Flow processes — gas expansion through a minute aperture When an ideal (perfect) gas is forced through an aperture of minute dimensions or a porous plug, the process is best described as which of the following?

Introduction / Context:
Gas expansions can occur in different ways: through turbines (doing work), through nozzles (kinetic energy rise), or through restrictions such as valves and porous plugs (throttling). Correctly naming the process is vital for enthalpy and temperature change predictions in refrigeration and gas processing.

Given Data / Assumptions:

• Aperture or porous plug of minute dimensions causing significant pressure drop.
• Steady-flow, adiabatic device; no shaft work.
• Negligible changes in kinetic and potential energies.

Concept / Approach:
For throttling (Joule–Thomson) processes, the steady-flow energy equation reduces to h1 ≈ h2 (enthalpy remains approximately constant). Temperature may change depending on the gas and inlet conditions (via Joule–Thomson coefficient). This differs from free expansion, which is a non-flow, sudden expansion into vacuum with no defined inlet/outlet states for steady flow.

Step-by-Step Solution:
Identify device: minute aperture/porous plug with pressure drop ⇒ throttling.Apply steady-flow energy balance with Q ≈ 0, W ≈ 0, ΔKE ≈ 0, ΔPE ≈ 0 ⇒ h2 ≈ h1.Conclude process name: Joule–Thomson (throttling) expansion.

Verification / Alternative check:
In refrigeration cycles using gas (e.g., Bell–Coleman alternatives for liquids use expansion valves), throttling is standard for liquids; for gases, temperature change depends on inversion temperature and initial state.

Why Other Options Are Wrong:
Isothermal and adiabatic reversible (isentropic) imply different energy changes; free expansion is unrestrained into vacuum, not through a tiny aperture; nozzle expansion produces large velocity rises, not enthalpy-constant throttling.

Common Pitfalls:
Confusing free expansion with throttling; assuming temperature must always drop—ideal gas shows no temperature change in throttling because h depends only on T.

Final Answer:
Throttling process (Joule–Thomson expansion)