisturbed flow was 4.1Ow4~ (H is the boundary layer depth) and the fric:ion velocity (u,)

was 0.05 U(H)

(U(H) being U at height H).

At a scale of 11500 the

wind tunnel flow corresponded to a 600 m atmospheric boundary layer above a surface of roughness length 20 cm.

In the undisturbed flow, experiments were

undertaken to study diffusion downwind of passive emissions concentrations being measured.

-

mean and fluctuating

In order to study scaling problems emissions from

a 0.20 m tall stack (100 m at full scale) were considered. The emissions were taken to be typical of a fossil fuel fired power station generating either 400 or 1000 MW of electricity.

(E W

=

=

The heat emitted from the stack was taken as E/6

station load) and the equivalent full scale conditions were: -1 (E = 400 MW), 53 ms-l (1000 Mw); 21 ms p , / p = 0.72; D = 6.4 m

The only adequate full scale experiment for present purposes is that undertaken downwind of the power stations at Tilbury and Northfleet, to the east of London (refs. 4 & 5).

The site is sketched in fig. 1, in which the positions of the

twenty-two sulphur dioxide measuring stations are shown, as are the extents of the regions modelled, at a scale of 11500, for use in the wind tunnel studies. Thus, at full scale, ground level concentrations (glc) could only be measured for winds from about S to SW, whereas plume rise could be measured for all wind directions. Approximately 5000 sets of mean hourly glc data were obtained and these have been categorised according to power station load, wind speed and atmospheric stability (ref. 6), meteorological information being obtained from a nearby 187 m tower and

119 from the Crawley radiosonde; plume rise measurements were categorised in the same manner.

The results revealed significant topographical effects influencing the

plume from Northfleet power station, and it was deduced that for winds which carried the plume across the river the effective stack height was 30 to 40 m less than the actual stack height.

Both power stations have twin stacks and operate with

emission temperatures of llO°C above ambient; for Tilbury A (oil burning):

E 5 360 MW, h(stack ht.)

=

101 m, D = 6 . 4 m

E 6 7 2 0 MW, h = 1 5 3 m, D = 7 . 4 m

for Northfleet (coal burning}:

In each case the heat emitted from the stacks was taken to be E / 6 . The wind tunnel techniques for mean concentration measurement are described in ref. 7 and the instrumentation used for studying concentration fluctuations in refs. 8 & 9. In the wind tunnel experiments averaging times

(T)

were chosen such

that the residual scatter in repeated measurements was no more than 10%; roughly speaking, this required: from about 30 (for U

=

T U / H = 4 0 0 , whereas at full scale this parameter varied

5 ms-')

to 150 (U = 25 ms

-1

) so that a degree of variance

must thereby result. Much more relevant sources of variance are unsteadiness in the mean flow and source conditions, even though results were only recorded when mean conditions were relatively steady over a period of one hour. None-the-less, in nominally similar conditions hourly average glcs between 0 times and 3 times the ensemble mean were observed (ref. 10). SCALING CONDITIONS (NEUTRALLY STABLE APPROACH FLOW) Assuming that the approach flow and surface conditions have been properly modelled it remains to model the emission and this requires conservation of the 2 following properties: p , / p ; W/U; gD/U This so called 'complete' scaling

.

condition will hereafter be designated condition C.

One approach to relaxing the

requirements is to consider the magnitudes of the important terms in the flow equations and this leads to a number of possible modelling conditions, i.e.: 2 2 2, 2, W/U or psW / P U and gD@/U2, where 8 = ( p - p ) / p and p is a reference density. These conditions will be abbreviated as FXY, where F denotes the conservation of the densimetric Froude number, X = V for conservation of the velocity ratio or M for the momentum ratio, and Y

=

2,

A if p

= p

or S for p,.

Another approach to

relaxing C scaling conditions follows from plume rise theory which gives the rise, 2 3 Az/D, as a function of x/D, psW /pU2 and g$DW/U The resulting conditions will

.

be abbreviated as BXY, where B denotes conservation of the nondimensionalised buoyancy flux. A variation on these methods can be derived from the specific form of plume rise formulae; eg. Briggs (ref. 11): 3 2 = a(p W2/pU2) x /D + b(g@DW/U )(x/D)

.

Modelling requires the reproduction of Az/h at given x/h and this may be achieved by conservation of:

p,/p,

2

3

DW/hU, gD W/hU ; i.e.

by distorting D/h - condition BMD.

Of course it is impossible to conserve the various Reynolds numbers of the

120 problem.

