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A series of experiments were conducted with PIV (Particle Image Velocimetry) in order to acquire the full view of primary and secondary flow fields, and investigate the interaction between them in the entry section of a rotating smooth channel with square cross section. An FPGA (Field-Programmable Gate Array) based trigger signal generator was developed to minimize the measurement errors caused by the relative movements between the rotating channel and the stationary PIV system. The measurements were conducted at three streamwise planes (X = 445 mm, 525 mm, 605 mm) and eight spanwise planes (Z = 40–75 mm) with the Reynolds number ( Re = U ¯ D / ν ) equaling to 4750 and the rotation number ( Ro = Ω D / U ¯ ) ranging from 0.25 to 0.75. Based on the experimental data, three interesting problems were discussed. Firstly, the secondary flow near the leading edge surprisingly points to the leading edge, this is attributed to the normal pressure gradient with the help of momentum integral equations. Secondly, the variations of the worst primary velocity deficit are analyzed by a method based on circulation. The results show that the trailing side secondary flow vortices can influence the variations of the worst primary velocity deficit near the leading wall. Thirdly, the critical rotation number phenomenon is analyzed. According to the force and velocity analyses, the critical rotation number phenomenon is attributed to the shift of balance between the normal pressure gradient term and Coriolis term. [ABSTRACT FROM AUTHOR]

A combined experimental and numerical study was conducted to better understand the 3-dimensional flow development in 120° curved rectangular ducts with continuously varying curvature ( Cr ) that are commonly used for blade profile design of solid–liquid two-phase centrifugal pumps. Three different types of curvature (double circular line, spiral line and involute line) and three different aspect ratios ( Ar = 0.4, 1 and 2.3) were studied to analyze the development of axial velocity in the horizontal mid-plane ( z = 0) and secondary flow patterns at various cross-sections along the ducts. The inlet flow satisfied fully developed turbulent condition, with Reynolds number ranged from Re b = 2.4 × 10 4 to 1.4 × 10 5 . The flow fields were measured using Particle Image Velocimetry (PIV) and numerical simulations were performed by solving the Reynolds Averaged Navier–Stokes (RANS) equations. The results reveal complex changes in the flow pattern with respect to both the flow and geometric parameters ( Re , Cr and Ar ) in terms of onset, development and disappearance of different types of Dean vortex. Comparison with the literature confirms that the flow development in these ducts with continuously varying curvature is quite different from that in conventional curved ducts with constant curvature. [ABSTRACT FROM AUTHOR]

This paper presents experimental and three-dimensional numerical study of early onset of separation with rarefied gas flow through a tube with single sharp 90° bend. Experiments are conducted for nitrogen gas flowing at low pressures in three conventional size tubes. The flow is dynamically similar to gas flow in a microchannel as the Knudsen number range (0.0003 < Kn < 0.0385) covers part of the continuum and the slip flow regime while maintaining the Reynolds number between 0.27 and 418.5. The static pressures along the inner, outer and top walls are measured for different mass flow rates and analyzed to understand the flow behavior. The static pressure measurement indicates adverse pressure gradient near the bend along the inner and outer walls of the tube at much lower value of Reynolds number as compared to conventional flow. The numerical solution of the Navier–Stokes equations with the Maxwell’s slip boundary condition shows good agreement with experimental data and helps bring out the complex flow behavior near the bend. The adverse pressure gradient, velocity profile, flow streamlines and velocity vectors in the bend plane clearly indicates secondary flows near the bend at as low a Reynolds number as unity. The flow acceleration and the presence of secondary flows near the bend causes a larger pressure drop as compared with a straight tube. Empirical correlations for Poiseuille number and additional pressure drop coefficient are proposed as part of this work. It is noted that limited experimental data exists in the literature for such flows; these results should therefore help enhance the fundamental understanding of gas flow in microchannels with bend. [ABSTRACT FROM AUTHOR]

To study the time-mean and temporal characteristics of secondary flow within a linear GE-E 3 high pressure turbine cascade, a planar Time-Resolved Particle Image Velocimetry (TR-PIV) system is used. In the double-passage cascade, a row of six converging slot-holes is placed upstream of center blade to generate film cooling effect, and different turbulence grids are replaced to create various free-stream turbulence ( Tu in ) levels. In this experiment, the time-mean characteristics of secondary flow, the fast switch process of unsteady leading edge horseshoe vortex (LEHV), and the temporal characteristics of corner vortices (CVs) are completely exhibited by the TR-PIV technique. The influences of the upstream coolant injection and Tu in level on the flow characteristics of LEHV and passage vortex (PV) are discussed. The discussion reveals that: (1) in the case of no coolant injection, a high Tu in level slightly moves the LEHV toward the blade, changes the shape of the PV, increases the fluctuations of the LEHV and PV, and reduces the frequency of the LEHV switch process; (2) at various Tu in levels, the coolant injections suppress the formation of the LEHV, and the LEHV disappears at a high coolant-to-mainstream blowing ratio (BR) of 1.5; (3) a high BR of 1.5 can greatly weaken the PV at various Tu in levels, and relative to the case of low Tu in , the high Tu in level induces a larger reduction; (4) for a low BR, at various Tu in levels, a slight change in the LEHV results in a distinct difference of the PV characteristics; (5) for a high BR, since the LEHV disappears, the Tu in effect on the secondary flow characteristics is slight. [ABSTRACT FROM AUTHOR]

• We experimentally study the axial heat-transfer in Taylor-Couette flow. • The axial heat-transfer and viscous-heating are decoupled using thermal waves. • The effects of boundary heat-loss on measured Nusselt number are modeled. • The measured Nusselt number is insensitive to oscillating period of thermal waves. • The correlations between axial Nusselt number and Taylor number, radius ratio are established. This study experimentally investigates the axial heat transfer of the secondary flow in the cylindrical annular gap around a flywheel within a canned reactor coolant pump, belonging to the Taylor-Couette (TC) flow. A dynamic method based on the transient temperature responses of the TC flow to the input thermal waves, where the axial heat transfer coefficient is determined by the amplitude decay and phase lag of axially propagating temperature waves, is proposed. The influences of viscous heating are distinguished using the mean temperature distribution, allowing for the performance of decoupling tests of axial heat transfer and viscous heating in TC flow. The modification of the boundary heat loss on the axial propagation of temperature waves in a rotor-fluid-stator TC system was modeled. The measured axial heat-transfer coefficient was insensitive to the oscillating periods of the input thermal waves within the maximum permitted period. The axial heat transfer coefficients with different Taylor numbers and radius ratios were measured based on the proposed method. The correlations between the axial heat-transfer coefficient and Taylor number satisfied the scaling law with an effective exponent of 0.5, but they change non-monotonically with respect to the radius ratio in highly turbulent TC flow. This study provides insight into the axial heat transfer characteristics of TC flow. [ABSTRACT FROM AUTHOR]