ABSTRACT INTRODUCTION

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1 Wind tunnel investigation of waste air re-entry with wall ventilation P. Broas Technical Research Centre of Finland, Ship Laboratory, Tekniikantie 12, SF-02150, Espoo, Finland ABSTRACT A wind tunnel investigation was carried out using tracer gas measurements with a 1:150 scale-model to determine how much waste air from a wall outlet is carried into the intakes in the same wall. The results were compared with full-scale experiments and with theories used with roof ventilators. These showed that if the ratio of the exhaust jet velocity to windspeed is kept constant, the dilution does not change even though the absolute velocities of the exhaust jet and wind tunnel flow are varied. Instead the ratio itself has an effect on the dilution, but this is not very clearly seen because of the turbulence of the surrounding wind field. Owing to the vortex flow and the wake of the building, the trajectories of exhaust jet plumes are much more complicated with wall ventilators than with roof ventilators. Further investigation is needed for the correct scaling of the exhaust jet momentum. The results indicate that in many cases wall ventilation is suitable for apartment houses INTRODUCTION The use of wall ventilation instead of roof ventilation would simplify ventilating channels and be of great help particularly in renovating old apartment houses where an apartment-based mechanical ventilation system could be used. However, it should be ensured that the expelled waste air does not pollute the intake air of the same or neighbouring apartments. The wind tunnel investigation described here was a part of an investigation where apartment-based mechanical ventilation was evaluated. Also full-scale experiments have been performed and odour treshold values of exhaust air during cooking and smoking have been determined. Ventilation with roof exhausts has been widely examined and formulas for estimating the contamination of intake air have been determined [1,2]. One reason for the wind tunnel tests was to find out if these formulas are applicable also for wall exhausts.

2 480 Air Pollution The wind tunnel tests were performed in the VTT Ship Laboratory, while the VTT Heating and Ventilation Laboratory was in charge of the investigation programme. TEST METHOD AND SCALE MODELS Test equipment The wind tunnel tests were carried out in the old VTT open-circuit tunnel, which has a test section 1.5 m wide, 1 m high and 6 m long. The atmospheric boundary layer was simulated to provide a sub-urban wind profile with a power law exponent of 0.28 and turbulence intensity 12 %. Sulphurhexafluorid (SFJi was used as a tracer gas, though at very small concentrations. The tracer was mixed with air in a ratio 1 : 100, the normal exhaust flow concentration being 20 ppm. Samples for measurements were gathered from ten points, of which one was a control point in the tunnel ahead of the model. A gas cromatograph, which due to its electronic capture detector (ECD) is capable of detecting very small concentrations (< 10 ppb), was used for determining the tracer gas concentration of the intake air. Smoke tests were performed to give an overall view of the wind field in the model area and to show how an exhaust jet becomes mixed with the surrounding flow. - L Side View o r J QO lo K2 0 K4 Ho o No So A 0 K6 0 ro & xo K8 0 K7 ^' o K9 Plan i i i i i Fig. 1. A schematic view of the model building. The positions of exhaust and intake openings and wind incidences. Exhaust openings G, H, N, S, X, BB, I, L, Q and T. Intake openings Kl - K9. The three-storey model building (fig. 1) was actually not a model of a residential building, but the actual building of the VTT Heating and Ventilation laboratory which was used in the full-scale tests. The model was made of wood on 1:150 scale. It also contained the surrounding houses and trees. The exhaust and intake openings were situated on the east wall of the building. Three exhausts were situated according to full-scale test positions, the others mainly on the ground floor. The intake openings from which the samples were gathered were both on the ground and the first floor. For comparisons with a conventional roof exhaust system, one exhaust point was placed on the roof. It consisted of a vertical pipe and

