Nader Karimi

Hydrodynamic Instabilities in Gaseous Detonations: Comparison of Euler, Navier–Stokes, and Large-Eddy Simulation

A large-eddy simulation is conducted to investigate the transient structure of an unstable detonation wave in two dimensions and the evolution of intrinsic hydrodynamic instabilities. The dependency of the detonation structure on the grid resolution is investigated, and the structures obtained by large-eddy simulation are compared with the predictions from solving the Euler and Navier–Stokes equations directly. The results indicate that to predict irregular detonation structures in agreement with experimental observations the vorticity generation and dissipation in small scale structures should be taken into account. Thus, large-eddy simulation with high grid resolution is required. In a low grid resolution scenario, in which numerical diffusion dominates, the structures obtained by solving the Euler or Navier–Stokes equations and large-eddy simulation are qualitatively similar. When high grid resolution is employed, the detonation structures obtained by solving the Euler or Navier–Stokes equations directly are roughly similar yet equally in disagreement with the experimental results. For high grid resolution, only the large-eddy simulation predicts detonation substructures correctly, a fact that is attributed to the increased dissipation provided by the subgrid scale model. Specific to the investigated configuration, major differences are observed in the occurrence of unreacted gas pockets in the high-resolution Euler and Navier–Stokes computations, which appear to be fully combusted when large-eddy simulation is employed.

Chaos in an imperfectly premixed model combustor

This article reports nonlinear bifurcations observed in a laboratory scale, turbulent combustor operating under imperfectly premixed mode with global equivalence ratio as the control parameter. The results indicate that the dynamics of thermoacoustic instability correspond to quasi-periodic bifurcation to low-dimensional, deterministic chaos, a route that is common to a variety of dissipative nonlinear systems. The results support the recent identification of bifurcation scenarios in a laminar premixed flame combustor (Kabiraj et al., Chaos: Interdiscip. J. Nonlinear Sci. 22, 023129 (2012)) and extend the observation to a practically relevant combustor configuration.

Temperature fields in a channel partially filled with a porous material under local thermal non-equilibrium condition – An exact solution:

This work examines analytically the forced convection in a channel partially filled with a porous material and subjected to constant wall heat flux. The Darcy–Brinkman–Forchheimer model is used to represent the fluid transport through the porous material. The local thermal non-equilibrium, two-equation model is further employed as the solid and fluid heat transport equations. Two fundamental models (models A and B) represent the thermal boundary conditions at the interface between the porous medium and the clear region. The governing equations of the problem are manipulated, and for each interface model, exact solutions, for the solid and fluid temperature fields, are developed. These solutions incorporate the porous material thickness, Biot number, fluid to solid thermal conductivity ratio and Darcy number as parameters. The results can be readily used to validate numerical simulations. They are, further, applicable to the analysis of enhanced heat transfer, using porous materials, in heat exchangers.

First and second laws of thermodynamics analysis of nanofluid flow inside a heat exchanger duct with wavy walls and a porous insert

This paper investigates the combined effects of using nanofluid, a porous insert and corrugated walls on heat transfer, pressure drop and entropy generation inside a heat exchanger duct. A series of numerical simulations are conducted for a number of pertinent parameters. It is shown that the waviness of the wall destructively affects the heat transfer process at low wave amplitudes and that it can improve heat convection only after exceeding a certain amplitude. Further, the pressure drop in the duct is found to be strongly influenced by the wave amplitude in a highly non-uniform way. The results, also, show that the second law and heat transfer performances of the system improve considerably by thickening the porous insert and decreasing its permeability. Yet, this is associated with higher pressure drops. It is argued that the hydraulic, thermal and entropic behaviours of the system are closely related to the interactions between a vortex formation near the wavy walls and nanofluid flow through the porous insert. Viscous irreversibilities are shown to be dominant in the core region of duct where the porous insert is placed. However, in the regions closer to the wavy walls, thermal entropy generation is the main source of irreversibility. A number of design recommendations are made on the basis of the findings of this study.

Thermodynamics Analyses of Porous Microchannels With Asymmetric Thick Walls and Exothermicity: An Entropic Model of Microreactors

This paper presents a study of the thermal characteristics and entropy generation of a porous microchannel with thick walls featuring uneven thicknesses. The system accommodates a fully developed flow while the solid and fluid phases can include internal heat sources. Two sets of asymmetric boundary conditions are considered. The first includes constant temperatures at the surface of the outer walls, with the lower wall experiencing a higher temperature than the upper wall. The second case imposes a constant heat flux on the lower wall and a convection boundary condition on the upper wall. These set thermal models for micro-reactors featuring highly exothermic or endothermic reactions such as those encountered in fuel reforming processes. The porous system is considered to be under local thermal non-equilibrium (LTNE) condition. Analytical solutions are, primarily, developed for the temperature and local entropy fields and then are extended to the total entropy generation within the system. A parametric study is, subsequently, conducted. It is shown that the ratio of the solid to fluid effective thermal conductivity ratio and the internal heat sources are the most influential parameters in the thermal and entropic behaviours of the system. In particular, the results demonstrate that the internal heat sources can affect the entropy generation in a non-monotonic way and, that the variation of the total entropy with internal heat sources may include extremum points. It is, further, shown that the asymmetric nature of the problem has a pronounced effect on the local generation of entropy.

