MEEN 315 Exam 2 T&F

For liquid water, the approximation v(T,p) ≈ v_f(T) is reasonable for many engineering applications.

True. Liquids are nearly incompressible, so pressure has little effect on specific volume. Using v_f(T) is a good approximation.

For gases modeled as ideal gases, the ratio c_v/c_p must be greater than one.

False. Since c_p = c_v + R, we always have c_p > c_v, so c_v/c_p < 1.

As pressure increases toward the critical pressure, the values of v_f and v_g approach each other.

True. At the critical point, saturated liquid and vapor become identical, so their specific volumes converge.

A two-phase liquid-vapor mixture with equal volumes of saturated liquid and saturated vapor has a quality of 0.5.

False. Quality is based on mass, not volume. Vapor has much larger specific volume, so equal volumes ≠ equal masses.

If a closed system consisting of a simple compressible substance is at equilibrium, only one phase can be present.

False. Multiple phases (like liquid-vapor mixtures) can coexist at equilibrium.

For a one-inlet, one-exit control volume at steady state, mass flow rates are equal but volumetric flow rates may differ.

True. Mass flow rates are equal, but volumetric flow rate = m_dot * v, so changes in specific volume cause differences.

For a control volume at steady state, mass can accumulate.

False. Steady state means no accumulation of mass or energy.

Flow work is work done by a paddle wheel or piston.

False. Flow work is pv work to push fluid into/out of a control volume; paddle wheel work is shaft work.

At steady state, energy in = energy out.

True. No energy accumulation means rates in and out must balance.

At steady state, mass can accumulate.

False. Steady state implies no mass accumulation.

As velocity decreases in a diffuser, pressure decreases.

False. Diffusers convert velocity to pressure, so pressure increases as velocity decreases.

A restriction increases pressure significantly.

False. Restrictions cause pressure drops due to friction and losses.

A pump increases pressure by doing work on a liquid.

True. Pumps add shaft work energy to raise the pressure of a liquid.

Negligible heat transfer assumptions (insulation, small area, small ΔT, short time) are valid.

True. All listed conditions justify approximating Q ≈ 0.

Neglecting KE and PE changes applies to all steady-flow devices including nozzles and diffusers.

False. Kinetic energy changes are crucial in nozzles and diffusers and cannot be neglected.

Throttling process: h_1 = h_2.

True. Throttling is isenthalpic — steady state, no work, negligible KE and PE changes.

Friction is an irreversibility.

True. Friction generates entropy and dissipates useful energy.

Carnot efficiency limits wind turbines.

False. Carnot efficiency applies to heat engines, not kinetic energy devices like wind turbines.

Max efficiency between 1000°C and 500°C is 50%.

False. Must use Kelvin: efficiency ≈ 39.3%, not 50%.

All power cycles operating between the same reservoirs have the same efficiency.

False. Only reversible cycles share the same maximum efficiency; real cycles are less efficient.

The Clausius statement forbids heat transfer from cold to hot without work.

True. This is exactly the Clausius statement of the second law.

Entropy change is the same for any process between two states.

True. Entropy is a state function, so it depends only on the end states.

Entropy of an incompressible substance increases when temperature increases.

True. ds = c * ln(T2/T1), which increases with temperature.

Violating the second law implies violating the first law.

False. Energy can be conserved while still violating entropy constraints of the second law.