| The natural convection heat transfer characteristics outside a circular tube with a large diameter have been more intensively studied comparing with that of microscale tube or wire,and many correlations of Nusselt number in a function of Rayleigh number have been proposed by different authors.Due to the complexity in geometry for microscale tube or wire,and more involved driving forces for the fluid flow outside the tube or wire,however,such as the larger ratio of surface area to volume,the stronger edge effect,the higher relative surface roughness,etc.,the natural convection heat transfer characteristics outside the microtube or wire,especially for those with a diameter less than 1mm,are not yet clear in some aspects,many problems remain unresolved.Based on the literature review for the natural convection outside the microscale tube or wire,the following aspects will be studied in this paper.First of all,the main reasons resulting in the large deviation between experimental and theoretical results at small Ra number will be analyzed.Secondly,to verify a special phenomenon that was observed previously that an obvious non-periodic oscillation of temperature of heated hotwire may occur at a certain condition.Those critical parameters at which this phenomenon occurs are recorded.Finally,a numerical simulation trying to explain this phenomenon was conducted by using Lattice Boltzmann Method（LBM）.In this paper,an experimental setup for a Joule heated fine wire using direct electric current was established at beginning.The platinum wire has a diameter of50.4 m and purity of over 99.6%,which was immersed horizontally in a rectangular container filled with liquid.The distilled water and the 50cs silicon oil were selected separately as the test liquid.The platinum wire can be used as a thermometer itself by circulating small electric current,and heating wire.The fluid temperatures were monitored with a high-frequency.The correlation between Ra and Nu numbers was obtained,and the critical Racr number while the non-periodic oscillation of hotwire temperature can be calculated,which indicates that the Ra-Nu curve may be not continuous.The results show that the obtained Nu numbers in a Ra number range of10-4<Ra<10-2 for the horizontal heated wire were more consistent with the Fand’s fitting curve with a maximum deviation of less than 10%.The maximum deviation from the Churchill fitting curve is more than 15%,which indicates that Fand’s fitting curve is more suitable under the low Ra number.The experimental data while using50cs silicone oil as the test fluid are closer to the Tsubouchi equation with a maximum deviation of 11.86%.The temperature of the platinum wire under the critical Racr has an obviously non-periodic oscillation phenomenon,which is much sensitive to the initial temperature of the fluid.In the present paper,it has been observed that it may only occur in a narrow temperature range of 17.5-18.0℃.According to our observations,this phenomenon may be contributed to the joint actions of two driving forces,the interface tension of heated wire surface and the buoyancy with the variation of temperature of water in the gravity field.A numerical simulation was conducted by using LBM coupling the two forces of interface tension between water and heated wire,and buoyancy.However,in the simulation a confined enclosure instead of infinite large space was assumed.The obtained results show that the Nu number without considering the interface tension has a significantly lower value than that with considering the interface tension by21.9%-29.8%.The maximum deviation from the experimental results was 29.8%.The results also shows that the experimental data are more consistent with the simulated results with considering the interface tension.The maximum deviation is within 22%,and with the increase of the Ra number,the deviation between the simulation results and the experimental results decreases rapidly.The deviation from the fitting curve of Churchill equation is within 6.5%,indicating that the interface tension should be taken into account for the natural convection of heated micro-wire in water with a low Ra number. |
