Search

A single‐row crossflow heat exchanger with concentric and eccentric circular fins is designed and shown in this paper using a constructal design approach. The transverse fins spacing, the spanwise fins spacing, and the fin‐tube eccentricity are varied inside a fixed volume. In addition to the concentric case (ε = 0), two tube‐fin eccentricities are considered with (ε = ±0.25). The (heat transfer/volume) heat density is to be optimized with respect to the transverse and the spanwise spacing. The height and the width of the heat exchanger occupation space in addition to the fin diameter are fixed as design constraints. The crossflow is motivated by constant pressure drop with two dimensionless pressure drop numbers (Bejan number) (Be = 103 and 105). Both the fins and the tubes are maintained at a constant temperature, and they are cooled by the cross air of ambient temperature. The ratio of the fin to tube diameter is kept at 0.5. Three‐dimensional equations for conservation of mass, conservation of momentum, and conservation of energy are solved using finite volume method for incompressible and steady flows. The heat density for concentric fins (ε = 0) and eccentric fins (ε = ±0.25) is maximized two times, one with respect to transverse spacing, and the other with respect to spanwise spacing at Bejan numbers (Be = 103 and 105). The highest double‐maximized heat density is attained at the fin eccentricity (ε = 0.25). The increase in the double maximized heat density when the fin eccentricity (ε = 0.25) is 7.65% for Be = 103, and it is 12% for Be = 105. [ABSTRACT FROM AUTHOR]

Flow maldistribution at the inlet of a heat exchanger (HX) is a significant parameter that needs to be considered to judge the performance of the same. In this paper, four arrangements of three‐fluid cross‐flow heat exchanger (3FCFHX) with different flow conditions are considered as shown in Figure 1 in which is the performance of cross‐countercurrent and cross‐cocurrent arrangements for uniform and flow maldistribution at the inlet. In addition, the effect of different inlet fluid flow models on the HX performance are investigated numerically. Among the three fluids in these arrangements, the central fluid is considered to be the hot fluid. From the principles of conservation of energy, the governing equations for three fluids are generated and solved using the finite element method. Four different inlet fluid‐flow models are considered for the analysis. Performance is judged using hot fluid effectiveness and the number of transfer units for a different range of governing parameters. The effects of inlet flow maldistribution (IFM) are measured using degradation factors. The results show that the performance of cross‐cocurrent arrangement is found to be superior to the cross‐countercurrent arrangement. In addition, the IFM enhances thermal performance. Further, it is determined that the flow maldistribution at the inlet will enhance the hot fluid effectiveness by 4%–4.5% and 1.8%–2% in cross‐cocurrent and countercurrent arrangements, respectively. The results give a thorough insight into the significant concerns involved in the design of such HXs. Application of the finite element method proves the ease of determining the exit temperatures of the fluids in the HX. This approach is indeed time‐saving and gives insights when compared to the CFD approach. [ABSTRACT FROM AUTHOR]

The main disadvantage of crossflow microfiltration processes is the occurrence of fouling, which impedes its application and economic efficiency. Unsteady crossflow, e.g., due to pulsatile or alternating operating mode, can reduce the formation of a deposit layer and therefore prevent strong flux decay and retention of solutes. With this technique, especially processes run at low crossflow velocity or high transmembrane pressure can be significantly improved. The degree of improvement also largely depends on the fouling propensity of the fluid to be filtered. [ABSTRACT FROM AUTHOR]

Resolving the flow field within the voids between particles of reacting granular assemblies is essential to obtain correct conversion rates. Due to the associated computational cost, a high spatial resolution cannot be employed on the device‐scale. A combination of a locally resolving and an averaging method is suggested. Namely, the so‐called blocked‐off method is used to resolve the voids between particles within prescribed regions, while the volume‐averaged method is applied in the remaining domain. For a nitrogen jet injected into a regular simple cubic packing of spherical particles passed by a cross‐flow of air, the jet dispersion is too low when applying the volume‐averaged method for the whole computational domain. The same is also observed in a reacting methane jet flowing into a bed of arbitrarily packed particles with air cross‐flow, where the locally resolved approach predicts significantly larger local reaction rates. [ABSTRACT FROM AUTHOR]

