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The heat transfer performance of an ionic jet impinging on the upper surface of a heated plate is investigated. Ions are injected by a point, set at high voltage. The working fluid is the viscous dielectric ester MIDEL 7131, a refrigerant for power transformers. Different tests are performed on the working fluid, varying the composition, the shape and the polarity of the point, the applied voltage, the point-to-plane distance, and the imposed heat flux. Heat transfer coefficients are augmented up to 310% with respect to natural convection, a higher enhancement than the one obtained with traditional transformer oil. The electrohydrodynamic flow can be generated without significant power input (<0.01 W in these experiments). Cooling of power transformers can be greatly enhanced by this technique. [ABSTRACT FROM AUTHOR]

The application of a strong electric field within a dielectric medium can produce a jet-like motion of ionized liquid from a high-voltage electrode to a grounded heated plate. Ionic jet impingement on the upper surface of a horizontal plate where boiling is occurring significantly modifies the boiling curve. The effect of ion injection on saturated boiling heat transfer, specifically on boiling inception, nucleate boiling, and CHF, in a small-size sample with a single point-electrode configuration, has been experimentally investigated; the working fluid is FC-72. Significant heat transfer enhancement has been observed for the entire nucleate boiling curve. Besides, CHF has been delayed to higher heat fluxes (up to a 40% increase) by means of the impinging ionic jet. This active technique requires a negligible power input (in the order of a few milliwatts) for the ionic current. A focus on the influence of the electrical parameters on the phenomenon has been performed and preliminary results are presented. Further tests and modeling effort are needed to optimize this very promising technique. [ABSTRACT FROM AUTHOR]

Laminar to weakly turbulent forced convection in a square duct heated from the bottom is strengthened by ion injection from an array of high-voltage points opposite the heated strip. Both positive and negative ion injection are activated within the working liquid HFE-7100 (C4F9OCH3), with transiting electrical currents on the order of 0.1 mA. Local temperatures on the heated wall are measured by liquid crystal thermography. The tests are conducted in a Reynolds number range from 510 to 12,100. In any case, heat transfer is dramatically augmented, almost independently from the flow rate. The pressure drop increase caused by the electrohydrodynamically induced flow is also measured. A profitable implementation of the technique in the design of heat sinks and heat exchangers is foreseen; possible benefits are pumping power reduction, size reduction, and heat exchange capability augmentation. [ABSTRACT FROM AUTHOR]

The heat transfer characteristics of an electrohydrodynamically induced submerged impinging liquid flow are studied in a point-plane configuration. The working fluid, HFE-7100, is a weakly polar dielectric. Thermofluid-dynamic conditions as close as possible to the microgravity ones are obtained by heating a confined, downward-facing plate. Heat transfer coefficients on the plate can be stably enhanced by over 10 times, with negligible additional work, by means of the ion injection technique. [ABSTRACT FROM AUTHOR]

With the present article, the authors summarize over 15 years of work dedicated to studying the effects of the electrical and the gravitational force fields on two-phase and single-phase thermofluid dynamics. Results obtained on several microgravity platforms are presented and the role played by the electric field in the different heat transfer processes is analyzed. In particular, the regimes of nucleate boiling and film boiling are treated. Also, the parameters controlling the liquid–vapor interface instability and the main consequences regarding the critical heat flux (CHF) are outlined. In the final section, the promising technique of ion injection for efficient heat transfer enhancement in single-phase liquids is described, together with the main results obtained under different flow regimes and geometries. The following dielectrics were compared: R113, Vertrel XF, and FC-72 for the pool boiling research; again FC-72 and HFE-7100 for the single-phase convection heat transfer experiments. [ABSTRACT FROM AUTHOR]

Abstract: The heat transfer characteristics of an electrohydrodynamically induced impinging liquid flow were studied in point-plane geometry. Points of four different metals were tested, varying the high voltage and the point-to-plane spacing. Heat transfer coefficients augmented more than 200%, with a negligible Joule dissipation. The Nusselt number on the stagnation point was successfully correlated to a dimensionless group, behaving similar to the Reynolds number in submerged jets. [Copyright &y& Elsevier]

The thermofluid-dynamic effects of ion injection from sharp metallic points added perpendicularly to the inner wire of a short horizontal annulus were experimentally investigated. A dielectric liquid (FC-72 by 3M) was weakly forced to flow in the duct, which was uniformly heated on the outer wall. A dc voltage as high as 22 kV was applied to the inner electrode, while the heated wall was grounded. Both the laminar and the turbulent mixed-convection regimes were obtained, varying the imposed flow rate. Once an electric field is applied, the flow is dramatically modified by the jets of charged particles, which transfer their momentum to the neutral adjacent ones. Different injection strengths were obtained on the emitters, because the shape of the point tips was not controlled at the microscale. Nusselt number distributions were obtained azimuthally and longitudinally, monitoring the wall temperatures. In all cases, heat transfer turned out greatly enhanced in the proximity of the emitters, without a significant increase in pressure drop through the test section and with a negligible Joule heating, making this technique very attractive for application in compact heat exchangers. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]