Oscillatory Flow Characteristics Research Articles

Nusselt and Reynolds numbers correlation for oscillatory flow of thermoacoustics over heated tube banks

The back-and-forth motion characteristic of the oscillatory flow brings complexity in predicting the heat transfer nature for thermoacoustic wave conditions. Investigating this is crucial as the flow forms the backbone for the thermodynamic cycles of its operation. Often, the steady approximation in calculating heat transfer leads to ambiguity in designing the system. This concern is addressed in this paper via an in-depth analysis of the fluid dynamic behavior and heat transfer characteristics of oscillatory flow within a thermoacoustic framework, spanning experimental works for Reynolds numbers ranging from 300 to 24,000. The investigation focused on a tube bank heat exchanger composed of nine tubes arranged in inline and staggered configurations. Experiments were conducted at constant tube surface temperatures treated at 40 °C and 80 °C, respectively. The distinguishing difference in heat transfer behaviors for oscillatory flow is visualized by the Computational Fluid Dynamics (CFD) models, employing the Reynolds-Averaged Navier Stokes equation with the k-omega turbulence shear stress transport model. The flow never leaves the system, and it cyclically crosses the tubes back and forth with a travel distance that depends on the drive ratio of the flow. The unique nature of the flow forms the foundation for the experimental findings that show a linear relationship between the Nusselt and Reynolds numbers regardless of the configuration of tube banks and tube bank temperature. The Pearson ruler regression analysis was conducted using Matlab R2022b and a Nusselt correlation, Nu = 0.000853RePr⅓, is proposed with a confidence level of 95 %. Notably, the correlation aligns with the constant value known as the Colburn-j factor, with a value of 0.00083. The small Colburn-j value is shown to be the influence of the log-mean temperature difference characteristic of oscillatory flow. It shows a consistent heat transfer process between the heated tubes and surrounding fluid, which is important to sustain the thermoacoustic effect in the system. For future thermoacoustic design, the use of steady heat transfer correlation should be avoided or at least used with caution as the comparative analysis with published works concluded that the Nusselt and Reynolds correlation for steady flow tends to overpredict the heat transfer by two to threefold.

Thermally oscillatory flow inside an irregular channel through a porous medium with velocity and temperature slips

The purpose of this exploration is to scrutinize the heat transfer characteristics of oscillatory flow on the magnetized second-grade fluid within an asymmetric porous wavy channel with velocity slip and temperature jump conditions. Flow in porous space is characterized by modified Darcy's law. The use of a magnetized second-grade fluid in an asymmetric porous wavy channel could potentially enhance heat transfer within the fluid, leading to better control over drug release rates and spatial distribution. The fundamental flow equations are converted into ordinary differential equations (ODEs) utilizing suitable non-dimensional variables and then resolved analytically. The outcomes are graphically represented to illustrate the physical impacts of governing parameters on the temperature, and velocity profiles. Streamlines are also constructed for specific physical parameters to investigate trapping phenomena. In addition, variations on the coefficient of skin friction, mass flux, and heat transfer coefficients are illustrated with different values of pertinent parameters. The outcomes indicate that fluid velocity is enhanced by increasing the values of Darcy's number and Grashof's number. The fluid temperature decreases by increasing the values of the temperature jump parameter.

Computational Fluid Dynamics modelling of hydrodynamic characteristics of oscillatory flow past a square cylinder using the level set method

This paper presents a numerical study of the three-dimensional hydrodynamic characteristics of oscillatory flow past a square cylinder including the effect of the variation of blockage ratio and the Keulegan–Carpenter (KC) number on those characteristics. The oscillatory flow is simulated using an open-source Computational Fluid Dynamics (CFD) modelling tool REEF3D. The CFD model solves the Reynolds-Averaged Navier–Stokes (RANS) equations in all the three dimensions. The location of the free-surface is represented using the level-set method (LSM), which calculates the complex motion of the free-surface in a very realistic manner. For the implementation of the waves, the CFD code is used as a numerical wave tank. First, the model has been extensively validated against experimental measurements present in the literature. Thereafter, further analysis is carried out using the CFD code to study the effect of blockage ratio on the hydrodynamic characteristics (drag coefficient, inertia coefficient, lift coefficient and coefficient of pressure) for different oscillatory flow regimes. To this effect KC numbers ranging between 3 and 13 (implying different flow regimes) have been selected for blockage ratios of 0.05, 0.0625, 0.1, 0.2, 0.3, 0.5 (for wall effect). The results indicate that the drag coefficient and the lift coefficient increases with the increase of blockage ratios for a particular KC number. Further, both these coefficients increases with the increase of KC number at a fixed blockage ratio. The coefficient of inertia, however, increases up to a blockage ratio of 0.2 and decreases thereafter for a particular KC number. The results also show that for a particular blockage ratio the coefficient of inertia decreases on escalating KC number and the coefficient of pressure increases for large blockage ratios at a fixed KC number on the upstream of the cylinder and reduces on the downstream side of the cylinder.

