Laminar Crossflow Research Articles

05/00920 Unsteady conjugate heat/mass transfer from a circular cylinder in laminar crossflow at low Reynolds numbers

05/00920 Unsteady conjugate heat/mass transfer from a circular cylinder in laminar crossflow at low Reynolds numbers

05/00497 Unsteady conjugate heat/mass transfer from a circular cylinder in laminar crossflow at low Reynolds numbers: Juncu, G. International Journal of Heat and Mass Transfer, 2004, 47, (10–11), 2469–2480

05/00497 Unsteady conjugate heat/mass transfer from a circular cylinder in laminar crossflow at low Reynolds numbers: Juncu, G. International Journal of Heat and Mass Transfer, 2004, 47, (10–11), 2469–2480

Conjugate heat/mass transfer from a circular cylinder with an internal heat/mass source in laminar crossflow at low Reynolds numbers

This paper presents a theoretical study of conjugate heat/mass transfer from a circular cylinder with an internal heat/mass source and a surrounding fluid flow. The heat/mass source consists of a constant temperature/concentration wire imbedded in the cylinder center. A finite difference method discretizes the equations. The multigrid method solves the discrete system. Numerical investigations were carried out for cylinder Re numbers equal to 2 and 20. The values of the Pr number were selected such that the product Re × Pr is constant and equal to 100. The main aspect analysed is the influence of the conductivity ratio on the local and average Nu numbers at different values of the wire diameter.

Unsteady conjugate heat/mass transfer from a circular cylinder in laminar crossflow at low Reynolds numbers

Unsteady conjugate heat/mass transfer between a circular cylinder and a surrounding fluid flow has been analysed. Numerical investigations were carried out for the cylinder Reynolds numbers equal to 2 and 20 and different values of the Prandtl/Schmidt numbers. The heat/mass balance equations were solved numerically in cylindrical coordinates by the ADI finite difference method. The computations yield information regarding the influence of the parameters that characterize the coupling features of the conjugate heat/mass transfer, i.e. the physical properties ratios, on Nusselt/Sherwood numbers and cylinder dimensionless average temperature/concentration. The thermal/mass wake phenomenon were also studied.

Analysis of heat transfer from single wires close to walls

Two-dimensional numerical investigations of the forced heat convection from a microcylinder in laminar cross-flow, both in free stream and in near-wall flow, were carried out aiming at a better understanding of the physics behind the wall effects on hot-wire near-wall measurements. In the physical model, an infinitely thin plate with the same properties as the fluid (air) was used as an artificial wall. The conjugate heat transfer between the flow regions on both sides of the plate was taken into account. The effect of the conjugate thermal conditions (temperature distribution and diffusive heat flux) at the interface of the two flow regions on the heat transfer from the wire was investigated by varying the flow conditions on the side opposite to the wire location. Careful energy balance analysis was performed for both the free-stream case and the near-wall case. This enabled the authors to verify their own understanding of the physical mechanism responsible for the wall effect on hot-wire measurements and to examine other mechanisms proposed in the literature. The numerical results showed that the heat diffusion from the wire is significantly enhanced in the case of small wire-to-wall distances (Y+<3). This is mainly caused by modifications of the thermal boundary condition (diffusive effect) at the fluid–wall interface. In contrast, the flow distortion (enhanced convection) was shown not to be the most important influencing factor for the heat transfer of a hot wire. Although the present model study was performed for a laminar flow, the results obtained are applicable to hot-wire measurements in turbulent flows, as stated in the literature.

Laminar Crossflow over Circular Cylinder with Inner Ducts

Laminar Crossflow over Circular Cylinder with Inner Ducts

On the linear heat transfer correlation of a heated circular cylinder in laminar crossflow using a new representative temperature concept

Determination of the reference temperature for the kinematic viscosity of Reynolds number is the key problem for describing the forced convection of a heated bluff-body. The heat transfer of a heated circular cylinder in laminar cross airflow is experimentally investigated in this paper. New correlation equations based on the effective temperature have been firstly derived. All experimental data collapsed excellently on the correlation curves. Moreover, the derived Nusselt and Reynolds number relations are independent of the vortex shedding patterns. The correlation equations have further been transformed to an explicit form without temperature ratio. By introducing a newly proposed representative Reynolds number concept, linear correlation equations have been successfully derived.

Momentum and heat transfer from cylinders in laminar crossflow at 10−4 ⩽ Re ⩽ 200

A summary of results of numerical investigations of the two-dimensional flow around a heated circular cylinder located in a laminar crossflow is presented. Numerical investigations were carried out for the Reynolds number range 10 −4 ⩽ Re ⩽ 200 and for temperature loadings of 1.003–1.5. The computations yield information on Nu and C D variation with Reynolds number. The temperature dependence of the fluid properties (air) was taken into account and this resulted in a temperature dependence of the Nu– Re and C D– Re results. Information is also provided on the Strouhal number dependence on the Re number and on the critical Re number where vortex shedding starts.