3 Air Pollution 481 a horizontal plate 3 mm above the pipe head so that the waste air was directed radially and horizontally in calm weather. Test programme First some full-scale test situations were simulated in the wind tunnel and the results were compared with each other. As the winds during the full-scale experiments were mild (< 5 m/s), not very many cases could be investigated. Hence a systematic series of wind tunnel tests was carried out with five different wind directions. In these tests the exhaust jet-to-wind velocity ratio was kept constant (M = V/U = 2) by using the exhaust velocity of 4 m/s and wind tunnel flow speed of 2 m/s. The exhausts from five points (H, N, S, X and BB, fig. 1), one at atime,were examined. In all cases the samples were gathered from 9 points (Kl - K9). Both the exhaust and intake openings were situated on the east wall of the building. Additional tests were also performed to determine the reliability of the test method and the effects of certain technical factors. For instance, the effect of two simultaneous exhausts and various opening diameters were examined. Numerous exhaust rates and exhaust velocity-to-wind speed ratios were tried to determine their effect on the intake air contamination. The dilution rate has been presented in tables and graphs as a function of the dimensionless exhaust-to-intake distance, which was determined as a ratio of the distance and the square root of the opening area. The results have been compared to the minimum dilution curve for roof exhausts presented by Wilson & Chui [ 1 ] by the equation: where and 0*.-[D?+0«f (1) D.=l + 7.0j8(U./U)* 0,=B,(U/U.)(S*/fl) where U = wind velocity, V, = exhaust velocity, S = exhaust-to-intake distance, A = exhaust-opening area, (3 and B, are constants. This equation gives the minimum dilution at a roof or wall intake for buildings with roof exhausts of zero stack height on a flat roof. It shows that the dilution is proportional to the velocity ratio M and the dimensionless distance S/VA. It gives the dilution for full-scale ten-minute averages. In figures 2-6 the minimum dilution curve is presented for the velocity ratio M = 2.

4 482 Air Pollution TEST RESULTS Full-scale comparisons With the north-west wind, the exhaust was placed in the middle of the east wall (point T), ground floor, and samples were gathered from the same wall (fig. 1) Dilution calculated from the wind tunnel test results varied between 100 and 250, while full-scale dilutions were higher, between 400 and 8000 (fig. 2). With the south-east wind, which is oblique and against the east wall, the measured concentrations in the wind tunnel led to dilution rates of , full-scale During the full-scale testing of the roof exhaust there was a south-west wind. Concentrations could be measured only at one point in the upper part of the east wall, the dilution being about In the wind tunnel test concentrations could be measured in several points, the dilution varying from 20 to 4000 depending on the point and the velocity ratio M. DilutiorI 0, 100 ^*~*Z- J- 0 7 f y * %" / / 4, % 1 n ] 10 1CK) 10( S/SQR(Ae) Fig.2. Dilution from wind tunnel and full-scale tests. Po = exhaust point code. Systematic test series with various wind directions Winds from south-west sector Wind conditions were very much alike with west, south-west and south winds because of the channelling of the wind flow, which was clearly visible in smoke tests. In these situations the exhaust jet is directed to the wake of the building and concentrations could be measured only at the points north of the exhaust opening. The highest concentrations were measured when the exhaust was from point H near the south end of the building. With west wind, the dilution increased only slightly with increasing distance, varying from 160 to 350 (fig. 3). This can be understood if one observes the flow pattern from the smoke

5 Air Pollution 483 tests. The exhaust jet is mixed with the vortex along the side of the building and therefore the waste air is carried to the intakes along the wall. Dilution 0 Westwind s Po-H a Po-N» Po-S O Po-X A Po-BB S/SQR(Ae) Fig. 3. Dilution with west wind for exhausts from points H, N, S, X and BB. With south-west wind, large deviations were found among the measured concentrations. Near the exhaust openings the concentration was quite low, although it increased farther away with the maximum being found near the middle of the wall. This is probably due to flow separation and conical vortex formation at the edges of the building, which causes high turbulence around the vent openings. With south wind, the flow pattern around the building was symmetrical and concentrations could be measured only in the cases where the exhaust was near the south end of the east wall, these being at the same level as with the previous wind incidences. Adverse winds South-east and east winds are against the east wall, the exhaust jet thus impinging on the wind flow. With south-east wind, the flow is directed partly upwards over the building and the waste air is carried with it away from the wall. Partly the flow goes towards the other end of the building and around it. Even the highest concentrations near the corner were quite low, the dilution being 640. With east wind the exhaust jet is directed straight against the wind and the results are symmetrical according to the symmetrical flow pattern around the building. The waste air is directed to the south or north end of the building according to the position of the exhaust vent. The distances where concentrations could be measured are quite short and the points in the graph lie above the calculated minimum curve (fig. 4).

