The trajectories, referred to as lifelines, of individual microorganisms in an industrial scale fermentor under substrate limiting conditions were studied using an Euler‐Lagrange computational fluid dynamics approach. The metabolic response to substrate concentration variations along these lifelines provides deep insight in the dynamic environment inside a large‐scale fermentor, from the point of view of the microorganisms themselves. We present a novel methodology to evaluate this metabolic response, based on transitions between metabolic “regimes” that can provide a comprehensive statistical insight in the environmental fluctuations experienced by microorganisms inside an industrial bioreactor. These statistics provide the groundwork for the design of representative scale‐down simulators, mimicking substrate variations experimentally. To focus on the methodology we use an industrial fermentation of Penicillium chrysogenum in a simplified representation, dealing with only glucose gradients, single‐phase hydrodynamics, and assuming no limitation in oxygen supply, but reasonably capturing the relevant timescales. Nevertheless, the methodology provides useful insight in the relation between flow and component fluctuation timescales that are expected to hold in physically more thorough simulations. Microorganisms experience substrate fluctuations at timescales of seconds, in the order of magnitude of the global circulation time. Such rapid fluctuations should be replicated in truly industrially representative scale‐down simulators.

in Wiley InterScience (www.interscience.wiley.com).The four-point optical probe is applied in a bubble column with an air-water system to investigate the bubble properties (local gas holdup, velocity, chord length, specific interfacial area, and frequency) over a range of gas superficial velocities. Both bubbles moving upward and downward are recorded and measured as opposed to only upward bubbles measured and reported in other studies involving probes. The probe worked efficiently in both bubbly flow and highly churn-turbulent flow at very high superficial gas velocities. Bubble properties at the conditions of churn-turbulent flow are obtained and investigated for the first time. The changes in the bubble velocity distribution, bubble chord length distribution, and specific interfacial area with superficial gas velocity, sparger design, and with axial and radial positions in the column are discussed.

The hydrodynamics of two-dimensional bubble columns operated in various flow regimes are studied using particle image velocimetry. Both averaged velocity profiles and Reynolds stress profiles are obtained and discussed in relation to large-scale structures present in the flow. The normal stresses, dominated by large-scale structures, are an order of magnitude higher than the shear stress. It is found that the contribution from the bubbles to the shear to the normal stresses is negligible. A time series of the flow field is studied, demonstrating that the flow could be split into a low-frequency contribution due to the uortical structures and a high-frequency fluctuatingpart. The latter gives rise to flat normal stress profiles, and the former is responsible for the original form of the normal stress profiles. The shear stress in the smaller columns investigated can be related to the averaged vertical velocity profile according to a Boussinesq approximation. Data on the eddy viscosity are presented.

■ Abstract Gravity-driven bubbly flows are a specific class of flows, where all action is provided by gravity. An industrial example is formed by the so-called bubble column: a vertical cylinder filled with liquid through which bubbles flow that are introduced at the bottom of the cylinder. On the bubble scale, gravity gives rise to buoyancy of individual bubbles. On larger scales, gravity acts on nonuniformities in the spatial bubble distribution present in the bubbly mixture. The gravity-induced flow and flow structures can increase the inhomogeneity of the bubble distribution, leading to a turbulent flow. In this flow, specific scales are identified: a large-scale circulation with the liquid flowing upward in the center of the column and downward close to the wall. On the intermediate scale there are vortical structures; eddies of liquid, with a size on the order of the diameter of the column, that stir the liquid and radially transport the bubbles. On the small scale there is the local stirring of the bubbles. We describe the ideas developed over time and identify some open questions. We discuss the experimental findings on the turbulence generated, the stability of the flow, axial dispersion, and the similarities between bubble columns and air lifts. Especially for higher gas fractions, many questions still lack accurate answers. The lateral lift force in bubble swarms and the structure of the turbulence in the bubbly mixture are important examples of inadequately understood physical phenomena, providing many challenges for fundamental and applied research on bubbly flows.

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The hydrodynamical similarities between the bubbly flow in a bubble column and in a pipe with vertical upward liquid flow are investigated. The system concerns air/water bubbly flow in a vertical cylinder of 14.9 cm inner diameter. Measurements of the radial distribution of the liquid velocity, gas fraction and the bubble velocity and size are performed using laser Doppler anemometry for the liquid velocity and a four-point optical fibre probe for the gas fraction, bubble velocity and size. The averaged gas fraction was 5.2% for the bubble column (with a superficial liquid velocity of zero) and 5.5% for the bubbly pipe flow at a superficial liquid velocity of 0.175 m s−1. From a hydrodynamical point of view, the two modes of operation are very similar. It is found that in many respects the bubbly pipe flow is the superposition of the flow in the bubble column mode and single-phase flow at the same superficial liquid velocity.The radial gas fraction profiles are the same and the velocity profiles differ only by a constant offset: the superficial liquid velocity. This means that the well-known large-scale liquid circulation (in a time-averaged sense) of the bubble column is also present in the bubbly pipe flow. For the turbulence intensities it is found that the bubbly pipe flow is like the superposition of the bubble column and the single-phase flow at the superficial liquid velocity of the pipe flow, the former being at least an order of magnitude higher than the latter. The large vortical structures that have been found in the bubble columns are also present in the bubbly pipe flow case, partly explaining the much higher ‘turbulence’ levels observed.