Thermodynamics Flashcards
Define Polytropic processes
For gases and superheated vapours, it is found experimentally that thermodynamic processes may often be represented by the equation Pv^n = constant. Where n is the index of expansion.
What are the assumptions behind an ideal gas? So when can we use these laws for practical purposes and when do real gases stop behaving like ideal gases?
There are three main ones. They are randomly moving particles with no forces between each other. They are point particles. They make perfectly elastic collisions. When the pressure is very high, the particles stop behaving like point particles. And Under conditions near to the liquid-gas transition, the forces the forces between particles are significant. For water however, most interactions occur near the liquid-gas transition so experimental measurements must be used to give relationships between the state properties.
Define sensible heat addition, Latent heat addition, saturated vapor.
Figure 2.5(a) shows the change in the water in moving from 1 to 2. Here heat is added and the liquid water expands. Since no phase change takes place this
is called sensible heat addition.
Point 2 represents the saturated liquid line. This is where the liquid is just at its boiling point (for a given pressure). Any additional heat added to the cylinder will be used in freeing the molecules from this liquid state to form a vapour phase – termed latent heat addition.
Ultimately though, as more and more heat is added, a point would be reached (point 3) where the liquid phase no longer exists – all the water would be vapour. Just at this point, the vapour is described as saturated vapour.
As further heat is added the ideal gas laws no longer apply because there will be some molecular interactions and zero volume particles are not valid.
Define Subcooled Liquid, Superheated, and supercritical.
This is water below its saturation temperature. Within this region, the specific volume of the liquid varies little with either temperature and pressure (i.e. water is almost incompressible). Hence in the liquid region of the P-v diagram, isotherms are (in reality) almost vertical and very close together;
At temperatures below the critical temperature (374.15oC) the vapour is usually termed superheated, at temperatures above the critical temperature the gas is
termed supercritical.
What is the dryness fraction. What is its other name? How do you calculate it and consequently, what would you use it to find?
Dryness fraction also called Quality factor describes the ratio of the vapour and liquid in a 2 phase state. x = mg/mtotal.
Consequently this is used to determine the specific volume of the mixture. The formula for it is:
v = xvg + (1-x)vf
Define Internal Energy.
Internal Energy is the energy associated with the Kinetic Energy, e.g. vibrations of the molecules and the potential energy, e.g. intermolecular forces. of the fluid molecules.
What is the specific heat at constant volume and how does one make use of it?
Experimentally derived graph of the relationship between internal energy and Temperature shows that the gradient of the graph for different variables remains constant and independent of the specific volume for perfect gases and for many real gases it is considered constant over a temperature range. We’ll call that constant the specific heat at constant volume. When Cv is taken to be constant the equation: u2 - u1 = Cv ( T2 - T1)
How to calculate the internal energy in mixed substances like saturated liquid and vapour phases.
similar to calculated the overall specific volume of with the dryness fraction. The equation is u = xug + (1-x)uf.
Define Enthalpy.
Enthalpy is a thermodynamic quantity equivalent to the total heat content of the system. It is equal to the internal energy + Pressure x Volume.
What is the specific heat at constant pressure.
Basically the enthalpy equivalent to the internal energy. Similar to internal energy, there is a specific relationship between enthalpy and temperature ie there is no pressure dependence. So it is the gradient of the H vs T graphs. so When Cp is constant, H2 - H1 = Cp ( T2 - T1 )
What is the relationship between Cp and Cv and r for ideal gases.
Cp - Cv = R and Cp/Cv = is gamma. It’s also a propertyof a gas.
What are the sign conventions on the transfer of energy into and out of a system. (Engine, heat pump etc).
Heat into the system = +
Heat out of the system = -
Work into system = -
Work out of the system = +
Define thermal efficiency. What is the equivalent of that for a reverse heat engine. And for combined heat engines what is the overall efficiency.
Thermal efficiency is the ratio between what we want/what we pay for. For a reverse heat engine or a refrigerant its called coefficient of performance given by again, what we want/what we paid for. Overall efficiency is Coefficient of performance x thermal efficiency.
