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Experimental and numerical investigations of the performance of fume cupboards
Building and Environment, Vol. 26, No. 2, pp. 153-164, 1991. Printed in Great Britain.
0366-1323/91 $3.00+0.00 © 1991 Pergamon Press plc.
Experimental and Numerical Investigations of the Performance of Fume Cupboards F. DURST* J. C. F, P E R E I R A * t The paper stresses the wide use of fume cupboards in different fields of industry, in research laboratories and in teaching institutions. The fume cupboards protect people outside the cupboard from hazards of experiments carried out inside. It is often assumed that protection exists when a sufficiently strong air stream is maintained from the laboratory through the fume cupboard to the exhaust air system. However, it is shown that this assumption is only valid for a few specially designed cupboards. For many cupboards in use it does not necessarily apply, even when the sash opening is nearly closed. An experimental test facility which was set up to test the performance of fume cupboards is described. Test results are given indicating the protection that the cupboard can provide for various front window openings. Numerical investigations of the .[low inside the cupboard are described. These were carried out to yield explanations for the experimentally obtained results. The computer programs used also permit optimization of the flow inside fume cupboards and are suggested for use in future developments.
NOMENCLATURE
carrying out chemistry experiments inside the cupboards. In this way dangerous gases, generated by the experiment, are kept away from the researchers and also prevented from diffusing into the laboratory. This also means that chemistry experiments can be carried out inside closed rooms, whilst maintaining strict requirements on the air quality in the rooms. However, to achieve this aim requires well-functioning fume cupboards. The design of these has become a major concern of designers, manufacturers and users. Their concern has created research programmes to develop experimental and numerical techniques to test the performance of fume cupboards under various operating conditions. In the present paper, such techniques and their application are described and results of performance studies utilizing the techniques are given. In Section 2 an experimental set-up is described which permits performance measurements of fume cupboards. The major part of the measuring system consists of an infra-red light absorption unit which permits a SFr-tracer gas to be continuously monitored in front of the cupboard. Concentration measurements can be performed with the system yielding information of tracer gas leakage from the inside of the cupboard to the outside. In this way the efficiency of fume cupboards can be reliably tested. The best cupboards will be those that yield the smallest concentration in front of the cupboard. Investigations of different fume cupboards showed that the concentration in the front region increased with increasing suction speed through small front window openings. This finding could not be readily explained from flow observations. It was therefore decided to carry
as elements of finite difference coefficient matrix C tracer gas concentration C' fluctuation of tracer gas concentration c~, c,i, c,2 coefficients in turbulence models k turbulence kinetic energy P pressure ek production of k S, source term U, /), W fluctuating velocities in x, y, z directions U , V mean velocities in x, y directions X, y Cartesian coordinates Xi coordinate in tensor notation
ap, aE, a w , aN,
Greek symbols
~b dependent variable U, V, k, e, C Ft turbulent diffusivity dissipation rate of turbulence energy 2 diffusivity of tracer gas concentration # dynamic viscosity #ee effective dynamic viscosity (/z + #t) v kinematic viscosity p fluid density ak, cry,at turbulent Prandtl numbers for diffusion of k, e and C 1. INTRODUCTION F U M E CUPBOARDS are nowadays widely used in university and industrial laboratories to protect researchers *LSTM-Erlangen, Lehrstuhl fiir Str6mungsmechanik, Universit~itErlangen-Niirnberg, Egerlandstrasse 13, 8520 Erlangen, F.R.G. tPresent Address: Instituto Superior T6enico, Mech. Engng Dept, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal. 153
F. Durst and J. C. F. Pereira
154
out numerical studies of the flow and concentration fields inside fume cupboards. These are described in Section 3. The equations for the flow and concentration fields are given together with the turbulence models employed in the computations. Some numerical results are provided and are used to interpret the performance of a number of fume cupboards. These are described in Section 3. In Section 3.3 some results of the containment measurements presented in Section 2 are explained using results of the numerical predictions. It is hoped that this will yield an improved understanding of the flow inside fume cupboards. Conclusions and final remarks are provided in Section 4 and suggestions for future developments are given. These suggest the use of the experimental and numerical techniques introduced in this paper.
1
I
2. EXPERIMENTAL INVESTIGATIONS Cupboards that provide the maximum containment are needed in practice. Previously it has been assumed that this can be achieved by having high air flow rates passing the cupboard from the laboratory into the air suction system. People trained in fluid dynamics know that high air flow rates through the cupboard will not necessarily result in high containment of fumes of hazardous gases. Only cupboards that are designed to provide the inherently assumed correlation between high containment and air flow rates will behave as required. Others will not, or only by accident, due to the rather complex flow structures inside fume cupboards. To study this in detail, an experimental test facility was set up at the "Lehrstuhl ffir Str6mungsmechanik" (Institute of Fluid Mechanics) of the University of Erlangen Nfirnberg (LSTM-Erlangen). Its essential parts and the use of this facility for fume cupboard investigations are described in the following section. A further need for this test facility emerged from the main author's work in connection with the new formulation of the "German Standard for Fume Cupboards", DIN 12 924-new, which appeared as a preprint in Spring 1988 [1]. The standards require containment tests to assure the correct fluid dynamic performance of cupboards prior to their installation in laboratories. The test facility described allows all performance tests required by DIN 12 924-new and, hence, represents an experimental set-up suited both for fume cupboard developments and performance tests. As will be explained, the test set-up will permit measurements of all properties that directly result from its aerodynamic performance. Safety aspects dominate the tests the authors and their colleagues usually perform but all the test equipment can also be used to yield information on installation requirements. As Fig. 1 shows, equipment exists for noise performance tests, containment measurements, pressure loss measurements, etc.
2.1. Facility for fume cupboard tests The LSTM-Erlangen test facility for fume cupboards is set up in a test room where 3 fume cupboards of a maximum total width of 5.0 m can be installed for parallel experimental studies. The complete size of the test room is 3.3 m by 5.5 m with an attached completely separate
®
\
i
®
Fig. 1. Major components facility, for fume cupboard flow and containment tests.
control room of roughly the same size. Inside both rooms the major components of the test facilities are set up. These components are shown in a form in Fig. 1. They consist of the following : (1) Fume cupboards to be tested (2) Air suction systems with air flow measuring device (3) Air supply system for laboratory air and supply air for fume cupboards (4) Laser light sheet illumination system and photographic recording system (5) Microphone and frequency analyser for noise production analysis (6) Pressure loss measuring system (7) SF6-tracer gas supply system to be placed inside fume cupboards (8) Data acquisition and control system to carry out and evaluate measurements. The air suction system is designed to allow flow rates of 10 m 3 h - ~ to 1200 m 3 h - z to be passed through each of the cupboards. This allows the system to supply the small and large fume cupboards. The supply system to the laboratory provides 3 times the amount of the suction system as there are 3 fume cupboards. The air supply system can be arranged in such a way that the supply air reaches the laboratory directly or through secondary air supply inlets provided by the cupboards. This air is normally brought in as sketched in Fig. 2. If the performance of fume cupboards is perfect, several experiments are carried out sequentially. The light
The Performance of Fume Cupboards
155
llllll
I
Air Supply fhrough Ceiling
l
.....Jl Air Supply through/ / (::upboo.rd Front ~1 l i
f';
rl
/l /
L
Fig. 4. Fume cupboard with suction nozzles and dummy.
