When the ideal gas assumption is valid?
a gas behaves more like an ideal gas at higher temperature and lower pressure
➢ The gas consists of a large number of molecules, which are in a random motion and
obey Newton’s law of motion
➢ The volume of the molecule is negligibly small
➢ All collisions between gas molecules are elastic and all motion is frictionless (no
energy is lost in collision or in motion)
➢ The distance between molecules are much larger than the size of the molecule. That
means, the intermolecular forces are negligible.
Why this assumption valid for combustion (pressure and temperature are high)?
Because combustion involves high temperatures which results in low densities (elastic
collision) even at high pressure.
Why understanding the phase change is important in gas dynamics?
it plays a significant role in determining the behavior of gases under different conditions
Gases changes their phases according to their physical condition.
- Water exists as a mixture of liquid and vapor in the boiler and the condenser of a
steam power plant.
- The refrigerant turns from liquid to vapor in the freezer of a refrigerator.
- Fluid flow in gathering lines.
- Distillation
- absorption
Define saturation temperature and saturation pressure?
At a given pressure, the temperature at which a pure substance changes phase is called the saturation temperature Tsat.
Likewise, at a given temperature, the pressure at which a pure substance changes phase is called the saturation pressure Psat.
Water: At a pressure of 101.325 kPa, Tsat=99.978C. Conversely, at a temperature of 99.978C, Psat=101.325 kPa.
Define the triple point, and a supercritical point?
Triple point: In thermodynamics, the triple point of a substance is the temperature and
pressure at which the three phases (gas, liquid, and solid) of that substance coexist in
thermodynamic equilibrium.
A supercritical point: at extremely high temperatures and pressures, the liquid and
gaseous phases become indistinguishable in terms of their thermodynamic properties.
Define the cricondentherm point?
maximum temperature at which liquids and vapor can coexist
Why the effect of temperature on specific heats in monotonic and non-monotonic
species?
Define the compressibility factor and when is used?
To compensate for these deviations, the ideal gas equation is modified by introducing
the compressibility factor (Z).
Compressibility factor is also known as gas deviation factor. It is the ratio of the volume occupied by a gas at given pressure and temperature to the volume the gas would occupy at the same pressure and temperature if it behaved like an ideal gas
Explain the following chart?
Is used to find the compressibility factor for all hydrocarbons. In order to use the chart, we need the reduced temperature (T/Tcr) and pressure (P/Pcr)
How to calculate the Tr (r für reduced) and Pr for a sample of gas?
(yi ist der volume fraction von dem gas)
What is a reversible and irreversible process?
Reversible process (ideal process):
- Is a process which can be reversed without leaving any trace on the surroundings.
On other words at the end, the process come back to its initial state.
- Represents the highest possible efficiency that a heat engine can achieve.
- Reversible processes are unreal. However, reversibility serves as a theoretical limit for
the irreversible ones.
- Reversible compression consumes the least work.
- Reversible expansion produces the most work.
What is an isentropic compression and expansion?
No heat transfer and adiabatic and therefore entropy = 0
What does expansion of gases mean physically?
When gases expand the molecules loses kinetic energy and does a positive work on
the system. Expansion work also means the temperature and pressure and volume( ich glaube das hat er in seinen lösungen falsch) go
down.
What is meant by the process is irreversible and reversible and what are the main
causes of irreversibility?
a. Reversible means ideal process b. Irreversible means real process. c. Common cause of irreversibility:
(certain amount of energy is lost in the friction).
Heat transfer across a finite temperature difference.
Mixing of two fluids
Chemical reaction (not possible to achieve the original products the same way)
Electric resistance
Fast compression
Fast expansion
What is the difference between exergy and energy?
Energy is the capacity to do work. It has different forms (kinetic, potential, thermal,
electrical, chemical, nuclear etc.). Als energy is conserved.
Why the work output is always smaller than the energy and exergy provided?
Because of the irreversibility effects or entropy generation.
Define entropy
It is the degradation of engineering devices because of irreversibility effects.
disorder or irregulaity in a system
Name and explain 3 methods for natural gas liquefaction and which one reduces the
fluids more?
a. Cooling the gas by heat transfer to a cold reservoir, e.g. refrigeration.
The temperature decrease is small. By using the refrigeration cycle.
b. Expanding the gas in a reversible manner, so that it does work.
temperature decrease is high. By expanding the gas in turbine. The turbine produces
work which can be used to compress a gas or derive shaft.
c. The Joule-Thomson effect
The temperature decrease is small. When a fluid passes through a restriction such a porous plug or a capillary tube, the temperature of the fluid increase or decrease.
How a turbo expander works.
When there is a fluid with high temperature the gas expands in turbine increases its volume and pressure which allow a connected shaft to
rotate. The rotation can be used to drive another compressor to increase T, P and
decreases its volume.
Define alkanes isomers?
Alkane isomers burn more efficiently.is a simple and cost-effective process for octane
enhancement. as the number of carbon atoms increases in the compound, the carbon
maybe connected as a continuous chain or connected as branches of isobutane. For
example, isobutane is a flammable gas that is converted from butane in a process
called isomerisation.
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