A modelling and experimental study to reduce boundary layer flashback with microstructure

Trying to improve gas turbines to be flexible to use different fuels requires a big challenge for gas turbine designers when working with current operation stability issues. Flashback is the major challenge for low NOx premixed combustion of high hydrogen content fuels. Flashback in gas turbine combustors is usually assigned to four mechanisms; core flow flashback, combustion instability flashback, boundary layer flashback (BLF) and combustion induced vortex breakdown (CIVB). The last two mechanisms are most common in swirl combustors, which are used to have better operation stability and low emissions. Improvement of swirl combustors against CIVB has been studied extensively to improve the combustion stability. The most promising solutions of CIVB are limited because the improvement against CIVB worsens the system against boundary layer flashback. Boundary layer flashback is theoretically based on the Lewis von Elbe’s formula for laminar flame, with formulas also used in turbulent flames (with some reservation) by most recent studies. However, the majority of studies take the flame side of the formula, parameters such as fuel type and blends, pressure and preheat temperature, and try to improve the understanding of the boundary layer flashback hoping to find ways to reduce its onset. However, the effect of the burner nozzle has not been studied in many types of research, especially the internal nozzle surfaces. Therefore, this work aims to study the effect of regular surface roughness on the boundary layer flashback in a 150 kW tangential swirl burner. The first part of this study is a numerical simulation using the in-house code Hydr3D to simulate the flow over riblets with different geometries (blade, triangular, scallop, diamond, lotus and sharkskin). The numerical results demonstrate that the blade riblets were the best at reducing the boundary layer thickness and consequently showed the best drag reduction around 11% compared to the smooth surface while the sharkskin geometry was the worse in drag reduction with only 0.5%. vi Although the blade showed the best drag reduction, its weak structure and complex machine specifications make the scallop, lotus, and diamond and sharkskin riblet to be chosen for manufacturing on small discs. The scallop riblet on the nozzle was manufactured using wire electrical discharge machining (WEDM). The second part of the study was isothermal experimental tests for manufactured surfaces. The flow structure was measured using a 1D LDA. The results show that the riblets alter the flow structure near the wall and increase the velocity gradient which helps the flow to reach a velocity 0.99 from mainstream velocity at y+ 30 for a smooth surface. The third part of the study was obtained with combustion and isothermal experiments using two different stainless steel woven meshes that served as a liner for the nozzle burner. A 50μm and a 150μm wire diameter meshes were used. The isothermal test showed that the 150μm mesh denoted the best shift of the velocity gradient close to the wall. The combustion experiments showed that the two meshes help to improve system against the boundary layer with the 150μm being the best. Thus, it was demonstrated that BLF could be reduced using microsurfaces, which in conjunction with other techniques, have the potential of increasing optionality, an essential feature for fuel flexibility in Gas Turbines

Experimental study to enhance swirl burner against boundary layer flashback

In gas turbine systems, operation stability represents the major challenge to any successful device deployment. Climate change combined with fossil fuel pollution has led to the need of considering high hydrogen content fuels, thus putting more pressure to stabilise gas turbines at operation conditions. Flashback is one of the main operation stability problems that represent a real challenge for gas turbine designers when using fast reacting fuels with high hydrogen content. One mechanism that has shown to contribute to flashback considerably is the propagation of the flame through its boundary layer. Although the latter has been studied, there are still several unknowns in its evolution through the system. Thus, boundary layer flashback of a swirling turbulent flame was investigated in a 150 kW tangential swirl burner previously characterised. To produce controlled changes to the boundary layer, the internal side of the burner was covered by woven wire steel mesh to mimic biological skin techniques in flow drag improvement. Two different wire meshes were used to study the effect of the regular roughness size on the boundary flashback. Moreover, the effects of using the wire mesh in such swirling flow with and without central air injection for reduction of other flashback phenomena were studied. The results show good enhancement of the system to boundary layer flashback, and a new map of the combustion stability of the rig has been produced

A study of fluid flow characteristics using microstructured surfaces produced by WEDM

This work presents an approach where biomimetic structures are tested for stabilisation and reduction of the boundary layer to reduce flashback propensity. Micro structures have been designed to have the shape of micro-riblets similar to those found in shark skin and other vegetable organisms. Various texture geometries were produced by micro Wire Electro Discharge Machining. These geometries were tested in a bespoke flame channel under isothermal conditions to observe the flow boundary profile with the aim of using combustion as a next step to determine the resistance to flashback propagation. CFD analyses were also performed using in-house simulation software Hydro3D utilising LES modelling. Experimental and simulation results were compared and showed an improvement of the boundary layer using the micro-riblets based on their structural patterns

Evaluating Antimicrobial Effectiveness of Gold Nanoparticles against Streptococcus oralis

Biofilm includes many microorganisms that causes the periodontal diseases. The increased drugs resistance against the infectious diseases is a major issue owing to excessive using of a broad spectrum of antibiotics. Recently, metallic nanoparticles (NPs) are being administered to control the growth of different types of microorganisms. For instance, gold nanoparticles (Au NPs) are found to be successful to control and limit the bacterial pathogenicity in the oral cavity without any cytotoxic effects on the human body. Aim. In this paper, it was aimed to detect the antibacterial effect of Au NPs and compare with chlorhexidine (CHX) against Streptococcus oralis (S. oralis) in dental plaque of patients with chronic periodontitis. Materials and Methods. First, supragingival and subgingival plaque samples were collected from the patients suffering from periodontal disease and incubated under aerobic or/and anaerobic conditions. Second, the morphological examination, and biochemical test by Vitec 2 machine are used to confirm the S. oralis species. Third, the synthesis of Au NPs was carried out by seed growth method and their properties were characterized. Finally, the antimicrobial effect of the Au NPs against S. oralis was evaluated by Agar well diffusion method for different Au NPs concentrations (100, 50, 25, 12.5, 6.25, 3.125, 1.562, 0.781, 0.391, 0.195, and 0.097 ppm). CHX was used as the positive control and distilled water as the negative control. The antibacterial activity data were statistically analyzed by least significant difference (LSD) using the Statistical Program for Social Science (SPSS) version 22. Results. The Au NPs with an average particles size of 43 nm, polycrystalline face-centered cubic structure were characterized. The Au NPs at 100 ppm concentration had similar antibacterial effect of CHX for inhibiting the growth of S. oralis, with no significant difference. Conclusions. The Au NPs as an antibacterial agent could be equally effective against S. oralis similar to the CHX when used at higher concentration

Optimizing the performance of the air conditioning system using an innovative heat pipe heat exchanger

This paper presents an experimental investigation to verify the thermal performance of a window-type air-conditioning system equipped with an innovative type of heat-pipe heat exchanger (HPHE). As the use of this type of heat exchanger leads to a reduction in the power consumed by the compressor. So, there is an increase in the coefficient of performance (COP) of the system which is the main measure for the efficiency of the air conditioning system. In this study, distilled water, acetone, and R-134a were selected as working fluids to fill the proposed HPHE with two various filling ratios (FR = 50% and 100%) to do the experimental investigation. The results indicated that equipping the system with the proposed type of heat exchanger gives a saving in the consumed power with (2.01, 2.195, and 1.33%) for 100% filling ratio of water, acetone, and R-134a as working fluids, respectively, compared to the regular system not equipped with the heat exchanger. Also, the results show that using innovative HPHE in the AC system led to an enhancement of refrigeration effect by about (3.5, 6.03, and 3.97%) at R134a, acetone, and water working fluids, respectively with (F.R = 100%