THE FIRST LAW OF THERMODYNAMICS
If we give heat to a system the heat will divided into two categories- some energy will change into work energy and remaining part is used to increase its internal energy.
FIRST LAW FOR A CLOSED SYSTEM
First law can be written for a closed system in an equation form as[ Heat supplied = Work done + increase internal energy]
Q = W + (E2 - E1)
Where the total energy content
Kinetic energy term is due to the system movement with a velocity C. For stationary systems this term will be zero. The term gc is a constant of value 1 in SI unit. It will be dropped here after since SI unit is followed throughout the book.
Potential energy term is due to the location of the system in the gravitational field. It remains constant for a stationary system. The unit of energy in SI is kJ.
The Thermodynamic Property Enthalpy
Enthalpy( H ) = U + pVh = u + pv
Where h is specific enthalpy in kJ/kg
u is specific internal energy in kJ/kg and
v is specific volume in m3/kg
The Steady-state Flow Process
When a flow process is satisfying the following conditions, it is known as a steady flow process. The mass and energy content of the control volume remains constant with time.
The state and energy of the fluid at inlet, at the exit and at every point within the control volume are time independent.
The rate of energy transfer in the form of work and heat across the control surface is constant with time.
Therefore for a steady flow process
For problem of single inlet stream and single outlet stream
This equation is commonly known as steady flow energy equation (SFEE).
Turbines
Turbines are devices used in hydraulic, steam and gas turbine power plants. As the fluid passesthrough the turbine, work is done on the blades of the turbine which are attached to a shaft. Due to the work given to the blades, the turbine shaft rotates producing work.
General Assumptions
1. Changes in kinetic energy of the fluid are negligible
2. Changes in potential energy of the fluid are negligible.
Compressors
Compressors (fans and blowers) are work consuming devices, where a low-pressure fluid is compressed by utilizing mechanical work. Blades attached to the shaft of the turbine impart kinetic energy to the fluid which is later converted into pressure energy.
General Assumptions
- Changes in the kinetic energy of the fluid are negligible
- Changes in the potential energy of the fluid are negligible
Governing Equation
Applying the above equations SFEE becomes
Pumps
Similar to compressors pumps are also work consuming devices. But pumps handle incompressible fluids, whereas compressors deal with compressible fluids.
General Assumptions
- No heat energy is gained or lost by the fluids;
- Changes in kinetic energy of the fluid are negligible.
Governing Equation
As the fluid passes through a pump, enthalpy of the fluid increases (internal energy of the fluid remains constant) due to the increase in pv (flow energy). Increase in potential energy of fluid is the most important change found in almost all pump applications.
Nozzles
Nozzles are devices which increase the velocity of a fluid at the expense of pressure. A typical nozzle used for fluid flow at subsonic speeds as shown in Figure.General Assumptions
- In nozzles fluids flow at a speed which is high enough to neglect heat lost or gained as it crosses the entire length of the nozzle. Therefore, flow through nozzles can be regarded as adiabatic. That is =0.
- There is no shaft or any other form of work transfer to the fluid or from the fluid; that is = 0.
- Changes in the potential energy of the fluid are negligible.
Governing Equation
Diffusers
Diffusers are (reverse of nozzles) devices which increase the pressure of a fluid stream by reducing its kinetic energy.General Assumptions
Similar to nozzles, the following assumptions hold good for diffusers.
- Heat lost or gained as it crosses the entire length of the nozzle. Therefore, flow through nozzles can be regarded as adiabatic. That is Q=0
- There is no shaft or any other form of work transfer to the fluid or from the fluid; that is = 0.
- Changes in the potential energy of the fluid are negligible
Governing Equation
First Law for a Cyclic Process
In a cyclic process the system is taken through a series of processes and finally returned to its original state. The end state of a cyclic process is identical with the state of the system at the beginning of the cycle. This is possible if the energy level at the beginning and end of the cyclic process are also the same. In other words, the net energy change in a cyclic process is zero.
Consider a system undergoing a cycle consisting of two processes A & B as shown in Figure 3.11 Net energy change
Hence for a cyclic process algebraic sum of heat tranfers is equal to the algebraic sum of work transfer.
This was first proved by Joule, based on the experiments he conducted between 1843 and 1858, that were the first quantitative analysis of thermodynamic systems.
ENERGY IS A PROPERTY OF SYSTEM
Consider a system undergoing a process from state1 to state2 along path A as shown in Figure 3.12. Let the system be taken back to the initial state 1 along two possible paths B and C. Process A, combined separately with process B and C forms two possible cycles.
From both equations it can be concluded that energy change in path B and path C are equal and hence energy is a point function depending only on the end states.
It has been already shown that all the properties are point functions and hence energy is also a property of the system.
SPECIFIC HEAT AT COSTANT VOLUME AND AT CONSTANT PRESSURE
Specific heat at constant volume of a substance is the amount of heat added to rise the temperature of unit mass of the given substance by 1 degree at constant volume.From first law for a stationary closed system undergoing a process
dQ = pdV + dU or dq = pdv + du
For a constant volume process
dQ = dU or dq = du
or du = CvdT
Similarly specific heat at constant pressure is the quantity of heat added to rise the temperature of unit mass of the given substance by 1 degree at constant pressure
where dQ = pdV + dU
dQ = pdV + d(H - PV)
dQ = pdV + dH - Vdp - pdV
dQ = dH - Vdp
For a constant pressure process dp = 0
Hence dQ = dH or dq = dh
or dh = CpdT
PERPETUAL MOTION MACHINE
An engine which could provide work transfer continuously without heat transfer is known as perpetual motion machine of first kind. It is impossible to have such an engine as it violates first law of thermodynamics.LIMITATIONS OF FIRST LAWS OF THERMODYNAMIC
If a well insulated tank of fluid is stirred by a rotating paddle wheel, the energy of the fluid increases. If the stirrer is stopped, however the energy of the fluid will not decrease and cause the stirrer to rotate in the opposite direction. The possibility of this process proceeding in the opposite direction is not excluded by the first law of Thermodynamics. Hence first law of thermodynamics does not allow us to predict whether a proposed conceived energy conversion is possible or not.In all the internal combustion engines fuel and air mixture is supplied at room temperature. This mixture undergoes combustion inside the engine and gives out work. Exhaust gases coming out of the engine are always at higher temperature, indicating that some heat is taken away into atmosphere. Hence, in all the IC engines only a part of the heat is converted into work. From our experience we know that if any attempt is made to convert all the heat into work, our effort will go in vain. This limitation in the extent of energy conversion has also not been addressed in first law of thermodynamics
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