Aalborg Universitet

Modelling of Air Flow trough a Slatted Floor by CFD

Svidt, Kjeld; Bjerg, Bjarne; Morsing, Svend; Zhang, Guoqiang

Publication date:1998

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Citation for published version (APA):Svidt, K., Bjerg, B., Morsing, S., & Zhang, G. (1998). Modelling of Air Flow trough a Slatted Floor by CFD. Dept.of Building Technology and Structural Engineering. Indoor Environmental Engineering Vol. R9833 No. 89

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Modelling of Air Flow

through a Slatted Floor

by CFD

K. Svidt, B. Bjerg. S. Morsing, G. Zhang

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The Indoor Environmental Engineering papers are issued for early dissemination of research results from the Indoor Environmental Engineering Group at the Department of Building Technology and Structural Engineering , Aalborg University. These papers are generally submitted to scientific meetings, conferences or journals and should therefore not be widely distributed. Whenever possible , reference should be given to the final publications (proceedings , journals , e~c . ) and notto the Indoor Environmental Engineering papers .

Pri 11ted at Aalborg U11iversi ty

Modelling of Air Flow

through a Slatted Floor

by CFD

K. Svidt, B. Bjerg, S. Morsing, G. Zhang

AgEng Oslo98

Paper no: 98-B-038

Title:

EurAgEng

Modellipg of Airf1ow through a Slatted Floor by CFD

Authors:

· Kjeld Svidt Aalborg University, Denmark, http://iee.civil.auc.dkl-i6ks/

; Bjarne Bjerg Royal Veterinary and Agricultural University, Denmark, http://www.husdyr.kvl.dk/htm/bb/

: Svend Morsing Danish Institute of Agricultural Sciences, Denmark, Svend.Morsing@agrsci.dk

Guoqiang Zhang Danish Institute of Agricultural Sciences, Denmark, http://www.sp.dk/-gqzfcb/

§~:~:rrl1'J'!(.!IJ!:~ .... . In this paper two different CFD-approaches are investigated to model the airflow through a . slatted floor. Experiments are carried out in a full-scale test room . The computer simulations . are carried out with the CFD-code FLOVENT, which solves the time-averaged Navier-Stokes equations by use of the k-epsilon turbulence model. The simulations are two-dimensional and the slatted floor has been modelled in two different ways: A resistance volume and a resistance plane. The resistance volume has been defined as a volume covering the outer dimensions of the slatted floor i.e. 1.17m wide, 5.00 m long and 0.02 m thick. The resistance plane is defined in the same way except that it has no thickness. In both cases two parameters are used to

· specify the characteristics of the slatted floor: a free area ratio and a resistance coefficient. At · an opening area of 39% the predicted airflow rate is in the range of 6 to 12% less than measured. At lower opening area ratios there is an increasing difference between the two

· models. The differences are due to different airflow patterns resulting from the models.

1. Introduction In this paper two different CFD-approaches are investigated to model the airflow through a slatted floor. Computational Fluid Dynamics (CFD) is a suitable method for a detailed computer prediction of air velocities and temperature distribution in a ventilated space. With a CFD model it is possible to include the effect of room geometry and heat sources in the calculation of air flow in a building.

However, CFD is also a very compute intensive type of calculation, so for practical purposes it will normally be too expensive to model the finest details of the room geometry. The slatted floor normally has a geometry which is too complex to model in details, so it would be preferable to have a suitable coarse-grid representation of the slatted floor. This means that the model should be able to predict how the slatted floor affects the overall airflow field and contaminant distribution even though the local flow field through the slatted floor is not calculated in every detail.

The airflow through the slatted floor is important with regard to air quality and contaminant distribution, since the slurry surface below the slatted floor is a major contaminant source. Figure 1 shows a comparison of the flow in a building with solid floor and a building with a fully slatted floor where the opening area is assumed to be 30% of the total floor area. The calculated flow field shows, that the space below the slatted floor forms an integrated part of the ventilated space. This implies that gaseous contaminants released here may be transported to the occupied zone by the recirculating airflow.

~! ( r ' r 1

r 1

r 1

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. . ... - - - - - - - ---' ~ - - ---- - - -- - - -- -/

Figure 1 Calculated flow field in case of solid floor and slatted floor. Bold arrows indicate air inlet and exhaust.

