A computational fluid dynamics (CFD) model is developed to study thermal performance of hollow autoclaved aerated concrete (AAC) blocks in wall constructions of buildings under hot summer conditions. The goal is to determine size and distribution of cavities (within building blocks) that reduce heat flow through the walls and thereby lead to energy savings in air conditioning. The model couples conjugate, laminar natural convective flow of a viscous fluid (air) in the cavities with long-wave radiation between the cavity sides. Realistic boundary conditions were employed at the outdoor and indoor surfaces of the block. A state-of-the-art building energy simulation programme was used to determine the outdoor thermal environment that included solar radiation, equivalent temperature of the surroundings, and convective heat transfer coefficient. The CFD problem is put into dimensionless formulation and solved numerically by means of the control-volume approach. The study yielded comprehensive, detailed quantitative estimates of temperature, stream function and heat flux throughout the AAC block domain. The results show a complex dependence of heat flux through the blocks on cavity and block sizes. In general, introducing large cavities in AAC blocks, being a construction material of low thermal conductivity, leads to greater heat transfer than the corresponding solid blocks. Several small cavities in a block may lead to small reductions in heat flux, but the best configuration found is a large cavity with a fine divider mesh in which case heat flux reductions of 50% are achievable.
The potential of currently used internal blade cooling technologies is nearly exhausted and the further improvements in the cooling efficiency can only be achieved through increases either in the air flow rate bled from the compressor or through reductions in the cooling passage size. This however results in the decrease of the gas turbine efficiency, and can cause dirt blockage problems. Several research programs were recently launched in the USA, UK and Germany to explore some novel concepts and to study innovative cooling techniques with improved parameters. One of them is an internal cyclone cooling technique, based on generation of swirl flow in blade cooling passages. Several designs of cyclone cooling, based on a two cooling passage configuration have been proposed and investigated in the USA and the former USSR over the last ten years, and these investigations have already demonstrated the superior heat transfer rate compared with conventional cooling techniques.
The novel three passage serpentine cyclone cooling scheme was proposed by the authors recently in which coolant moves successively through three cooling passages in two of those swirling flow is generated. New experimental data and the experimental correlations regarding heat transfer and fluid flow parameters in passages of an innovative cyclone cooling scheme have been obtained. The diagram of thermal-hydraulic performance is presented and comparison with conventional cooling techniques is given. The three passage cyclone cooling scheme has demonstrated the high rate of heat transfer and thermal-hydraulic performance at acceptable energy losses.
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