Jeroen De Ridder

Enthalpy and Entropy Barriers Explain the Effects of Topology on the Kinetics of Zeolite-Catalyzed Reactions

The methylation of ethene, propene, and trans-2-butene on zeolites H-ZSM-58 (DDR), H-ZSM-22 (TON), and H-ZSM-5 (MFI) is studied to elucidate the particular influence of topology on the kinetics of zeolite-catalyzed reactions. H-ZSM-58 and H-ZSM-22 are found to display overall lower methylation rates compared to H-ZSM-5 and also different trends in methylation rates with increasing alkene size. These variations may be rationalized based on a decomposition of the free-energy barriers into enthalpic and entropic contributions, which reveals that the lower methylation rates on H-ZSM-58 and H-ZSM-22 have virtually opposite reasons. On H-ZSM-58, the lower methylation rates are caused by higher enthalpy barriers, owing to inefficient stabilization of the reaction intermediates in the large cage-like pores. On the other hand, on H-ZSM-22, the methylation rates mostly suffer from higher entropy barriers, because excessive entropy losses are incurred inside the narrow-channel structure. These results show that the kinetics of crucial elementary steps hinge on the balance between proper stabilization of the reaction intermediates inside the zeolite pores and the resulting entropy losses. These fundamental insights into their inner workings are indispensable for ultimately selecting or designing better zeolite catalysts.

Fluid-Elastic Instabilities of Clamped-Clamped Aligned and Inclined Cylinders in Turbulent Axial Flow

Fluid-elastic instabilities arise due to the coupling of structural motion and fluid flow. In the specific case of a clamped-clamped cylinder in axial flow, it will buckle at a sufficiently high flow velocity and start to flutter at even higher flow velocities. This dynamic behavior is of importance to nuclear reactor core design, undersea pipe lines and devices for energy harvesting. In this contribution, the fluid forces and the dynamics of a flexible clamped-clamped cylinder in turbulent axial flow are computed numerically. In contrast to present analytical approaches, this numerical model does not require to tune parameters for each specific case or to obtain coefficients from experiments.To provide insight in the way viscous fluid forces affect the dynamics of a cylinder in axial flow, fluid forces are computed on rigid inclined cylinders, mimicking the damping force experienced by the same cylinder moving perpendicular to the axial flow. The computations showed the existence of two different flow regimes. Each regime gave rise to a different lift force behavior, which will also influence the damping of the coupled system. Furthermore it is shown that the inlet turbulence has a non-negligible effect on these forces and thus on the dynamics of the cylinder.Next, the dynamics of a flexible cylinder clamped at both ends in axial water flow are computed by means of a methodology developed earlier. The results are successfully compared with dynamics measured in experiments available in literature. Computationally it was found that the cylinders natural frequency decreases with increasing flow velocity, until it loses stability by buckling. The threshold for buckling is in quantitative agreement with experimental results and weakly nonlinear theory. Above this threshold, the amplitude of the steady deformation increases with increasing flow speed. Eventually, a fluttering motion is predicted, in agreement with experimental results. It is also shown that even a small misalignment (1°–2°) between the flow and the structure can have a significant impact on the coupled dynamics.Copyright

Large-eddy simulations of turbulence-induced vibrations in annular pipe flow

This paper investigates the turbulence-induced vibration of a circular beam in annular pipe flow. Vibrations induced by turbulence are one of the causes of fatigue and fretting wear in process environments. Although the small-scale vibrations are normally not leading to immediate failure of structural components, they typically result in long term damage. To predict the amplitude of these subcritical vibrations, current methods require an accurate description of the incident pressure field. However, measurements of cross-spectral pressure fields in annular geometries are rare. Models to describe the pressure field have a tendency to provide only descriptive information, after a series of experiments have been performed. Therefore this paper aims to predict the pressure field numerically, by means of wall-resolved large-eddy simulations. In order to validate this approach the flow field of an experiment available in literature is computed. In the conditions simulated, water is flowing at 10 m/s in an annulus with a hydraulic diameter of 1.27cm. Pressure correlations obtained from the computations are compared to descriptive models such as the Corcos and Chase models. The numerical power spectra are also compared to experimental spectra.

Numerical Computation of Modal Characteristics of a Clamped-Clamped Cylinder in Turbulent Axial Pipe Flow

In this paper the modal characteristics of a flexible cylinder in turbulent axial flow are investigated with partitioned fluid-structure interaction simulations. In these simulations a computational fluid dynamics calculation to resolve the flow field is coupled with a computational structure mechanics calculation to compute the structural behavior. The cylinder is initially deformed according to an eigenmode in vacuo and then released. From this free vibration decay of the cylinder in the turbulent axial flow, modal characteristics are determined. To assess the accuracy of these calculations, the same configuration is computed as in an experiment with a solid brass cylinder mounted in a water-conveying pipe. The natural frequency appears to be relatively insensitive to an increase in flow velocity in this case. Both experiments and computations show the same trend of slightly decreasing natural frequency with increasing flow velocity. The damping, on the other hand, is very sensitive to the flow velocity A change in flow velocity from 10m/s to 30 m/s results in a modal damping increase from 1.4% to 3.0%. Changes in molecular viscosity due to temperature differences had only a small effect on modal damping.