Convective Modes Research Articles

Effect of pretreatments on drying of red dacca in a single slope solar dryer

AbstractPretreatment of fruits normally increases moisture loss and reduces the overall drying period. The combination of pretreatments and drying assures the least color difference, excellent preservation, and better antioxidant activity. Pretreatment of Red Dacca is soaked in four different solutions, namely honey, lemon juice, sugar syrup, and ascorbic acid, for 10 min. The best solution for pretreatment is investigated among the selected samples. The results indicated that dipping the red dacca in ascorbic acid and lemon juice had a higher impact on moisture loss. The moisture content of the red dacca control samples reduced from 74 to 14% on a wet basis within 8 hr, whereas pretreated red dacca reaches an equilibrium moisture content of 14% within 6 hr. The ascorbic acid dipped samples have shown the best drying rate. The lemon juice samples have the best sensory appeal, whereas honey samples have the best taste over the other samples. The effect of pretreatment on drying characteristics is experimentally investigated on a single slope dryer in both natural and forced convection mode. Pretreated red daca drying is investigated for the first time and the enhancement in performance is compared with three kinds of drying modes.Practical applicationsDue to a lack of processing facilities, the farmers have to sell their products at very low prices. Red dacca is a popularly consumed fruit and it contains various nutritional and health benefits to humans. The drying process is playing an important role in heat‐sensitive product dehydration. Solar drying is a better choice to reduce the loss and produce good quality red dacca after harvesting. In this research, the best drying and pretreatment method for red dacca is identified from existing literature. The finding obtained from this research work will help to increase the shelf life of food samples by reducing the overall drying time without altering the dryer design.

Application of Cylindrical Fin to Improve Heat Transfer Rate in Micro Heat Exchangers Containing Nanofluid under Magnetic Field

In this study, the convective mode heat transfer phenomena of bi-phase elasticoviscous (non-Newtonian) nanofluid is quantified by forcefully flowing it through a specially designed microchannel test section. The test section, which is rectangularly cross-sectioned and annexed internally with cylindrical needle ribs is numerically investigated by considering the walls to be maintained at a constant temperature, and to be susceptible to a magnetizing force field. The governing system-state equations are numerically deciphered using control volume procedure and SIMPLEC algorithm. With the Reynolds number (Re) varying in the turbulent range from 3000 to 11,000, the system-state equations are solved using the Eulerian–Eulerian monofluid Two-Phase Model (TPM). For the purpose of achieving an apt geometry based on the best thermo-hydraulic behavior, an optimization study must be mandatory. The geometry of the cylindrical rib consists of h (10 × 10−3, 15 × 10−3, 20 × 10−3), p (1.0, 1.5), and d (8 × 10−3, 10 × 10−3, 12 × 10−3), which, respectively, defines the height, pitch, and diameter of the obstacles, with the dimensions placed within the braces being quantified in mm. The results demonstrated that the magnetic field leads to an enhanced amount of average Nusselt number (Nuav) in contrast with the occurrence at B = 0.0. This is due to the that the magnetic field pushes nanoparticles towards the bottom wall. It was found that B = 0.5 T has the maximum heat transfer compared with the other magnetic fields. The channel with h = 15 μm height leads to the maximum value of Nuav at all studied Re for constant values of d and h. The channel with p = 1.5 μm results in the maximum value of Nuav at all studied Re for constant values of d and h. The microchannel with d = 8 μm, p = 1.5 μm, and h = 15 μm in the presence of the magnetic field with B = 0.5 T is the best geometry in the present work.

Effect of evaporator numbers on water production of a free convection air-water harvester

An experimental study was performed to determine the effect of the number of evaporators on the water production and coefficient of performance of an air-water harvester. Water was obtained on the evaporator walls in free convection and condensation heat transfer modes. The evaporator dimension was 480 mm × 285 mm × 6.25 mm, and it was made of copper pipes. The evaporators were installed in a parallel arrangement in an open box (top and bottom open) with a size of 500 mm × 500 mm × 500 mm. The refrigerant R134a was used as the working fluid. The experiments were performed at a low pressure of 40 psi and a high pressure of 180 psi. In the outside evaporator walls, air flowed naturally and part of the water vapour in the air condensed. Three evaporator variations were tested: Case A (one evaporator), Case B (two evaporators), and Case C (three evaporators). Each evaporator contained 25 copper pipes and the length of each pipe was 285 mm. The maximum water production and the evaporator efficiency attained were 0.51 L a day and 13 % respectively. The COP of the machine was ranging from 5.2 to 13.3. Increasing the evaporator number elevated the water production.