Experimental investigation of oscillating flow characteristics at the exit of a stacked mesh grid regenerator.

The aim of this study is to investigate the oscillating flow velocity field at the exit of different stacked mesh grid regenerators using Particle Image Velocimetry measurements. Twelve different experimental cases are discussed, yielding oscillating flow fields at the exit of four kinds of regenerators for different acoustic levels. The regenerators are classified according to the mesh wire size to viscous penetration depth ratio and according to the method of stacking the mesh grids. Based on the analysis of the vorticity fields at the exit of the regenerator, three groups of flow patterns are identified. This classification is correctly verified by using the Reynolds number (based on the acoustic amplitude and wire diameter) and the Strouhal number (based on the acoustic displacement amplitude and wire diameter). The characteristics of the fluctuating velocity components are investigated for these various flow patterns. The critical Reynolds number, past which the flow is highly dissipative, is determined. The dissipation timescale is investigated and the quasi-steady approximation is found to be valid for the analysis of the oscillating flow at the exit of the regenerator mesh.

Numerical and experimental investigation on the effect of the two-phase flow pattern on heat transfer of piston cooling gallery

To study factors affecting the formation and conversion of two-phase flow pattern as well as the heat transfer of piston cooling gallery, a transient visual target test bench was set up to research the oscillatory flow characteristics in the cooling gallery under idle condition of the engine. The computational fluid dynamics (CFD) was employed while dynamic mesh technology, SST k–ω turbulence model and volume of fluid (VOF) two-phase flow model were applied to simulate the flow process of piston cooling gallery so as to predict the distribution pattern of two-phase flow. Simulation results were in good agreement with that experimentally obtained. It was observed that in the reciprocating movement of the piston, the action of two-phase flow oscillation was severe, forming some unstable wave flows and slug flows. Results show that under the same pipe diameter, the increase of fluid viscosity results in the decrease of amplitude and the increase of the liquid slugs number as well as the enhancement on heat transfer effect. In addition, it was revealed that injection pressure has little effect on the two-phase flow pattern. However, when the pressure is reduced, the change of the liquid phase is weakened and the locations of flow pattern transition move towards to the behind, thus the impact on the heat transfer is also faint.

Pulsatile flow of Jeffrey fluid in a porous elastic tube with variable cross-section under the effect of magnetic field

Pulsatile flow of a conducting Jeffrey fluid through a porous elastic tube with variable cross-section is investigated in this paper. The Jeffrey fluid flow under the effect of transverse magnetic field is considered to understand the nature of conductive physiological fluids such as movement of blood in small arteries of heart muscle. The problem is formulated by considering the radial motion of the tube wall. The analytical expressions of the axial and transverse velocity for both steady and fluctuating parts are derived. The influence of different pertinent parameter on axial velocity distribution for different instants of times is analyzed through graphs. The variation in the velocity field for two different geometries is discussed. The results reveal that the effect of frequency parameter on fluctuating axial velocity is significant. The influence of magnetic field and Jeffrey parameter has pronounced effect on variation of fluctuating axial velocity. Also it is observed that the geometry has a remarkable effect on axial velocity distribution. The results obtained for the present oscillatory flow characteristics reveal many interesting behaviors that warrant further study of conductive physiological fluid flows in an elastic tube with variable cross-section. In the absence of Magnetic field and Jeffrey parameter the present results are similar to the observations of Misra and Ghosh [8].

Numerical study on flow characteristics and heat transfer enhancement of oscillatory flow in a spirally corrugated tube

