Net Flow Reynolds Number Research Articles

Influence of imposed oscillatory frequency on mass transfer enhancement of grooved channels for pulsatile flow

The present experimental study describes mass transfer enhancement in grooved channels with different cavity lengths for pulsatile flow. Overall and local mass transfer rates were measured by the electrochemical method with a high Schmidt number and also the vortical motion within the groove was visualized by the electrolytic precipitation method. We especially focused on the influence of oscillation frequency on mass transport enhancement. Transport enhancement by means of fluid oscillation is found to be higher in laminar flow than in turbulent flow. There is a noticeable enhancement at intermediate Strouhal numbers, depending on the cavity length and the net flow Reynolds number. It is revealed that the mechanism for a peak transport enhancement factor against Strouhal number is not explained by the hydrodynamic resonance proposed by Patera and Mikic, under a certain condition.

The Effects of Oscillatory Flow and Bulk Flow Components on Residence Time Distribution in Baffled Tube Reactors

A characteristic of oscillatory flow mixing in a baffled tube is that the residence time distribution performance can be affected independently of net flow conditions. The effects of both the oscillatory velocity and the throughput velocity on the residence time distribution performance has been investigated in a 24 mm diameter, 2.8 m long, baffled tube oscillatory flow reactor. The experiments were performed by applying the tanks-in-series model to ’perfect pulse’ tracer experiments over a wide range of oscillatory conditions and flow rates. An optimum set of oscillatory and net flow conditions was found which resulted in near plug flow behaviour, usually 50 tanks-in-series ( N ) or greater. For a throughput velocity, N was found to be sensitive to both the amplitude and frequency of oscillation, and this dependence could be expressed by means of the oscillatory flow Reynolds number ( Re 0 ), which combines both parameters. A unique value for Re 0 for each value of the net flow Reynolds number ( Re n ) gave the closest approach to a plug flow RTD. To relate the oscillatory and net flows, a dimensionless velocity ratio, ψ, was used, defined as Re o / Re n . In order to relate the RTD performance for different flow rates, the tanks-in-series model was non-dimensionalized by the use of a stage-wise efficiency term,η, defined as the ratio of N to the theoretical number of stages. Over the range of oscillatory and net flow conditions studied, it was found that the range 2 ≤ψ ≤ 4 corresponded to the optimum RTD conditions, where efficiencies close to 1 were achieved. It was concluded that these dimensionless parameters were sufficient to select, a priori, the oscillatory parameters necessary to obtain the optimum RTD in an oscillatory flow reactor based on a desired throughput specification.

Numerical Study of Pulsatile Flow through a Wavy Channel. Vortex Dynamics and Fluid Mixing.

Numerical analysis is performed to study the dynamical behavior of vortices generated in a sinusoidal wavy channel for pulsatile flow. This system is employed in order to enhance heat and mass transfer under laminar flow conditions. A pair of counter-rotating vortices periodically moves between the upper and lower walls with one wavelength. The vortex strength has a maximum value at a certain Womersley number, which increases with the net flow Reynolds number. A particle advection procedure also indicates that the vortex dynamics lead to efficient fluid mixing with increasing Womersley number.

Heat transfer and associated energy dissipation for oscillatory flow in baffled tubes

We report experimental data on the heat transfer performance of a periodically baffled tube subject to both steady (net) flow and oscillatory flow. The data show that, in particular, at a low net flow Reynolds number, significant heat transfer enhancement can be achieved with the superposition of fluid oscillations. A general correlation is derived for the measured Nusselt number as a function of both net flow and oscillatory Reynolds number. Dynamic pressure drop data for oscillatory flow are also reported, and estimates of energy efficiency for obtaining heat transfer enhancement made from these measurements are compared with smooth wall turbulent flow equations. For large amplitudes of oscillation (equivalent to half the tube diameter) the overall power dissipation follows the quasi-steady theory. At smaller amplitudes of oscillation the power dissipation was larger than predicted by the quasi-steady theory, indicating an increased eddy interaction.

Residence time distribution measurements in a pulsed baffled tube bundle

AbstractExperimental flow pattern and associated residence time distribution measurements are reported for a tube bundle where periodic baffles and fluid oscillation may be present. When there is no fluid oscillation, high Reynolds number flow conditions are required to give sharp residence time distributions. When baffles are present, fluid oscillation can give sharp residence time distributions for modest low net flow Reynolds numbers. These observations extend our previous results and also show the viability of the system, for example in use as a reactor or heat exchanger where a multi‐tube configuration might be required.

Pulsatile Flow in a Symmetric Sinusoidal Wavy-Walled Channel.

Pulsatile laminar flow is examined experimentally in a two-dimensional furrowed channel with varying flow parameters. We present flow diagrams in which five flow modes are classified in terms of the Womersley number and oscillatory fraction of the flow rate for each net flow Reynolds number, on the basis of the process of vortex formation, growth and subsequent disappearance. A striking feature of pulsatile flow in the wavy-walled channel is that after the vortex ejects from the furrow, the vortex does not always disappear in the mainstream, but reattaches to the wall and eventually disappears under certain conditions, in contrast to oscillatory flow. We also show the transition to three-dimensional flow and a new flow structure in which a wave appears in the mainstream during the flow deceleration phase due to a shear layer instability.