Compact Two-Phase Heat Transfer Surfaces

The Naval Nuclear Laboratory (NNL) is interested in innovative water-to-water latent heat transfer surfaces and flow paths for boiling and condensing that maximize heat transfer while minimizing pressure drop, volume, and weight. Latent heat transfer is one of the most effective ways to transfer heat. The heat transfer coefficients (HTC) for boiling and condensing are orders of magnitude higher than the HTC found for sensible heat transfer. Packaging this capability on a small footprint is proven difficult due to the large velocity gradients found in the two-phase side of the heat exchanger. These large velocity gradients also add additional pressure drop, which can affect the local HTC. Another challenging characteristic is channel size. During channel size selection for boiling surfaces, the designer must keep in mind the average bubble detachment radius to determine the average bubble size, also determine that coalescence of such bubbles will not blanket the surface and prevent heat transfer. For the purpose of this request for proposal, a compact heat exchanger is defined as having a beta value of at least 1000 m2/m3. Compact two-phase heat exchanger design is a challenging process due to the importance of pressure drop in the system. Apart from the pressure drop affecting the steam outlet pressure, boiling regimes and velocities must be managed during phase change to have an effective design. Optimizing the pressure drop, flow regimes, and boiling regimes to make the latent heat transfer as efficient as possible is the biggest challenge presented during the design. Optimizing other heat transfer bottlenecks, such as but not limited to single-phase side HTC coefficients is also of interest. New heat transfer surfaces and flow paths to address the challenges described above are requested for submission. These submittals should also take into consideration hermeticity, manufacturability, fouling, fatigue limits, repairability, in-service inspectability, performance of the surface at different orientations, performance at a design pressure of 2400 psig, and a design temperature of 600 °F. For boiling, NNL is interested in an outlet condition of low steam quality to superheat, but there is a preference for high steam quality. These considerations also apply for condensing surfaces and flow paths. The condenser pressure should be in between 1.5 to 7.0 in Hg absolute, with seawater as the heat sink. All proposals should specifically identify the technological gap and address how the work proposed may lead to the implementation or development of an improved latent heat transfer surface and/or flow path. Any ideas on how to make the entire heat transfer process more power dense without adversely impacting the flow will be considered.