State Laminar Flow Research Articles

Characteristics of temperature distribution in atmospheric pulsed surface dielectric barrier discharge for ozone production

The temperature field distribution in air SDBD (surface dielectric barrier discharge) is investigated by measuring the temperatures of reactive plasma volume (Tp), electrode (Te) and gas (Tg) using the fiber optic thermometer. Results show that Tp is almost the same as Te, while Tg is lower than Tp. Both Tp and Tg follow a first-order model. The temperature in the area between the boundary of the plexiglass surfaces and the electrodes is higher than the temperature in the center. The gas flow rate has little effect on Tp due to that air flow is in steady laminar flow state.

Analysis of pressure characteristics under laminar and turbulent flow states inside the pilot stage of a deflection flapper servo-valve: Mathematical modeling with CFD study and experimental validation

Electro-hydraulic servo-valves are widely used components in the mechanical industry, aerospace and aerodynamic devices which precisely control the airplane or missile wings. Due to the small size and complex structure in the pilot stage of deflection flapper servo-valves, accurate mathematical models for the flow and pressure characteristics have always been very difficult to be built. In this paper, mathematical models for the pilot stage of deflection flapper servo-valve are investigated to overcome some gaps between the theoretical formulation and overall performance of the valve by considering different flow states. Here, a mathematical model of the velocity distribution at the flapper groove exit is established by using Schlichting velocity equations for in-compressible laminar fluid flow. Moreover, when the flow becomes turbulent, a mathematical model of pressure characteristics in the receiving ports is built on the basis of the assumption of the collision between the liquid and the jet as the impact of the jet on a moving block of fluid particles. To verify the analytical models for both laminar and turbulent flows, the pressure characteristics of the deflection flapper pilot stage are calculated and tested by using numerical simulation and experiment. Experimental verification of the theory is also presented. The computed numerical and analytical results show a good agreement with experimental data.

Nonlinear stability of laminar flows in an inclined heated layer with an imposed magnetic field

Nonlinear stability of stationary laminar flow solutions of two inclined parallel planes filled with a hydromagnetic fluid heated from below is studied via Lyapunov direct method. In order to determine the energy stability bound for the critical Rayleigh number ( ) explicitly, for , the laminar flow solutions of the problem are nonlinearly unconditionally and exponentially stable, and we consider just transverse perturbations. Compared with the results in the literature, conditions for the nonlinear stability obtained in this article have no restriction on the magnetic Prandtl number Pm.

Heat Transfer Characteristics of Microjet Impingements with Flow Extraction

The present study proposes a novel microjet impingement-effusion model for electronics cooling. The model is realized on a silicon substrate with the arrangement of impinging and extraction holes. A conjugate heat transfer analysis is carried out which includes heat transfer from the substrate base to the impinging fluid which subsequently taken away through the extraction holes. The impingement microjet is surrounded by six micro holes distributed hexagonally to extract the spent flow. A three-dimensional (3-D) numerical analysis is performed for a single-phase steady state laminar flow in view of the small flow rates and micro scales. The proposed model is investigated for local temperature and heat transfer coefficient distribution. The highest temperature is observed near the impingement, and the lowest temperature is observed on the surface below the extraction holes. With the increase of jet-to-effusion hole diameter square ratio, the high-temperature region is reduced. However, there is no significant improvement is detected in the local heat transfer coefficient.

A laminar-jet-discharging method for measuring the interfacial tension of deformable surfaces

We propose a laminar-jet-discharging method to measure the interfacial tension of deformable surfaces such as soap bubbles. This method avoids the need to measure the small pressure difference inside and outside a soap bubble in a static state. By allowing the air in a soap bubble to discharge in a laminar flow state through a long tube, we show that the surface tension of the soap bubble is proportional to the rate of change of the fourth power of the bubble radius. Experimentally, we verify that this linear relationship is valid over a long period and thus can be measured with cameras at slow recording rates. Our method offers a straightforward and accurate way to measure the surface tension of soap bubbles using easy-to-obtain devices. In addition, we propose a new development of the pendant drop method based on the silhouette of the drop, which does not require any pressure transducer or computation of the second-order derivatives of the drop profile and hence is easier to implement and less sensitive to the accuracy of the drop profile determination. For pure liquids, results from the new pendant drop method compare well with standard values. This method is thus used to verify the laminar-jet-discharging measurements.

