Wicking Structures

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Aravamudhan, Rahman, and Bhansali. [70] developed a micro direct ethanol fuel cell with silicon diffusion layers. Each silicon substrate had a number of straight micropores or holes that were formed using microelec-tromechanical system (MEMS) fabrication techniques. The pores acted both as microcapillaries/wicking structures and as built-in fuel reservoirs. The capillary action of the microperforations pumps the fuel toward the reaction sites located at the CL. Again, the size and pattern of these perforations could be modified depending on the desired properties or parameters. Lee and Chuang [71] also used a silicon substrate and machined microperforations and microchannels on it in order to use it as the cathode diffusion layer and FF channel plate in a micro-PEMFC. 

Several wick structures are in common use. First is a fine-pore (0.14—0.25 mm (100-60 mesh) wire spacing) woven screen which is rolled into an annular structure consisting of one or more wraps inserted into the heat pipe bore. The mesh wick is a satisfactory compromise, in many cases, between cost and performance. Where high heat transfer in a given diameter is of paramount importance, a fine-pore screen is placed over longitudinal slots in the vessel wall. Such a composite structure provides low viscous drag for liquid flow in the channels and a small pore size in the screen for maximum pumping pressure. 

A heat pipe is a simple device with no moving parts that can transfer large quantities of heat over fairly large distances essentially at a constant temperature without requiring any power input. A heat pipe is basically a sealed slender tube containing a wick structure lined on the inner surface and a small amount of fluid such as water at the saturated stale, as sliown in Fig. 10-36. It is composed of three sections the evaporator section at one end, where heat is absorbed and ilie fluid is vaporized a condenser section at the other end, where the vapor is condensed and heat is rejected and the adiabatic section in between, where the vapor and the liquid phases of the fluid flow in opposite directions through the core and the wick. 

Theoretical simulation of mHPs with different wick structures (sintered powder, mesh structure, wire bundle) is an efficient tool to perform the comparisons of mHP efficiency. Experimental verification of mHP parameters proves validity of the simulation software. 

Kim S.J., Seo J.K., Do K.H., (2003), Analytical and experimental investigation on the operational characteristics and the thermal optimization ob a miniature heat pipe with a grooved wick structure, International Journal of heat and Mass Transfer, Vol.46, 2051 - 2063... 

Technology of the reverse meniscus consists in imposition of micro-porous layer to the PIN-structure that provides high intensity of heat transfer. Using of bidisperse wick structure is also instrumental in intensification of heat transfer. 

Values for the effective capillary radius rc are given in Table 12.1 for some of the more common wicking structures [7]. In the case of other geometries, the effective capillary radius can be found theoretically using the methods proposed by Chi [9] or experimentally using the methods described by Ferrell and Alleavitch [10], Freggens [11], or Tien [12]. In addition, limited information on the transient behavior of capillary structures is also available [13]. 

TABLE 12.1 Effective Capillary Radius for Several Wick Structures [7]... 

TABLE 12.3 Effective Thermal Conductivity for Liquid-Saturated Wick Structures [7,9]... 

Composite wicking structures accomplish the same type of effect in that the capillary pumping and axial fluid transport are handled independently. In addition to fulfilling this dual purpose, several wick structures physically separate the liquid and vapor flow. This results from an attempt to eliminate the viscous shear force that occurs during countercurrent liquid-vapor flow. 

Liquid-flow-modulated heat pipes have two separate wicking structures, one to transport liquid from the evaporator to the condenser and one that serves as a liquid trap. As the temperature gradient is reversed, the liquid moves into the trap and starves the evaporator of fluid. In addition to these liquid-vapor control schemes, the quantity and direction of heat transfer can also be controlled through internal or external pumps, or through actual physical contact with the heat sink. 

G. P. Peterson and X. F. Peng, Experimental Investigation of Capillary Induced Rewetting for a Flat Porous Wicking Structure, ASME J. Energy Resources Technology (115/1) 62-70,1993. 

J. K. Ferrell and J. Alleavitch, Vaporization Heat Transfer in Capillary Wick Structures, preprint no. 6, ASME-AIChE Heat Transfer Conf., Minneapolis, MN, 1969. 

Although this work only evaluated the performance of the anode electrode, a complete fuel cell is envisioned that would be cofired into the LTCC, consisting of the required cavities for fuel storage, channels, and wick structures for fuel deliver, a porous Ag structure for the electrodes, separated by a cavity with nanoporous surfaces that will allow the PEM organics to be added after firing and polymerized in situ. This work only considers the development of the porous cermet films for the Ag-based anode structure. 

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