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Thermodinamica > Lecture 9 - Reversible Work > Flashcards

Lecture 9 - Reversible Work Flashcards

1
Q

What is the equation for work of a closed system?

A

Wb = PdV

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2
Q

Use the first law to write an expression for reversible steady work (in an open system).

A

Assuming no heat transfer or change in kinetic or potential energy:

Q̇ - Ẇ = ṁ[h2-h1 + (V2^2 - V1^2)/2 + g(z2-z1)]
-Ẇ = ṁ[h2-h1]

Dividing by the mass-flow rate:

-w = h2-h1 [in kJ/kg]

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3
Q

Use the first law to write an equation for work in a steady flow device undergoing a reversible process.

A

δq(rev) - δw(rev) = dh + dke + dpe

with entropy, δq(rev) = Tds
from Gibb’s second equation, Tds = dh - vdP

Combining the two equations we get:
δq(rev) = dh - vdP

SUBSTITUTING THE ABOVE INTO THE FIRST LAW,

dh - vdP - δw(rev) = dh + dke + dpe

WE NOW HAVE THE FOLLOWING:

  • δw(rev) = vdP + dke + dpe

INTEGRATING between two states WE GET:

-wrev = v(P2P1) + Δke + Δpe [kJ/kg]

IGNORING KINETIC AND POTENTIAL ENERGY:

wrev = -v(P2P1)

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4
Q

Give the expression for work in an open reversible system.

A

wrev = -v(P2P1)

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5
Q

Give the expression for boundary work in a closed system.

A

Wb = 1∫2 Pdv

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6
Q

In pumps and compressors, we do work …

A

on the system, which means we do negative work

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7
Q

Write the expression for work in a pump or compressor.

A

w(rev) = -[-v(P2-P1)] = 1∫2 vdP

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8
Q

The amount of work done in a compressor depends on …

A

the type of process

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9
Q

In compression, we move from …

A

a lower pressure, P1 to a higher pressure, P2

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10
Q

Isentropic processes are those that …

A

are both adiabatic and reversible

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11
Q

In an isentropic (adiabatic and reversible) compression process, we relate pressure and volume by the relation …

A

Pv^k = cte

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12
Q

In isothermal compression, we relate pressure and volume by the expression …

A

Pv = cte

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13
Q

Real compression processes are …

A

polytropic

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14
Q

A polytropic process is …

A

somewhere between isentropic and isothermal

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15
Q

In a polytropic process, we relate pressure and volume by the equation …

A

Pv^n = cte

where n = some power between 1

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16
Q

In a polytropic process, we relate pressure and volume by the equation …

A

Pv^n = cte

where n = some power between 1 and k (1

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17
Q

W(rev)in = +1∫2 vdP. Draw this on a P-v diagram. What does W(rev)in = +1∫2 vdP represent on the diagram?

A

P
2
dP =========\
│ 1
v (represents the specific volume; an
actual volume value!)

W(rev)in = +1∫2 vdP represents the area to the left of the P-v curve of an isentropic, polytropic, or isothermal process

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18
Q

_____________ process has the least amount of work, and ___________ has the most work.

A

Isothermal

Isentropic

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19
Q

Heat exchangers (el termocambiador/ intercambiador de calor) _______ gases

A

cool

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20
Q

When you cool a gas, the specific volume __________.

A

decreases

21
Q

Density, ρ, =

A

ρ = P/(RT)

22
Q

Specific volume, v, =

A

v = 1/ρ = RT/P

23
Q

Intercoolers reduce the temperature of the gas.

A

True

24
Q

Intercoolers are …

A

gas to liquid or gas to air heat exchangers

25
Q

reversible means ….

A

no friction, viscous dissipation, rapid expansion, or heat transfer across finite temperatures

26
Q

Write the first law for a static, adiabatic open system.

A
  • Ẇ = ṁ[h2-h1]
27
Q

Write the first law for a static, adiabatic turbine.

A

Ẇ = -ṁ[h2-h1] bcz for a turbine, work is positive

28
Q

What is the adiabatic efficiency of a turbine?

A

η(Turbine) = Actual work/Isentropic work

where the mass flow rate cancels because it is the same for both turbines

actual work = -ha+h1 (on mollier diagram for the real entropy)
isentropic work = -h2+h1 on the Mollier diagram

29
Q

What is the Mollier Diagram?

A

A plot of enthalpy vs entropy where we go from high pressure to low pressure in an expansion device

30
Q

In reality, when we go through a turbine, we have irreversibilities, and the entropies increase.

A

True

31
Q

In isentropic expansion, we can say that the entropy …

A

remains the same; s1 = s2

32
Q

Work in a compressor is positive or negative?

A

negative because work is done on the system

33
Q

What is the equation for work in a compressor?

A

w = h2-h1

34
Q

What is the first law for a static adiabatic compressor?

A

-Ẇ = ṁ[h2 - h1]

35
Q

What is the efficiency of a compressor?

A

η = Isentropic Work/Actual Work

where isentropic work = h2s - h1 (on the mollier diagram)
actual work = h2a-h1 on the mollier diagram

36
Q

When dealing with an incompressible fluid, W(rev) =

A 
W(rev) = 1∫2 vdP
W(rev) = v(P2-P1)
37
Q

The efficiency of a pump, ηP, =

A 
ηP = Isentropic work/Isentropic work
ηP = v(P2-P1)/(h2a-h1)
38
Q

What is a nozzle (boquilla/tobera)?

A

a device that accelerates a flow by converting high pressure in a static fluid into kinetic energy

39
Q

What is the first law with respect to a nozzle and why?

A
  • Δh = ΔKE
  • h2+h1 = V2^2/2

because there is no heat transfer, so it is adiabatic, and all of the energy is kinetic, so we can drop the potential energy

Also, a nozzle does not do work, so we drop Ẇ, and ṁ.

WE ALSO ASSUME THAT VELOCITY ON THE INLET ~ = 0 & VELOCITY AT THE EXIT IS NOT 0.

40
Q

Does a nozzle do work?

A

NO

41
Q

What is the isentropic efficiency of a nozzle?

A

ηN = Actual kinetic energy @ exit/Isentropic kinetic energy @ exit

ηN = V2actual^2/V2isentropic^2 = -h2a+h1/ -h2s+h1

42
Q

-h2a+h1 = V2actual^2/2 & -h2s+h1 = V2isen^2/2 only works when …

A

V1 ~= 0

43
Q

In a nozzle, we go from high entropy to low entropy.

A

True

we also go from high pressure to low pressure

44
Q

State the entropy generation equation.

A

Ṡgen = Σṁoutsout - Σṁinsin - ΣQ̇k/Tk

45
Q

We treat air like an ideal gas.

A

True

46
Q

We account for changing specific heats in a process by using …

A

s2^0 - s1^0

47
Q

A watt is the unit of measurement for power. What is the conversion in joules per second?

A

1 Watt = 1 J/s

48
Q

1MPa = ? bar

A

1MPa = 10 bar

49
Q

Power in Spanish.

A

Poder
energía
Potencia

Thermodinamica (10 decks)

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