Investigation On Natural Convection Around A Heated Wire At Low Ra Number

Up to date, the existing correlations for Nusselt number prediction of heated wire are not consistent or accepted in public, especially at very low Ra number. Firstly, it’s still not sure whether the heat transfer at a very low Ra number is in a state of conduction or natural convection. Secondly, when Ra number is infinitely small, or approaching to zero, the Nu number also become zero as well? or a limited number? At last, what is the main reason for the difference between those experimental data and the theoretical solution.In order to resolve these raised problems above, an experimental study was conducted for Joule heated wire in fluids covering a low Ra number range from conduction to convection in the paper. The initial fluids’ temperature was measured by the fine platinum wire itself circulating with a low electric current in order to reduce the deviation of the measured temperature difference between the wire and the surrounding bulk fluids. Compared with the horizontal platinum wire, a vertical heated platinum wire was also installed in a separated container but in the same electric circulation loop.As a result, a nonlinear heat transfer behavior was observed, i.e. there was a transition stage before natural convection starting in water. The critical Ra numbers that natural convection set in for two platinum wires with diameters of φ50.8μm and φ20.0μm were respectively determined experimentally. The experimental results show that a local minimum Nu number happens at the transition stage to convection. The frequently measured temperature of platinum wire indicates that the temperature fluctuates with a relative very large magnitude in the transition stage from conduction to convection. However, the local minimum Nu number is not zero but about 0.36, which is in general agreement with the previous measured value using silicon oil by Tsubouchi.Compared with the water, the 50 cs silicon oil was used in the experiment. However, the phenomenon didn’t happen. It is reasonable to believe that the geometrical effect due to microscale and the fluid intrinsic property play important roles in the experiment, for example, the wire vibrating due to some forces.A vibrating heated wire was numerically investigated by the dynamic mesh technology of FLUENT. The effects of the natural convection and vibrating parameters on the natural convection heat transfer of the heated wire were analyzed.On one hand, the comparison between the simulated numerical results of natural convection with those former experimental results shows that the horizontal wire has a larger Nu number than that of the vertical one. As a whole, the numerically simulated Nu numbers of natural convection are less than those of experimental data, which indicates that the governing N-S equation in previous model may not interpret the natural convection around the heated microwire at low Ra number correctly. On the other hand, an enhanced heat transfer mode may exist in practice that the numerical simulation of natural convection was not taken into account. Therefore, an enhanced heat transfer mode due to the weak wire vibration is proposed, which is much possible filling the gap between the simulated and experimental results. The numerical results obtained shows that there is a critical Rec number at low Ra number, and under which the Nu number is independent of Re number, and identical to the experiment results of 50 cs silicon oil.