Applied thermo Flashcards
(28 cards)
power cycle define
thermodynamic cycles that convert HEAT to WORK
W(exp) =
W(exp) = - PdV
power cycle P vs V graph - what does the enclosed area represent?
the net expansion work
define thermal efficiency of power cycle
n = net work out / heat in
= 1 - (heat out / heat in)
isobaric expansion: expressions for work and heat
Constant P:
W = -PdV = -P(V1-V2)
Q = ∆H = #mols * integral of specific Cp between T1 –> T2
isothermal expansion: expressions for work and heat
constant T. IG: PV = constant.
W = - integral of P dV between V1 and V2 as P is now changing
IGL: = -nRT ln(V2/V1)
U constant for same T, so ∆U = Q+W=0
Q = -W = + nRT ln(V2/V1)
adiabatic expansion: expressions for work and heat
PV^y = 0
Q = 0
W(exp) = ∆U.
U=f(T)
W = #mols * integral of specific Cv dT
= n Cv (T2-T1)
what is the Carnot cycle? what two types of compression/expansion are involved?
ideal reversible cycle
gives max possible efficiency of any power cycle
isothermal and adiabatic compression and expansion
efficiency of Carnot cycle
n = 1 - T(C) / T(H)
Temperatures of source (H) and sink (C)
From Q(out)/Q(in), but ∆S cancels out top and bottom
recall dS = 𝛿Q/T for reversible processes so Q = T(S2-S1) at const T
what is the Otto cycle? what types of compression/expansion are involved?
ideal cycle for spark-ignition petrol engines
adiabatic (Q=0) and isochoric (const V, W=0) steps
what is r in the Otto cycle?
the compression ratio:
r = V1/V2 = V4/V3
for step 1–>2 adiabatic exp
3–>4 adiabatic comp
adiabatic gas law
PV^𝛾 = constant
efficiency Otto cycle
n = 1 - 1 / r^(𝛾-1)
depends only on compression ratio r for ideal gas
purpose of a pump
increase the pressure of a liquid
pumps and compressors require the input of
shaft work
ideal pumps and compressors are considered to be
adiabatic and reversible, hence isentropic
isentropic efficiency factor for pumps and compressors
n = ideal (isentropic) work needed / actual work needed
ideal valves are considered to be
adiabatic, with no shaft work
SFEE –> ∆H = 0, ISENTHALPIC
what is a turbine?
extracts mechanical work (P drop, W out)
opposite of a compressor
ideal turbines are considered to be…
isentropic.
adiabatic and reversible if no frictional losses.
efficiency of a turbine
n.= actual work out / ideal work out
energy balance for pumps, compressors, turbines:
m ∆h = Q + Ws
all in flow rates (dot on m, Q, Ws)
for continuous process at SS.
(this is lowkey from the SFEE)
compare a heat pump / refrigeration cycle to a heat engine
opposite to a heat engine.
uses the INPUT of work to move heat
heat pump vs refrigerator
heat pump uses work to deliver heat energy to a location
refrigerator uses work to remove heat AWAY from location
