• Numerical simulation of spray combustion of conventional fuels and Biofuels

      Alajmi, Ayedh; Abdalla, Ibrahim E.; Bengherbia, T.; Yang, Zhiyin; De Montfort University, UK; University of Derby, UK; Faculty of Technology, De Montfort University, UK; Faculty of Technology, De Montfort University, UK; Faculty of Technology, De Montfort University, UK; Department of Engineering and Design, University of Sussex, UK (WIT Press, 2014-07-01)
      Numerical studies based on steady Computational Fluid Dynamics (CFD) for reactive flows were performed with the objective of validating advanced reaction mechanisms used to study spray combustion for both conventional and Biofuels. The SST-4 equation model was used to model turbulence, while more than one (comprehensive) reaction mechanisms were used to model the combustion of methanol, diesel and biodiesel using CHEMKIN-CFD and Fluent CFD code. Some of the reaction mechanisms used in modelling the current reactive flow simulation was already tested while others were developed during the course of this work. The computational results have shown good agreement with the available experimental data ofWidmann and Presser (Combustion and Flame, 129, 47–86, 2002) with the developed reaction mechanism slightly over predicting the temperature range. The CFD results have also shown that most of the harmful emission of the combustion of liquid fuels is less for Biodiesel compared to conventional diesel with the exception of CO2. This is in line with the finding of many experimental data. Keywords: combustion, biofuels, emissions.
    • Numerical study of the combustion of conventional and biofuels using reduced and advanced reaction mechanisms

      Abdalla, Ibrahim E.; Alajmi, Ayedh; Yang, Zhiyin; University of Derby (Vinča Institute of Nuclear Sciences, Belgrade, 2015-04-04)
      Combustion process of conventional liquid fuels and BioFuels depend on many factors including thermo - physicochemical properties associated with such fuels, their chemical structure and the combustion infrastructure used. This manuscript summarises the computational results of a steady cfd simulation for reactive flows performed to validate advanced reaction mechanisms for both conventional and BioFuels. The computational results have shown good agreement with the available experimental data with the differences thoroughly discussed and explained. An important observations and findings reported in this work was that when comprehensive reaction models were used, the injected fuels burned at a slower rate compared to the situation when reduced models were employed. While such comprehensive models predicted better flame structure and far better biproducts compared to the existing experimental results, it has also led to over-predicting the temperature field. The computational results have also shown that BioDiesel produces a marginally higher rate of CO2 compared to Diesel. Such results are thought to be due to the Oxygenated nature of the fuel and how such feature influences the development of a comprehensive reaction mechanism for such fuels.
    • On secondary instability of a transitional separation bubble.

      Yang, Zhiyin; Abdalla, Ibrahim E.; University of Derby; Jubail University College (Elsevier, 2018-12-01)
      It is well established in the natural transition of an attached boundary layer that the transition process starts with a two–dimensional primary instability (Tollmien–Schlichting wave, denoted as TS wave), followed by usually a three-dimensional secondary instability (fundamental mode or subharmonic mode) leading to the breakdown to turbulence. However, the transition process of a separation bubble (laminar flow or laminar boundary layer at separation and transition occurs downstream of the separation, leading to turbulence at reattachment) is less well understood, especially on the nature of secondary instability. The focus of this paper is on trying to advance our understanding of secondary instability of a transitional separation bubble on a flat plate with a blunt leading edge (separation is induced geometrically at the leading edge) under a very low free-stream turbulence level (< 0.1%). Large-Eddy Simulation (LES) is employed in the current study with a dynamic sub-grid-scale model. The numerical flow visualisation together with the spectral analysis has indicated that a three dimensional secondary instability, the elliptical instability, which occurs for fundamental frequency is the main mechanism at work whereas the subharmonic mode in the form of vortex-pairing is hardly active. There is no evidence for the existence of hyperbolic instability in the braid region either.