Developed Flow Region Research Articles

Effects of geometrical parameters and Reynolds number on the heat transfer and flow characteristics of rectangular micro-channel using nano-fluid as working fluid

The combined effects of Reynolds number and geometrical parameters on heat transfer and flow of Al2O3–H2O nanofluid under laminar flow condition in single rectangular microchannel was investigated numerically. In the process of the actual flow and heat transfer of nanofluids, the nanoparticles concentration distribution is nonuniform in the flow field. A proposed simulation method carefully considering the nanofluids thermophysical properties are nonuniform and dynamic in the channels was adopted to improve the simulation accuracy. The results show that the local heat transfer coefficients increase with the aspect ratio and design variable β (the ratio of the microchannel width to the wall thickness) at the fully developed flow region because the nanoparticles enriched near the solid-fluid interface and then heat transfer is enhanced as the aspect ratio and design variable β increase. The effect of aspect ratio on heat transfer coefficient is remarkable. But design variable β has a very mild effect on heat transfer compared with aspect ratio. The difference of heat transfer performance between a conventional channel and a microchannel is decreased with the increase of aspect ratio. The Poiseuille number of the microchannel is related to not only aspect ratio but also the design variable β and Reynolds number, and it is smaller than that of the conventional scale rectangular channels when the Reynolds number is less than about 1000.

Numerical study on bubble-liquid two-phase turbulent hydrodynamics in extremely narrow shape bioreactor

The main objective of this work was to determine the influence of the bubble-liquid two-phase turbulent hydrodynamics on cell culture in extremely narrow shape bioreactor. An improved second-order moment bubble-liquid turbulent model and built in-house calculation source code were developed to numerically simulate the hydrodynamics that related to cell damage. Two higher gas entrance velocities of 78.3 m/s and 104.4 m/s were employed and their effects on hydrodynamic parameters, such as bubble rising and liquid flow velocities, bubble normal and shear stresses, correlation of bubble-liquid normal stresses, turbulent kinetic energy and turbulent energy dissipation rate of bubble and liquid were investigated details. All increased with gas entrance velocities increase but failed to explain an experimental observation that higher cell death at jetting region. A new correlation definition, bubble-liquid two-phase turbulent energy production term was proposed to successfully elaborate it, as well as the effects of high gas entrance velocities. Simulation results showed turbulent energy production terms are the largest at near gas jetting regions in comparison to those of developed flow regions, which are in good agreements with experimental result. Furthermore, extremely narrow shape deteriorated the cell living environment.

Conjugate Heat Transfer in a Microchannel Simultaneously Developing Gas Flow: A Vorticity Stream Function-Based Numerical Analysis

A simultaneously developing microchannel gas flow is analyzed numerically, using the vorticity–stream function form of the Navier–Stokes equation, together with the fluid energy equation and the solid wall heat conduction equation. Rarefaction, shear work, viscous dissipation, pressure work, axial conduction, and conjugate effects on heat transfer characteristics are investigated. The shear work contribution to the wall heat flux is evaluated in both the developing and the fully developed flow regions and compared with the conductive wall heat flux. The assumption of hydrodynamically fully developed, thermally developing flow—normally used in the analysis of channel heat transfer—is assessed and compared with the simultaneously developing flow case. Analytical expressions for the fluid flow and heat transfer parameters under fully developed conditions are also derived and compared with the numerical results for verification. The analysis presented shows that the shear work and the combined viscous dissipation and pressure work result in extending the thermal entrance length by far. Heat conduction in the wall also contributes to increase the thermal entry length. The results presented also demonstrate the shear work contribution to heat transfer in the slip flow regime, although minor in the very first portion of the thermal entrance length, and it becomes progressively more significant as the flow thermal development conditions are approached and turns out to be exactly equal in magnitude to the conductive wall heat flux in the thermally fully developed region, resulting in a zero Nusselt number, as verified by both the exact and numerical solutions.

