  Context: A radial temperature gradient together with an inhomogeneous radial
electric field gradient is applied to a dielectric fluid confined in a vertical
cylindrical annulus inducing thermal electro-hydrodynamic convection.
  Aims: Identification of the stability of the flow and hence of the line of
marginal stability separating stable laminar free (natural) convection from
thermal electro-hydrodynamic convection, its flow structures, pattern formation
and critical parameters.
  Methods: Combination of different measurement techniques, namely the
shadowgraph method and particle image velocimetry, as well as numerical
simulation are used to qualify/quantify the flow.
  Results: We identify the transition from stable laminar free convection to
thermal electro-hydrodynamic convective flow in a wide range of Rayleigh number
and electric potential. The line of marginal stability found confirms results
from linear stability analysis. The flow after first transition forms a
structure of vertically aligned stationary columnar modes. We experimentally
confirm critical parameters resulting from linear stability analysis and we
show numerically an enhancement of heat transfer.
