Processes under high pressure occur in many fields of
nature and technology such as geology, oceanic biology,
mechanical and chemical engineering, and high pressure
treatment of foods (up to 1000 MPa). The last mentioned
research field is a promising technology, but basic
understanding of physico-chemical and thermofluiddynamical
processes during the application of high pressure are
needed for future advancement. This requires in-situ
measuring techniques not available as yet.
This contribution deals with in-situ methods for diagnosing
milieu conditions to which biotechnological materials are
exposed. In the literature the milieu is usually
characterised only by determining the pressure, the density
and the temperature at a fixed point of the pressurised
cell. But this appears to be not sufficient as, in general,
physico-chemical processes in the measuring chamber must be
described by fields which depends on extensives and
intensives quantities as well as on transport parameters.
In this sense the work in progress aims to measure not only
the density and compressibility of liquid food ingredients,
but also their viscosity and thermal conductivity. As shown
in /1 / the temperature and velocity field have to be taken
into consideration, too, because they may effect processes
of heat and mass transfer decisively. Last but not least,
measuring the pH-values is of decisive importance when
describing the biochemical milieu conditions under high
pressure.
Regarding the viscosity new experimental data for water at
high pressures up to 700 MPa in the temperature range of -
13 C to 20 C are presented. The measurements were carried
out with a rolling ball type viscometer /2/. The set-up of
the viscometer and its pressure dependent calibration is
briefly described here. The applicability of the system for
the use under pressure is shown by comparing it with an
absolute testing method. The viscosity data are compared to
available literature data. Possible deviations are
discussed. Models to describe pressure-viscosity behaviour
of water are applied to the data. The applicability of the
models to the low temperature viscosity data is discussed,
and checked whether the range of validity of the model can
be extended to temperatures bellow that of the triple point.
In order to visualise the temperature and velocity fields
in pressurised water up to 700 MPa thermochromic liquid
crystals (TLC) have been used to visualize temperature and
velocity fields. The effect of the pressure on the change
of the reflected wavelength of the used TLCs due to
pressure is inverse to the effect of temperature. Hence,
the detectable temperature range shifts to a higher
temperature level as pressure increases. The colour change
keeps reversible and a good temperature resolution is
achievable. In order to select TLCs for high pressure
investigations a simple empirical relation between the
start temperature at normal pressure and the slope of the
isochromes in the pressure-temperature plane was found.
Furthermore, experiments have been performed for analysing
the pressure induced convection in the high pressure cell.
It has been found that the movement of the fluid in the
measuring chamber is produced not only by the temperature
due to dissipation during fluid compression, but also by
the forced convection generated by the entrance of fluid
into the chamber when the density is increased. The
velocity field is determined by using image processing.
In the future, image processing is planned to be used for
measuring the temperature distribution, too. For
completeness, also the works in progress regarding the
experimental determination of the thermal conductivity and
the pH-values are described briefly in this contribution.
Literature
/1/ Pehl, M.; Delgado, A.: An In-situ Technique to
Visualize Temperature and Velocity Fields in Liquid
Biotechnical Substances at High Pressure. In: Proc. of
IVth joint meeting of Japanese and European Seminars on
High Pressure Bioscience and Biotechnology, August 30 -
September 3, 1998. Ed: H. Ludwig, Heidelberg,.S. 52.
/2/ Först, P.; Delgado, A.: In-situ Viscosity Measurement
during the High Pressure Treatment of Fluid Food
Ingredients. - In: Proc. of IVth joint meeting of
Japanese and European Seminars on High Pressure Bioscience
and Biotechnology, August 30 - September 3, 1998. Ed: H.
Ludwig, Heidelberg,.S. 50.
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