Characterization of fluid-thermal transport and boiling in micro/nano-engineered porous structures

Justin A Weibel, Purdue University

Abstract

The principal objective of the current work is to further understand the fundamental fluid and heat transport mechanisms in heat pipe wick materials and to thereby improve design, modeling, and performance capabilities. The investigations presented herein examine wicking properties and two-phase transport in capillary-fed porous structures. Determination of porous wick properties is critical for development of heat pipe and microscale transport models. Experimental facilities are developed to measure the permeability and the capillary pressure of sintered wick materials and it is shown that analytical expressions alone cannot be relied upon for prediction of these properties. Additionally, the thermal resistance across the evaporator section of heat pipes plays a dominant role in governing overall performance. To quantify this resistance for common sintered copper powder wick surfaces, a novel test facility is developed which feeds fluid to the wick by capillary action. Flow visualization of the wick surfaces during evaporation and boiling allows the thermal performance to be correlated with the observed regimes. A significant reduction in the evaporator thermal resistance is observed corresponding to a transition from evaporation to boiling. In order to extend heat pipe performance beyond state of the art, capillary-fed carbon nanotube (CNT) boiling surfaces are investigated. The wicking properties of CNT-based structures are not well understood; in order to be incorporated into heat pipe devices, their wetting properties, specifically with water, must be investigated. The effects of a novel copper functionalization technique on CNT wicking ability are studied and shown to decrease the wetting contact angle with water. Given these results, evaporators composed entirely of copper functionalized CNTs are evaluated. While these CNT evaporators are able to support high heat fluxes, the central portion of the CNT nanowick area may dry out due to the density of the array. A numerical model is developed and demonstrates that evaporator surfaces composed of nanostructured wicks fed by interspersed conventional wick materials can provide the required permeability for fluid flow while simultaneously decreasing the evaporator thermal resistance. Wicks of this type are fabricated and tested in the capillary-fed boiling facility to quantify the effects of patterning and CNT-coating of a sintered powder wick. The measured thermal performance enhancement is explained in terms of the observed vapor formation characteristics. Identification of operating regimes and critical transitions along the boiling curve provides the first step toward developing performance prediction tools. The current research addresses all the relevant regimes by exploring empirical prediction of the evaporation regime performance, incipience superheat, boiling performance, and dryout heat flux. A detailed investigation is performed to quantify and predict the incipience superheat in nanostructured and bare sintered powder wicks. Also, available predictive methods for the surface superheat under evaporation and capillary-fed boiling are reviewed and compared to the current results. Lastly, the dryout heat flux, and its dependence on the heat input area, is identified for thin sintered powder wicks. Plans for future work are outlined based on the current findings. The next required step is to develop generalized, analytical models that can be used to match the experimental predictions of performance and critical regime transitions in porous structures developed in this work and by others. The ultimate engineering objective is to develop a unified model for each of these regimes that is applicable over a wide range of operating parameters.

Degree

Ph.D.

Advisors

Garimella, Purdue University.

Subject Area

Mechanical engineering

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