Displacement ventilation

Figure 6.6 and Tables 6.4-6.6 give ranges for local thermal discomfort parameters for the three categories listed in Table 6.3. The acceptable mean air velocity is a function of local air temperature and turbulence intensity. 7 he turbulence intensity may vary between 30% and 60% in spaces with mixed flow air distribution. In spaces with displacement ventilation or without mechanical ventilation, the turbulence intensity may be lower. 

In rooms where air and contaminant movement is dominated by thernnal energy of heat sources (e.g., in rooms with natural or displacement ventilation), temperature and contaminant stratification along the room height is created. Air supply and exhaust in such rooms are designed not to disturb the natural pattern of air movement created by heat sources cooled air enters the room in... 

The criteria K is similar to the Archimedes number introduced in 19.30 liy Baturin and Shcpelev to characterize air jets influenced by buoyancy, or to ihe Richardson criteria used in meteorology to characterize rhe ratio of the mrbu-lence suppression by rhe buoyancy forces over the turbulence generation by the Reynolds tension, In the case of displacement ventilation, the Richardson criteria can be defined by rhe relationship -... 

One of the unidirectional flow system modifications is air supply through diffusers located above the occupietl zone. The supply air temperature is lower than the desired room air temperature in the occupied zone, and air velocity is lower compared to a mixing-type air supply, bur higher than for a thennal displacement ventilation. Polluted air of the occupied zone is suppressed by an tiverlying air cushion that displaces the contaminated air toward floor-level exhausts (Fig. ". 12). 

Kristeiisson, J. A., and O. A. Lmdqvist. 1993. Displacement ventilation systems in industrial buildings. ASHRAE Transactions, vol. 99, no. 1. 

Nielsen, P. V. 1992. Velocity distribution in the flow from a wall-mounted diffuser in rooms with displacement ventilation. In Koomvent 92 Proceedings of the Third International Conference on Air Distribution in Rooms, vol. 3. Aalborg, Denmark. 

Mundt, E. 1996. The performance of displacement ventilation systems Experimental and theoretical studies. Ph.D. thesis. Bulletin No. 48, Building Services Engineering, KTH, Stockholm. 

Nielsen, P. V. 1993. Displacement Ventilation Theory and Design. Aalborg Universitv, Denmark. 

J2. Skistad, H. 1994. Displacement Ventilation. Research Studies Press, John Wiley k Sons, Ltd., West Sussex, UK. 

Zhivov A. M., E.O, Shilkrot, P. V. Nielsen, and G. L. Riskowski. 1997, Displacement ventilation design. In Ventilation 97 Proceedings of the 5th International Symposium on Ventilation for Contaminant Control, vol. 1, pp. 427-438. Ottawa, Canada. 

Zhivov A. M., G. L. Riskowski, T. W. Ruprecht, L. L. Christianson, P. V. Nielsen, E. O. Shilkrot, and A. A. Rymkevich. 1997. Design Guide for Displacement Ventilation. Research Project for Philip Morris Management Corporation. lAT, Savoy, IL. 

Natural or displacement ventilation systems creating temperature stratification (Fig. 7.109(i)... 

FIGURE 7.111 Contaminant and heat transfer due to turbulent exchange between building zones (o> contaminant movement ablest the airflow near the vicinity of local exhaust (b) heat and contaminant transfer between the lower and upper zones of the building with displacement ventilation. 

E. Mundt, The Performance of Displacement Ventilation Systems Flxperimental and Theoretical Studies, Stockholm,. Sweden Royal Institute of Technology, 1996... 

E. Shilkrot and A. Zhivov, Zonal model for displacement ventilation design, in Roonwcnt 96 vol. 2. Yokohama, Japan, 1996. 

For ideal displacement ventilation, e. Values for the other indices depend on the location of the zone in question. 

German guidelines base the division on the resulting airflow pattern within the room rather than distribution methods. They suggest that airflow patterns be divided into four categories hall-filling mixed flow zonewise mixed flow low-momentum, low-turbulence flow for the air supply in the work region and zonewise displacement ventilation. 

F. Alamdari, Displacement ventilation and cooled ceiling.s, in Proceedings of Spacevent 95, Stockholm 1998. 

El. Brohus, Influence of the cooled ceiling on indoor air quality in a displacement ventilated space examined by means of computational fluid dynamics, in Proceedings of Spacevent 9,>i, Stockholm 1998. 

H. Tan, T. Murata, K. Aoki, and T. Kutabuchi, Cooled ceilings/displacement ventilation hybrid air conditioning system—Design criteria, in Proceedings of Spacevent 98, Stockholm 1998. 

