The problem of liquid expansion energy
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I have a question for everyone: when high-pressure gas undergoes isentropic expansion through an expander, it does work on the outside world, its enthalpy decreases, and shaft power is generated. So, for high-pressure liquids such as water, does the temperature of the liquid itself decrease after energy is recovered through a hydraulic turbine? After the liquid passes through the valve and is throttled, its pressure decreases. Where does the reduced hydraulic energy go? ? ?
Students, who haven’t mastered the basics yet and also tend to get stuck on trivial details; their mindset doesn’t shift well when it comes to working. W=FS, force*distance; gases can be compressed to produce distance, thereby doing more work. The liquid compression distance is extremely small and can be ignored; S is minimal, so no large value can be obtained as a result of multiplication
Generally speaking, a hydraulic turbine converts pressure energy into mechanical energy. If the fluid remains liquid after pressure reduction, the temperature change is not significant. However, for fluids that are at critical conditions, such as ammonia, Freon, and liquefied gases, they turn into gas upon pressure reduction, resulting in a substantial drop in temperature – this is the principle behind refrigeration. The liquid has its pressure reduced through a throttle valve, and the energy is lost to frictional resistance and temperature changes; all throttle valves with a large pressure difference generate significant noise. Personal opinion.
In liquid expansion, potential energy is converted into mechanical energy, with only a slight change in internal energy; therefore, the temperature change is small. In gas expansion, both potential energy and internal energy are converted into mechanical energy, resulting in a significant change in internal energy; hence, the temperature change is large.
Gases have compressible properties; expansion under reduced pressure performs volume work, so pressure can be a source of energy. Liquids are difficult to compress; what is known as high pressure essentially refers to potential energy or kinetic energy that can be used to do work, such as in hydroelectric power generation and the turbines in cooling towers.