Choked Flow, Entrainment und Schockstrukturen – was in der Dampflanze passiert

Choked flow, entrainment and shock structures – what happens in the steam wand

Frothing milk looks simple from the outside: inject steam into milk, done. But inside the steam wand, the physics looks more like a jet engine nozzle than kitchen equipment. Three key concepts explain why some wands produce silky microfoam and others just noisy bubbles: choked flow, entrainment, and shock structures.

Choked Flow – the “Throttle Point” of Steam

  • From a critical ambient pressure/boiler pressure ratio of approximately 0.55, the flow in the nozzle throat reaches Mach 1.
  • At a certain pressure ratio, the velocity in the nozzle throat reaches the speed of sound. From this point on, the steam mass flow is fixed by boiler pressure and nozzle size, not by what happens in the milk.
  • This effect, called choked flow, is why steam wands provide a consistent, predictable amount of steam – the foundation for reproducible frothing.

Entrainment – Dragging Air and Milk Along

Once the high-speed jet exits the nozzle, it drags surrounding air and liquid with it – this is entrainment.

  • Stretching (pulling): With the wand tip just under the milk’s surface, air is pulled in, creating small bubbles.
  • Rolling: With the wand deeper, no new air is drawn in, but the milk circulates strongly, breaking down bubbles and distributing them evenly.
  • Milk surface: This is the critical interface. Too high above it, and you get noisy big bubbles; too deep, and no air is added at all.

Shock Structures – Miniature Waves in the Steam Jet

When steam expands beyond the critical point, it can reach supersonic speeds. To adjust back to ambient pressure, the jet forms compression shocks – similar to the “shock diamonds” you see in rocket exhausts.

  • Low-pressure zones between shocks enhance air entrainment during the stretching phase.
  • Compression zones increase turbulence, shredding larger bubbles into finer ones.

Takeaway

  • Choked flow gives a stable, consistent steam output.
  • Entrainment explains how air is introduced during stretching and how milk is circulated during rolling.
  • Shock structures act like an invisible mixer, refining bubbles into silky microfoam.

This combination of high-speed physics explains why a simple steam wand can behave like a miniature jet engine — and why it sometimes hisses and growls while creating that glossy milk foam essential for latte art.

Back to blog