In situ investigation of the turbulent–laminar transition of temperature fluctuations during pressure building up to 300 MPa

Abstract

The investigation of fluid flow under high pressure up to 300 MPa has been developing extensively during recent years [M. Pehl, F. Werner, and A. Delgado, First visualization of temperature fields in liquids at high pressure using thermochromic liquid crystals, Exp. Fluids 29(3) (2000), pp. 302–304; P. Kitsubun, C. Hartmann, and A. Delgado, Numerical investigations of process heterogeneities during high hydrostatic pressure treatment with turbulent inflow conditions, Proc. Appl. Math. Mech. 5 (2005), pp. 573–574; K. Song, C. Rauh, and A. Delgado, Experimental in-situ investigations on fluid flow during high pressure processing by means of LDA and HWA, Proc. Appl. Math. Mech. 8 (2008), pp. 10603–10604; K. Song, A. Al-Salaymeh, J. Jovanovic, C. Rauh, and A. Delgado, Experimental in-situ investigations of turbulence under high pressure, Ann. NY Acad. Sci. (2010), in press]. The range of application of high-pressure technologies spreads from food processing through chemical, bio-chemical and pharmaceutical applications, to mechanical engineering. This results in a high demand for experimental data as well as theoretical backgrounds of various phenomena occurring in the fluids during high-pressure processes. Up-to-date research shows that the properties of many substances and the phenomena occurring under increased pressure differ significantly from their properties in ambient conditions. Song et al. showed for the first time that a sudden increase of pressure in liquids can completely re-laminarize the turbulent flow [K. Song, C. Rauh, and A. Delgado, Experimental in-situ investigations on fluid flow during high pressure processing by means of LDA and HWA, Proc. Appl. Math. Mech. 8 (2008), pp. 10603–10604; K. Song, A. Al-Salaymeh, J. Jovanovic, C. Rauh, and A. Delgado, Experimental in-situ investigations of turbulence under high pressure, Ann. NY Acad. Sci. (2010), in press]. This paper focuses on the investigation of the fluctuations in a liquid's temperature field during the pressure build-up phase. A 1.5 L pressure autoclave is fed with silicon oil, hexamethyldisiloxane, via a nozzle with a diameter of 1.6 mm. The investigation using high-pressure-hotwire anemometry concerns the free jet flowing out from the nozzle into the vessel. The pressure of 300 MPa is reached within 52 s, the temperature due to the compression increases from 18 °C to 50 °C, and the Reynolds number varies from 10,000 to 2000. Initially, the fully turbulent flow starts to be damped by increasing viscous forces, which grow due to the pressure ramp. When viscous forces prevail over fluid's inertia, the turbulence can break down. The transition in the temperature field from turbulence to laminar occurs at the level of Re ≈ 3600, which is typical for turbulent flows. The results show great regularity in the occurrence of the turbulence break-down due to the increase of viscosity and compressibility effects. The other critical parameters for the turbulence–laminar transition are also obtained. The knowledge gathered in the experiments could be adapted in various fields of industry. For example, in IC-engines, it would enable keeping a fully laminar flame, thus controlling the combustion better and obtaining better efficiency. On the other hand, when the laminarization is undesired, e.g. in food processing, the turbulence in a high-pressure vessel assures more uniform pasteurization, thus such conditions could be avoided.