2.2. Containment measurementsfor various window Fig. 2. Air supply system to laboratory.
sheet illumination facility shown in Fig. 1, in conjunction with the photographic recording equipment, is firstly used to carry out visualization studies of the air flow inside the cupboard. This is followed by quantitative measurements of the concentration of the smoke containment followed by an isodensometric evaluation of the photographic record in order to provide information on the region in the cupboard where the smoke concentration is maintained at its highest level. This yields information as indicated in the photograph of Fig. 3 which shows the high concentration load in the left and right points of the cupboard. It is in this region that SF6-tracer gas concentration measurements are performed to assess the explosion safety of the cupboard. This explosion test is part of the new "German Standard for Fume Cupboard Tests", DIN 12 924-new [1]. When the cupboard is operating under set conditions, usually specified by the manufacturer, total pressure loss measurements are performed, followed by noise production measurements using the system parts 5 and 6 respectively as shown in Fig. 1. All these studies are followed by containment measurements using SF6-tracer gas supply system indicated in Fig. 1 and given in detail in Ref. [1]. With this supply system, SF6-tracer gas in a concentration of 10% in 90% N2 is entered into the cupboard at a rate of 3.3 1 minper linear meter fume cupboard, specified in DIN 12 924new. With this tracer gas supply and the appropriate air flow rate through the cupboard, tracer gas measurements are performed in front of the cupboard using the sampling device shown in Fig. 4. To simulate operating conditions, a dummy is placed in front of the cupboard, (see Fig. 4). In this way containment measurements are performed in a way partially suggested by the proposed ASHRAE-test for fume cupboards and the DP80-test of the British Standards Institute. Elements of both these tests were entered into the DIN 12 924-new containment test. It suggests an integral value of the tracer gas emerging from the cupboard to be used to assess satisfactory containment as a measure of the required protection of the tested cupboard.
openings Employing the test facility of Fig. 1, the authors and their colleagues at LSTM-Erlangen have carried out extensive studies of the aerodynamic performance of various types of fume cupboards. The results of these studies are summarized in the rest of this section. These relate mainly to fume cupboards with internal air motions as those given in Fig. 5. Such motions are mainly controlled by the inlet flow passing the cupboard flow opening. The average face velocity is given by the flow rate divided by the sash opening. The inflow requires a strong entrainment of air which is responsible for the large eddy motion moving from the back of the cupboard towards the front. This motion towards the front is inherently present in nearly all the cupboards used nowadays in laboratories, in spite of the suction through the bottom and top openings shown in Fig. 5. It is this eddy motion and the superimposed small fluid rotation in corner regions inside the cupboard that characterize the outbreak of tracer gas into the front region, in spite of the relatively strong flow through the front sash opening. If the large scale motion inside the cupboard is responsible for the outbreak of tracer gas into the front region of the cupboard, the sampling facility of Fig. 4 will collect SFr-gas of concentration given in Fig. 6. This figure shows concentration traces as a function of time, indicating that there is a time delay until the gas has been
t
Fig. 5. Sketch of air flow in a conventional fume cupboard.
156
F. Durst and J. C. F. Pereira
sampled and passed through the mixing unit mounted on the back of the dummy shown in Fig. 5. The concentration increases and reaches a nearly constant value after a maximum time of about 500 s. All measurements in Fig. 6 have been carried out with a one third window opening. With this window opening, as well as with the closed window, most cupboards show a surprising feature as far as the outbreak of SF6-tracer gas is concerned. The higher the flow rate through the cupboard, the more tracer gas leaves the cupboard. This readily indicates that high suction rates are not necessarily correlated with high tracer gas containment, as was assumed in most designs and operations of fume cupboards. If small corner separation regions of the flow in a cupboard are responsible for the outbreak of SF6-tracer gas, the time variation of concentration in the sampling unit shows characteristic features as those shown in Fig. 7. The "micro circulation" in small separated flow regions puts out small amounts of tracer gas which are then sampled and passed through the infra-red detector yielding short spikes of tracer gas concentration. Hence, already the time variation of the tracer concentration in the sampling unit of Fig. 2 indicates the cause for reduced containment, providing hints where flow improvements might be needed for a better performance of a fume cupboard. The spikey traces of concentration of tracer gas can be avoided if smooth inlets are chosen for all points of the cupboard front inlet opening. If a fume cupboard is designed to have tracer gas outbreaks only by small recirculating regions in the corners of the cupboards, it tends to behave well for all window openings. Of course, as the window is opened, the outbreaks increase because their number usually increases linearly with the sash opening. This is readily seen from Fig. 8 showing time variations of measured tracer gas concentration for a complete sash opening. The increased amount is seen by the increased magnitude of the peaks and by their longer duration. In connection with the above-mentioned containment measurements, the question of repeatability of these measurements arose. For this purpose the above-described measurements were repeated several times for the same experimental settings. Figure 9a indicates the repeatability of the tracer gas measurements for a cupboard with a closed window. In Fig. 9b the same kind of measurements are reported for a complete open window. Again, these tests confirm that through the concentration measurements, a good repeatability of the performance assessment of cupboards is found. This readily suggests that the containment measurements carried out in this study can be used to reliably assess the fluid dynamic performance of fume cupboards. The tests readily provide information to what extent sufficient containment is provided by a fume cupboard. 2.3. Dynami¢ flow investigations In addition to measurements under static window conditions, DIN 12 925-new also requires containment measurements when the window is moved. In practice, window motions are usually performed to provide access to the inside of the cupboard to set up test facilities and/or
to control experiments. DIN 12 924-new suggests that the cupboard should reliably perform under the following front window motions : • Starting out from a closed sash position, a one third opening is maintained for about 10 s. Thereafter the window is closed again, i.e. put back into the starting position. • Starting from the closed sash position the window is fully opened and after 10 s closed again, i.e. put back into the starting position. For both window motions, DIN 12 924-new suggests similar concentration measurements to be performed as for the static tests. Performing these for a cupboard that only has tracer gas outbreaks due to small recirculating regions in corner parts, results such as those in Figs 10a and b will be obtained. It is indicated that the window opening and closing will cause some increased outbreaks after the usual delay period of the sampling device. Thereafter, the outbreaks go back to the results obtained for static containment measurements. If the window is completely opened the outbreak is larger, as shown in Fig. 8b. Nevertheless, as soon as the window is closed again, the cupboard suction rapidly changes the measured concentration to the level typical for the static measurements of the closed window case. Such a cupboard performs well and provides the protection expected by the person carrying out the in-cupboard experiments.
3. NUMERICAL INVESTIGATIONS
3.1. Governing equations and their solution Flow investigations are nowadays increasingly performed, employing computer codes solving time-averaged equations of fluid mechanics. If the flow under consideration is assumed to be two-dimensional, steady and incompressible, these time-averaged equations reduce to the following set of partial differential equations :
Continuity equation : a(pU) a(pV) + =0. Ox ~y
(1)
Momentum equations ." ~(pu:) o(puv) + Ox ~y
(2) O(pUV) + c~(pV:) ~x ~y OP • ( ¢3V ) ~3 t" OV ) Oy + ~x "ffxx -Ou'v; + ~y~#fffy _p~.2 . Scalar transport equation : a(puc) ax
a(pvC) ay
(3)
"lhe Per.]brmance of Fume Cupboards
Fig. 3. The distribution of concentration inside a cupboard.
