## 4 Results and Discussion
Figure 4:2 The comparison of the distributions of the heat transfer coefficient versus coordinate x along the length of the minichannel obtained from three calculation methods (FEM using Trefftz functions, ADINA software and the 1D method) (**a,c,e**) and relative differences (\(rd\)) between the results (**b,d,f**). The data collected for: (**a,b**) \(Q_{m}=5\;[\mathrm{kg/h}]\), (**c,d**) \(Q_{m}=30\;[\mathrm{kg/h}]\), (**e,f**) \(Q_{m}=55\;[\mathrm{kg/h}]\). The data corresponding to ONB points are indicated by black arrows in Figure 4:3  
vs. 1D was noted (Figure 7b,f, but for \(Q_{m}=30\) [kg/h], the trend is not clear (Figure 7d). Furthermore, lower _rd_ values were achieved close to the minichannel inlet. Figure 8 illustrates the heat transfer coefficient versus fluid velocity/mass flow rate for three selected distances from the minichannel inlet defined as _x/L_, in detail as follows: _x/L_ = 0.25, 0.5 and 0.75. This figure also refers to the data captured during boiling incipience. The results are obtained by calculations performed with the aid of ADINA software and (Figure 8a) and using Trefftz functions (Figure 8b). Upon analysis of the results shown in Figure 8, it was observed that, in general with increasing mass flow, the heat transfer coefficient increased. However, the results obtained for the distance defined as _x/L_ = 0.5 showed several extremums, while the maximum heat transfer coefficient occurs at \(Q_{m}=10\) [kg/h]. Furthermore, the greatest data scatter was noticed in the dependence constructed for _x/L_ = 0.75. It can be noted that the dependence obtained for _x/L_ = 0.5 corresponds to the lowest values of the heat transfer coefficient. The distributions of the heater temperature versus the coordinate \(x\) along the channel length and time, obtained from the infrared thermography measurement performed on the outer foil surface, for 11 values of mass flow rate, are presented in Figure 9a-k. Furthermore, the fluid temperature on the reference line, as the result of the calculations resulting from the ADINA software (on the basis of data for \(Q_{m}=55\) [kg/h]) is additionally shown in in Figure 9l. Sharp differences between values are characteristic for lower mass flow rates, Figure 9a-e. For higher flow rates, the recorded data are blurred, Figure 9f-k. Furthermore, changes in fluid temperature are uniform and stable, Figure 9l. It can be added that the lowest temperature of the heated foil near the channel outlet was noticed for \(Q_{m}=10\) [kg/h], Figure 9b. It resulted in the highest values of the heat transfer coefficient, achieved for \(Q_{m}=10\) [kg/h] in Figure 4:4 The time intervals considered in FEM simulations relate to the boiling incipience and the subcooled boiling region. In numerical computation, it was assumed that the initial time is zero. In Figure 9k, the numbering of moments of time, related to the whole experiment, was also shown. Figure 4:5 The heat transfer coefficient versus fluid velocity/mass flow rate for three selected distances from the minichannel inlet was defined as _x/l_, the results obtained from the calculations: (**a**) with the help of ADINA software and (**b**) using Trefftz functions. The data corresponding to ONB points are indicated by black arrows in Figure 4:6  
Figure 10 shows distributions of the heat transfer coefficient versus coordinate \(x\) and time \(t\), obtained using ADINA software, Figure 10a,d,g and Trefftz functions, Figure 10b,e,h, based on data collected for three values of the mass flow rate.