Transient Effectiveness Methods for the Dynamic Characterization of Heat Exchangers

This chapter introduces transient effectiveness methods for dynamic characterization of heat exchangers. The chapter provides a detailed description and review of the transient effectiveness methodology. In this chapter, all the transient effectiveness–related knowledge/works are summarized. The goal of this chapter is to provide a thorough understanding of the transient effectiveness for the reader and to provide guidance for utilizing this methodology in related heat exchanger transient characterization studies. Basically, there are three important applications for transient effectiveness methodology: (1) characterization of heat exchanger dynamic behaviors; (2) characterization of the transient response of closed-coupled cooling/heating systems with multiple heat exchanger units; and (3) development of compact transient heat exchanger models. This innovative modeling method can be used to assist in the development of physics-based predictive, capabilities, performance metrics, and design guidelines, which are important for the design and operation of highly reliable and energy efficient mechanical systems using heat exchangers

The Effect of Polydispersivity on the Thermal Conductivity of Particulate Thermal Interface Materials

A critical need in developing thermal interface materials (TIMs) is an understanding of the effect of particle/matrix conductivities, volume loading of the particles, the size distribution, and the random arrangement of the particles in the matrix on the homogenized thermal conductivity. Commonly, TIM systems contain random spatial distributions of particles of a polydisperse (usually bimodal) nature. A detailed analysis of the microstructural characteristics that influence the effective thermal conductivity of TIMs is the goal of this paper. Random microstructural arrangements consisting of lognormal size-distributions of alumina particles in silicone matrix were generated using a drop-fall-shake algorithm. The generated microstructures were statistically characterized using the matrix-exclusion probability function. The filler particle volume loading was varied over a range of 40-55 %. For a given filler volume loading, the effect of polydispersivity in the microstructures was captured by varying the standard deviation(s) of the filler particle size distribution function. For each particle arrangement, the effective thermal conductivity of the microstructures was evaluated through numerical simulations using a network model previously developed by the authors. Counter to expectation, increased polydispersivity was observed to increase the effective conductivity up to a volume loading of 50%. However, at a volume loading of 55%, beyond a limiting standard deviation of 0.9, the effective thermal conductivity decreased with increased standard deviation suggesting that the observed effects are a trade-off between resistance to transport through the particles versus transport through the inter-particle matrix gap in a percolation chain

BioMed2008-38096 APPLICATION OF SPLIT FLOW DESIGN TECHNIQUE TO SIMPLE MICROCHANNEL GEOMETRIES FOR ENHANCED MIXING

ABSTRACT The ability to control mixing of reagents in MEMS systems is crucial for many biological and chemical analysis applications. However mixing in these microfluidic devices is a challenge because the flows are laminar corresponding to very low Reynolds number. In this paper mixing of such reagents in simple microchannel geometries is investigated computationally. A novel concept of "split flow design" is applied to these simple microchannel configurations. Significant improvement in mixing is seen by employing the split flow design technique

Chemical vapor-deposited carbon nanofibers on carbon fabric for supercapacitor electrode applications

Entangled carbon nanofibers (CNFs) were synthesized on a flexible carbon fabric (CF) via water-assisted chemical vapor deposition at 800A degrees C at atmospheric pressure utilizing iron (Fe) nanoparticles as catalysts, ethylene (C2H4) as the precursor gas, and argon (Ar) and hydrogen (H-2) as the carrier gases. Scanning electron microscopy, transmission electron microscopy, and electron dispersive spectroscopy were employed to characterize the morphology and structure of the CNFs. It has been found that the catalyst (Fe) thickness affected the morphology of the CNFs on the CF, resulting in different capacitive behaviors of the CNF/CF electrodes. Two different Fe thicknesses (5 and 10 nm) were studied. The capacitance behaviors of the CNF/CF electrodes were evaluated by cyclic voltammetry measurements. The highest specific capacitance, approximately 140 F g(-1), has been obtained in the electrode grown with the 5-nm thickness of Fe. Samples with both Fe thicknesses showed good cycling performance over 2,000 cycles