Measurement of turbulence properties

The aim of the research is to investigate anisotropic turbulence intensities, id est to investigate the distribution of Reynolds stresses and energy spectra in a square cross-section channel, downstream of a semi-active jet turbulence grid generating anisotropic turbulent airflow. In addition to the semi-active jet turbulence grid, another type of turbulence grid was developed and experimentally investigated. This grid contains vertical, flexible strips of aluminum (in this case, there are no perpendicular (horizontal) grid elements), which vibrate at a frequency depending on the velocity of the main airflow. Besides the investigation of the velocity- and turbulence intensity distributions, another main objective of the research is to measure the von Kármán energy spectrum when the turbulence cannot be considered isotropic. This aspiration of ours is justified by the knowledge gap present in the literature in this specific field. Monin has carried out a theoretical study to extend and generalize the von Kármán – Howarth isotropic principal stress equation to the anisotropic regime. The proposed new experimental work aims to provide a solid experimental background for verifying and validating the physical correctness of the Monin equation, which may result in a new theoretical understanding and perception of the major issues and the nature of anisotropic turbulence. Since the anisotropic energy spectra are expected to exhibit different characteristics from the isotropic Kolmogorov spectra, these new experimental results may contribute to the development of new anisotropic and engineering turbulence models that can be used in industrial applications.

An Experimental Study of the Effect of a Turbulence Grid on the Stack Performance of an Air-Cooled Proton Exchange Membrane Fuel Cell

In a previous numerical study on heat and mass transfer in air-cooled proton exchange membrane fuel cells, it was found that the performance is limited by heat transfer to the cathode side air stream that serves as a coolant, and it was proposed to place a turbulence grid before the cathode inlet in order to induce a mixing effect to the air and thereby improve the heat transfer and ultimately increase the limiting current and maximum power density. The current work summarizes experiments with different turbulence grids which varied in terms of their pore size, grid thickness, rib width, angle of the pores, and the distance between the grid and the cathode inlet. For all grids tested in this study, the limiting current density of a Ballard Mark 1020 ACS stack was increased by 20%. The single most important parameter was the distance between the turbulence grid and the cathode inlet, and it should be within 5 mm. For the best grid tested, the fuel cell stack voltage and thus the efficiency were increased by up to 20%. The power density was increased by more than 30% and further improvements are believed to be possible.

Large Eddy Simulation of Acoustical Sources in a Low Pressure Axial-Flow Fan Encountering Highly Turbulent Inflow

A large eddy simulation (LES) was applied to predict the unsteady flow in a low-speed axial-flow fan assembly subjected to a highly “turbulent” inflow that is generated by a turbulence grid placed upstream of the impeller. The dynamic Smagorinsky model (DSM) was used as the subgrid scale (SGS) model. A streamwise-upwind finite element method (FEM) with second-order accuracy in both time and space was applied as the discretization method together with a multi-frame of reference dynamic overset grid in order to take into account the effects of the blade-wake interactions. Based on a simple algebraic acoustical model for axial flow fans, the radiated sound power was also predicted by using the computed fluctuations in the blade force. The predicted turbulence intensity and its length scale downstream of the turbulence grid quantitatively agree with the experimental data measured by a hot-wire anemometry. The response of the blade to the inflow turbulence is also well predicted by the present LES in terms of the surface pressure fluctuations near the leading edge of the blade and the resulting sound power level. However, as soon as the effects of the turbulent boundary layer on the blades become important, the prediction tends to become inaccurate.