Frictional losses and convective heat transfer in sparse, periodic cylinder arrays in cross flow

A numerical study of fluid flow and heat transfer around periodic cylinder arrays under laminar cross flow conditions is presented. The investigation considers flow in sparse square and triangular arrays, with fluid fractions ranging from 0.80–0.99 and particle Reynolds numbers from about 3–160. Volume averaging of the microscopic flow field variables is used to ascertain the functional form of coefficients appearing in the macroscopic (porous media) equations. Frictional losses are shown to follow Darcy's law when the Darcian Reynolds number is of the order of one, while significant non-Darcy effects are seen at higher Reynolds numbers. The validity of the Forchheimer and Ergun correlations is shown to be suspect for flow in this highly porous media. Local thermodynamic equilibrium is relaxed in order to explore cylinder-to-fluid convective heat transfer. Power-law relationships for frictional losses and Nusselt number are shown to correlate well with detailed simulation results.

Evaluation of crossflow filtration models based on shear-induced diffusion and particle adhesion: Complications induced by feed suspension polydispersivity

Specific flux data were obtained during the transient period of flux decline in laminar crossflow filtration. Effects of hydrodynamics on cake parameters such as specific resistance, mass and particle size distribution were studied experimentally. An evaluation of crossflow filtration models suggests that a model based on shear-induced diffusion [1] is a better predictor of specific flux decline than a particle adhesion model [2]. Even for relatively narrowly distributed suspensions, polydispersivity complicates analyses in a manner that is not adequately addressed by these models. Changes in experimental specific cake resistances with module hydrodynamics coupled to the inadequacy of these models for accurately predicting time-dependent specific flux profiles, cake specific resistances, and mass suggests that cake morphology is a key variable that needs to be incorporated in future modeling efforts.

Migration and Deposition of Submicron Particles in Crossflow Microfiltration

The migration and deposition of submicron particles in laminar crossflow microfiltration is simulated by integrating the Langevin equation. The effects of operating conditions on the particle trajectories are discussed. It is found that the Brownian motion of particles plays an important role in particle migration under a smaller crossflow velocity of suspension or a smaller filtration rate. Based on the simulated trajectories of particles, the transported flux of particles arriving at the membrane surface can be estimated. The particle flux increases with an increase of filtration rate and with a decrease of particle diameter; however, the effect of crossflow velocity on the particle flux is not obvious. The forces exerted on particles are analyzed to estimate the probability of particle deposition on the membrane surface. The probability of particle deposition increases with an increase of filtration rate, with a decrease of crossflow velocity, with a decrease of particle diameter, or with an increase of zeta potential on the particle surfaces. The simulated results of packing structures of particles on the membrane surface at the initial stage of filtration show that a looser packing can be found under a larger crossflow velocity, a smaller filtration rate, or a smaller diameter of filtered particles. Crossflow micro-filtration experiments are carried out to demonstrate the reliability of the proposed theory. The deviation between the predicted and experimental data of filtration rate at the initial period of filtration is less than 10% when the Reynolds number of the suspension flow ranges from 100 to 500.

A laboratory study of buoyant plumes in laminar and turbulent crossflows

The results of laboratory experiments of buoyant plumes in crossflows are presented. Specific comparisons are made of plume evolution for laminar and weakly turbulent crossflows for otherwise identical flow parameters to establish that plume dilution rates differ significantly even for weakly turbulent crossflows. Decaying, grid-generated turbulence constituted the flow field of the weakly turbulent crossflow. Measurements comprised analyses of flow visualization, hot-film anemometry and conductivity probe data. Plume thicknesses and widths are found to be greater for turbulent crossflows, although they both grow as χ 2/3 for laminar and turbulent crossflows. The effect of turbulence in the crossflow is to increase entrainment and dilution with the consequence of lower plume trajectories. It is found, relative to the values of dilution for buoyant plumes in a laminar crossflow, that dilutions of plumes in a weakly turbulent crossflow (with kinetic energy dissipation rates two orders of magnitude smaller than internal plume dissipation rates) were greater by 33%. Trajectories can be predicted by an integral model with a modified β, the entrainment coefficient, which relates the ratio of dissipation rates within and outside the plume.

Effect of transmembrane pressure pulsing on concentration polarization

Previous research [Rodgers and Sparks, AIChE J., 37 (1991) 1517] on transmembrane pressure pulsing indicated that pulsing increased solute flux by as much as two orders of magnitude in crossflow ultrafiltration. While it was clear that pulsing reduced membrane fouling resistance, it was not certain how pulsing effected concentration polarization resistance. The objective of this study was to determine how pulsing affected concentration polarization resistance. This was done by examining the effects of oscillating transmembrane pressure in the ultrafiltration of a totally retained solute. Transmembrane pressure was oscillated by applying pulsatile positive pressure on the filtrate side of the ultrafiltration cell. Solutions of 1% BSA at 7.4 pH in 0.15 M NaCl buffered solution were ultrafiltered in a crossflow module using Amicon YM-30 cellulosic membranes. The studies were carried out at crossflow shear rates between 160 sec−1 and turbulent conditions. The operating pressures were 75 and 140 kPa. Transmembrane pressure pulsing was generated by increasing the filtrate pressure from 5 to 30 kPa above the respective operating pressure at frequencies ranging from 0 to 5 Hz. The results indicated that pulsing increased solvent flux in the ultrafiltration process with laminar crossflow. It was determined that transmembrane pressure pulsing altered the concentration polarization boundary layer by translation of body forces through the membrane and minute but significant membrane motion. This was not the mechanism observed for reduction of membrane fouling in the previous study.

