MENG 260 Midterm 2

closed system

no mass crosses the system boundary

control volume

mass can cross the system boundary and we have to account for the energy balance equation

mass flow rate

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

steady state theory

all properties are unchanging in time dmcv/dt = 0

entropy (s)

-an extensive property
-non-conserved it's produced
- it is impossible for any system to operate in a way that entropy is destroyed

entropy and heat

entropy accompanies heat transfer
-direction is the same as the heat transfer +Q (in, s is going to increase) and -Q(out, s is going to decrease)

sigma

entropy production

isentropic process

a constant-entropy

adiabatic process

has no heat transfer Q=0

specific entropy

s = s(f) + x(s(g)-s(f))

2nd Law of Thermodynamics allows us to...

-predict the direction of processes
-establish conditions for equilibrium
-determines the best theoretical performance of cycles
-define a temp scale
-develop means for evaluating properties (u and h)

2nd law statements

-Clausius
-Kelvin-Planck
-Entropy

Clausius Statement

specifies the limitations of refrigeration cycles and heat pumps
-impossible for energy transfer by heat to go from cooler to hotter body

Kelvin-Planck Statement

specifies the limitation of power cycles (aka heat engines)
-impossible for us to convert all heat to work W(cycle) it has be W(cycle)= Q(in)-Q(out)

Thermodynamic cycle

is a sequence of processes that begins and ends at the same state( change in energy = 0)
-power cycles
-refrigeration cycles
-heat pump cycles

Power Cycle efficiency (Eta)

n = W(cycle)/Q(in)
W(cycle) = Q(in) - Q(out)

Refrigeration Cycle efficiency (Beta)

B = Q(in)/W(cycle)
W(cycle) = Q(out) - Q(in)

Heat Pump Cycle efficiency (Gamma)

y = Q(out)/W(cycle)
W(cycle) = Q(out) - Q(in)

Thermal Reservoir

is a system that always remains at constant temperature even though energy is added or removed by heat transfer

Entropy Statement

it is produced within systems whenever non-idealities (such as friction) are present
-impossible for any system to operate in a way that entropy is destroyed

Irreversibilities

Actual processes are distinguishable from such idealized processes by the presence of non-idealities. All actual processes are irreversible
-ex. heat transfer, spontaneous chem reaction, friction

reversible process

when no irreversibilites are present within the system and its surroundings

internally reversible process

is a quasiequilibrium process

Power Cycles interacting with 2 thermal reservoirs

n=W(cycle)/Q(H)
n =1 - (Q(C)/Q(H))