John H. Rosenfeld

Porous media heat exchangers for cooling of high-power optical components

Technologies based on porous media can be used in several classes of heat exchangers that can be used to meet the cooling needs of high heat load optical components as well as other high heat flux applications. These include mechanically pumped single-phase and two-phase porous media heat exchangers, as well as capillary pumped (heat pipe) two-phase designs. A brief overview of each of these classes of heat exchangers is given, and several applications representative of the state of the art are described. Various specific technologies are discussed that have demonstrated the capability to dissipate heat fluxes greater than 1000 W/cm 2 over areas of interest in optics applications. Finally, an assessment is given of the capabilities of each approach to meet the needs of specific applications.

High heat flux loop heat pipes

Loop Heat Pipes (LHPs) can transport very large thermal power loads, over long distances, through flexible, small diameter tubes and against high gravitational heads. While recent LHPs have transported as much as 1500 W, the peak heat flux through a LHP’s evaporator has been limited to about 0.07 MW/m2. This limitation is due to the arrangement of vapor passages next to the heat load which is one of the conditions necessary to ensure self priming of the device. This paper describes work aimed at raising this limit by threefold to tenfold. Two approaches were pursued. One optimized the vapor passage geometry for the high heat flux conditions. The geometry improved the heat flow into the wick and working fluid. This approach also employed a finer pored wick to support higher vapor flow losses. The second approach used a bidisperse wick material within the circumferential vapor passages. The bidisperse material increased the thermal conductivity and the evaporative surface area in the region of highest heat ...

An Overview of Long Duration Sodium Heat Pipe Tests

High temperature heat pipes are being evaluated for use in energy conversion applications such as fuel cells, gas turbine re‐combustors, and Stirling cycle heat sources; with the resurgence of space nuclear power, additional applications include reactor heat removal elements and radiator elements. Long operating life and reliable performance are critical requirements for these applications. Accordingly long‐term materials compatibility is being evaluated through the use of high temperature life test heat pipes. Thermacore, Inc. has carried out several sodium heat pipe life tests to establish long term operating reliability. Four sodium heat pipes have recently demonstrated favorable materials compatibility and heat transport characteristics at high operating temperatures in air over long time periods. A 316L stainless steel heat pipe with a sintered porous nickel wick structure and an integral brazed cartridge heater has successfully operated at 650C to 700C for over 115,000 hours without signs of failure...

Test results from a helium gas-cooled porous metal heat exchanger

A helium-cooled porous metal heat exchanger was built and tested, which successfully absorbed heat fluxes exceeding all previously tested gas-cooled designs. Helium-cooled plasma-facing components are being evaluated for fusion applications. Helium is a favorable coolant for fusion devices because it is not a plasma contaminant, it is not easily activated, and it is easily removed from the device in the event of a leak. The main drawback of gas coolants is their relatively poor thermal transport properties. This limitation can be removed through use of a highly efficient heat exchanger design. A low flow resistance porous metal heat exchanger design was developed, based on the requirements of the Faraday shield for the International Thermonuclear Experimental Reactor device. High heat flux tests were conducted on two representative test articles at the Plasma Materials Test Facility at Sandia National Laboratories. Absorbed heat fluxes as high as 40 MW/m2 were successfully removed during these tests without failure of the devices. Commercial applications for electronics cooling and other high heat flux applications are being identified.

Test results from a pumped single-phase porous metal heat exchanger

A pumped single-phase porous metal cooled microwave cavity design is being evaluated for use in a high-power gyrotron. A small-scale porous metal cooled test article was designed, built, and tested on a Phase I SBIR program. The program was funded by the United States Department of Energy. A copper/water porous metal heat exchanger test article was fabricated and was subsequently tested at absorbed heat fluxes up to 7.4 +/- 0.3 kW/cm2 before failure occurred. Multiple tests were successfully completed at heat fluxes of 4.0 to over 6.0 W/cm2 with no signs of failure. The test article design, coolant parameters, test methodology, and test results are presented. The results of this work show the potential of porous metal cooling to solve a number of high heat flux cooling problems; several such applications are described.

