Thermal Buoyancy Effects Research Articles

Thermal radiation and buoyancy effects on MHD free convective heat generating flow over an accelerating permeable surface with temperature-dependent viscosity

Thermal radiation and buoyancy effects on MHD free convective heat generating flow over an accelerating permeable surface with temperature-dependent viscosity

Thermal radiation and buoyancy effects on hydromagnetic flow over an accelerating permeable surface with heat source or sink

Similarity equations governing steady hydromagnetic boundary-layer flow over an accelerating permeable surface in the presence of such effects as thermal radiation, thermal buoyancy, and heat generation or absorption effects are obtained. These equations are solved numerically by an implicit finite-difference method. Favorable comparisons with previously published work are obtained. The effects of the various parameters on the velocity and temperature profiles as well as the skin-friction coefficient and wall heat transfer are presented graphically and in tabulated form.

Natural Convection from Combined Thermal and Mass Diffusion Buoyancy Effects in Micropolar Fluids

Natural Convection from Combined Thermal and Mass Diffusion Buoyancy Effects in Micropolar Fluids

Prediction of natural ventilation in buildings with large openings

This paper first presents a consistent pressure-based formulation for natural ventilation of single-zone and multi-zone buildings with multiple openings. Pressure-based multi-zone formulation is made easier to implement by introducing an auxiliary concept of external pressure, which allows all the formulas to be presented in an integrated form. Multi-zone situations considered include vertically interconnected zones, and horizontally interconnected zones with same heights and different heights. The formulation includes the combined effect of wind, thermal buoyancy and mechanical ventilation, and it can be used for both external and internal large openings. A simple and easy implementation method was then presented. Single-zone and multi-zone analytical solutions are revisited or developed by the pressure-based formulations and used for the validation of the implementation method. A CFD method is also used to cross-check the implementation method in a single-zone building with very large external openings. A reasonable agreement has been found between the results predicted by the pressure-based formulation and those predicted by the analytical solutions and CFD methods.

Flow of Two-Immiscible Fluids in Porous and Nonporous Channels

This study considers steady, laminar flow of two viscous, incompressible, electrically-conducting and heat-generating or absorbing immiscible fluids in an infinitely-long, impermeable parallel-plate channel filled with a uniform porous medium. A magnetic field of uniform strength is applied normal to the flow direction. The channel walls are assumed to be electrically nonconducting and are maintained at two different temperatures. When present, the porous medium is assumed to act as an electrical insulator and that it is in local thermal equilibrium with the fluid. The transport properties of both fluids are assumed to be constant. This study is expected to be useful in understanding the influence of the presence of slag layers on the flow and heat transfer aspects of coal-fired Magnetohydrodynamic (MHD) generators when the porous medium is absent and the effects of thermal buoyancy and a magnetic field on enhanced oil recovery and filtration systems where the porous medium is present. The problem is formulated by employing the balance laws of mass, linear momentum, and energy for both phases. Continuous conditions for the velocity and temperature as well as the shear stress and heat flux of both phases at the interface are employed. The resulting governing ordinary differential equations are solved numerically subject to the boundary and interface conditions for the velocity and temperature distributions of both fluids in the channel. Analytical solutions for a special case of the problem where the porous medium is absent or only its inertia effect is neglected are obtained. Comparisons with previously reported velocity profiles are performed and excellent agreements are obtained. A parametric study illustrating the influence of the physical parameters involved in the problem is conducted and the results are presented graphically and discussed. [S0098-2202(00)02101-5]

Non-Darcy natural convection heat and mass transfer along a vertical permeable cylinder embedded in a porous medium

Combined heat and mass transfer in non-Darcy natural convection flow along a permeable vertical cylinder embedded in a saturated porous medium is studied. The boundary layer analysis is formulated in terms of the combined thermal and solutal buoyancy effect. The flow field characteristics are analyzed using the implicit finite difference method as well as the local nonsimilarity method. The effect of the curvature, the buoyancy ratio, the Lewis number and the transpiration parameter on the local Nusselt number and the local Sherwood number are also studied. The results are presented in tabular form as well as graphically. Comparisons of the results obtained by the local nonsimilarity method are in excellent agreement with finite difference solution up to χ (the curvature parameter) of 10. The effects of different pertinent parameters on the velocity, temperature and species concentration profiles are also shown graphically.

