Reaz Hasan

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-e 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.

Comparative LCA of technology improvement opportunities for a 1.5-MW wind turbine in the context of an onshore wind farm

This paper presents life cycle assessment (LCA) results of design variations for a 1.5-MW wind turbine due to the potential for advances in technology to improve their performance. Five LCAs have been conducted for design variants of a 1.5-MW wind turbine. The objective is to evaluate potential environmental impacts per kilowatt hour of electricity generated for a 114-MW onshore wind farm. Results for the baseline turbine show that higher contributions to impacts were obtained in the categories of ozone depletion potential, marine aquatic eco-toxicity potential, human toxicity potential and terrestrial eco-toxicity potential compared to technology improvement opportunities (TIOs) 1–4. Compared to the baseline turbine, TIO 1 with advanced rotors and reduced tower mass showed increased impact contributions to abiotic depletion potential, acidification potential, eutrophication potential, global warming potential and photochemical ozone creation potential, and TIO 2 with a new tower concept involving improved tower height showed an increase in contributions to abiotic depletion potential, acidification potential and global warming potential. Additionally, lower contributions to all the environmental categories were observed for TIO 3 with drivetrain improvements using permanent magnet generators while increased contributions towards abiotic depletion potential and global warming potential were noted for TIO 4 which combines TIO 1, TIO 2 and TIO 3. A comparative LCA study of wind turbine design variations for a particular power rating has not been explored in the literature. This study presents new insight into the environmental implications related with projected wind turbine design advancements.

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.

A new effective viscosity model for nanofluids

Purpose At present, no unified model predicts the viscosity of nanofluids. This paper sets out a novel viscosity model designed to resolve this issue. Design/methodology/approach The model draw upon regression analysis of carefully selected several published papers covering experimental, numerical and theoretical findings to propose the effective viscosity model. Findings Unlike some other models, this one is reliable and has a good level of accuracy. This model has been assessed in a numerical investigation using a 3D horizontal pipe and presenting the results. Originality/value This is a new model for predicting the effective viscosity of nanofluids. The proposed model has been tested in 3D horizontal pipe, and the predicted results for viscosity and Nusselt number show good agreement with available data.