MEEN 315 exam 1 conceptual

initial step 1

define system. list assumptions

initial step 2

identify type of system

initial step 3

list properties of a system

System

region we want to study

Surroundings

everything external to the system

Boundary

separates the system from the surroundings.

can be at rest OR in motion

Types of system

isolated, closed, or open

Isolated system

no mass transfer or energy transfer across boundary (ex: perfect insulator)

Closed system

no mass transfer across boundary, but there is energy transfer across the boundary (ex: a closed cold drink cup in a warm room)

Open system

there is mass and energy transfer across the boundary (ex: a classroom door opening)

Macroscopic properties

characterize the gross/total behavior of the entire system

Intensive properties

size independent, not additive

Temperature, pressure, density

Extensive properties

size dependent, additive

Volume, mass, energy (typically converted to intensive)

Specific volume

volume/unit mass

v = V/m = 1/density

Specific energy

energy/unit mass

e = E/V

Equilibrium

state of balance

Process

action causing change in a system from one state to another

Zeroth law of thermo

if 2 bodies are in equilibrium with a 3rd body, they are all in equilibrium with each other, even if not in direct contact

Pressure

normal force exerted by fluid per unit area

Absolute pressure

actual pressure

measured with respect to an absolute vacuum

Gage/Vacuum pressure

used for measuring devices

measured with respect to the absolute pressure of the atmosphere

is everything above sea level

Pressure relations

Pgage + Pother = Pabs

Temperature

degree of hotness and coldness

measured to a definite scale

empirical has limitations, like range

Problem solving steps

1. known and find

   1. identify what is known

   2. state what needs to be determined

2. schematic and given data

   1. draw a sketch of the system

   2. label with relevant info

3. engineering model

   1. list all assumptions and idealizations

4. analysis

   1. reduce equations to get results

Transient

properties at the same location change with respect to time

Steady state

all properties of a system at the same location do NOT change with time

Forms of energy

macroscopic and microscopic

Macroscopic energy

processed as a whole with respect to an outside source frame

scaled we can directly see

Microscopic energy

processed by molecular structure of a system

thermal, chemical, and nuclear

independent of outside reference frames

sum equals to internal energy (U)

Potential energy

position of a system in earth’s gravitational field

Kinetic energy

motion of a system as a whole

Total energy of a system

Esys = KE + PE + U

kJ, Btu

Specific energy of a system

esys = ke + pe + u

kJ/kg, Btu/lbm

Modes of energy transfer

Heat, mass, work

only recognized at the system boundary

NOT property

depend on process paths

Heat

transferred by a temperature difference

moves from high T to low T

Q

Adiabatic process

NO heat transfer

Tsystem = Tsurroundings

only energy transfer is considered at the boundary

Work

energy transfer associated with force exerted on a system in the direction of a distance

W = F*S

Wb = integral( P dV ) ,(expansion and compression)

Quasi-equilibrium

moves very slowly

basically stays at equilibrium state at all times

Isochoric

Volume stays constant

makes work always = 0

Polytropic

pressure and volume are related

n = polytropic index between 0 and infinity

p*V^n = constant

Modes of work

boundary work

shaft work: Ws = T(2 pi n)

electrical work: We = V*I

Total energy

static

property

KE, PE, U

belongs to a system

point function (independent from process path)

exact differential

Transferred energy

dynamic

mode of energy transfer

Q, Emass, W (Wb like isobaric, isochoric, etc)

recognized at boundary

NOT properties

path function (depends on process path)

inexact differential

Process paths

area under curves show changes in work (P-V graph)

First law of thermo

Energy can be transferred between systems, but not created or destroyed

delta Esys = delta Ein - delta Eout

Sign conventions of energy transfer

positive is in/to/on

negative is out/by/from

State principle

state of a simple compressible system is completely specified by 2 independent, INTENSIVE (size indep., not additive) properties

Thermodynamic cycle

system that returns to the initial state at the end of the process

Power cycle

delivers net work transfer to surroundings

Refrigeration/Heat pump cycle

transfers heat from a cold body to a hot body by an addition of work input

Pure substance

constant chemical composition throughout

cannot be separated by physical separation

homogenous

may exist in more than one phase (ex: h2o can be solid, liquid, gas, or mix)

Latent heat

amount of heat energy absorbed or released during a phase change process

Latent heat of fusion (hif)

energy absorbed during melting

equals energy released during freezing

Latent heat of vaporization (hfg)

energy absorbed during vaporization

equal to energy released during condensation

Phase changes

oscillation changes phases

processes are reversable

Phase diagram (P-T)

supercritical: liquid and vapor do not differentiate past critical point

triple point: all 3 phases coexist in equilibrium

melting line shows whether a substance expands or contracts with freezing (moves left = expands, moves right = contracts)

Liquid-vapor phase change process states

1. compressed liquid: substance NOT about to vaporize

2. saturated liquid: liquid about to vaporize

3. saturated mixture: liquid and vapor states coexist in equilibrium

4. saturated vapor: vapor about to condense

5. superheated vapor: vapor NOT about to condense

T-V diagram

saturation temp (Tsat): temp when a pure substance changes phase at a given pressure

saturation pressure (Psat): pressure when a pure substance changes phase at a given temp

Tsat = f(Psat): there are multiple Tsats that depend on the pressure!

Tsat2 >Tsat1 and P2 > P1

saturated liquid and saturated vapor curves meet at the critical point

the linear sections go from low to high from L to R

P-V diagram

Psat is the line in the sat mix region

T1 is isotherm (constant temperature)

relies on the state principle

the linear sections go from high to low from L to R

Quality (x)

mg/mt

x = 0: saturated liquid

x = 1: saturated vapor

x between 0 and 1: saturated mixture

Linear interpolation

finds a value between 2 known values

can only do this between 2 points, do NOT extrapolate

v < vf

compressed liquid

vf < v < vg

saturated mixture

v > vg

superheated vapor

T < Tsat at a given P OR P > Psat at a given T

compressed liquid

T > Tsat at a given P OR P < Psat at a given T

superheated vapor

Ideal gas equation of state

relates pressure, temp, and specific volume

pv = RT

pV = mRT

pV - nRT

For ideal gas, R = Rbar/MW

Real gases

hypothetical gas with molecules that take up negligible space and have no interactions

low density, high T, low P

Compressibility factor (Z)

accounts for deviation of real gases from ideal gas behavior at a given T and P

Z = vactual / videal

Principle of corresponding states

Z is the same for all gases at the same values of the reduced temperature and reduced pressure

Tr = T/Tc

Pr = P/Pc

Specific heat capacity

heat energy required to raise the temperature of the unit mass of a substance by 1 degree

constant volume specific heat (cv)

constant pressure specific heat (cp)

Cp is ALWAYS greater than Cv for all substances

Incompressible substance

substance whose specific volume or density is constant

solids and liquids