Thermophoretic transport and deposition of particles in vertical tube flow with variable wall temperature and thermal radiation

A particulate two phase flow with variable wall temperature has been studied for examining the deposition of particles in the thermal radiation and mixed convection flow associated with the manufacture of optical fiber preforms. The two-dimensional governing equations of continuity, momentum and energy have been solved numerically including the effects of thermal radiation and buoyancy (upward or downward flow) in the vertical tube flow. A particle trajectory model has been adopted to predict the particle transport, and P-1 approximation has been used to evaluate the radiation heat transfer. In the upward flow case, a high deposition efficiency is obtained and the deposition zone of the downward flow is broader than that of the upward flow. Thermal radiation makes the deposition zone broader and the deposition efficiency smaller.

A study of a control strategy utilizing outdoor air to reduce the wintertime carbon dioxide levels in a typical Taiwanese bedroom

A CO 2 concentration of more than 1000 ppm has been monitored in Taiwanese bedrooms during sleeping hours in the wintertime. The high indoor CO 2 levels were caused by poor ventilation due to insufficient ventilation rates. This study sought to reduce the wintertime CO 2 concentration level in a typical Taiwanese bedroom with less outdoor air to maintain thermal comfort. CO 2 was used as an indicator to assess whether an adequate ventilation rate has been obtained to dilute or remove harmful pollutants. With the help of the thermal buoyancy effect, the CO 2 generated in the bedroom was effectively removed by means of less outdoor air. Through computational fluid dynamics simulations, the appropriate window and transom locations with the corresponding outdoor air supply volume, as well as the lowest possible outdoor air temperature were identified.

A numerical study of solar chimney for natural ventilation of buildings with heat recovery

The performance of a glazed solar chimney for heat recovery in naturally-ventilated buildings was investigated using the CFD technique. The CFD program was validated against experimental data from the literature and good agreement between the prediction and measurement was achieved. The predicted ventilation rate increased with the chimney wall temperature. The effects of solar heat gain and glazing type were investigated. It was shown that in order to maximise the ventilation rate in a cold winter, double or even triple glazing should be used. Installing heat pipes in the chimney for heat recovery not only increased the flow resistance but also decreased the thermal buoyancy effect. To achieve the required air flow rates in naturally-ventilated buildings with heat recovery, use should be made of wind forces.

Similarity solutions of mixed convection heat and mass transfer in combined stagnation and rotation-induced flows over a rotating disk

The objective of the present study is to develop a novel similarity model for analysis of mixed convection heat and mas transfer in combined stagnation and rotation-induced flows over a rotating disk. Thermal and concentration (solutal) buoyancy effects stemmed from temperature and concentration gradients in rotational as well as gravitational forces fields are all taken into account. The influences of the forced flow, disk rotation, thermal buoyancy, buoyancy ratio and the fluid properties, i.e. Prandtl and Schmidt numbers, on the flow, temperature and concentration fields and the associated friction factors, heat and mass transfer rates are investigated. The present results reveal the effects of various buoyancy modes with combined forces on the transport phenomena in rotating-disk flows, and the analysis is also useful in understanding the mechanisms of mixed convection in the class of rotating fluids.

On the Boussinesq approximation for fluid systems with deformable interfaces

The assumptions underlying the Boussinesq approximation place restrictions on its applications to systems with free surfaces and interfaces. In this paper we reconsider the limits in which the Boussinesq approximation is valid and develop a generalization of the approximation which allows for self-consistent application to such systems. The Boussinesq limit is characterized two parameters, G = gL 3 ν 2 , and ε = βθ, where G is a dimensionless measure of gravitational acceleration or system size and ε is the product of the fluid's coefficient of thermal expansion with a characteristic temperature difference. The Boussinesq limit, in general, corresponds to G → ∞ and ε → 0 while the product Gε (equal to the familiar Grashof number) remains finite. We consider two problems involving deformable boundaries: the stability of a two-layer fluid system heated from above and below and the influence of buoyancy on long-wavelength Marangoni instability. In the first two examples, we examine the conditions required to consistently account for the effects of a deformable surface on thermal convection while simultaneously applying the Bousinesq approximation. In particular, the effect of the deformable surface can be included through a term proportional to the product of Gδ with the deflection, ζ, of the interface from planarity, but it is required to treat the density of the two fluids as equal in the equations of motion (i.e., δ → 0). In the second example, that of long-wavelength Marangoni instability, we find that to simultaneously consider the effects of thermal buoyancy together with a deformable surface we must either treat the surface as undeformable ( G → ∞ and ε → 0) or consider finite G and take Gε and ε to be independent parameters in the equations of motion. This leads to a result which explicitly reveals the role of buoyancy as a destabilizing influence on long-wavelength instability.