The present paper investigates a three‐dimensional simulation of film cooling on a C3X turbine blade with a single hole at a suction surface. The Reynolds averaged Navier–Stokes approach with k–ε realizable turbulence model and enhanced wall function are used for the numerical simulation. To simulate the jet flows, the length of the jet input approximately 4.5 times the diameter of the hole is added to the geometry so that the jet outlet flow is closer to the actual condition. The density ratio of the cooling flow to the mainstream flow is assumed about 2. The numerical results in four blowing ratios of 0.5, 0.7, 1.0, and 1.4, and at the low turbulence intensity (0.02%), and high turbulence intensity (12%) are extracted and compared for the turbine blade with a single hole. The results show that the turbulence intensity has a dual effect on the film cooling effectiveness and a higher blowing ratio increases the strength of the jet against the cross‐flow. Moreover, it is illustrated that the distribution of the film cooling effectiveness in higher blowing ratios and high turbulence intensity is more uniform than the low blowing ratios and low turbulence intensity. [ABSTRACT FROM AUTHOR]

The maximization of volumetric heat transfer density from radially finned tubes in cross‐flow is investigated in this study based on the constructal design method. A row of radially finned tubes is placed in cross‐air flow. The tubes and the radial fins are heated at uniform temperatures and cooled by the air cross‐flow. The cross‐air flow is generated by a finite pressure difference. Two dimensionless pressure differences (Bejan number) are considered (Be = 103 and Be = 105). The objective function, the degrees of freedom, and the constraints in the constructal design method should be identified. The objective function is the maximization of the heat transfer density from the finned tubes. The degrees of freedom are; the fin tip‐to‐fin tip spacing, the number of fins, the tube diameter, the fin thickness, and the angle between the fins. The constraints are the length and height of the space occupied by the finned tubes. The pressure‐driven flow and energy equations (steady, two‐dimensional, and incompressible) are solved by means of the finite volume method. The ranges of the dimensionless fin tip‐to‐fin tip spacing are (0.2 ≤ S ≤ 1 for Be = 103 and 0.05≤ S ≤ 0.3 for Be = 105). The number of fins is changed as (N = 2, 4, 6, 8, 10, and 12). The dimensionless tube diameter is changed as (D = 0.25, 0.5, and 0.75). The dimensionless fin thickness is changed as (T = 0.001, 0.01, and 0.05). The results showed that for both (Be = 103) and (Be = 105), the highest value of the maximum volumetric heat transfer density is for (N = 2) and decreases as the number of fins increases. In addition, the minimum values of the maximum volumetric heat transfer density occur when the vertical fins exist at (N = 4, 8, and 12). [ABSTRACT FROM AUTHOR]

The steady flow of non‐Newtonian power‐law fluids across a periodic square array of infinitely long circular cylinders is studied numerically using an unstructured finite volume method. The local and global flow characteristics have extensively been explored by the systematic variations of the pertinent dimensionless parameters as follows: fluid volume fraction (ϕf =.70–.99), Reynolds number (Re = 1–40), and power‐law index (n =.4–1.8). Qualitatively, the dense and curved streamlines are seen with the increasing inertial forces and shear‐thinning behavior across all the fluid volume fractions. Further, the pressure coefficient over the surface of periodic cylinders is significantly influenced by the governing parameters and found to be maximum and minimum for the upstream and downstream cylinders, respectively. The individual and total drag coefficients have shown complex dependence on n, ϕf, and Re. For shear‐thinning fluids (n < 1), the pressure drag coefficient dominates over the friction drag coefficient, whereas an opposite response is seen for the shear‐thickening fluids (n > 1) except at Re = 40. Further, both individual and total drag coefficients are observed to increase and decrease in the ranges of ϕf of.70–.90 and.92–.99, respectively, with increasing n. The strong interactions between the periodic cylinders, at smaller ϕf, diminish with a corresponding increase in ϕf. In addition, simple predictive correlations for the pressure, friction, and total drag coefficients have been developed to gain further physical insights into the detailed flow kinematics. Finally, the present findings display a good agreement with the available literature. [ABSTRACT FROM AUTHOR]

