###### Abstract
This leads to an improvement of the volumetric efficiency of the engine, which is a measure of the effectiveness of the air intake system [20]. In addition, this temperature is also important during the combustion process. If the ambient temperature lowers, the maximum temperature during the combustion decreases, which reduces the NOx formation [21]. However, this also increases the emission of hydrocarbons, due to lower combustion efficiency and a higher quantity of fuel attached to the cylinder walls [22]. In the case of the humidity, its effect on the combustion process has not been studied in much detail, but some studies on gas turbines [23] show the negative effects of the increase of water content in the efficiency of the combustion, and thus on the efficiency and performance of the engine. With respect to controlling the humidity for the combustion air, two technologies are mostly used: atomizing water or an air current or passing air through a bubbling reactor. The atomizing allows the amount of water injected in the air to be controlled accurately, but it needs piping length to let enough time for the water droplets to break up and dissolve into the air [24]. Contrarily, the reactor size is limited by the amount of air that has to pass through it, to control the maximum air velocity [25] and avoid an excessive increase in the water level that could lead to overflowing. The reactor solution is quite similar to fluidizedbed technologies, widely used in the chemical industry. However, the procedure of design of this kind of reactor is focused on the interaction between the particles and the fluid [26]. Instead, the increase in height of the emulsion, which is the main factor that leads to the reactor overflowing is not studied in detail [27]. Moreover, these existing technologies are focused on large-scale installations, as industrial chemistry plants, with a high working mass flow rate over 1 kg/s [28], or small installations such as dedicated laboratories with mass flow rates below 0.01 kg/s [29]. Hence, the utility of the development of the installation is described in this paper. It covers the medium installation range, in which almost all the propulsion systems for land transport are located. Either fuel cells, ICE with liquid or gas fuels, or other kind of propulsion systems that require an air stream provided to the engine in order for it to work. Therefore, a detailed analysis of a bubble reactor has been carried out. The objective of the reactor is to humidify an air stream up to a water content set point, which afterwards is going to be expanded and cooled to reach temperature and pressure set points. In Section 2 the design requirements for the installations will be explained. In Section 3 the designed experimental facility and the testing methodology will be described. Afterwards, the set-up parameters for the CFD code used will be shown in Section 4. Also, the study of the effect of the mesh size and air-water interaction models is explained, as well as the results of the parametric studies carried out to obtain the final design of the bubble reactor. Lastly, the conclusions obtained through this work will be presented in Section 5.