Experimental and LES investigation of flame propagation in a hydrogen/air mixture in a combustion vessel

This work presents an experimental and numerical investigation of premixed flame propagation in a hydrogen/air mixture in a closed combustion vessel.In the experiment,high-speed schlieren video photography and pressure sensor are used to examine the flame dynamics and pressure transient.In the numerical study,a large eddy simulation(LES)based on a RNG sub-grid approach and a LES combustion model is applied to reproduce experimental observations.The effects of four physical phenomena on the burning velocity are considered in the combustion model,and the impact of grid type on the combustion dynamics is examined in the LES calculations.The flame experiences four stages both in experiment and LES calculations with structured and unstructured grids,i.e.,spherical flame,finger-shaped flame,flame with its skirt in contact with the sidewalls,and tulip-shaped flame.The flame speed and pressure in the vessel develop with periodical oscillations in both the experiment and LES simulations due to the interaction of flame front with pressure wave.The numerical simulations compare well with the detailed experimental measurements,especially in term of the flame shape and position,pressure build-up,and periodical oscillation behaviors.The LES combustion model is successfully validated against the bench-scale experiment.It is put into evidence that mesh type has an impact to a certain extent on the numerical combustion dynamics,and the LES calculation on structured grid canpredict the flame dynamics and pressure rise more accurately than that on unstructured grid with the same mesh resolution.The flame shape is more asymmetrical in the LES on an unstructured grid than that on a structured grid,and both the flame speed and the pressure rise at the later flame stage are underestimated in the LES on the unstructured grid.

Performance analysis of reconfigurable intelligent surface assisted systems under channel aging

Reconfigurable intelligent surfaces(RISs)have attracted significant attention due to their capability in customizing wireless communication environments to improve system performance.In this study,we investigate the performance of an RIS-assisted multi-user multiple-input single-output wireless communication system,considering the impact of channel aging caused by users’relative movements.In particular,first,we propose a model incorporating the joint effects of channel aging and channel estimation errors to investigate the performance of the RIS-assisted system.Then,we derive novel closed-form expressions for characterizing the sum spectral efficiency with zero-forcing precoding.From our analysis,we unveil that an increase in the temporal channel correlation coefficient,the number of base station antennas,and the received power at the users could help improve system performance.Furthermore,increasing the number of reflecting elements M of the RIS generally yields a good system performance,but with a diminishing return when M is sufficiently large.Finally,simulation results are presented to validate the accuracy of the analytical results.