Nature of throttling (Joule–Thomson) process Classify a throttling expansion (flow through a partially closed valve or porous plug) in terms of reversibility and flow type.

Introduction / Context:
Throttling is ubiquitous in refrigeration, gas distribution, and pressure reduction stations. Understanding its thermodynamic character clarifies why enthalpy is (approximately) conserved while entropy increases, and why it is modeled as an irreversible steady-flow process.

Given Data / Assumptions:

• Steady flow through a valve/orifice/porous plug.
• Negligible heat transfer and shaft work; kinetic/potential energy changes typically small.
• Large pressure drop across a localized restriction.

Concept / Approach:

In throttling, the control volume analysis (steady-flow energy equation) with Q ≈ 0, W_s ≈ 0, and negligible KE/PE changes yields h1 ≈ h2. Large pressure gradients, viscous dissipation, and mixing cause internal entropy generation, so the process is inherently irreversible. Because inlet and outlet conditions are steady, throttling is a steady-flow process even though the fluid experiences a non-equilibrium drop through the restriction.

Step-by-Step Solution:

Verification / Alternative check:

Joule–Thomson coefficient experiments (μ_JT) measure temperature changes at constant enthalpy, confirming h ≈ const while entropy rises in real throttling valves.

Why Other Options Are Wrong:

Assuming ideal-gas exception: even ideal gases experience irreversible throttling (though ΔT ≈ 0 for an ideal gas because h = h(T)).Entropy decreases claim contradicts the Second law for adiabatic throttling of real fluids.

Common Pitfalls:

Confusing throttling with isentropic expansion in turbines; turbines produce work with near-reversible expansion, whereas throttling destroys availability without producing work.

Final Answer:

Correct