Degassed Boundary Layer for Propeller Cavitation Prevention
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Solution Overview
Problem
Cavitation in hydraulic systems, such as impellers and propellers, leads to efficiency losses and material degradation due to the formation of vapor cavities and subsequent shock waves, limiting the speed of vessels and causing wear on surfaces moving relative to fluids.
Innovation Solution
Directing a partially degassed fluid towards the surface to form a boundary layer that increases the negative pressure required for cavitation inception, thereby reducing its occurrence, using a degasser to produce a degassed fluid that is then directed towards the surface at a controlled volumetric flow rate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the shaft speed of impellers or propellers is increased to improve productivity, then the speed and efficiency of vessels or fluid transmission is improved, but cavitation occurs more frequently causing material degradation and wear
Solution Approach 1:
A layer of degassed fluid is introduced as an intermediary substance between the solid surface (impeller/propeller blade) and the bulk fluid. This degassed boundary layer acts as a mediator that prevents cavitation nucleation at the surface by eliminating gas pockets and impurities that would otherwise serve as cavitation initiation sites, thereby allowing higher shaft speeds without cavitation damage
Solution Approach 2:
The invention applies a localized treatment (degassed fluid layer) specifically at the surface boundary where cavitation nucleation occurs, rather than treating the entire bulk fluid. This creates a zone of different quality (degaussed) at the critical surface region, preventing cavitation at the most vulnerable location while maintaining normal fluid properties elsewhere
2Reliability
If dissolved gases are present in the fluid, then the fluid maintains its natural composition and lubrication properties, but gas molecules create nucleation sites that promote cavitation formation
Solution Approach 1:
The invention creates a localized zone with different gas concentration properties - a degassed boundary layer at the surface with very low dissolved gas content, while the bulk fluid maintains its normal dissolved gas concentration. This spatial variation in gas concentration prevents cavitation at the surface without altering the overall fluid composition
Solution Approach 2:
Rather than completely removing all dissolved gases from the entire fluid system (which would be excessive and potentially harmful), the invention applies partial degassing only to the boundary layer where it is needed for cavitation prevention. This partial action achieves the necessary cavitation resistance while preserving the natural fluid properties in the bulk
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method effectively reduces cavitation, improving the performance and efficiency of vessels, minimizing material degradation, and reducing noise associated with cavitation, allowing for higher operational speeds and reduced wear on surfaces.
Implementation Method 1
Cavitation is the formation of vapour cavities in hydraulic systems due to a consequence of forces acting upon the hydraulic system. Cavitation generally occurs when the local pressure falls sufficiently far below a saturated vapour pressure of a fluid
Implementation Method 2
directing a second fluid that is at least partially degassed towards the surface such that the second fluid is able to form a boundary layer at the surface
Implementation Method 3
Both aqueous and non-aqueous fluids show similar effects, and, in addition, non-polar fluids have an increased capacity to dissolve gases relative to water. Therefore, degassing the second fluid can also help to reduce the number of gas-derived nucleation sites
Implementation Method 4
In order for cavitation inception to occur, the vapour cavities generally need a surface on which they can nucleate. This surface can be provided by the sides of a container, pipe or valve, by impurities in the fluid, or by small undissolved microbubbles within the fluid
Implementation Method 5
When subjected to higher pressure, the cavities implode and can generate an intense shock wave
Implementation Method 6
Cavitation can also cause wear on materials in a hydraulic system. The most common examples of cavitation wear are to pump impellers, propellers, and bends in fluid circuits
Data Source
AI summary
This disclosure relates to a system for reducing cavitation at a surface that moves relatively with respect to a first fluid. The system comprises a degasser configured to at least partially degas a second fluid. The system also comprises a reservoir in communication with the degasser and configured to house the at least partially degassed second fluid, the reservoir having an outlet that is arranged for directing the second fluid towards the surface. The system is configured such that the directing of the at least partially degassed second fluid towards the surface forms a boundary layer at the surface. The boundary layer is adapted to at least partially increase the negative pressure required to initiate cavitation at the surface so as to reduce the occurrence of cavitation during such relative movement.