Energy transfer 2 Flashcards
What is a mole fraction equal to ?
volumetric fraction (for an ideal gas)
How is partial pressure found?
Mole fraction * total pressure
no. of moles =
mass/molar mass
How is the mass fraction of a gas found from the volumetric analysis?
Multiply mole fraction of each compound by the molar mass of each compound. Find the sum of the masses. Divide the indervidual masses by the total mass.
How is R(specific) found?
R/molar mass
What does the octave rating indicate?
The resistance to auto-ignition
What does a certane number indicate?
Measure of how readily the fuel auto-ignites
What is Φ?
Stoichometic air/fuel ration / actual air fuel ratio Φ<1 lean Φ=1 stoichometric Φ>1 Rich mixture
What is λ?
1/Φ
What is the first law of thermodynamics?
ΔU=Q+W
What is the relationship between the internal energy and enthalpy of reactants and products?
In an exothermic reaction, the enthalpy and internal energy of the products is less than the reactants.
What is the molar mass of air?
29 Kg/Kmol
What is the adiabatic flame temperature?
Max temperature obtained as a result of combustion in adiabatic conditions without dissociation effects. (temp achieved when no heat is lost) Q=0
What is Vc?
Clearence volume. Volume with piston at TDC
What is Vd?
Displacment volume or swept volume
How is the compresstion ratio found?
Vd+Vc / Vc
Vc is the clearance volume e.g. volume above piston at TDC
Vd is the swept volume.
What is indicated mean effective pressure?
Indicated work output per unit swept volume
How is power found?
Power=torque*ω
What is a heat engine?
Where heat goes from a high temperture thermal reservoir Qh to a low temperture thermal reservoir, Ql while also producing an amount of work, Wnet
Ql+Wnet=Qh
How do you find the efficiency of a heat engine?
1-(Ql / Qh)
What is the difference between a heat pump and a fridge?
A fridge if you care about Ql and a heat pump if you care about Qh
How does a heat pump work?
When work is used to move heat from a low temp heat source to a high temp.
Qh=Ql+Wnet
How is the COP found for a fridge?
COPR=Ql/ (Qh-Ql)
Ql is the temperture of the cold internal (K)
Qh is the temperture of the hot surroundings (K)
How is the COP found for a heat pump?
COPHP=Qh/ (Qh-Ql)
Ql is the temperture of the cold internal (K)
Qh is the temperture of the hot surroundings (K)
What is the nature of the carnot cycle?
How do you find the efficiency of a carnot cycle?
1-Tl/ Th
How do carnot, reversible and irreversible cycles compare?
What is the ps diagram of an otto cycle?
How does the compresstion ratio affect the efficiency of an otto cycle?
What are the PV and TS diagrams of a diesel engine?
At the same compresstion ratio, is otto or diesel more efficient?
Otto. Though diesdel can go to higher ratios
How does a closed brayton cycle work?
How does an open brayton cycle work?
What is the TS and PV diagrams of an open brayton cycle?
How do you find the theorectical effiency of a disel cycle?
efficiemcy is also given by 1-(qout/qin)
What is the efficiency of the ideal otto cycle?
1-(1/R^ (υ-1))
where υ is Cp/Cv
What is Rc?
The cut of ratio. Ratio of specific volume at 3 / 2.
Also equal to T3/T2
It is the change in volume where fuel is added.
what is saturated liquid?
heated liquid on the point of boiling.
What is saturated vapour?
Vapur which is at its boiling point.
What is the critical point?
temperture where a fluid can’t be liquified no matter how much pressure you apply.
How do you find the dryness fraction?
Mass of vapour / total mass.
x=v-vg / vg-vf
Where all v are specific volumes.
Vg and vf are found from the tables.
Upon having found the dryness fraction, how do you find the mass of vapour and mass of liquid?
mass of gas=total mass*dryness fraction
mass of liquid=total mass*(1-dryness fraction)
Carnot Cycle Applied For Steam Power Plant
1►2 Steam is compressed isentropically in a compressor to produce saturated liquid at high pressure and temperature. In this process work is needed to drive the compressor.
Heat added at the boiler to evaporate the saturated liquid to saturated vapour 2 ►3 at constant temperature Th •
Saturated vapour is then expanded isentropically to state 4 while doing work in a turbine.