157
Static Measurements
158
I/3 - Sash Opening
1.5 (6)
--
250 rn3/h
- -
400 m 3 / h
--
- 600 m3,'n
u i,.. °
L Fh 03 .5
0
100
200
300
---> Fig. 6. C o n c e n t r a t i o n
400
590
T i m e [s]
v a r i a t i o n as a f u n c t i o n o f t i m e a n d f o r v a r i o u s suction f l o w rates t h r o u g h the
cupboard,
Static Measurements Closed
Sash
.4"
(?) "E .3 Gi. n o) .2 o a
Ff.O .1 h O3 A I I I
\
0
1 oo
o
200
300
--->
T i m e [s]
400
soo
Fig. 7. Time variations of sampled tracer gas concentration for a closed sash fume cupboard with micro motion tracer gas outbreaks.
Static Measurements Complete
Sash Opening
1.5
(8) o. o. i.+
1¸
o ¢D h O3 A I I I
0
-.5 aoo 4o0 50o ---> Time Is] Fig. 8. Time variations of measured tracer gas concentration for complete sash opening. 0
1o o
200
The Performance of Fume Cupboards
159
Static Measurements Closed Sash .009
(a) E ~..006
=.= 0
A
.003 ILl-
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-°003 0
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300
400
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O
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.5
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0
0
/
J
100
200
--->
300
400
500
Time [sl
Fig. 9. Repeatability of concentration measurements for: (a) cupboard with closed window; (b) cupboard with completely open window.
160
F, Durst and J. C. F. Pereira
Dynamic Measurements 1 / 3 Sash Opening .08
(a) .06
(3. Q. 0
.04-
u I,-cO .o2 LL A
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300 ------>
400
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T i m e [s]
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2
(b) CL CL O
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1
It
/L
¢D .5 LL O3 A I I I
0
-.5
o
1 oo
200 --->
300 Time [s]
~o
soo
600
Fig. 10. Dynamic concentration measurements: (a) one-third of window opened and closed after 10 s (b) complete window opened and closed after 10 s.
The Performance of Fume Cupboards where U and V are the mean velocities in the x- and ydirection, respectively, and C denotes the smoke or tracer gas concentration. Fluctuating velocities or fluctuating concentration are represented by the corresponding lower case symbols ; the overbars indicate the time averages of the products of the corresponding fluctuations. The mean pressure is denoted by P, and the density by p. The smoke concentration was assumed a passive scalar without interference with the flow, i.e. no buoyancy effects are assumed to be introduced by the scalar quantity. The latter could be introduced in principle but would add another parameter to the numerical studies and is presently not needed in fume cupboard design and testing. Turbulence model. The above set of differential equations contains more unknowns than the equations. This requires additional information to be introduced using turbulence models. In the present study the Reynolds stresses appearing on the right-hand side of the momentum equations (2) and (3) are expressed by means of the Ice eddy-viscosity turbulence model [2]. The turbulent stresses and the mean strain rates are related by an eddy viscosity St, namely : au
2
-pu '2 = 2S, ~x - ~pk,
(5)
aV 2 PV'2 = 2St fffy - - 5 pk,
(6)
t f dV+ ~-xV).
(7)
-
= s tj-y
In the above equations, the quantity S, is the so-called turbulence viscosity and k the turbulence kinetic energy. In direct analogy to turbulent momentum transport, it was assumed that turbulent mass transport can be approximated utilizing an eddy diffusivity concept, namely : aC
-pu'c' = rtT~x , aC
- p~'c' = r,~y,
(8)
(9)
where Ft is the turbulent diffusivity and like the eddy viscosity, Ft is not a fluid property but depends on the local state of turbulence. The Reynolds analogy between mass transport and momentum transport suggests that Ft is closely related ~ by : r t = -~ , (7t
(10)
where (7t is called the turbulent Prandtl (heat transfer) or Schmidt (mass transfer) number. The eddy viscosity is made up of the turbulent kinetic energy k = 1/2(u2+v2+w 2) and its dissipation rate, namely : k2 S t = C/x P - - ' e,
(1 1)
where c, is an empirical constant whose value is given below. Local values of k and e, are calculated from the following transport equations of the k-e, turbulence
D~ 24h2-F
161
model:
a(pUk) + O(p~) ax By
St Ok ] OYY a L~ ll.h fffy]+ pk_pe, ' ax [ O kOxx]+ (12)
a(pue,) a(pve,) ax ay
(13)
ax The production rate given by :
Pk of the turbulent kinetic energy is
r2/au 2+2(aV12+ (aU+ a,,.'lq kay/ kay ax/1
Pk = st L k?--xx]
(14)
For the empirical constants appearing in equations (11)(13) the following standard values suggested by Launder and Spalding [3] are chosen : cv = 0.09, ak = 1.0, a,. = 1.3, C,t = 1.44 and C~2 = 1.92, and the turbulent Prandtl number a, was made equal to 1 for all the calculations presented here. Numerical solution procedure. The conservative finite volume method embodied in the TEACH program [4] was used to solve the flow and turbulence model equations. The finite difference mesh employed consisted of many rectangular control volumes and was made up of a staggered grid system. For such a grid all scalar quantities are computed at the intersection of any two grid lines. The U and V velocities are also computed on the scalar control volume faces solving the following general transport equation which summarizes each of equations (1)-(4) :
= a
a4~
ay\
ay] +S*" (15)
Integration of this equation over the control volume followed by discretization applying central differencing for the diffusion terms and hybrid central/upwind differencing for the convective terms [5, 6] yields the following finite difference equations :
apdPi,j =
aEq~j+ ,.j+ aw~bi_ t.jWaN~i.j+,+asdp,,j- i + S , , (16)
where a's are the elements of the five diagonal coefficient matrix and S denotes the source term. The solution of the pressure momentum equations, using the above discretization, is based on the SIMPLE method (Semi-Implicit Method for Pressure-Linked Equations) [7]. For this method the solution of the momentum equations is based on a guessed pressure field. The pressure is corrected in order to satisfy the continuity equation at the end of each iteration, yielding a new pressure estimate for the next iteration. The coupled linear equations arising from discretization were solved in an iterative, sequential manner by applying a modified version of the strongly implicit method of Stone [8] to each sub-system (also see Durst et al. [9] and Azevedo et al. [10]). Two inner iterations were performed for U, V, k and e, per outer solution. The
F. Durst and J. C. F. Pereira
162
t~v o
r, : J
I U=cte
"t!
