## 4 Results and Discussion  
In general, the results are presented as boiling curves and local heat transfer coefficients. Moreover, experimental data, covering the temperature of the heater and the heat flux values, are also visualized. Figure 4a-k present boiling curves, i.e., dependencies between heat flux (density) and the temperature difference between the heated wall and the bulk fluid temperature, for selected mass flow rates \(Q_{m}\) (see Table 1). Each graph illustrates three curves, depending on the location along the minichannel length from which temperatures were taken: \(x/L=1/3\) (green curve), \(x/L=1/2\) (blue curve) and \(x/L=2/3\) (red curve). The data presented in these graphs were taken from experimental measurements for the entire experiment. The black arrows indicate the moments of the experiments taken into consideration in further analyses, with regard in detail to the points referring to subcooled boiling initiation, for curves marked in red. Furthermore, Figure 4l shows the heat flux as a function of time for the entire experiment at \(Q_{m}=55\)\([\mathrm{kg/h}]\). The red ellipse indicates a part of the experiment while the subcooled boiling starts, which was considered in the FEM simulations. It should be explained that, in calculations, only data from the experiment concerning boiling incipience and subcooled boiling region were considered. Figure 3: A typical FE mesh used in the computation in ADINA software. Figure 4: (**a–k**) The boiling curves obtained for selected distances from the minichannel inlet: \(x/L=1/3\) (green curve), \(x/L=1/2\) (blue curve) and \(x/L=2/3\) (red curve), corresponding to following mass flow rates: (**a**) \(Q_{m}=5\) [kg/h]; (**b**) \(Q_{m}=10\) [kg/h]; (**c**) \(Q_{m}=15\) [kg/h]; (**d**) \(Q_{m}=20\) [kg/h]; (**e**) \(Q_{m}=25\) [kg/h]; (**f**) \(Q_{m}=30\) [kg/h]; (**g**) \(Q_{m}=35\) [kg/h]; (**h**) \(Q_{m}=40\) [kg/h]; (**i**) \(Q_{m}=45\) [kg/h]; (**j**) \(Q_{m}=50\) [kg/h] (**k**) \(Q_{m}=55\) [kg/h]; (**l**) heat flux versus time for the whole experiment, \(Q_{m}=55\) [kg/h]; other data and parameters of the experimental series are listed in Table 1. The red ellipse indicates subcooled boiling region, whereas black arrows indicate onset of boiling (ONB). During the construction of the boiling curves, the experimental data were plotted as a full line in Figure 4a to show a typical trend to emphasise the continuity of measurement. At the beginning, while increasing the heat flux (from point A to point ONB--onset of boiling), the heat transfer between the heated wall and the fluid proceeds by single phase forced convection. In the foil adjacent area, the liquid becomes superheated, whereas, in the flow core, it remains subcooled. The increase in activation of the vapour nuclei on the heated wall and their spontaneous nucleation cause a temperature drop on the heater surface, resulting from the spontaneous formation of vapour bubbles in the adjacent layer to the heated wall. Bubbles act as internal heat sinks and absorb a significant amount of energy transferred to the liquid. It is visible as a drop from ONB to point C, which is evidence that 'nucleation hysteresis' occurred [6]. A further increase in the heat flux supplied to the heater leads to developed nucleate boiling, Section C-D. When analysing the results presented in Figure 4i-k, it was observed that the temperature difference between the heated wall and the bulk fluid temperature at the ONB point was the lowest compared to all other boiling curves presented. The curves shown in Figure 4i-k relate to the highest value of the mass flow rate. Furthermore, the highest temperature difference was gained for the boiling curve corresponding to \(Q_{m}=25\)\([\)kg\(/\)h\(]\), Figure 4e. When comparing the boiling curves with the distance from the inlet, it was observed that the curves generated at \(x/L=2/3\) were characterized by the lowest temperature difference at the ONB, which was related to the lowest mass flow rate set, Figure 4a-c.