Joule–Thomson (porous plug) experiment — Which characterization best describes the expansion process in this classic experiment?

Introduction:
The Joule–Thomson experiment investigates temperature change when a gas is forced through a restriction (porous plug/valve) without external heat exchange. It forms the basis of throttling in refrigeration and liquefaction processes.

Given Data / Assumptions:

• No shaft work and negligible kinetic/potential energy changes across the porous plug.
• System is well-insulated (no heat transfer with surroundings).
• Single steady flow of gas.

Concept / Approach:
Under these conditions, the steady-flow energy equation reduces to h_in ≈ h_out. No heat is added or removed (adiabatic), and with no work, the enthalpy remains constant (isenthalpic). The resulting temperature change depends on the Joule–Thomson coefficient, which varies with gas type and inlet conditions.

Step-by-Step Solution:
Apply steady-flow energy balance with Q̇ ≈ 0 and Ẇ_s ≈ 0.Neglect Δ(KE) and Δ(PE) to obtain h_in = h_out.Conclude: the process is adiabatic and isenthalpic; temperature change is determined by gas properties.

Verification / Alternative check:
Measured temperature drops or rises match predicted signs of the Joule–Thomson coefficient, confirming the isenthalpic nature of throttling.

Why Other Options Are Wrong:

• Isobaric/isothermal do not describe throttling; pressure falls across the plug, and temperature generally changes.
• Adiabatic alone ignores enthalpy constancy; isenthalpic alone omits the insulated condition central to the experiment.

Common Pitfalls:
Confusing throttling with isentropic expansion in an expander (which produces work and is not isenthalpic).

Final Answer:
Both adiabatic and isenthalpic