It
m
t'
/"/
"
..,,1111
I ¢I''
Fig. I 1. Computational domain for the fume cupboard.
llI pressure correction was iterated four times per outer iteration. Convergence was controlled by monitoring the absolute value of the residuals in each of the momentum and continuity equations. These resultant quantities were normalized by the inlet mass and momentum fluxes. The iterations were stopped after each residual sum was below 0.001. About 600 iterations were necessary to achieve this on the 40 x 40 Control Volume grid employed in the present study. These computations were performed on a Micro-VAX-II computer and took about 3,600 CPU seconds per flow case. 3.2. Flow configuration and boundary conditions The geometry of the two-dimensional fume cupboard, chosen for the present numerical study, is shown in Fig. 11. The flow was assumed to enter through the front window opening and to leave the cupboard through two back openings, one on the upper corner and the other at the bottom of an extension channel 287 mm above the ground plate. The flow configuration also shows that the inlet and outlet flow conditions were specified through a constant flow velocity at the inlet and a gradient boundary condition at the outlet. Standard wall functions were employed in the present prediction to prescribe the boundary conditions along solid walls. 3.3. Prediction of flow and concentration fields The authors have performed quite a number of computations for fume cupboards. In the present sections the results are described that correspond to the cupboard geometry given in Fig. 11. Solving the equations described above for this flow geometry, flow fields such as the one given in Fig. 12 resulted for a window opening of 40 mm. A clearer picture is obtained if the streamlines of this flow field are drawn as given in Fig. 13a. This figure clearly indicates the strong inflow through the small opening of the front window, driving the large eddy motion inside the cupboard. The strength of this eddy motion is reduced if the window is opened to 287 mm as shown in Fig. 13b. A further reduction of the strength of the recirculating flow is obtained if calculations are performed for a window opening of 685 mm as shown in
'
t
k
, llll 11
I I I~ 1
i I i
,.,,I ~
,
.
,
,
t
, I
II
i
I
II
i
r
J .
c(M/S)
Fig. 12. Predicted flow patterns corresponding to window opening of 40 mm.
Fig. 13c. This complete opening causes a free stagnation point to develop at the back wall of the cupboard at a height of approximately 400 mm above ground level. Such a free stagnation point tends to oscillate, i.e. is sensitive to flow disturbances caused in the front part of the cupboard. Although the fluid flows indicated in Figs 12 and 13 are of interest to people who design fume cupboards, the more relevant information is obtained from the computed concentration fields. These are given in Figs 14a and b for the window openings of 40 and 287 mm, respectively. For the low front window opening the strong recirculating flow region, indicated in Fig. 13a, causes the front region of the fume cupboard to be covered with a concentration of tracer gas of 0.2 relative to the concentration of unity at the source. This will cause an outbreak of tracer gases in spite of the high flow rate through the small cupboard sash opening. If the front window is opened to 287 mm, the C = 0.2 concentration line shifts
The Performance of Fume Cupboards (a)
163
b)
!
(c)
1.
Y(M~.~
Y(M)
Y(M) 1.2
1.2
1.0
1.0-
/~1!
0.8-
OB
06
Q6- /
0~.
0.~~-
02
0 . 2 ~ ~
0 0.0
1.0
02
O.t~
0.6
~
0.0 . . . . . (12 O0
X(M}
O.t,,.
0.6
X(M)
0,0 02 0~.X(M)0~
Fig. 13. Predicted streamlines corresponding to window openings of: (a) 40; (b) 287 ; (e) 685 mm.
backwards. This indicates a reduction of the tracer gas concentration in the front inside the cupboard. The cupboard will therefore work better with a larger opening as far as tracer gas containment is concerned. It is only at low window openings, where the high inlet velocity through the sash opening yields a strong flow from the back to the front, that high concentration of tracer gas is convected to the front region of the cupboard. The above computations indicate why the experimental results yielded a higher outbreak of tracer gases at higher flow rates for the closed sash opening. High flow rates yield high face velocities at the small sash
opening. This drives a jet flow close to the bottom part inside the fume cupboard. The entrainment of air into this jet drives the recirculating flow region indicated in Fig. 13a. This flow has a strong forward component driving tracer gas from the back to the front part of the cupboard. Hence, along the entire sash of the window high concentration of tracer gas has to be expected, yielding high outbreaks. If the flow is reduced the driving power for the recirculating flow region is also reduced. Hence, reduced tracer gas outbreaks occur. This readily suggests that the performance of fume cupboards can be drastically improved by controlling the front face velocity by suitable sensors. 4. CONCLUSIONS AND FINAL REMARKS
(b)
025
Fig. 14. Predicted concentration field (source = 1 unit) corresponding to window openings of: (a) 40; (b) 287 mm.
The performance of fume cupboards does not necessarily improve iftbe suction rate of air from the laboratory through the cupboard to the exhaust system is increased. This has been shown in the paper by containment measurements for a SFr-tracer gas using an infra-red concentration measuring system. Hence, for many cupboards, increased safety could be achieved by reducing the air suction through the cupboard to an optimum value which provides the highest containment. This yields the highest suction rates for the highest sash opening and a continuous reduction when the sash opening is reduced. This not only increases the required containment but is also accompanied by large energy savings. 50% of the air consumption of most conventional fume cupboards can be reduced, at the same time providing increased containment. An experimental facility was set up at LSTM-Erlangen in Germany to carry out containment measurements as a major performance test for fume cupboards. Such measurements are complemented by measurements for explosion safety, noise reduction, pressure measurements, etc. These measurements are part of the DIN 12 924-new performance test of fume cupboards. This German standard appeared in Spring 1988 as a preprint.
164
F. D u r s t a n d J. C. F. P e r e i r a
The test facility is also suitable for carrying out performance tests as a basis for cupboard improvements or the developments of new cupboards. Such developments can be supported by numerical prediction procedures that allow the fluid motion inside the cupboard to be numerically predicted. Results of the flow motion can be obtained in this way and concentration distribution inside the cupboards can be computed. If these computations yield regions of high concentration in the front of the cupboard, concentration measurements in front
of the cupboard sash are likely to be high. Reduced concentrations in the front region of the cupboard should be the aim of new developments. In addition, completely new designs should also be attempted. The introduced experimental and numerical method can and will support such developments. Acknowledgements--This work has been sponsored by LSTMErlangen and Waldner GmbH. Thanks are also due to Ms. Sofia Arrunda for the speedy typing of the manuscript.
REFERENCES 1. DIN 12 924-neu, Tell I, Anforderungen an Abziige, Norm-Gelbdruck. DIN Deutsches Institut fiir Normung e.V., Marz (1988). 2. W.P. Jones and B. E. Launder, The prediction of laminarization with a two-equation model of turbulence. Int. J. Heat Mass Transfer 15, 301-314 (1972). 3. B.E. Launder and D. B. Spalding, Mathematical Models of Turbulence. Academic Press, New York (1974). 4. A.D. Gosman, and W. H. Pun, Lecture notes for course entitled : Calculation of Recirculation Flows. Report No. HTS/74/2, Mech. Engng Dept, Imperial College, London (1974). 5. D.B. Spalding, A novel finite-difference formulation for differential expressions involving both first and second derivatives. Int. J. Numer. Methods Engn# 4, 551-559 (1972). 6. S.V. Patankar, Numerical Heat Transfer and Fluid Flow. Hemisphere, Washington, D.C. (1980). 7. S.V. Patankar and D. B. Spalding, A calculation procedure for heat, mass and momentum transfer in three-dimensional parabolic flows. Int. J. Heat Mass Transfer 15, 1787-1806 (1972). 8. H. L. Stone, Iterative solution of implicit approximations of multidimensional partial functions. S I A M J. Numer. Anal. 5, 530-558 (1968). 9. F. Durst and J. C. F. Pereira, The calculation of some steady laminar flows using various finitedifference schemes. Numerical Methods in Laminar and Turbulent Flow (edited by C. Taylor, M. D. Olson, P. M. Gresho and W. G. Habashi), pp. 551-562. Pineridge Press, Swansea (1985). I0. J . L . T . Azevedo, F. Durst and J. C. F. Pereira, Comparison of strongly implicit procedures for the solution of the fluid flow equations in finite difference form. AppL Math. Modelling 12, 51-62 (1988).
