Transient natural convection and conjugate transients around a line heat source

Transient natural convection in liquid nitrogen around a heated wire is studied experimentally and numerically. A thin bronze wire of 40 (mu)m in diameter and 5.1 cm in length heated by Joule effect is used in experiments and time evolutions of the wire temperature are measured through its electrical resistance. Numerical simulations of such flows are performed by using velocity–pressure formulation, spectral methods and domain decomposition technique. Experimental data and numerical results are compared and show that although the wire is extremely thin its thermal inertia plays an important role during early transients. Scaling analyses performed for the convection set-up and transients yield t~q^-1/2 and (delta)T~q^3/4, which are confirmed by numerical simulations. For wires heated by Joule effect hybrid thermal conditions––combined Dirichlet and Neumann conditions are proposed on the wire surface and validated by numerical simulations. On the other hand flow conditions to be imposed on the outer artificial boundary are not well known and the answer to the question remains still open: two types of flow conditions are tested and yield different velocity fields.