Calculate flows easily: Reference work on hydraulic transients published
ZHAW researcher Zhengji Zhang published a monograph. His work deals with flow processes in hydroelectric power plants and reveals a simple solution to a complex calculation problem.
Pressure surges in hydroelectric power plants are unavoidable. They always occur when pumps or turbines are started, stopped, or regulated. As a result, the pressure in water pipes suddenly rises or falls. "This is why the occurrence is also called water hammer," explains Zhengji Zhang from the ZHAW Institute of Energy Systems and Fluid Engineering (IEFE), adding: "The flow processes that occur with pressure surges are called hydraulic transients."
It is precisely these hydraulic transients that Zhang's monograph deals with. He says, "The transients not only determine the design of a hydropower plant, but also its daily operation." That is why transient computations are so critical. Thanks to them, shock pressures in pipelines can be brought under control.
"It's an unpleasant fact that pressure surges in the field of hydropower cannot easily be calculated," says Zhengji Zhang. Programs available on the market today are based on different and sometimes controversial algorithms. "They are difficult to access and unreliable for us engineers." Moreover, in reality, calculation results often have to be validated by measurements.
The basis used today for transient computations is the Method of Characteristics, or MOC for short. "This method requires large amounts of data, which is why it cannot be implemented with programs like Excel," says Zhengji Zhang. In addition, MOC calculations have various weaknesses. "Firstly, not all influences on transient computations have been fully clarified to date, and secondly, the computational methods are quite chaotic when it comes to application and interpretation in the case of pumps and turbines.” The fact that most of the hydraulic systems in hydroelectric power plants have surge tanks installed further complicates the computations.
"Developments from my personal involvement have been incorporated into the monograph," says IEFE researcher Zhengji Zhang. His book shows that hydraulic transients and thus flow processes can be calculated easily, for instance, with Excel. "It doesn’t matter how complex the present system is," adds Zhang. The simple calculation makes it possible to investigate influences on system pressure and system stability. "The calculation makes no controversial assumptions," explains Zhengji Zhang. This eliminates the need for experimental validation of the calculation results.
"My monograph describes the fundamentals of hydraulic transients and computational methods," says Zhang. His work includes the Wave Tracking Method (WTM), precise initial and boundary conditions, the influence of valves, generation of shock pressure waves, conservation and traveling laws of pressure waves, friction losses and the dynamic behavior of surge tanks. From all these aspects, Zhang has also developed the direct calculation method MDC. "Calculation examples are also included in the book," the researcher reveals.
"With my book I want to offer a reference for self-programming and calculation of hydraulic transients", says Zhengji Zhang. The book will thus serve as a reference for all engineers and scientists in the field. "I also hope that the calculation methods will one day be standardized."
Zhengji Zhang's monograph entitled "Hydraulic Transients and Computations" was published by Springer.