Experimental and numerical investigation of Helmholtz resonators and perforated liners as attenuation devices in industrial gas turbine combustors

This paper reports upon developments in the simulation of the passive control of combustion dynamics in industrial gas turbines using acoustic attenuation devices such as Helmholtz resonators and perforated liners. Combustion instability in gas turbine combustors may, if uncontrolled, lead to large-amplitude pressure fluctuations, with consequent serious mechanical problems in the gas turbine combustor system. Perforated combustor walls and Helmholtz resonators are two commonly used passive instability control devices. However, experimental design of the noise attenuation device is time-consuming and calls for expensive trial and error practice. Despite significant advances over recent decades, the ability of Computational Fluid Dynamics to predict the attenuation of pressure fluctuations by these instability control devices is still not well validated. In this paper, the attenuation of pressure fluctuations by a group of multi-perforated panel absorbers and Helmholtz resonators are investigated both by experiment and computational simulation. It is demonstrated that CFD can predict the noise attenuation from Helmholtz resonators with good accuracy. A porous material model is modified to represent a multi-perforated panel and this perforated wall representation approach is demonstrated to be able to accurately predict the pressure fluctuation attenuation effect of perforated panels. This work demonstrates the applicability of CFD in gas turbine combustion instability control device design

Energy Efficient Uplink Transmissions in LoRa Networks

LoRa has been recognized as one of the most promising low-power wide-area (LPWA) techniques. Since LoRa devices are usually powered by batteries, energy efficiency (EE) is an essential consideration. In this paper, we investigate the energy efficient resource allocation in LoRa networks to maximize the system EE (SEE) and the minimal EE (MEE) of LoRa users, respectively. Specifically, our objective is to maximize the corresponding EE by jointly exploiting user scheduling, spreading factor (SF) assignment, and transmit power allocations. To solve them efficiently, we first propose a suboptimal algorithm, including the low-complexity user scheduling scheme based on matching theory and the heuristic SF assignment approach for LoRa users scheduled on the same channel. Then, to deal with the power allocation, an optimal algorithm is proposed to maximize the SEE. To maximize the MEE of LoRa users assigned to the same channel, an iterative power allocation algorithm based on the generalized fractional programming and sequential convex programming is proposed. Numerical results show that the proposed user scheduling algorithm achieves near-optimal EE performance, and the proposed power allocation algorithms outperform the benchmarks. © 2020 IEEE

FOCUSING MAGNETIC FIELD DESIGN FOR A FEL LINAC

Abstract A linac-based Free Electron Laser is planned to be built in Huazhong University of Science and Technology (HUST). As an important component of the linac, the focusing magnetic field is carefully designed. Spacecharge force is calculated at first to give a rough estimation of the focusing field. Start-to-end simulation shows that the magnetic field has only significant effect on spot size and phase space. With the final designed field, 3-ps-length (FWHM) pulse containing 200pC electrons can be obtained and the corresponding RMS normalized emittance and RMS radius are 7 mm·mrad and 0.25 mm, respectively. Finally, a new double-peak scheme is discussed and excitation current is proposed as the evaluation index

Synthesis and photocatalytic properties of a new paddle-wheel Cu(II) complex: An integrated experimental and theoretical investigation

A new paddle-wheel Cu(II) complex having formula [Cu(L)2(DMF)] (1)  (HL = 2-fluorobenzoic acid) has been synthesized and characterized using FTIR and single crystal X-ray diffraction. The single crystal X-ray diffraction study indicates that the carboxylic acid ligand adopt bridging bidentate modes to coordinate Cu(II) centers thereby forming a binuclear copper(II)carboxylate type structure. The complex has been utilized as photocatalyst to photodegrade model aromatic dyes viz. methyl violet (MV) and rhodamine B (Rh B) in UV light. The photocatalytic results indicated that1 offered moderate photocatalytic activity and offers good stability after performing photocatalysis. The plausible mechanism through which 1 exerted photcatalytic property had been proposed by using density of states (DOS) and partial DOS calculations.           KEY WORDS: Photocatalysis, Aromatic dyes, 2-Fluorobenzoic acid, Calculation Bull. Chem. Soc. Ethiop. 2019, 33(2), 285-292.DOI: https://dx.doi.org/10.4314/bcse.v33i2.