Analysis of airflow in a full-scale room with non-isothermal jet ventilation using PTV techniques

ASHRAE Transactions, Jan, 2007 by Lingying Zhao, Yuanhui Zhang, Xinlei Wang, Gerald L. Riskowski

The temperature difference is caused by ambient weather conditions and indoor thermal comfort requirements. Decreasing temperature difference is an effective strategy to solve winter ventilation difficulties. Warming up the inlet air by means of a heated hallway and passing the ambient air through an attic are practical ways to increase inlet air temperature, thus decreasing the difference in air temperature. Air-to-air heat exchangers or geothermal heat pumps may also be feasible options with today's high energy cost environment.

To evaluate the strategy for improving airflow patterns under winter ventilation conditions by reducing the inlet opening and simultaneously increasing the inlet air velocity, Test 5 was designed. Test 5 ventilation settings are similar to those of Test 3, except that the inlet opening decreased to a minimum limit of 6 mm (0.24 in.) so that the inlet air velocity was increased to 3.3 m/s (650 fpm), similar to that of a typical winter inlet opening of a swine building.

Figure 12 shows the airflow patterns of Test 5. Contrasting with the airflow patterns of Test 3 (Figure 8), it was found that the air jet was lifted up to the ceiling and traveled about 1.5 m (5 ft) from the inlet wall before it began to drop. Due to insufficient jet momentum and temperature difference with the resulting buoyancy effects, however, the inlet air jet still separated shortly after it was discharged into the test room and the air jet fell into the animal occupied zone. The draft was strong in the room. Two major rotating zones formed: one a clockwise rotating flow and the other a counterclockwise rotating flow. In the clockwise rotating zone, there were a few weak vortexes showing that the airflow was more turbulent. The overall floor-level velocity was also larger than that which occurred under isothermal ventilation conditions. A clear vortex formed on the other side of the test room.

[FIGURE 11 OMITTED]

By analyzing the winter non-isothermal ventilation cases, one can appreciate that it is not easy to design and manage winter ventilation systems for animal facilities. From Test 5, which was designed to improve airflow patterns in winter ventilation conditions, it was found that there is a limit to increasing inlet air velocity by decreasing the inlet openings. Inlet openings of 6 mm (0.24 in.), the minimum in realistic buildings, resulted in a maximum inlet air velocity of 3.3 m/s (650 fpm) with a typical 8.6 ach air exchange rate. The 3.3 m/s (650 fpm) inlet air velocity is not large enough to form a stable airflow pattern. An inlet air velocity of 1.6 m/s (315 fpm) was reported in Zhang et al. (1992) as the limit in forming stable airflow patterns under non-isothermal ventilation conditions with an 8[degrees]C (15[degrees]F) temperature difference. Therefore, inlet air velocities forming stable airflow patterns depend on the temperature difference. Because of the minimum inlet opening limit, this strategy did not resolve the winter ventilation challenge.