Effect Of Entrance Geometry Research Articles

Optimization of entrance geometry and analysis of fluid distribution in manifold for high-power proton exchange membrane fuel cell stacks

Uniformity of fluid distribution in the manifold is extremely essential to enhance the output performance and prolong the lifetime of high power proton exchange membrane (PEM) fuel cell stack. The entrance effects are usually ignored in the existing studies focusing on the fluid distribution at stack level, which cannot thoroughly guide the high-power fuel cell stack development. In this study, the effects of entrance geometry on the fluid distribution in manifold for a high-power fuel cell stack are investigated using computational fluid dynamics (CFD) method. Optimizations of the intermediate zone configuration in upper endplate and inlet tube diameter are conducted under different current densities. Results show that the fluid distribution in manifold is strongly influenced by entrance geometry which determines the generation of vortexes. The mass flow rates in unit cells near the entrance of the stack with diffuser-type intermediate zone are enhanced compared to the non-diffuser-type intermediate zone. The coefficient of variation (CV) of mass flow rate decreases dramatically as the ratio of inlet tube to manifold hydraulic diameter (RITMHD) increases and then raises. The experimental and simulation results are useful in guiding the design of high-power PEM fuel cell stacks to seek higher power density.

Flow fluctuations in wormlike micelle fluids.

We investigate the unstable flow of wormlike micelle solutions in pressure driven capillary flow, with a focus on the effect of entrance geometry on the fluid fluctuations. The flow is measured at different points in the capillary using particle image velocimetry while simultaneously measuring the pressure drop across the entire capillary. The fluctuations are characterized by rapid flow rate jumps that correspond with a decrease in the pressure drop followed by a longer recovery period. Velocimetry measurements in the entrance region show a transition to unstable flow above a critical flow rate, where large flow circulations are observed in the tapered geometry and localized jets are observed in an abrupt contraction. The transition to this unstable flow is shown to occur at a similar dimensionless extension rate normalized by the micelle relaxation time. A rapid breakdown in micelle alignment is observed in polarized light microscopy at the onset of the flow rate jump, indicating the importance of rapid micelle structural changes on the fluctuations. We characterize the system by analyzing the power spectral densities and develop a dynamical systems model to describe the relationship between pressure and flow rate. These developments provide understanding to control flow fluctuations and motivation for more detailed study of the coupling of fluid microstructure transitions and flow fluctuations.

Numerical and experimental study of gas flows in 2D and 3D microchannels

In the experiments conducted at Purdue, the air flow in rectangular cross-section microchannels was investigated using pressure sensitive paint. The high resolution pressure measurements were obtained for inlet-to-outlet pressure ratios from 1.76 to 20 with the outlet Knudsen numbers in the range from 0.003 to 0.4 based on the hydraulic diameter of 151.7 µm and the length-to-height ratio of about 50. In the slip flow regime, the air flow was simulated by the 2D and 3D Navier–Stokes equations with no-slip and slip boundary conditions. For various pressure ratios, the entrance flow development, compressibility and rarefaction effects were observed in both experiments and numerical simulations. It was found that the accurate modeling of gas flows in finite-length channels requires the inlet and outlet reservoirs to be included in computations. Effects of entrance geometry on the friction factor were studied for 3D cases. In both experiments and numerical modeling, significant pressure drop was found starting at the inlet chamber. The numerical modeling also predicted an apparent temperature drop at the channel exit.

Influence of Entrance Geometry on Heat Transfer in Rotating Rectangular Cooling Channels (AR=4:1) With Angled Ribs

The effect of entrance geometry on the heat transfer in rotating, narrow rectangular cooling channels is investigated in this study. Both smooth channels and channels with angled ribs are considered with three different entrance conditions: fully developed, sudden contraction, and partial sudden contraction. The rectangular channel has as aspect ratio of 4:1, and it is oriented at 135° with respect to the plane of rotation. In the test section with angled ribs, the ribs are angled at 45° to the mainstream flow. The rib height-to-hydraulic diameter ratio e/Dh is 0.078, and the rib pitch-to-height ratio P/e is 10. The range of flow parameters includes Reynolds number (Re=5000–40,000), rotation number (Ro=0.0–0.302), and inlet coolant-to-wall density ratio (Δρ/ρ=0.12). The heat transfer at the entrance of the heated portion of the smooth channel is significantly enhanced with the sudden contraction and partial sudden contraction entrances. In the smooth rotating channels, the effect of the entrance geometry is also present; however, as the rotation number increases, the effect of the entrance geometry decreases. It was also found in this study that the sudden and partial sudden contraction entrances provide higher heat transfer enhancement than the fully developed entrance through the first three to four hydraulic diameters of the channels with angled ribs. Again, the effect of the entrance geometry is greater in the stationary channels with angled ribs than the rotating channels with ribs. In both stationary and rotating channels, the influence of the entrance geometry on the heat transfer is more apparent in the smooth channels than in the ribbed channels.

Rheological behavior and fiber orientation in slit flow of fiber reinforced thermoplastics

AbstractThe flow behavior and fiber orientation in slit flow of a short fiber reinforced thermoplastic composite melt are investigated. A slit die with adjustable gap and interchangeable entrance geometries was designed and built. The slit die is fed by a single screw extruder. The bulk viscosity is calculated from the axial pressure profiles measured using three flush mounted pressure transducers. The effect of entrance geometry and gap dimensions on the fiber orientation and bulk flow behavior is specifically considered. A skin‐core composite fiber orientation is observed in the thickness direction. Fibers are oriented in the flow direction and parallel to the walls in the skin region irrespective of the entrance geometry. Different fiber orientation distributions in the core region can be realized by using different entrance geometries. However, the changes in the core fiber orientation are not fully reflected by the measured viscosities, due to highly oriented skin layer. Exit pressures obtained by extrapolation of linear pressure profiles are found to be all positive, but dependent on the die geometry and entrance conditions, even for the unfilled melts.

Effect of entrance geometry on the capillary flow of cellulose acetate–acetone solution

AbstractCapillary flow data were obtained for a 27.5% solution of cellulose acetate in acetone. The solution temperature was 50°C, and the range of apparent shear rates investigated was 1.7 × 105 to 1.7 × 106 sec−1. Capillaries having tapered entrance angles of 37.88° to 120.63° were used. A power‐law model was adequate to describe the shear stress at the wall (τw) and the corrected shear rate \documentclass{article}\pagestyle{empty}\begin{document}$(\dot \gamma )$\end{document} relationship. Entrance angle affected the entrance pressure drop corrected for kinetic energy, (ΔP0,c); ΔP0,c increased as the angle widened. Treating the entrance flow as an elongational flow situation facilitated superposition of the Delta;P0,c data on a single curve. Estimated elongational viscosities decreased with increasing applied stress.