Final answer:
The critical heat flux in a frictionless duct is the maximum heat transfer rate without causing a critical transition in the flow pattern.
Step-by-step explanation:
The critical heat flux in a frictionless duct refers to the maximum heat transfer rate that can be achieved without causing a critical transition in the flow pattern.
When the critical heat flux is reached, the flow pattern changes from a single-phase (liquid-only) flow to a two-phase (liquid-vapor) flow.
This transition can lead to reduced heat transfer efficiency and potential damage to the system.