Thermal performance investigation of a novel heating terminal integrated with flat heat pipe and heat transfer enhancement

Comfort heating with fast thermal response rate, which is applicable for intermittent heating, requires of small thermal inertia and a large heat transfer coefficient for heating terminals. However, existing heating terminals are generally inconsistent in terms of the response rate and thermal comfort. Therefore, a novel radiant-convective heating terminal integrated with a flat heat pipe and heat transfer enhancement is proposed in this research. The flat heat pipe, which acts as the radiant panel, provides quick thermal response and high radiation uniformity. Additionally, fins are attached to the back of the flat heat pipe to enhance the heat transfer ability via both natural and forced convection. The heating performance, including the response rate, radiation temperature, heating power was studied experimentally, and influential factors, such as heat source temperature, ambient temperature, and forced convection, were also considered. The results suggested that the heating terminal had a response rate of 920–1125 s, which is superior to other conventional radiant terminals, and the surface temperature distribution exhibited high uniformity. Furthermore, the heating power in the natural convection mode was approximately 498–724 W at the heat source temperature of 50 °C. When the forced convection was activated, the heating power and exergy efficiency increased by up to 62% and 25% respectively. Overall, the novel flat heat pipe heating terminal showed great potential for intermittent heating applications.

The Horton–Rogers–Lapwood problem in a Jeffrey fluid influenced by a vertical magnetic field

In this work, the impact of a magnetic field on the onset of the Jeffrey fluid convection through a porous medium is investigated theoretically. The layer of Jeffrey fluid is heated from below and is operated by a consistent upright magnetic field. Using the normal mode procedure, a dispersion equation is obtained analytically and this dispersion relation is utilized to derive the critical conditions for the onset of stationary and oscillatory patterns of convection. The results reveal that the stability of the system diminished with the augmentation of the Jeffrey parameter, while an opposite result is obtained with magnetic field parameters (magnetic Chandrasekhar–Darcy number and magnetic Prandtl number). The size of convective cells decreases with Jeffrey and magnetic field parameters. It is also found that the existence of a magnetic field indicates the possibility of the survival of the oscillatory mode of convection.

Spatiotemporal correlation and control of wall pressure fluctuations induced by subsonic cavity flows

Strong wall pressure fluctuations are generated mostly by a high-speed flow passing over a cavity and should be accurately predicted and controlled for the structural design of aircraft. In this work, a spatiotemporal analysis was performed to estimate wall pressure fluctuations and reveal the mechanism of flow control by a sawtooth-like vortex generator installed on the leading edge or by the use of a rounded trailing edge in suppressing wall pressure fluctuations. Both the cavity tones and overall sound pressure level were analyzed under different flow speeds and flow control approaches. To investigate the power transfer in the temporal and spatial domains, power spectral density contours were drawn and discussed. For the mode separation, the phased array technique with an improved beamforming algorithm was adopted to calculate the wavenumber map and then to separate the acoustic modes from the convective modes by utilizing the spatial correlation of the wall pressure fluctuations. The mode separation results show that the convective modes contribute mainly to the low-frequency pressure fluctuations, while the acoustic modes contribute to both the low-frequency and high-frequency pressure fluctuations. Flow control by the rounded trailing edge approach is more effective than that by the serrated structure approach in reducing the cavity acoustic environment, which enables wall pressure fluctuations to transfer from the high-frequency range to the low-frequency range compared with the “no control” case.

RETRACTED ARTICLE: Modeling of MHD influence on convection of nanomaterial utilizing melting effect

Flow between two rotating plates with inclusion of nanomaterial was scrutinized in current article. The below plate is not only the melting surface but also it is stretching sheet. Two-phase approach for nanomaterial leads to adding new scalar for concentration and extra terms as source terms in energy equation. Inclusion of magnetic force as well as centrifugal force affects this system. Final equations have one variable (η) and solved via RK4 to achieve four parameters. Active parameters affect the velocity distributions and velocity boundary layer and these influences were shown in variation of Cf. Lorentz force has direct relation with surface tension and Cf for M = 6 is 1.2 times higher than that of M = 0. Impact of this external force grows with increase of R and δ while it declines with rise of Kr. Growth of Kr from 1 to 3 offers augmentation in Cf about 0.12% and this percentage reaches to 0.07% when M = 6. Cf declines about 1.59% with rise of δ when Lorentz force is zero. All factors reduce the negative impact of M except for R which provides greater negative effect. As M augments from 0 to 6, Nu reduces by 0.67% and 2.08% when R = 0.2 and 0.8, respectively. Nu augments about 16.9% with rise of δ when M = 6. Both Nt and Nb have positive impacts on nanomaterial convective mode and Nu increases with augment of these factors. With growth of Kr, Nu declines about 0.19% and this impact reduces as magnetic force declines.