0366-1323/91 $3.00+0.00 © 1991 Pergamon Press plc.
Experimental and Numerical Investigations of the Performance of Fume Cupboards F. DURST* J. C. F, P E R E I R A * t The paper stresses the wide use of fume cupboards in different fields of industry, in research laboratories and in teaching institutions. The fume cupboards protect people outside the cupboard from hazards of experiments carried out inside. It is often assumed that protection exists when a sufficiently strong air stream is maintained from the laboratory through the fume cupboard to the exhaust air system. However, it is shown that this assumption is only valid for a few specially designed cupboards. For many cupboards in use it does not necessarily apply, even when the sash opening is nearly closed. An experimental test facility which was set up to test the performance of fume cupboards is described. Test results are given indicating the protection that the cupboard can provide for various front window openings. Numerical investigations of the .[low inside the cupboard are described. These were carried out to yield explanations for the experimentally obtained results. The computer programs used also permit optimization of the flow inside fume cupboards and are suggested for use in future developments.
NOMENCLATURE
carrying out chemistry experiments inside the cupboards. In this way dangerous gases, generated by the experiment, are kept away from the researchers and also prevented from diffusing into the laboratory. This also means that chemistry experiments can be carried out inside closed rooms, whilst maintaining strict requirements on the air quality in the rooms. However, to achieve this aim requires well-functioning fume cupboards. The design of these has become a major concern of designers, manufacturers and users. Their concern has created research programmes to develop experimental and numerical techniques to test the performance of fume cupboards under various operating conditions. In the present paper, such techniques and their application are described and results of performance studies utilizing the techniques are given. In Section 2 an experimental set-up is described which permits performance measurements of fume cupboards. The major part of the measuring system consists of an infra-red light absorption unit which permits a SFr-tracer gas to be continuously monitored in front of the cupboard. Concentration measurements can be performed with the system yielding information of tracer gas leakage from the inside of the cupboard to the outside. In this way the efficiency of fume cupboards can be reliably tested. The best cupboards will be those that yield the smallest concentration in front of the cupboard. Investigations of different fume cupboards showed that the concentration in the front region increased with increasing suction speed through small front window openings. This finding could not be readily explained from flow observations. It was therefore decided to carry
as elements of finite difference coefficient matrix C tracer gas concentration C' fluctuation of tracer gas concentration c~, c,i, c,2 coefficients in turbulence models k turbulence kinetic energy P pressure ek production of k S, source term U, /), W fluctuating velocities in x, y, z directions U , V mean velocities in x, y directions X, y Cartesian coordinates Xi coordinate in tensor notation
ap, aE, a w , aN,
Greek symbols
~b dependent variable U, V, k, e, C Ft turbulent diffusivity dissipation rate of turbulence energy 2 diffusivity of tracer gas concentration # dynamic viscosity #ee effective dynamic viscosity (/z + #t) v kinematic viscosity p fluid density ak, cry,at turbulent Prandtl numbers for diffusion of k, e and C 1. INTRODUCTION F U M E CUPBOARDS are nowadays widely used in university and industrial laboratories to protect researchers *LSTM-Erlangen, Lehrstuhl fiir Str6mungsmechanik, Universit~itErlangen-Niirnberg, Egerlandstrasse 13, 8520 Erlangen, F.R.G. tPresent Address: Instituto Superior T6enico, Mech. Engng Dept, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal. 153
F. Durst and J. C. F. Pereira
154
out numerical studies of the flow and concentration fields inside fume cupboards. These are described in Section 3. The equations for the flow and concentration fields are given together with the turbulence models employed in the computations. Some numerical results are provided and are used to interpret the performance of a number of fume cupboards. These are described in Section 3. In Section 3.3 some results of the containment measurements presented in Section 2 are explained using results of the numerical predictions. It is hoped that this will yield an improved understanding of the flow inside fume cupboards. Conclusions and final remarks are provided in Section 4 and suggestions for future developments are given. These suggest the use of the experimental and numerical techniques introduced in this paper.
1
I
2. EXPERIMENTAL INVESTIGATIONS Cupboards that provide the maximum containment are needed in practice. Previously it has been assumed that this can be achieved by having high air flow rates passing the cupboard from the laboratory into the air suction system. People trained in fluid dynamics know that high air flow rates through the cupboard will not necessarily result in high containment of fumes of hazardous gases. Only cupboards that are designed to provide the inherently assumed correlation between high containment and air flow rates will behave as required. Others will not, or only by accident, due to the rather complex flow structures inside fume cupboards. To study this in detail, an experimental test facility was set up at the "Lehrstuhl ffir Str6mungsmechanik" (Institute of Fluid Mechanics) of the University of Erlangen Nfirnberg (LSTM-Erlangen). Its essential parts and the use of this facility for fume cupboard investigations are described in the following section. A further need for this test facility emerged from the main author's work in connection with the new formulation of the "German Standard for Fume Cupboards", DIN 12 924-new, which appeared as a preprint in Spring 1988 [1]. The standards require containment tests to assure the correct fluid dynamic performance of cupboards prior to their installation in laboratories. The test facility described allows all performance tests required by DIN 12 924-new and, hence, represents an experimental set-up suited both for fume cupboard developments and performance tests. As will be explained, the test set-up will permit measurements of all properties that directly result from its aerodynamic performance. Safety aspects dominate the tests the authors and their colleagues usually perform but all the test equipment can also be used to yield information on installation requirements. As Fig. 1 shows, equipment exists for noise performance tests, containment measurements, pressure loss measurements, etc.
2.1. Facility for fume cupboard tests The LSTM-Erlangen test facility for fume cupboards is set up in a test room where 3 fume cupboards of a maximum total width of 5.0 m can be installed for parallel experimental studies. The complete size of the test room is 3.3 m by 5.5 m with an attached completely separate
®
\
i
®
Fig. 1. Major components facility, for fume cupboard flow and containment tests.
control room of roughly the same size. Inside both rooms the major components of the test facilities are set up. These components are shown in a form in Fig. 1. They consist of the following : (1) Fume cupboards to be tested (2) Air suction systems with air flow measuring device (3) Air supply system for laboratory air and supply air for fume cupboards (4) Laser light sheet illumination system and photographic recording system (5) Microphone and frequency analyser for noise production analysis (6) Pressure loss measuring system (7) SF6-tracer gas supply system to be placed inside fume cupboards (8) Data acquisition and control system to carry out and evaluate measurements. The air suction system is designed to allow flow rates of 10 m 3 h - ~ to 1200 m 3 h - z to be passed through each of the cupboards. This allows the system to supply the small and large fume cupboards. The supply system to the laboratory provides 3 times the amount of the suction system as there are 3 fume cupboards. The air supply system can be arranged in such a way that the supply air reaches the laboratory directly or through secondary air supply inlets provided by the cupboards. This air is normally brought in as sketched in Fig. 2. If the performance of fume cupboards is perfect, several experiments are carried out sequentially. The light
The Performance of Fume Cupboards
155
llllll
I
Air Supply fhrough Ceiling
l
.....Jl Air Supply through/ / (::upboo.rd Front ~1 l i
f';
rl
/l /
L
Fig. 4. Fume cupboard with suction nozzles and dummy.
