In tube injection systems applied in high-pressure processing of packed biomaterials and foods, the pressure-transmitting medium is injected into the vessel to increase the pressure up to 1000 MPa, generating a submerged liquid-free jet. The presence of a turbulent-free jet during the pressurization phase and its positive influence on the homogeneity of the product treatment has already been examined by computational fluid dynamics investigations. However, no experimental data have supported the existence and properties of turbulent flow under high-pressure (HP) conditions up to 400 MPa. This contribution presents the development of two experimental setups: HP-laser Doppler anemometry and HP-hot wire anemometry. For the first time the time-averaged velocity profiles of a free jet during pressurization up to 300 MPa at different Reynolds numbers (Re) have been obtained. In this article, the dependence of the velocity profiles on the Re is discussed in detail. Moreover, the relaminarization phenomenon of the turbulent pipe flow most likely caused by the compressibility effects and viscosity changes of the pressure-transmitting medium is examined.

In a tube injection system for High Pressure Processing (HPP) of foods and biomaterials, pressure transmitting medium is injected into the vessel to increase the pressure up to 1000 MPa generating a submerged liquid jet. The presence of a turbulent free jet during pressurizing and its positive influence on the homogeneity of the product treatment have already been examined by computational fluid dynamics investigations. However, up to now there is no experimental evidence, supporting the existence and properties of turbulence of a free jet during HPP due to difficulties in gaining in–situ insight into the high pressure vessel and the lack of high pressure withstanding probes. Hence, we developed two experimental setups: (1) HP–Laser Doppler Anemometry (HP–LDA), and (2) HP–Hot Wire Anemometry (HP–HWA). This paper presents first results of velocity profiles measured by LDA and HWA under high pressure conditions up to 300 MPa. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reverse transition from the turbulent towards the laminar flow regime was investigated experimentally by progressively increasing the pressure up to 400 MPa in a fully developed pipe flow operated with silicone oil as the working fluid. Using hot-wire anemometry, it is shown indirectly that at low Reynolds numbers a rapid increase in pressure modifies the turbulence dynamics owing to the processes which induce the effects caused by fluid compressibility in the region very close to the wall. The experimental results confirm that under such circumstances, the traditional mechanism responsible for self-maintenance of turbulence in wallbounded flows is altered in such a way as to lead towards a state in which turbulence cannot persist any longer.

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