In the presence of a sharp HV electrode, free charge can build up in a single-phase liquid by ion injection at the metal/liquid interface. Electrophoretic forces acting on ions can generate strong convective motion, thus augmenting the heat transfer rate. In a vertical annular duct, uniformly heated on the outer wall, a dielectric liquid is weakly forced to flow upward. Sharp points are added perpendicularly to the inner cylinder and d.c. voltages as high as 22 kV are applied to it, while the outer wall is grounded. Prior to the application of the electric field, a regime of turbulent aided mixed convection is obtained. Being in a region of thermally developing flow, laminarization effects are observed, with local heat transfer coefficients depending non-trivially on longitudinal position, heat flux, and flow rate. With the electric field on, heat transfer turns out to be only weakly influenced by the heat flux and the flow rate. The heat exchange reveals to be highly enhanced by this technique and can be modulated by varying the applied voltage. In all cases, the heat transfer improvement is accompanied by a slight increase in pressure drop through the test section and a negligible Joule heating.
Heat Transfer Enhancement in a Vertical Annulus by Electrophoretic Forces Acting on a Dielectric Liquid
TESTI, DANIELE;
2005-01-01
Abstract
In the presence of a sharp HV electrode, free charge can build up in a single-phase liquid by ion injection at the metal/liquid interface. Electrophoretic forces acting on ions can generate strong convective motion, thus augmenting the heat transfer rate. In a vertical annular duct, uniformly heated on the outer wall, a dielectric liquid is weakly forced to flow upward. Sharp points are added perpendicularly to the inner cylinder and d.c. voltages as high as 22 kV are applied to it, while the outer wall is grounded. Prior to the application of the electric field, a regime of turbulent aided mixed convection is obtained. Being in a region of thermally developing flow, laminarization effects are observed, with local heat transfer coefficients depending non-trivially on longitudinal position, heat flux, and flow rate. With the electric field on, heat transfer turns out to be only weakly influenced by the heat flux and the flow rate. The heat exchange reveals to be highly enhanced by this technique and can be modulated by varying the applied voltage. In all cases, the heat transfer improvement is accompanied by a slight increase in pressure drop through the test section and a negligible Joule heating.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
