Circle grid fractal plate as a turbulent generator for premixed flame: an overview

This review paper focuses to ascertain a new approach in turbulence generation on the structure of premixed flames and external combustion using a fractal grid pattern. This review paper discusses the relationship between fractal pattern and turbulence flow. Many researchers have explored the fractal pattern as a new concept of turbulence generators, but researchers rarely study fractal turbulence generators on the structure premixed flame. The turbulent flow field characteristics have been studied tand investigated in a premixed combustion application. In terms of turbulence intensity, most researchers used fractal grid that can be tailored so that they can design the characteristic needed in premixed flame. This approach makes it extremely difficult to determine the exact turbulent burning velocity on the velocity fluctuation of the flow. The decision to carry out additional research on the effect circle grid fractal plate as a turbulent generator for premixed flame should depends on the blockage ratio and fractal pattern of the grid. 1.Introduction Turbulent flow is defined as formation of chaotic and eddies motions which do not contribute to the volume flow rate. It’s also described as moving in irregular patterns, while the overall flow is in one direction . Turbulent flow is common in nonviscous fluids moving at high velocities. Turbulent flow considered as most exited topic to explore, there are many researchers had been studied about it [813], over past decade or so. They purpose fractal grid/multiscale space-scale structure as a new type of turbulent flow generator. These new approach depend on correlation between fractal behaviour and turbulent flow. The turbulent flow field characteristics have been studied to be used to investigate in a premixed combustion application. Turbulent flows were distinguished from laminar flow by Reynolds number [14] characterized qualitatively as irregularity in space and time (chaotic behaviour with coherent structures) ,continuous spectrum of time and length scales, higher Reynolds number, domination of vortical motion and enhance both, mass and heat transfer [15]. The idea of the fractal/multiscale as a turbulent generator establish from a fractal object obstructing a steady laminar stream that turbulence force by a 3-D fractal grid [16]. Fractal can create space-filling fractal devices for high efficient fluid mixing. The invention was changed in the fractal dimension of the grid alters the scaling of the structure function of the structure functions of turbulence generated by fractal grid. The fractals potential also identified by distinguishes between the scaling properties of turbulence stirred by fractal object [16, 17]. 2nd International Conference on Mechanical Engineering Research (ICMER 2013) IOP Publishing IOP Conf. Series: Materials Science and Engineering 50 (2013) 012050 doi:10.1088/1757-899X/50/1/012050 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 In the structure of premixed flame, fractal grid produced different characteristic compared to the regular grid. At the same burner, same mean and turbulent condition, the turbulent burning velocity fractal grid produced is higher than standard turbulent grid. Although with same turbulent intensity background, the turbulent burning velocity is different depends on the different decay law of turbulent. [18]. 2. Relation between fractal pattern and turbulence 2.1 Blockage Ratio Blockage ratio is defined as ratio of the area blocked by the grid to the whole area of the channel/tunnel square or circle section before it become turbulent or laminar flow. Experiment investigation on a turbulence generation system with high blockage ratio that compared two families of turbulent generator (multi circular jet plate and non circular jet plate) show the multi circular jet have high turbulent level at same blockage ratio, flow rate and preserving radial uniformity.[8]. Figure 1. Families multi-circular jet (MCJ) plate (top left) and non-circular jet (NCJ) plates (single-jet (top center); central-jet (top right); central wake (bottom left), multi-jet (bottom center), wake (bottom right))[8]. Non-circular geometry considered as a good candidate for turbulence generation in a system compared to the circular geometry. It is because, in the same percentage of blockage ratio, the maximum turbulence intensity of non-circular geometry is much higher than circular geometry. There have been several studies looking at the effect of blockage ratio [19]. Depending on blockage ratio Castro demonstrated experimentally that two well distinguishable flow regimes exist. As the blockage ratio decreases, more bleed air is allowed to pass resulting in the re-circulation region moving further downstream until at a certain blockage, it disappears all together. As the blockage ratio increased from 70% to 80% suggesting that at some point in this range, the bleed air is not allowing the flow to come around to create the re-circulating region. To increase the blockage ratio, some geometry altered such as decreasing the bar thickness ratio of the fractal grid, but when lower blockage close to the grid centre and therefore produce large velocities in the central region of the grid [20]. 2nd International Conference on Mechanical Engineering Research (ICMER 2013) IOP Publishing IOP Conf. Series: Materials Science and Engineering 50 (2013) 012050 doi:10.1088/1757-899X/50/1/012050