THE CASE-STUDY: THE NUMERICAL CFD SIMULATIONS

In the first stage, a very common configuration (both for new buildings and for existing ones) was hypothesised:

• stairwell door at base closed; operable hinged hopper type window mounted at the top of the main entrance (generally closed in winter);

• small operable hopper window at each half landing (partially open);

• stairwell door at roof closed; exhaust air device mounted above this door;

• full louvered opening at the roof level (wind sheltered);

• full louvered narrow openings above the entrance door of each flat (all openings are characterized by the same dimensions and pressure losses).

The CFD simulations highlight that (figures 4, 5):

• the neutral pressure level (NPL) is located near the mid-height of the building;

• air is extracted from the flats below the neutral plane and is supplied to the apartments above the neutral plane; this mechanism involves a different airflow rate on each floor and poor indoor air quality at the higher levels;

• cold air enters the stairwell from the windows at the half landing level causing thermal discomfort and thermal losses.

In the second step, the windows at the half landing levels were considered closed. The temperature in the stairwell is higher, but the main behaviour is the same as the first case (see fig 5, step 2).

SHAPE * MERGEFORMAT

Fig.5: Velocity and temperature fields of steps 1 and 2.

3

Speed

iTempeiature

Speed

(rn/s)

ШВ?1-

,n. V, ■

2.4385

■ за?:’.-;-

2.15756

■ 177778

: 8Э661

■ 15 5556

: 62567

Щ13 3333

1.35472

11 1111

1.03378

Mjf 8. S8E89

0.812834

^ 6.66667

0.54189

■ 4/4444

0.270945

■ 2.22222

0

Я Q.

Fig.6: Velocity and temperature fields of steps 3 and 4.

In the third step the window at roof level was oversized in order to move the neutral pressure level upward. The results shown in fig. 6 highlight that the air is extracted from all the dwellings. The air exhausted from dwellings rises in the stairwell flowing along two main patterns: the first flow path is helicoidal in shape (just below the flights of stairs and landings), the second one is a vertical path along the core of the stairwell. The velocities are low and the exhausted air seems not to interfere with the occupied zone.

The calculated flow rates (see table 1) range from 0.027 to 0.440 m3/s, that is about 0,33­5.28 ACH (dwelling net volume = 300 m3). The above values are evaluated in favourable conditions (minimum external temperature in winter), so the minimum air change rate may be not enough for ventilation purposes under higher external temperatures.

STEP 1

STEP 2

STEP 3

STEP 4

STEP 5

T ext = 0°C

T ext = 0°C

T ext = 0°C

T ext = 0°C

T ext = 8°C

Opening

H [m]

Q

[mA3/s]

P

[Pa]

Q

[mA3/s]

P

[Pa]

Q

[mA3/s]

P

[Pa]

Q

[mA3/s]

P

[Pa]

Q

[mA3/s]

P

[Pa]

flat 1st floor, sx grille 100 X 20 cm

5.60

Q1

0.289

-3.717

0.396

-6.988

0.437

-8.507

0.177

-9.513

0.141

-5.986

flat 1st floor, dx grille 100 X 20 cm

5.60

Q’1

0.291

0.399

0.440

0.177

0.140

half landing hopper 100 X 30 cm

6.10

Q1-2

0.318

flat 2nd floor, sx grille 100 X 20 cm

8.90

Q2

0.235

-2.456

0.325

-4.676

0.376

-6.280

0.222

-7.550

0.175

-4.700

flat 2nd floor, dx grille 100 X 20 cm

8.90

Q’2

0.236

0.324

0.381

0.222

0.175

half landing hopper 100 X 30 cm

9.40

Q2-3

0.255

flat 3rd floor, sx grille 100 X 20 cm

12.20

Q3

0.162

-1.162

0.235

-2.460

0.300

-3.989

0.245

-5.370

0.193

-3.320

flat 3rd floor, dx grille 100 X 20 cm

12.20

Q’3

0.165

0.238

0.303

0.245

0.193

half landing hopper 100 X 30 cm

12.70

Q3-4

0.167

flat 4th floor, sx grille 100 X 20 cm

15.50

Q4

-0.024

0.057

0.081

-0.292

0.212

-1.989

0.231

-3.191

0.182

-1.984

flat 4th floor, dx grille 100 X 20 cm

15.50

Q’4

-0.043

0.088

0.217

0.231

0.184

half landing hopper 100 X 30 cm

16.00

Q4-5

-0.016

flat 5th floor, sx grille 100 X 20 cm

18.80

Q5

-0.180

0.524

-0.210

0.702

0.027

-0.037

0.153

-1.044

0.121

-0.650

flat 5th floor, dx grille 100 X 20 cm

18.80

Q’5

-0.188

-0.218

0.030

0.158

0.124

half landing hopper 100 X 30 cm

19.30

Q5-top

-0.248

roof — full louvered 260 X 90 cm

22.45

Qtop

2.562

3.755

-2.844

0.822

-2.224

0.515

-1.721

0.311

roof sx — full louvered 100 X 90 cm

22.45

Q’top

roof dx — full louvered 100 X 90 cm

22.45

Q»top

-1.456

-1.763

1.591

Further simulations were run in order to equalize and optimise the flow rates, by adding pressure losses at the unit entrance ventilation openings (step 4). The main results are presented in fig. 6. In addition the external temperature was set to 8°C in order to evaluate the airflow-rate changes (see step n.5 in table 1).

Tablel: Airflow rate from each opening and pressures.

The minimum air change rate is about 1.8-1.5 ACH, above the normal ventilation needs in winter for residential units [1]. Each occupant can manually adjust the air-inlets of some or all the rooms to suit personal requirements.