External natural convection from a Joule heated horizontal platinum wire in water at low Rayleigh number

We report an experimental study of external natural convection from a Joule heated horizontal platinum wire immersed in water in a relative large cavity. Through that the platinum wire was used both as a heater and a thermistor for measuring a small temperature increment, our experiments were performed with much lower Rayleigh numbers in a range from 10−5 to 10−2 for water. Contrary to the general expectation, it is found that a local minimum Nusselt number happens at a critical Rayleigh number, which are about 5.0 × 10−4 and 1 × 10−4 for the platinum wires with diameters of 50.8 μm and 20.1 μm, respectively. The Nusselt number decreases rather than increases as usual with Rayleigh number before reaching the critical value, and the temperature of platinum wire fluctuates wildly over time at the critical condition. Corresponding experiments by using silicon oil instead of water, however, did not show such phenomena, which indicate that natural or free convection from a micro object may be fluid dependent.