A Thermal Balance Model for Livestock Buildings for use in Climate Change Studies

Abstract A model for estimating the mean internal temperature of livestock buildings was developed for use in climate change studies. The model, which calculates a steady-state heat balance every hour, combines wind, artificial ventilation, thermal buoyancy effects and solar radiation, based on the established theoretical work. The temperatures of the building are solved analytically. Results are presented which compare the model with experimental measurements made in a naturally ventilated building. Predictions give a good representation with mean absolute error of 0·41 K.

Helium-Based Simulation of Smoke Spread Due to Fire in Enclosed Spaces

This study concerns helium smoke simulators which have been used for simulating smoke spread in aircraft cabins during inflight tests. The smoke from cabin fires is buoyant due to higher temperatures, compared to the surroundings. However, the smoke from commercially available smoke generators is nonbuoyant and an addition of helium is employed to make it buoyant so that the actual circumstance is closely represented. The dependence of buoyancy on the concentration of helium and Ihe validity of this approach to represent thermal buoyancy effects are important questions that need to be answered. In this technique, helium is added to smoke from commercially available smoke generators in order to provide buoyancy in the flow due to the lower density of helium. The mixture is then used to simulate the flow of smoke and toxic gases in the cabin of an aircraft, or in other enclosed spaces, without employing heated air to obtain the buoyancy. Since buoyancy plays a very important role in the flow and spread of smoke and hot gases, it is important that the simulation include this effect in order to closely represent the actual flow An experimental investigation is undertaken to explore the validity of simulating thermally buoyant gases, from a fire, wilh a helium and air mixture. The experimental flow consisted of a ceiling jet with a corner flow. This same configuration was used in previous studies which were directed at thermally buoyant flows. Thus, a basis for comparison between the two buoyant flows can be established. Parameters, such as the volume flow rate of the air and helium and the initial helium concentration, were varied over wide ranges. Temperature profiles from the thermally buoyant case were compared to helium concentration profiles. In addition, velocity profiles from the two studies were compared to determine the flow spread. From these results, the characteristics of the buoyant helium-air mixture were obtained. It is found that, though this mixture can be used to qualitatively model thermally buoyant flows, large quantitative differences exist between the two and must be taken into account for an accurate simulation.

Wind action and temperature difference effects on the ventilation rate of a two-storey building communicating with the outside environment by a chimney

In this paper a simplified analytical model to predict the ventilation rates, internal pressures and temperatures as influenced by the combined effects of thermal buoyancy and wind action is applied to a simple example of practical importance. It consists of a two-storey building communicating with the outside environment through a semi-opened window at each storey and a common chimney. The rational approach formulation of the problem combines experimental information on pressure distributions due to wind, on pressure drop coefficients of the air circulation through wall openings and on the performance of a chimney as a natural ventilator, with the numerical solution of the thermal and dynamic governing equations. The results emphasize the relative importance of thermal and dynamic variables, including wind incidence and the role of the chimney.

Wind Induced Air Exchange Rates in a Greenhouse Tunnel with Continuous Side Openings

To study the influence of wind speed on ventilation rates, experimental trials were carried out in a plastic tunnel with continuous side ventilators. This type of greenhouse is the most representative of Mediterranean and arid regions. The injection peak technique using N 2O as tracer gas was used to measure air exchange rates. Simultaneously air pressure difference between inside and outside at ground level was recorded together with outside wind velocity and direction and inside and outside air temperatures. Several combinations of side ventilator openings were studied under different wind velocities and directions. Results from pressure measurements show that the thermal buoyancy effect has no influence on the pressure field when the wind velocity exceeds 1·5 m s −1and that the turbulent wind pressure coefficient increases when adjacent obstacles perturb the wind. Results from gas measurements show that the air exchange rate is strongly dependent on wind velocity and total ventilator area, as expected, and that it can be expressed as a function of a global wind coefficient having properties similar to the turbulent wind coefficient. This indicates that the induced turbulent component of the air exchange rate can be significant. The reduced form of the ventilation flux, defined as the ventilation flux per unit ground area and average wind speed is used to compare the present results with those of other authors on different types of greenhouses and ventilation systems. It appears that roof openings are more effective than side openings and that the effectiveness of the ventilation vents depends on the type of opening. The effectiveness appears to be less when the vents are equipped with a side-mounted flap on their frame.