The effects of internals and low aspect ratio on the fully developed flow region and bubble properties in a pilot-plant bubble column

This work investigates, for the first time, the effects of the presence of internals and low dynamic liquid levels on the bubble dynamics in industrial-size pilot plant bubble columns. Experimental work that conducted in a bubble column of 0.6 m inner diameter and 3.9 m height with an air-water system was utilizing our advanced four-point fiber optical probe technique to measure the radial profiles of the bubble properties. The superficial gas velocity varied from 0.2 to 0.45 m/s to cover the churn turbulent flow regime. PVC pipe of 0.06 m diameter used to represent the heat exchanging internals, occupying 24% of the column cross-section area, and three different aspect ratios (H/D = 3, 4, and 5) were used. Data obtained show that the presence of internals slightly increases the overall gas holdup and significantly affects the radial profiles and distribution of the bubble properties, particularly in the fully developed flow region. However, the presence of internals increases the local gas holdup, the bubble pass frequency, and the interfacial bubble area, especially in the wall region, while decreases the bubble chord length and the bubble rise velocity. The variation in the aspect ratio (H/D) and the presence of internals exhibited a slight impact on the bubble dynamics in the sparger region, whereas the effect of internal on the local gas holdup was concentrated in the wall region. Meanwhile, the axial location (Z), where the fully developed flow region occurs, appears a high sensitivity toward the internals existence and the variations in the aspect ratio and the superficial gas velocity. However, the presence of internals and the increase in aspect ratio both show that the fully developed flow region begins at lower axial locations, while an increase of the superficial gas velocity delays the transition to fully developed flow to a higher axial location.

Analytic representations of dissipation in partially-submerged reciprocating pipe flow

Abstract A series of analytic models are presented of a partially-submerged, straight circular cross-section pipe subjected to reciprocating flow, incorporating viscous and turbulent boundary-layer dissipation. This pipe is the most elementary paradigm for a fixed-type Oscillating Water Column (OWC), and its analysis more generally addresses one aspect of reciprocating pipe flow. The derivation of the dissipation terms in the momentum conservation equations from their origin in the Navier–Stokes equation are explicitly identified. The contribution of different damping sources to the overall energy loss is compared. The flow inside the pipe is assumed fully developed along the entire length, neglecting the effect of the flow development region. For relevance to engineering applications, a power take-off system is modelled assuming that the air compresses and expands isentropically in an air chamber at the top of the pipe. Existing theory is adapted into the current models for computing the hydrodynamic coefficients related to the scattered wave and radiation wave. A novel contribution of this paper is the inclusion of damping due to the wall shear stress, modelled for the reciprocating flow system inside the pipe. Comparison between the damping factors in relatively short OWCs confirms the findings in the literature that the major part of the damping is due to the radiation wave. However, it is found that with the increase of the draft length, the wall shear stress damping may also become an important factor in the resulting dynamics.

Experimental and predictive research on solids holdup distribution in a CFB riser

The present work focuses on the axial and radial distribution of the solids holdup in the flow development region of a CFB riser. The effects of operating conditions (superficial gas velocity and solids circulation rate) and particle properties (particle density and size) on solids holdup are investigated by an optical fiber probe (PC6M). The solids holdup is higher in the lower section than that in the upper section of the riser, and is lower in the center than in the wall region of the riser. Increasing the solids circulation rate or decreasing the gas velocity results in the increment of the solids concentration and radial non-uniformity. With the increase of the particle density and size, the solids concentration increases, which is more evident at the lower part of the riser. Empirical correlations for predicting the cross-sectional averaged solids holdup and local solids holdup are developed respectively based on lots of experimental data. A good agreement is obtained between the predicted results and experimental data of the present work and many other data reported in the literature.

Regulating flow of vapor to enhance pool boiling

High heat flux dissipation can be achieved in pool boiling by regulating vapor removal to induce a streamlined motion of liquid over the heater surface. In the present work, a novel approach is presented by directing the vapor through specific structures to generate separate liquid-vapor pathways. A hollow conical structure (HCS) is printed above the heater surface using metal additive manufacturing to induce this effect. As the liquid boils and vapor accumulates inside the HCS, independent liquid and vapor flow fields are formed at the top and side holes of the HCS. The direction of liquid and vapor flow is determined by the size of the top hole in the HCS. If the top hole is larger than the departure bubble diameter, vapor naturally escapes from the top and liquid enters through the side holes. A smaller top hole induces a reversal of this motion and causes the vapor to exit through the side holes and liquid to enter from the top hole. To isolate the convective effects on boiling enhancement from the fin effect, the HCS was thermally insulated from the heater surface. The results indicated that 74% of the boiling enhancement achieved with these structures resulted from the convective heat transfer in the developing flow region. The boiling performance was further increased using multiple HCS of a smaller footprint over microchannels to improve heat transfer in the entrance region. This resulted in a 2-fold increase in CHF and a 4-fold increase in HTC compared to plain surface.

