Magneto-fluid-mechanic heat transfer from hot film probes

Abstract

This paper presents results of an experimental investigation of magneto-fluid-mechanic heat transfer from quartz-coated hot film probes (0.015 cm and 0.005 cm dia) traversed vertically through a tank filled with mercury and aligned axially with a horizontal magnetic field. Findings exhibit a reduction in the probe's heat transfer due to the magnetic field in the Reynolds number range from 0 to 130 and Hartmann number range from 0 to 4.68 for magnetic interaction parameter values of order one. Four regions of heat transfer are identified, each associated with a specific flow configuration about the probe, and are described as follows: 1. (1) In the free convection region the magnetic field suppresses the free convective heat transfer, eventually limiting the heat transfer to thermal conduction. Heat transfer in this region is examined through an approximate solution of the momentum and energy equations. 2. (2) For very low Reynolds numbers of about three there is an increase in the heat transfer from the free convection state. The Reynolds number at which this initial region of forced convection becomes important is predicted by a theoretical criterion which compares free and forced convection velocities. This increase becomes detectable at a lower Reynolds number in the presence of a magnetic field. 3. (3) Beginning at a Reynolds number of about five, the Nusselt number can be expressed in a power law relation with the Reynolds number where the power is independent of magnetic field strength. Also a theoretical correlation is presented which determines the Reynolds numbers for various Hartmann numbers at which this stationary Föppl vortex pair initiates. The magnetic field inhibits the formation of this pattern. 4. (4) The generation of von Karman vortex shedding behind the probe at a Reynolds number of about 34 yields a slight increase in the power law of the heat transfer relation which is again independent of magnetic field strength. Reynolds numbers for various Hartmann numbers at which shedding begins are represented mathematically. The magnetic field delays the onset of the vortex street and suppresses its size.