**2. Materials and Methods**

The original model discussed in [26] assumed adiabatic flow, that is, no heat transfer was allowed on the walls of the parallel flow channels in the distribution system. The model was shown in the same article to provide data with relative errors of at most 4% compared to detailed transient CFD simulations even in the case of highly turbulent flows. Such accuracy was achievable due to the relative simplicity of the flow systems for which the respective model has been intended (e.g., tube bundles in heat recovery steam generators). The overall conclusion, therefore, was that, in terms of application in preliminary analyses of selected heat transfer equipment or for shape optimization of the mentioned equipment, the model was suitable for engineering practice.

Because of the nature of the model, its performance in case of laminar flow was a priori expected to be acceptable. Although several tests were carried out earlier even with low total mass flow rates to make sure this really was the case, no example was given in [26]. To remedy this, let us mention, for instance, one of the test flow distribution systems (see Figure 1) used in the original article and the respective laminar flow distribution data and relative errors. For convenience's sake, parameters of the flow system are listed in Table 1. The obtained mass flow rates are compared in Figure 2a, while Figure 2b shows the corresponding relative errors. It can be seen that the error values generally were in a ±1% band with only two of them being at around 1.2%. Relative errors obtained using other test flow systems were of similar magnitudes. Thus, one could conclude that, in the case of laminar flow, the accuracy was even better than when the flow was highly turbulent.

**Figure 1.** Schematic of the flow distribution system from Table 1.

**Table 1.** Parameters of the flow distribution system used to obtain the laminar flow-related data shown in Figure 1.


**Figure 2.** (**a**) Tube mass flow rates obtained for the flow system from Table 1 using the model based on Finite Element Analysis (FEA) discussed in [26] and a transient Computational Fluid Dynamics (CFD) simulation. Average tube Reynolds number was ca. 1500. (**b**) The corresponding relative tube mass flow rate errors (FEA vs. CFD simulation). Tube numbers correspond to Figure 1. For the details regarding the CFD model, the reader is kindly referred to [27].