This is of little consequence in

far as the approach flow is concerned

so

(as it is a fully turbulent flow) providing the surface is fully rough (u,zo/v

3 0(1)),

and in the present case this implies U(H) 5 0.6 ms-'.

The internal

flow in the stack must be 'tripped' in order to reproduce an appropriate exit velocity profile and this was affected by the insertion of 'porous plugs' a few diameters below the stack tops - in the experiments WD/v stack Reynolds numbers were between whereas they exceeded

lo6

lo3 and

4.10

3

= LO3

4

to 10

.

The external

in the wind tunnel studies,

However, as entrainment into the

at full scale.

recirculating flow immediately downwind of the stack was not a significant feature of the problem this discrepancy was unimportant. RESULTS Isolated Stack in Undisturbed Flow That the flow within the wind tunnel was an adequate model of the neutrally stable abE, and that the behaviour of plumes resulting from passive emissions was acceptably modelled have already been established (refs. 3, 12 & 1 3 ) .

Figs. 2 & 3 show typical

results for buoyant emissions of the variation of maximum glc with wind speed nondimensionalised as CU(H)h Az/h.v. l/U(H).

2

/Q.v.U(H),

and plume rise with wind

speed, shown as

Q is the volume emission rate and the results have been converted

to equivalent full scale values, assuming

W

refer to full scale conditions of:

=

E

1/500. -1

=

21 ms

, ps/p

The data shown in fig. 2 = 0.72.

If any of the

relaxed scaling conditions are to be considered realistic then results obtained by their application must satisfy two conditions: a) they must be relatively insensitive to the choice of model scale emission density, b) they must agree tolerably well with the results obtained by complete scaling. It can be seen that, of the scaling conditions tested, only BMA and DMS are acceptable on the above basis.

It is interesting to note that although the BMA and

BMD scalings conserved the same parameters, enhancement of the source diameter

proves to be unrealistic. Measurements revealed that with BMD scaling the initial dimensions of the plume were considerably exaggerated and,

as

a result, the

development of the plume was not properly modelled. Tilburv-Northfleet Site

It was observed that the chief effect of the topography upwind of the stacks was to increase the lateral turbulence intensity (also, presumably, the scale). near linear plume lateral growth,

(r

agreement with the full. scale data.

Y

A

(x), was measured (fig. 4) which was in good

Observations revealed that the mean streamline

through the top of the Northfleet stack descended by 30 to 40 m in a 2 km fetch downwind;

this agreed with deductions from full scale plume rise data.

Measurements of plume rise and ground level concentration were consistent with those in the undisturbed flow in that conditions DMS and BMA produced plume

121 behaviour close to that observed for complete scaling, whereas the other methods (BMD was not used) did not.

Further experiments were undertaken to investigate

the possibility of even greater relaxation of complete scaling.

If it is assumed

that the plume is entirely buoyancy driven then it may be argued that it is only necessary to conserve the densimetric Froude number, or buoyancy flux parameter. It was found that this was indeed feasible providing the momentum ratio was not

greatly enhanced (say by no more than a factor of two).

However, it would perhaps

be unwise to recommend the adoption of such a technique as the exaggeration of emission momentum could have important consequences to the near field plume path which would result in under-estimation of entrainment effects etc. A major difficulty in model full scale comparisons arises because of the almost

inevitable scatter in field data.

In the present case this was compounded by the

relative sparseness of the SO measuring network in comparison with the wind tunnel 2 gas sampling system. None-the-less, it was possible to demonstrate reasonable -1 simulation of plume rise and vertical spread for wind speeds greater than 6 ms , by using scaling conditions C, DMS and BMA.

Fig. 5 is a comparison of the variation

of maximum glc with wind speed for emissions from Tilbury; the model data are a composite of the results obtained by C and DMS scaling. At first sight it appears as though the model overpredicts the glc, but this is not a correct interpretation since the field data refer to the mean maximum observed glc (i-e. at one o f the 22 monitoring sites) for all wind directions in which the plume passed over the network. The equivalent data can be derived from the wind tunnel measurements and this results in good model full scale agreement - this also holds for the position of the maximum glc.

An alternative approach is to attempt to derive centre-line glc

profiles from the field data and to compare the resulting maxima with the model results.

This procedure shows reasonable agreement between the model and the upper

range of the full scale data.

