1,721,283 research outputs found
Analysis of combined forced and free flow in a vertical channel with viscous dissipation and isothermal-isoflux boundary conditions
No full textFully developed and laminar mixed convection in a parallel-plate vertical channel is investigated in the case of non-negligible viscous heating. The channel walls are subjected to asymmetric boundary conditions: One wall experiences a constant and uniform heat flux, while the other is kept at a uniform and constant temperature. The velocity field and the temperature field are evaluated analytically by means of perturbation expansions with respect to a buoyancy parameter, i.e., the ratio between the Grashof number and the Reynolds number. The Nusselt numbers and the friction factors are obtained as functions of the buoyancy parameter. © 1999 by ASME
Fully developed laminar forced convection in circular ducts for power-law fluids with viscous dissipation
No full textThe asymptotic behaviour of the temperature field for the laminar and hydrodynamically developed forced convection of a power-law fluid which flows in a circular duct is studied. The effect of viscous dissipation is taken into account, while the axial heat conduction in the fluid is considered as negligible. First, it is shown that the sufficient condition for the existence of an asymptotically invariant value of the Nusselt number is less restrictive than that usually employed in the previous literature. Then, both the asymptotic Nusselt number and the asymptotic behaviour of the temperature field for the axial wall heat flux distributions which yield a thermally developed region are determined. Finally, the asymptotic Nusselt number and the asymptotic temperature distribution are evaluated analytically in the cases of either uniform wall temperature or convection with an external isothermal fluid. Copyright © 1996 Elsevier Science Ltd
Heat transfer by fully developed flow and viscous heating in a vertical channel with prescribed wall heat fluxes
No full textAn analysis of combined forced and free convection in a vertical parallel-plate channel with prescribed wall heat fluxes is performed by considering a fully developed flow and by taking into account the effect of viscous dissipation. It is shown that the condition of fully developed flow cannot be accomplished if a streamwise change of the wall heat fluxes occurs. An analytical solution of the momentum balance and energy balance equations is found by a perturbation method. In particular, the forced convection flow with viscous heating is treated as the base heat transfer process. Then, the effect of buoyancy is accounted for by expressing the fluid velocity and temperature as power series with respect to the ratio between the Grashof number and the Reynolds number
On fully-developed mixed convection and flow reversal of a power-law fluid in a vertical channel
No full textThe fully-developed mixed convection of power-law fluids is investigated for laminar flow in a parallel-plate vertical channel. The boundary condition of uniform and unequal temperatures prescribed at the channel walls is considered. The velocity field, the viscous stress field and the temperature field are obtained by solving analytically the momentum and energy balance equations as well as the stress-strain constitutive equation. An expression which allows the evaluation of the friction factors is presented. The condition for the occurrence of flow reversal is obtained
Evolution of massive true vacuum bubbles
No full textThe various trajectories of a massive true vacuum bubble in a preexisting false vacuum are obtained by Israel's matching conditions between the bubble's interior (anti-De Sitter) and exterior (Schwarzschild) line elements. © 1988 Società Italiana di Fisica
Combined forced and free flow of a power-law fluid in a vertical annular duct
No full textAn investigation of mixed convection and flow reversal in a vertical annular duct is presented with reference to laminar and fully-developed flow of a power-law fluid. The boundary surfaces are supposed to be isothermal, with unequal temperatures. The momentum balance and the energy balance equations as well as the viscous stress constitutive equation are solved analytically in order to obtain the velocity field, the viscous stress field and the temperature field. First, two special cases are analyzed: mixed convection of a Newtonian fluid; forced convection of a power-law fluid. Then, in the general case, the evaluation of the friction factors is employed to determine the conditions for the occurrence of flow reversal, for fixed values of the power-law index and of the ratio between the duct inner and outer radii
Laminar mixed convection with viscous dissipation in a vertical channel
No full textCombined free and forced convection flow in a parallel-plate vertical channel is analyzed in the fully developed region by taking into account the effect of viscous dissipation. The two boundaries are considered as isothermal and kept either at equal or at different temperatures. The velocity field, the temperature field and the Nusselt numbers are obtained by a perturbation series method which employs a perturbation parameter proportional to the Brinkman number. Dimensionless coefficients which allow the evaluation of the dimensionless mean velocity, of the dimensionless bulk temperature and of the Nusselt numbers are determined
Hyperbolic propagation of an axisymmetric thermal signal in an infinite solid medium
No full textThermal wave propagation in an infinite solid medium which surrounds an infinitely long cylindrical surface is considered. This surface transfers a prescribed and time-dependent heat flux to the solid medium. The non-stationary heat conduction problem is studied by assuming a non-vanishing value of the thermal relaxation time for the solid medium, i.e. by employing the hyperbolic heat conduction equation. An analytical expression of the temperature field in the solid is determined. Examples are provided for heat fluxes which vary with time as a square wave pulse or as a triangular wave pulse. Comparisons with the solutions obtained for parabolic heat conduction are performed. Copyright © 1996 Elsevier Science Ltd
Temporal to Spatial Instability in a Flow System: A Comparison
Full text linkThe definitions of temporal instability and of spatial instability in a flow
system are comparatively surveyed. The simple model of one-dimensional Burgers'
flow is taken as the scenario where such different conceptions of instability
are described. The temporal analysis of instability stems from Lyapunov's
theory, while the spatial analysis of instability interchanges time and space
in defining the evolution variable. Thus, the growth rate parameter for
temporally unstable perturbations of a basic flow state is to be replaced by a
spatial growth rate when a coordinate assumes the role of evolution variable.
Finally, the idea of spatial instability is applied to a Rayleigh-B\'enard
system given by a fluid-saturated horizontal porous layer with an anisotropic
permeability and impermeable boundaries kept at different uniform temperatures.Comment: 19 page
Analysis of flow reversal for laminar mixed convection in a vertical rectangular duct with one or more isothermal walls
No full textAn investigation of laminar and fully developed mixed convection in a vertical rectangular duct is presented. The analysis refers to thermal boundary conditions such that at least one of the four duct walls is kept isothermal. The evaluation of the velocity field and of the temperature field is performed analytically. The limiting case of free convection, i.e. the case of pure buoyancy-driven flow, is discussed. Special attention is devoted to the following sets of thermal boundary conditions: (A) two facing duct walls are kept isothermal with different temperatures and the others are kept insulated; (B) two facing duct walls have a uniform wall heat flux and the others are kept isothermal with the same temperature. In both cases, the conditions for the onset of flow reversal are obtained. The friction factor is evaluated. It is shown that this parameter depends only on the duct aspect ratio in case (A), while it depends also on the ratio between the Grashof number and the Reynolds number in case (B). © 2001 Elsevier Science Ltd. All rights reserved
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