2

1.0 ----~

1.5

------------------- ~ 1: ~:e~ ~-- ~

( 10% open)

~~-=-=-=-=-=-=-<~~~~- \-- --

Figure 2 Calculated contaminant concentrations in a room with the air exhaust above the slatted floor and below the slatted floor, respectively. The free area ratio of the slatted floor is 10%.

Contaminant concentrations are studied in figure 2. It is assumed that a contaminant is released at a constant rate from a surface 0.4 m below the slatted floor. Concentration levels are normalised with the exhaust concentration, i.e. a concentration level of 1.0 is the concentration that would be in a situation with completely mixed air. The contaminant distribution has been calculated with the exhaust opening above the slatted floor and below the slatted floor respectively. The figure shows that in this case with a free area ratio of 10 %, the exhaust position clearly affects the air quality in the occupied zone.

2. Methods Two different CFD-approaches to model the airflow rate through a slatted floor are compared with a full-scale laboratory measurement. Experiments are carried out in a full-scale test room as shown in figure 3. The room dimensions are 3.00 x 8.50 x 5.00 m. An inlet slot in the end wall covers the full width of the room. The inlet is located immediately below the ceiling and the inlet height is 19 mm. The air flow rate is 1300 m3/h. For the CFD-calculations an effective height of 15 mm was specified. An exhaust slot is located in the floor near the inlet-wall. The room is equipped with guiding plates below the ceiling to ensure a two-dimensional flow field (see Bjerg et al 1998). A channel covered by a slatted floor is located from 4.76 m to 5.93 m from the inlet-wall. The bottom of the channel is 0.42 m below floor level. The slatted floor is a 2 cm thick plastic type with 39% open area.

3

~ B.50 ___________,

Inlet slot "'.,..

T 3.00

l Exhaust slot

Figure 3 The full-scale test room has a slot inlet and exhaust, guiding plates below the ceiling and a channel-.covered with slatted floor.

The airflow rate through the slatted floor is measured by a tracer gas method with C02 as tracer gas. The measuring procedure is as follows: 1. An airtight cover is placed on the slatted floor. 2. While the ventilation system is running, C02 is supplied to the room above the slatted floor. The exhaust air is reused as inlet air to maintain a constant C02 concentration. 3. When the concentration is stable the cover is removed and concentration history is recorded in the channel (0.29 m below floor level) as well as in the room (0.20 m above floor level).

By this procedure the concentration progress is expected to follow equation 1:

eh- eo 1

-nr = -e ea -eo

where Ca is the concentration above the floor

eb is the concentration below the slatted the floor

eo is the background concentration

t is elapsed time in hours n is the number of air changes per hour

(1)

The computer simulations are carried out with the CFD-code FLOVENT, which solves the time-averaged Navier-Stokes equations by use of the k-epsilon turbulence model. The simulations are two-dimensional. The slatted floor has been modelled in two different ways: A resistance volume and a resistance plane. The resistance volume has been defined as a volume covering the outer dimensions of the slatted floor i.e. 1.17m wide, 5.00 m long and 0.02 m thick. The resistance plane is defined in the same way except that it has no thickness. In both cases two parameters are used to specify the characteristics of the slatted floor: a free area ratio and a resistance coefficient. The slatted floor affects the air flow by a pressure drop as used by Nielsen et al (1996, 1998), Svidt et a! (1998):

where (Jpj(Jx

f p

V

a P 1 2 -p V

dx 2 is the pressure drop [Palm]

is the loss coefficient is the air density [kg/m3

]

( 2)

is the local air velocity taking into account the free area ratio[m/s]

4

0.9

0.8

0.7 ~

't 0.6 (.) ;:;;:: 0.5 0 1 0.4

(.)

-- 0.3

0.2

0.1

0 0 0 0

o Measured concentrations

--Air exchange rate: 30 per hour

0~~~-+------+------+------+-----~

00:00:00 00:03:00 00:06:00 00:09:00 00:12:00 00:15:00 Time [hh.mm.ss]

Figure 4 The air exchange rate below the slatted floor is ~determined from the concentration history.

