Begell House Inc.
International Journal of Fluid Mechanics Research
FMR
2152-5102
22
2
1995
Superadiabatic Combustion in Porous Media: Wave Propagation, Instabilities, New Type of Chemical Reactor
1-26
Lawrence A.
Kennedy
Mechanical Engineering Department, College of Engineering, University of Illinois at Chicago, 842 W.Taylor Str.,M/S 251, Chicago 60607, IL, USA; Department of Mechanical Engineering The Ohio State University Columbus, Ohio 43210
Alexei V.
Saveliev
Mechanical Engineering Department, College of Engineering, University of Illinois at Chicago, 842 W.Taylor Str.,M/S 251, Chicago 60607, IL, USA; North Carolina State University, Raleigh, NC 27695, USA
This paper discusses theoretical and experimental results of filtration combustion of methane, hydrogen, and acetylene in porous media. Two general cases were studied: linear propagation of a slow, thermal combustion wave during fast fuel filtration, and a reverse unsteady-state combustion process, when the fuel flow direction is periodically switched from one end to the other.
The intensive heat transfer between the heat releasing filtrating gas and the high thermal capacity, porous medium (through the highly developed internal solid surface) results in energy accumulation in the solid body and in the so called superadiabatic effect, when gas temperatures in the porous combustor can significantly exceed the adiabatic temperature of a feeding gas fuel.
It was found that in such a superadiabatic combustor (SAC): (1) a very fuel lean gas mixture (e.g. very small concentration of CH4, C2H2, or H2 in air) can be burned, and (2) conversely even a very small amount of oxygen in such gas fuels as CH4 or H2S can support a combustion process associated with the high temperature pyrolysis, hydrogen production and hydrocarbon or hydrogen sulfide partial oxidation.
The physical phenomena of superadiabatic combustion examined included: superadiabatic (SAC) wave propagation, SAC-wave velocity dependence on fuel concentration and gas flow rate, gasdynamic and kinetic thermodiffusion instabilities of SAC-waves, and cellular and double wave structures occurring as a result of the overheating instabilities.
Possible SAC applications are discussed. These include: applications in chemistry and energy systems (particularly for a lean gas fuel burning), partial oxidation of very fuel rich gas mixtures, hydrogen production from fossil fuels, and applications for environmental control, in particular for air purification of volatile organic compounds (VOC).
On Intrinsic Errors in Turbulence Models Based on Reynolds-Averaged Navier-Stokes Equations
27-55
James M.
McDonough
Department of Mechanical Engineering, University of Kentucky, Lexington, KY 40506-0046, USA
Results from the recent literature are reviewed to demonstrate the prevalence of modeling errors throughout the construction of eddy viscosity and Reynolds stress turbulence closure models for the Reynolds-averaged Navier-Stokes equations of incompressible flow. A straightforward analysis is presented to show the effects of these errors on the temporal development and spatial distribution of errors in mean flow quantities. This analysis indicates that solutions to the Reynolds-averaged equations can coincide with correspondingly averaged solutions of the Navier-Stokes equations only in the case that exact Reynolds stresses are employed in solving the Reynolds-averaged equations. In all other cases an O(1) error is shown to occur. It follows that computed results obtained from the Reynolds-averaged equations with typical turbulence closure models cannot be equivalent to correspondingly averaged solutions of the Navier-Stokes equations.
The Near Wall Behaviour of Particles in a Simple Turbulent Flow with Gravitational Settling and Partially Absorbing Walls
56-65
Michael W.
Reeks
School of Mechanical and Systems Engineering, Newcastle University, Stephenson Building, Newcastle upon Tyne NE1 7RU
D. C.
Swailes
Dept. of Engineering Maths, Newcastle University, Newcastle upon Tyne, Northumbria, NE1 7RU, U.K.
The paper describes how a pdf model for particle dispersion with gravitational settling in turbulent flows can be used to model the natural boundary conditions arising from partially or perfectly absorbing boundaries. The validity of so called continuum models is shown to breakdown for wall distances less than or of the order of a particle mean free which is a natural length scale for the pdf equation.
Heat Transfer in Forced-Convection Film Boiling
66-72
A. A.
Vasil'yev
Engineering Thermophysics Institute, Ukrainian Academy of Sciences, Kiev
A mathematical model of the problem of forced-convection film boiling in tubes is developed and an approximate solution is obtained. Design equations for determining the heat transfer coefficients at supercritical heat flux densities in tube flows of subcooled and saturated water at p = 20 to 21.5 MPa, ρW = 1000 to 8000 kg/m2-sec, ΔT = 0 to 140 K, temperature of the inner surface of tube wall TW ≤ 800 K, and tube diameter d between 4 and 8 mm are presented.
Thermoacoustic Vibrations in Subcooled Boiling Flows
73-88
E. D.
Domashev
Institute of Technical Thermophysics, National Academy of Sciences of Ukraine, Kiev, Ukraine
N. I.
Antonyuk
Polytechnic Institute
It is demonstrated that thermoacoustic oscillations in tubes carrying boiling flows are oscillations at the natural acoustic frequency of the tube. They arise due to transfer of acoustic energy from condensing bubbles to the flow. Experimental data are explained and an analytic proof of the effect of changes in the flow variables on excitation of self-sustained oscillations is presented.