Interaction effects between surface radiation and double-diffusive turbulent natural convection in an enclosed cavity filled with solid obstacles

The work reported here is a 2D numerical study on the buoyancy-driven low speed flow of humid air inside a rectangular cavity partially filled with solid cylindrical objects and whose vertical walls are maintained at 1.2 and 21 oC. This is a case of double diffusion where both temperature and concentration gradients are significant. Detailed calculations were carried out and results compared with reliable data, with the aim of investigating the influence of surface emissivity on heat and moisture transport. The Rayleigh number of the fluid mixture (air and water vapour) based on the height of the vertical wall is found to be 1.45 x 109. In the computations, turbulent fluxes of the momentum, heat and mass were modelled by low-Re (Launder-Sharma) k-ε eddy viscosity model. The effect of radiation has been found to be significant even for the moderate temperature difference of 19.8 oC between the hot and the cold walls with the humid air participating in the radiation heat transfer. Variations of average Nusselt number and buoyancy flux are analysed and profiles of turbulent quantities are studied in order to observe the net effect of the intensity of turbulence. It has been found that a change in surface emissivity influences the humidity distribution and heat transfer within the cavity. It was also observed that during natural convection process the air/water vapour combination results in an increase in the heat transfer as compared to pure natural convection. An increase in heat transfer is observed using thermo-physical materials of higher surface emissivity. It can thus be implied that with the appropriate choice of components, the fluid flow, heat and mass transfer due to natural convection can be increased passively

A Study on Buoyancy Driven Turbulent Flow Associated with Radiation in Cavities Partially Filled with Blockages

Fluid flow and heat transfer in cavities partially filled with disconnected blockages is important in the design of a wide range of industrial and engineering applications such as thermal management of indoor environments, cooling of electronic panels, drying of agricultural products, stacking of items in cold storage etc. The flows in such confined spaces develop as a result of temperature and concentration gradient which is further complicated by the interactive effects of turbulence and radiation. The aims of this research are to explore the detailed heat transfer and flow field inside cavities partially filled with solid blockages and, in particular, to address the uncertainties associated with turbulence models, to quantify the influence of double diffusion and to study the effect of surface properties. To achieve the above aims, a systematic numerical investigation has been carried out by validating the computational results against reliable experimental data available in open literature. A selection of turbulence and radiation models has been employed to scrutinise the effects of the above flow physics. An experimental set up capable of establishing low Rayleigh number buoyancy driven flow in a rectangular cavity containing cylindrical blockages was designed and fabricated to obtain temperature data. A series of experiments was conducted to obtain reliable temperature distribution at various positions in the flow domain and on the surfaces of the blockages. This set up also allowed us to study the proximity effect of blockages which has not been reported elsewhere. It has been found that the choice of turbulence model remains to be an important issue and should be given due consideration for natural convection flow with a high Rayleigh number. The results from the parametric study on the specification of passive thermal boundary conditions reveal that the experimental temperature profile is the most accurate boundary condition for passive walls in relation to the adiabatic and linear temperature profiles. Experimental benchmark temperature data evaluated at various positions in the cavity with and without blockages are presented and some of them are compared with CFD simulations. Finally, as an example of the application of the research methodology, a detailed numerical modelling was conducted on a Double-Skin-Façade which is known to reduce energy consumption in building and has become popular in recent years. The current methodology has been applied to establish a number of parameters in connection with the design and performance of DSF which are believed to be useful to practitioners

Natural convection flow and heat transfer in an enclosure containing staggered arrangement of blockages.

The work reported in this paper is a numerical study of airflow and heat transfer for low turbulence buoyancy-driven flow in a rectangular cavity partially filled with solid objects. The two vertical walls were maintained at constant temperatures giving a temperature differential of 42.2 °C resulting in a characteristic Rayleigh number of 1.45×109. Two different types of blockage arrangements were considered for analysis, and these consist of In-line and Staggered arrangements of 12×6 and 12×3 objects. In all cases, steady state flow and wall heat transfer data at the mid-height and mid-width of the cavity are presented. The flow domain displayed a stable core region and the average core temperature was found to be strongly influenced by different stacking arrangement of solid objects. In general, the staggered arrangement resulted in lower heat transfer through the surfaces which is linked with the suppression of turbulence within the boundary layers close to the surfaces

Numerical simulation of 2D turbulent natural convection of humid air in a cavity filled with solid objects.

