How water flows through the puck: pressure and speed.

At first glance, making espresso seems simple: you press hot water through finely ground coffee at high pressure. But what actually happens in the espresso puck is a highly complex interplay of pressure, flow and particle dynamics. Thanks to a flow simulation, we can better understand how water flows inside the puck - and why this is crucial for the perfect espresso.

The CFD simulations show the physical principles of extraction and explain why uniform compaction and a well-thought-out filter system are so important.

pressure distribution in the espresso puck

For most baristas it is clear: the pressure gauge must show 9 bar during extraction, otherwise something is wrong. On closer inspection it becomes clear that the coffee itself is only exposed to 9 bar (relative pressure) in a very thin zone. In the vertical direction within the puck the pressure drops from 10 bar (absolute pressure) to the atmospheric pressure at the bottom of the filter . This happens due to the friction between water and coffee particles, which leads to an intense pressure gradient .

What does this mean for extraction?

• A uniform pressure drop ensures that the water is pressed evenly through the puck.
• If there is less resistance in an area (e.g. due to uneven tamping), the water will flow preferentially there - a problem known as channeling .
• Homogeneous compaction, such as that achieved with the Barbro tamper , is crucial to ensure consistent extraction.

flow in the corners of the filter

The simulation results show that the water flow is weaker in the corners of the filter than in the center . This means that less extraction takes place in these areas. This is true for most filters because the ring of holes is usually smaller in diameter than the screen itself.

🔎 Why does this happen?

• The water seeks the path of least resistance.
• The middle of the filter has the most holes, which is why more water flows through here.
• Dead zones (green) appear at the edges, which lead to insufficient extraction . These zones cannot be prevented by tamping or any other type of preparation.

Solution:

• Some manufacturers are experimenting with filter screens that have more holes around the edges to minimize this effect.
• Barbro builds a variable, perfectly cylindrical sieve with interchangeable homogeneous porosities at the bottom.

Why does a donut-shaped flow occur?

The flow simulation shows that an annular flow is created at the filter outlet.

Why does this happen?

• Due to the lack of holes on the outer edge of the sieve, the flow from the edge area collects at the outermost holes, resulting in a higher flow velocity here.
• Directly next to the areas with low extraction there are areas with too high extraction.
  
• Incidentally, this phenomenon also occurs between the filter holes.

The simulation discussed here is based on the simplified assumption of a perfectly homogeneous porosity as an espresso puck with simultaneous homogeneous inflow of water from above. It therefore represents the ideal case.

The reality

In reality, the flow fields are far more complex due to an extraction-related phenomenon. The formation of the crema is based on the outgassing of CO2 from the water/espresso (sparkling bottle effect). However, as the pressure inside the puck decreases and water can absorb less CO2 at lower pressure, most of it escapes in the lower part of the puck. Here, the water flow is influenced or accelerated by the additional CO2 that is escaping. These are highly complex fluid mechanics phenomena that are very difficult to depict with models.