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

ABSTRACT

Draft and non-uniform fresh air distribution are common problems in winter ventilation, especially for large animal buildings. Thermal-based anemometers have difficulties in accurately measuring non-isothermal and low-speed indoor airflow. A new technology, particle tracking velocimetry (PTV), which uses particles and their images to study indoor airflow, can overcome the traditional limitations in indoor airflow measurement. A PTV system was used to characterize indoor airflow in a full-scale ventilated room under nonisothermal ventilation conditions. Non-isothermal mild weather and winter ventilation conditions were simulated to analyze their effects on indoor airflow and air velocities in animal occupied zones and human breathing zones. It is found that winter ventilation created a totally reversed rotating airflow pattern. Air velocities in the animal occupied zone increased substantially compared with the corresponding isothermal ventilation conditions. Winter ventilation strategies for improvement of airflow distribution were studied: (1) increasing inlet air velocity, (2) increasing inlet air jet momentum by use of air recirculation devices, and (3) decreasing ventilation temperature difference.

INTRODUCTION

Indoor air quality of large-scale animal production facilities is increasingly recognized as important to the health, well-being, and productivity of building occupants. Ventilation is one of the major means of controlling the indoor environment and indoor air quality. In summer, indoor air temperatures of animal buildings are generally within 3[degrees]C of inlet air temperatures. It is generally accepted that summer ventilation can be assumed "isothermal" for most practical engineering purposes. In mild and cold weather, room air temperatures tend to be much higher than the inlet air temperature due to heat production from animals and equipment. Supply air is not heated during winter for most production animal facilities and thus is much colder than the room air. Therefore, non-isothermal ventilation is typical in realistic buildings during heating seasons. Draft and non-uniform fresh air distribution are common problems with winter ventilation, especially for large-scale animal buildings. Understanding airflow patterns under non-isothermal ventilation conditions, especially those typical in winter, is important in evaluating ventilation system effectiveness and in developing environmental control strategies.

Ventilation research has been extensively studied over the past two decades (Nielsen et al. 1978; Timmons 1984; Sandberg 1987; Zhang et al. 1992; Jin and Ogilvie 1992; Riskowski et al. 1993; Heber and Boon 1993; Wang and Ogilvie 1996). It should be noted that most ventilation study cases simulated isothermal ventilation because it is simpler to measure, model, and analyze. Few studies were conducted on non-isothermal ventilation.

Under non-isothermal ventilation conditions, separation of the inlet air jet from the ceiling is mainly affected by the inertial force of the air jet, which is the result of jet momentum and buoyancy force caused by heating loads. The room inlet Archimedes number (Ar), defined in Equation 1, is the ratio of thermal buoyancy force to inertial force. Air jet separation behavior is determined by Archimedes number. Critical Archimedes number is a limit value at which the diffuser air jet drops immediately after entering the room. Zhang et al. (1992) indicated an equivalent critical Ar value of 0.005 for the test room.

[A.sub.r] = [g[beta][DELTA][T.sub.0][D.sub.0]]/[U.sub.0.sup.2] (1)

where

[beta] = thermal expansion coefficient of air, 1/K;

g = acceleration of gravity, 9.8 m/[s.sup.2];

[DELTA][T.sub.0] = air temperature difference of inlet air and room air, K;

[U.sub.0] = inlet air velocity, m/s; and

[D.sub.0] = inlet width, m.

Randal and Battams (1979) found that a corrected Archimedes number (Arc) of inlets can predict airflow patterns of buildings under non-isothermal ventilation conditions. Zhang et al. (1992) intensively studied non-isothermal ventilation with slot inlet and outlet and revealed that both the critical Archimedes number (Ar) and the critical Arc for full-scale building ventilation were affected by room height. Heber and Boon (1993) studied air velocity characteristics in a full-scale simulation livestock building with non-isothermal jet ventilation and a high ventilation rate by using three-dimensional ultrasonic anemometry. Wang and Ogilvie (1996) further studied non-isothermal ventilation and developed critical wall jet Archimedes numbers (Arm), which directly reflect the balance of initial force and thermal buoyancy force for full rotating airflow patterns. However, in previous research, non-isothermal ventilation was simulated merely by adding a heat load onto the floor while keeping a normal inlet temperature. The temperature differences were 7[degrees]C-15[degrees]C (45[degrees]F-59[degrees]F). Therefore, typical winter ventilation of animal facilities has not been fully studied. Zhang and Strom (1999) developed jet drop models for non-isothermal free jets without detailed airflow measurement.