CFD ANALYSIS OF LAMINAR FORCED CONVECTIVE HEAT TRANSFER FOR TiO2/WATER NANOFLUID IN A SEMI-CIRCULAR CROSS-SECTIONED MICRO-CHANNEL

In this study, forced convection flow and heat transfer characteristics of TiO2/water nanofluid flow with different nanoparticle volume fractions (1.0%, 2.0%, 3.0% and 4.0%) in semi – circular cross – sectioned micro – channel was numerically investigated. The three - dimensional study was conducted under steady state laminar flow condition where Reynolds number changing from 100 to 1000. CFD model has been generated by using ANSYS FLUENT 15.0 software based on finite volume method. The flow was under hydrodynamically and thermally developing flow condition. Uniform surface heat flux boundary condition was applied at the bottom surface of the micro – channel. The average and local Nusselt number and Darcy friction factor values were obtained using numerical results. Also, the effects of using nanofluid on local values of Nusselt number and Darcy friction factor were investigated. Numerical results indicate that the increasing of nanoparticle volume fraction of nanofluid, the average Nusselt number increases; however, there is no significant variation in average Darcy friction factor.

Liquid jet breakup: A new method for the preparation of poly lactic-co-glycolic acid microspheres

The purpose of this study was to apply the phenomenon of liquid jet breakup to the preparation of sustained-release microspheres. The mechanisms of liquid jet breakup in different jet states were investigated and the single factor method was used to study the effect of each process parameter on the particle size and size distribution of microspheres. Meantime, the prepared microspheres were characterized by morphology, drug loading, encapsulation efficiency and in vitro release. The results indicated that the process of liquid jet breakup could have 5 different states. The laminar flow state dominated when the Reynolds number (Re) was low, and the prepared microspheres had larger particle sizes. When the Re was high, the turbulent state was dominant and the microspheres had smaller particle sizes. And during the transition state from the laminar flow to the turbulence, the microspheres had a wide particle size distribution. Different process parameters could affect the particle size and distribution of microspheres by changing the Re, surface tension coefficient and viscosity. The microspheres prepared by liquid jet breakup were smooth and round with the drug loading of 35% and the encapsulation efficiency of 88%. In addition, when the polymeric carrier materials were different, the microspheres could have various drug release models such as sustained release with a lag phase, sustained release with no lag phase, pulsed release and so on, which could be applied widespread in the future.

Slip theory-based numerical investigation of the fluid transport behavior on a surface with a biomimetic microstructure

This paper presented a numerical simulation of drag reduction on a superhydrophobic surface with a groove structure. The computational fluid dynamics (CFD) method was used to analyze the effect of the groove microstructure when droplets impacted a superhydrophobic surface. The simulation results revealed three main characteristics: (1) The distance the droplet spread was larger along the direction parallel to the groove and smaller perpendicular to the groove; (2) Two protruding small spheres were formed at the edge of the droplet along the groove direction; (3) During retraction, the droplets presented a narrow, cross-shaped morphology. The effect of the groove structure in the two-dimensional microchannel on drag reduction near the wall was analyzed based on slip theory by the coupled level set/volume of fluid (CLSVOF) method. The air in the superhydrophobic pit formed a low-velocity vortex, which made the fluid roll on the air surface. The rolling on the surface produced a velocity slip at the gas–liquid interface. In addition, the superhydrophobic surface had an obvious drag reduction effect in the laminar flow state, but the drag reduction effect in the turbulent state was not ideal and even increased the flow resistance at the wall.

