###### Abstract
The main idea of PV is to provide clean air close to the face of each occupant and to give some control to the user to improve comfort in their microenvironment. However, PV systems might be used as protective shields against cross-infection in crowded environments, such as an aircraft. In this case, the concept of protective personalized ventilation (PPV) is to supply clean air directly to the breathing zone of a seated person and to allow some control of the flow rate and flow direction. ASHRAE and REHVA position documents on airborne infectious diseases [31,32] recommend PV and other ventilation strategies (such as dilution ventilation, local exhaust, and source-control ventilation) for use as effective measures to control and prevent SARS-CoV-2 transmission. This proposed strategy is in keeping with evidence that PV, in conjunction MV, provides better protection against airborne infection than systems relying solely on MV. [PERSON] et al. [33] evaluated the performance of a desk-mounted PV diffuser and found that the PV could reduce both the peak aerosol concentration and the exposure time of droplets that were released during coughing. Displacement ventilation (DV) systems have also been widely used in buildings and have been shown to be more effective than MV systems in removing contaminants [30]. There have been some attempts to propose new strategies based on the displacement concept in order to reduce the transmission of airborne infectious disease viruses and/or to improve cabin air quality. [PERSON] [34] and [PERSON] [35] compared MV and DV in a section of an A320 cabin mockup. [PERSON] [34] found that an MV system had higher draft risk, while a DV system could result in an unpleasant sensation of overheating. In [36], [PERSON] proposed an under-aisle air distribution system with the purpose of improving relative humidity from 10% to 20%. [PERSON] [37] also proposed a PV system with air terminals embedded in chair armrests on commercial airplanes. Cleaner air in the breathing zone demands a certain degree of control over the flow features of the breathing zone, allowing for the interaction of PV flow with the free convection flow of the human boundary layer. Any attempt to supply a fresh flow of air to the BZ via PV needs to penetrate this boundary layer (i.e., velocities over 0.3 m/s). Previous studies showed that the PV flow strongly interacts with the convective boundary layer of the human body [18]. Figure 2: Classical mixing ventilation air distribution schemes for aircraft: (**a**) general mixing ventilation scheme, (**b**) general mixing ventilation scheme and inlet nozzles above the heads of the passengers, (**c**) schematic of the proposed solution with additional PV nozzles. PV designs, such as furniture-incorporated air diffusers (in tables or chairs) [19] or air-terminal devices (ATDs) [20], are usually situated at a certain distance away from occupants' breathing zone. Attempts have been made to dissipate the boundary layer through forced ventilation [21] or to diminish the strength of its flow by cooling the desk where the occupant is seated [22] in the hopes of improving PV efficiency. Results indicated improved levels of clean air for the occupant using reduced PV airflow (25% less), i.e., weakening of the convective boundary layer reduced required PV airflow from 8 L/s to 6 L/s. Another method to improve PV performance is to reduce the distance between the occupant and the PV diffuser [23; 24]. This can lower the PV flow rates to as little as 0.5 L/s, resulting in most of the air inhaled (>90%) being clean air. Other factors that influence this approach are the initial velocity and equivalent diameter of the diffuser, as well as its geometry [25]. [PERSON] et al. [38] conducted a detailed experimental study to explore the flow interactions between personalized air and the thermal boundary layer of an occupant and indicated that concerns about PV jet penetration of the thermal plume are not as challenging as expected.