Turbulence structure and similarity in the separated flow above a low building in the atmospheric boundary layer

Separated and reattaching flows over sharp-leading-edge bluff bodies are important to investigate in order to improve our understanding of practical flows such as the case of low-rise buildings in the atmospheric boundary layer. In this study, Particle Image Velocimetry measurements of the separated-reattaching flows over the roof surface of a low-rise building model were taken for six different turbulent boundary layer conditions. The results were analyzed to understand how the incident turbulence affects the flow field of the separation bubbles above the low-rise building roof. The mean flow field above the roof-surface was found to be approximately similar across the six terrain conditions using the mean reattachment length in the streamwise direction and the maximum mean thickness of the separated shear layer in the vertical direction. However, the turbulence stresses are not similar which is attributed to high levels of initial turbulence kinetic energy in the separated shear layer. This leads to fundamental differences in the initial development of the separated flow when compared to flows with lower turbulence in the incident stream. The results indicate that the Kelvin-Helmholtz instability may be altered, or perhaps even suppressed, in the initial flow development region. This leads to substantially different turbulence statistics and characteristics within the separated shear layers.

Effect of aspect ratio on developing and developed narrow open channel flow with rough bed

This study attempts to unravel the effect of aspect ratio on the turbulence characteristics in developing and fully developed narrow open channel flows. In this regard, experiments were conducted in a rough bed open channel flow. Instantaneous 3D velocities were acquired using an acoustic Doppler velocimeter at various locations along the centerline of the flume. The variables of interest include the mean components of the flow velocity, turbulence intensity, wall normal Reynolds shear stress, correlation coefficient, turbulence kinetic energy, and anisotropy. A new correlation between the equivalent roughness and velocity shift from the smooth wall logarithmic equation as a function of aspect ratio is proposed. Aspect ratio was found to influence the velocity characteristics throughout the depth in the developing flow region, while the effects are confined to the outer layer for fully developed flows. New equations to describe the variation of turbulence intensities and turbulent kinetic energy are proposed for narrow open channel flows. Reynolds stress anisotropy analysis reveals that level of anisotropy in narrow open channel flow is less than in wide open channel flows. Finally, a linear regression model is proposed to predict flow development length in narrow open channel flows with a rough bed.

Coupling of Lattice Boltzmann and Volume of Fluid Approaches to Study the Droplet Behavior at the Gas Diffusion Layer/Gas Channel Interface

With the increased concern about energy security, global warming, and air pollution, the possibility of using polymer electrolyte fuel cells (PEFCs) in approaching renewable and sustainable energy systems has reached significant momentum [1,2]. A typical PEFC flow field consists of a series of micro/minichannels. The continuous removal of liquid water from the cathode channels is a critical issue, as water droplets forming in the channels can block the transport of gaseous oxygen to the active sites. This phenomenon gives not only an uneven current distribution, substantial loss of performance, but also, unstable operation and increased degradation rates [1]. Water generated by the electrochemical reactions often condenses into liquid form, potentially flooding the GC (gas channel), the GDL (gas diffusion layer), the micro porous layer (MPL) and the catalyst layer. Insight into the fundamental processes of liquid water transport and evolution is still not complete, preventing further FC development [1]. Computational fluid dynamics (CFD) models make it possible to reduce the number of experiments needed for cell design and development [1,3]. The aim of this work is to obtain an increased understanding of the droplet behavior at the GDL/GC interface by coupling of Lattice Boltzmann and Volume of Fluid (VOF) approaches, for gas channels relevant for PEFCs. The VOF model is an interface-resolving method and has become a widespread methodology for solving two-phase flow phenomena inside PEFCs. The volume fraction of liquid water at the computational cell volume schemes, are used to track the interface between the phases [1,3]. Input parameters in the VOF model are extracted from our in-house Lattice Boltzmann calculations [4], i.e., establishing a multiscale environment. It is clear that the contact angle as well as the size of the liquid droplet vary with positions at the interface, depending on the stochastic GDL geometry. A model describing one straight channel with one gas inlet, one liquid inlet (at the GDL surface) and one two-phase outlet is developed. The operating parameters belong to the laminar flow regime. Notice that a developing flow region covers the entire channel. The uniqueness of this work includes a calculation of contact angles at the GDL interface as well as the size of the liquid inlet, by the authors, with Lattice Boltzmann calculations (3D nineteen-velocity (D3Q19) pseudopotential multiphase multicomponent isothermal LBM) [4].

