Global Pressure Drop Research Articles

Constructal design of a comb-like channel network for self-healing and self-cooling

The present work shows how to minimize the global pressure drop in a comb-like channel network with self-healing and self-cooling functionalities implementing the concept of constructal design. A thick channel distributes the fluid flow to many thin channels that are perpendicular to the thick channel. The flow regime throughout the network is assumed to be fully developed laminar flow with negligible local losses. We systematically investigated the degrees of freedom of the fluid channel network, and determined the optimal internal and external aspect ratios of the flow architecture such that the total pressure drop is minimum.

Experimental and Numerical Investigation of the Gas Side Heat Transfer and Pressure Drop of Finned Tubes—Part II: Numerical Analysis

The heat transfer and pressure drop behavior of segmented circular and helical as well as solid finned tubes are investigated in a three-dimensional numerical study. The simulation is carried out using a finite volume method for calculating the steady-state temperature and flow field of the fluid as well as the temperature distribution of the tube material. For modeling the turbulence, the k-ε turbulence model based on the renormalization group theory (RNG) is used to resolve the near-wall treatment between adjacent fins. All simulations are performed in the Re range between 3500 ≤ Re ≤ 50,000. The influence of Reynolds number and fin geometry (segmented or solid and circular or helical) on the local and global averaged heat transfer and pressure drop was studied. A comparison between solid and segmented finned tube has shown that the heat transfer and pressure drop for the segmented finned tubes is higher. The numerical results are compared with experimental data.

관군 배열에서의 종간 간격이 열전달에 미치는 영향에 대한 수치 해석적 연구

In this study, a longitudinal pitch effect on in-line tube bank heat transfer has been analyzed numerically. To verify the accuracy of the solver model and boundary conditions, global Nusselt number(Nu) and pressure drop across the 2 row tube bank are compared with the existing experimental correlations under 500 ~ 2,000 Reynolds number(Re) range. By changing transverse pitch(<TEX>$S_T$</TEX>) or longitudinal pitch(<TEX>$S_L$</TEX>) separately in tube bank, we're trying to identify the each effect on heat transfer. We found that the effect of transverse pitch can be accounted for Reynolds number evaluated with maximum velocity(<TEX>$V_{max}$</TEX>) at the smallest flow area similar to most existing correlations. Variation of the longitudinal pitch(<TEX>$S_L$</TEX>) has a greater impact on the heat transfer compared to the transverse pitch(<TEX>$S_T$</TEX>). Overall Nusselt number increases with larger longitudinal pitch(<TEX>$S_L$</TEX>), however individual Nusselt number of the tube row has significant difference after the first row.

Slope effect on core-annular flow

To facilitate the flow of heavy viscous oils in a pipe, a water-lubricated transport is generally used. The water migrates into the regions of high shear at the pipe wall where it lubricates the flow. The pumping pressures are balanced by wall shear stresses in the water, the process therefore requires pressures comparable to pumping water alone, with no dependence on the viscosity of the oil. This means that significant savings in pumping power can be derived from this process, provided that it is well monitored. Indeed, the flow of a water/oil mixture in a pipe has two main characteristics. First, the fluids can adopt different spatial arrangements called flow regimes, and second, the presence of a water layer at the channel wall significantly reduces the global pressure drop. In this paper, an experimental investigation was performed on the effect of pipe slope and fluids flow rates on flow regimes, pressure drop and interfacial instability.

Experimental Investigation on Pressure Drop and Heat Transfer Characteristics of Copper Metal Foam Heat Sink

The effect of the cooling performance of a copper metal foam heat sink under buoyancy-induced convection is investigated in this work. Experiments are conducted on copper metal foam of 61.3% porosity with 20 pores per inch. The pressure drop experiment is carried out to find the permeability and foam coefficient of the porous media. It is found that the property of porous media changes by changing the angle of inclination of the porous media from a horizontal to a vertical position while keeping the orientation and porosity the same. The Hazen-Dupuit Darcy model is used to curve-fit the longitudinal global pressure drop versus the average fluid speed data from an isothermal steady-flow experiment across the test section of the porous medium. The study concludes that the permeability and foam coefficient for copper foam is found to be 1.11 × 10−7 m2 and 79.9 m−1, respectively. The heat transfer study shows that the thermal performance of copper metal foam is 35–40% higher than the conventional aluminum metal heat sink under an actual conventional mode.