Подавление турбулентности в течениях с вращением

The possibilities of turbulence control in spherical Couette flow were examined experimentally. It was shown, that with increasing of outer sphere rotational rate modulation amplitude, the suppression of turbulence is possible with transition to laminar flow state. The reverse process - turbulence recovery - is available with amplitude decreasing. It was established that turbulence breaking and recovering is accompanied by hysteresis. It was shown, that at small values of modulation amplitude, suppression of turbulence is available only in narrow frequency band.

Thermal hydraulic performance of micro-channel heat sink with composite metal foam-solid fin design

<p indent="0mm">Three-dimensional (3D) interconnect technology promises significant performance advantages for the next generation of high speed and multi-functional electronic devices, however, the large-scale integration of electronic circuit and vertical stacking of multi-functional dice have been demonstrated to result in substantially increased waste-heat generation rates and localized hotspots. Efficient thermal management solutions are imperative to ensure the operating temperature of components below a reasonable level. Among various cooling techniques, metal foam heat sink has been identified as a promising alternative for electronic cooling, because of the large specific surface area, high effective thermal conductivity and intensive fluid mixing capability that foam material provided. In this study, a novel micro-channel heat sink with combined metal foam-solid fin, namely combined fin heat sink (CFHS), was established for electronics cooling. Numerical simulations based on Brinkman-Darcy extended momentum equation and local thermal non-equilibrium energy equations were carried out to investigate the fluid flow and heat transfer characteristics. To simplify the computations, a single channel was used as the computational domain with a dimension of 20 mm×2.4 mm× <sc>6 mm.</sc> The constant heat flux of <sc>100 W/cm²</sc> was supplied at the bottom wall to simulate the high-powered electronic device. Metal foam and fin were made of high thermal conductivity copper, and pure water was used as the working coolant. It was assumed that the metal foam was fully saturated with fluid in laminar flow state and the thermal resistance between the metal foam and fin was neglected. Effects of combined fin configuration on the flow and heat transfer performances were studied to determine the optimal porosity and dimensionless metal foam layer thickness for designing the CFHS. The thermal performance of the novel heat sink was compared to that of the conventional heat sink, and the total thermal resistance was analyzed based on the simplified thermal resistance network. The results indicate that the CFHS is more favorable in promoting thermal performance that the average Nusselt number of CFHS is 2.53 times higher than that of the conventional heat sink, while accompanied by the high pressure drop as a penalty. The flow resistance and specific area are two conflicting parameters that affect the thermal and hydraulic performances of the CFHS, our results show that there exists a critical dimensionless metal foam thickness and optimal porosity, which are 0.9 and 0.3, respectively. Compared with the conventional heat sink, the total thermal resistance of CFHS is significantly reduced by 56%, and the thermal performance factor of CFHS performs 1.32 times higher. The presented CFHS thus provides a feasible solution for thermal management of electronic devices with high-powered density.

Double diffusive mixed convection study in a vertical annulus at different aspect ratio and Richardson number

The present study has been considered in heat, mass transfer and the reversal flow occurrence inside the annulus of concentric vertical cylinders numerically. Both the thermal buoyancy and the buoyancy due to different concentrations are of equal magnitude and direction. The forced flow direction effect is investigated. The problem is considered a laminar flow and steady state. The influencing parameters are identified for the problem are Richardson number, Lewis number, and length to gap ratio and Radius ratio. The dimensionless gap is kept constant at unity. The Richardson number is varied from 0.1 to 10, dimensionless length from 5 to 25, the dimensionless radius ratio from 1.5 to 11 and the lewis number from 0.1 to 10. The dimensionless temperature and concentration distributions are illustrated at various range of parameters. In addition, the average values concerning Nusselt as well as Sherwood numbers are correlated. The reverse flow is examined at both aiding and opposing flows. The reverse flow existence is affected by the Richardson number and the length to gap ratio. The value concerning thermal Grashof number is stable at 104.