Mantle overturn and thermochemical evolution of a non-plate tectonic mantle

We use coupled thermomechanical and thermodynamic modelling to investigate the evolution of a heating mantle below a stagnant lithosphere that leads to overturn events as proposed for Venus. Eclogitized crust entrenched into the upper mantle accumulates in a gravitational trap at the lower-upper-mantle boundary, where basaltic compositions display an intermediate density between upper and lower mantle. While the convecting upper mantle remains at a stable mantle potential temperature, the lower mantle progressively heats. Over 900 Ma, the basalt-rich layer develops into a thermal boundary layer with an offset in potential temperatures as high as 300°C. Occasional plumes breaching the layer from below cause local increased magmatism at the surface but do not induce complete mantle overturn. Once the basalt-rich layer, however, reaches a critical thickness, so that the crust in its lowermost part experiences a transition into assemblages denser than lower mantle a runaway process is initiated. The entire crustal-rich layer is dragged into the lower mantle, switching the convective mode to whole mantle convection. Fertile, superheated lower mantle replaces the depleted cold upper mantle and magmatic production increases by orders of magnitude for ∼50 Ma. During this period, crustal production-rates are so high that not only a new surface in generated but also the delamination rate of lower crust into the upper mantle is increased dramatically with the hot low-viscosity crust now forming much larger delaminating bodies. About 50 Ma after the onset of the overturn, the magmatic production subsides and crust starts to accumulate at the lower-upper mantle boundary again. The overturn event creates a mantle that is chemically heterogeneous and intensely foliated at the model-resolution scale with intermingled crust, depleted and undepleted mantle. The phase-changed induced collapse of the basalt-layer and subsequent global overturn happen at mantle temperatures considerably lower than predicted by previous models for Venus.

Numerical Study on Natural Convection of Alumina-water Nanofluid in a Square Cavity with Two Localized Heat Sources on Adjacent Surfaces

In the present work, results of a numerical study carried out using finite volume method, to investigate the fluid flow and heat transfer characteristics of Alumina ( Al2O3 ) nanoparticles in the base fluid (water) in a square cavity under natural convection mode are presented. The Semi Implicit Method for Pressure Linked Equations (SIMPLE) algorithm was used to solve the discretized momentum and energy equations. Constant temperature heat sources of same strength are placed on bottom and left vertical surfaces whereas the right surface was kept cold, while the top surface was maintained as adiabatic. The impact of Rayleigh number (RaN) ( 1000 to 106 ) and nanoparticles volume fraction (Φ = 0 %, 5 %, 10 %, 15 % and 20 %) on fluid and heat flow characteristics were numerically investigated and presented in the form of streamlines, isothermal lines, mid line horizontal and vertical velocity components, local Nusselt number ( Nuloc ) and average Nusselt number ( Nuavg ). The obtained results indicate, for lower RaN ( i.e; 103 ), conduction dominates over convection near heated surfaces and results in lower fluid velocities and poor heat transfer. For higher values of RaN ( RaN = 105 and 106 ) and volume fraction of nanoparticles, there was a significant increase in mid horizontal and vertical velocity components, Nuloc and Nuavg due to increase in convective heat transfer and thermal conductivity of nanofluid.

On trigonometric cosine, square, sawtooth, and triangular wave‐type rotational modulations on triple‐diffusive convection in salted water

AbstractThe effect of sinusoidal (trigonometric cosine) and nonsinusoidal (square, sawtooth, and triangular) rotational modulations on triple‐diffusive convection in a Newtonian fluid is studied in this paper. The derived Ginzburg–Landau equation for the stationary mode of convection is used to quantify heat and mass transport. The enhancement of heat and mass transport in water due to the addition of different salts is explained in terms of the thermophysical properties of the fluid and the aqueous solutes. The flow equations are reduced to a Lorenz‐type scheme of nonlinear evolution equations using a truncated Fourier series. The equations describing the growth of convection amplitudes are solved using the multidomain spectral collocation method. With the aid of thermophysical properties of the fluid, numerical computations are performed for different values of the revised Rayleigh number (), Taylor number (), the amplitude of modulation , and modulation frequency . A comparison of heat and mass transport in different combinations of aqueous solutions is made. The study reveals that all modes of rotational modulation can control the rate of heat and mass transport. It is noted that amongst all other modulations, the square wave modulation is the most destabilizing one.

Selection of a turbulence model to calculate the temperature profile near the surface of VVER-1000 fuel assemblies in the NPP spent fuel pool

The paper considers the problem of selecting a turbulence model to simulate natural convection near the surface of a VVER-1000 fuel assembly unloaded from the reactor by computational fluid dynamics (CFD simulation) methods. The turbulence model is selected by comparing the calculated data obtained using the Ansys Fluent software package with the results of experimental studies on the natural convection near the surface of a heated vertical plate immersed in water, which simulates the side face of the VVER-1000 fuel assembly in a first approximation. Two-parameter semi-empirical models of turbulence, k-ε and k-ω, are considered as those most commonly used in engineering design. The calculated and experimental data were compared based on the excessive temperature of the plate surface and the water temperature profiles in the turbulent boundary layer for convection modes with a Rayleigh number of 8∙1013 to 3.28∙1014. It has been shown that the best agreement with experimental data, with an average deviation not exceeding ~ 8%, is provided by the RNG k-ε model which is recommended to be used to simulate natural convection near the surface of VVER-1000 FAs in the NPP spent fuel storage pool.

Application of compositional data analysis to determine the effects of heating mode, moisture status and plant species on pyrolysates

Pyrolysate gas mixtures are multivariate and relative in nature. Statistical techniques applied to these data generally ignore their relative nature. