| Natural convection and forced convection are the most common heat transfer forms in life.Natural convection doesn't require external driving force,and it only relies on the temperature difference or density difference of the fluid to drive the flow.Forced convection uses external driving forces,such as fans or pumps,to drive the flow.The advantages and disadvantages of natural convection and forced convection are obvious.The advantages of natural convection are safety,quietness and low cost,but the disadvantages are also obvious,the heat transfer effect is poor.However,the forced convection is reversed.The advantage is that the heat transfer effect is better and can be controlled by humans,but the disadvantages are that the noise is larger and the energy consumption is higher.Since nanofluids were put forward,many scholars have done a lot of researches on the thermal properties and applications of nanofluids.Nowadays,the research and application of nanofluids in the field of heat transfer enhancement is the advanced problem of engineering thermophysics,chemistry,materials and energy utilization,which is of great significance for improving the heat exchanger efficiency and increasing the heat transfer load.In this paper,TiO2-water nanofluids are prepared,and natural convection and forced convection heat transfer are experimentally studied respectively:the natural convection heat transfer of nanofluids in a closed cavity and a closed cavity filled with mental foam are investigated;the forced convection heat transfer and flow characteristics in the built-in twisted tape tube and corrugated tube are studied.It provides an experimental basis for the studies of natural convection and forced convection heat transfer of TiO2-water nanofluids.The main research work and conclusions of this paper are as follows:?1?Preparation of nanofluids:stable TiO2-water nanofluids are prepared by a two-step method and the thermal properties are measured.In this paper,stable TiO2-water nanofluids are prepared by adjusting the pH value of the nanofluids,the dispersant ratio and the stirring oscillation time.The nanofluids can remain unchanged for 7 days witout obvious precipitation,so the stability of the TiO2-water nanofluids prepared in this paper is good and can meet the experimental requirements.?2?Natural convection heat transfer of nanofluids in closed cavities:an experimental system for natural convection heat transfer of cavity with different rotation angles is established.The effects of TiO2-water nanofluid concentrations,cavity rotation angles,heating power and aspect ratios on natural convection heat transfer are studied.The enhanced heat transfer mechanism of nanofluids in natural convection heat transfer system is analyzed.The experimental results show that the heat transfer effect increases with the concentration of nanofluids,but the heat transfer enhancement decreases when the fluid concentration increases from 0.3%to 0.5%.As the placement angle of the closed cavity is different,the natural convection heat transfer effect is quite different.The heat transfer effect is best at?=0°,followed by?=45°and90°,and the worst heat transfer effect is?=-45°.In addition,the influence of the aspect ratios of the closed cavity on the heat transfer is extremely obvious.With the increase of the aspect ratio,Nusselt number is significantly improved,and the natural convection heat transfer effect is also significantly enhanced.?3?Natural convection heat transfer of nanofluids in closed cavities filled with mental foam:an experimental system for natural convection heat transfer of cavity filled with metal foam copper?PPI=5,15,25?is established.The natural convection of TiO2-water nanofluids in the cavity filled with metal foam copper is studied,and the effects of concentrations and pores per linear inch?PPI?on natural convection heat transfer are investigated.The experimental results show that when the cavity is filled with metal foam copper PPI=5 and PPI=25,the heat transfer effect of natural convection is basically the same,which is better than the heat transfer effect when PPI=15.In addition,the Nusselt number increases with the heating power,but the heat transfer enhancement efficiency decreases with the heating power.?4?Forced convection heat transfer of nanofluids in a circular tube with built-in twisted tapes:the experimental system for forced convection heat transfer of a circular tube with built-in twisted tapes is established.The heat transfer and flow characteristics of TiO2-water nanofluids in tubes with twisted tapes?rotating and static?are investigated.The effects of concentrations and the state of the twisted tapes?rotating and static?on the flow and heat transfer are studied.The experimental results show that compared with the coupled heat transfer of deionized water and round tube,the coupled of TiO2-water nanofluids with rotating and static twisted tapes can enhance the heat transfer by 53.1-101.6%and 40.1-81.7%respectively.Under the same experimental conditions,the rotating twisted tapes in the round tube can enhance the heat transfer by 13.1%compared with static twisted tapes.In addition,for all the conditions of the tube with built-in rotating twisted tapes,the maximum comprehensive performance index of nanofluids?concentration:0.3%?can reach1.519.?5?Forced convection heat transfer of nanofluids in corrugated tubes:an experimental system for forced convection heat transfer of corrugated tube?circular tube?is established.The flow and heat transfer of TiO2-water nanofluids in different pitch corrugated tubes are studied.The effects of nanofluids concentrations,tube types?circular tube,corrugated tube?and corrugation pitchs on flow and heat transfer of the system are investigated.The experimental results show that the heat transfer of nanofluids can be enhanced by 2.64-22.62%compared with water in the laminar flow,but it can be enhanced by 4.3-66.34%in the turbulent flow.The heat transfer characteristics of the experimental system are improved with the decrease of the corrugation pitch and the increase of the concentration,but at the same time,the flow resistance coefficient also becomes larger.In addition,the maximum comprehensive performance indexs of the circular tube and corrugated tube can reach 1.156 and1.5637 respectively. |
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