###### Abstract  
In this paper, a procedure for the design of a bubble reactor which allows the control of the humidity of a gas stream used as combustion air is presented. This reactor is designed to be used as a component of an altitude simulator test facility for the optimization, homologation and calibration of new hybrid engines. The design has been carried out by means of Computational Fluid Dynamics (CFD) multi-phase models and validated against the experimental data obtained from the developed prototype. A discussion about the adequate mesh topology and cell size is presented, as well as a comparison between the two available models for the air-water interphase. Lastly, a validation of the CFD results using experimental data shows that the model that should be used is the multi-regime interaction model, from which the final design for the bubble reactor was obtained. hybrid power plant; real drive; CFD; multi-phase; bubble reactor; experiments; atmospheric simulator 2020 rights rightsC  
On the other hand, the decrease of air density caused by the altitude conditions [9] affects greatly the performance and emissions of the engine [10], due to the change in the combustion chamber and boundary conditions of the engine, which leads to changes in the behavior of some processes. One of them is the increase in backpressure generated by the variable geometry turbine, due to a higher need in compression rate to achieve the same boost pressure than at sea level. The increase in back pressure can lead to backflows in the exhaust valves, increasing the internal EGR (exhaust gases recirculation) rate as well as the pumping losses of the engine, which reduces the performance [11]. Moreover, the lower density generates smaller fuel drop size inside the cylinder, which reduces the coalescence [12; 13] thus decreasing the jet-to-wall impingement [14] as the fuel mixes better with the air and evaporates faster inside the combustion chamber. However, the penetration of the fuel spray increases, due to the decrease of the spray angle caused by the low air density [14]. In order to study the effect of the altitude on the engine performance, the use of a hypobaric chamber is the solution commonly used during the early stages of the calibration process instead of real driving tests, more used at the final stages of the engine calibration. The use of hypobaric chambers has become a standard due to the high costs and low repeatability of real driving tests. Nevertheless, the hypobaric chambers have some important drawbacks. This kind of installations are expensive and voluminous and requires extensive planning to build one. This has meant that few hypobaric chambers are available at research and development (R&D) centers, which increases greatly the development time. Moreover, the human body cannot withstand continuous pressure changes [15] so operating inside hypobaric chambers entails a serious health risk what limits the number of testing hours in chassis dyno test cells. Therefore, to avoid the drawbacks of hypobaric chambers, devices that generate altitude only inside the intake and exhaust of the engine have been developed. These devices, called altitude simulators (AS), can generate the altitude for the engine while keeping the rest of the test cell at room pressure. Moreover, some AS have additional conditioning modules that allow for controlling control temperature and humidity for a complete simulation of ambient conditions. One example of this family of equipment is the Multifunctional Efficient and Dynamic Altitude Simulator (MEDAS), MEDAS Temperature Module (MTM) and MEDAS Humidity Module (MHM) product from HORIBA [16; 17; 18; 19], which allows for simulating the atmospheric conditions for almost every location on the planet in which a vehicle can be driven. These two features of the ambient conditions are also important with respect to the effect on the performance of an ICE. The ambient temperature has a great effect on the performance of the engine, since the lower the temperature of the air the higher its density.