This work examines the thermosolutal convection of a horizontal layer of an incompressible viscous magnetohydrodynamic fluid of variable permeability for the cases when the fluid is heated and soluted from above, heated and soluted from below, heated from below and soluted from above and heated from above and soluted from below. The model has been proposed by Roberts in the context of neutron stars but the results obtained are also relevant to the area of ferromagnetic fluids. The pressence of the variable magnetic permeability has no effect on the development of instabilities through the mechanism of stationary convection but influences the threshold of overstable convection which is often the preferred mechanism in nonterrestrial applications. In the context of ferromagnetic fluids, both stationary and overstable instability can be expected to be realisable possibilites. When the fluid is heated from above and soluted from below the only possible mode of instability is overstability. However, for the other three cases stationary convection and overstability are possible under some conditions. Numerical results were obtained using series expansion of Chebyshev polynomials.
Convective heat transfer in turbulent flow from an array of blunt plates is numerically studied. The flow is assumed to be steady, two dimensional, incompressible and turbulent. A modified two equation k-epsilon model with the preferential dissipation modification is incorporated to determine accurately turbulent flow field, as well as the recirculation pattern along the entrance region of the plates. To predict the local variations of turbulence quantities in the k equation, a three-layer, near wall turbulence model was examined based on the wall function. The governing equations are solved using finite volume technique based on the bounded skew hybrid difference scheme BSHD, and the PISO algorithm to iterate for pressure corrections. The solutions were obtained using a two-pass procedure, devised to allow for the correct use of the wall functions. Computations for Re-D, were obtained in the range 2.5.10(4) to 10(6): Prandtl numbers of 1, 2, 5, and 10 and blockage ratios of 5 % through 30 %. Results of friction coefficient, and Nusselt number distribution for the combined entry length problem are presented for different flow conditions and plates thickness. These findings are in accord with previously published experimental and theoretical results of a single plate. (C) 2000 Editions scientifiques et medicales Elsevier SAS.
A two-dimensional mixed convection flow of a viscous incompressible fluid of temperature dependent viscosity past a vertical impermeable fluid is considered. The governing equations for the flow are transformed for the regions appropriate to the forced convection, free convection and forced-free convection regimes. Solutions of the reduced equation appropriate in the forced convection and free convection regime are obtained using the perturbation technique treating ξ, the buoyancy parameter, as the perturbation parameter and those for the forced-free convection regime are obtained by the implicit finite difference method. Numerical results thus obtained are presented in terms of the local shear stress and local surface heat-flux. Effect of the viscosity variation parameter, ε, on the surface shear stress and the surface heat-flux for the fluid appropriate for Prandtl number ranging from 0.02 to 100 is shown. The perturbation solutions obtained for small and large values of ξ are found in excellent agreement with the finite difference solutions for the entire ξ regime.
Heat transfer coefficients in horizontal flow boiling of the wide-boiling binary mixture of C2F6/C2H2F4 and of the ternary mixture C2F6/SF6/C2H2F4 are measured. The experiments are carried out with the condition of uniform inner wall temperature at the tube perimeter. Results of the two boiling regions—forced convective evaporation and nucleate boiling—are obtained. In the forced convective evaporation region the results are predicted best using a pure fluid correlation which takes mixture properties into account. In the nucleate flow boiling region a degradation of the heat transfer coefficients occurs. This degradation has to be predicted with a suitable mass transfer controlled model, based on the ideal heat transfer coefficient, provided that a certain heat flux limit is not exceeded. In the region of high heat fluxes an “effective” heat transfer coefficient, based on local total vaporization, is more suitable than the previously cited ideal heat transfer coefficient.
In electron beam devices, the free surface of the liquid metal is submitted to high temperature gradients which, owing to the Marangoni shear at the surface, give rise to an intense recirculating meridional flow. This flow takes an important part of the heat given by the electron beam hitting the surface near its center and transports it towards the external cooled walls. This gives a poor energetic efficiency. The present work considers the possibility of using a rotating magnetic field to improve the energetic efficiency. It is shown numerically that a rotating magnetic field generates a meridional cell which flows in the opposite direction to the Marangoni flow. As a consequence, for a given heat flux, the temperature surface is increased. (C) 2000 Editions scientifiques et medicales Elsevier SAS.
