Numerical and Experimental Study of Various Innovative Jet Mixing Ventilation Concepts for Improved Ventilation Efficiency

Ventilation has an important impact on the quality of indoor environments, regarding e.g. the air quality or thermal comfort, but may contribute considerably to the energy use as well. In particular in airplane cabins, ventilation is indispensable due to the high occupant density within a confined air volume. Typically, the cabin main air distribution system supplies fresh (conditioned) ventilation air above the passenger zone (mixing ventilation), often through opposing diffusers located at both sides of the cabin. The diffusers provide high-momentum supply jets that should ensure a sufficient mixing between fresh and stale air. However, even in the case of perfect mixing, the overall mixing ventilation performance may still be low compared to other (non-conventional) ventilation methods and, in addition, may strongly deteriorate when stagnant zones or short-circuiting flows are present, usually compensated for by an (undesired) increase in the supply flow rate. This necessitates the search for novel mixing ventilation concepts.

The ventilation performance depends on the induced complex airflow patterns constituting the overall flow field, which – in an opposing-jet configuration – appear to be mainly driven by the supply jets but are also affected by other factors such as obstacles (seats, passengers), the specific cabin geometry, passenger’s movements and thermal buoyancy flows (passengers, equipment). Improving the ventilation performance therefore requires detailed insights into these flow patterns in the first place, and more specifically demands a thorough analysis of their underlying fundamental flow components (the supply jets, their interaction into a merged jet, recirculation cells, etc.), which for enclosed opposing-jet flows is scarcely represented in the literature.

This dissertation comprises a systematic investigation following the five main objectives: (1) development of a new experimental set-up consisting of a simplified cabin model that incorporates the fundamental flow components of isothermal airplane cabin opposing-jet mixing ventilation, (2) measuring and analysing the overall flow field and its flow components, (3) provision of high-resolution measurement data serving as benchmark for numerical simulations, (4) performance analysis of different numerical simulation methods in the prediction of the overall flow field and its flow components, (5) numerical evaluation of time-periodic supply flow rates as novel concept for an improved cabin ventilation performance regarding the ventilation efficiency (air mixing/replacement and contaminant removal). The measurements are carried out with particle image velocimetry (PIV), whereas computational fluid dynamics (CFD) is used for the numerical analyses.

Three main parts are considered. Part I covers the measurements in the simplified – reduced-scale, water-filled, empty and isothermal – cabin model. Both the mean and instantaneous flow field are analysed for two configurations of opposing jets, i.e. plane wall jets (ceiling diffusers) and plane free jets (lateral diffusers). Different supply flow Reynolds numbers in the transitional regime are evaluated and specific attention is devoted to the characterisation of the fundamental flow components. In Part II, the focus is on the validation of (unsteady) Reynolds-averaged Navier-Stokes (RANS) simulations and large eddy simulations (LES) with different turbulence models and subgrid-scale models, respectively. The measured mean velocity and turbulence data/characteristics of Part I are used for this purpose. Overall, LES demonstrates the highest accuracy. Finally, in Part III, based on sub-configuration validation, LES is performed in a realistic model of a single-aisle airplane cabin to assess the mixing ventilation with time-periodic supply flow rates compared to conventional (statistically) steady supply flow rates. Two time-periodic ventilation strategies are evaluated and the potential for an improved ventilation efficiency is illustrated.