Experiments have shown that electrohydrodynamic pumping can be utilized to increase the heat-transport capacity of a heat pipe. More specifically, electrohydrodynamic pumping can be used to increase the flow of liquid to the evaporator section of the heat pipe; the increased flow helps to prevent dryout of the evaporator during a startup transient or during operation at a high heat-transport rate. Electrohydrodynamic pumping can also accelerate recovery from dryout.
The electrohydrodynamic (EHD) phenomenon is a result of interactions between a dielectric fluid and an applied electric field. These interactions give rise to the following body forces on the fluid:
- The Coulomb force - the force attributable to the electric field acting on free charges in the fluid;
- The dielectrophoretic force, which is proportional to the square of the magnitude of the electric field and to the gradient of permittivity of the fluid (for example, the sharp gradient at a boundary between the liquid and vapor phases of the fluid); and
- The electrostrictive force, which is proportional to the gradient of the square of the magnitude of the electric field. The dielectrophoretic and electrostrictive forces are both denoted as polarization forces because they are attributable to the interaction between the electric field and polarization electric charges in the molecules of the fluid.
The particular EHD-pumping design chosen for the experiments utilizes the Coulomb force with no direct injection of electric charges. The EHD pump (see figure) was incorporated into the liquid channel of the adiabatic section (the section between the evaporator and condenser sections) of a monogroove heat pipe in which the working fluid was dichlorotrifluoroethane. In the experiments, the heat-transport capacity was increased by more than 200 percent when a potential of 20 kV was applied to the EHD-pump electrodes. The electric power consumed under this condition was only 0.08 W. In one experiment in which the heat pipe had undergone progressive evaporator dryout for 70 minutes at a heat-transport rate of 400 W, the application of voltage to the EHD pump resulted in essentially instantaneous recovery from dryout.
This work was done by J. Seyed-Yagoobi and J. E. Bryan of Texas A&M University for Johnson Space Center.