In the bearings of an aero-engine, oil is supplied not only for lubrication, but mostly for cooling, where heat is removed from the hot walls of the chambers by the oil films that are formed there. Based on prior work, it is observed that air bubbles can become trapped within liquid films potentially affecting fluid behavior.
This paper reports on experiments conducted in a horizontal rectangular duct, where the liquid phase was sheared by a strong gas stream. In this configuration, air bubbles are seen to be entrapped in a similar manner to what was observed within prior bearing chamber visualization research. Of particular interest in this paper, is the effect of surface tension on air bubble generation in the liquid phase and its consequences for appropriate oil selection criteria. The surface tension was controlled by testing solutions with reduced surface tension by up to 47% of that of water, while maintaining the density and viscosity parameters constant. The results were obtained using a Brightness-Based Laser-Induced Fluorescence technique.
Results show that reducing surface tension leads to a higher number of bubbles being generated, thus increasing the level of aeration of the liquid phase. Additionally, the velocity of the bubbles was also measured and seen to be lower for the lower surface tension cases.
The interfacial wave structure of the liquid phase in upwards annular gas-liquid flow in a 11.7 mm pipe was studied using the brightness based laser induced fluorescent technique (BBLIF). Due to the versatility of the BBLIF technique, film thickness measurements were carried out over different axial locations covering a measurement area with very high spatial and temporal resolution. These comprise measurements performed close to the inlet section (between 0 to 10 z/D), where the dynamics of the small wavelets that are created due to the gas-liquid interaction and that subsequently will lead to the formation of the disturbance waves was analysed. Furthermore, using a tracking algorithm to detect disturbance waves, a full characterization in terms of their velocity, distance and time separation (both in absolute and in terms of their distribution), frequency, intermittency and wavelength was carried out based on measurements conducted further downstream (between 28 to 37 z/D). An Omega-K analysis was also conducted to these latter measurements showing a constant increasing ratio between the ripple and disturbance wave velocity. Although these measurements were conducted well below the predicted length for a fully developed annular flow to occur, it was observed that the behaviour of the structures present in the film during these undeveloped stages plays a crucial role on the overall events that occur further downstream.
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