The current investigation deals with the numerical simulation of cross‐flow of non‐Newtonian fluid. Flow dynamics are studied by considering a uniform channel bounded by porous walls. Transversely acting magnetic fields have also been taken into account on the two‐dimensional flow of Tangent‐Hyperbolic fluid. Skin friction is addressed by employing the lubrication effects on the porous channel. In addition to this, Fourier's law of conduction is incorporated to highlight the heating effects which attenuates the shear thickening posture of the fluid. Highly nonlinear and coupled differential equations are solved numerically by using the Runge‐Kutta method with shooting technique. Thermal and momentum transport is simulated by generating the numerical MATLAB codes. Study reveals that slippery porous walls/channels can effectively be used in chemical and mechanical industries which deal with various kinds of highly viscous flows. Moreover, more energy is contributed by Peclet number and viscous heating parameter and improve the heat transfer rate. [ABSTRACT FROM AUTHOR]

The flow through asymmetric nonuniform (convergent) channels with the effect of the magnetic field have a pronounced impact in engineering and biological fields such as chemical and food industries, blood flow through capillaries, and arteries, and so forth. With this motivation, the present study focuses on convective hydromagnetic particulate suspension flow in an asymmetric convergent channel under the heat generation effect. The numerical method is applied to solve the nondimensionalized equations governing the transport process of fluid and particle flow and its heat. To check the convergence of the computational results, a grid independence test has been performed. A comparison test has been made to validate the results and an admirable agreement is noticed with published results. Computation results are reported for the influence of emerging parameters on the fluid as well as particle velocity and temperature profiles through graphs and tables. A method of slope linear regression through data points is presented to study the impact of various parameters on skin friction and Nusselt number. The study pioneers the investigation on the significance of the combined influence of cross‐flow Reynolds number and magnetic field on fluid and particle in the convergent channel and also reports its importance on drag coefficient and rate of heat transfer at the walls. It is perceived that a reduction in fluid velocity takes place with an increment in Magnetic parameter, Grashof number, and Reynolds number. An augmentation in fluid temperature is noted with an increment in Prandtl number and heat source parameter. [ABSTRACT FROM AUTHOR]

To verify the applicability of upstream ribs in film cooling, the present numerical study examines heat transfer characteristics and flow field for ribs located upstream of the film hole. Five ribs including bilaterally truncated ribs, centrally truncated ribs, and continuous ribs are explored with the smooth case at two blowing ratios and fixed crossflow Reynolds number. The results show that the film cooling effectiveness of cases with ribs outperforms the case without rib at a low blowing ratio. Centrally truncated ribs and continuous ribs provide superior cooling effectiveness than bilaterally truncated ribs and smooth cases. The introduction of ribs makes the distribution of the heat transfer coefficient (HTC) uneven after the hole. Among these, centrally truncated ribs increased the HTC, while bilaterally truncated ribs reduce the HTC in the far hole area at a high blowing ratio. It is found that anti‐kidney‐shaped vortex pairs are generated between two adjacent jets for centrally truncated rib cases, while they are generated in front of the jets for bilaterally truncated rib cases. For continuous rib, the impingement of the mainstream gas on the jet leads to a reduction in strength of the kidney‐shaped vortex, which allows the coolant to form a better coverage. [ABSTRACT FROM AUTHOR]