6 484 Air Pollution Dilution 0 - Eastwind S/SQR(Ae) Fig. 4. Dilution with east wind for exhausts from points H, N, S, X and BB. Comparison with roof exhaust/inlet system As a curiosity, a usual system where the exhaust and intake are situated in the opposite walls of a special ventilation room on the roof of the building was also tested. The results were quite obvious; with the wind against the exhaust opening, very high concentrations were measured at the intake and with opposite wind direction the waste air was directed to the wake without polluting the intake air. The effect of velocities The exhaust jet-to-wind velocity ratio (M) in equation 1 is one parameter, to which the dilution is proportional. Velocity ratios 1-4 were examined by changing the exhaust jet velocity in two cases. The results did not indicate significant differences. Beside varying the velocity ratio the absolute velocities were also varied so that the ratio was kept constant. From figure 5 it can be seen that the dilution level is the same. The effect of opening diameter Since at the beginning of the tests it was not exactly known how small the concentrations to be measured would be, both the exhaust and intake openings were made clearly larger than the geometric scale (3 mm). Hence we had to determine the effect of the hole diameter. It was found that the size of the exhaust opening has a considerable effect on the dilution rate. The measured concentrations decreased because of the flow rate reduction when the diameter of the exhaust opening was reduced to 1 mm although the velocity ratio was kept constant (fig. 6). The differences in concentrations between 1 and 3 mm openings were not similar for all intakes and hence the results could not be corrected using the area ratio, and they were not proportional to the dimensionless distance. As the exhaust flow volume was much smaller with smaller openings, larger tracer concentrations

7 Air Pollution 485 (about 50 times) had to be used in the exhaust, this making the comparison even more difficult. On the other hand, changing the volume flow rate by changing the exhaust velocity did not have a considerable effect. These changes are still subjects for further research. Varying the size of the intake opening did not greatly change the measured concentrations, although the dimensionless distance became different. 0 = U= Im/s O U = 2 m/s O U = 4 m/s M= S/Sqr (A) Fig. 5. Dilution with three sets of exhaust and wind velocities with the velocity ratio being a constant M = c 1 1 " 7 / u "/ r c / ~*/ MMM= jjj^ 1 10 S/Sqr (A) 100 ]000 Fig. 6 Dilution with different sizes of exhaust openings.

8 486 Air Pollution Two simultaneous exhausts In practice there are usually more than one simultaneous exhaust sources to pollute the intake air. As the first step in investigating numerous exhausts, two simultaneous exhausts were tested to discover if summarizing the results of separate tests could be used. The comparison of the results showed that summarizing is not possible in this case. The concentrations were at the same level both for one and two exhausts, hence summarizing would have given double concentrations. CONCLUSIONS Wind tunnel tests can be used especially for comparative tests for different ventilation systems and exhaust positioning because both wind conditions and test arrangements can easily be varied. The results are also reliable for these kinds of investigations. For example, the absolute velocities of the wind and the exhaust jet may differ from practice provided that the velocity ratio is correct. However, the determination of absolute dilution values needs further research and more knowledge is required about the dynamics of mixing and the scaling of the momentum of the exhaust jet. The effect of sampling time was studied on fullscale, and it was also found to have a considerable effect on the concentrations. In spite of these uncertainties, the following conclusions could be drawn: The equation of Wilson & Chui [ 1 ] can also be used for wall exhaust for short distances. For longer distances (>5-10m) the wind's effect on the flow is so strong that the dilution does not increase according to the equation. With certain wind directions, the polluting of intake air by the waste air from roof exhausts is greater than from wall exhausts. It is recommended that the exhaust openings should be placed above the intake openings and above 1/3 of the height of the wall (also mentioned by Wilson [2]) if possible. REFERENCES 1. ASHRAE Fundamentals Handbook Chapter 14, "Air flow around buildings." American Society of Heating, Refrigerating and Air Conditioning Engineers. Atlanta Wilson, D. J. Ventilation Intake Air Contamination By Nearby Exhausts. Proc. Air Pollution Control Assoc. Conf. on Indoor Air Quality in Cold Climates, Ottawa, April 28 - May