Airflow Concepts in Building Design

Cooling Methods

• Two main cooling strategies exist: passive natural ventilation and active mechanical ventilation.

Passive Natural Ventilation

• Methods include openings such as doors, windows, and shafts.
• Advantages of natural ventilation: inexpensive, energy-efficient and healthy.

Active Mechanical Ventilation

• Methods include air conditioning, fans, and air coolers.
• Disadvantages of mechanical ventilation: expensive and requires energy.

Natural Ventilation Principle

• Natural ventilation involves the exchange of air between the inside and outside of a building.

Natural Ventilation Use in Bahrain

• Hourly effective temperature (ET) data for Bahrain indicates optimal natural ventilation use from October to April.

Comfort Zone

• The comfortable temperature range in Bahrain is 22-27°C.

Airflow Principles

• Air always flows from high to low pressure.
• Air has mass and continues in its path until obstructed.
• Smooth airflow (laminar flow) occurs when adjacent air moves similarly; opposed currents become separated by intermittent eddies.
• Sharp changes in airflow produce turbulent flow.
• Bernoulli effect: accelerated air causes decreased pressure; greater distance covered compared to adjacent airflow.
• Venturi effect: constricted airflow increases speed.

Stack Effect

• Building air warms, density decreases, and the warmer air rises, which creates a pressure difference and encourages airflow toward open high points.

Cross Ventilation

• Cross ventilation requires an inlet and an outlet.

Factors Affecting Airflow

• Low-pressure regions form along windward construction sides and the leeward side of buildings.
• Tall structures in the wind path cause turbulence near them.
• Multiple low-pressure regions occur along linearly arranged buildings, resulting in ventilation shadow.
• High-pressure regions on windward sides are reduced when structures allow air to pass underneath.
• Airflow and ventilation improve when structures have openings on opposing sides.

Window Placement

• Best airflow arises when openings are on opposite walls.
• Ventilation can improve with openings on adjacent walls via wing walls.

Effective Window Open Area

• Measurements exist showing open area effectiveness of different window types (single-hung, double-hung, sliding, awning, casement, jalousie, hopper).

Inlet and Outlet Sizing

• A smaller inlet compared to the outlet enhances airflow. If the inlet is larger, the velocity of air within the building decreases.

Double Roof Strategies

• Roofs are primary sources of unwanted heat gain.
• Introducing a ventilated air space between the roof and the ceiling can reduce that gain.

Inlet Centering

• Inlets positioned centrally in walls restrict side airflow, making the direction of flow more linear.

Inlet Positioning

• Lower inlets maximize airflow across the occupied portion of a building.
• Outlet placement has minimal influence on airflow within the structure.

Interior Partitions

• Partition sizes affect airflow velocity near the inlet; larger partitions can increase the speed of air entering the building.

Case Study: Center Zion National Park (ZNP)

• The visitor center is located in Utah, 150 miles outside Las Vegas.
• Cooling primarily relies on natural ventilation via operable clerestory windows.
• Passive cooling is augmented by evaporative cool towers.
• Cool towers operate through natural convection primarily driven by buoyancy.

Cool Tower Operation

• Water is pumped to pads at the tower top.
• Cool air descends the tower through openings.
• Cool air enters the building, and hot air rises and exits via the clerestory windows.
• Energy requirements for the pumps are 249W per tower.