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Heat transfer enhancements

What is the EHD in heat transfer? by Gianfranco
Posted: 2 Feb 02

Introduction to Electro Hydro Dynamic(EHD)in Heat Transfer

EHD is a technique of augmentig of surface convective heat transfer coefficients (1), (2).  For instance, experimental results have shown an increasing > 1000% in heat transfer coefficients for the refrigerant R-134a.

The EHD technique works by applying a high-voltage electrostatic potential field across a heat transfer fluid, such as a refrigerant or refrigerant mixture. The cause of phenomenon is due to the Coulomb's force exerted by an electric field on free charge into a Liquid Dielectric (LD) causes motion of that liquids (3).  The applied electric field serves to destabilize the thermal boundary layer, increasing boiling or condensation of the fluid near the heat transfer surface, and producing better mixing of the bulk fluid flow.

Although the EHD technique can be applied to both single phase and phase change heat transfer, it is more effective when applied to phase change processes (boiling and condensation).

A common method for using the EHD effect is to suspend a charged electrode (for example, a straight wire running parallel to the tube) in the fluid medium and to electrically ground the heat transfer surface. The reverse case (charging the surface and grounding the wire) is also possible. The applied electric field can be either direct or alternating. The field polarity in most cases has little effect on the enhancement mechanism, particularly for the phase change processes.

For shell-and-tube heat exchangers, the tube and shell sides can be simultaneously energized by placing electrodes in both the tube and shell sides.
For plate heat exchangers, there is no need for an external wire electrode, as the plates themselves can serve as the electrodes by charging one plate and grounding the other.

The improvements in heat transfer are dramatic, especially at lower refrigerant qualities (more liquid and less vapor). This may allow manufacturers to produce highly compact heat exchangers with less complicated surfaces without sacrificing heat transfer efficiency.

It is suggested that this phenomenon be used to electronically control the capacity of a heat exchanger by raising the applied voltage when additional heat transfer is needed.  This can lead to improved efficiencies by using smaller capacity equipment, without EHD most of the time, and then utilizing the EHD effect during peak loads.

Another potential application is to use continuously EHD enhancement with smaller, less costly heat exchangers. The EHD effect would offset the loss of heat transfer capacity experienced from the smaller heat exchanger.
Similarly, EHD could be used in place of, or in conjunction with, enhanced surface heat exchangers.

From a safety and cost point of views, to use EHD seems to have some troubles because of an electrical voltage needs to be added to the heat transfer device (it depens on the application, anywhere from a few volts to thousands of volts are used). However, because the heat transfer fluids are typically dielectric (low electrically conductive) materials, very little current is generated, despite the high voltage. This low current helps keep the power (voltage times current) and the associated energy penalty small. The electronics needed also represent an increased material cost.

More significantly, applying the electronics would require a more complicated manufacturing process. This aspect may be the biggest current obstacle to employment of EHD technology to air-conditioning and refrigeration equipment.

Another important question is the long-term effects of the EHD equipment on the system. For instance, will the application of large voltages cause the refrigerants and lubricants to break down? Will the electronics used accelerate chemical reactions of the refrigerant and lubricant? Furthermore, the long-term reliability of the EHD equipment needs to be assessed. Whatever electronics are used will need to last for the life of the air-conditioning system, or will need to be easily repaired by a service technician.

New design of a geometrically integrated EHD heat transfer surface-electrode offers greater prespective for existing heat exchangers. Typical applications of the new design include both single-phase and phase-change internal/external heat transfer processes, external/internal boiling or condensation, and convective flow in compact channel geometries such as those used in automotive application.
Patents coming from Univ. of Mariland (see below) has now under technology development into industrial field: see for example http://www.atec-ahx.com/mission.htm with the picture of an EHD-Enhanced Oil Cooler.

Research efforts are in progress in U.S. (1)(2) and Japan.
A study by Toshiba on an EHD condenser involved thorough testing of the unit after one thousand hours of continuous operation. It is reported that this research showed that the EHD did not leave any side effects on the refrigerant or heat exchanger components. However, to achieve this, careful design of the electrode was essential.
The U.S. Air Force has also initiated a project to study the reliability and long term performance of EHD heat transfer for use in aircraft environmental control systems.

EHD may become viable for use in mass-produced air-conditioning and refrigeration equipment. But whether or not it is employed will depend on manufacturers' analyses of its benefits and costs as compared to other means of increasing efficiencies.

(1) http://www.wam.umd.edu/~ohadi/
(2) http://www.ari.org/rt/tu/1996/9601b.html
(3) http://www.eng.uts.edu.au/~jafarm/ehd/EHD.htm

Patents
--------------
Multi-Step Helical Electrode for Electronic Control and Enhancement of Condensation Heat Transfer
M.M. Ohadi, S.V. Dessiatoun, and K.H. Cheung,
US Patent Pending, filed Sep 1997.

EHD-Enhanced Gas-to-Gas Recuperator
M.M. Ohadi
US Patent Pending, filed Jun 1995.

Electronically-Controlled Heat Transfer Surface for Enhanced Boiling Heat Transfer
M.M. Ohadi, S.V. Dessiatoun
US Patent Pending, filed Jun 1995.

-------------------------
Gianfranco Padovan
lopad@tin.it
info@sistemazienda.it

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