Mechanical Engineering: Thermodynamics, Fluids, Machine Design and Plant
Mechanical Engineering in IESL Part III tests thermodynamic cycle analysis, fluid flow and turbomachinery, heat transfer, strength and machine element design, manufacturing tolerances, and plant equipment such as boilers and refrigeration systems.
Core rules and concepts
- Use absolute pressure in gas laws and cycles: p_abs = p_gauge + p_atm. Cycle efficiency for Rankine and Brayton cycles comes from the enthalpy differences at the actual states, not the ideal ones.
- Bernoulli with losses: p1/ρg + v1²/2g + z1 = p2/ρg + v2²/2g + z2 + h_L, with h_L from Darcy–Weisbach, h_f = f(L/D)(v²/2g).
- Pump power input P = ρgQH/η, and the affinity laws: Q ∝ N, H ∝ N², P ∝ N³; throttling a pump wastes the difference between the two head curves.
- NPSH available must exceed NPSH required by a margin; NPSH_A = (p_suction − p_vapour)/ρg + z − h_L.
- Machine design: fatigue uses alternating and mean stresses with a Goodman-type diagram and stress concentration factors; ball bearing life L10 is quoted in millions of revolutions and scales as (C/P)^3.
Worked example
A pump delivers 0.02 m³/s against 30 m head with 0.75 overall efficiency and water density 1000 kg/m³. Hydraulic power = ρgQH = 1000 × 9.81 × 0.02 × 30 = 5886 W. Shaft power = 5886/0.75 = 7848 W, so select a motor of about 7.5–11 kW to allow margin. If speed dropped to 80%, flow would fall to 0.016 m³/s and power to 0.8³ = 0.512 of the original, confirming the cube law.
Traps that cost marks
Using gauge pressure in gas calculations; omitting the loss term from Bernoulli; applying the affinity laws to a throttled system as if it were a single curve; mixing isentropic and overall efficiency; and quoting bearing life without stating millions of revolutions and the load basis.