Modulated Enhancement of Heat EXchangers - meHEX
Abstract
A myriad of systems depend on compact heat exchangers to effectively promote heat transfer between different streams, while minimizing the impact of their integration. Conventionally, heat exchangers utilize either smooth walls or intentionally roughened walls, in which perturbators increase the wetted surface and turbulence, which leads to a penalty seen in increased pressure drop. The current aerial platforms carry hundreds of heat exchangers, significantly contributing tothe dry weight of the aircraft. This, in turn, limits range due to reduced fuel capacity of the aircraft at take-off conditions. The proposed work aims to augment the heat transfer in both smooth and roughened heat exchange passages without additional pressure drop, via acoustic modulation to promote enhanced convective heat transfer, resulting in compact heat exchangers with reduced size and weight. Nowadays, with evolutionary improvements to current engine technologies, allnations are converging towards similar performance propulsion units. Therefore, there is a critical need to create new heat exchange technologies that reduce the unit weight and size, without additional pressure penalty in the system.The efficiency of smooth wall heat exchangers is dictated by the behavioral characteristics of the boundary layer. For high-frequency oscillations, a relatively thin Stokes layer is developed, rendering the near-wall region independent from the outer steady mean flow. The comparison between time-averaged unexcited and excited flow velocities, reveals the contribution of the acoustic excitation to the time-averaged flow field, and hence to the convective heat transfer. The literature on forced convection related to acoustically enhanced heat transfer in attached smoothwall boundary layer is scarce. The majority of the investigations do not extend beyond purely aerodynamic case-studies analyzed at asymptotic conditions of extremely limited real life utility, preventing a fundamental understanding of the dominant physical mechanisms and their respective contribution to heat transfer enhancement. In roughened heat exchangers, as acoustic waves travel along the passages, a standing wave can be formed, and consequently the heat exchange passages act as resonators. By superimposing this standing wave on the separated and reattaching flow, significant heat transfer modulation can be achieved. On a fundamental level, prior investigations have demonstrated that periodic forcing ofseparated shear layers can be conducive towards alteration of the unsteady vortex dynamics. However, the underlying physical mechanisms are characterized from an aerodynamic perspective, only a small number of studies relate to the ensuing convective heat transfer ramifications. Moreover, the literature on separated flow control mostly pertains to local excitations via mechanical devices, which deliver a large concentrated and localized energy input. However, this is impractical in closely confined heat exchange surfaces. The proposed study will investigate the convective heat transfer ramifications of sound excitation on roughened flowtopologies. Approved for Public Release.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jun 13, 2019
- Source ID
- N000141912433
Entities
People
- Guillermo Paniagua
Organizations
- Office of Naval Research
- United States Navy
- University of Virginia