Thermodynamics 1: cram pack

Formula sheets, the key method steps for every topic and 8 practice problems, with full solutions at the back.

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1Formulas and methods

Ideal gas law

Ideal gas law: pV = nRT
Gas constant: R = 8.314 J/(mol·K)
Combined gas law: p₁V₁/T₁ = p₂V₂/T₂
Moles: n = m/M
Temperature: T[K] = T[°C] + 273.15

Energy balance

Sensible heat: Q = ṁ·c_p·ΔT
Latent heat: Q = ṁ·h_fg
Mixing: Σṁc_pT_in = Σṁc_pT_out
Water: c_p ≈ 4.18 kJ/(kg·K)

Heat engines & refrigerators

Efficiency: η = W/Q_H = 1 − Q_C/Q_H
Carnot efficiency: η_C = 1 − T_C/T_H (kelvin)
Refrigerator COP: COP_R = Q_C/W
Heat pump COP: COP_HP = Q_H/W = COP_R + 1
Energy balance: Q_H = W + Q_C

Gas power cycles

Isentropic, volume ratio: T₂/T₁ = r^k−1
Isentropic, pressure ratio: T₂/T₁ = r_p^(k−1)/k
Otto efficiency: η = 1 − 1/r^k−1
Diesel efficiency: η = 1 − (1/r^k−1)·(r_c^k − 1)/(k(r_c − 1))
Brayton efficiency: η = 1 − 1/r_p^(k−1)/k
Heat added: Otto: c_v(T₃ − T₂) Diesel, Brayton: c_p(T₃ − T₂)
Brayton net work: w = c_p[(T₃ − T₄) − (T₂ − T₁)]
Isentropic efficiencies: w_c = w_c,s/η_c, w_t = η_t·w_t,s
Cold air: k = 1.4, c_p = 1.005, c_v = 0.718 kJ/(kg·K)

2Practice problems

1.Ideal gas lawMedium

What volume V does 350 g of He occupy at 350 kPa and 20 °C? Treat it as an ideal gas.

m = 350 gM = 4.003 g/molP = 350 kPaT = 20 °CR = 8.314 J/(mol·K)

V = L

2.Ideal gas lawHard

Find the density ρ of Ar at 350 kPa and 100 °C, treating it as an ideal gas.

M = 39.95 g/molP = 350 kPaT = 100 °CR = 8.314 J/(mol·K)

ρ = kg/m³

3.Energy balanceMedium

Hot light oil cools water in a counter-flow heat exchanger with no heat loss. Find the water outlet temperature t_out.

ṁ_h = 2 kg/sc_p,h = 2.1 kJ/(kg·K)T_h: 120 → 60 °Cṁ_c = 1.5 kg/sc_p,c = 4.18 kJ/(kg·K)t_in = 30 °C

t_out = °C

4.Energy balanceHard

Hot light oil heats water in an exchanger that loses 5% of the heat released by the hot stream to the surroundings. Find the water flow rate ṁ_c needed.

ṁ_h = 3 kg/sc_p,h = 2.1 kJ/(kg·K)T_h: 130 → 90 °Ct: 30 → 65 °Cc_p,c = 4.18 kJ/(kg·K)loss = 5 %

ṁ_c = kg/s

5.Heat engines & refrigeratorsMedium

A heat engine works between a hot reservoir at 625 °C and a cold reservoir at 20 °C. It takes in 800 kJ of heat. Find the most work W_max it could possibly produce.

T_H = 625 °CT_L = 20 °CQ_H = 800 kJ

W_max = kJ

6.Heat engines & refrigeratorsHard

A refrigerator keeps its cold space at -10 °C in a kitchen at 30 °C, removing heat from the cold space at 2 kW. Find the highest possible coefficient of performance COP_max.

T_H = 30 °CT_L = -10 °CQ̇_L = 2 kW

COP_max =

7.Gas power cyclesMedium

An ideal Diesel cycle has a compression ratio r = 20 and a cutoff ratio r_c = v₃/v₂ = 1.5. At the start of compression the air is at 320 K. Use the cold-air-standard assumptions: air is an ideal gas with constant specific heats, k = 1.4, c_p = 1.005 kJ/(kg·K) and c_v = 0.718 kJ/(kg·K). Find the temperature T₃ at the end of heat addition, in K.

r = 20r_c = 1.5T₁ = 320 Kk = 1.4

T₃ = K

8.Gas power cyclesHard

A gas-turbine plant runs on a Brayton cycle with a compressor pressure ratio of 8. Air enters the compressor at 288 K and the turbine at 1400 K. The compressor has an isentropic efficiency of 88% and the turbine 90%. Use the cold-air-standard assumptions: air is an ideal gas with constant specific heats, k = 1.4, c_p = 1.005 kJ/(kg·K) and c_v = 0.718 kJ/(kg·K). Find the net work output w_net per kilogram of air, in kJ/kg.

r_p = 8T₁ = 288 KT₃ = 1400 Kη_c = 88%η_t = 90%k = 1.4c_p = 1.005 kJ/(kg·K)

w_net = kJ/kg

3Solutions

1.Ideal gas law

Answer: V = 608.9 L

Ideal gas law: PV = nRT, with n = m/M.

