Study On Heat Transfer And Flow Characteristics Of TiO₂-H₂O Nanofluids In Elliptical Tubes

Forced convection heat transfer is the most important heat transfer model in high-intensity heat exchange field.The combination of traditional heat transfer fluids（pure liquids,such as water,alcohol,oil,etc.）and conventional heat transfer structures（such as circular tubes）is no longer sufficient for higher heat transfer intensity in smaller space.The nanofluids are solid-liquid two-phase suspensions,which are formed by adding nano-level solid particles into the base liquid in a certain proportion and way and distributing them uniformly in the base liquid.Many studies have shown that nanofluids have superior thermal conductivity and can enhance heat transfer.However,there are many numerical studies and relatively few experimental studies on nanofluids at present.Therefore,this paper takes TiO₂-H₂O nanofluids as the research object to carry out the experimental study on forced convection heat transfer of TiO₂-H₂O nanofluids.At the same time,the effects of enhanced heat transfer tubes（elliptical tubes）and built-in inserts（twisted tape）on the flow and heat transfer performances of TiO₂-H₂O nanofluids are investigated.The main research work of this dissertation is summarized as follows:（1）An experimental platform of forced convection of nanofluid in elliptical tube is set up to study the flow and heat transfer characteristics of the nanofluids.The convective heat transfer characteristics of deionized water and nanofluids with different mass fractions in circular and elliptical tubes are investigated respectively.It is found that TiO₂-H₂O nanofluids with 0.5wt%can increase the Nusselt number by9.7-16.1%and 25.8-32.9%compared with deionized water in circular and elliptical tubes respectively.TiO₂-H₂O nanofluids with 0.1wt%,0.3wt%and 0.5wt%in the elliptical tube can increase the Nusselt number by 21.8%,24.8%and 27.9%compared with the Nusselt number in the circular tube under the same condition respectively.（2）An experimental platform of forced convection of nanofluid in built-in twisted-tape elliptical tube is set up to investigate the influence of the insert（twisted tape）and the axial ratio of the elliptical tubes on the flow and heat transfer performances of the nanofluids.Studies have shown that the twisted tape is a simple and effective way to improve the heat transfer.The heat transfer performance can be enhanced by 2.7%-4.3%under the same conditions.As an enhanced heat transfer tube,the larger the axial ratio of the tube,the better the heat transfer effect.Under the same conditions,the Nusselt number of the TiO₂-H₂O nanofluids in the elliptical tube with an axial ratio Z=1.706 is 18.28%more than that in an elliptical tube with an axial ratio Z=1.235.（3）The heat transfer performance,resistance performance and comprehensive performance of the test fluid are evaluated by three parameters:Nusselt number（Nu）,flow resistance（f）and comprehensive performance evaluation index（η）.The results show that there is a critical Reynolds number in the range of the experiment in this paper,which makes the system in the best heat transfer condition.