2.2. Containment measurementsfor various window Fig. 2. Air supply system to laboratory.
sheet illumination facility shown in Fig. 1, in conjunction with the photographic recording equipment, is firstly used to carry out visualization studies of the air flow inside the cupboard. This is followed by quantitative measurements of the concentration of the smoke containment followed by an isodensometric evaluation of the photographic record in order to provide information on the region in the cupboard where the smoke concentration is maintained at its highest level. This yields information as indicated in the photograph of Fig. 3 which shows the high concentration load in the left and right points of the cupboard. It is in this region that SF6-tracer gas concentration measurements are performed to assess the explosion safety of the cupboard. This explosion test is part of the new "German Standard for Fume Cupboard Tests", DIN 12 924-new [1]. When the cupboard is operating under set conditions, usually specified by the manufacturer, total pressure loss measurements are performed, followed by noise production measurements using the system parts 5 and 6 respectively as shown in Fig. 1. All these studies are followed by containment measurements using SF6-tracer gas supply system indicated in Fig. 1 and given in detail in Ref. [1]. With this supply system, SF6-tracer gas in a concentration of 10% in 90% N2 is entered into the cupboard at a rate of 3.3 1 minper linear meter fume cupboard, specified in DIN 12 924new. With this tracer gas supply and the appropriate air flow rate through the cupboard, tracer gas measurements are performed in front of the cupboard using the sampling device shown in Fig. 4. To simulate operating conditions, a dummy is placed in front of the cupboard, (see Fig. 4). In this way containment measurements are performed in a way partially suggested by the proposed ASHRAE-test for fume cupboards and the DP80-test of the British Standards Institute. Elements of both these tests were entered into the DIN 12 924-new containment test. It suggests an integral value of the tracer gas emerging from the cupboard to be used to assess satisfactory containment as a measure of the required protection of the tested cupboard.
openings Employing the test facility of Fig. 1, the authors and their colleagues at LSTM-Erlangen have carried out extensive studies of the aerodynamic performance of various types of fume cupboards. The results of these studies are summarized in the rest of this section. These relate mainly to fume cupboards with internal air motions as those given in Fig. 5. Such motions are mainly controlled by the inlet flow passing the cupboard flow opening. The average face velocity is given by the flow rate divided by the sash opening. The inflow requires a strong entrainment of air which is responsible for the large eddy motion moving from the back of the cupboard towards the front. This motion towards the front is inherently present in nearly all the cupboards used nowadays in laboratories, in spite of the suction through the bottom and top openings shown in Fig. 5. It is this eddy motion and the superimposed small fluid rotation in corner regions inside the cupboard that characterize the outbreak of tracer gas into the front region, in spite of the relatively strong flow through the front sash opening. If the large scale motion inside the cupboard is responsible for the outbreak of tracer gas into the front region of the cupboard, the sampling facility of Fig. 4 will collect SFr-gas of concentration given in Fig. 6. This figure shows concentration traces as a function of time, indicating that there is a time delay until the gas has been
t
Fig. 5. Sketch of air flow in a conventional fume cupboard.
156
F. Durst and J. C. F. Pereira
sampled and passed through the mixing unit mounted on the back of the dummy shown in Fig. 5. The concentration increases and reaches a nearly constant value after a maximum time of about 500 s. All measurements in Fig. 6 have been carried out with a one third window opening. With this window opening, as well as with the closed window, most cupboards show a surprising feature as far as the outbreak of SF6-tracer gas is concerned. The higher the flow rate through the cupboard, the more tracer gas leaves the cupboard. This readily indicates that high suction rates are not necessarily correlated with high tracer gas containment, as was assumed in most designs and operations of fume cupboards. If small corner separation regions of the flow in a cupboard are responsible for the outbreak of SF6-tracer gas, the time variation of concentration in the sampling unit shows characteristic features as those shown in Fig. 7. The "micro circulation" in small separated flow regions puts out small amounts of tracer gas which are then sampled and passed through the infra-red detector yielding short spikes of tracer gas concentration. Hence, already the time variation of the tracer concentration in the sampling unit of Fig. 2 indicates the cause for reduced containment, providing hints where flow improvements might be needed for a better performance of a fume cupboard. The spikey traces of concentration of tracer gas can be avoided if smooth inlets are chosen for all points of the cupboard front inlet opening. If a fume cupboard is designed to have tracer gas outbreaks only by small recirculating regions in the corners of the cupboards, it tends to behave well for all window openings. Of course, as the window is opened, the outbreaks increase because their number usually increases linearly with the sash opening. This is readily seen from Fig. 8 showing time variations of measured tracer gas concentration for a complete sash opening. The increased amount is seen by the increased magnitude of the peaks and by their longer duration. In connection with the above-mentioned containment measurements, the question of repeatability of these measurements arose. For this purpose the above-described measurements were repeated several times for the same experimental settings. Figure 9a indicates the repeatability of the tracer gas measurements for a cupboard with a closed window. In Fig. 9b the same kind of measurements are reported for a complete open window. Again, these tests confirm that through the concentration measurements, a good repeatability of the performance assessment of cupboards is found. This readily suggests that the containment measurements carried out in this study can be used to reliably assess the fluid dynamic performance of fume cupboards. The tests readily provide information to what extent sufficient containment is provided by a fume cupboard. 2.3. Dynami¢ flow investigations In addition to measurements under static window conditions, DIN 12 925-new also requires containment measurements when the window is moved. In practice, window motions are usually performed to provide access to the inside of the cupboard to set up test facilities and/or
to control experiments. DIN 12 924-new suggests that the cupboard should reliably perform under the following front window motions : • Starting out from a closed sash position, a one third opening is maintained for about 10 s. Thereafter the window is closed again, i.e. put back into the starting position. • Starting from the closed sash position the window is fully opened and after 10 s closed again, i.e. put back into the starting position. For both window motions, DIN 12 924-new suggests similar concentration measurements to be performed as for the static tests. Performing these for a cupboard that only has tracer gas outbreaks due to small recirculating regions in corner parts, results such as those in Figs 10a and b will be obtained. It is indicated that the window opening and closing will cause some increased outbreaks after the usual delay period of the sampling device. Thereafter, the outbreaks go back to the results obtained for static containment measurements. If the window is completely opened the outbreak is larger, as shown in Fig. 8b. Nevertheless, as soon as the window is closed again, the cupboard suction rapidly changes the measured concentration to the level typical for the static measurements of the closed window case. Such a cupboard performs well and provides the protection expected by the person carrying out the in-cupboard experiments.
3. NUMERICAL INVESTIGATIONS
3.1. Governing equations and their solution Flow investigations are nowadays increasingly performed, employing computer codes solving time-averaged equations of fluid mechanics. If the flow under consideration is assumed to be two-dimensional, steady and incompressible, these time-averaged equations reduce to the following set of partial differential equations :
Continuity equation : a(pU) a(pV) + =0. Ox ~y
(1)
Momentum equations ." ~(pu:) o(puv) + Ox ~y
(2) O(pUV) + c~(pV:) ~x ~y OP • ( ¢3V ) ~3 t" OV ) Oy + ~x "ffxx -Ou'v; + ~y~#fffy _p~.2 . Scalar transport equation : a(puc) ax
a(pvC) ay
(3)
"lhe Per.]brmance of Fume Cupboards
Fig. 3. The distribution of concentration inside a cupboard.
