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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]

Highlights: [•] Experimental investigation of pressure drop and drag reduction in helical coils by injection microbubbles. [•] The drag reduction ratio decreases with increasing Reynolds number in the constant void fraction. [•] The effect of drag reduction on lower Reynolds number is greater than higher Reynolds number. [•] The maximum ratio of drag reduction was 25% occurred in the void fraction of 0.09 and Reynolds of 11,000. [•] The drag reduction ratio decreases with an increase in the curvature ratio of helical coil. [Copyright &y& Elsevier]

Abstract: To improve understanding of the heat transfer enhancement for shell side of a double-pipe heat exchanger with helical fins and pin fins, the three-dimensional velocity components for the shell side with and without pin fins were measured experimentally by using laser Doppler anemometer (LDA) under cylindrical coordinate system and the fluid flow characteristics, i.e., the distributions of the axial velocity, secondary flow, stream function as well as vorticity were analyzed by employing orthogonal helical coordinate system. The results showed that, for the shell side only with helical fins at large pitch, there was a pair of vortex near the upper and lower edge of the rectangular cross section; the weakest secondary flow occurred at the center. By pin fins being installed, the three-dimensional velocity components in the helical channel were strongly changed. The extra two vortices emerged at the center of the cross sections. The fluctuation of the velocity at the center of the cross sections before and behind a pin was increased sharply by the pin, especially for the situation behind a pin. [Copyright &y& Elsevier]

Abstract: This investigation presents detailed experimental measurements of an active tip-clearance control method based on tip injection in a high-turning axial turbine cascade. Besides that, numerical investigations are also conducted to study phenomena which are not easily measured in the experiments. It aims to study the influence of tip injection on tip clearance flow, with emphasis on the effects of injection locations. Detailed flow field measurements were made downstream of the cascade using a three-hole probe. Static pressure distributions were also measured on the blade surface at 50% and 97.5% span, respectively. The results suggest that tip injection can weaken tip clearance flow, reducing the tip clearance mass flow and its associated losses. Meanwhile, the heat transfer condition on the blade tip surface can be also improved significantly. It also can be found that injection chordwise location plays an important role in the redistribution of secondary flow within the cascade passage. When the same number of injection holes and injection mass flow are applied, holes located in the aft part of blade can perform much better than that in the front part. [Copyright &y& Elsevier]

Abstract: The temperature and velocity field in a horizontal convergent–divergent rectangular channel heated from below is studied experimentally for a Reynolds range 8–120, Grashof numbers from 0.44×105 to 2.56×105 and Richardson numbers from 3 to 4000, using water as working fluid. The duct aspect ratio (width/height) varies from 1 at its inlet to 2.28 at the stenosis neck, and both the upper and bottom walls are tilted with an angle of ±15.7° with respect to the horizontal. The temperature of the bottom wall is kept constant above that of the issuing fluid. The temperature field is recorded by liquid crystals in the vertical mid plane whereas the velocity field is measured in this plane as well as in four cross sections of the divergent passage by planar PIV, revealing the characteristics of the secondary velocity field. For all the examined cases the flow in the convergent passage is free of thermal plumes, and the thermal boundary layer is thin. In contrast, the divergent passage is characterized by a thermal plume which is shifted upstream with increasing Gr or reducing Re. Both transversal and longitudinal rolls emerge in this diffuser the strength of which depend on Re and Gr influencing the streamwise distribution of Nusselt which for low Re presents a minimum. [Copyright &y& Elsevier]

• 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]

The flow characteristics in rectangular ducts with aspect ratio of 1:8 were investigated experimentally using particle image velocimetry to reveal the function of the inclined ribs as turbulators or generators of favorable flows. Circular ribs were attached on the wide walls and the rib height-to-hydraulic diameter ratio was kept as 0.06 with an attack angle of 60°. The obtained results showed strong effects of the inclined ribs on the flow behavior by the development of secondary flows. The magnitude of the streamwise velocity component was detected to have spanwise variation. The flow characteristics were correlated with the previous corresponding heat transfer results obtained by the same authors. [Copyright &y& Elsevier]

Three-dimensional laser Doppler anemometry measurements are performed on developed laminar flow in three helical pipes. The experimental observations are compared to results of numerical calculations employing the fully elliptic numerical method. Good agreement is found between measured data and numerical results. The three helical pipes, with curvature ratios of 0.0734 and 0.1374 and non-dimensional pitches of 0.0793 and 0.193, are adopted to study the effects of curvature and pitch on laminar flow in the experimental approach. The range of Reynolds numbers is 500–2000 to ensure laminar flow in the entire helical pipe. Both the profile shapes of the normal components of the secondary flow and those of the axial flow along the same centerline present not only similar patterns but also similar change when pitch, curvature ratio, and Reynolds number vary. The results demonstrate comprehensive relationships between the axial flow and the secondary flow. [Copyright &y& Elsevier]

Turbulent viscous fluid flow is always accompanied by a secondary flow if the flow is not exactly two-dimensional or exactly axisymmetric. The classic example is duct flow, where counter-rotating pairs of vortices appear in each corner, distorting the isoaxialvelocity contours. The present investigation of the turbulent boundary-layer development along a streamwise external corner (chine) also shows distortion of the isoaxialvelocity contours which predicts the presence of a pair of counter-rotating vortices. These vortices or secondary flow are induced by inequality of Reynolds stresses around the corner. The direct effect of this corner flow is to increase the drag force. A series of experimental investigations at the University of Melbourne provided, in qualitative terms, the general behaviour expected from this class of flow as revealed by Elder [J. Fluid Mech. 9 (1960) 133] and Xu and Pollard [Phys. Fluids 13 (2001) 3321]. This flow is also expected to exhibit symmetry about the corner bisector. Our measurements indicate quasi-symmetry about the bisector all along the corner. [Copyright &y& Elsevier]