Date of Award

5-2010

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Beasley, Donald

Committee Member

Miller , Richard

Committee Member

Ochterbeck , Jay

Committee Member

Ma , Lin

Abstract

This thesis involves the modeling of cryogenic fluids through a porous metal foam counter flow heat exchanger. Although much research has been completed involving light weight high porosity metal foams in recent years, research performed on the subject of mid range porosities in metal foams and their use in heat exchangers has been relatively untouched. Even less literature is available pertaining to the effects of cryogenic fluids in porous metal foams.
Much about heat exchanger performance and pressure drop depends on the structure and relative density of the metal foams. This has been elaborated in several sources, but only for foams with high porosities (φ≥85%). Literature available on porous media with moderate porosities (45%<φ<85%) has mostly been performed on packed beds of granular spheres. The internal geometry of a porous medium composed of packed spheres is different than porous foams with similar porosity ranges. Consequently available moderate porosity information is inaccurate when attributed to metal foams, due to their complex cellular geometry.
Further review of literature showed a discrepancy in numerical models used to determine pressure drop and heat transfer of metal foams inside heat exchangers. Pressure drop through foams depends on foam properties such as permeability and inertial resistance. The latter is complicated to predict, which affects model accuracy, particularly in non-linear laminar flow ranges. Heat transfer may be defined using the local thermal equilibrium model, which is most commonly used in commercial analysis software. Alternatively, the local thermal non-equilibrium model improves upon accuracy, but has closure issues.
A micro-scale porous metal foam heat exchanger was developed and initial testing for cryogenic applications was completed at Kennedy Space Center. Data from this experiment was provided and an analytical model was created to characterize it. Two and three-dimensional models were created in FLUENT 6.3 based on helium gas and two-phase liquid/gaseous nitrogen used during testing. This model utilized the local thermal equilibrium model for heat transfer and attempted to correlate moderate porosity metal foams using experimental data. Results showed that models produced in FLUENT corresponded reasonable well to experimental data when fluid velocities were low, between 0 and 0.5 m/s. When the velocities increased to 5.0 m/s, the models became less accurate and showed a greater pressure drop in the flow than was recorded during the experiments. Further work is needed to characterize the porous metal foams at these conditions.

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