A single‐row cross‐flow heat exchanger with wing‐ shaped tubes is designed in this paper using the constructal design method. The wing‐to‐wing spacing and the wing thickness are free to morph while the chord of the wings remains constant. Wing‐to‐wing spacing optimization or heat transfer density maximization from the wings is the objective function in this design. Two directions of the free‐stream (incoming) flow are considered. The right‐ and left‐flow directions are considered with a constant drop in pressure. All wings are heated at a uniform temperature, and the air is used to cool these wings. The two‐dimensional conservation of mass, conservation of momentum, and the conservation of energy equations are solved by means of the finite volume method for steady and incompressible flow. The ratio of the wing thickness to the chord (dimensionless wing thickness) is changed from 0.2 to 1. For each wing thickness, three Bejan numbers (Be = 10³, 104, and 105) are used in the numerical simulation. The results revealed that in the case of the flow direction to the left, the maximum heat transfer density is higher than that in the case of the flow direction to the right for all wing thicknesses and all Bejan numbers. [ABSTRACT FROM AUTHOR]

Many aquatic processes (i.e., organism locomotion, macrophyte response to unsteady flow and mass/heat transfer) involve oscillatory or undulatory movements perpendicular to the direction of flow and perpendicular to the plane of oscillation (i.e., spanwise). The reduction of boundary layer (BL) thickness (δ) and increased turbulence have been identified as the mechanisms for faster heat and mass transfer on these moving surfaces. We examined the hydrodynamics of a submerged spanwise‐oscillating plate in a crossflow within turbulent open channel flow at different oscillation frequencies (f) and channel velocity (U). The BL over the plate was characterized using moving particle image velocimetry to understand the effects of f and U on boundary stress, vorticity, and transfer processes. Changes in the velocity gradient at the plate surface depend on the interactions between spanwise vorticities of opposite directions that reversed downstream along the plate. The vorticity structure reduced shear stress at the surface of the plate resulting in > 65% drag reduction near the upstream edge. Whereas δ was reduced in comparison to a stationary plate, vorticity increased the diffusive sublayer thickness (δDSL) over the oscillating plate (≲1.75 thicker than stationary), which would reduce rather than increase mass transfer based on the film model. Conversely, oscillations would enhance the mass transfer by a factor of ≤ 7 due to periodic renewal of water at the DSL. Similar arguments indicate that renewal would lead to increased convective heat transfer along the plate. Results of this work will increase our understanding of the physical and biological processes that occur in environmental boundary layers. [ABSTRACT FROM AUTHOR]

We apply the method of lines and the backward difference method to solve highly nonlinear governing mean flow equations of the Ekman boundary‐layer flow over a rough disk with a wavy surface profile. These equations are derived using the YHP roughness model that imposes a wavy surface profile in terms of the radial position r, assuming rotational symmetry. Our main interest is to determine the effect of roughness on the local linear convective instability of the Ekman flow, and we solve the local linear stability equations of the disturbances stationary relative to the disk using the Chebyshev polynomials. The crossflow and viscous types of instability modes, the Type I and Type II modes respectively, are analyzed. Our theoretical findings prompts that an adequately designed rough disk can be a useful tool for a passive flow‐control method for the Ekman flow. The underlying physical mechanisms are investigated applying a fundamental energy analysis. [ABSTRACT FROM AUTHOR]

In the current numerical study, the thermal and flow field performance of an array of confined multiple jets with air, water, and water‐Al2O3 nanofluid in the maximum crossflow configuration over the target plate with and without pin fins is investigated. The numerical results are validated with the experimental data; it is found that a reasonable prediction related to heat transfer can be made. For this study, steady‐ state Reynolds‐averaged Navier‐Stokes simulations with the shear‐stress transport k‐ω turbulence model in ANSYS Fluent were performed. The simulations are performed with volumetric concentration ϕ = 0.2% to 3% and the jet's Reynolds number Re = 15 000 to 35 000. In all cases, the jet outlet‐to‐target plate distance Z D/ is 3. It is found that the increase in values of the volumetric concentration of nanoparticles results in a decrease of the Nusselt number and an increase of the convective heat transfer coefficient. This is because there is an increase in thermal conductivity of the working fluid with the increase in the volumetric concentration of nanoparticles for the same Reynolds number. About 81.5% and 89.1% enhancement in the average heat transfer flux values is observed for flat and pin fin‐roughened target plates, respectively, for ϕ = 3%. [ABSTRACT FROM AUTHOR]