Numerical Visualization of Plunging Water Jet using Volume of Fluid Model

A plunging liquid jet is defined as a moving column of liquid passing through a gaseous headspace, air in our case, before impinging a free surface of receiving liquid pool. The mechanism of air entrainment due to plunging liquid jets is very complex and the complete mechanism of air entrainment is not fully understood so far. The present paper is an unsteady numerical simulation of air entrainment by water jet plunging, using the Volume Of Fluid (VOF) model. The piece wise linear interface construction algorithm (PLIC) for interface tracking is used, to describe the phase distributions of entirely immiscible air and liquid phases. The aim of this work is to investigate the performance and accuracy of the VOF method in predicting the initial impact between the descending jet and water free surface, air entrainment and the developing flow region under free surface. Three scale models based on geometric similarities (Froude number and dimensionless free jet length) are used for validation according to Chanson (2004) experience. The simulations show with accuracy, the air cavity formation steps, caused by the initial jet impact, its deep stretching under the pool free surface, until breakdown due to the shear created by a toroidal vortex. In terms of, air entrainment estimation, bubble dispersion and radial distribution of air volume fraction, large-scale models present a good agreement with the experience. However, for the smallest scale model, the results lead to suggest that air entrainment is governed by more parameters than the geometric similarities.

Pressure Work and Viscous Dissipation Effects on Heat Transfer in a Parallel–Plate Microchannel Gas Flow

ABSTRACTConvective heat transfer in a parallel plate microchannel gas flow is investigated analytically and numerically, considering the effects of viscous dissipation, pressure work, shear work, axial conduction and rarefaction. Analysis is performed with constant wall temperature and constant wall heat flux boundary conditions for both gas cooling and heating. The results presented demonstrate the significance of the combined effect of pressure work and viscous dissipation, shear work, rarefaction degree and axial conduction on microchannel convective heat transfer, in both the thermally developing and fully developed flow regions. Viscous dissipation and pressure work in a pressure-driven microchannel gas flow are of comparable magnitudes and may not be neglected from the energy equation. The shear work at the wall, which is effectively the combined effect of viscous dissipation and pressure work, needs to be included in the Nusselt number for better predictions of heat transfer.

Intrusive probes in riser applications

Many of the probes used to understand hydrodynamics in circulating fluidized bed risers intrude into the environment they are measuring, although assumptions are typically asserted that the intrusive probes do not affect the data collected. This could be a poor assumption in some cases and conditions. We found that intrusive fiber‐optic probe measurements consistently mis‐predicted the solids concentration compared to the nonintrusive pressure drop measurements outside the fully developed flow region of a riser containing fluid catalytic cracking catalyst or glass bead particles. The discrepancy was sensitive to superficial gas velocity, solid circulation rate, probe position, and flow direction. Barracuda VR™ computational fluid dynamics simulations confirmed this, and indicated that particle momentum was lost at the leading edge of the probe and particles were spilling over to the probe tip. Accordingly, new probe designs were proposed to mitigate the intrusiveness of a fiber‐optic probe for more accurate characterization. © 2017 American Institute of Chemical Engineers AIChE J, 63: 5361–5374, 2017

On the Calibration of Irwin Probes for Flow in Rectangular Ducts With Different Aspect Ratios

