Fusion Science and Technology / Volume 61 / Number 1T / January 2012 / Pages 322-328
Modeling and Simulations / Proceedings of the Fifteenth International Conference on Emerging Nuclear Energy Systems
Refractory metallic foams can increase heat transfer efficiency in gas-to-gas and liquid metal-to-gas heat exchangers by providing an extended surface area for better convection, i.e. conduction into the foam ligaments providing a “fin-effect,” and by disruption of the thermal boundary layer near the hot wall and ligaments by turbulence promotion.
We present the relative contributions of the heat transfer mechanisms stated above, and show how the design of a gas regenerator or liquid metal-to-gas heat exchanger can be optimized for use in high-temperature Brayton cycle applications for nuclear power generation or hydrogen production. Our results include temperature and thermal stress distributions for several densities of Nb1Zr, Mo and W foams compared to Cu. For instance, the simulations reveal that unconnected W foam can increase the convective heat transfer coefficient by almost a factor of two compared to an open rectangular channel and a factor of three if the foam ligaments are thermally connected to the sidewalls under the same flow conditions.
The effect of ligament thermal conductivity is also highlighted by comparing the performance of W foams to identical Cu foams and the use of SiC foams in thermal barrier applications. The studies indicate that thermal stresses increase with foam density, but are not clearly correlated with pore cell size.
For thermal management applications, the presence of the connected foam minimizes the thermal stresses in the wall, by concentrating them in the ligaments where the temperature gradients are higher. In addition, the large number of small connected ligaments provides a modest degree of compliance for thermal expansion of the hotter walls in relation to the colder portions of the heat exchanger. These CFD studies have led to design strategies for creating compact, high-temperature, high-pressure heat exchangers that are easily fabricated and perform better than plate-type heat exchangers.