The diameter of the cooling channel is required, together with its cross sectional area, to calculate the pressure drop through the circuit.
For round elements the relationship is easily calculated, but the wetted perimeter and cross sectional area are required in order to calculate the hydraulic diameter of the element for non-round elements. The length of the element and the friction factor are also required. The friction factor depends upon the Reynolds number in the fluid and the relative roughness of the pipe or duct material. The fluid properties, density and viscosity, also need to be specified, however these apply to the entire network and are element independent.
The loss coefficients K need to be specified in order to solve the flow rate pressure loss equation including frictional and minor losses. Although the solver calculates generic K factor values for bends, elbows, tee junctions, changes in diameter and cross sectional area, if you provide accurate data specific to your channel geometry, the solver ignores the generic calculations in preference for the more accurate data.