Numerical treatment of mass continuity in pressure-correction methods for patient specific flow simulations

PurposeThe purpose of this paper is the development of a robust numerical scheme for fluid flow simulations in complex domains with open boundaries.Design/methodology/approachA modified pressure correction algorithm is presented. The proposed modifications are derived through a step-by-step analysis of the importance of mass continuity enforcement in pressure correction methods, the boundary conditions of the pressure correction equation and the special nature of open boundaries.FindingsThe algorithm is validated by performing steady state laminar flow simulations in two backward facing step geometries with progressively truncated outlet channels. The efficiency of the methodology is demonstrated by simulating the pulsatile flow field in a patient specific iliac bifurcation reconstructed by medical imaging data.Originality/valueThe proposed numerical scheme provides accurate and mass conserving solutions in complex domains with open boundaries. The proposed methodology may be directly implemented in any computational domain without any prerequisites regarding the location or type of domain boundaries.

Numerical simulations of the Cu/Al composite plate continuous cast-rolling process

A two dimensional steady state laminar flow model was established using finite volume software to study the effects of rolling speed and melt pouring temperature on the bonding performance of a copper/aluminum composite plate manufactured using a cast-rolling process. The boundary conditions were optimized in two aspects by writing C language programs. Meanwhile, outlet temperature was measured by thermocouple in the experiment, the maximum error of the simulation and experiment was 2.8%, so the reliability of the simulation analysis was verified. The study found that semi-solid/solid contact time and rolling reduction percentage are the key factors affecting bonding, with pouring temperature and rolling speed changing which factor plays a leading role. The calculated results show that the optimal pouring temperature and casting speed are 963 K and 0.5 m min−1, respectively. Processing under these conditions results in semi-solid/solid contact time of 2.2 s and rolling reduction of 40% based on simulation results, with experimental peeling strength of 86 N mm−1. Optical images of surface morphology after peeling show that the failure mechanism of the Cu/Al composite plate is plastic and brittle fracture. This study provides guidance for optimizing cast-rolling processing parameters.

Notes on factitious shear work of slip flow in a channel

Gas slip flow is observed in a micro scale channel whose characteristic length is less than about 10 μm under atmospheric conditions. To analyze the slip flow, the energy equation with the viscous dissipation term is solved with a boundary condition which includes a factitious sliding shear work term due to the slip at the wall to compensate the energy balance. Recently, an alternate method which uses the boundary condition without inclusion of the factitious sliding shear work term has been proposed. However, it seems that physics of the slip flow has not been properly understood by researchers. In this paper to clarify the issue, a very simple configuration, a steady state laminar slip flow in a hydro-dynamically fully developed region of a circular micro-tube with an adiabatic wall, is considered. Also all thermo-physical properties of the fluid are assumed to be constant. Theoretical justification to the boundary condition of the energy equation is provided using the discontinuity of the velocity of the slip flow on the wall.

Experimental study on two-phase flow pressure drop during ethanol–water vapor mixture condensation in microchannels

Ethanol-water vapor mixtures condensation shows different flow patterns in microchannels due to the equivalent surface free energy differences. The transition point of streak flow pattern was introduced in the paper which was found closely related to the frictional pressure drop of ethanol–water vapor mixtures condensation flow in microchannels. Characteristics of two-phase pressure drop for ethanol–water vapor mixtures condensation flow in silicon microchannels of streak and smooth annular flow pattern were investigated. Four types of multi-port trapezoidal and triangular microchannels with hydraulic diameter in the range of 126–155 μm were employed. The pressure drop was determined under steam mass flux between 259.2 and 504.8 kg m−2 s−1 when the inlet ethanol weight concentration varied from 1% to 60%. The experimental pressure drop results were compared with existing correlations of flow condensation. The transition point basing on the streak flow pattern of binary vapor condensation was proposed to replace the traditional Reynolds number criterion of the laminar flow state to turbulent flow state for the frictional factor and the C parameter for Kim and Mudawar’s universal pressure drop model, which is in good agreement with the results the experiment results.

