Improved evaporators for loop heat pipes have been developed by incorporating bidisperse structures (in place of older monodisperse structures) into evaporator wicks. As explained in more detail below, the bidisperse structures feature two distinct pore sizes (see Figure 1), which helps to prevent vapor blanketing that can limit heat-flux capacities to unacceptably low values.

Loop heat pipes are important parts of systems for cooling electronic components that dissipate heat at flux densities up to 100 W/cm2. Loop-heat-pipe evaporators of older design do not work at heat-flux densities in excess of 12 W/cm2 because vapor blanketing of the wicks in those evaporators blocks the flow of heat-transfer liquids into the evaporators. These wicks have monodisperse micron-size features.

Figure 1. Bidisperse Wicks exhibit two distinct pore sizes.

The present improved evaporators are designed (see Figure 2) to prevent vapor blanketing of the wicks. The wicks in these evaporators include bidisperse structures at the interfaces between the heated evaporator walls and core wicks. The bidisperse structures contain both micron-size pores for the liquid supply and larger pores for venting of vapor. The bidisperse structures are in contact with the core wicks, which contain monodisperse micron-size pores. In a given evaporator, the bidisperse structures can be sintered in circumferential grooves in the evaporator wall and/or sintered in circumferential grooves on the outer surface of the core wick.

Figure 2. In the New Design, the circumferential grooves of the evaporator body are filled with sintered bidisperse wick. Effectively, this creates an efficient extended surface evaporator because bidisperse wick and the triangular groove lands both function as fins.

Because vapor can leave the wicks through the larger pores, vapor blanketing of the wicks does not occur, even at evaporator-wall heat-flux densities greater than 12 W/cm2. Tests of loop heat pipes equipped with the bidisperse wick structures demonstrated good performance at evaporator-wall heat-flux densities up to 100 W/cm2.

This work was done by John H. Rosenfeld, David B. Sarraf, Dmitry K. Khrustalev, Peter J. Wellen, and Mark T. North of Thermacore, Inc., for Goddard Space Flight Center. GSC-14225

NASA Tech Briefs Magazine

This article first appeared in the November, 1999 issue of NASA Tech Briefs Magazine.

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