Local Thermal Non-equilibrium Analysis of Cu-Al2O3 Hybrid ‎Nanofluid Natural Convection in a Partially Layered Porous ‎Enclosure with Wavy Walls

A numerical study is performed to investigate the local thermal non-equilibrium effects on the natural convection in a two-dimensional enclosure with horizontal wavy walls, layered by a porous medium, saturated by Cu-Al2O3/water hybrid nanofluid. It is examined the influence of the nanoparticle volume fraction, varied from 0 to 0.04, the Darcy number (10-5 ≤ Da ≤ 10-2), the modified conductivity ratio (0.1 ≤ ϒ ≤ 1000), the porous layer height (0 ≤ Hp ≤ 1), and the wavy wall wavenumber (1 ≤ N ≤ 5) on natural convection in the enclosure. Predictions of the steady incompressible flow and temperature fields are obtained by the Galerkin finite element method, using the Darcy-Brinkman model in the porous layer. These are validated against previous numerical and experimental studies. By resolving separately the temperature fields of the working fluid and of the porous matrix, the local thermal non-equilibrium model exposed hot and cold spot formation and mitigation mechanisms on the heated and cooled walls. By determining the convection cell strength, the Darcy number is the first rank controlling parameter on the heat transfer performance, followed by N, Hp and γ. The heat transfer rate through the hybrid nanofluid and solid phases is highest when N = 4 at a fixed value of nanoparticle volume fraction

Experimental Investigation on effects of bluff-body size and axial air injection on blowoff limits in swirl burners

The stability limits of swirl combustors have been considered as a crucial factor for obtaining a wide stability operation map. The present global consideration is towards using low-carbon emission fuel in gas turbine production sector and, many other combustion systems. However, the demands of introducing lowcarbon emission fuels impose a considerable modification in the combustor hardware; consequently, the variation of burner stability operation map. Blowoff and flashback are two parameters that determined the margins of stability operation in swirl burners, when correlated with equivalence ratio and inlet tangential or bulk velocity. This study investigates the effect of hardware modification with different bluff-body sizes (external diameter) and flow-field manipulation like using axial air injection on blow-off limits in swirl combustors. The first part of this study has demonstrated that variation of bluff-body diameter alters the blowoff limits significantly. Small central injector (bluff-body) diameter displaces blowoff limits towards leaner equivalence ratios with (Φ= 4 to 4.2); which is favourable for low emission demands. However, the stability map became narrower regarding inlet tangential velocities with (w=2.7 to 4.2), consequently reducing output power. In contrast, bigger injector diameter leads to having blowoff limits occur at a wider range in term of inlet tangential velocity(w=2.5-4.5) which means high output power, despite slight displacement to the rich region, Φ= 0.5 at high tangential velocity. The second part of this work has proposed, a new technique that can replace hardware (bluff-body) by axial air-jets which can simulate the physical shape of bluff-body. Using axial air jets results in wider operation map, the inlet tangential velocity range is (w=2-8 m/s) compare with bluff-body case (w=2.5-4.5 m/s), hence increasing the burner output power while keeping its size. The position of air-jet opening inside burner plenum alter blowoff limits, baseline Lo=0 and Lo=150 extend the range of inlet tangential velocity at which the blowoff occurs, almost (2-8 m/s). While the other three positions revealed less range of inlet tangential velocities, as the affected by aerodynamic perturbations arise from the clash between axial jets and inlet tangential flo