Hybrid solutions obtained via integral transforms for magnetohydrodynamic flow with heat transfer in parallel-plate channels

PurposeThe purpose of this study is to show the procedure, application and main features of the hybrid numerical-analytical approach known as generalized integral transform technique by using it to study magnetohydrodynamic flow of electrically conductive Newtonian fluids inside flat parallel-plate channels subjected to a uniform and constant external magnetic field.Design/methodology/approachThe mathematical formulation of the analyzed problem is given in terms of a streamfunction, obtained from the Navier–Stokes and energy equations, by considering steady state laminar and incompressible flow and constant physical properties.FindingsConvergence analyses are performed and presented to illustrate the consistency of the integral transformation technique. The results for the velocity and temperature fields are generated and compared with those in the literature as a function of the main governing parameters.Originality/valueA detailed analysis of the parametric sensibility of the main dimensionless parameters, such as the Reynolds number, Hartmann number, Eckert number, Prandtl number and electrical parameter, for some typical situations is performed.

Three-dimensional printed microfluidic modules for design changeable coaxial microfluidic devices

Coaxial microfluidic devices have been widely used for the preparation of monodisperse droplets and multi-layered hydrogel fibers. However, in the formation of the coaxial microfluidic devices, as the configurations of the coaxial channels become larger and more complicated, the production time is prolonged, the yield is reduced, and there are difficulties associated with rinsing the channels. In this paper, we propose three-dimensional (3D) printed microfluidic modules as connectable platforms for the preparation of coaxial microfluidic devices. Since the microfluidic modules have simple channel configurations, we can easily fabricate them with a 3D printer and rinse their channels after use. The combination of the microfluidic modules allows the formation of various types of coaxial microfluidic devices, such as axisymmetric flow-focusing devices and co-flow devices with specified dimensions for their channels. Therefore, devices comprising these microfluidic modules have succeeded in production of monodisperse droplets and hydrogel fibers with various dimensions and different numbers of layers. Furthermore, as a demonstration of biofabrication applications, we show that the device comprising the microfluidic modules enables us to fabricate a multi-layered cell-laden fiber by gelation of cell-laden collagen solutions in a multi-layered laminar flow state. We believe that the microfluidic modules will be useful for preparing coaxial microfluidic devices for various applications in analytical chemistry, biology, and tissue engineering.

Heat transfer enhancement strategies in a swirl flow minichannel heat sink based on hydrodynamic receptivity

The swirl flow minichannel heat sink has shown to be a promising alternative for thermal management of high heat flux applications, such as electronics and concentrated photovoltaics. Effective heat transfer enhancement strategies for this device are identified by studying the receptivity of temperature disturbances to a momentum forcing input. Steady state laminar flow is calculated numerically and experimental measurements are carried out to validate the results for subcritical Reynolds numbers. Using the framework of nonmodal stability theory, a harmonically driven linear perturbation problem is formulated, and the methodology to apply the local and parallel flow approximations based on order of magnitude arguments is detailed. The input-output response of temperature perturbations to forcing of the radial, azimuthal, and wall-normal momentum components is calculated for a range of wavenumbers, waveangles and temporal frequencies. The largest amplification is presented by streamwise vortices and streaks, followed by axisymmetric inward travelling waves, and then by streamwise propagating waves. Micromachining the channel walls with streamwise spiral grooves is proposed as a heat transfer enhancement technique. Excitation of streamwise independent structures in the wall-normal direction is expected, therefore maximum amplification should be obtained. Due to its simple implementation, we also propose using a pulsating flow rate as a heat transfer enhancement technique. Receptivity results for streamwise propagating waves of radial forcing show a response curve with moderate amplification for a wide range of actuation frequencies. Experimental work is conducted to measure the performance of the swirl flow channel heat sink using flow pulsations at the range of forcing frequencies suggested by the receptivity study. Compared to the unforced case, a lower wall temperature (up to 5°C cooler) was observed with pulsations, at the same imposed heat flux and flow rate. To get the same wall temperature as in the unforced case, a pumping power reduction of up to 26.6% was observed, and using the same pumping power resulted in up to a 10.3% Nusselt enhancement. Hydrodynamic receptivity was successfully used to identify effective heat transfer enhancement strategies, resulting in a significant performance improvement for the swirl flow channel heat sink. This physics based approach can be extended to other techniques, for instance, to select the wavelength of a wavy surface, the periodicity of surface roughness elements, or the frequency of acoustic vibrations.

