Mechanical Engineering: Thermodynamics, Fluids and Strength of Materials
Mechanical Engineering in the paper concentrates on thermodynamics and power-plant cycles, fluid mechanics and pumping, strength of materials, mechanics of machines and maintenance practice, with Sri Lankan plant examples such as the Lakvijaya coal station, the Mahaweli and Uma Oya hydro schemes, and boiler and chiller plant in industry.
Thermodynamics
- Rankine cycle efficiency = (W_turbine − W_pump)/Q_in; superheating and regenerative feed heating raise it, and lowering condenser pressure helps but is limited by cooling-water temperature.
- Boiler efficiency by the indirect (loss) method accounts for dry flue gas, moisture and unburnt carbon losses.
Fluids
- Continuity: Q = A1V1 = A2V2. Bernoulli for ideal flow; add head loss for real flow.
- Darcy–Weisbach: hf = fLV²/(2gD). Pump shaft power P = ρgQH/η.
Worked example
A pump delivers 0.05 m³/s against 30 m head with overall efficiency 0.75. Hydraulic power = ρgQH = 1000 × 9.81 × 0.05 × 30 = 14 715 W. Shaft power = 14 715/0.75 = 19.6 kW, so a motor above this with margin is required. Multiplying by 0.75 instead gives 11 kW - an overloaded motor.
Strength and machines
- Bending: σ = My/I; for a rectangle I = bd³/12, so σmax = 6M/(bd²).
- Gear ratio N1/N2 = T2/T1; belt slip reduces transmitted speed.
- Slenderness ratio governs column buckling, not compressive stress alone.
Traps
- Gauge versus absolute pressure in gas-law problems.
- Using g = 10 in one step and 9.81 in another.
- Confusing mass flow (kg/s) with volume flow (m³/s) for non-water fluids.
- Reporting indicated power when brake power is asked.