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

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The general structure of these airflow patterns agrees with those of Zhang et al. (1992), which had exactly the same experimental settings but used a hot-wire anemometer for air velocity and a smoke gun for airflow pattern. One exception is that the PTV airflow patterns have more detailed structure and definitive airflow direction data. Another exception is that Zhang et al. (1992) showed that a reverse airflow pattern formed below the air jet under the isothermal ventilation, whereas the PTV-measured airflow pattern (Figure 6) showed not reverse airflow but a stagnant zone. This discrepancy may be caused by limitations of the smoke gun flow observation in low airspeed zones and the high turbulence level of the low airspeed zone. In summary, these results confirmed that the room airflow PTV system overcomes difficulties of traditional airflow measurement and visualization, yielding clearer and more quantitative airflow pattern results. It can effectively be used to study airflow of non-isothermal ventilation.

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Air Velocity Distribution. Figure 6 shows the air velocity distribution of Tests 1 and 2 over the center section of the test room. The non-isothermal ventilation case with a small temperature difference had the same range of air velocities in the air jet and in the animal occupied zone as its isothermal counterpart. However, because of the temperature difference, the air jet was heavier than the room air. The air jet did not travel the whole room width and dropped early. According to ASHRAE (2005), air velocity greater than 25-30 fpm (0.1-0.15 m/s) with 60% turbulence intensity will cause cold draft conditions for 15% of the population. Indoor airflow of large animal buildings is fully turbulent, with turbulence intensity of up to 97% (Zhang et al. 1992). Therefore, when air velocity in the human breathing zone increases to 0.3 m/s (59 fpm) due to the air jet drop, it is likely that a cold draft will form at the human breathing zone in the center of the room. This also can be verified from temperature distribution (Figure 3). However, the jet drop did not significantly affect air velocities near the floor, which is normally recognized as the animal occupied zone in animal buildings.

Typical Winter Ventilation Conditions

Figure 7 shows the temperature spatial distribution of Test 3, which represents typical winter ventilation conditions. The inlet air temperature was only -2[degrees]C (28[degrees]F) when it entered the room. The temperature distribution pattern with low air temperature near the inlet wall and floor showed that the cold air jet dropped and traveled in a counterclockwise direction. The air temperature at the center space of the room rose quickly due to turbulent mixing and thermal diffusion between cold jet air, the heated floor, and warm indoor air. There is a high-temperature zone in the room near the air inlet.

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Airflow Patterns. Figure 8 shows the airflow patterns of Test 3, a typical winter non-isothermal ventilation test case. Figure 9 shows the airflow pattern of Test 4, which is the correlated isothermal ventilation case of Test 3. When a heated hallway is used in a swine building to preheat the inlet air, the winter ventilation case will likely be as in Test 4, an isothermal ventilation case with a low air ventilation rate.