After expansion, the steam is then partially condensed at constant pressure and temperature while heat is rejected. Condensation is stopped at state 1 such that s1=s2
Basic rankine cycle
For an ideal pump, how do you find the change in enthalpy between the fluid coming in and out?
wp=h2-h1=υ(P2-P1)
υ for water=0.001
wp is the specific work in to the pump
What is 1 bar equal to?
1 bar= 100,000 Pa
=100KPa
What is the TS diagram of an isentropic process?
How do you find the specific volume of wet steam?
ν = Xυg + (1 - X)υf
How do you find the specific enthalpy of wet steam?
h = hf + Xhfg
X=dryness fraction
hfg=Specific enthalpy of saturated steam-specific enthalpy of saturated water
How do you find the specific entropy of wet steam?
s = sf + X(sg-sf)
X = Dryness (% / 100)
sf = Specific Entropy of Saturated Water
sg=specific entropy of saturated vapour
How do you analyise the rankine cycle?
How does the ideal TS diagram for a simple rankine steam cycle compare will the real TS diagram?
How do you find the efficinecy of a pump or turbine?
How can you increase rankine cycle efficiency?
Lower condenser pressure
Superheating steam to higher temperatures
Increasing the boiler pressure
How is the ideal reheat Rankine cycle organised?
What is the TS diagram of the ideal reheat rankine cycle?
What should be the outlet dryness fraction of a turbine?
No less than 90%
How can you find the heat removed from a refigerated area?
m*(h1-h4)
m=mass flow rate of refrigerate
The h’s are the different enthalpies of the refirgerate before and after.
What is an equation that relates changing temperture and pressure together?
T2/T1=(P2/P1)(υ-1)/υ
What is the work back ratio?
The fraction of the work produced by the turbine that is consumed by the compressor.
-
For fixed maximum and minimum temperatures, what is the effect of the pressure ratio on:
- thermal efficiency
- net work output
- The thermal efficiency increases with increasing the pressure ratio steeply at low pressure ratios and slowly at high pressure ratios.
- The net work output increases to a maximum value and then reduces down as the pressure ratio increases.
What is a simple equation for the efficiency of a brayton cycle?
1-(T1/T2)
When analysing a rankine cycle, how do you find h1 and h3?
h1 is the enthalpy of liquid at the lower pressure
h3 is the enthalpy of super heated steam before it enters the turbine.
How do you find h2 in a rankine cycle?
What is the specific heat capacity of water?
4181 j/kg c
How do you find specific steam consumption?
3600/power output(kJ/kg) = specific steam consumption (kg / kW hour)
What is the gravimetric analysis?
The relative mass analysis.
How do you find molar analysis from gravimetic analysis?
Divide the mass fraction by the mass of the molecule
Find the sum of this for all the molecules
Divide each of the first numbers by the total.
How would you find the costant volume specific heat of a mixture?
Multiply Cv for each molecule by the mass fraction.
What is the universal gas constant?
Ru=8.314 kJ/k mol
Cv and Cp for air
Cv=0.718 kJ/ kg K
Cp=1.005 kJ/ kg K
What is the equation for internal energy?
U=mCvT
units kJ/kg
How do you find the internal energy for a mixture?
Find the internal energy for all the indervidual molecules and add together.
What is the equation for enthalpy?
U=mCpT
units kJ/kg
For every mole of O2 in the air, how many moles of N2 are there?
79/21= 3.762
What is an ultimate gravimetic analysis?
One where you find the relative weights of the elements.
What is the first law of thermodynamics?
ΔU=Q+W
where W is work done on the system
Q is heat supplied to the system
What are the definitive equations containing dryness fractions?
What is the relationship between Cp and Cv?
Cp=Cv+R
What is the definition of γ?
γ=Cp/Cv
What is the second law of thermodynamics?
The second law of thermodynamics says that the entropy of any isolated system always increases. Isolated systems spontaneously evolve towards thermal equilibrium—the state of maximum entropy of the system. More simply put: the entropy of the universe (the ultimate isolated system) only increases and never decreases.
How might you find mean effective pressure?
MEP=Wnet / Vmax-Vmin
How would you find the real coefficient of perfomrance of a fridge?
Rate of engergy removed / rate of energy put in.
What is a heat engine?
Allow heat to go from a hot to a cold place and produce work in the process.
How do you find the energy supplied by a cylinder in the otto cycle?
Energy=Ql+Qh
Qh=(u3-u2)
Qi=u1-u4
Ql will be negative and it is the engery that has to be allowed to go free and not do any useful work.