Moles

n = 350/4.003 = 87.43 mol

T = 20 + 273.15 = 293.1 K

Volume

V = nRT/P = 87.43·8.314·293.1/350 = 608.9 L

Answer

V = 608.9 L

2.Ideal gas law

Answer: ρ = 4.507 kg/m³

Ideal gas density: ρ = PM/(RT).

Rearrange the gas law

PV = (m/M)RT → ρ = m/V = PM/(RT)

T = 100 + 273.15 = 373.1 K

Density

ρ = 350·39.95/(8.314·373.1) = 4.507 kg/m³

Answer

ρ = 4.507 kg/m³

3.Energy balance

Answer: t_out = 70.19 °C

Exchanger energy balance: with no losses, ṁ_hc_p,h(T_h,in − T_h,out) = ṁ_cc_p,c(t_out − t_in).

Heat released by the hot stream

Q = 2·2.1·(120 − 60) = 252 kW

Heat picked up by the water

t_out = t_in + Q/(ṁ_cc_p,c) = 30 + 252/(1.5·4.18) = 70.19 °C

Answer

t_out = 70.19 °C

4.Energy balance

Answer: ṁ_c = 1.636 kg/s

Energy balance with losses: heat released = heat absorbed + heat lost.

Heat released

Q_h = 3·2.1·(130 − 90) = 252 kW

Heat that reaches the water

Q_c = (1 − 0.05)·252 = 239.4 kW

Water flow rate

ṁ_c = Q_c/(c_p,c·Δt) = 239.4/(4.18·35) = 1.636 kg/s

Answer

ṁ_c = 1.636 kg/s

5.Heat engines & refrigerators

Answer: W_max = 538.9 kJ

Second-law limits: no cycle working between reservoirs at absolute temperatures T_H and T_L can beat the reversible (Carnot) cycle: η_max = 1 − T_L/T_H, COP_R,max = T_L/(T_H − T_L) and COP_HP,max = T_H/(T_H − T_L). Energy is always conserved: Q_H = W + Q_L.

Absolute temperatures

T_H = 625 + 273.15 = 898.1 K

T_L = 20 + 273.15 = 293.1 K

Carnot efficiency

η_max = 1 − T_L/T_H = 1 − 293.1/898.1 = 67.36 %

Maximum work

W_max = η_max·Q_H = 0.6736·800 = 538.9 kJ

Answer

W_max = 538.9 kJ

6.Heat engines & refrigerators

Answer: COP_max = 6.579

Absolute temperatures

T_H = 30 + 273.15 = 303.1 K

T_L = -10 + 273.15 = 263.1 K

Best possible COP

COP_R,max = T_L/(T_H − T_L) = 263.1/(303.1 − 263.1) = 6.579

Answer

COP_max = 6.579

7.Gas power cycles

Answer: T₃ = 1591 K

Diesel cycle (compression-ignition engine): 1→2 isentropic compression, 2→3 heat addition at constant pressure, 3→4 isentropic expansion, 4→1 heat rejection at constant volume. With compression ratio r = v₁/v₂ and cutoff ratio r_c = v₃/v₂: T₂ = T₁·r^k−1, T₃ = r_c·T₂, q_in = c_p·(T₃ − T₂) and η = 1 − (1/r^k−1)·(r_c^k − 1)/(k·(r_c − 1)).

End of compression

T₂ = 320·20^0.4 = 1061 K

Heat addition at constant pressure

T₃ = r_c·T₂ = 1.5·1061 = 1591 K

Answer

T₃ = 1591 K

8.Gas power cycles

Answer: w_net = 300.4 kJ/kg

Brayton cycle (gas turbine): 1→2 isentropic compression, 2→3 heat addition at constant pressure, 3→4 isentropic expansion in the turbine, 4→1 heat rejection at constant pressure. With pressure ratio r_p = P₂/P₁: T₂ = T₁·r_p^(k−1)/k, T₄ = T₃/r_p^(k−1)/k and η = 1 − 1/r_p^(k−1)/k. Net work w = c_p·[(T₃ − T₄) − (T₂ − T₁)]. With isentropic efficiencies, the compressor needs w_s/η_c and the turbine gives η_t·w_s.

Isentropic temperatures

T_2s = 288·8^0.2857 = 521.7 K

T_4s = 1400/8^0.2857 = 772.9 K

Compressor and turbine work

w_c = 1.005·(521.7 − 288)/0.88 = 266.9 kJ/kg

w_t = 0.9·1.005·(1400 − 772.9) = 567.2 kJ/kg

Net work

w_net = 567.2 − 266.9 = 300.4 kJ/kg

Answer

w_net = 300.4 kJ/kg

Back-work ratio: w_c/w_t = 47.05%.

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