tailieunhanh - TWO PHASE FLOW, PHASE CHANGE AND NUMERICAL MODELING_2

Tham khảo sách 'two phase flow, phase change and numerical modeling_2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Part 3 Nanofluids 17 Nanofluids for Heat Transfer Rodolphe Heyd CRMD UMR6619 CNRS Orléans University France 1. Introduction A need for energy saving The global warming and nuclear or ecological disasters are some current events that show us that it is urgent to better consider renewable energy sources. Unfortunately as shown by figures of the International Energy Agency IEA clean energies like solar geothermal or wind power represent today only a negligible fraction of the energy balance of the planet. During 2008 the share of renewable energies accounted for 86 Mtoe only of the 12 267 Mtoe of global consumption. Unfortunately the vital transition from fossil fuels to renewable energies is very costly in time and energy as evidenced by such high costs of design and fabication of photovoltaic panels. Thus it is accepted today that a more systematic use of renewable energy is not sufficient to meet the energy challenge for the future we must develop other ways such as for example improving the energy efficiency an area where heat transfers play an important role. In many industrial and technical applications ranging from the cooling of the engines and high power transformers to heat exchangers used in solar hot water panels or in refrigeration systems the low thermal conductivity k of most heat transfer fluids like water oils or ethylene-glycol is a significant obstacle for an efficient transfer of thermal energy Table 1 . liquids Ethylen Glycol EG Glycerol Gl Water Wa Thermal Compound TC k W mK tt metals Iron Aluminium Copper Silver CNT k W mK 80 237 400 429 tt 2500 Table 1. Thermal conductivities k of some common materials at RT. The improvement of heat transfer efficiency is an important step to achieve energy savings and in so doing address future global energy needs. According to Fourier s law jo kVT an increase of the thermal conductivity k will result in an increase of the conductive heat flux. Thus one way to address the challenge .

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