Posted on Categories:Thermodynamics, 热力学, 物理代写

# 物理代写|热力学代写Thermodynamics代考|Knowing How Phase Changes Occur

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## 物理代写|热力学代写Thermodynamics代考|Knowing How Phase Changes Occur

A phase change in a material is one of the most frequently encountered processes in thermodynamics. Melting and freezing between solid and liquid phases are found in thermal energy storage systems, which are used for energy conservation purposes. In an electric power plant, water is boiled in a steam generator and condensed back into a liquid in the condenser (see Chapter 12). The refrigerator in your kitchen has a refrigerant that boils in the evaporator and condenses from a vapor to a liquid in the condenser (see Chapter 13).
Figure 3-5 shows a phase change from liquid to vapor on a temperature-volume $(T-v)$ diagram. As you add heat to a liquid, both the temperature and volume of the liquid increase until the liquid reaches the boiling point. During boiling under constant pressure, the temperature remains constant until all the liquid is turned into vapor. The volume for a given mass of a fluid increases a lot as it becomes a vapor. Often the volume of a vapor is 1,000 times more than the volume of a liquid. Additional heating of the vapor continues to increase both temperature and volume. In this section, I focus on the liquid-to-vapor phase change and introduce you to thermodynamic lingo on phase changes so you can talk like an engineer.

Volume changes due to a temperature change are small in a liquid compared to a vapor.

## 物理代写|热力学代写Thermodynamics代考|From compressed liquid to saturated liquid

Have you ever made a boiled egg? To properly prepare a boiled egg, you start with just enough water to cover the egg. The water in the pot is considered a compressed liquid. Even though nothing appears to be compressing the water, atmospheric pressure does a great job of compressing it. Without atmospheric pressure, the water in the pot would boil immediately and continue boiling until it all evaporated (without being heated at all).
Whether the water is in a pot on your stove or in a pipe feeding the boiler in a power plant, it’s a compressed liquid. As the water in your pot warms up, the volume of the water increases slightly. You can try this at home to see that it’s true. If the cold water barely covers the eggs, you’ll see that the eggs are completely submerged when the water is really hot because the volume of the water increases.

When the water is boiling, it’s a saturated liquid. A saturated liquid is a liquid that’s able to generate vapor if you add more heat to it. So you see bubbles forming in the liquid. The pressure inside the vapor bubbles is equal to the pressure that’s compressing the liquid. This pressure is called the saturation pressure. The temperature of the water when vapor bubbles are formed is called the saturation temperature.
The difference between saturation pressure and saturation temperature can be a bit tricky. Saturation pressure corresponds to a particular temperature, and saturation temperature corresponds to a particular pressure. You can think of it this way: For water, the saturation temperature at 1 atmosphere pressure is 100 degrees Celsius. The saturation pressure at 100 degrees Celsius is 1 atmosphere. After the water starts to boil, set your egg timer for 10 minutes to get perfect hard-boiled eggs.

If you know the temperature and the pressure of the water at any instant in time while the water is warming up, you can find thermodynamic properties like specific volume, internal energy, and enthalpy by using property tables like those in the appendix. You can also use thermodynamic properties to figure out how much energy it takes to make the water reach the boiling point, as shown in this example.

## MATLAB代写

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