The flow and heat transfer characteristics of oscillatory flow in a smooth tube and two-start spirally corrugated tube were investigated by using a three-dimensional unsteady numerical simulation. The simulation was performed under variable properties, pressure and velocity conditions, with helium (He) as the working medium. The results indicated that longitudinal swirl vortexes were generated with the guidance of a spirally corrugated channel in a two-start spirally corrugated tube in which the fluid flowed spirally forward. The heat absorption (Q) of a two-start spirally corrugated tube exceeded that of a smooth tube (Q0). The difference in the average heat absorption was 561 W and the average of Q/Q0 was 1.36. At the same inlet and wall temperatures of 575 K and 1000 K, the outlet temperature of the smooth and spirally corrugated tube increased by 205 K and 365 K, respectively. Therefore, the spirally corrugated tube presented a superior heat transfer performance when compared to that of the smooth tube. Two types of performance evaluation criterion (PEC) calculation methods in a cycle were used to evaluate the heat transfer enhancement of a spirally corrugated tube. The average value of transient PEC in a cycle was 1.69, and the PEC calculated by average parameters in a cycle was 1.38. When compared with the change of pressure in a MPa level in a heating tube, the pressure drop of hundreds of Pa was negligible. However, the outlet temperature of the heating tube could be subject to an additional temperature increase of hundreds of K with the negligible pressure drop of hundreds of Pa. Thus, the benefits were apparent for the heat transfer enhancement by using spirally corrugated tubes in the heater.

Higher order turbulent flow characteristics of oscillatory flow over a wall-mounted obstacle

The paper presents an experimental study in a laboratory flume to investigate the combined wave–current flow over a wall-mounted cubic obstacle. Detailed velocity data were collected using three-dimensional (3D) micro-acoustic Doppler velocimeter from upstream to downstream of the cube along the centerline of the flume. This study examines the spectra, co-spectra, and turbulence kinetic energy flux analysis under the influence of cube in the turbulent boundary layer. It is observed that stream-wise spectral and co-spectral energy distributions vary with the addition of surface wave. Particular attention is also given to understand the behavior of higher order moments to explicitly account for turbulent momentum transport characteristics over wall-mounted cube under wave–current interactions.

Characterization of the frictional losses and heat transfer of oscillatory viscous flow through wire-mesh regenerators

In this paper, new relations for calculating heat transfer and pressure drop characteristics of oscillatory flow through wire-mesh screen regenerator such as Darcy permeability, Forchheimer’s inertial coefficient, and heat transfer area per unit volume, as a function of the wire diameter are presented. According to the derived relations, thinner wires have higher pressure drop and higher heat transfer rate. The relations are applicable for all regenerative cryocoolers. Embedding the new relations into a numerical model, three Stirling-type orifice pulse tube cryocoolers with three regenerators different in length and diameter but same volume in a variety of wire diameters, have been modeled. The results achieved by the model reveal that the local heat transfer coefficient decreases with increase of the wire diameter and the length-to-diameter ratio. In addition, it was shown that the mean absolute gas–solid wire temperature difference is a linear function of wire diameter in the range investigated. The results show that for larger length-to-diameter ratios, Forchheimer’s effect will dominate frictional losses, and the variations of the frictional losses are proportional to the inverse of the wire diameter. Wire diameter has been optimized to maximize the coefficient of performance of the cryocooler. Shorter regenerators have thinner optimum wires.

Analysis of signal characteristics of swirlmeter in oscillatory flow based on Hilbert–Huang Transform (HHT)

In the paper, firstly, on an experimental facility, we investigated the measurement characteristics of a diameter 50mm swirlmeter in uniform flow and oscillatory flow. At the same time, the interference characteristics of oscillatory flow were studied. Then, the signal characteristics of swirlmeter in oscillatory flow were analyzed by Hilbert–Huang Transform (HHT) method. Results show that the response characteristics of swirlmeter in oscillatory flow are addition of that of swirlmeter in uniform flow and the interference characteristics of oscillatory flow. They further prove the conclusions which suppose that the correlation between the velocity pressures of fluid disturbs wave and that of vortex precession in swirlmeter is linear in the literature, and a new method for the oscillatory flow swirlmeter noise removal on HHT was provided.

Fabrication and evaluation of a slicon-based micro pulsating heat spreader

A silicon-based micro pulsating heat spreader (MPHS) was fabricated and its thermal performance was experimentally evaluated. The width and height of the channel are 1 mm and 0.4 mm, respectively. The MPHS has length of 50 mm, width of 15.5 mm, and thickness of 1.5 mm. Ethanol was used as a working fluid. The MPHS achieved maximum effective thermal conductivity of 600 W/m K and a maximum heat transport capability of 4 W. In order to investigate the oscillating flow characteristics, the frequency and the amplitude of the liquid oscillation were measured. As the input power increases, the frequency and the amplitude are increased, which results in an increase in the thermal performance of the MPHS. The effective thermal conductivity was evaluated from a theoretical model in which the thermal performance of the MPHS is assumed to be similar to that of oscillating flow in a channel completely filled with liquid. When the input power is low, the calculated values are in close agreement with the measured values. When the input power is high beyond 2 W, the measured values are slightly higher than the calculated values, which is due to the contribution of heat transport by phase change. The MPHS transports heat mainly by liquid oscillation and the contribution of latent heat transport increases with increasing input power; however, the effect of the latent heat transport is not significant.

