No abstract

Mechanisms behind the pressure distribution and skin friction within a laminar separation bubble (LSB) are investigated by large-eddy simulations around a 5% thickness blunt flat plate at the chord length based Reynolds number 5.0 × 103, 6.1 × 103, 1.1 × 104, and 2.0 × 104. The characteristics inside the LSB change with the Reynolds number; a steady laminar separation bubble (LSB_S) at the Reynolds number 5.0 × 103 and 6.1 × 103, and a steady-fluctuating laminar separation bubble (LSB_SF) at the Reynolds number 1.1 × 104, and 2.0 × 104. Different characteristics of pressure and skin friction distributions are observed by increasing the Reynolds number, such that a gradual monotonous pressure recovery in the LSB_S and a plateau pressure distribution followed by a rapid pressure recovery region in the LSB_SF. The reasons behind the different characteristics of pressure distributions at different Reynolds numbers are discussed by deriving the Reynolds averaged pressure gradient equation. It is confirmed that the viscous stress distributions near the surface play an important role in determining the formation of different pressure distributions. Depending on the Reynolds numbers, the viscous stress distributions near the surface are affected by the development of a separated laminar shear layer or the Reynolds shear stress. In addition, we show that the same analyses can be applied to the flows around a NACA0012 airfoil.

We present a new concept of a structured surface for enhanced boiling heat transfer that is capable of self-adapting to the local thermal conditions. An array of freestanding nanoscale bimorphs, a structure that consists of two adjoining materials with a large thermal expansion mismatch, is able to deform under local temperature change. Such a surface gradually deforms as the nucleate boiling progresses due to the increase in the wall superheat. The deformation caused by the heated surface is shown to be favorable for boiling heat transfer, leading to about 10% of increase in the critical heat flux compared to a regular nanowire surface. A recently developed theoretical model that accounts for the critical instability wavelength of the vapor film and the capillary wicking force successfully describes the critical heat flux enhancement for the nanobimorph surface with a good quantitative agreement.

We report combined electrochemical double-layer capacitance (EDLC) and pseudocapacitance in reduced graphene oxide (rGO) thick film like paper due to annealing temperature variations. The influence of annealing temperature (from room temperature (RT) to 1000 C) on the structural, morphological, electrical, and electrochemical properties of rGO paper was evaluated. Upon increasing the annealing temperature, shifting of the dominant (002) X-ray diffraction (XRD) peak to a higher degree, volume expansion, and red-shifting of the G band in Raman spectra were observed. High-resolution transmission electron microscopy (HRTEM) images showed a reduction in the interlayer distance in rGO sheets from 0.369 to 0.349 nm as the annealing temperature increased from RT to 1000 C; these results were congruent with the XRD results. According to X-ray photoelectron spectroscopy (XPS), the presence of hydroxyl, carboxyl, and other oxygen-containing groups decreased in samples annealed at higher temperatures. The attached functional groups, the electrical conductivity, and the supercapacitance of rGO papers were found to be mutually interrelated and could be tuned by varying the annealing temperature. The rGO paper annealed at 200 C in a 1 M H 2 SO 4 electrolyte at a scan rate of 50 mV s À1 exhibited a maximum specific capacitance of 198 F g À1 .

No abstract