Friction between skis and snow was studied in a variety of field and laboratory measurements. Whilst field tests have the drawback of changing conditions, in laboratory tests sport-specific sample sizes and speeds could not be measured up to now. Hence, a novel linear tribometer was developed allowing studies with whole skis at sportspecific speeds. The precision of the tribometer was better than 2.2 %. The dominant cause for the imprecision was the variability of the single snow tracks at lower speed, whilst at higher speeds also the determination of normal and friction force and speed became relevant. The precision is high enough for discriminating differences needed for the analysis of different ski and snow conditions and the study of friction processes.
Abstract. "Homogeneity-time" is defined and introduced as the criterion for mixing quality in bioreactors. The criterion could replace the mixing time, in the case, when more than one measuring point (sensors) is included in the measuring system. Results based on the homogeneity-time and the temperature pulse method, achieved in stirred tank reactors under aerated conditions as well as in a jet-mixed tank, are presented.
A new scale-up concept based upon mixing models for bioreactors equipped with Rushton turbines using the tanks-in-series concept is presented. The physical mixing model includes four adjustable parameters, i.e., radial and axial circulation time, number of ideally mixed elements in one cascade, and the volume of the ideally mixed turbine region. The values of the model parameters were adjusted with the application of a modified Monte-Carlo optimization method, which fitted the simulated response function to the experimental curve. The number of cascade elements turned out to be constant (N = 4). The model parameter radial circulation time is in good agreement with the one obtained by the pumping capacity. In case of remaining parameters a first or second order formal equation was developed, including four operational parameters (stirring and aeration intensity, scale, viscosity). This concept can be extended to several other types of bioreactors as well, and it seems to be a suitable tool to compare the bioprocess performance of different types of bioreactors. (c) 1994 John Wiley & Sons, Inc.
The construction of the horizontal rotating tubular bioreactor (HRTB) represents a combination of a "thin-layer" bioreactor and a "biodisc" reactor. The bioreactor was made of a plastic tube whose interior was divided by the O-ring shaped partition walls. For the investigation of mixing properties in HRTB the temperature step method was applied. The temperature change in the bioreactor as a response to a temperature step in the inlet flow was monitored by six Pt-100 sensors (60 response time 0.08 s and resolution 0.002 ~ which were connected with an interface unit and personal computer. Mixing properties of the bioreactor were modeled using the modified "tank in series" concept which divided the bioreactor into ideally mixed compartments. A mathematical mixing model with "simple flow" was developed according to the physical model of the compartments network and corresponding heat balances. Numerical integration of an established set of differential equations was done by the Runge-KuttaFehlberg method. The final mathematical model with "simple flow" contained four adjustable parameters (N1, Ni, For and Fp) and five fixed parameters.
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