Coexistence of Solidlike and Fluidlike States in a Deep Gas-Fluidized Bed

Characterizing regime transition in gas-fluidized beds is of fundamental importance for the successful applications of fluidization technology. In this study, we show that a state-of-the-art two-fluid model has the ability to correctly predict the transition from packed bed to fully bubbling fluidized beds. To this end, we have studied a deep gas fluidized bed, in which the process of transition is shown much more clearly (see Figure 1) compared to relatively shallow fluidized beds. Particularly, we show that in the process of regime transition, the solidlike and fluidlike states can coexist. We conclude this on the basis of visual observation, global bed pressure drop signals, local solid volume fraction signals, cross-sectional averaged axial granular temperature profiles, and radial particle velocity profiles. The presence of the coexisting phases can be easily understood from the fact that the gas phase is compressible.

Flow simulation in a complex fluidics using three turbulence models and unstructured grids

PurposeThe purpose of this paper is to simulate the behaviour of the symmetrical turn‐up vortex amplifier (STuVA) to obtain insight into its maximum through‐flow operation within the eight‐port STuVA, and understand the relation between its design parameters and flow characteristics. Furthermore, it is to test the performance of different turbulent models and near‐wall models using the same grid, the same numerical methods and the same computational fluid dynamics code under multiple impingement conditions.Design/methodology/approachThree turbulence models, the standard k‐ε, the renormalization group (RNG) k‐ε model and the Reynolds stress model (RSM), and three near‐wall models have been used to simulate the confined incompressible turbulent flow in an eight‐port STuVA using unstructured meshes. The STuVA is a special symmetrical design of turn‐up vortex amplifier, and the simulation focused on its extreme operation in the maximum flow state with no swirling. The predictions were compared with basic pressure‐drop flow rate measurements made using air at ambient conditions. The effect of different combinations of turbulence and near‐wall models was evaluated.FindingsThe RSM gave predictions slightly closer to the experimental data than the other models, although the RNG k‐ε model predicted nearly as accurately as the RSM. They both improved errors by about 3 per cent compared to the standard k‐ε model but took a long time for convergence. The modelling of complex flows depends not only on the turbulence model but also on the near‐wall treatments and computational economy. In this study a good combination was the RSM, the two layer wall model and the higher order discretization scheme, which improved accuracy by more than 10 per cent compared to the standard k‐ε model, the standard wall function and first order upwind.Research limitation/implicationsThe results of this paper are valid for the global pressure drop flow rate. It should be desirable to compare some local information with the experiment.Originality/valueThis paper provides insight into the maximum through‐flow operation within the eight‐port STuVA to understand the relation between its design parameters and flow characteristics and study the performance of turbulence and near wall models.

Simulations of extensional flow in microrheometric devices

We present a detailed numerical study of the flow of a Newtonian fluid through microrheometric devices featuring a sudden contraction–expansion. This flow configuration is typically used to generate extensional deformations and high strain rates. The excess pressure drop resulting from the converging and diverging flow is an important dynamic measure to quantify if the device is intended to be used as a microfluidic extensional rheometer. To explore this idea, we examine the effect of the contraction length, aspect ratio and Reynolds number on the flow kinematics and resulting pressure field. Analysis of the computed velocity and pressure fields show that, for typical experimental conditions used in microfluidic devices, the steady flow is highly three-dimensional with open spiraling vortical structures in the stagnant corner regions. The numerical simulations of the local kinematics and global pressure drop are in good agreement with experimental results. The device aspect ratio is shown to have a strong impact on the flow and consequently on the excess pressure drop, which is quantified in terms of the dimensionless Couette and Bagley correction factors. We suggest an approach for calculating the Bagley correction which may be especially appropriate for planar microchannels.