R V.. Nielsen, Vertical temperature distribution in a space with displacement ventilation. In lEA Annex26 Elnergy Efficient Ventilation of Large Enclosures, Rome 1995. 

The dimensioning of displacement ventilation is shown in the example in Section 7.5.4. That given above is an example of displacement ventilation with weak thermal stratification. Even though the stratification is weak, the contamination in the lower, cleaner zone is normally on the order ol onc-rhird of the contamination in the upper zone. 

Figure 8.30 shows an example of displacement ventilation in a silicon carbide furnace room. Ihe thermal stratification is very strong, as indicated in the graph on the right-hand side of the figure. 

An example of displacement ventilation with cold contaminants is shoun in Fig. 8.31. The contaminants, i.c., the fumes from the sewer, are colder than the room air, and thus tend to stay low due to the negative buoyancy. 

The general rule is that when cuntaminants are warm, the exhaust should be located as high as possible in the room. Obviously, this applies in the case ot displacement ventilation, as shown in Fig 8.43. 

Heavy fumes or gases (i.e., negatively buoyant contaminants should be exhausted at floor level. Tbis applies to displacement ventilation as well as to mixing ventilation (Fig. 8.45). 

FIGURE 8.4S Wieo the contaminants are heavier than the room air, the exhaust should be at floor level, for displacement ventilation as well as for mixing ventilation. 

A amimon heating method in industry is using blowers that blow heated air into the room. See Fig. 8.57, An advantage is that the installation costs are low. A possible disadvantage is that it mixes the air in the room, which may not be desirable in combination with displacement ventilation. 

Horizontal displacement ventilation (see Chapters 7 and 8) is a ventilation principle mainly applied to general ventilation of workrooms. In some instances, a local ventilation problem may be solved by building a separate enclosure or a room around the workstation and arranging for a general ventilation in that enclosure. An example where that principle has been utilized is the control of emissions of and worker exposure to styrene vapors during... 

In displacement ventilation systems air is supplied to the room at low velocity, with a volume flow rate near the floor, and is extracted near the ceiling. The temperature of the supplied air is slightly lower than that of the room. Air is heated by the objects in the room, e.g., computer terminals and photocopying machines, and it rises due to buoyancy. 

One of the first simulations of displacement ventilation was presented in refs, 34 and 35. The predictions were compared with water-model experiments, and hence radiation was not taken into account. In ventilated rooms radiation should be taken into account. In ref. 36 plumes related to displacement ventilation were numerically studied. 

In displacement ventilation, there are regions with very low turbulence, and the flow can even be laminar. Hence it is important to use a turbulence model which can handle these regions. The k-f model gives rise to large numerical problems in regions of low turbulence. The reason is thar as k goes to zero, the destruction term in the e equation goes to infinity. The c equation is... 

Jacobsen, T. Airflow and remperature distribution in rooms with displacement ventilation. Ph.D. thesis. Dept, of Building Technology and Structural Engineering, Aalborg University, Aalborg, 1993. 

Most room models contain only one zone air node, thus assuming perfect mixing of the zone air and a homogenous temperature distribution in the space. Spatial temperature variations, such as vertical temperature gradients, are not considered. For specific applications such as displacement ventilation or atria, models with several zone air nodes in the vertical direction have been developed. ... 

Perfect mixing of the room air is assumed in the individual zone, except in room models with more than one air node (displacement ventilation model, atria model). 

Kosehenz M. Simulation of displacement ventilation in building energy codes [in German]. HI.H, No. 9, 1993. 

The model experiments were carried out in a model on the scale tjf I to iO. Experience with measurements on flow from wall-mounted dif(users for displacement ventilation indicates that it is possible to ignore the level of the Reynolds number at the given dimensions, which will enable reasonable temperature differences in the model experiments. 

Figure 12.41 shows the results of three experiments with a similar Archimedes number and different Reynolds numbers. The figure shows vertical temperature profiles in a room ventilated by displacement ventilation. The dimensionless profiles are similar within the flow rates shown in the figure, although the profile may involve areas with a low turbulence level in the middle of the room. A test of this type could indicate that further experiments can be performed independently of the Reynolds numbers. 

Figure 12.42 shows another example of the use of similarity principles in experiments. The temperature effectiveness ej is measured in a room ventilated by displacement ventilation. The measurements are made at different flow rates to the room, at different loads (from 100 W to 500 W) and by... 

P. V. Nielsen. Displacement ventilation in a room with low-level diffusers. Kdlte-Klima-Tagung. Deutscher Kalte- und Klimatechnischer Verein e.V, 1988. 

Displacement ventilation Room ventilation created by room air displacement, by introducing air at a low level in a space at a lower temperature than the room air. 

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