In this work, the suitability of pressure probes, commonly known as Irwin probes, to determine the local wall shear stress was evaluated for steady turbulent flow in rectangular ducts. Pressure measurements were conducted in the fully developed flow region of the duct and both the influence of duct aspect ratio (AR) (from 1:1.03 to 1:4.00) and Reynolds number (from 104 to 9 × 104) on the mean characteristics of the flow were analyzed. In addition, the sensitivity of the longitudinal and transversal placement of the Irwin probes was verified. To determine the most appropriate representation of the experimental data, three different characteristic lengths (l*) to describe Darcy's friction coefficient were investigated, namely: hydraulic diameter (Dh), square root of the cross section area (√A), and laminar equivalent diameter (DL). The comparison of the present experimental data for the range of tested Re numbers against the results for turbulent flow in smooth circular tubes indicates similar trends independently of the AR. The selection of the appropriate l* to represent the friction coefficient was found to be dependent on the AR of the duct, and the three tested scales present similar performance. However, the hydraulic diameter, being the commonly employed to compute turbulent flow in rectangular ducts, is the selected characteristic length scale to be used in the present study. A power function-based calibration equation is proposed for the Irwin probes, which is valid for the range of ARs and Reynolds numbers tested.

Interaction of Two Unequal Turbulent Jets with Different Temperature

AbstractThis paper examines the control and heat transfer process of the mixing of a strong jet and a weak jet. The control of the jet diffusion may be required in many devices such as cooling systems, combustors, gas turbines, heating or cooling surfaces, cooling problem techniques, mixing flows, etc. A steady three‐dimensional problem is solved by the finite volume method using RANS modeling. The validation confirms that both RSM and k‐ε realizable models predict more accurately this flow configuration than the standard k‐ε model. A parametric study is conducted on the basis of ratios of the momentum of the two jets. Due to the Coanda effect, the deviation of the weak jet has been highlighted for several momentum ratios. In the fully developed flow region, after the confluence point, the structure of a single jet flow is recovered. However, the point of maximum velocity is shifted toward the side of the strong jet. For a given velocities ratio of 0.25, the temperatures ratio between the two jets is checked. The location of maximum temperature of the jet is almost constant for diverse values of 0.8 ≤ λT ≤ 1. The location of the point of maximum temperature varies linearly according to λT.

Comparative analyses of phase-detective intrusive probes in high-velocity air–water flows

A comparative analysis of a wide range of air–water flow properties was conducted for two types of phase-detection intrusive probes including fiber-optical and conductivity probes. Experiments were conducted on a stepped spillway model for a skimming flow discharge q = 0.478m2/s and for Re = 4.7 105 in a flow region just downstream of the inception point of free-surface aeration and in the fully developed flow region. The comparison of a large number of key air–water flow properties showed a very close agreement for the two sensor types including void fraction, interfacial velocity and equivalent clear water flow depth enabling a direct comparison of past and future data collected with either phase-detection probe type. Minor differences were observed in terms of chord sizes, clustered properties and interparticle arrival times linked with the slightly smaller sensor size of the fiber-optical probe. The in-line positioning of the leading and trailing tips of the fiber-optical probe affected the trailing tip properties resulting in elevated turbulence intensities. An optimum dual-tip phase-detection probe design should consist of small probe tips positioned side-by-side.

Performance and Regression Rate Characteristics of 5-kN Swirling-Oxidizer-Flow-Type Hybrid Rocket Engine

Burning tests were carried out for the 5-kN-thrust swirling-oxidizer-flow-type hybrid rocket engine to establish the engine technology and to obtain the fuel regression characteristics. The engine attained stable burning without any combustion oscillation and efficiencies of more than 98% under the 5-kN-thrust conditions. The maximum time-averaged thrust realized was 4.4 kN, which was lower than 5 kN. This was attributed to the regression rate being lower than the value estimated by regression rate correlation. The fuel regression behavior of this engine was characterized as the flow-development region near the grain leading edge and the fully developed turbulent flow region. The axial length of the flow-development region varied from that observed in the laboratory-scale engine, and it depended on the oxidizer mass flow rate, grain initial port diameter, and swirl strength. The controlling parameter of the regression rate in the fully developed flow region was modified to correlate with the grain port diameter. By effecting these modifications, the accuracy of the regression rate prediction was improved. The range of the applicable controlling parameter of the modified regression rate correlations was extended for use for larger-scale engines.

