Study on the gas-liquid two-phase flow properties inside a liquid ring pump
The liquid ring pump is a universal mechanical device that uses liquid as an intermediate medium for energy transfer to pump gases. Due to its advantages of large flow capacity, compact structure, and isothermal compression, the liquid ring pump is widely used in petroleum, metallurgy, pharmaceuticals, coal mining, power generation, and food processing industries. The liquid ring pump is particularly suitable for handling flammable, explosive, moisture-laden, and dust-laden gases.
However, despite its widespread application, the liquid ring pump faces significant operational challenges. The internal flow within a liquid ring pump constitutes a complex gas-liquid two-phase flow with free interfaces. The flow patterns in both the pump cavity and impeller exhibit non-through characteristics, with complex multi-scale vortical structures developing in the impeller passages. These intricate flow dynamics result in substantial hydraulic losses and relatively low efficiency—typically ranging from 30% to 45%.
This article provides a comprehensive analysis of the gas-liquid two-phase flow properties inside a liquid ring pump, examining the underlying flow mechanisms, the sources of energy loss, the latest research advancements, and practical performance improvement measures. For B2B buyers, plant engineers, and maintenance professionals, understanding these internal flow characteristics is essential for selecting, operating, and optimizing liquid ring pumps in demanding industrial applications.
The Working Principle of a Liquid Ring Pump
Before examining the complex internal flow properties, it is helpful to understand the fundamental operating principle of a liquid ring pump.
A liquid ring pump operates by using a rotating liquid ring—typically water or another compatible fluid—as both a sealing and compression medium. An eccentrically mounted impeller rotates within a cylindrical casing. Centrifugal force throws the sealing liquid outward against the casing wall, forming a rotating liquid ring. Because the impeller is mounted off-center, the space between the impeller blades and the liquid ring varies continuously during rotation, creating expanding chambers that draw gas into the pump and contracting chambers that compress and discharge the gas.
This design gives the liquid ring pump several inherent advantages: isothermal compression (minimal temperature rise), the ability to handle wet and dirty gases, oil-free operation, and robust construction. However, the same design also creates the complex gas-liquid two-phase flow characteristics that are the subject of intensive research.
The Complexity of Gas-Liquid Two-Phase Flow Inside a Liquid Ring Pump
Free Interfaces and Non-Through Flow Characteristics
The internal flow within a liquid ring pump is characterized by a complicated gas-liquid two-phase flow distribution accompanied by complex space-time evolution characteristics. The gas-liquid two-phase flow has a free interface—the boundary between the gas phase and the liquid ring—which is not fixed but continuously evolves as the impeller rotates.
The flow patterns in both the pump cavity and the impeller passages exhibit non-through characteristics. This means that the flow does not simply pass through the pump in a straightforward manner. Instead, the gas and liquid phases interact in complex ways, with the liquid ring continuously circulating and the gas being periodically trapped, compressed, and discharged.
Multi-Scale Vortical Structures
One of the most significant features of the flow inside a liquid ring pump is the presence of complex multi-scale vortical structures developing in the impeller passages. These vortices occur at various scales and intensities, contributing significantly to hydraulic losses and pressure fluctuations.
Research using large-eddy simulation coupled with the volume-of-fluid method (LES-VOF) has revealed detailed vortex dynamics inside the liquid ring pump. The results show that the channel vortex inside the impeller on the pump suction side gradually flows out of the flow passage with the rotation of the impeller and continuously mixes with the wake vortex at the impeller outlet and the separation vortex at the inner wall of the casing.
The evolution and development of vortex structures with different intensities and scales near the inner wall of the suction side casing have a relatively large contribution to pressure fluctuation. On the exhaust side, the wake vortex at the impeller outlet gradually enters the flow passage and dissipates progressively with time. There is an obvious high-intensity backflow vortex in the exhaust section due to the pressure difference between the flow channels near the leading edge of the exhaust port, which leads to obvious rotor-stator interaction in the exhaust section.
Secondary Flow Structures
The liquid ring pump also contains complex secondary flow structures within the impeller and pump cavity. These secondary flows—which include recirculation zones, backflow, and leakage flows—are a major contributor to the pump's relatively low efficiency. The secondary flow structures are caused by the non-uniform pressure distribution around the circumference of the pump, the interaction between the rotating impeller and the stationary casing, and the free interface between the gas and liquid phases.
Axial and Radial Clearance Leakage
The clearances between the impeller and the pump casing—both axial and radial—create additional pathways for gas-liquid leakage flow. The axial clearance exists between the impeller blade tip and the pump casing, and gas-liquid leakage flow in this clearance will significantly reduce the vacuum degree and efficiency of the liquid ring pump.
Research has shown that axial leakage flow reduces the inlet vacuum and efficiency of the liquid ring pump. The gas-liquid two-phase flow within the axial clearance region is completely separated. Several droplets are scattered outside the suction region, some of which flow back into the low-pressure suction region along the wall of the suction port. These leakage flows represent a direct loss of pumping capacity and contribute to the overall hydraulic losses.
Efficiency Challenges – Why Liquid Ring Pumps Have Low Efficiency
The Fundamental Limitation
For a long time, the efficiency of liquid ring pumps has been low—generally 30% to 45%. This low efficiency is not accidental but is inherent to the pump's operating principle. The liquid ring pump suffers from significant energy losses due to its reliance on gas-liquid two-phase flow, resulting in lower efficiency compared to other pump types. The hydraulic loss is serious, resulting in low energy efficiency.
Sources of Energy Loss
Research based on entropy production theory has identified several key sources of energy loss in liquid ring pumps. The results indicate that turbulent entropy production and wall entropy production dominate the energy losses of liquid ring pumps. The entropy production caused by the wall effect mainly occurs at the shell of the gas compression region.
Additional sources of energy loss include:
Fluid friction between the liquid ring and the stationary housing
Over-compression and under-compression effects
Outlet backflow and clearance leakage
Complex secondary flow structures within the impeller and pump cavity
The Vapor Pressure Limitation
The liquid ring pump also faces a fundamental limitation related to the vapor pressure of the working liquid. Since the liquid ring pump operates using a liquid as the sealing and compression medium, its achievable ultimate vacuum is limited by the vapor pressure of that liquid. As the vacuum level increases (pressure decreases), the liquid begins to vaporize, creating additional gas load and reducing pumping capacity. This is why the liquid ring pump cannot be used for high-vacuum applications.
Recent Research Advancements in Gas-Liquid Two-Phase Flow Properties
Numerical Simulation Methods
Understanding the complex gas-liquid two-phase flow inside a liquid ring pump has been a major focus of research. Numerical simulation has become an essential tool for studying these flows.
The Volume of Fluid (VOF) method has been widely used to model the gas-liquid two-phase flow in liquid ring pumps. The VOF model can effectively simulate complex free-interface gas-liquid two-phase flows and accurately capture the free interface. Researchers have also employed large-eddy simulation (LES) coupled with VOF to study unsteady gas-liquid flow characteristics in liquid ring pumps.
These numerical methods have enabled researchers to analyze the streamline distribution, velocity distribution, phase distribution, and pressure distribution within the liquid ring pump. The simulation results have been validated against experimental data and shown to accurately describe the gas-liquid two-phase flow规律 and predict the hydraulic performance of the liquid ring pump.
Key Research Findings
Recent research has yielded several important findings about the gas-liquid two-phase flow properties inside liquid ring pumps:
Pressure pulsation characteristics: The gas-liquid flow-induced hydraulic excitation characteristics have been analyzed. The evolution and development of vortex structures near the casing inner wall contribute significantly to pressure fluctuations.
Rotor-stator interaction: There is obvious rotor-stator interaction in the exhaust section, driven by the pressure difference between flow channels near the exhaust port.
Central symmetry in double-acting pumps: The gas-liquid two-phase flow field within double-acting liquid ring pumps exhibits central symmetry, with the vorticity intensity of the gas phase exceeding that of the liquid phase.
Altitude effects: As altitude increases, the outlet initial region state gradually transitions from over-compression to under-compression, with additional efficiency losses caused by over-compression, outlet backflow, and clearance leakage.
Breakthroughs in Performance Optimization
Significant breakthroughs have been achieved in liquid ring pump performance optimization. Researchers have proposed control methods for tip clearance leakage flow, including tip groove and tip jet control methods. Methods for matching ejectors with liquid ring pump impeller parameters and multi-stage liquid ring wheel matching optimization have been established.
The first monograph on liquid ring pumps in China, "Gas-Liquid Two-Phase Flow and Performance Optimization in Liquid Ring Pumps," has been published. Multiple invention patents have been authorized, and research achievements have been successfully applied to vacuum pump manufacturing enterprises. The research on "Flow and Performance Optimization in Liquid Ring Pumps" has received the second prize of the Gansu Provincial Science and Technology Progress Award.
Practical Improvements – Working Fluid Cooling and Performance Enhancement
The Importance of Working Fluid Temperature
The temperature of the working liquid in a liquid ring pump directly affects its performance. As the working liquid temperature increases, its vapor pressure rises, reducing the pump's achievable vacuum and increasing the risk of cavitation. Cavitation can cause impeller damage, increase maintenance costs, and significantly reduce pump efficiency.
Cooling System Modifications
One practical improvement that has been successfully implemented is the modification of the working fluid cooling system for liquid ring pumps. By implementing a closed-loop circulation system with an added cooler to reduce the liquid ring temperature, operators can significantly improve pump performance.
A documented case study shows that after the working fluid supplementary cooling modification of a liquid ring pump, the equipment was monitored and measured over a period of more than three years. The results showed that the front and rear bearings, shaft, balance ring, and other components of the liquid ring pump were not adversely affected. The liquid ring pump operated with excellent vacuum performance, fully meeting process requirements.
This practical example demonstrates that targeted improvements—particularly in working fluid temperature management—can significantly enhance the performance and reliability of liquid ring pumps without compromising equipment integrity.
Additional Performance Enhancement Strategies
Other strategies for improving the performance of liquid ring pumps include:
Optimizing radial clearance: Adjusting the radial clearance and vane wrap angle can effectively improve the flow situation inside the pump and enhance suction efficiency and overall performance.
Plasma excitation control: Research has shown that plasma excitation can increase the efficiency of liquid ring pumps by 3.6% to 4% under various flow conditions.
Structural parameter optimization: Using entropy production theory, researchers have developed optimization models that achieve an increase in suction capacity of 8.74% and an increase in isothermal compression efficiency of 3.75%
Shandong Zhangqiu Blower Co., Ltd. – Expertise in Liquid Ring Pump Technology
Shandong Zhangqiu Blower Co., Ltd. (often referred to as "Zhanggu" or "SDZG"), founded in 1968, has accumulated over 50 years of experience in the design, production, and manufacturing of industrial blowers and vacuum equipment. The company has conducted in-depth research on the gas-liquid two-phase flow properties inside liquid ring pumps and has applied these insights to product development.
The company's understanding of the complex internal flow dynamics—including free interface characteristics, secondary flow structures, and clearance leakage mechanisms—has informed the design of its liquid ring pump products. By optimizing impeller geometry, clearance settings, and flow passages, Shandong Zhangqiu Blower Co., Ltd. has developed liquid ring pumps that deliver reliable performance, reduced energy consumption, and extended service life.
Key support services include:
Technical consultation on liquid ring pump selection and application
Original equipment manufacturer (OEM) spare parts
Performance testing and optimization support
On-site commissioning and training
The company's commitment to understanding the fundamental flow physics of liquid ring pumps and applying this knowledge to practical product improvements makes it a trusted partner for industrial facilities seeking reliable and efficient vacuum solutions.
Conclusion – The Path Forward for Liquid Ring Pump Performance
The gas-liquid two-phase flow properties inside a liquid ring pump are extraordinarily complex, characterized by free interfaces, non-through flow patterns, multi-scale vortical structures, secondary flows, and clearance leakage. These flow dynamics are the primary cause of the liquid ring pump's relatively low efficiency—typically 30% to 45%—and its limitation to low-vacuum applications.
However, significant progress has been made in understanding and optimizing these flow properties. Advanced numerical simulation methods—including VOF and LES-VOF models—have enabled researchers to visualize and quantify the complex gas-liquid two-phase flow inside liquid ring pumps. Breakthroughs in performance optimization, including tip clearance control methods, structural parameter optimization, and plasma excitation control, have demonstrated tangible improvements in efficiency and suction capacity.
Practical improvements—particularly in working fluid temperature management through cooling system modifications—have been validated through long-term field operation, showing that liquid ring pumps can achieve excellent vacuum performance without compromising equipment integrity.
For B2B buyers and plant engineers, understanding the gas-liquid two-phase flow properties inside a liquid ring pump is essential for:
Selecting the right liquid ring pump for specific applications
Implementing effective maintenance and optimization strategies
Achieving maximum efficiency and service life from the equipment
As research continues to advance and manufacturers like Shandong Zhangqiu Blower Co., Ltd. apply these insights to product development, the performance of liquid ring pumps will continue to improve—delivering greater efficiency, reliability, and value for industrial users.