A l s o shown in the figure is the variation in maximum

glc due to changes in boundary layer height and load division between the two stacks. Concentration Fluctuations Wind tunnel measurements of concentration fluctuations in the Northfleet plume showed that DMS scaling produced the same fluctuation levels as C scaling (ref. 4 ) . Unfortunately, it was not possible to obtain adequate statistics from the full scale 3 minute mean data in order to attempt model-field comparisons. An interesting

feature to emerge from subsequent laboratory experiments was the importance o f initial source size in determining fluctuation levels (fig. 6).

This reinforces

the previous observation that stack diameter exaggeration is not a reliable scaling procedure because it results in incorrect initial plume geometry. DISCUSSION It has been demonstrated that adequate modelling can be obtained with C, DMS or

122 BMA scaling. This is not entirely consistent with previous less extensive studies (refs. 1 & 2), though the accuracy of BMA scaling has been demonstrated elsewhere (Prof. D. J. Wilson, private communication). In practice it is to be recommended that C and either DMS or BMA are used in conjunction and that some situations are modelled in both ways in order to demonstrate the performance of the relaxed scaling condition. The situation is less clear in

so

far as building entrainment and other

near-field studies are concerned and in such cases it seems prudent to use only C scaling (Prof. W. Melbourne, private communication).

A necessary preliminary is a

demonstration of the adequacy of the modelled boundary layer; velocity, turbulence and dispersion data are necessary for this.

Providing the above recommendations are

followed it is to be expected that reasonably accurate modelling of the behaviour of buoyant emissions in the neutrally stable abl is possible. ACKNOWLEDGEMENTS This work was undertaken at the Marchwood Engineering Laboratories and is published by permission of the Central Electricity Generating Board. REFERENCES 1 N. Isyumov & H. Tanaka, Proc. 5th Int. Conf. on Wind Engineering, Colorado S t a t e University, 1979 paper VIII-3. 2 D.P. Hoult, S.R. O'Dea, G.L. Touchton & R . J . Ketterer, J. Air Poll. Control Assoc. 27 (1977) 56-60. 3 A.G. Robins, J. Industrial Aero., 4(1979)71-100. 4 D.H. Lucas, K.W. James & I. Davies, Atmos. Environ. 1(1967)333-365. 5 D.J. Moore, Atmos. hnviron., 1(1967)389-410. 6 D . J . Moore ti P.A. Roberts, CEGB Report RD/L/M469 (1974). 7 A.G. Robins, Proc. Inst. Mech. Engrs., 189(1973)44-54. 8 J.E. Fackrell, J. Phys. E:Sci. Instrum., 11(1978)1015-1022. 9 J.E. Fackrell, CEGB Report R/M/N1056 (1979). 10 D.J. Moore, Proc. Inst. Mech. Engrs., 189(1975)33-43. 11 G.A. Briggs, USAEC (1969) TID-25075. 12 A.G. Robins, Atmos. Environ., 12(1978)1033-1044. 13 A.G. Robins & J.E. Fackrell, in C.J. Harris (Ed.) Mathematical Modelling of Turbulent Diffusion in the Environment, Academic Press. London, 1979, pp.55-114. 14 J.E. Fackrell, CEGB Report (1978) R/M/N1016. FIG. 1 Tilbury (T) Northfleet (N) site showing SO2 monitors (X) and model outline. Land over 100 ft shaded.

123

.

10 FIG. 2

UCH) m/s

a

- -

-

40

10

Maximum glc.v.wind speed; E = 400 MW. Shaded area is C scaling (p / p = 0.72), scatter typical of all data. Relaxed sczlings p / p = 0.27 ( A ) , 0.17 ( B ) .

DMA//BMD

28

0',0

0

// a

BMA

c

C shaded area-shows scatter typical o f all results.

. I

%

N

Q

0

u

FIG. 3

I/UCHI

s/m

0-1

Plume rise.v.wind speed at x = 1 0 h ; E = 1000 MW. For C scaling p / p = 0.72, otherwise 0.27.

124

Q

I

8

7

0 FIG. 4

8

x

m.

8

5000

Lateral spread downwind of Tilbury A

Full scale data mean f I stdv. Source 1360

-10 FIG. 5

UH)

D/H 0*029+ Ooof

0.003 0

+ I

0

m/s

30

Maximum glc.v.wind speed, Tilbury, E = 250 MW. Solid line is actual max.; 1, H = 600; 2, 300 m; 3 , single stack. Dashed line is max. observed at site of field SO2 monitors.

dn

1

0

FIG. 6

Fluctuating (rms) concentrations downwind of source at h/H = 0.2. c' is max. fluctuation and C is max. mean concentration in plane x = const.