Induced pulsing in trickle beds — Particle shape and size effects on pulse characteristics

Periodic liquid feeding of the ON–OFF type is investigated — at sufficiently high frequencies to be classified as “fast” mode of induced pulsing — in the range of mean gas and liquid flow rates corresponding to the steady “trickling flow” regime. Two of the most common types of catalyst-support particles, i.e. porous spherical and cylindrical extrudates, are employed to study the imposed pulse characteristics. Detailed information is obtained, on the axial propagation and attenuation of pulses, from instantaneous, cross-sectionally averaged holdup measurements. Key fluid-mechanical parameters studied include, aside from dynamic holdup and pressure drop, pulse celerity and intensity, as a function of fluid feed rates ( G , L ) and liquid cyclic frequency. Similar published data, for 6 mm glass spheres, are employed for comparison; it is shown that, for the particles examined, particle size has a pronounced effect, but not as significant as that of particle shape. For particles of comparable size, the cylindrical shape is associated with much greater global dynamic holdup and pressure drop, and with increased pulse attenuation. Moreover, packed extrudates exhibit a significant increase of holdup in the axial direction, recently also observed in steady trickling flow. For spherical particles, both time-average holdup and pulse celerity are practically constant along the bed for fixed L , G . Pressure drop, global holdup and pulse celerity are not affected by cyclic liquid feeding frequency, for both spherical and cylindrical extrudate particles. Based on the pulse attenuation characteristics, for the three particle types examined, recommendations are made on preferred conditions for induced pulsing (from the fluid dynamics point of view) which would maximize benefits. Overall, it appears that spherical packings hold significant advantages over cylindrical extrudates of comparable size. Finally, in view of the observed significant decay of imposed pulses along the bed, care should be exercised to properly interpret data obtained in short laboratory reactors (where pulse attenuation is limited) for scale-up of the much longer industrial beds.

Experimental investigation of water flow in smooth and rough silicon microchannels

Water flow in smooth rectangular cross-section microchannels and microchannels with discrete rectangle roughness elements was experimentally investigated. The microchannels were microfabricated on silicon wafers and the hydraulic diameter ranged from 153 µm to 191 µm. The rectangle roughness elements mounted on the sidewall of the microchannel were 50 µm high and 50 µm wide. The global pressure drop and velocity field were measured for Reynolds numbers between 100 and 2300. For the smooth rectangular microchannels, the friction factor of the laminar flow agreed with the value predicted by classical theory. The transition from laminar to turbulent flow occurred at about Re = 2100. For the microchannels with roughness elements, the friction factor approaches the conventional value for Re < 900 and is higher than the conventional value when Re > 900. It indicates that the presence of roughness elements leads to the early transition in the microchannels. The streamwise mean velocity profiles and turbulent intensities obtained from microscopic particle image velocimetry (micro-PIV) show that the transition from laminar flow to turbulent flow occurred for Reynolds numbers ranging between 900 and 1100. This result was consistent with the experimental data of the friction factor. The experimental results of flow fields also suggest that the flow became fully developed turbulence for Re > 1400 in the rough microchannel. In the microchannel with roughness elements, large scale coherence structures are observed in the flow fields for Re < 2000.

Flow characteristics in a trapezoidal silicon microchannel

In this paper, global pressure drop and velocity field were experimentally investigated for water flow in a trapezoidal silicon microchannel with a hydraulic diameter of 237 µm. Microscopic particle image velocimetry (micro-PIV) is used to obtain the velocity profiles at different locations of the microchannel for Reynolds numbers between 50 and 2800. For Re > 700, Δp–Re relationship deviates from the linear behavior and the deviation depends on the Re. The experimental results of the velocity field show that the deviation is due to the entrance effect in microchannel, and the entrance length in our experiments can be predicted byLe/Dh = (0.08–0.09)Re. Velocity profiles and root mean square values of the fluctuating velocity along the centerline obtained by micro-PIV indicate that the transition from laminar flow to turbulent flow occurs for Reynolds number ranging between 1500 and 1800. This result is consistent with the global measurements of the pressure drop.

