Investigation of heat transfer around microwire in air environment using 3ω method

The 3ω principle is presented for the measurement of the heat transfer coefficient (h) based on natural convection model and heat conduction model. The 3ω technique is used at room temperature to measure h over the surfaces of microwires of the diameters 10–100 μm at horizontal and vertical orientations. The fitted results show that the heat loss from the microscale platinum wire to the air is dominated by heat conduction and the natural convection contribution is negligible. The comparison of the measured third harmonics for horizontal and vertical wires justifies that the orientation effect is negligible at microscales. The measured value of h is nearly two orders larger than that at macro scale and of the similar order to those from other literatures. Based on the 3ω principle, an explicit expression with a heat conduction shape factor is introduced and can predict the heat transfer coefficient reasonably in the validated range of frequency. Both the experimental results and the theoretical analysis conclude that the scale effect of heat transfer may be contributed to two factors: the effect of buoyancy, the driving force at microscales may be negligible; the heat loss is enhanced mainly by high ratio of surface to volume at microscales. It also shows the validation of the 3ω principle for thin wire if the heat loss to surrounding gas is dominated by heat conduction at microscale.

► The 3ω method is used to measure heat transfer coefficient around microwires.
► Heat loss from microwires to air is dominated by heat conduction.
► An expression proposed predicts heat transfer coefficients reasonably.