Electrical, Thermodynamics & Fluid Systems Flashcards

A 3-phase balanced load draws 30 kW at 0.8 power factor lagging from a 480 V (line-to-line) source. What is the line current?

Answer: 45.1 A

P = √3 × V_L × I_L × PF → I_L = 30000 / (√3 × 480 × 0.8) ≈ 45.1 A.

Which thermodynamic cycle best approximates the operation of a gas turbine engine?

Answer: Brayton cycle

Gas turbines operate on the Brayton cycle, which consists of isentropic compression, constant-pressure heat addition, and isentropic expansion.

For fully developed laminar flow in a circular pipe, the friction factor f is related to Reynolds number Re by:

Answer: f = 64/Re

The Hagen-Poiseuille result gives the Darcy friction factor f = 64/Re for laminar pipe flow.

A capacitor is charged to 100 V and then connected in series with a 10 kΩ resistor. After one time constant, the voltage across the capacitor is approximately:

Answer: 36.8 V

For a discharging RC circuit, V(τ) = V₀ e^(−1) ≈ 0.368 × 100 V = 36.8 V.

Steam enters an adiabatic turbine at 3 MPa and 350°C and exits at 100 kPa. If the isentropic efficiency is 85%, the actual work output compared to ideal is:

Answer: 85% of the isentropic work

Isentropic (turbine) efficiency η_t = w_actual / w_isentropic, so actual work = 0.85 × ideal work.

Bernoulli's equation is strictly valid for flow that is:

Answer: Steady, incompressible, and inviscid along a streamline

Bernoulli's equation assumes steady, inviscid, incompressible flow along a single streamline.

In a DC circuit, two resistors R₁ = 4 Ω and R₂ = 12 Ω are connected in parallel across a 24 V source. What is the total current drawn from the source?

Answer: 8 A

I₁ = 24/4 = 6 A, I₂ = 24/12 = 2 A, total I = 6 + 2 = 8 A.