A numerical study of a three‐dimensional, turbulent, forced convection flow around a hot cubic block exposed to cross‐flow and an impinging jet is carried out. The standard k‐ε turbulence model is used to study the effects of Reynolds number ratio on the flow and heat transfer. For each value of the Reynolds number of the jet, the Reynolds number ratio is equal to 1, 1.5, and 2. The influence of the channel height and the jet axis location are also examined. The governing equations are solved by using Ansys Fluent software 14.5. Results show that the heat transfer increases with the increase in the Reynolds number ratio. At the top of the cube, better cooling occurs with an increase in the speed of the impinging jet. A reduction in the height of the channel and the displacement of the axis of the jet toward the channel inlet improve the heat transfer. Our simulations are compared with experimental data found in the literature, using different turbulence models. [ABSTRACT FROM AUTHOR]

To commercialize large‐scale and high‐density microalgae cultivation, a 10L cross‐flow flat‐plate airlift photobioreactor (CFA‐PBR) was designed and amplified to two types of 40L CFA‐PBRs via different scaling‐up methods. The multiphase flow characteristics and bioreaction process of these CFA‐PBRs were studied by experiments and computational fluid dynamics (CFD) simulation. The results show that the parameters of CFA‐PBRs, such as bubble size, gas holdup, mass transfer, and photosynthetic reaction efficiency, mainly conform to the principle of geometric similarity under the same light thickness and superficial gas velocity, which is significant for large‐scale industrial applications of the efficient high‐density algae cultivation. [ABSTRACT FROM AUTHOR]

The optimal configuration of two‐scale elliptic tubes in crossflow is found on the basis of the constructal design. The larger tubes are installed inside a domain of fixed length and height. In the same domain, smaller tubes are inserted between the larger tubes in the entrance region at the mid leading edge to leading edge distance of the larger tubes. The spacing between the larger tubes, the semiminor axis of the larger tubes, the major axis of the smaller tubes, and the semiminor axis of the smaller tubes are varied inside the domain freely to find the optimal configuration. There are two optimal configurations: one without the smaller tubes and the other with the presence of the smaller tubes. Both the larger and the smaller tubes are heated at a constant surface temperature. The flow is induced by a fixed pressure difference. The equations for steady, laminar, two‐dimensional, and incompressible flow are solved by finite volume method. In the absence of the smaller tubes, the range of Bejan number (dimensionless pressure drop) is 103

The wet classification of particles < 10 μm is a complex process that has been researched for many years. In this study, the usage of a modified cross‐flow filtration process as a classification process was investigated. With this process, particles in a fine micrometer range can be separated from suspensions. The upper particle size is dependent on hydrodynamic forces. The experimental results were compared with different hydrodynamic force models to predict upper size. The influence of the permeate flux and the particle concentration in the feed on the upper particle size is studied. [ABSTRACT FROM AUTHOR]

Based on the Tesla‐type turbomachinery principle of operation, a fan consisting of flat, round discs arranged in between two separate channels, which generate two air flows in opposite directions, is investigated. Simulations of a model with one disc as well as a five‐channel model at different grids were performed. With almost unthrottled operation, secondary flows could be determined at velocity magnitudes of up to 20 % of the mean main flow velocity, with secondary currents reaching up to 50 % in throttled operation. Besides high dissipation and recirculation, these secondary currencies are found to be capable of reducing the overall efficiency of the system. Thus, topic of further investigations is the potential of increasing efficiency by means of straighteners and geometric adaptions. [ABSTRACT FROM AUTHOR]

Abstract: Membrane solute permeability plays a role in the buildup of concentration polarization in pressure‐driven crossflow filtration processes, and thus in the determination of the permeate flux, solute rejection, retentate flux, and concentration. Reverse‐osmosis desalination with membranes of fixed solvent permeability but of variable selectivity with respect to the solute is numerically examined. The study highlights an intricate coupling between retentate and filtrate properties. In particular, it reveals that, for given values of solute permeability and feed concentration, there is a maximum operating pressure that optimizes solute rejection regardless of the feed salinity. The conditions leading to this and to other peculiar behaviors for permeation fluxes and concentrations are identified. [ABSTRACT FROM AUTHOR]

