Hello,
First of all, make sure you follow the workflow example in the documentation when modeling a refrigeration system: https://www.mathworks.com/help/hydro/ug/modeling-a-refrigeration-cycle.html
The key idea here is to test each component in individual test harness models before assembling them into a refrigeration loop.
In a test harness model of a condenser or an evaporator, you can drive both the refrigerant side and the external fluid side using reservoirs and ideal flow rate sources. I usually keep the boundary temperature/states and the flow rate the same as the nominal flow rate of the heat exchanger, just to make sure I get the nominal heat transfer I expect.
Now to the question on how to set the nominal values. The nominal values we provide to the block is supposed to describe one set of typical working condition for this heat exchanger. This set of condition may come from
(1) design requirement: e.g. a description like this heat exchanger should allow ?kW heat transfer for coolant at temperature ??deg and refrigerant at ??MPa, given ?? and ?? flow rate for each side. Such requirement usually are derived from the cooling and heating need of the cycle and the temperature conditions it's designed to operate around. If you are following the work flow I mentioned earlier, you should have such information at this stage.
(2) a set of test data or supplier data. Usually you'll have more than one test conditions. I just pick one set somewhere in the middle. The Colburn coefficients will allow you to do a parameter estimation exercise to match other conditions.
