ME200 Purdue Exam 2

open system

mass crosses the system boundary; a control volume analysis is needed

one dimensional flow

normal to the boundary at locations where mass enters or exits the control volume

uniform flow (uniform state)

all intensive properties are uniform with position over each inlet and exit area though which mass flows (temperature does not vary)

mass flow rate

mdot = (density)(area)(velocity)

= (area) (velocity) / (specific volume)

= (volumetric flow rate) / (specific volume)

volumetric flow rate

vdot = mdot / density = area * velocity

steady state

change in energy and change in mass are zero

steady flow

change in mass is zero

flow energy

the amount of work required to push mass into and out of the given system to maintain a continuous flow

Wflow = pressure * specific volume

internal energy

u

for energy contained within the system

enthalpy

h = u + Pv

for energy accompanying mass flow

nozzle

accelerates fluid flow

A2 < A1

V2 > V1

P2 < P1

hout < hin

diffuser

decelerates fluid flow

A2 > A1

V2 < V1

P2 > P1

hout > hin

assumptions for nozzles and diffusers

SSSF

adiabatic

deltaPE = 0

Wdot = 0

W = 0

turbine

produces work by expanding the working fluid to a lower pressure, density, and enthalpy

produce power

hout < hin

Pout < Pin

vout > vin

Wdot > 0

assumptions for turbines

SSSF

adiabatic

deltaPE = 0

neglect deltaKE

1DUF

compressor

increases pressure, density, and enthalpy of a gas through a work input

working fluid is in vapor or gas phase

fans work similar to compressors but for small pressure rises

hout > hin

pout > pin

vout < vin

Wdot < 0

assumptions for compressors

SSSF

adiabatic

deltaPE = 0

neglect detlaKE

1DUF

pump

similar to compressors fundamentally

working fluid in liquid phase

hout > hin

pout > pin

vout < vin

assumptions for pumps

SSSF

adiabatic

deltaPE = 0

neglect deltaKE

working fluid incompressible

1DUF

heat exhanger

non-mixing flow streams are allowed to exchange heat within the control volume

no energy enters or leaves the heat exchanger as heat or work

mass flow rate of each stream must be conserved independently

assumptions for heat exchangers

SSSF

externally adiabatic

deltaPE = 0

deltaKE = 0

fluid pressure doesn't change (deltap = 0)

Wdot = 0

mixing device

similar to heat exchanger in that 2 or more streams exchange heat with each other

heat transfer occurs through direct mixing internally

assumptions for mixing devices

SS

adiabatic

deltaPE = 0

deltaKE = 0

deltap = 0

1DUF

output flow is fully mixed

Wdot = 0

throttling valve

flow restricting devices that cause a pressure drop without a work output

used to:

-control flow

-provide temperature drop (refrigeration, air conditioning)

-measure flow rates

assumptions for throttling valve

SSSF

adiabatic

deltaKE = 0

1DUF

deltaPE = 0

Wdot = 0

system analysis

neglect losses in connections: fluid state exiting one component is equal to the state entering the next component

enthaply

represents energy accompanying mass flow

specific energy

represents energy contained within a system

thermodynamic cycle

sequence of processes that begins and ends at the same state; there is no change in system energy

power cycle

objective is to generate power

Wcycle = Qin - Qout

heating/cooling cycles

objective is to generate heating or cooling

Wcycle + Qin = Qout

thermal efficiency

desired output / required input

transient behavior

state is changing as a function of time

opposite of SS

must integrate

if something is filling

uniform flow

when the thermodynamic state at all inlets and exits is constant with respect to time

hi and he are constant

uniform state

when the intensive properties within a control volume are uniform with respect to position at a particular time t

V = m * specificv

U = m * specificu

ideal gas model

Pv = RT

PV = mRT

T-v diagram

temperature increases with constant pressure from CL to SHV

P-v diagram

pressure decreases with constant temperature from CL to SHV

quality

mvapor / mtotal

1DUF

mdot = (density)(area)(velocity)

adiabatic

Qdot = 0

ignore KE

Vi = 0

Ve = 0

ignore PE

zi = 0

ze = 0

ideal gas

both internal energy and enthalpy depend only on temperature

cp(T) = cv(T) + R

h(T) = u(T) + RT