# Impact of Mesh Resolution and Temperature Effects in Jet Ejector CFD Calculations
## 1 Introduction  
Jet ejectors are devices that utilize a high-pressure (primary) flow to entrain a lower-pressure (secondary) flow. This process relies on the momentum transfer from the high-velocity primary stream to the secondary stream, resulting in fluid mixing and pressure recovery in the combined flow. Their simplicity, durability, and low-maintenance nature make them highly multipurpose in applications such as refrigeration, chemical processes, and energy recovery systems. The performance of jet ejectors is dependent on their geometry, as they are static devices governed entirely by fluid- and thermodynamics [1]. Thus, understanding the dependencies between geometry, boundary conditions, and operational regimes is key for optimizing their design and performance [2, 3]. To do so, the operating conditions and the performance of a jet ejector have to be defined. One way to achieve this can be by describing using parameters such as the efficiency of individual modules. In the bibliography there have been ways of defining them throughthe efficiencies of particular points along the jet ejector (primary nozzle [4], diffuser [5], or mixing chamber [6]). These efficiencies can be estimated through CFD simulations. Performance can also be assessed based on the variation in the enthalpy through each section [7], along with the secondary flow's behavior during mixing. Efficiencies can be derived from changes in the measured velocity and pressure [8]. [PERSON] et al. [9] first categorized ejector operations into critical (double-choking), subcritical (single-choking), and malfunction (backflow) modes, distinguishing the on- and off-design performance based on the entrainment ratio and pressure. This is still a commonly used division, taken up, for example, by [PERSON] et al. [10]. Choking in a jet ejector refers to a condition where the flow velocity in a particular section of the ejector reaches the local speed of sound, creating a sonic barrier. Once choking occurs in the primary nozzle, the mass flow rate through that section becomes constant, regardless of further decreases in downstream pressure. The high-pressure motive flow is accelerated through a convergent-divergent nozzle to supersonic speeds. When choking occurs at the throat of the nozzle, it results in a fixed mass flow rate. The low-energy secondary flow is drawn into the ejector by the entrainment effect of the motive stream. If the secondary flow also chokes, its mass flow rate stabilizes, and the overall entrainment ratio is determined. After the mixing of the primary and secondary flows, choking may occur in the mixing chamber. This is important for ensuring an efficient recompression and determining whether the ejector operates in a choked (the mixing chamber or diffuser operates at sonic conditions, and the mass flow rate of the secondary flow is maximized and fixed) or unchoked mode (when the downstream pressure increases beyond a certain threshold, the flow can become subsonic in the mixing chamber, leading to lower entrainment and poorer performance). Other studies from [PERSON] et al. have further analyzed design and off-design conditions, especially concerning the motive pressure and secondary flow visualization [11]. The latter method results in creating ejector working maps, which graphically depict its behavior. In on-design conditions, both primary and secondary flows are choked, maintaining a maximum and constant entrainment ratio, and are represented on the map as a straight, horizontal line. In off-design operations, only the primary flow is choked, leading to a linear reduction in the entrainment ratio with increasing back pressure, and are graphically depicted as a line, descending at a constant angle. The point of transition is recognized as a critical point. Starting from it the performance degrades, as ejectors are designed for given parameters which in that case are exceeded. This can cause flow separation, and the ejector operates unstably.