Concentration profile within non-uniform mudcakes

The concentration profiles within filtercakes of bentonite particles have been studied. A theoretical analysis of filtration is presented, and is compared to results obtained by cake dissection experiments. The constitutive relations which control short-term compaction of the cake are determined. Long-term cake-creep is also observed, and it is shown that the very simplest viscoelastic models of cake compaction exhibit some, but not all, features of the observed behaviour. When the experimental results are combined with known values for the yield stress within the cake, the analysis predicts the hydraulic resistance of cakes which have been eroded by cross-flow: this is used to explain the apparent absence of erosion when cakes are subjected to laminar cross-flow.

Slow bimolecular reactions in a deflected turbulent jet

An integral method that incorporates the entrainment hypothesis of Hoult, Fay and Forney (JAPCA, 19 (1969) 585) is used to predict the concentration of product formed within a turbulent jet in a laminar crossflow. The analysis is restricted to a slow irreversible bimolecular reaction and asymptotic expressions are derived for the product formed in both the near and far field. Reliable laboratory data are presented for the oxidation of nitric oxide by the reaction 2NO + O 2 → 2NO 2 with excess O 2 and the results are correlated with the theory in the far field.

Solidification around a cylinder in laminar cross flow

Liquid-solid phase change problems in convection heat transfer have many engineering applications. The Navier-Stokes and energy equations have been solved for transient laminar two-dimensional flow past a horizontal subcooled cylinder with outward solidification. The irregular, moving solid-liquid regions have been transformed into uniform squares for easier numerical problem solution using an ADI finite difference scheme. Results discussed are the unsteady thermal flow patterns, solid-liquid interface locations, Nusselt number distributions, and rates of ice formation for different Reynolds numbers, superheat parameters and Stefan numbers. After the initial phase of conduction-controlled ice layer growth, forced convection heat transfer may strongly affect the freezing process, potentially causing irregular shapes of ice formation.

Development of a fiber anemometer for direct measurement of low average velocity inside ducts

A fiber anemometer for direct (without traversing) measurement of average velocities, 0<u<40 cm/s, of air in laminar flow inside ducts is developed. Three experimental configurations representing different locations of the fiber, shapes of flow cross section (circular and rectangular), and duct heating are considered. The relationship between average mass velocity and fiber deflection is established by calibration and calculations, which used a local drag coefficient for thin cylinders in laminar cross flow. The effect of velocity profile on the relationship was found to be significant. The effect of fluid temperature was also significant, and the anemometer is capable of measuring the average mass velocity only in connection with a thermometer. For the present applications, however, the uncertainty of the method due to these effects was found to be small.

Air conditioning systems.

There are some basic requirements for air conditioning systems in operating theatres. 1. Obviously the regulation of temperature and humidity is necessary, but cleaning of fresh air and circulating air is much more important. 2. Concerning cleaning, a high standard of filtration of air is demanded to avoid contamination of wounds. 3. So far only HEPA-filters, a technical description, give security and safety. They filter particles down to under 1 micrometer and have a filtration rate of more than 99.9%. 4. The ventilation rate is directly related to the germ concentration in the air. The higher the ventilation rate the cleaner the air during working. 5. To avoid turbulences unidirectional or uniform flow is necessary. This technique has become popular as the laminar flow technique. To get such a flow an air flow velocity of 0 45 +/- 0.1 m/s is necessary. 6. The arrangement of the different working places in an operating theatre is very important. Instrument tables and the operation field itself have to be in the zone of first air. 7. One of the commonest germ dispersion sources is the human body. Proper dressing and behaviour have to be observed. The airflow behind a person (in laminar cross flow) or under a person (in laminar down flow for instance when the neurosurgeon bends his head over the operating field) has to be regarded as contaminated. 8. Hygienic controls must be made in the operation theatre when unused and also under working conditions, in a sophisticated manner. If all these points are respected a real clean room technique is possible with considerable lowering of the risk of infection for the patient. Obviously all the other rules of hygiene, i.e., asepsis and antisepsis, elaborated since the days of Lister, have to be strictly respected.

Turbulent plume in a laminar cross flow

A simple entrainment hypothesis, a synthesis of previous models, is presented to explain the rise and growth of a turbulent plume in a laminar cross flow. The entrainment rate is assumed proportional to the plume parameter and the tangential and normal velocity differences between the plume and the surrounding fluid. Because a linear combination of the entrainment due to each velocity difference is assumed, two constants of proportionality or entrainment parameters are introduced in the model. Sources of momentum, buoyancy or combinations of the two are accounted for by the theory. The major purpose of this research was to determine the entrainment parameters. Experiments done in neutrally stable ambient fluids show that the entrainment coefficients have, within the scatter of available data, constant values for a fully turbulent plume and small density differences. Superposition of the two entrainment effects was verified by experiments on pure jets in a cross flow.