## 1 Introduction
The thickness of this ITO coating is 0.7 \(\upmu\)m and is heated by the direct Joule effect. They managed to visualize the bubbles through the sapphire beneath the heater and collect the data related to the heated plate temperature and heat flux. An infra-red high-speed camera measures the temperature of the heated plate on its external face and the temperature of the wet side of the plate is obtained by solving an inverse conduction-radiation problem-solution. Despite the obvious interest in such an experiment, the representativeness of the heated surface is questionable, in particular, due to its thickness. One should notice that for classical industrial plates (e.g., metallic plates), the characteristic size of an active nucleation site is of the same order of magnitude, not to say larger, than the thickness of the ITO layer used by [PERSON] et al. [6]. This may influence the correct development of a bubble. To illustrate this influence, let us consider the work of [PERSON] [4]. He designed an experimental setup to study the two-phase flow characteristics for PWR thermal-hydraulic conditions. The test section was composed of a rod bundle with nine rods heated by the direct Joule effect. Those rods were placed inside a vessel that could support the pressure. The vessel was equipped with two sapphire windows that allowed direct visualization of the two-phase flow along the heated rods, particularly the bubbles that were created at the surface of the rods. Pictures of the flow were obtained using shadowgraphy. Experiments were performed within the following thermal-hydraulic parameter range (i) pressure from 26 bars up to 155 bars, (ii) mass velocity ranging from 1000 to 3500 kg\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), and (iii) equilibrium exit quality \(x_{eq}\) corresponding to sub-cooled conditions (from ONB up to \(x_{eq}\approx 0\), where \(x_{eq}\) is defined by \(x_{eq}=\frac{h_{m}-h_{sat}}{h_{l_{2}}}\), where \(h_{l_{I}}\) is the latent heat and \(h_{l,sat}\) is the saturated liquid enthalpy). During the tests, [PERSON] [4] focused on the impact of the roughness of the heated surface on nucleate boiling (especially the sizes of the bubbles). He observed strong changes in the flow structure, especially in the bubble size as a function of the roughness. For a smooth surface, which can be characterized by a roughness close to 0.2 \(\upmu\)m, and for the same set of thermal-hydraulic conditions, the bubbles are quite larger than the bubbles created at the surface of the rough rod (characterized by a higher roughness close to 3.9 \(\upmu\)m). It appears that the roughness largely influences the nature of the boiling flow, particularly in terms of bubble size [7]. This result confirms that it is necessary to consider a realistically heated plate in order to ensure good representativeness. If [PERSON]'s work [4] clearly demonstrates the technical feasibility of such measurements, some limitations should be pointed out. The influence of optical distortion due to the thermal gradient (refractive index) in the vicinity of the heated wall was not studied and very little data were collected. The resulting database needs to be completed. One can also notice that there were no thermal measurements concerning the heater. Finally, to the best of our knowledge, data collected by [PERSON] [4] were unique but have not been challenged by anyone else; moreover, reproducibility still needs to be demonstrated. Figure 2: Sapphire substrate coated with ITO. Reprinted with permission from [PERSON] et al. [6]. The main objective of the present work was to design a setup to measure the previously mentioned parameters for operating conditions that are representative of a pressurized water reactor and, therefore, extend the initial work performed by [PERSON] [4]. A realistic heater needs to be designed, taking into account the previous work described ahead. To achieve this goal, a specific study of the thermal behavior of the heater is presented.