Aircraft crown compartment thermal management: the influence of internal heat dissipating elements

The content of this thesis describes the interaction of internal heat dissipating elements with their heated enclosure with the applied case of a crown compartment in a single aisle commercial aircraft. The crown compartment is defined as the confined area between the passenger cabin and the upper external fuselage. Aircraft compartments are subject to a wide variety of boundary conditions during operation which leads to the setting up of highly complex internal thermal environments. These compartments require strict thermal management to ensure safe and reliable operation of the installed systems. The work conducted firstly examines the fundamental fluid flow and natural convection heat transfer inside a differentially heated square cavity with a centrally located heated horizontal cylinder. There is a distinct lack of information available in literature which focuses on the change in heat transfer experienced by the cylinder due to the interaction with the enclosure. An increase in the cylinder heat transfer is observed due to this interaction. Experimental thermocouple and PIV measurements confirmed the presence of a transition process whereby the flow transitions from being dominated by the temperature difference across the cavity to that dominated by the temperature difference due to the cylinder for the range 2 ×104 < Racyl < 8 ×104. A similar investigation was then conducted for the crown compartment. A study of the unpopulated case, i.e. no internal heat dissipating elements, for the range 323.15K Tfus 378.15K revealed a stratified high temperature region in the upper area and a low temperature forced flow region in the bottom of the compartment due to the cabin ventilation system entering the crown. With the inclusion of representative heat dissipating elements, namely the SEPDC avionic system and Route G electrical conduit, the populated compartment flow and thermal fields become dominated by the SEPDC, and no transition process, as found for the square cavity, was observed. Design of Experiments and regression analysis techniques, combined with numerical and experimental testing, resulted in the generation of thermal models capable of predicting the heat transfer of the surfaces of the elements with the average differences between the predictions and experiments ranging from 7.24 - 26.38%. The optimal placement of the elements, from a thermal management point of view, were found to be when they are positioned as close to the crown floor as possible where they come in contact with the forced convective cooling flow from the inlet, and when they are positioned towards the centreline of the compartment where the hot buoyant air does not become trapped between the elements and the fuselage insulation. The methodology employed allows aircraft thermal engineers to optimise equipment placement in all compartments during the design phase of an aircraft. The determined models can be included in the global aircraft numerical models to improve accuracy and reduce model size and computation time. This in turn reduces the need for expensive physical testing and redesign, which inevitably results in delays to aircraft development time.