Thermodynamics revision

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System: A quantity of matter or a region in space chosen for study.

Surroundings. The mass or region outside the system

Boundary The real or imaginary surface that separates the system from its surroundings

System

Closed

(Fixed Mass)

(Control Mass)

No Mass Transfer Cross Boundry

Energy Can be Transfered

Open

(Fixed Volume)

(Control Volume)

Mass Transfer CrossBoundry

Energy can be Transfered


Isolated system Energy is not allowed to cross the boundary

State A set of properties uniquely defining the conditions of the substance

Process Any change from one equilibrium state to another

Cycle A series of connected processes in which the initial state is the final state.

Special Types of Processes:

Isothermal process A process during which the temperature T remains constant.

Isobaric process A process during which the pressure P remains constant.

Isochoric (or isometric) process A process during which the specific volume (v) remains constant.

Adiabatic process No heat transfer occurs

Isentropic Process No entropy change occurs

Density is defined as mass per unit volume


Specific Volume v The reciprocal of density is, which is defined as volume per unit mass

Pressure Force per unit area is called pressure, and its unit is the Pascal, N/m2 in the SI system

Special Types of Processes:

Isothermal process (C): A process during which the temperature T remains constant.

Isobaric process (D):A process during which the pressure P remains constant.

Isochoric (or isometric) process (A):A process during which the specific volume v remains

constant.

Adiabatic, B


Properties of Pure Substances

Pure Substance A substance that has a fixed chemical composition throughout.

Compressed Liquid (Sub-Cooled Liquid) No vapor, all liquid, NOT about to vaporize

Saturated Liquid About to vaporize

Saturated Vapor: About to condense

Superheated Vapor: No longer any liquid, NOT about to condense.

Saturation Temperature: The boiling temperature at a given pressure

Saturation Pressure: The pressure at which boiling occurs at a given T

Enthalpy, H another property of pure substances (like internal energy, U) U is a function ONLY of the temperature of the substance, f(T) H is formed by combining the internal energy, U, and work done, f(T,P) on a per unit mass basis: h = u + Pv (specific enthalpy)


PROPERTY TABLES used to look up the properties for various substances at many temperatures and

pressures vf = specific volume of the saturated LIQUID vg = specific volume of the saturated VAPOR vfg = the difference between the specific volume of the saturated vapor and

saturated liquid = (vg - vf)

QUALITY, x, is never used to describe compressed liquid or superheated vapor, it may

be expressed as a percentage: from 0% to 100%, where 0% is a saturated liquid and 100% is a saturated vapor.

Quality has significance for saturated mixtures only. It has no meaning in the compressed liquid or superheated vapor regions. Its value is between 0 and 1.


1- For saturated The values of the average properties of the mixtures are always between the values of the saturated liquid and the saturated vapor properties

2- For Compressed Liquid

Usually we approximate compressed liquid behavior evaluated at the given

TEMPERATURE (don’t use the pressure)

v = vf @ T h = hf @ T u = uf @ T s = sf @ T


3- For Super heated vapor

Ideal Gases:

The Ideal Gas Equation is:

where: Ru …… the universal gas constant

Ru = 8.314 kJ/kmole-K (for all gases)

M …… the molecular weight of the gas (kg/kmole )

N……..the number of moles of the gas molecules

For changes of state (with a fixed mass of Ideal gas; closed system):

P1 v1 = R T1 and P2 v2 = R T2


Specific heat at constant volume (Cv)

The amount of energy needed to raise the temperature of a unit of mass of a

substance by one degree in a constant-volume process

Specific heat at constant volume (Cp)

The amount of energy needed to raise the temperature of a unit of mass of a

substance by one degree in a constant-pressure process


FIRST LAW OF THERMODYNAMICS Law of Conservation of Energy: "Energy cannot be created or destroyed, it can only change forms"

The First Law of Thermodynamics

Closed (Fixed Mass)

(Control Mass)

no mass transfer (in/from the System

Examples:

- Tank

- Vessile

- Piston Cylinder Device

Open (Fixed Volume)

(Control Volume)

Examples:

- Compressor

- Turbine

- Nozzle

- Diffuser

- Mixing Chamber

- Throttle vavle

- Heat Exchanger

- Pump


For a closed system: Energy can cross the boundaries of a closed system in the form of heat or work.

Energy transfer across the boundaries of a closed system is due to a temperature difference, it is heat; otherwise, it is work.

Remember, SIGN CONVENTIONS:

+Q is heat ADDED TO a system FROM the surroundings (Energy IN) +W is work done BY a system ON the surroundings (Energy OUT) Q - W = ∆E = ∆U + ∆KE + ∆PE

∆U = m ( u2 - u1) ∆KE = ½ m ( V2

2 - V12 )

∆PE = mg ( z2 - z1)

If NO Work is done ON or BY the system (W = 0): Q = ∆E

If NO Heat is exchanged between the system and its surroundings: (Q = 0, ADIABATIC = perfectly insulated) -W = ∆E

If the system is stationary (no motion), ∆KE = ∆PE = 0 Q - W = ∆U

ALTERNATE Form (more convenient): Q - WOTHER - Wb = ∆E

In terms of RATE:

- = dE/dt

In terms of per unit mass: q - w = ∆e

In a CYCLE, the system returns exactly to its initial state: ∆E = 0 because E1 = E2 Q - W = 0

For an Open System:

A control volume differs from a closed system in that it involves mass transfer

MASS CONSERVATION: The Continuity Equation Σ m in - Σ m out = Σ m cv

(mass coming in) - (mass going out) = (change of mass in the C.V.)


= amount of mass/unit time If the flow velocity, V, and the cross-sectional area, A, of the channel are given:

= .V(Velocity). A since = 1 / V Flow is also measured as a VOLUMETRIC Flow Rate (m3/sec)

= V(Velocity). A

Giving: = = / v

MASS CONSERVATION (the amount of mass in the C.V. is CONSTANT):

I = E

(V A) I = ( V A) E


Steady Flow Devices

NOZZLE

For adiabatic

DIFFUSER

For adiabatic

TURBINES:

For adiabatic

COMPRESSORS (or PUMPS): Require Work (Work Done ON Fluid)

For adiabatic

THROTTLING VALVES:

MIXING CHAMBERS:


Heat Engine Air conditioning,

Refrigerator Heat Pump


CARNOT PRINCIPLES

(Use ABSOLUTE T)

THE CARNOT HEAT ENGINE

for a reversible heat engine

The Carnot efficiency is the highest possible efficiency for a heat engine operating

between thermal energy reservoirs at TH and TL.

CARNOT Refrigeration and Heat Pumps


WHAT IS ENTROPY? Entropy is a thermodynamic property that measures the degree of randomness, disorder or uncertainty

S = total entropy in kJ/oK

s = specific entropy in kJ/kg oK

For a process, the change in entropy is

Entropy Change in Ideal Gases (Assuming Constant Specific Heats):

Using ideal gas relationships: CP = CV + R and k = CP/CV

For isentropic process:


The Efficiency of Compressor and Turbine

Thermodynamics revision

Engineering

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