Why Fluid Dynamics Determines Separation Performance

An air wall does not work simply because air exits an opening at high speed. The decisive question is what remains of that flow at the critical point of the opening.

Critical Areas of the Opening

The relevant point is not only the discharge velocity directly at the nozzle. What matters is how much usable flow energy remains where shielding is actually required. At open doors or entrances, this is often the lower part of the opening, because cold air enters there and drafts are felt there. If air is blown from the side, the middle area of the opening is also relevant.

Wind presses against the opening from outside, and depending on the building condition, more or less air enters. If the wind speed is about 3 m/s, the air jet must still have significantly more velocity at this point. Based on practical experience, we typically work with around 5 m/s at the critical area.

The critical areas also depend on the application. In cold storage, the objective is often to prevent warm outside air from entering the cooled area. In a heated building, the focus is more on warm indoor air escaping and cold outdoor air entering. In waste treatment, composting or process air applications, the critical point may be where odors, dust or contaminated air would leave the area.

Extraction systems, fans and temperature differences can also influence the critical area.

Pressure, Velocity and Momentum

The Navier-Stokes equations describe the movement of air as a real fluid. For an air wall, they are more relevant than simple ideal models because they do not only consider pressure and velocity, but also friction, shear, turbulence, density and external forces such as gravity.

In simple terms: pressure differences accelerate air. Air density influences mass flow and momentum. Velocity creates movement and effect. Friction, shear and turbulence reduce this effect. This is exactly what matters for an air wall.

At the edge of the free air jet, a boundary or mixing layer forms. Fast jet air meets slower ambient air. The jet transfers momentum to the surrounding air, entrains it and loses velocity, momentum and usable flow energy.

These losses are reduced by uniform pressure distribution, a defined discharge direction, calmer flow before the nozzle and suitable nozzle geometry.

In practice, the air jet must be generated so that it retains as much direction, velocity and momentum as possible over the required distance. Measuring high velocity at the outlet is not enough. If the jet slows down too much, becomes turbulent or deviates sideways on its way through the opening, too little usable flow energy reaches the critical area to act against incoming or outgoing air.

Pressure Chamber and Nozzle

We work with significantly higher pressures than many conventional air curtains.

High pressure is the prerequisite for accelerating air in a controlled way and discharging it through a defined nozzle as a directed air jet. The fan builds up the pressure; the pressure chamber distributes it along the length of the nozzle. The nozzle converts this pressure into directed flow velocity. Its geometry helps ensure that the air jet exits as uniformly and orderly as possible and that the streamlines follow the intended direction.

For an air wall, this is decisive because the goal is not to move as much air as possible. The goal is to create a stable air jet. The air jet should exit uniformly along the full length of the nozzle, have a defined direction and still carry enough flow energy and momentum at the critical point of the opening.

The SAW-Airwall therefore uses a pressure module with a defined plenum chamber and nozzle geometry. The plenum chamber has two main functions.

First, it should provide a pressure level that is as uniform as possible along the entire nozzle. Only then can the air jet exit evenly along the full length. Without this pressure distribution, local differences occur: more air exits at some points, less at others. The air barrier becomes uneven and loses effect.

Second, the plenum chamber aligns the air before the nozzle. The air should not exit randomly, diagonally or with local turbulence. It should be guided outward in a defined way. The direction of the air jet is decisive because an air wall can only separate effectively if the flow exits as orderly as possible and with low transverse components.

High pressure is not an end in itself. The decisive point is how efficiently and uniformly this pressure is converted into an effective air jet through the nozzle geometry. Our optimized nozzles are patent-pending and achieve up to 40% higher discharge velocity at the same energy input compared with the previous nozzle design, measured according to ISO 27327-1:2009.

The continuously narrowing cross-section gradually accelerates the air before it leaves the nozzle at the narrowest point with the highest velocity.

We have already realized and measured 50 m/s. This corresponds to about Mach 0.15. The flow therefore generally remains approximately incompressible, but it is already in a range where further velocity increases must be considered carefully from an airflow engineering perspective.

Why a Narrow Slot Alone Is Not Enough

A narrow discharge slot helps create a concentrated air jet. Mixing starts at the outer layers of the jet. In the core of the jet, kinetic energy can be retained for longer. This allows the air jet to remain more stable over a greater distance.

However, this only applies within a sensible design. An ever smaller slot and ever higher discharge velocity do not automatically create greater reach. Very small slots can generate high velocities, but they can also lose energy very quickly if air volume, jet shape and pressure management do not fit together.

Very fast, small air jets can be strong over a short distance, but they are not automatically suitable for creating a stable air barrier over several meters. For an air wall, a single parameter such as slot width or discharge velocity is not decisive.