Xiang-Qi Wang

A review on nanofluids - part I: theoretical and numerical investigations

Research in convective heat transfer using suspensions of nanometer-sized solid particles in base liquids started only over the past decade. Recent investigations on nanofluids, as such suspensions are often called, indicate that the suspended nanoparticles markedly change the transport properties and heat transfer characteristics of the suspension. This first part of the review summarizes recent research on theoretical and numerical investigations of various thermal properties and applications of nanofluids.

A review on nanofluids - part II: experiments and applications

Research in convective heat transfer using suspensions of nanometer-sized solid particles in base liquids started only over the past decade. Recent investigations on nanofluids, as such suspensions are often called, indicate that the suspended nanoparticles markedly change the transport properties and heat transfer characteristics of the suspension. This second part of the review covers fluid flow and heat transfer characteristics of nanofluids in forced and free convection flows and potential applications of nanofluids. Opportunities for future research are identified as well. Keywords: Nanofluids; Nanoparticles; Heat transfer; Thermal conductivity.

Numerical Analysis of Blockage and Optimization of Heat Transfer Performance of Fractal-like Microchannel Nets

The conjugate fluid flow and heat transfer characteristics of fractal-like microchannel nets embedded in a disk-shape heat sink are investigated using a three-dimensional computational fluid dynamics (CFD) approach. A constant heat flux is applied to the top wall of the heat sink. The intrinsic advantages of fractal-like microchannel nets such as low flow resistance, temperature uniformity, and reduced danger of blockage compared with the traditional parallel channel nets are demonstrated. In addition, various optimized designs with parameters such as the number of branches, number of branching levels, and number of channels that reach the center of the disk are addressed in this context.

Effect of bifurcation angle in tree-shaped microchannel networks

Tree-shaped microchannel networks are being considered for thermal designs that require high heat transfer densities for the cooling of modern electronics. Here we investigate the effect of the bifurcation angles in the constructal nets on the fluid flow and heat transfer characteristics of such networks using a three-dimensional computational fluid dynamics approach. Results show that the bifurcation angle is an important factor which determines the performance of such cooling nets. Surface temperature distributions and pressure drop along the flow paths are analyzed and compared. For the same boundary conditions, a lower temperature and pressure variation is observed at lower bifurcation angles.

Laminar Heat Transfer in Constructal Microchannel Networks With Loops

Heat sinks with radial and constructal branching microchannel networks with loops are examined numerically. Radial and constructal networks are embedded in disk-shaped heat sinks. Constructal nets with loops are found to be more robust than the radial ones, when one or more channel segments are blocked. Since complex constructal networks would involve problems in manufacturing, constructal channel nets with loops may be a better choice in engineering applications. Networks with loops and without loops are compared. Results show that the constructal nets with loops provide a great advantage when the structure experiences accidental damage in one or more subchannel segments, since the loop assures the continuity of flow. In spite of blockage, the performance of the network has only a small drop considering the increased pressure drop.

Near infrared excited micro-Raman spectra of 4:1 methanol–ethanol mixture and ruby fluorescence at high pressure

Near infrared (NIR) lasers, as a new excitation source for Raman spectroscopy, has shown its unique advantages and is being increasingly used for some special samples, such as those emitting strong fluorescence in the visible region. This article focuses on some issues related to high-pressure micro-Raman spectroscopy using NIR excitation source. The Raman spectra of 4:1 methanol–ethanol mixture (4:1 M–E) show a linear variation in both Raman shifts and linewidths under pressure up to 18 GPa. This result is useful in distinguishing Raman scattering of samples from that of the alcohol mixture, an extensively used pressure-transmitting medium. The R1 fluorescence in the red region induced by two-photon absorption of the NIR laser is strong enough to be used as pressure scale. The frequency and line width of the R1 lines are very sensitive to pressure change and the glass transition of the pressure medium. Our results manifest that it is reliable and convenient to use NIR induced two-photon excited fluoresce...

High-pressure Raman study and pressure-induced phase transitions of sodium niobate NaNbO3

Raman spectroscopic studies have been carried out on antiferroelectric sodium niobate NaNbO3 under pressures up to 22 GPa. The Raman spectra show a rich variety of changes with pressure and evidence for several phase transitions were observed. A first-order phase transition was found at 7.0 GPa. A low wavenumber mode at 42 cm−1 appears at 6.0 GPa as a precursor, which softens at higher pressure. This band is obscured above 7.0 GPa after the first order phase transition. Another possible phase transition is proposed at 12 GPa, indicated by the sign change of the gradient of wavenumber-pressure plots for several modes and the disappearance of the Raman band for the strong ν1 mode. The spectral changes above 6.0 GPa closely resemble those of KNbO3, and the results are interpreted in comparison with LiNbO3 and KNbO3. The high-pressure phase after the first transition at 7.0 GPa may be ferroelectric and the phase above 12 GPa is probably the paraelectric phase. Copyright

Micro-Raman studies of substrate temperature effects on pulsed laser deposition fabricated YBa2Cu3O7-x epitaxial thin films

Micro-Raman spectroscopy has been used to analyse the substrate temperature effects on high temperature superconducting YBa2Cu3O7-x thin films grown by the in situ pulsed laser deposition method. In particular, the dependence of the epitaxy quality on substrate temperature Ts is studied. Our results reveal that films with the highest epitaxy degree are grown with substrate temperature Ts between 680 and 720 °C, in good agreement with previous superconducting transition temperature measurements and x-ray diffraction analyses. In addition, the formation and distribution of impurities produced during growth are identified by Raman spectra and Raman imaging, and they are found to be more pronounced at higher substrate temperatures. Finally, localized Raman analysis has been performed to investigate the orientation of the single crystal grains in films grown at substrate temperature Ts>760 °C. It is confirmed that, within the experimental error of 3°, the crystal a/b-axes of these single crystal grains have a strong tendency to orient themselves along the crystal axis of the substrate, independent of their size, shape and orientation of the domain boundaries.

Two-band model of spin-polarized tunneling incorporating discrete charging energy

Tunneling transport across a double-junction system, consisting of a small magnetic metallic island, coupled to ferromagnetic contacts by tunnel barriers, is studied by incorporating the effects of source–drain Va and gate Vg voltages, and the island charging energy into the model Hamiltonian. The transmission coefficients and current across the double barrier are evaluated using quantum mechanical transfer matrix method. The tunneling J–Va characteristic exhibits a staircase pattern, while the tunneling current oscillates with the gate voltage. The device also exhibits a bias-dependent tunneling magnetoresistance with a peak value exceeding 35%. We attribute these behaviors to the combined effect of spin-polarized tunneling and discrete charging of the island.

Raman study of laser irradiation effects on YBa2Cu3O7-x thin films

The effects of laser irradiation on high-temperature superconducting YBa2Cu3O7-x thin films have been investigated by micro-Raman spectroscopy. Laser heating affects YBa2Cu3O7-x thin films to varying degrees at different laser power levels. For a laser power density below 1 mW µm-2, laser irradiation causes only thermal expansion within the focused area. When the laser power density is increased to about 1.7 mW µm-2, oxygen out-diffusion occurs. The orthorhombic to tetragonal structural phase transition was observed at 3.1 mW µm-2. Above 4.3 mW µm-2, chemical decomposition is inferred from the appearance of Raman bands due to the impurity phase BaCuO2. Comparison between the laser-heated results and the furnace-heated results was made, and the role played by temperature gradient and stress in the laser-heated sample is discussed in detail.