The gas turbine is a continuous-flow engine which develops steady aerodynamics and flame kinetics during stationary operation. These favourable features limit the constraints placed on fuel properties as to the performance of combustion and provide a considerable margin for devising clean combustion designs. This is why gas turbines have by essence access to a broad range of primary energies. However, due to the high specialisation of aero- and aeroderivative engines, this advantage is essentially exploitable by the Heavy Duty branch which, thanks to moderate compression ratios, robust mechanical designs and versatile combustion systems, can utilise a wide series of commercial and process by-products fuels: natural gas, petroleum distillates, gasified coal or biomass, gas condensates, alcohols, ash-forming fuels etc. In this context, a thorough knowledge of the multiple fuel/machine interdependences, which seem insufficiently covered by the existing literature on gas turbines, is of prime interest. This paper offers a consistent approach to three selected aspects of importance for gas turbine designers and users, which are: energy conversion performances; combustion and emissions. This approach includes: (i) a review of main primary energies accessible to stationary gas turbines, (ii) a new, differential method for assessing the influence of fuel on machine thermodynamics, with original equations accounting for performance changes versus fuel, (iii) a comprehensive discussion of how fuel properties impact on NOx emission and combustion design. Finally, the main trends and prospects of gas turbine technology evolution, in terms of efficiency enhancement and low NOx achievement, are tentatively outlined.
Transient natural convection boundary layer flow of an incompressible viscous fluid past an impulsively moving semi- infinite vertical cylinder is considered. The temperature and concentration of the cylinder surface are taken to be uniform. The unsteady, nonlinear and coupled governing equations of the flow are solved using an implicit finite difference scheme. The finite difference scheme is unconditionally stable and accurate. Numerical results are presented with various sets of parameters for both air and water. Transient effects of velocity, temperature and concentration profiles are analyzed. Local and average skin friction, rates of heat and mass transfer are shown graphically.
The data of Kutateladze related to mean condensation heat transfer coefficients of pure vapors of steam flowing in a horizontal tube are correlated considering the phenomenon as a homogenous flow. Colburn's analogy assuming to hold good the mean friction coefficients for a homogenous flow conditions are evaluated making use of heat transfer data. By extending the model further the two-phase frictional pressure drop multipliers are established. Predictions from these correlations have reasonably agreed with the magnitudes obtained from Lockhart and Martinelli correlations which are conventionally used in the design.
Trying to achieve the optimal utilisation of oil shale resources, and the production of a more environmentally acceptable fuel, as well as a solid waste with a low a sulphur content, thereby reducing acidic leachates, has led to this investigation. In this article, the influences of the bed temperature and the choice of fluidising gas on the quality and quantity of the produced fuel gas, using a continuous feed fluidised bed reactor, were investigated. The composition and calorific value of the generated gas were determined. There was a nearly linear increase in the amount of fuel gas produced as a function of temperature, reaching ∼350 kg per 103 kg of dry raw shale and the gross calorific value ranged from 15 to 23 MJ·kg−1 with CO and H2 being the major constituents. Subsequent experiments were carried out, employing a thermogravimetric analyser under similar conditions as applied to the fluidised bed reactor. Gasification of the investigated shales complied with first-order kinetics within the limits of experimental error and the activation energy and temperature at which maximum reactivity rate occurred decreased slightly as the shale particle size was reduced.
An investigation was carried out to study the effect of flow pulsation on the characteristics of a planar air jet impinging normally on a heated surface. Such information was further utilized to determine the influence of flow characteristics in the plane of impingement on Nusselt number distribution. Time-resolved system properties were investigated with modern instrumentation that allowed instantaneous heat transfer and flow velocity measurements to be performed simultaneously. Based on good coherence function estimates between the signals, heat transfer measurements were used in return to infer flow dynamics near the impingement surface. Experiments were performed for steady and pulsating jets at jet Reynolds numbers of 1000, 5500, and 11000, pulse frequencies up to 82 Hz (corresponding to Strouhal numbers below 0.13), and pulse amplitude at the nozzle exit up to 50% of the mean flow velocity. Special techniques commonly used for periodically disturbed flow fields elucidated the dynamics of the pulse and associated coherent flow structures. Results indicated the parametric conditions for which alterations are expected in time-averaged heat transfer from the surface. Engineering applications include cooling of electronic packages and heat transfer to gas turbine blades.
This work takes place in an engineering global study of incineration processes (rotary kiln, bubbling fluidised bed); evaluation of pollutants emission at the exit of the primary processing fire chamber is one of its objective. The example developed here focuses on the impact of the swirl number of an industrial burner on pollutants emission at the supply of the post combustion chamber. The burner works with a rotary kiln fed with liquid waste. Fluent(R) UNS is used to achieve simulations, both For inert and reactive flows, taking into account turbulence and combustion interaction with probability density function model and 11 chemical species. Thermal NO production is evaluated using Zeldovich mechanism. (C) 2000 Editions scientifiques et medicales Elsevier SAS.