Use Cv for calculations
Equation for internal energy?
u=mass*cv*t
What is an equation relating the temperture and volume in a adiabatic process?
T4/T3=(V3/V2)^(γ-1)
What is the PH graph of the refrigeration cycle?
Sorption refrigerators
in Sorption refrigerators and heat pumps high grade heat is supplied from burning primary fuels.
How do sorportion refrigerators work?
- Utilize a chemical substance that has large affinity to the refrigerant to pump the refrigerant in the cycle. That is the mechanical compressor of the vapour compression system is replaced by a chemical compressor driven by thermal energy.
- If the chemical substance is in liquid phase, the system is called absorption refrigeration/heat pump. Examples of absorption systems are lithium bromide/water and water/ammonia.
If the chemical substance is in solid phase, the system is called adsorption refrigeration / heat pump. Examples of adsorption systems are silica gel /water and activated carbon/ammonia
Absorption Refrigeration System - Lithium Bromide/Water versus Water/Ammonia
- Lithium bromide / water system is used for water chillers employed in the air conditioning industry. It can not be used for cooling below 4C due to the freezing of the refrigerant water. Generator temperatures used are 90-120C.
- Lithium bromide / water system has the disadvantage of corrosiveness and crystallisation that occur at high concentration.
- Water / Ammonia system can be used for both refrigeration and air conditioning applications. The generator temperatures needed in this system is relatively high 120-160C.
- Water / Ammonia system has the disadvantage that ammonia is poisonous and flammable. Also water has high affinity to ammonia therefore rectifying columns are needed.
Silica Gel / Water Versus Activated Carbon / Ammonia
Silica gel / Water system is suitable for air conditioning applications
Activated carbon / ammonia is suitable for refrigeration as well as air conditioning
Silica gel / water requires low desorption temperature (60-80C) while activated carbon ammonia requires high desorption temperatures (120-150C).
Ammonia is high pressure refrigerant while water is low pressure one.
Ammonia is poisonous and flammable.
Absorption versus Adsorption
- Absorption systems requires solution circulating pump and hence electrical power consumption.
- Adsorption systems requires complex valve control system.
- Adsorption systems have more tolerance to vibration hence recommended for automotive applications.
- Adsorption systems tend to be more bulky (heat transfer performance of liquids is better than solids) and hence more expensive.
Vapour Sorption Refrigeration versus Mechanical Vapour Compression Refrigeration
- Vapour sorption systems use environment friendly refrigerants (No ozone depletion and No/Low global warming).
- Vapour sorption utilise heat hence they can use waste heat from internal combustion / gas and steam power plants, use solar energy, use waste heat from chemical processes.
- Vapour sorption systems tend to be bulky and more expensive than the mechanical vapour compression ones.
- Vapour sorption systems have lower COP than the mechanical vapour compression ones, however, when waste heat or solar energy is used, the savings in operating costs can overcome both the high capital cost and the low COP.
How do you interpolate adiabatic flame temp?
(Hactual-Hsmall / Hlarge-Hsmall )*100 + Tsmall
How would you find H2 for a refrigeration cycle?
If a system contains liquid and fluid and is in equilibrium, what does it mean?
The liquid and vapour is saturated.
Why is S4 greater than S3 in the refrigeration cycle?
Irreversibiltes in the throttle.
What is good about the rankine reheat cycle?
Lower moisture content at the exit of turbines especially the low-pressure turbine and thus higher isentropic efficiency.
Higher specific work output and lower specific steam consumption.
Effective use of combustion energy output.
Improves the ability of using high-pressure boilers. I
ncreasing the work output and reducing the specific steam consumption results in reducing the size of equipment and their cost. However, this does not offset the complexity of reheating. Therefore, the main reason for reheating is increasing the dryness fraction of steam leaving the turbines. Thus avoiding turbine blades eroding and increasing the turbine isentropic efficiency.
REGENERATIVE CYCLE WITH OPEN TYPE FEED HEATER
What is are the advantages of the closed type feed heater
• High efficiency and low specific steam consumption • By bleeding, the volume flow rate at the low-pressure end is considerably reduced. The size of the condenser is also reduced which partially compensates for the additional feed heaters. • The best compromise between the additional capital cost and the improved efficiency is obtained when a train of eight heaters is used.
Open type feed heater TS diagram.
Closed type feed heater?
