Surreal illustration of annular seal dynamics with swirling oil and air bubbles.

Seal the Deal: How Understanding Leakage & Rotordynamics Can Keep Your Machinery Running Smoothly

Mira Elwood in Tech & Innovation September 2025 • 4 min read.

"Dive into the world of two-phase mixtures and smooth seals to uncover the secrets of optimal performance and stability in industrial pumps and turbomachinery."

In the intricate world of industrial machinery, smooth operation hinges on components performing as expected. Annular seals, essential in pumps and turbines, face a constant challenge: maintaining stability and preventing leaks. These seals, often dealing with complex mixtures of fluids, can significantly impact the overall performance and reliability of the equipment they serve.

Recent research has shed light on the behavior of smooth seals when handling two-phase mixtures—specifically, mixtures of oil and air. These findings, stemming from experiments conducted on a specialized 2-phase annular seal stand (2PASS), offer valuable insights into how these mixtures affect the seal's leakage and rotordynamic characteristics. The implications of this research extend to industries where maintaining the integrity and efficiency of fluid handling systems is paramount.

This article explores the experimental study, translating technical data into understandable concepts. By examining the effects of adding air to oil flows within smooth seals, we aim to provide practical knowledge that can help in troubleshooting, optimizing system design, and reducing operational risks. Whether you're an engineer, technician, or simply someone keen to understand the mechanics of industrial systems, this guide offers a clear path through the complexities of seal technology.

Decoding the Dynamics: How Air Affects Oil-Based Seals

Surreal illustration of annular seal dynamics with swirling oil and air bubbles.

The study focuses on a smooth annular seal with specific dimensions: an inner diameter of 89.306 mm, a length-to-diameter ratio (L/D) of 0.65, and a radial clearance of 0.188 mm. The test fluid is a blend of silicone oil (PSF-5cSt) and air, created using spargers to inject air bubbles into the oil flow. Tests were run under varying conditions, including different inlet gas-volume-fractions (GVF), rotor speeds, inlet temperatures, and pressure drops.

The primary measurements focused on the complex dynamic stiffness coefficients of the test seal. These coefficients, which describe the seal's response to dynamic forces, were characterized by stiffness (Kij), damping (Cij), and virtual-mass (Mij) coefficients. The analysis of these measurements revealed several key insights:

Leakage: Adding air to the oil flow doesn't significantly alter the seal's mass flow leakage (ṁ).
Rotordynamics: Air significantly impacts the seal's rotordynamic characteristics.
Stiffness: At lower speeds and pressure drops, stiffness (K) decreases from positive to negative as inlet GVF decreases.
Stiffness Changes: For most conditions, K increases as inlet GVF increases, except at higher pressure drops where it may decrease.
Other Coefficients: Cross-coupled stiffness (k) and direct damping (C) remain relatively unaffected by the addition of air.

These findings indicate that while the physical leakage of the seal remains stable, the dynamic behavior is significantly altered by the presence of air. The changes in stiffness, in particular, can have profound effects on the stability and performance of the machinery.

Practical Steps Forward: Optimizing Seal Performance in Real-World Applications

The insights from this experimental study provide a foundation for improving the design and operation of annular seals in various industrial applications. Further research is needed to refine predictive models and account for factors such as temperature variations and mixture inhomogeneities. By integrating these findings into engineering practices, it’s possible to enhance the reliability and efficiency of machinery, reducing downtime and operational costs. Keeping abreast of advancements in seal technology will ensure optimized performance and contribute to sustainable industrial operations.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1115/gt2018-75328, Alternate LINK

Title: Experimental Study Of The Leakage And Rotordynamic Coefficients Of A Long Smooth Seal With Two-Phase, Mainly-Oil Mixtures

Journal: Volume 7B: Structures and Dynamics

Publisher: American Society of Mechanical Engineers

Authors: Min Zhang, Dara W. Childs, James E. Mclean, Dung L. Tran, Hari Shrestha

Published: 2018-06-11

Everything You Need To Know

How does adding air to oil flow affect the performance of smooth seals, according to the 2-phase annular seal stand (2PASS) experiments?

The 2-phase annular seal stand (2PASS) experiments revealed that while adding air to the oil flow doesn't drastically change the seal's mass flow leakage (ṁ), it does significantly alter its rotordynamic characteristics. Specifically, the stiffness (K) of the seal can decrease from positive to negative as the inlet gas-volume-fraction (GVF) decreases at lower speeds and pressure drops. Under most conditions, the stiffness (K) increases as the inlet GVF increases, except at higher pressure drops where it may decrease. However, cross-coupled stiffness (k) and direct damping (C) remain relatively unaffected by the addition of air.

What are 'complex dynamic stiffness coefficients,' and why are they important in understanding annular seal behavior?

The 'complex dynamic stiffness coefficients' describe how the seal responds to dynamic forces. These coefficients include stiffness (Kij), damping (Cij), and virtual-mass (Mij). Changes in these coefficients, particularly stiffness, can significantly impact the stability and performance of the machinery using these seals. Accurately measuring and understanding these coefficients allows for better prediction of seal behavior under various operating conditions.

What were the specific parameters and materials used in the experimental study of the smooth annular seal?

The experimental study focused on a smooth annular seal with a specific geometry: an inner diameter of 89.306 mm, a length-to-diameter ratio (L/D) of 0.65, and a radial clearance of 0.188 mm. The fluid used was a blend of silicone oil (PSF-5cSt) and air, introduced through spargers to create air bubbles. Tests were run under various conditions, including different inlet gas-volume-fractions (GVF), rotor speeds, inlet temperatures, and pressure drops. These parameters allowed researchers to analyze how different operational conditions affected the seal's leakage and rotordynamic characteristics.

How can understanding the effects of air on oil-based seals improve the stability and efficiency of industrial equipment?

The stability and efficiency of industrial pumps and turbomachinery can be enhanced by carefully considering the impact of two-phase mixtures (like oil and air) on smooth seals. Understanding how air affects the leakage and rotordynamic coefficients allows engineers to optimize system design, predict seal behavior under various operating conditions, and mitigate potential issues that could lead to downtime. By integrating these findings, the reliability and efficiency of machinery can be improved, reducing operational costs and promoting sustainable industrial operations.

Besides stiffness (K), which rotordynamic coefficients are least affected by the addition of air, and why is this important?

The research indicated that cross-coupled stiffness (k) and direct damping (C) are relatively unaffected by the addition of air. This is important because it suggests that some aspects of seal dynamic behavior are more robust to changes in the fluid mixture. However, the significant changes observed in stiffness (K) highlight the need to focus on this parameter when designing and operating systems with two-phase mixtures to avoid instability.