Use of non-crystallising particles to mitigate crystallisation fouling

Fouling in heat exchangers is of paramount importance considering its impact on the overall performance of a process plant. Fouling can be controlled by using different methods that affect the deposition rate and/or the removal rate. The use of larger particles (of the order of several millimetres) in fluid bed heat exchangers has been quite successful for heat transfer with severely fouling fluids. In this study, we will investigate the role of much smaller (micron size), non-crystallising particles on crystallisation fouling in a heat exchanger. This will be done by measuring global fouling resistance and pressure drop across the heat exchanger. The smaller particles have been found to be effective in mitigating the crystallisation fouling substantially by lowering the deposition rate and by increasing the removal rate.

General Model for Plate Heat Exchanger Performance Prediction

A general heat transfer and pressure drop model is presented for plate heat exchangers. For many years we noticed a strong advancement in the field of plate heat exchangers. These developments were due to new materials, manufacturing technology, and chevrons configurations, which make the heat exchanger more and more compact. But to determine the heat transfer and pressure drop performances, we still need empirical correlations established as a function of geometric parameters variations, like chevron angle, amplitude, and pitch. The new model presented in the paper, called the universal model, is based on the theory of porous media and includes the simultaneous influence of 11 geometric parameters of the plate. Two unique correlations for single-phase flow are proposed to simulate the heat transfer coefficients and friction factors as well as the global heat load and pressure drop for the wide range of Reynolds numbers. The comparison with many experimental results shows the accuracy of the model.

Relation between pressure and fractional flow in two-phase flow in porous media.

We study average flow properties in porous media using a two-dimensional network simulator. It models the dynamics of two-phase immiscible bulk flow where film flow can be neglected. The boundary conditions are biperiodic, which provide a means of studying steady-state flow where complex bubble dynamics dominate the flow picture. We find fractional flow curves and corresponding pressure curves for different capillary numbers. In particular, we study the case of the two phases having equal viscosity. In this case we find that the derivative of the fractional flow with respect to saturation is related to the global pressure drop. This result can also be expressed in terms of relative permeabilities or mobilities, resulting in an equation tying together the mobilities of the two phases.

Modified Hazen-Dupuit-Darcy Model for Forced Convection of a Fluid With Temperature-Dependent Viscosity

We investigate numerically the global pressure-drop of fluids with temperature dependent viscosity, flowing through a porous medium channel bounded by two parallel isoflux surfaces. By reviewing the development of the Hazen-Dupuit-Darcy (HDD) equation we bring to light the inappropriateness of the model in estimating the global pressure-drop of fluids with temperature dependent viscosity. Albeit this observation, we tested the accuracy of the HDD model in comparison with numerical results by using three alternatives, namely (1) fluid viscosity determined at the average bulk temperature, (2) fluid viscosity determined at the log-mean bulk temperature and (3) fluid viscosity replaced by a channel-length averaged fluid viscosity. The HDD model is inadequate because the temperature dependent fluid viscosity surprisingly affects both, viscous and form, global drag terms. We propose and validate a new global model, which accounts for the effects of temperature dependent viscosity in both drag terms of the original HDD model. Based on our new model, two regimes are discovered as the surface heat flux increases. In the first regime both drag terms are affected, while in the second regime only the form drag term is affected, prior to the model reaching an inviscid limit. Predictive empirical relations correcting the viscous and form drag terms, complementing the new model, are obtained as functions of the surface heat flux.

Liquid Distribution in Trickle-Bed Reactor

The aim of this study is to develop techniques to qualify the efficiency of liquid distribution in trickle-bed reactors, using cold mockups. The experimental setup consists mainly in a 0. 3-m-ID packed-bed column with three different plates used to vary the quality of inlet liquid distribution. Liquid distribution has been qualified using several techniques: global pressure drop measurements, global RTD (Residence-Time Distribution) of the liquid, local heat transfer probes, capacitance tomography, collector at the bottom of the reactor with nine equal zones. The bed pressure drop and the overall external liquid saturation decrease when the maldistribution increases; quantitative information is however difficult to obtain this way. Global RTD of the liquid allows quantifying of the average liquid distribution in the bed. The local thermal sensors give an indication of local liquid velocity and indicate possible local maldistribution of the liquid (scale mm) even when global distribution is good. Concerning the results obtained with the collector, a maldistribution index is defined, ranging from 0 (ideal distribution) to 1 (worst possible distribution), and the influence of the different operating parameters (gas and liquid velocities, particle shape) is discussed.

