Bubble column reactors are widely used in many industrial applications due to their simplicity and safety of operation. Despite these advantages, the design and scale‐up of bubble column reactors is still challenging especially for industrial conditions at elevated pressure and temperature. One reason is the uncertainties concerning the specific interfacial area which is directly dependent on the bubble size distribution, bubble velocity, and gas hold‐up. All these parameters are difficult to measure under industrial conditions due to the opaqueness of the bubbly flow and the safety risks of using organic solvents at elevated pressures and temperatures. This article introduces endoscopic bubble image velocimetry, a new measuring method that enables the detection of bubble sizes and bubble velocities in organic solvents at elevated pressure and temperature (pmax = 1.85 MPa and Tmax = 70 °C) for maximal gas hold‐ups of 16 %. For this system it becomes evident that the bubble size distribution for low superficial gas velocities is almost unaffected by pressure and temperature, whereas the bubble velocity decreases slightly.
This work demonstrates how the interaction between particle image velocimetry (PIV) and robotics can massively increase measurement efficiency. The interdisciplinary approach is shown using the complex example of an automated, large scale, industrial environment: a typical automotive wind tunnel application. Both the high degree of flexibility in choosing the measurement region and the complete automation of stereo PIV measurements are presented. The setup consists of a combination of three robots, individually used as a 6D traversing unit for the laser illumination system as well as for each of the two cameras. Synchronised movements in the same reference frame are realised through a master-slave setup with a single interface to the user. By integrating the interface into the standard wind tunnel management system, a single measurement plane or a predefined sequence of several planes can be requested through a single trigger event, providing the resulting vector fields within minutes. In this paper, a brief overview on the demands of large scale industrial PIV and the existing solutions is given. Afterwards, the concept of RoboPIV is introduced as a new approach. In a first step, the usability of a selection of commercially available robot arms is analysed. The challenges of pose uncertainty and importance of absolute accuracy are demonstrated through comparative measurements, explaining the individual pros and cons of the analysed systems. Subsequently, the advantage of integrating RoboPIV directly into the existing wind tunnel management system is shown on basis of a typical measurement sequence. In a final step, a practical measurement procedure, including post-processing, is given by using real data and results. Ultimately, the benefits of high automation are demonstrated, leading to a drastic reduction in necessary measurement time compared to non-automated systems, thus massively increasing the efficiency of PIV measurements.
The present work introduces an extension of the shadowgraphy method by differently colored oblique light sources for the observation of the three-dimensional spatio-temporal dynamics of gas–liquid interfaces. The proposed expanded approach is tested and elaborated with the example of a droplet during impingement. Particularly, it is elaborated in a combined experimental/theoretical approach, how well glare points from differently colored oblique light sources can be used to encode additional 3D information of the droplet shape within a single shadowgraph image. Narrow-banded LEDs with distinct spectra and maxima in the visible light illuminate the droplet from different angles in red, green and blue light, respectively, while a high-speed RGB camera captures the images produced by each light source in the corresponding image channel, therefore creating three unique views of the droplet. In order to compensate for the mutual perturbation of the images resulting from cross-talk between the channels and the polychromatic light of the LEDs, a color correction is introduced, which is based on the transfer function between the light sources and the channels of the RGB camera. In experiments with the proposed measurement setup of a water droplet impinging onto a flat substrate it is successfully demonstrated that three unique and independent grayscale images can be reconstructed with this color correction function. The optimal illumination angles for the lateral light sources are determined experimentally, which lead to consistent glare points on the deforming gas–liquid interface throughout the dynamic process of the drop impact. An ellipsoidal droplet is considered to derive information on orientation and three-dimensional shape of a non-axisymmetrical droplet from the relative positions of the glare points and the shadowgraph contour. Thereby it is successfully demonstrated that the additional three-dimensional information encoded in the glare points can lay the groundwork for the volumetric reconstruction of the deforming gas–liquid interface during the impingement of a droplet.
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