Unsteady shock-flow characteristics in an over-expanded rocket nozzle

A numerical investigation of transient side-loads in an axisymmetric over-expanded thrust optimized contour nozzle is presented. These nozzles experience side-loads during start-up and shut-down operations, because of the flow separation at nozzle walls. Two types of flow separations such as FSS and RSS shock structure occur. A two-dimension numerical simulation has been carried out over an axisymmetric TOC nozzle to validate present results and investigate oscillatory flow characteristics for start-up processes. Reynolds Averaged Navier-Stokes equations are numerically solved using a fully implicit finite volume scheme. Governing equations are solved by coupled implicit scheme. Reynolds Stress turbulence model is selected. Present computed pressure at the nozzle wall closely matched with experiment data. A hysteresis phenomenon has been observed between these two shock structures. The transition from FSS to RSS pattern during start-up process has shown maximum nozzle wall pressure. Nozzle wall pressure and shear stress values have shown fluctuations during the FSS to RSS transition. The oscillatory pressure has been observed on the nozzle wall for high pressure ratio. Present results have shown that magnitude of the nozzle wall pressure variation is high for the oscillatory phenomenon.

Numerical investigation of transient side-loads in the start-up process of a rocket nozzle

A numerical investigation of transient side-loads in axisymmetric over-expanded thrust optimized contour nozzles is presented. These nozzles experience side-loads during start-up and shut-down operations because of flow separation at the nozzle walls. Two types of flow separations, FSS and RSS shock structures, also occur. A two-dimension numerical simulation was carried out over axisymmetric TOC nozzles to validate the present results and investigate oscillatory flow characteristics for start-up processes. Reynolds Averaged Navier-Stokes equations are numerically solved using a fully implicit finite volume scheme. Governing equations are solved via the coupled implicit scheme. The Reynolds Stress turbulence model is selected for this work. It was found that the present computed pressure at the nozzle walls closely matched the experimental data. The phenomenon of hysteresis was also observed between these two shock structures. The transition from an FSS to RSS pattern during the start-up process showed maximum nozzle wall pressure and fluctuations in shear stress values. Oscillatory pressure was observed on the nozzle walls with high pressure ratio. The present results show that the magnitude of nozzle wall pressure variation is high for the phenomenon of oscillation.

Numerical simulation of transient flows in a rocket propulsion nozzle

A numerical investigation of transient flows in an axisymmetric over-expanded thrust-optimized contour nozzle is presented. These nozzles experience side-loads during start-up and shut-down operations, because of the flow separation at nozzle walls. Two types of flow separations such as free shock separation (FSS) and restricted shock separation (RSS) shock structure occur. A two-dimensional axisymmetric numerical simulation has been carried for a thrust-optimized contour nozzle to validate present results and investigate oscillatory flow characteristics during the start-up processes. Reynolds-Averaged Navier–Stokes equations are numerically solved using a fully implicit finite volume scheme. Governing equations are solved by coupled implicit scheme. The present work is concerned with comprehensive assessment of the flow features by using Reynolds stress turbulence model. Computed pressure at the nozzle wall closely matched with the experimental data. A hysteresis phenomenon has been observed between these two shock structures. The transition from FSS to RSS pattern during start-up process has shown maximum nozzle wall pressure. Nozzle wall pressure and shear stress values have shown fluctuations during the FSS to RSS transition. The oscillatory pressure has been observed on the nozzle wall for high pressure ratio. Present results have shown that magnitude of the nozzle wall pressure variation is high for the oscillatory phenomenon.

Comparative study of oscillating flow characteristics of cryocooler regenerator at low temperatures

A brief review is presented on previous experimental results and correlations on the friction factor of cryocooler regenerators operating at oscillating flow and pulsating pressure conditions, for different mesh sizes of packed woven screens, focusing on the effects of different operating frequencies ranging from 20 to 80 Hz, at room and cryogenic temperatures. A comparison of the friction factor data with those of other studies is presented to clarify the reason for the difference. Finally, a new oscillating flow correlation of regenerators, in terms of several nondimensional parameters, is discussed and compared.

1918 多孔質体間隙部およびその近傍で生じる乱流場の計測(S21-4 噴流,後流及びはく離流れの流動解析と応用(4),21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Turbulence in pores of porous media in oscillating flow was measured by using a Particle Tracking Velocimetry with the Refraction index matching method. Relationship between turbulent energy and oscillating flow characteristics such as the Reynolds number and KC number is presented. Variation of generated turbulent energy in pores and vicinity of porous media in a phase is presented in detail.