Sensitivity analysis of geometric parameters upon the aerothermodynamic performances of Mars entry vehicle

Aerothermodynamic environment predictions play an important role in the heatshield design of Mars entry vehicle. This article investigates and presents the influences of geometric parameters of the heatshield on its aerothermodynamic performances. The three-dimensional coupled implicit compressible Reynolds Averaged Navier–Stokes (RANS) equations and perfect gas model with the specified effective specific heat ratio have been applied to numerically simulate the flow fields around the vehicle. Menter’s shear stress transport (SST) turbulence model with compressible correction is implemented to take account of the turbulent effect. The laminar and turbulent heating rates are demonstrated and analyzed in detail. Furthermore, a non-intrusive polynomial chaos (NIPC) method with Latin hypercube sampling (LHS) is utilized to establish the functional relationship between the aerothermodynamics and geometric parameters. In addition, Sobol indices as global sensitivity metrics have been introduced to investigate the relative contribution of each geometric parameter. The results show that for the maximum heat flux, the value of the cone angle (αc) with a high index is the top contributor to the both laminar and turbulent flow state, thus the geometric parameter αc should be considered firstly in the material design process of thermal protection system. Moreover, in the most region of MSL heatshield, cone angle (αc) also became the major influence factor. However, in a relatively small region, aerothermodynamics exhibits a great sensitivity to the change of nose radius (Rn). In all regions of heatshield, the parameter of shoulder radius (Rs) is always at a low level of Sobol index.

Numerical research of stream analysis on helical baffles heat exchangers

In this paper, the numerical simulations for a helical baffles heat exchanger and a segmental baffles heat exchanger with component clearance are performed to reveal the features of leakage streams and their effect on heat exchanger performance. Helical baffles heat exchanger models with four different angles and segmental baffles heat exchanger model were established and calculated via Gambit and Fluent software. The results reveal that the heat exchanger with a 40° helix angle shows the best comprehensive heat transfer performance in turbulent state, and the heat exchanger with a 50° helix angle shows better comprehensive heat transfer performance in laminar flow state. The leakage streams proportion of the helical baffles heat exchanger varies from 5.5% to 6.1%, compared with the leakage streams proportion changes from 16.6% to 21.0% in the segmental baffles heat exchanger. In both turbulent flow state and laminar flow state, with the rise of shell-side Reynolds number, the main spiral stream B proportion decreases and the leakage streams proportion increases in the segmental baffles heat exchanger, while the stream B proportion increases and the leakage streams proportion decreases in helical baffles heat exchanger. The proportion of stream B increases with the increase of the helix angle β and the proportions of leakage streams decrease with the increase of β. The proportion of the tube-baffle leakage stream A increases in radial direction. Both the proportion of stream A and that of the baffle-shell leakage stream E fluctuate in the intermediate zone in axial direction; the stream A proportion decreases at the inlet and outlet zone, while the stream E proportion increases at the inlet and decreases at the outlet zone. The results of this paper could be of great significance in the optimal design and manufacture of the helical baffles heat exchanger.