This situation is generated where the controlling thermal capacity (C minimum) between that on the air side and that on the fluid side inside the tubes, is the latter. In these conditions, the exchanger does not work properly, and unbalanced situations can be generated inside the calculation engine when you go to perform the calculation using the delta T.

In addition to the warning message related to laminar flow, it could be present a second message related to the exchange coefficients, where it is highlighted that the exchange coefficient on the internal side of the tubes, divided by the ratio internal/external surface, is lower than the exchange coefficient on the air side (similar situation to the one described above, and in any case to be avoided for a correct functioning of the exchanger).

In this situation it's suggest to try to increase the internal surface of the tube or to decrease the finned surface, so as to increase the coefficient on the tubes and make the one on the air side the lower of the two.

Another suggestion would be to try to perform the calculation by setting the fluid flow rate instead of the outlet temperature, so that the calculation engine can proceed directly, without having to trigger iterative mechanisms to find the fluid flow rate that cause these fluctuations on the results.