The increasing demand for cooling aboard future commercial aircrafts and the need for more efficient cooling technologies require a review of existing aircraft cooling concepts. State-of-the-art for the discharge of waste heat is the use of heat exchangers integrated into ram air channels located in the belly fairing. Ram air channels are afflicted with a significant drag increase and imply challenging certification issues. In order to reduce the additional drag and to simplify the certification, waste heat could possibly be removed from a liquid cooling loop through the aircraft skin using Skin Heat Exchangers (SHX).
The study is concerned with a joint experimental/numerical investigation of the performance of liquid-to-air SHX prototypes. Accordingly, SHX-designs are investigated by means of wind-tunnel experiments (cf. Fig. 1). The extremely flat device provides a flat surface on the air-side and inheres approx. 50 liquid channels. The device is flush-mounted to the wind-tunnel floor in the centre of the 3/4-open measurement section.
When attention is given to the liquid-side, the flow distribution is most crucial. Hence, the respective focal points of the employed experimental techniques is on wall shear stress and temperature-distribution measurements which indicate the flow homogeneity. Thermal imaging techniques in addition to thermocouples are utilised to assess the temperature distribution (cf. Fig. 1).
As regards the air-side, two challenges are addressed. Bearing in mind the limited attainable speed range of the employed wind tunnel (15-35m/s, Ma=0.04-0.11) and the proximity of the SHX position to the beginning of the test section (about 2m), the first challenge refers to the experimental modeling of different representative full-scale scenarios. Based upon a similarity analysis and empirical design formulae for boundary-layer simulation devices (aka. spires, cf. Fig. 1), a range of full-scale scenarios, which involve values for the boundary-layer thickness up to 0.5m, can be simulated in the wind tunnel.
The second major air-side issue is associated to accurate wall shear-stress measurements. Low shear-stress values of about 1N/m² impede the use of direct measurements by means of shear-stress balances. Indirect measurement techniques - which conclude the wall shear stress assuming a universal velocity behaviour - are also inappropriate due to imposed disturbances of the approach flow. The adopted approach refers to a direct optical interferometry method developped at the DLR. The latter observes the thickness change of of a thin oil film driven by the wall shear stress. The temporal thickness evolution is measured via its interference pattern (cf. Fig. 1), which are analysed over a finite area. Using low-cost, of-the-shelf consumer equipment, a sufficient experimental accuracy of 3% can usually be realised.
Fig. 1: Employed wind tunnel setup (left: view out of the nozzle with floor-mounted SHX, left-centre: spires mounted aft of the nozzle, right-centre: thermografic imaging, right: oil-film pattern).
The STELLA project is funded under the aegis of the 3rd BMWi LUFO4 call. The work is performed in colaboration with AIRBUS and TUHH Inst. of Thermo-Fluid Dynamics under the framework of TP2/AP2.