Turbulence modulation in jet diffusion flames: Modeling and experiments

Turbulence in a diffusion flame is modulated by thermal expansion, buoyancy effects and lift-off. Aside from simple shear-generated sources density-velocity correlations, π′u ″ i represent additional source terms in the Reynolds-stress equations that distort the shear-generated turbulence anisotropy. A modeled transport equation and several zero-order models of π′u ″ i are analyzed and their effect on flame predictions with a Favre averaged second-order moment closure for velocity and scalar transport is investigated. The chemical reaction is described by chemical equilibrium and laminar flamelet modeling. The latter is shown to have limitations in application due to differential diffusion. Two attached, vertical H 2/N 2-air flames with the same Reynolds numbers but different Froude numbers are investigated numerically and experimentally. The desired data base for an overall comparison is provided by comprehensive 3D-LDV, coherent anti-Stokes Raman spectroscopy and spontaneous Raman spectroscopy measurements. The calculations yield correct results in all measured profiles of velocity, temperature and species concentrations. It is shown that only one zero-order model and the transport equation of π′u ″ i are adequate. The neglect of those terms will falsify the prediction of decay rates, fluctuations and flame shapes. The magnitude of errors depends on the local Froude number which decreases downstream. The increase in the influence of buoyancy leads to smaller decay rates of axial velocity and to enhanced scalar mixing. Furthermore, turbulence intensities are reduced, and scalar fluctuations and anisotropy are enlarged. The experimentally observed visible flame length shortening with decreasing, density weighted Froude number is reproduced by the presented model.

Numerical study of thermal buoyancy effect in an operating state of clean rooms

Numerical study of thermal buoyancy effect in an operating state of clean rooms

Measurement and Analysis of Air Exchange Rates in a Greenhouse with Continuous Roof and Side Openings

The dresults of natural ventilation measurements carried out in a plastic greenhouse with continuous roof and side openings are presented. Both the static and turbulent pressure differences were taken into account. The results show a strong dependence of air exchange rate on wind velocity and total ventilator opening area as expected, but no important dependence on wind direction could be found for the conditions in which the experiments were conducted. Wind induced ventilation seems to become dominant when the wind speed exceeds about 1·8 m/s that is, the thermal buoyancy effect can be neglected. When the wind speed is lower than 1·8 m/s, although the thermal buoyancy effect is minor it cannot be neglected. Roof only and roof and side ventilator configurations are always more efficient than side only ventilator configuration. The best results for the calculated ventilation flux were obtained by a model that takes into account both thermal buoyancy and wind effects.

Simulation of the indirect natural convection solar drying of rough rice

A mathematical model to simulate the indirect natural convection solar drying of rough rice is presented. The temperatures along the collector are described by a numerical solution. The drying of the grain is described by the introduction of a deep-bed solution procedure. The variation of the air temperature in the collector and across the air bed is incorporated in the prediction of the thermal buoyancy effect. The results show that, at the low air flow rates produced by natural convection, the drying wave moves slowly and the layer of grain cannot be considered uniform, with serious overdrying taking place in the bottom layer. The deep bed model is therefore essential in order to obtain a realistic simulation. Also, due to the temperature drop across the bed, the chimney height makes little contribution to buoyancy. Sensitivity analysis showed the system to be most sensitive to grain bed depth and collector length and least to chimney height and cover to absorber spacing. The simulation results have a number of practical implications. The grain bed should be turned regularly to avoid over-drying and give a tempering effect. The collector and plenum chamber upstream of the grain bed should be well sealed to contain the buoyancy pressure. The effect on performance of reducing or removing the chimney should be examined.

Heat transfer from continuously moving material in channel flow for thermal processing

The conjugate mixed convection and conduction transport that arises due to the continuous movement of a heated material in a parallel channel flow of a cooler fluid has been numerically investigated. The basic problem studied is of interest in several manufacturing processes, ranging from crystal growing and continuous casting to extrusion, hot rolling, wire drawing, and glass fiber drawing. A flat plate, or sheet, moving in a wide channel is considered in this article. A two-dimensional, steady circumstance, with laminar flow in the fluid, is assumed. The full, elliptic, governing equations are solved, employing finite difference techniques. Numerical results are obtained for various values of the important physical parameters such as plate speed US9 the forced flow velocity in the channel UX9 and temperature T0 of the material upstream of the cooling region. The effect of the geometry, or configuration, of the system is also investigated. The results obtained indicate that, as expected, the effect of thermal buoyancy is more significant when the plate is moving vertically upward than when it is moving horizontally, and also when the forced flow velocity is small. A strong forced flow leads to a fairly uniform velocity distribution across much of the channel. The plate speed substantially affects the overall heat transfer rate. Although this work is focused on convective cooling of a heated moving material, the basic considerations and many of the results can easily be extended to a moving material which is being heated through convection, i.e., in a heat treatment process.