Evaluation of porous media heat exchangers for fusion applications

Several types of porous media heat exchangers are being evaluated for use in fusion applications. Broadly, these devices can be classified as capillary-pumped (heat pipes) or mechanically-pumped heat exchangers. Monel/water thermosyphon heat pipes with a porous metal wick are being evaluated for use in Faraday shields. A subscale prototype has been fabricated, and initial tests at Oak Ridge National Laboratory have shown favorable results. Alkali metal heat pipes have demonstrated absorbed heat flux capability of over 1000 MW/m{sup 2}. An advanced gyrotron microwave cavity is being developed that uses water cooling in a mechanically-pumped copper porous metal heat exchanger. Tests on a prototype demonstrated absorbed heat flux capability in excess of 100 MW/m{sup 2}. Porous metal heat exchangers with helium, water, or liquid metal coolants are being evaluated for plasma-facing component cooling. Tests on a helium/copper porous metal heat exchanger demonstrated absorbed heat flux capability in excess of 15 MW/m{sup 2}. Applications, conceptual designs, fabricated hardware, and test results are summarized. 22 refs., 5 figs., 2 tabs.

Metal/ceramic composite heat pipes for a low‐mass, intrinsically‐hard 875 K radiator

Thermacore, Inc. of Lancaster, Pennsylvania has recently completed Phase I of a development program to investigate the use of layered metal/ceramic composites in the design of low‐mass hardened radiators for space heat rejection systems. This effort evaluated the use of layered composites as a material to form thin‐walled, vacuum leaktight heat pipes. The heat pipes would be incorporated into a large heat pipe radiator for waste heat rejection from a space nuclear power source. This approach forms an attractive alternative to carbon/carbon, or silicon‐carbide fiber reinforced metal heat pipes by offering a combination of low mass and improved fabricability. Thermacore and United Technologies Research Center have jointly developed an approach for fabrication of layered composite thin‐walled heat pipes for use in hardened space radiators. Potassium heat pipes with wall thicknesses as low a 0.3 mm have been built and tested. Wall thicknesses as low as 0.13 mm are believed to be achievable with this approach.

Innovative technologies for Faraday shield cooling

Alternative advanced technologies are being evaluated for use in cooling the Faraday shields used for protection of ion cyclotron range of frequencies (ICR) antennae in Tokamaks. Two approaches currently under evaluation include heat pipe cooling and gas cooling. Both of these technologies offer attractive alternatives to water-cooled designs primarily because they remove the time-consuming cleanup required in the event of tube weld failures and because they increase the operating temperature range for the shield. A Monel/water heat pipe cooled Faraday shield has been successfully demonstrated. Heat pipe cooling offers the advantage of reducing the amount of water discharged into the Tokamak in the event of a tube weld failure. The device was recently tested on an antenna at Oak Ridge National Laboratory. The heat pipe design uses inclined water heat pipes with warm water condensers located outside of the plasma chamber. This approach can passively remove absorbed heat fluxes in excess of 200 W/cm/sup 2/. Helium-cooled Faraday shields are also being evaluated. This approach offers the advantage of no liquid discharge into the Tokamak in the event of a tube failure. Innovative internal cooling structures based on porous metal cooling are being used to develop a helium-cooled Faraday shield structure. This approach can dissipate the high heat fluxes typical of Faraday shield applications while minimizing the required helium blower power. Preliminary analysis shows that nominal helium flow and pressure drop can sufficiently cool a Faraday shield in typical applications. Plans are in progress to fabricate and test prototype hardware based on this approach.

Sulfur heat pipes for 600 K space heat rejection systems

A preliminary investigation was performed to study the use of sulfur heat pipes in a lightweight space radiator for waste heat rejection at 600 K. Several space power concepts have a need for heat rejection at 600 K. Heat pipes have been shown in previous studies to be useful in reducing the mass of radiators; however, few high‐performance, lightweight working fluids are available near 600 K. Sulfur has not been previously suggested as a heat pipe working fluid for this application because of its high liquid viscosity, which reduces heat pipe transport capability. However, it has been shown that the addition of several weight percent iodine to the sulfur has the effect of reducing dynamic viscosity by three to four orders of magnitude near 600 K. The addition of iodine significantly reduces the liquid pressure drop flow resistance in sulfur heat pipes. This appears to make sulfur heat pipes a viable approach for 600 K space heat rejection systems. Preliminary design calculations were performed to determin...

Advances in High Temperature Titanium‐Water Heat Pipe Technology

A development program has been completed to design, assemble, and test high‐performance titanium/water heat pipes for spacecraft heat rejection applications. Three variations of wick designs were designed, assembled, tested, and delivered to NASA GRC. The heat pipe length was 115.5 cm, the outer diameter was 1.27 cm, and the charged devices weighed 285 g. All wick designs were based on porous‐walled axially grooved titanium wick structures. All three variations exhibited high heat transport capability at adverse tilt conditions. Transport capability of the devices at 225 °C ranged between 800 W and 1000 W against gravity. The results represent a significant advance in the technology capability for passive heat rejection in this temperature range.