157
Static Measurements
158
I/3 - Sash Opening
1.5 (6)
--
250 rn3/h
- -
400 m 3 / h
--
- 600 m3,'n
u i,.. °
L Fh 03 .5
0
100
200
300
---> Fig. 6. C o n c e n t r a t i o n
400
590
T i m e [s]
v a r i a t i o n as a f u n c t i o n o f t i m e a n d f o r v a r i o u s suction f l o w rates t h r o u g h the
cupboard,
Static Measurements Closed
Sash
.4"
(?) "E .3 Gi. n o) .2 o a
Ff.O .1 h O3 A I I I
\
0
1 oo
o
200
300
--->
T i m e [s]
400
soo
Fig. 7. Time variations of sampled tracer gas concentration for a closed sash fume cupboard with micro motion tracer gas outbreaks.
Static Measurements Complete
Sash Opening
1.5
(8) o. o. i.+
1¸
o ¢D h O3 A I I I
0
-.5 aoo 4o0 50o ---> Time Is] Fig. 8. Time variations of measured tracer gas concentration for complete sash opening. 0
1o o
200
The Performance of Fume Cupboards
159
Static Measurements Closed Sash .009
(a) E ~..006
=.= 0
A
.003 ILl-
A
0o
! I I
-°003 0
I O0
200 --->
300
400
500
Time Is]
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O
I--~O ~'l
.5
I I I
0
0
/
J
100
200
--->
300
400
500
Time [sl
Fig. 9. Repeatability of concentration measurements for: (a) cupboard with closed window; (b) cupboard with completely open window.
160
F, Durst and J. C. F. Pereira
Dynamic Measurements 1 / 3 Sash Opening .08
(a) .06
(3. Q. 0
.04-
u I,-cO .o2 LL A
I I I
~
~
,~lt~LJ~=.. i ~ A ~ . . I A
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- .... 'v,,,',-y rv'~'," V~
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300 ------>
400
500
600
T i m e [s]
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2
(b) CL CL O
1.5
1
It
/L
¢D .5 LL O3 A I I I
0
-.5
o
1 oo
200 --->
300 Time [s]
~o
soo
600
Fig. 10. Dynamic concentration measurements: (a) one-third of window opened and closed after 10 s (b) complete window opened and closed after 10 s.
The Performance of Fume Cupboards where U and V are the mean velocities in the x- and ydirection, respectively, and C denotes the smoke or tracer gas concentration. Fluctuating velocities or fluctuating concentration are represented by the corresponding lower case symbols ; the overbars indicate the time averages of the products of the corresponding fluctuations. The mean pressure is denoted by P, and the density by p. The smoke concentration was assumed a passive scalar without interference with the flow, i.e. no buoyancy effects are assumed to be introduced by the scalar quantity. The latter could be introduced in principle but would add another parameter to the numerical studies and is presently not needed in fume cupboard design and testing. Turbulence model. The above set of differential equations contains more unknowns than the equations. This requires additional information to be introduced using turbulence models. In the present study the Reynolds stresses appearing on the right-hand side of the momentum equations (2) and (3) are expressed by means of the Ice eddy-viscosity turbulence model [2]. The turbulent stresses and the mean strain rates are related by an eddy viscosity St, namely : au
2
-pu '2 = 2S, ~x - ~pk,
(5)
aV 2 PV'2 = 2St fffy - - 5 pk,
(6)
t f dV+ ~-xV).
(7)
-
= s tj-y
In the above equations, the quantity S, is the so-called turbulence viscosity and k the turbulence kinetic energy. In direct analogy to turbulent momentum transport, it was assumed that turbulent mass transport can be approximated utilizing an eddy diffusivity concept, namely : aC
-pu'c' = rtT~x , aC
- p~'c' = r,~y,
(8)
(9)
where Ft is the turbulent diffusivity and like the eddy viscosity, Ft is not a fluid property but depends on the local state of turbulence. The Reynolds analogy between mass transport and momentum transport suggests that Ft is closely related ~ by : r t = -~ , (7t
(10)
where (7t is called the turbulent Prandtl (heat transfer) or Schmidt (mass transfer) number. The eddy viscosity is made up of the turbulent kinetic energy k = 1/2(u2+v2+w 2) and its dissipation rate, namely : k2 S t = C/x P - - ' e,
(1 1)
where c, is an empirical constant whose value is given below. Local values of k and e, are calculated from the following transport equations of the k-e, turbulence
D~ 24h2-F
161
model:
a(pUk) + O(p~) ax By
St Ok ] OYY a L~ ll.h fffy]+ pk_pe, ' ax [ O kOxx]+ (12)
a(pue,) a(pve,) ax ay
(13)
ax The production rate given by :
Pk of the turbulent kinetic energy is
r2/au 2+2(aV12+ (aU+ a,,.'lq kay/ kay ax/1
Pk = st L k?--xx]
(14)
For the empirical constants appearing in equations (11)(13) the following standard values suggested by Launder and Spalding [3] are chosen : cv = 0.09, ak = 1.0, a,. = 1.3, C,t = 1.44 and C~2 = 1.92, and the turbulent Prandtl number a, was made equal to 1 for all the calculations presented here. Numerical solution procedure. The conservative finite volume method embodied in the TEACH program [4] was used to solve the flow and turbulence model equations. The finite difference mesh employed consisted of many rectangular control volumes and was made up of a staggered grid system. For such a grid all scalar quantities are computed at the intersection of any two grid lines. The U and V velocities are also computed on the scalar control volume faces solving the following general transport equation which summarizes each of equations (1)-(4) :
= a
a4~
ay\
ay] +S*" (15)
Integration of this equation over the control volume followed by discretization applying central differencing for the diffusion terms and hybrid central/upwind differencing for the convective terms [5, 6] yields the following finite difference equations :
apdPi,j =
aEq~j+ ,.j+ aw~bi_ t.jWaN~i.j+,+asdp,,j- i + S , , (16)
where a's are the elements of the five diagonal coefficient matrix and S denotes the source term. The solution of the pressure momentum equations, using the above discretization, is based on the SIMPLE method (Semi-Implicit Method for Pressure-Linked Equations) [7]. For this method the solution of the momentum equations is based on a guessed pressure field. The pressure is corrected in order to satisfy the continuity equation at the end of each iteration, yielding a new pressure estimate for the next iteration. The coupled linear equations arising from discretization were solved in an iterative, sequential manner by applying a modified version of the strongly implicit method of Stone [8] to each sub-system (also see Durst et al. [9] and Azevedo et al. [10]). Two inner iterations were performed for U, V, k and e, per outer solution. The
F. Durst and J. C. F. Pereira
162
t~v o
r, : J
I U=cte
"t!
