Issam Lakkis

Numerical simulation of the dynamics of large fire plumes and the phenomenon of puffing

A vortex-based computational simulation of an axisymmetric fire plume is presented. The physical model accounts for unsteady buoyancy dynamics, conductive and convective heat transfer, and fast combustion. The effect of radiation is modeled by reducing the effective heat release from the combustion zone and raising the temperature of the evaporate from the pool. Numerical solutions are obtained using a modified vortex method in which the element vorticity is updated every time step according to source terms modeling the impact of gravity and pressure gradients in variable density flows. Attention is focused on the unsteady dynamics of the fire plume, or puffing, its dependence on the pool diameter, and the observed similarity between the unsteady behavior of isothermal and fire plumes. The numerical results show that, except for very low Reynolds numbers, fire and isothermal plumes oscillate at a frequency that depends most strongly on the pool diameter. The oscillations are accompanied by the shedding of large burning structures at a point located approximately one pool diameter above the ground. Most of the burning occurs within these structures, whose sizes are of the order of magnitude of the pool diameter. We find that the origin of the instability is a Kelvin-Helmholtz type mechanism of the vortex sleeve that forms at the interface between the fuel and air sides of the plume. The numerical results agree with experimental data on the shedding frequency, the average size of the structures and average flame height. We also find that while the buoyancy flux variation with height is different in fire and isothermal plumes, over a range of conditions, both cases have similar density fields near the pool and thus similar shedding characteristics.

Natural-Convection Heat Transfer in Channels with Isothermally Heated Convex Surfaces

This article reports on a numerical investigation conducted to study laminar natural-convection heat transfer in channels with convex surfaces that are isothermally heated. Six Grashof number (Gr) values (10 ≤ Gr ≤ 104) and 11 radii of curvature (1 ≤ κ ≤ ∞) are considered. The results are displayed in terms of streamline and isotherm plots, centerline pressure profiles, inlet mass flow rates, and local and average Nusselt number estimates. At the lowest radius of curvature (κ = 1), computations reveal the formation of recirculation zones in the exit section for all values of Grashof number considered. As the radius of curvature increases, the Gr value at which recirculation occurs also increases, until it disappears at κ values greater than 1.5. For all configurations studied, the average Nusselt number results indicate an increase in heat transfer with increasing Grashof number values. Moreover, the value of κ at which peaks increases with increasing Gr. Inlet volume flow rate and average Nusselt number correlations are presented.

A high resolution spatially adaptive vortex method for separating flows. Part I: Two-dimensional domains

A grid-free high-resolution spatially-adaptive vortex method for two-dimensional incompressible flow in bounded domains is presented. The computational algorithm is based on operator splitting in which convection and diffusion are handled separately every time step. In the convection step, computational elements are convected with velocities obtained by fast approximations of the Biot-Savart superposition with second-order Runge-Kutta time integration scheme. Diffusion is performed using the smooth redistribution method that employs a Gaussian basis function for vorticity in the interior. Near solid walls, the core functions are modified to conserve circulation. The no-slip boundary condition is enforced by creating of a vortex sheet that is redistributed to neighboring elements using the redistribution method. The proposed method enables accurate and smooth recovery of the vorticity and does not require explicit use of vortex images or occasional re-meshing. Algorithms for reduction in computational cost by accurately removing elements in overcrowded regions and for spatial adaptivity that allows for variable core sizes and variable element spacing are presented. Computations of flow around an impulsively started cylinder for Reynolds number values of 1000, 3000, and 9500 are preformed to investigate various aspects of the proposed method.

DSMC Simulations of Squeeze Film Between a Micro Beam Undergoing Large Amplitude Oscillations and a Substrate

This paper investigates the behavior of a gas film in a micro RF switch. A Two-dimensional numerical study of the flow field is performed as the micro-beam oscillates harmonically between its equilibrium position and the fixed substrate underneath. Unlike previous work in literature, the beam undergoes large displacements throughout the film gap thickness and the behavior of the gas film along with its impact on the moving RF switch (force exerted by gas on the beam’s front and back faces) are discussed. Since the gas film thickness is of the order of few microns (i.e. 0.01<Kn<1), the rarefied gas exists in the non-continuum regime and, as such, the Direct Simulation Monte Carlo (DSMC) method is used to simulate the fluid behavior. The impact of the squeeze film on the beam is investigated over a range of frequencies, velocity amplitudes, and for different film gases, corresponding to ranges of dimensionless flow parameters such as the Reynolds (Re), Strouhal (St) and Mach (Ma) numbers on the gas film behavior.© 2012 ASME

Reduced-Order Modeling of Low Mach Number Unsteady Microchannel Flows

We present reduced-order models of unsteady low-Mach-number ideal gas flows in twodimensional rectangular microchannels subject to first-order slip-boundary conditions. The pressure and density are related by a polytropic process, allowing for isothermal or isentropic flow assumptions. The Navier–Stokes equations are simplified using lowMach-number expansions of the pressure and velocity fields. Up to first order, this approximation results in a system that is subject to no-slip condition at the solid boundary. The second-order system satisfies the slip-boundary conditions. The resulting equations and the subsequent pressure-flow-rate relationships enable modeling the flow using analog circuit components. The accuracy of the proposed models is investigated for steady and unsteady flows in a two-dimensional channel for different values of Mach and Knudsen numbers. [DOI: 10.1115/1.4026199]