3. Results and discussion Figure 4 shows the normalised concentration history for the full-scale measurement. A theoretical curve based on equation ( 1) with n = 30 is shown together with the measured values. From this figure it is concluded that the air change rate below the slatted floor is 30 times per hour. This corresponds to 88.6 m3/h for the simulated channel.

The simulated air flow pattern using a volume resistance with a free area ratio of 0.39 and resistance coefficient f = 1 is shown in figure 5. The calculated airflow through a horizontal plane 0.02 m below ceiling level (i .e. the lower side of the slatted floor) is 83.2 m3/h for this case, which is 6% lower than the measured airflow rate.

To illustrate the sensitivity to the value of free area ratio and resistance coefficient, a number of simulations have been performed. Figure 6 and 7 show the calculated air change as a function of the two parameters. The two methods (resistance volume and the resistance plane) perform surprisingly different when the free area ratio is varied between 0.1 and 0.5. The difference may be explained by figure 8 which shows that the flow near the slatted floor is different for the two models with a free area ratio of 0.1. The resistance volume causes a local airflow immediately below the slatted floor which does not penetrate to the bottom of the channel, while the resistance plane results in a recirculating flow in the channel.

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... ] \ I

': ',! 1

0\[ '

=~- \ '!'!'. : •

I I I I I

. . ' · ... • ' · · 'f~\~~::~~~~~ :;:,~:;::·'·,';, l r · ~::-::.:::--;-.... ~;

Figure 5 An example of the simulated airflow pattern.

5

· . .,. · ....

I \

1. I

.. '

' i

:i i !

' I

!

! i l I

100

90 1

80

.t: 70 ..,-E 60 ai Cl 50 c::: 111 .t:

40 (J ... < 30

-o- Resistance volume

-o- Resistance plane 20

--Measurement 10

0 0 0.5 1.5

Resistance coefficient, f

Figure 6 Calculated air change rate or the two models at a free area ratio of 0.39.

160.0

140.0

120.0

..,rE 100.0 E

Cli Cl 80.0 c::: I'll .t: (J 60.0 ... Ci: -o- Resistance volume

40.0 -o- Resistance plane

20.0 :t:: Measurement

2

0.0 --~-------~-------r-------+-------~

0 0.2 0.4 0.6 0.8

Free area ratio

- -·--- - ----

Figure 7 Calculated air change rate or the two models with the resistance coefficient!= 1.0

6

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, 1 r I

, 1 I I

" - . - .... .. _,. / ·' I

Vector

0.500

0.429

0.357

0. 2 86 ""

0.21-:1

0.143

0. 071

0.000

Figure 8 Flow field below the slatted floor with a free area ratio of 0.1 . Top: Resistance plane model. Bottom: Resistance volume model.

4. Conclusions A volume resistance model and a plane resistance model to simulate the airflow through a slatted floor have been compared. At an opening area of 39% the predicted airflow rate is in the range of 6 to 12% less than measured. At lower opening area ratios there is an increasing difference between the two models. The differences are due to different airflow patterns resulting from the models. Further experiments with different types of slatted floor, including different opening area ratios, are necessary to determine the possibilities of the models.

5. References Bjerg, B., Morsing, S., Svidt, K., Zhang, G.: Measurement and Simulation of three-Dimensional Airflow in a Livestock Test Room with Two-Dimensional Boundary Conditions. Paper 98-B-037, Proceedings of AgEng98, International Conference on agricultural engineering, Oslo, Norway, 1998.

Nielsen, J.R., Nielsen, P.V., Svidt, K.: Obstacles in the Occupied Zone of a Room with Mixing Ventilation. Proceedings of RoomVent'96, Fifth International Conference on Air Distribution in Rooms, Yokohama, Japan, July 17-19, 1996, pp. 137-144.

Nielsen, J.R., Nielsen, P.V., Svidt, K.: The influence offurniture on air velocity in a room- an isothermal case. Proceedings of RoomVent'98, Sixth International Conference on Air Distribution in Rooms, Stockholm, Sweden, 1998.

Svidt, K., Zhang, G.,Bjerg, B.: Simulation of air velocity distribution in occupied livestock buildings. Proceedings of RoomVent'98, Sixth International Conference on Air Distribution in Rooms, Stockholm, Sweden, 1998.

7

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Complete list of papers: http://iee. civil.auc. dk/i6/publ/iee. h tml

ISSN 1395-7953 R9833

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