The "breathing effect" of dimethyl silicone oil in hydraulic systems - air solubility and cavitation risk
Introduction
Aviation and high ferroelectric hydraulic servo systems extensively use 20-500 cSt dimethyl silicone oil as flame retardant hydraulic fluid. The low-pressure area of the system may experience cavitation due to air release. This article is based on the core physical and chemical research of Peking University, providing quantitative data and engineering strategies for air solubility.
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Quick overview of experimental data
• The intrinsic solubility of 500 cSt silicone oil is α=0.085 (P/100 kPa) cm ³ (STP)/cm ³ at temperatures of 293-353 K and pressures of 0-350 kPa;
The difference in solubility of air between 20 cSt and 500 cSt silicone oils is less than 6%, indicating that the dissolution behavior is less affected by side chains and mainly driven by temperature and pressure.
Cavitation criterion
When the local pressure is lower than the "gas separation pressure" P_out=α · R · T · P ₀, bubbles appear in the system. Cooling 500 cSt silicone oil from 80 ℃ to 40 ℃ resulted in a decrease of 8 kPa in P_out and a significant increase in cavitation risk.
Engineering countermeasures
Add a back pressure valve to the fuel tank to maintain an absolute pressure of ≥ 50 kPa;
Using a 0.1 bar degassing tank for online defoaming, with a circulating flow rate of 5% of the fuel tank volume per minute;
5 minutes before system shutdown, increase the oil temperature to 60 ℃ to reduce air residue.
conclusion
The "respiratory effect" of dimethyl silicone oil is not a defect, but a predictable and manageable physical phenomenon. Mastering solubility data can minimize risks during the hydraulic topology design phase.