S. Morsing

Modeling of air inlets in CFD prediction of airflow in ventilated animal houses

Abstract This study investigates different methods to model wall inlets in computational fluid dynamics (CFD) simulations of airflow in livestock rooms. The experiments were carried out in an 8.5 m long, 3 m high and 10.14 m wide test room equipped with a forced ventilation system. Four wall inlets were distributed symmetrically along an end wall 0.5 m beneath the ceiling. To obtain uniform and easily modeled boundary conditions the inlets were designed as rectangular frames with an elliptic profile in the contraction section following the ISO standard for a long-radius nozzle. Vertical and horizontal air speed profiles in the jets were measured with thermistor speed sensors at four distances from the inlets and an ultrasonic sensor was used for measurement of air velocity in the occupied zone close to the floor. CFD-simulations with the k–e turbulence model were carried out with a number of different grid constructions. Both measurement and CFD simulations showed that two different airflow patterns occurred in the test room. In airflow pattern 1 the jets beneath the ceiling turned towards the symmetry plane of the room and above the floor the air flowed away from the symmetry plane. In air flow pattern 2 the jets turned away from the symmetry plane and above the floor the air flowed towards the symmetry plane. The findings in this study indicate that assuming two dimensional (2-D) inlet conditions might be a useful way to simplify inlet boundary conditions and grid constructions for prediction of air flow in the occupied zone of livestock rooms with many wall inlets. However, more work must be done to evaluate this statement in other arrangements, including changed orientation and locations of inlets, unattached jets and non-isothermal conditions.

BUOYANT FLOW GENERATED BY THERMAL CONVECTION OF A SIMULATED PIG

The effects of internal occupants and supplement heating make up an essential issue for the prediction and control of fresh ventilating air distribution in an enclosure. The influence from livestock is complex, since they are mobile obstacles, producing heat and contaminants in irregular geometry. As a part of the basic studies of these influences, the investigations of air motion in a thermal buoyant flow caused by free convection around a livestock body are reported in this article. A simulated pig, made of a painted metal tube (1 m long and 0.5 m in diameter) with covered ends and heat elements inside, was used as the heat source in the experiments. The experiments were carried out in a full-scale room, 5 m iA11 m in floor area, with a 2.4 m side wall height, and sloped ceiling to center (height: 4.8 m). The simulated pig was placed near the center of the floor. The vertical temperature difference in the room space was less than 0.3iaC. The velocity and temperature in the thermal plume were measured with six sensors (each has both temperature and velocity elements) placed at 0.2 m horizontal intervals. Data were acquired at 14 levels from 0.2 to 2.4 m above the top surface of the simulated pig. The data-sampling period was 30 min in steady state for each measurement. The results show that the plume was quite thin at the beginning (yd iU 0.6 m) in the central radial plane of the model. Observations showed that the laminar flow at the beginning remained for some distance before it became turbulent and spread. When the distance from the top of the model increased (yd iÝ 0.8 m), the temperature and velocity profile of the jet fit Gaussian distributions. The temperature profiles were slightly wider than the velocity profiles. Numerical simulations (Computational Fluid Dynamics) were applied for the same experimental set-up in computing the airflow over the pig simulator. Transient simulation with fixed time stepping provided similar results to the measurements, indicating the CFD simulation method used in the study has potential for prediction of buoyant flow generated by this type heat source.

PREDICTING NEAR–FLOOR AIR VELOCITIES FOR A SLOT–INLET VENTILATED BUILDING BY JET VELOCITY DECAY PRINCIPLES

Air velocities near the floor of slot–ventilated buildings are the result of interaction between the supply air jet and the geometry of the confined space. To predict air velocity, the effect of the inlet air jet momentum and the room geometry has to be included. Air movements in a 5 m wide × 3 m high × 8.5 m deep empty slot–ventilated room were studied for two different jet momentum situations under isothermal conditions. A negative–pressure ventilation system supplied air through a slot wall inlet with an adjustable flap. The inlet was installed beneath the ceiling. The exhaust air was taken out through a section of slatted floor along the inlet wall. Air velocity distribution in the room was recorded with a multi–channel measurement system. To study the center velocity decay of the incoming jet and the return airflow, the data used in this report are concentrated on the measurement points near the ceiling and the floor. The experiment showed that a given near–floor air velocity may be generated by choosing a proper supply jet momentum. Three–dimensional effects occurred in the symmetrical, empty room. Mean velocity prediction in the near–floor region was analyzed on the basis of supply jet momentum and room geometry. The prediction was accurate for the maximum near–floor air velocity that occurred near the end wall opposite the inlet. As the air returned along the floor towards the inlet wall, the measured velocity became consistently lower than the predicted value.

Modeling Jet Drop Distances for Control of a Nonisothermal, Flap-adjusted Ventilation Jet

In order to find criteria which can be used to control the trajectory of the air jet into a ventilated airspace, modeling the drop distance using the Archimedes number is proposed. The term “jet drop distance”, defined as the horizontal distance from the inlet to the point where the jet reaches the occupational zone, is used to describe the status of the jet. Following an analysis of jet trajectory models for round inlet openings, jet drop distance models for a rectangular inlet with a bottom hinged flap are described. The models are functions of the inlet Archimedes number, the inlet opening hydraulic diameter and the inlet height above the floor.

AIR VELOCITY AND TEMPERATURE DISTRIBUTION IN A COVERED CREEP FOR PIGLETS

A covered creep is primarily used to protect temperature-sensitive piglets. Knowledge of the air motion and the temperature distribution in a covered creep space is important for the optimization of the creep configuration. Experiments were carried out to investigate the temperature distribution in a covered creep and the velocity generated in the front opening as a function of heat level, heat location, and front opening configuration. Heating panels on the floor simulated the heat produced by piglets. The inside dimensions of the experimental creep were 1 m wide, 0.6 m high, and 1.8 m long. It was made from 50 mm polystyrene foam panels for floor, walls, and ceiling. The front configuration was either open or restricted with a top plate and bottom board. The experiments showed that significant vertical temperature stratification took place in the creep. The degree of temperature stratification depended on heat level, heat location, and front opening configuration. The maximum temperature rise, defined as the temperature difference between the creep air and the surrounding room air, was in the range 4.1.C to 8.8.C for heat levels of 140 to 420 W with open front. Location of the heat resulted in local effects within the covered creep, but had only minor effects on the velocity and temperature profiles at the front opening. Different front opening configurations influenced the temperature rise. The maximum temperature rise recorded was in the range 9.6.C to 14.8.C for the restricted fronts, and thus considerably higher than with an open front. Measured air velocities at the front opening were generally not in excess of 0.2 m/s, except near the ceiling. Both theoretical estimation and measurements showed that the air velocities out of the upper part of the front opening were notably higher than the air entering into the creep near the floor level. The estimated height of the neutral axis was close to the measured results.