Two-Phase Flow Pressure Drop in Right Angle Bends

Gas-liquid two-phase bubbly flows in right angle bends have been studied. Numerical predictions of the flow in right angle bends are made from first principles using an Eulerian-Eulerian two-fluid model. The flow geometry includes a sufficiently long inlet duct section to assure fully developed flow conditions into the bend. The strong flow stratification encountered in these flows warrant the use of Eulerian-Eulerian description of the flow, and may have implications for flow boiling in U-bends. The computational model includes the finer details associated with turbulence behavior and a robust void fraction algorithm necessary for the prediction of such a flow. The flow in the bend is strongly affected by the centrifugal forces, and results in large void fractions at the inner part of the bend. Numerical predictions of pressure drop for the flow with different bend radii and duct aspect ratios are presented, and are in general agreement with data in the literature. Measurements of pressure drop for an air-water bubbly flow in a bend with a nondimensional bend radius of 5.5 have also been performed, and these pressure drop measurements also substantiate the computations described above. In addition to the global pressure drop for the bend, the pressure variations across the cross section of the duct that give rise to the fluid migration (due to centrifugal forces), and stratification of the phases are interesting in their own right. [S0098-2202(00)01004-X]

Particle fouling of a rotating membrane disk

Fouling of rotating disk membranes as function of operating conditions and feed suspension characteristics was investigated in a series of laboratory experiments. A single-disk laboratory unit was operated over a range of disk rotation speeds and permeation rates (as controlled by the global transmembrane pressure drop) that were calculated to favor various conditions of particle transport and deposition. Permeate flux was found to be relatively insensitive to particle concentration in the feed stream. Thus, this membrane configuration appears to be well suited to the treatment of suspensions at high concentrations. However, the interplay between appropriate operating conditions to maintain permeate flux and the size of particles in the feed suspensions must be taken into account. Fouling was found to decrease with rotation rate and increase with transmembrane pressure (and therefore the initial permeate flux). Reductions in fouling observed at higher rotational speeds are attributed primarily to a high centrifugal force and radial component of drag on particles near the membrane surface. However, a trade off exists between the generation of high shear rates and centrifugal accelerations via high rotation rates and the radial distribution of the transmembrane pressure drop across the membrane that may locally reduce or reverse the flow of permeate across the membrane.

Numerical study of a low permeability microporous heat sink for cooling phased-array radar systems

Microporous media is being used to develop an improved forced convection cold plate device for removing waste heat from high frequency phased-array radar apertures. The waste heat, generated by transmit and receive microwave functions mounted in separate electronic modules, is conducted to the surfaces of a thin rectangular enclosure (cold plate) through which coolant flows. The performance of the phased-array radar is known to deteriorate very rapidly when the difference in operating temperatures of identical electronic components within each module increases. The cold plate device investigated here, designed to minimize this temperature difference, consists of a microporous layer placed (brazed) within the cold plate. A theoretical transport model is developed around an extended system of two-dimensional equations obtained by considering a thin enclosure and integrating the original three-dimensional equations along the direction of smaller dimension. Thermo/hydraulic characteristics are obtained through numerical simulations considering a low permeability aluminum alloy porous layer, and air, water and PAO as coolants. A theoretical estimate of the global pressure drop across the cold plate is also obtained and compared with the numerical results. The microporous cold plate provides substantially more uniform operating temperature for identical components in all module housings than a cold plate without porous layer. Results also suggest an increased global heat transfer coefficient reducing the operational (junction) temperature of the electronics for the same waste heat.

Modeling of Twofold Flame Behavior of Ceramic Foam Surface Burners

A model for the flame behavior of a ceramic foam surface gas burner is developed. The burner plate is divided into small segments. Each segment is assumed to behave independently and the transport phenomena in each segment are assumed to be one-dimensional. Relations between the local gas velocity and global pressure drop are derived. The model predicts the pressure drop over the hot burner plate and the radiating fraction of the plate reasonably well.