Slip-Flow and Heat Transfer of a Non-Newtonian Nanofluid in a Microtube

The slip-flow and heat transfer of a non-Newtonian nanofluid in a microtube is theoretically studied. The power-law rheology is adopted to describe the non-Newtonian characteristics of the flow, in which the fluid consistency coefficient and the flow behavior index depend on the nanoparticle volume fraction. The velocity profile, volumetric flow rate and local Nusselt number are calculated for different values of nanoparticle volume fraction and slip length. The results show that the influence of nanoparticle volume fraction on the flow of the nanofluid depends on the pressure gradient, which is quite different from that of the Newtonian nanofluid. Increase of the nanoparticle volume fraction has the effect to impede the flow at a small pressure gradient, but it changes to facilitate the flow when the pressure gradient is large enough. This remarkable phenomenon is observed when the tube radius shrinks to micrometer scale. On the other hand, we find that increase of the slip length always results in larger flow rate of the nanofluid. Furthermore, the heat transfer rate of the nanofluid in the microtube can be enhanced due to the non-Newtonian rheology and slip boundary effects. The thermally fully developed heat transfer rate under constant wall temperature and constant heat flux boundary conditions is also compared

Radiative Properties of Emerging Materials and Radiation Heat Transfer at the Nanoscale

A negative index material (NIM), which possesses simultaneously negative permittivity and permeability, is an emerging material that has caught many researchers attention after it was first demonstrated in 2001. It has been shown that electromagnetic waves propagating in NIMs have some remarkable properties such as negative phase velocities and negative refraction and hold enormous promise for applications in imaging and optical communications. This dissertation is centered on investigating the unique aspects of the radiative properties of NIMs. Photon tunneling, which relies on evanescent waves to transfer radiative energy, has important applications in thin-film structures, microscale thermophotovoltaic devices, and scanning thermal microscopes. With multilayer thin-film structures, photon tunneling is shown to be greatly enhanced using NIM layers. The enhancement is attributed to the excitation of surface or bulk polaritons, and depends on the thicknesses of the NIM layers according to the phase matching condition. A new coherent thermal emission source is proposed by pairing a negative permittivity (but positive permeability) layer with a negative permeability (but positive permittivity) layer. The merits of such a coherent thermal emission source are that coherent thermal emission occurs for both s- and p-polarizations, without use of grating structures. Zero power reflectance from an NIM for both polarizations indicates the existence of the Brewster angles for both polarizations under certain conditions. The criteria for the Brewster angle are determined analytically and presented in a regime map. The findings on the unique radiative properties of NIMs may help develop advanced energy conversion devices. Motivated by the recent advancement in scanning probe microscopy, the last part of this dissertation focuses on prediction of the radiation heat transfer between two closely spaced semi-infinite media. The objective is to investigate the dopant concentration of silicon on the near-field radiation heat transfer. It is found that the radiative energy flux can be significantly augmented by using heavily doped silicon for the two media separated at nanometric distances. Large enhancement of radiation heat transfer at the nanoscale may have an impact on the development of near-field thermal probing and nanomanufacturing techniques.Ph.D.Committee Chair: Zhang, Zhuomi

A full-range analytical solution of the critical velocity for smoke control in tunnel fires

The critical velocity plays a significant role in the design of the ventilation system in tunnels, from which the ventilation velocity of fresh air for preventing the smoke back-layering in tunnel fires can be determined. In this work, a full-range analytical solution to the equation for the critical velocity was derived. Specifically, we first proved that the unconstrained equation for the critical velocity has and only has one positive real solution, and then we proved that the positive real solution has a unified expression for different cases of tunnel fires. Finally, the analytical solution that satisfies the constraints was obtained, which was also compared with the iterative solution. The result shows that the analytical solution can avoid the negative and complex solutions automatically, which is more stable than the iterative solution. Furthermore, the analytical solution was compared with the simulation and experimental results, which were found to be in excellent agreements with each other. For tilted tunnels, the influence of the Froude number on the analytical solution of the critical velocity was also investigated by comparing the critical velocities obtained based on two different correlations of the Froude number

Semiconductor Thin Films Combined With Metallic Grating for Selective Improvement of Thermal Radiative Absorption/Emission

We propose in this work a structure of semiconductor thin films combined with a one-dimensional metallic grating which allows for selective improvement of thermal radiative absorptivity (also emissivity) of the structure. We numerically demonstrate with a 2-D rigorous coupled-wave analysis (RCWA) algorithm that the proposed structure exhibits enhanced spectral absorptivity (for p-polarization) for photon energy slightly above the gap energy of the semiconductor (silicon in this work). The enhanced absorptivity is explained as due to excitations of surface polaritons (SPs) in the grating region, along with interactions of multiple-order diffracted waves in the semiconductor layer. Furthermore, the enhanced absorptivity of the structure can be achieved for a wide range of incidence angles so that it may have potential applications in energy conversion purposes.Engineering, MechanicalNanoscience & NanotechnologyPhysics, AppliedCPCI-S(ISTP)

Modeling calcium wave based on anomalous subdiffusion of calcium sparks in cardiac myocytes.

Ca(2+) sparks and Ca(2+) waves play important roles in calcium release and calcium propagation during the excitation-contraction (EC) coupling process in cardiac myocytes. Although the classical Fick's law is widely used to model Ca(2+) sparks and Ca(2+) waves in cardiac myocytes, it fails to reasonably explain the full-width at half maximum(FWHM) paradox. However, the anomalous subdiffusion model successfully reproduces Ca(2+) sparks of experimental results. In this paper, in the light of anomalous subdiffusion of Ca(2+) sparks, we develop a mathematical model of calcium wave in cardiac myocytes by using stochastic Ca(2+) release of Ca(2+) release units (CRUs). Our model successfully reproduces calcium waves with physiological parameters. The results reveal how Ca(2+) concentration waves propagate from an initial firing of one CRU at a corner or in the middle of considered region, answer how large in magnitude of an anomalous Ca(2+) spark can induce a Ca(2+) wave. With physiological Ca(2+) currents (2pA) through CRUs, it is shown that an initial firing of four adjacent CRUs can form a Ca(2+) wave. Furthermore, the phenomenon of calcium waves collision is also investigated

Validation of our algorithm.

<p>We compare our local Nusselt number of a non-slip Newtonian fluid in a tube with that in the previous work for testing the algorithm.</p

The heat transfer rates corresponding to different nanoparticle volume fractions.

<p>The heat transfer curves are calculated for 0, 3% and 5% with a fixed dimensionless slip length 0.01 and pressure gradient Pa/m, showing the effect of nanoparticle volume fraction on the heat transfer rate; the non-slip curve for 0 is also plotted for revealing the slip effect.</p