Self-sustained oscillations between two tandem cylinders at Reynolds number 1,000

This study focuses on the self-sustained oscillatory flow characteristics between two tandem circular cylinders of equal diameter placed in a uniform inflow. The Reynolds number (ReD), based on the cylinder diameter, was around 1,000 and all experiments were performed in a recirculating water channel. The streamwise distance between two tandem cylinders ranged within 1.5 ≤ Xc/D ≤ 7.0. Here Xc denotes the center-to-center distance between two tandem cylinders. For all experiments studied herein, quantitative velocity measurements were performed using hot-film anemometer and the LDV system. The laser sheet technique was employed for qualitative flow visualization. The wavelet transform was applied to elucidate the temporal variation and phase difference between two spectral components of the velocity signals detected in the flow field. The remarkable finding was that when two tandem circular cylinders were spaced at a distance within 4.5 ≤ Xc/D ≤ 5.5, two symmetrical unstable shear layers with a certain wavelength were observed to impinge onto the downstream cylinder. The responding frequency (fu), measured between these two cylinders, was much higher than the natural shedding frequency behind a single isolated cylinder at the same ReD. This responding frequency decreased as the distance Xc/D increased. Not until Xc/D ≥ 6.0, did it recover to the natural shedding frequency behind a single isolated cylinder. Between two tandem cylinders, the Strouhal numbers (Stc = fu Xc/Uc) maintained a nearly constant value of 3, indicating the self-sustained oscillating flow characteristics with a wavelength Xc/3. Here Uc is the convection speed of the unstable shear layers between two tandem cylinders. At ReD = 1,000, the self-sustained oscillating characteristics between two tandem circular cylinders were proven to exhibit a sustained flow pattern, not just a sporadic phenomenon.

Heat Transport Characteritics of Oscillatory Flow in a Channel with a Rectnagular Cross Section

The axial heat transfer characteristics of oscillatory flow of water in a channel with a rectangular cross section are investigated experimentally and analytically. Experiments are conducted for various channel aspect ratios and heights, changing the frequency and the stroke of oscillatory flow. A sophisticated numerical simulation technique based on the profile method using the velocity profile for two-dimensional oscillatory Poiseuille flow is proposed to realize a practical way of studying the effects of flow parameters on the heat transfer. Numerical results show good agreements with the experimental results except for high Womersley number flow. The effects of various parameters on the heat transfer enhancement such as the channel aspect ratio, channel height, wall thickness and the wall materials are elucidated.

Oscillatory flow in thermoacoustic sound wave generator

Oscillatory flow in a thermoacoustic sound wave generator is described. The thermoacoustic sound wave generator plays an important role in thermoacoustic equipment. The heat exchange between the working fluid and the stack, the acceleration and deceleration of the working fluid and viscous friction loss both in the stack and in the resonance tube influence the performance of the thermoacoustic sound wave generator. Particularly, oscillatory flow significantly influences the heat exchange mechanism between the working fluid and the stack. Temporal changes in pressure and velocity are sinusoidal inside the resonance tube. Flow forms an oscillatory jet just behind the tube outlet, and becomes intermittent far downstream outside the resonance tube. The open-end corrections of 0.63R, that is, the region where oscillatory flow characteristics are maintained downstream in spite of being outside the tube outlet, are confirmed by velocity measurements and flow visualization. Also, they are almost equal to acoustical theoretical results.

Characteristics of oscillating flow through a channel filled with open-cell metal foam

An experimental study was performed to investigate the characteristics of oscillating flow through a channel filled with open-cell metal foam with a fully inter-connected pore structure. Detailed experimental data of oscillating flow pressure drops and velocities for a wide range of oscillatory frequency and the maximum flow displacement were presented. A correlation equation for the maximum friction factor of metal foams subject to oscillating flow was obtained and compared with the results for channels inserted with wire-screens obtained by other investigators. The results showed that oscillating flow characteristics in an open-cell metal foam are governed by a hydraulic ligament diameter based kinetic Reynolds number Re ω( Dh) and the dimensionless flow displacement amplitude A Dh . The effects of kinetic Reynolds number on the variations of pressure drop and flow velocity in metal foam are more significant than that of the dimensionless flow displacement amplitude. The maximum friction factor of oscillating flow in open-cell metal foams is much smaller than that of oscillating flow in wire-screens for large flow displacement amplitudes.