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

Air Velocities at the Human Breathing Zone and Animal Occupied Zone. Air velocity directly affects temperature distribution, thermal comfort, and air quality. Ultimately, the goal in studying airflow patterns of typical animal buildings is to optimize the thermal and air quality environments around animals and human workers. Regional characteristics of airflow, especially in the animal occupied zone and at the human breathing level, are most critical.

The ASAE (2001) Standard D321.2DEC94 provides typical dimensions for various animal species. For pigs, 0.7 m is defined as the highest point of a standing pig. Many researchers used 0.0 to 0.6 m (0 to 2 ft) as the animal occupied zone in swine buildings. Based on the ASAE standard and other research literature, the animal occupied zone was defined as 0.0 to 0.6 m (0 to 2 ft) from the floor and the human breathing level 1.4 to 1.7 m (4.6 to 5.6 ft) above the floor in this study.

Figure 11a shows a comparison of air velocities at the human breathing level under winter non-isothermal and isothermal conditions. It is clear that under non-isothermal ventilation conditions, due to cold drafts and floor heat, the air velocities at the human breathing level were much higher than for the isothermal ventilation cases. In winter, this velocity increase is not desirable. Because of counter primary rotating airflow patterns in the non-isothermal ventilation cases, the human breathing level has a reverse airflow. The reverse airflow resulted in air entrainment and higher air velocities at the human breathing level as it traveled toward the inlet wall. The air velocity fluctuated due to turbulence activities. Air velocities dropped dramatically at one place near the opposite wall because of the large vortex. Under the correlated isothermal ventilation case, because of primary rotating airflow patterns, air jets attached to the ceiling and gradually entrained room air. Therefore, air velocities at the human breathing level increased as the air jet traveled from the air inlet along the width of the room.

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Figure 11b shows a comparison of air velocities in the animal occupied zone under winter isothermal ventilation and the correlated isothermal ventilation. Large differences in air temperature resulted in higher air velocities and large velocity fluctuations in the animal occupied zone. Because of a counterclockwise rotary airflow pattern, air velocities at the animal occupied zone decreased as the airstream traveled from the inlet wall to the opposite wall. In the isothermal ventilation case, airflow in the animal occupied zone was dominated by reverse airflow of the primary rotating airflow patterns. Because of friction of the floor and viscosity of airflow, the air velocity decreased as the reverse airflow progressed.

Evaluation of Strategies to Improve Winter Ventilation

Room airflow patterns are mainly affected by the air jets present in the mixing ventilation. Under non-isothermal ventilation conditions, air jet separation behavior is determined by Archimedes number. Critical Archimedes number (Ar) is a limit Ar value at which the diffuser air jet drops immediately after entering the room. An Ar less than the critical Ar is needed to create full rotary airflow in a ventilation air space. By analysis of the Archimedes number definition (Equation 1), we can learn that there are three ways to decrease Archimedes number: (1) decreasing temperature difference, (2) decreasing inlet opening to increase air velocity, and (3) increasing inlet air velocity by increasing airflow rate. In winter, the minimum ventilation rate is determined by maximum conservation of heat energy and optimum control of moisture and air quality.