Thermo Concepts (Ch.3 - Ch.4)

phase

a quantity of matter that is homogeneous throughout in both chemical composition and physical structure

pure substance

is uniform and invariable in chemical composition

state principle

thermo state of a simple compressible system is completely defined by two independent, intensive propertiesn

simple compressible system

intensive states of systems of commonly encountered pure substances

p-v-t digram

3d model

phase diagram

property diagram

two-phase regions

two phases exist in equilibrium

• liquid–vapor

• solid–liquid,

• solid–vapor.

saturation state

state at which a phase change begins or ends

vapor dome

dome-shaped region composed of the two-phase liquid–vapor states

saturation temperature

temperature at which a phase change takes place at a given pressure

saturation pressure

pressure at which a phase change takes place at a given temperature

p–v diagram

pressure–specific volume plane

⭐T–v diagram

temp vs volume

compressed liquid (subcooled liquid)

temperature at these states is less than the saturation temperature at the given pressure

two-phase liquid–vapor mixture

distinguished from one another by the quality, an intensive property.

quality (x)

• ratio of the mass of vapor present to the total mass of the mixture

• only for saturated

• x= 0 👉saturated liquid

• x=1 👉 saturated vapor

$$x=\frac{v-v_{f}}{v_{g}-v_{f}}$$

v= given velocity

superheated vapor

states that would be attained by further heating while keeping the pressure constant.

enthalpy (hf, hg, hfg) (unit: kJ/kg)

constant pressure

$$h=h_{f}+x(h_{g}-h_{f})$$

specific internal energy(u, uf, ug) (unit: kJ/kg)

constant volume

$u=u_{f}+x(u_{g}-u_{f})$

specific volume (vf, vg, v) (unit: m³/kg)

$v=v_{f}+x(v_{g}-v_{f})$

specific heats (cυ, cp )

relate the temperature change of a system to the amount of energy added by heat transfer

universal gas constant R

8.314 kJ/mol

compressibility factor (Z)

$\frac{pv}{\overline{R}T}$

ideal gas model

2

For a simple compressible system, how many independent intensive properties must be specified to fix the state?

temperature

The ideal gas model includes the ideal gas equation of state to relate pressure specific volume and temperature, and the fact that the specific internal energy and specific enthalpy each depend only on ___________.

Intensive Properties

• Temperature

• Pressure

• Specific heat cap (c )

• specific internal energy (u)

• specific enthalpy (h)

• specific volume

True

T/F: For pure, simple, compressible systems, temperature and specific volume can be regarded as independent intensive properties.

False

T/F: If a closed system consisting of a simple compressible substance is at equilibrium, only one phase can be present.

True

T/F: The pressures listed in thermodynamic tables are absolute pressures, NOT gage pressures.

True

T/F: When an ideal gas undergoes a polytropic process with n = 1 the gas temperature remain constant.

False

T/F: A two-phase liquid-vapor mixture with equal volumes of saturated liquid and saturated vapor has a quality of 0.5.

True

T/F: As pressure increases toward the critical pressure, the values of vf and Ug approach each other.

False

T/F: For simple compressible systems, any two intensive thermodynamic properties fix the state.

True

A polytropic process for an ideal gas with n = k is adiabatic.

mass flow rates (m dot)

time rate of change of mass within the control volume

\frac{dmcv}{\differentialD t}=m_{i}-m_{e}

one-dimensional flow

• The flow is normal to the boundary at locations where mass enters or exits the control volume.

• All intensive properties assumed to be uniform across the inlet or exit area.

$$\displaylines{\dot{m}=\rho VA\\ \dot{m}=massflowrate\\ \rho=density\\ A=area}$$

volumetric flow rate (units: m3/kg or ft3/s)

flow of volume

• v= specific volume (m³/kg)

• V= volume (m³)

• m= mass (kg)

steady state

all properties are unchanging in time

mass flux

gives the time rate of mass flow per unit of area

flow work

account for the work associated with the pressure at the inlet and exit

energy rate balance

$0=Q_{cv}-W_{cv}+\dot{m}\left(\left(h_{i}-h_{E}\right)+\frac{v_{i}^2-v_{e}^2}{2}+g\left(z_{i}-z_{e}\right)\right)$ Qcv= heat transfer

Wcv= work

m-dot= mass flow rate

hi= specific enthalpy (in)

he= specific enthalpy (out)

vi= velocity (in)

ve= velocity (out)

g= gravity

zi= heigth (in)

ze= height (out)

nozzle

• a flow passage of varying cross-sectional area in which the velocity of a gas or liquid increases in the direction of flow.

• accelerates flow

diffuser

the gas or liquid decelerates in the direction of flow

turbine

a device in which power is developed as a result of a gas or liquid passing through a set of blades attached to a shaft free to rotate

compressor (gas)

devices in which work is done on the gas flowing through them in order to change the state of the gas

pump (liquid)

devices in which work is done on the liquid flowing through them in order to change the state of the liquid

heat exchanger

used in

• home ac systems

• automotive systems

throttling calorimeter

a device for determining the quality of a two-phase liquid–vapor mixture

throttling process

When the flow through the valve or other restriction is idealized

system integration

combine components to achieve some overall objective

transient operation

state changes with time.