Measurement of transient natural convection in non-ventilated aircraft compartments

The present study focuses on heat transfer behaviour in aircraft compartments. The objectives of the analysis were to investigate the transient effects on fluid flow structures and heat transfer mechanisms in aircraft wing boxes relative to aircraft turnaround times. Experimental methods employed were flow visualisation and thermocouple measurements. A simplified air filled aluminium rectangular enclosure of aspect ratio 0.25 was constructed to replicate an aircraft wing box. Rubber insulation was used between wall surfaces to maintain separate thermal boundary conditions at each wall. Flow visualisation was used to illustrate the transient evolution of full field flow structures and thermocouple measurements were recorded to investigate the full field transient temperature distribution. Experiments were carried out from time zero to steady state and were conducted for Rayleigh numbers of 2.87×10⁶, 4.81×10⁶ and 7.39×10 ⁶ based on enclosure height. Fluid flow patterns revealed the presence of two counter acting flows in the cavity with a downward motion adjacent to the front and rear sides of the enclosure. The downward motions were present due to the cooling of the warm air inside the cavity by the adjacent cooler aluminium side walls. Secondary flows existed in the lower region of the cavity specifically prominent to the front and rear surfaces where the height of detachment of the flow varied with time and temperature of the adjacent side walls. Transient spatial temperature distribution plots confirmed the approach to steady state was gradual, that a vertically thermal stratified distribution existed in the cavity and the presence of the thermal boundary layers along the horizontal and vertical walls were clearly evident. It was concluded that transient effects are significant relative to aircraft turnaround times as was observed that the flow structures change over the initial transient phase of analysis and remain similar thereafter. Wing box material thermophysical properties influenced the developed flow structures causing two secondary flows to exist along the front and rear spar walls. Vertical temperature distribution results were generalised using a power law based on dimensionless time (t*), dimensionless temperature (T*) and Rayleigh number.