Natural convection flow in enclosures containing solid objects is important in the design of a wide range of industrial and engineering applications. Numerical calculations were performed for low turbulence double-diffusion convection for humid air inside a rectangular cavity with an aspect ratio of 2:1(height/width) and partially filled with disconnected solid cylindrical objects occupying 15% of the total cavity volume. The vertical walls are maintained at 1.2 and 21 °C and a Rayleigh number of the fluid mixture based on the height of the vertical wall is 1.45 × 109. In the computations, turbulent fluxes of momentum, heat and mass were modelled by a low-Re (Launder-Sharma) κ-□ eddy diffusivity model. Radiation equation was discretised using the discrete ordinate method. Detailed analysis was performed on the flow and heat transfer and on the turbulence quantities within the cavity. The effect of 2D simplification of inherently 3D radiation modelling was carefully scrutinised and calculations carried out with an equivalent emissivity. Variations of average Nusselt number and buoyancy flux are analysed. Profiles of turbulent kinetic energy and turbulent viscosity are studied in detail to observe the net effect on the intensity of turbulence caused by the interactions of radiation with double-diffusive natural convection heat and mass transfer. Particular emphasis was placed on quantifying the proximity of the solid objects to the active walls. It has been found that turbulence is suppressed as the objects get closer to the walls

Double skin façade: Modelling technique and influence of venetian blinds on the airflow and heat transfer

The demand to reduce building cooling load and annual energy consumption can be optimised with the use of Double Skin Facade (DSF). Computational Fluid Dynamics (CFD) methods are frequently used for the analysis of heat transfer through DSF. However, considerable uncertainty exists regarding few key parameters, such as modelling strategies and the solar heat transmitted to the indoor space as a function of the blind tilt angles and positioning within the façade channel. In this paper we have investigated four modelling strategies and the influence of blind tilt angle and their proximity to the façade walls. The DSF system used in this investigation is equipped with venetian blinds and facades that absorb and reflect the incident solar radiation and transfer the direct solar heat gain into the building. A finite volume dis- cretization method with the SIMPLE solution algorithm of the velocity-pressure coupling involving the low-turbulence keε model is used. A ray-traced solar model is coupled with long wave radiation model to solve the complete solar and radiation fields along with convection and conduction fields. On the modelling strategies, three dimensional domains were cast over three computational zones; external zone with solar radiation entering the outer skin of glass; buoyancy-driven air cavity zone with convection and transmitted solar radiation; and an internal zone. Also investigated is the thermal behaviour of the DSF due to the blind tilt angles (30o , 45o , 60o , and 75o ) and its position from the facade walls (104 mm, 195 mm, 287 mm and 379 mm). Validations of the results are based on experimental data from the literature and the predicted trends compared very well with the experimental measurements. The heat gain due to direct solar radiation and convection through the facades to the internal space are presented. Comparative analysis of the four modelling strategies shows little variation of the results. The implication is a reduction in complexity and cost of modelling, since the additional effort requires in the CFD modelling is not justified by a significant improvement of the results. The variations of the blinds tilt angles as well as its proximity to façade walls significantly influences the convective flow within the façade cavity and the heat gains to the indoor spac

Numerical Modelling of the Effect of Wettability, Interfacial Tension and Temperature on Oil Recovery at Pore-Scale level

A numerical investigation into the effect of wettability and temperature on oil recovery with a hot water injection at different temperatures is reported in this paper. The computational domain is a two-dimensional porous medium (reservoir) maintained at a fixed temperature with pore spaces of varying sizes and interconnected pore-throats. ANSYS-Fluent VOF (volume of fluid) model was used to simulate the two-phase transport through the reservoir with hot water injections at varying temperatures (20, 40 and 60 °C) and wettability contact angles of 45°, 90° and 150°. In addition, an investigation was conducted on the effect of combined interfacial tension and matrix wettability on oil recovery process at low and high interfacial tension of 0.025 N/m and 0.045 N/m respectively for the three different wettability contact angles. The results showed that, the displacement behaviour of water and oil-wet system is affected significantly by the contact angle with a profound effect on the oil recovery factor. In the water-wet case (with the water wetting the matrix wall and the oil phase surrounded by water), relatively more oil is displaced from the domain thereby improving the oil recovery factor. The water-wetter system resulted in about 35–45% oil recovery than the oil-wet system, with the unrecovered oil mainly adhering to the wall region of the pore bodies for oil-wet system. For the intermediate wet case, initial fluid distribution is seen to have a more significant effect on the displacement behaviour than the contact angles. In conclusion, by altering the wettability from oil-wet to water-wet condition, the oil recovery rate is improved. The results from this study are consistent with the experimental and numerical studies in literature and it will further enhance the understanding of the phenomenon that is critical to the mechanism of recovery such as surfactant and polymer flooding process

Effect of emissivity on the heat and mass transfer of humid air in a cavity filled with solid obstacles.

The work reported here is a 2D numerical study on the buoyancy-driven low-speed turbulent flow of humid air inside a rectangular cavity partially filled with solid cylindrical objects for a Rayleigh number of 1.45 × 109. Variations of Nusselt number, buoyancy flux, vapor mass fraction, and turbulence viscosity ratio are presented for various emissivity values of wall surfaces. It was observed that during the natural convection process, radiation effects are very significant and the air/water vapor combination results in a small increase in heat transfer as compared with the pure natural convection of dry air

Design of solar powered charging backpack

This paper demonstrated a step by step process in designing a solar powered charging backpack that is capable of charging a mobile phone efficiently. A selection of existing products available on the market were reviewed and compared to ascertain the cost, size, and output capabilities. Next, the solar cell types and regulators were compared and their respective merits were also investigated. The charging system was then designed and tested before being integrated with the backpack. The results clearly showed that the system managed to charge the mobile phone. However, it was found that the excessive power dissipation has caused the linear regulator to generate significant heat