It
m
t'
/"/
"
..,,1111
I ¢I''
Fig. I 1. Computational domain for the fume cupboard.
llI pressure correction was iterated four times per outer iteration. Convergence was controlled by monitoring the absolute value of the residuals in each of the momentum and continuity equations. These resultant quantities were normalized by the inlet mass and momentum fluxes. The iterations were stopped after each residual sum was below 0.001. About 600 iterations were necessary to achieve this on the 40 x 40 Control Volume grid employed in the present study. These computations were performed on a Micro-VAX-II computer and took about 3,600 CPU seconds per flow case. 3.2. Flow configuration and boundary conditions The geometry of the two-dimensional fume cupboard, chosen for the present numerical study, is shown in Fig. 11. The flow was assumed to enter through the front window opening and to leave the cupboard through two back openings, one on the upper corner and the other at the bottom of an extension channel 287 mm above the ground plate. The flow configuration also shows that the inlet and outlet flow conditions were specified through a constant flow velocity at the inlet and a gradient boundary condition at the outlet. Standard wall functions were employed in the present prediction to prescribe the boundary conditions along solid walls. 3.3. Prediction of flow and concentration fields The authors have performed quite a number of computations for fume cupboards. In the present sections the results are described that correspond to the cupboard geometry given in Fig. 11. Solving the equations described above for this flow geometry, flow fields such as the one given in Fig. 12 resulted for a window opening of 40 mm. A clearer picture is obtained if the streamlines of this flow field are drawn as given in Fig. 13a. This figure clearly indicates the strong inflow through the small opening of the front window, driving the large eddy motion inside the cupboard. The strength of this eddy motion is reduced if the window is opened to 287 mm as shown in Fig. 13b. A further reduction of the strength of the recirculating flow is obtained if calculations are performed for a window opening of 685 mm as shown in
'
t
k
, llll 11
I I I~ 1
i I i
,.,,I ~
,
.
,
,
t
, I
II
i
I
II
i
r
J .
c(M/S)
Fig. 12. Predicted flow patterns corresponding to window opening of 40 mm.
Fig. 13c. This complete opening causes a free stagnation point to develop at the back wall of the cupboard at a height of approximately 400 mm above ground level. Such a free stagnation point tends to oscillate, i.e. is sensitive to flow disturbances caused in the front part of the cupboard. Although the fluid flows indicated in Figs 12 and 13 are of interest to people who design fume cupboards, the more relevant information is obtained from the computed concentration fields. These are given in Figs 14a and b for the window openings of 40 and 287 mm, respectively. For the low front window opening the strong recirculating flow region, indicated in Fig. 13a, causes the front region of the fume cupboard to be covered with a concentration of tracer gas of 0.2 relative to the concentration of unity at the source. This will cause an outbreak of tracer gases in spite of the high flow rate through the small cupboard sash opening. If the front window is opened to 287 mm, the C = 0.2 concentration line shifts
The Performance of Fume Cupboards (a)
163
b)
!
(c)
1.
Y(M~.~
Y(M)
Y(M) 1.2
1.2
1.0
1.0-
/~1!
0.8-
OB
06
Q6- /
0~.
0.~~-
02
0 . 2 ~ ~
0 0.0
1.0
02
O.t~
0.6
~
0.0 . . . . . (12 O0
X(M}
O.t,,.
0.6
X(M)
0,0 02 0~.X(M)0~
Fig. 13. Predicted streamlines corresponding to window openings of: (a) 40; (b) 287 ; (e) 685 mm.
backwards. This indicates a reduction of the tracer gas concentration in the front inside the cupboard. The cupboard will therefore work better with a larger opening as far as tracer gas containment is concerned. It is only at low window openings, where the high inlet velocity through the sash opening yields a strong flow from the back to the front, that high concentration of tracer gas is convected to the front region of the cupboard. The above computations indicate why the experimental results yielded a higher outbreak of tracer gases at higher flow rates for the closed sash opening. High flow rates yield high face velocities at the small sash
opening. This drives a jet flow close to the bottom part inside the fume cupboard. The entrainment of air into this jet drives the recirculating flow region indicated in Fig. 13a. This flow has a strong forward component driving tracer gas from the back to the front part of the cupboard. Hence, along the entire sash of the window high concentration of tracer gas has to be expected, yielding high outbreaks. If the flow is reduced the driving power for the recirculating flow region is also reduced. Hence, reduced tracer gas outbreaks occur. This readily suggests that the performance of fume cupboards can be drastically improved by controlling the front face velocity by suitable sensors. 4. CONCLUSIONS AND FINAL REMARKS
(b)
025
Fig. 14. Predicted concentration field (source = 1 unit) corresponding to window openings of: (a) 40; (b) 287 mm.
The performance of fume cupboards does not necessarily improve iftbe suction rate of air from the laboratory through the cupboard to the exhaust system is increased. This has been shown in the paper by containment measurements for a SFr-tracer gas using an infra-red concentration measuring system. Hence, for many cupboards, increased safety could be achieved by reducing the air suction through the cupboard to an optimum value which provides the highest containment. This yields the highest suction rates for the highest sash opening and a continuous reduction when the sash opening is reduced. This not only increases the required containment but is also accompanied by large energy savings. 50% of the air consumption of most conventional fume cupboards can be reduced, at the same time providing increased containment. An experimental facility was set up at LSTM-Erlangen in Germany to carry out containment measurements as a major performance test for fume cupboards. Such measurements are complemented by measurements for explosion safety, noise reduction, pressure measurements, etc. These measurements are part of the DIN 12 924-new performance test of fume cupboards. This German standard appeared in Spring 1988 as a preprint.
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The test facility is also suitable for carrying out performance tests as a basis for cupboard improvements or the developments of new cupboards. Such developments can be supported by numerical prediction procedures that allow the fluid motion inside the cupboard to be numerically predicted. Results of the flow motion can be obtained in this way and concentration distribution inside the cupboards can be computed. If these computations yield regions of high concentration in the front of the cupboard, concentration measurements in front
of the cupboard sash are likely to be high. Reduced concentrations in the front region of the cupboard should be the aim of new developments. In addition, completely new designs should also be attempted. The introduced experimental and numerical method can and will support such developments. Acknowledgements--This work has been sponsored by LSTMErlangen and Waldner GmbH. Thanks are also due to Ms. Sofia Arrunda for the speedy typing of the manuscript.
REFERENCES 1. DIN 12 924-neu, Tell I, Anforderungen an Abziige, Norm-Gelbdruck. DIN Deutsches Institut fiir Normung e.V., Marz (1988). 2. W.P. Jones and B. E. Launder, The prediction of laminarization with a two-equation model of turbulence. Int. J. Heat Mass Transfer 15, 301-314 (1972). 3. B.E. Launder and D. B. Spalding, Mathematical Models of Turbulence. Academic Press, New York (1974). 4. A.D. Gosman, and W. H. Pun, Lecture notes for course entitled : Calculation of Recirculation Flows. Report No. HTS/74/2, Mech. Engng Dept, Imperial College, London (1974). 5. D.B. Spalding, A novel finite-difference formulation for differential expressions involving both first and second derivatives. Int. J. Numer. Methods Engn# 4, 551-559 (1972). 6. S.V. Patankar, Numerical Heat Transfer and Fluid Flow. Hemisphere, Washington, D.C. (1980). 7. S.V. Patankar and D. B. Spalding, A calculation procedure for heat, mass and momentum transfer in three-dimensional parabolic flows. Int. J. Heat Mass Transfer 15, 1787-1806 (1972). 8. H. L. Stone, Iterative solution of implicit approximations of multidimensional partial functions. S I A M J. Numer. Anal. 5, 530-558 (1968). 9. F. Durst and J. C. F. Pereira, The calculation of some steady laminar flows using various finitedifference schemes. Numerical Methods in Laminar and Turbulent Flow (edited by C. Taylor, M. D. Olson, P. M. Gresho and W. G. Habashi), pp. 551-562. Pineridge Press, Swansea (1985). I0. J . L . T . Azevedo, F. Durst and J. C. F. Pereira, Comparison of strongly implicit procedures for the solution of the fluid flow equations in finite difference form. AppL Math. Modelling 12, 51-62 (1988).