Secondary flow and heat transfer coefficient distributions in the developing flow region of ribbed turbine blade cooling passages

This paper reports an experimental and numerical study of the development and coupling of aerodynamic flows and heat transfer within a model ribbed internal cooling passage to provide insight into the development of secondary flows. Static instrumentation was installed at the end of a long smooth passage and used to measure local flow features in a series of experiments where ribs were incrementally added upstream. This improves test turnaround time and allows higher-resolution heat transfer coefficient distributions to be captured, using a hybrid transient liquid crystal technique. A composite heat transfer coefficient distribution for a 12-rib-pitch passage is reported: notably the behaviour is dominated by the development of the secondary flow in the passage throughout. Both the aerodynamic and heat transfer test data were compared to numerical simulations developed using a commercial computational fluid dynamics solver. By conducting a number of simulations it was possible to interrogate the validity of the underlying assumptions of the experimental strategy; their validity is discussed. The results capture the developing size and strength of the vortical structures in secondary flow. The local flow field was shown to be strongly coupled to the enhancement of heat transfer coefficient. Comparison of the experimental and numerical data generally shows excellent agreement in the level of heat transfer coefficient predicted, though the numerical simulations fail to capture some local enhancement on both the ribbed and smooth surfaces. Where this was the case, the coupled flow and heat transfer measurements were able to identify missing velocity field characteristics.

Thermal performances of tubular flows enhanced by twin and four spiky twisted fins on rod

An experimental study measuring the axial Nusselt number (Nu) distributions, averaged Fanning friction factors (f) and thermal performance factors (TPF) of the tubes fitted with swirl generator configured by twin or four spiky twist-fins along a rod was performed at Reynolds numbers (Re) of 750–70,000. A set of test results acquired from the swirl tubes with a spiky twin- or four-twist-fin insert was selected to illustrate the interdependent y and Re impacts on Nu, f and TPF by the aid of flow snapshots collecting from the smoke flow visualization tests. Comparative heat transfer enhancements (HTE), f augmentations and TPF properties attributed to previous smooth and present spiky twin- and four- twisted fins were analyzed. The turbulent and laminar Nusselt numbers detected from present swirl tube (Nu) were normalized by the referenced plain-tube turbulent and laminar Nusselt numbers (Nu∞) as the Nu/Nu∞ ratio to quantify the HTE impact. The laminar and turbulent Nu/Nu∞ ratios of present swirl tubes in the developed flow region are respectively raised to 2.2–5.08 and 2.44–2.96 for the tubes with spiky twin-fins and 3.31–7.62 and 2.04–3.23 for the tubes with spiky four-fins. The HTE benefits with noticeable f augmentations have led the laminar and turbulent TPF to the respective ranges of 0.94–1.79 and 0.99–1.07 for spiky twin-fins and 0.98–1.82 and 0.82–0.94 for spiky four-fins. Two sets of empirical Nu and f correlations are devised to assist the relevant applications.

CPFD modeling and experimental validation of gas–solid flow in a down flow reactor

This manuscript reports the fluid dynamics in the developed flow region of a cocurrent gas–solid down flow fluidized bed unit. Gas–solid flows are simulated using a Computational Particle Fluid Dynamics (CPFD) numerical scheme and experimental data from the CREC-GS-Optiprobes. This model represents clusters in a downer unit. It is hypothesized that in downers, clusters are formed via a Random Particle-Selection Method (RPSM) ensuring cluster dynamic stability. To accomplish this, a statistical particle-selection of clusters (SPSC) method is developed. This hybrid model is validated with experimental data obtained in a 2m height and 2.57cm diameter column. Observed time-averaged axial and radial velocities and solid concentration profiles are successfully simulated by the Hybrid CPFD/CREC-GS-Optiprobes Data Model. These findings support: (a) a narrow distribution of particle cluster residences, (b) the relatively flat radial solid concentrations and solid cluster velocities, and (c) a valuable approach for establishing slip velocities in downer units.