A nonlinear inverse heat conduction problem is resolved by using a formulation of the Kalman filter based on a statistical approach and extended to nonlinear systems. The time evolution of a surface heat flux density is reconstructed from a numerical simulation which allowed us to analyse the influence of some parameters, that condition the running of the filter, on the estimation result. A suitable choice of these parameters, guided by the filter behaviour observations, leads to a solution that remains stable when using noisy data, but that is slightly time-lagged compared to the exact function. This time-lag depends on the location of the interior temperature measurement needed for the inversion and on the model error caused by the approximation of the heat flux with a piece-wise constant function. The application of the extended Kalman filter with real measurements recorded from an experimental set-up, shows that this technique fits the stochastic structure of experimental measurements. The provided results are validated by using the Raynaud's and Bransier's inverse method and are in good agreement with the heat flux density estimated with th is method. (C) 2000 Editions scientifiques et medicales Elsevier SAS.
Impinging jets are used in many applications for cooling or heating systems, for example cooling gas turbine blades. The jet impact on a surface and the interaction between the jet and the flow determine a complex flow field which leads to a high heat transfer coefficient. The study of this flow field is thus very important. An experimental flow visualisation study has been conducted using several techniques (smoke technique, oil and pigment and the thermotropic liquid crystal technique) to determine the flow pattern for a row and a system of jets impinging on a flat plate with subsonic velocity. Some secondary vortexes are shown, thus allowing the heat transfer coefficient distribution to be understood.
This work concerns the modelling of heat and mass transfer in the boundary layer and inside a plane porous plate which is below a hot fluid flow and submitted to cold fluid blowing. A preliminary study of the heat transfer rates in the boundary layer without blowing permits us to validate, comparing with experimental results, the RNG k - epsilon model. The RNG k - epsilon model, with kinematic and thermal laws for the wall, linked with a model of blowing, is then used to study the heat and mass transfer rates at the wall when the main flow and the injected fluids are of the same species - air - but at different temperatures. The comparison between calculated friction factors, Stanton numbers and published results confirms the validity of our model. We also show the strong influence of the injection rate on the thermal convective coefficient of the wall. In the last part, results on cooling by blowing with water vapour in a main flow of air are given. Comparisons of the evolution of Stanton numbers and friction factors show that blowing with water vapour is more efficient than air injection in terms, of momentum transfer and thermal protection of walls. (C) Elsevier, Paris.
The diffusional method, a new, simple and natural concept, is introduced for solving convection-diffusion equations. The inherent formulation leads to the variational scheme for one-dimensional steady-state problems; however, the formulation is general and can be directly used in multidimensional analysis. Additionally, a lumped capacitance (mass) matrix first-order time integration technique is presented that is unconditionally stable in its implicit form and conditionally stable for Courant numbers less than one in its explicit form. The explicit form is shown, by comparison with a two-step Taylor-Galerkin scheme, to present an excellent performance for solving transient boundary layer problems and Burger's non-linear equation. Because of dampening due to first order accuracy of the time integrator, the method is not well suited to solve advection dominated problems involving travelling waves at high Péclet numbers; in this respect, a brief analysis is made on the explicit form of the Taylor-Galerkin scheme. This work also presents a performance analysis of the method when used in conjunction with adaptive time stepping procedures. The resulting adaptive PMGV scheme works very well for boundary layer problems and for solving Burger's non-linear viscous and non-viscous equations.
In this study, heat and mass transfer in a horizontal-tube falling-film evaporator, destined to be used in a desalination plant functioning by aero-evapo-condensation process, is studied theoretically. The present exchanger is made of polypropylene and it was designed to work at low temperatures (60°C–90°C) utilising geothermal energy. The one-dimensional model developed, uses basic aerodynamic, hydrodynamic and heat/mass transfer information to predict the performance of the exchanger. The predicted transfer characteristics obtained from the simulations are compared with experimental data. From this comparison it can be learnt that the model is well able to predict the trends of heat and mass characteristics of the evaporator. However, in low liquid film flow rate conditions, the model was found to overpredict the transfer characteristics slightly and the reasons for the differences are discussed. The influence of the different thermal, hydrodynamic and geometric parameters on the evaporator performances was investigated. The variations of the distilled water amount inside the exchanger are analysed. The results are used to determine the operational conditions corresponding to a maximum evaporative performances. The model was also used to optimise the different components of the exchanger.
This is a theoretical and experimental study of the time-optimisation of condensation on a vertical wall when the film is removed periodically by a mechanical device. The objective is to fine-tune the periodic process so that the production of condensate and the heat transfer rate are maximal. The first part of the paper develops the scales of the periodic condensation process, and predicts the existence of an optimal condensation time interval. The analysis also predicts that the augmentation of condensation increases as the mechanical cleaning time decreases. These design optimisation opportunities are confirmed in the second part of the paper, which reports measurements of steam condensation on a vertical surface scraped periodically. (c) Elsevier, Paris. (C)Elsevier, Paris.