This study presents the successful application of the bees algorithm ( BA) for optimal design of a cross-flow plate fin heat exchanger by offset strip fins. The ε - NTU method is used to approximate the heat exchanger effectiveness and pressure drop. Two different objective functions including the minimization of total annual cost (sum of investment and operational costs) and total number of entropy generation units for certain heat duty required under given space constraints are considered as targets of optimization separately. Based on the applications, seven design parameters (heat exchanger length at hot and cold sides, fin height, fin frequency, fin thickness, fin-strip length, and number of hot side layers) are selected as optimization variables. Two examples from the literature are presented to illustrate the efficiency and accuracy of the proposed algorithm. Results showed that the BA can detect an optimum configuration with higher speed (short computational time) and accuracy compared to the imperialist competitive algorithm ( ICA) and the genetic algorithm ( GA). © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(5): 427-446, 2014; Published online 3 October 2013 in Wiley Online Library (). DOI 10.1002/htj.21087 [ABSTRACT FROM AUTHOR]

Concentration polarization in narrow reverse osmosis channels with parabolic crossflow was numerically simulated with finite different equations related to permeate velocity, crossflow velocity, average salt concentration, and wall salt concentration. A significant new theoretical development was the determination of two correction functions, F2 and F3, in the governing equation for average salt concentration. Simulations of concentration polarization under various conditions were then presented to describe the features of the new model as well as discussions about the differences of concentration polarizations of the more realistic parabolic flow with those when plug flow or shear flow was assumed. The situations in which the simpler models based on shear or plug flow can be used were indicated. Concentration polarization was also simulated for various conditions to show the applicability of the model and general features of concentration polarization in a narrow, long reverse osmosis channel. [ABSTRACT FROM AUTHOR]

Obtaining pectin through traditional precipitation processes requires a large amount of organic solvent. A reduction in solvent consumption may be achieved by incorporating a cross-flow microfiltration step in which the extraction solution is removed and pectin is concentrated. In this study, we used α-alumina tubular membrane (0.44 μm) in two operation modes: total recycle mode to evaluate the effect of temperature, initial concentration of pectin and transmembrane pressure on the permeate flux and pectin coefficient rejection; and batch mode to evaluate the degree of concentration and loss of pectin. It was observed that pectin coefficient rejection varied from 93.4 ± 0.7% to 97.8 ± 0.3%, and a maximum permeate flux of 238.69 ± 6.48 kg m−2 h−1 with 0.12 MPa at 50 °C and 1.0 g kg−1. Using the optimum conditions, the flux observed at the end of the process was 32.8% lower than the flux in the total recycle mode, enabling a final concentration of pectin in the retained solution of 61.04 ± 0.87% with an average pectin loss in the permeate stream of 8.18%. [ABSTRACT FROM AUTHOR]

Flange spread is one of the most important factors in the production of H-beams with the universal rolling mill. Although some prediction models for flange spread have been proposed, the constants in the model equations should be determined for each product size, which requires much experimental work. In this research, finite element analysis was carried out in various rolling conditions and a technique for deciding the model constants by the analysis was investigated. First, rolling experiments using pure lead were carried out to confirm the correctness of the flange spread model. Finite element analyses were then executed for rolling conditions corresponding to the experiments. The flange spread in the numerical analyses showed very good agreement with the experimental results, confirming the accuracy of the numerical simulation. At the same time, the possibility of determining the model constants by numerical analysis was demonstrated. Other related properties of universal rolling were also investigated with the data from the finite element analysis. Changes in the web and flange thicknesses after exiting the roll gap were quantitatively simulated. The cross-flow behavior between the web and flange was investigated, and the effect of rolling conditions on the cross-flow ratio was obtained. This research demonstrates that finite element simulation is a powerful tool for investigating and modeling deformation in H-beam universal rolling. [ABSTRACT FROM AUTHOR]