Characterizing the signature of flame flashback precursor through recurrence analysis

In this paper, it is shown that prior to flashback, small dynamical changes appear in the system. These changes appear as a drift in the recurrence plots and are found to be associated with a gradual increase in the determinism and recurrence rate. Thus, this study indicates that precursors to flame flashback exist and can be detected in the multidimensional phase space reconstructed from pressure measurements acquired during flashback. This observation could have broad academic as well as industrial implications.

On the Hydrodynamics and Heat Convection of an Impinging External Flow Upon a Cylinder with Transpiration and Embedded in a Porous Medium

This paper extends the existing studies of heat convection by an external flow impinging upon a flat porous insert to that on a circular cylinder inside a porous medium. The surface of the cylinder is subject to constant temperature and can include uniform or non-uniform transpiration. These cylindrical configurations are introduced in the analyses of stagnation-point flows in porous media for the first time. The equations governing steady transport of momentum and thermal energy in porous media are reduced to simpler nonlinear differential equations and subsequently solved numerically. This reveals the dimensionless velocity and temperature fields of the stagnation-point flow, as well as the Nusselt number and shear stress on the surface of the cylinder. The results show that transpiration on the surface of the cylinder and Reynolds number of the external flow dominate the fluid dynamics and heat transfer problems. In particular, non-uniform transpiration is shown to significantly affect the thermal and hydrodynamic responses of the system in the circumferential direction. However, the permeability and porosity of the porous medium are found to have relatively smaller influences.

Experimental and Theoretical Investigation of the Flashback of a Swirling, Bluff-Body Stabilised, Premixed Flame

Abstract Flashback of an open turbulent, premixed flame in a swirl burner with central bluff-body is considered. The aim is to obtain further understanding of the physical mechanisms responsible for the upstream flame propagation. Previous studies on the same configuration hypothesised that there is an adverse pressure gradient in the direction of flame propagation. In this paper this is further investigated experimentally and theoretically. Static gauge pressure is measured on the surface of the bluff-body during flame flashback. Simultaneously, flame luminosity is imaged at 5 kHz. The results indicate that the static pressure rises downstream of the propagating reactive front. This is, then, discussed in the context of the theory of vortex bursting. An existing theory of flame propagation in the core flow is extended to a configuration similar to that investigated experimentally. The theory, although highly simplified, explains the generation of adverse pressure gradient across the flame and is qualitatively consistent with the experiment.

Mixed convection and thermodynamic irreversibilities in MHD nanofluid stagnation-point flows over a cylinder embedded in porous media

The impingement of CuO-water nanofluid flows upon a cylinder subject to a uniform magnetic field with constant surface temperature and embedded in porous media is investigated for the first time in literature. The surface of the cylinder can feature uniform or non-uniform mass transpiration and is hotter than the incoming nanofluid flow. The gravitational effects are taken into account and the three-dimensional governing equations of mixed convection in curved porous media, under magnetohydrodynamic effects, are reduced to those solvable by a finite difference scheme. Through varying a mixed convection parameter, the situations dominated by forced, mixed and free convection are examined systematically. The numerical solutions of these equations reveal the flow velocity and temperature fields as well as the Nusselt number and induced shear stress. These are then used to calculate the rate of entropy generation within the system by viscous and heat transfer irreversibilities. The results show that Nusselt number increases with increasing the concentration of nanoparticles, while it slightly deceases through intensifying the magnetic parameter. Non-uniform transpiration is shown to strongly affect the average rate of heat transfer. Importantly, it is demonstrated that the specific mode of heat convection can majorly influence the intensity of entropy generation and that the irreversibilities are much larger under natural convection compared to those in mixed and forced convection. Calculation of Bejan number shows that this is due to more pronounced relative contribution of viscous irreversibilities when free convection effects dominate the mixed convection process.

A concise review on the role of nanoparticles upon the productivity of solar desalination systems

In recent years, nanofluids have been widely used to improve the performance of various energy systems due to their favourable thermo-physical and optical characteristics. In particular, solar distillation, as an affordable and reliable technique to provide freshwater, has benefited from nanofluid technology. This article performs a review of the literature on the implementation of nanofluid technology in active and passive solar distillation systems. The progress made and the existing challenges are discussed, and some conclusions and suggestions are made for future research. The review indicates that the daily productivities of solar distillation systems enhance by using nanofluid and increasing the volume fraction of nanoparticles. However, long-term operational stability and life cycle assessment remain critical issues. These factors should be considered for future research in this field.