REDUCED ORDER MODELS OF LOW MACH NUMBER ISOTHERMAL FLOWS IN MICROCHANNELS

We present reduced order models of unsteady low Mach number isothermal ideal gas flows in two-dimensional rectangular microchannels subject to first order slip boundary conditions. The Navier-Stokes equations are simplified using Low Mach Number expansions of the pressure and velocity fields. This approximation allows decoupling the density from spatial pressure variations, thus simplifying the momentum equation. The resulting diffusion equation and the subsequent pressure-flow-rate relationship enables modeling the flow using analog circuit components. The accuracy of the proposed models is investigated for steady and unsteady flows in a two-dimensional channel for different values of Reynolds and Knudsen numbers.Copyright

Natural-Convection Heat Transfer in Channels With Isoflux Convex Surfaces

Numerical solutions are presented for laminar natural convection heat transfer in channels with convex surfaces that are subjected to a uniform heat flux. Simulations are conducted for several values of Grashof number (10 to 104) and radius of curvature (1 to ∞). The governing elliptic conservation equations are solved in a boundary-fitted coordinate system using a collocated control-volume-based numerical procedure. The results are presented in terms of streamline and isotherm plots, inlet mass flow rates, curved wall temperature profiles, maximum hot wall temperature estimates, and average Nusselt number values. At the lowest radius of curvature, computations reveal the formation of recirculation zones in the exit section for all values of Grashof number considered. For a radius of curvature equal to or greater than 2, recirculation does not occur at any Grashof number. For values of radius of curvature between 1 and 2, the value of Grashof number at which recirculation occurs decreases with increasing values of the former. The variation in the buoyancy-induced volume flow rate is highly nonlinear with respect to the radius of curvature, and the value of the radius of curvature at which the volume flow rate is maximum increases with increasing Grashof number. The value of radius of curvature at which the maximum hot wall temperature is minimized increases with Grashof number. For all configurations studied, the average Nusselt number increases with increasing Grashof number values. Correlations for maximum wall temperature and average Nusselt number are provided.

TRANSIENT THERMAL PERFORMANCE OF AXIALLY AND RADIALLY DILUTED NUCLEAR FUEL CELLS

A numerical investigation is presented of steady and unsteady heat transfer in axially and radially diluted nuclear fuel rods. The transient performance is assumed to follow a sudden and complete loss of coolant starting from steady state operation. Steady state conditions are obtained from solving numerically a conjugate conduction problem in the fuel rod and a turbulent forced convection problem in the coolant section. To model turbulence, the mixing length model is used. Dilution is accomplished by adding high thermal property materials, either axially or radially, to the original fuel rods with the intention of increasing the time delay before melting of the reactor in case of loss of coolant. The effects of the amount, distribution, and material of added diluent on steady and unsteady heat transfer are studied. Results indicate that axial dilution has negligible influence on the thermal performance of the reactor. Radial dilution, however, holds great promise and shows a reduction in the maximum wall...

Computer-Aided Analysis of Centrifugal Compressors

This paper describes computer-aided analysis of centrifugal compressors (CAACC), a micro-computer-based, interactive, and menu-driven software package for use as an educational tool by mechanical engineering students studying radial flow compressors. CAACC is written in the Pascal computer language and runs on IBM PC, or compatible, computers. In addition to solving for any unknown variables, the graphical utilities of the package allow the user to display a diagrammatic sketch of the compressor and to draw velocity diagrams at several locations. Furthermore, the program allows the investigation and plotting of the variation of any parameter versus any other parameter. Through this option, the package guides the student in learning the basics of centrifugal compressors by the various performance studies that can be undertaken and graphically displayed. The comprehensive example presented demonstrates the capabilities of the package as a teaching tool.

Computer-Aided Analysis of Gas Turbine Cycles

This paper describes a microcomputer-based, interactive, and menu-driven software package designed to help mechanical engineering students to understand gas turbines and to allow them to conduct more analysis of gas turbine cycles than they would normally be able to do by hand-calculation. The program deals with gas turbine cycle analysis so the acronym GTCA is used. GTCA is written in the Pascal computer language and runs on IBM PC, or compatible, computers. Improvements to the basic Brayton cycle, including three compressor and turbine stages, reheater, heat exchanger, intercooler, and precooler are incorporated into the program. The package is highly flexible and allows the user to model cycle schemes formed of any combination of these elements and to handle both shaft power turbines and aircraft turbojet and turbofan turbines. An important feature of the program is its ability to solve for any unknown variables. In addition to this, the program provides a schematic of the turbine plant layout and a temperature-entropy diagram of the cycle, and permits the plotting of the variation of any quantity versus any other quantity. This option enables the student to easily study and understand the effects of changing design variables on the overall performance of the cycle and permits its optimization. The statistical survey conducted along with the examples presented demonstrate the capabilities of the package as a teaching tool.