The low-pressure environment of an RH vacuum furnace fundamentally alters jet dynamics by creating an extreme pressure differential. Because the ambient pressure is remarkably low (typically 1,000–4,000 Pa), the oxygen jet becomes "extremely under-expanded" immediately upon exiting the nozzle. Instead of maintaining a focused stream as it would in the atmosphere, the jet is forced to expand rapidly to equalize with the vacuum environment.
Core Takeaway The vacuum environment prevents the oxygen jet from dissipating normally; instead, the massive pressure drop triggers the formation of Mach disks. These shock wave structures paradoxically extend the jet’s supersonic core, allowing it to maintain high impact capability over much longer distances than possible under standard atmospheric conditions.
The Mechanics of Under-Expansion
The Pressure Differential
In an RH vacuum refining furnace, the environment is maintained at a very low pressure, specifically between 1,000 and 4,000 Pa.
This creates a massive contrast with the high pressure of the oxygen inside the lance.
This difference is the primary driver of the jet's morphology.
Rapid Volumetric Expansion
When the high-pressure oxygen exits the nozzle into this vacuum, it cannot remain contained in a narrow column.
The jet expands rapidly outwards in an attempt to equalize with the low surrounding pressure.
Technically, this state is defined as being extremely under-expanded.
Structural Changes to the Jet
Generation of Shock Waves
The rapid expansion of the gas is violent enough to generate strong shock wave structures within the jet itself.
The most critical of these structures are known as Mach disks.
These disks are essentially standing shock waves that occur when the flow is highly supersonic and under-expanded.
Extension of the Supersonic Region
While expansion typically implies a loss of focus, the formation of Mach disks alters this dynamic.
These shock structures significantly extend the potential flow core of the jet.
This elongation effectively stretches the supersonic region of the gas flow further away from the nozzle exit.
Understanding the Physical Trade-offs
Expansion vs. Impact
It is a common misconception that a rapidly expanding jet loses its kinetic energy quickly.
In this specific low-pressure context, the trade-off works in favor of the process.
While the jet does expand radially, the accompanying shock structures preserve the jet's coherence along its centerline.
The "Long Distance" Phenomenon
The presence of Mach disks creates a unique operational characteristic: maintained impact capability over long distances.
Without this vacuum-induced shock structure, the jet might disperse too quickly to be effective.
The vacuum environment, therefore, acts as a mechanism to preserve the jet's striking power against the molten bath, despite the physical distance between the lance and the surface.
Making the Right Choice for Your Goal
To optimize the refining process, you must recognize that this jet does not behave like a standard atmospheric gas stream.
- If your primary focus is Lance Positioning: Ensure your standoff distance accounts for the extended potential core; the jet remains supersonic further than standard calculations might suggest.
- If your primary focus is Process Efficiency: Rely on the Mach disk formation to deliver kinetic energy to the bath, even if the jet appears to expand visually.
The vacuum environment is not just a passive container; it is an active aerodynamic force that extends the lethal range of your oxygen jet.
Summary Table:
| Feature | Atmospheric Environment | RH Vacuum Environment (1,000-4,000 Pa) |
|---|---|---|
| Expansion State | Balanced / Slightly Under-expanded | Extremely Under-expanded |
| Core Structure | Standard Potential Core | Extended Supersonic Core via Mach Disks |
| Jet Morphology | Narrow and Gradual Dissipation | Rapid Initial Expansion with Shock Structures |
| Impact Range | Short to Medium | Long-Distance Impact Preservation |
| Shock Waves | Weak or Absent | Prominent Mach Disks |
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References
- Yue Tian, Zhangfu Yuan. Characteristics of Supersonic Oxygen Jet in RH Vacuum Refining Furnace. DOI: 10.2355/isijinternational.isijint-2024-392
This article is also based on technical information from Kintek Furnace Knowledge Base .
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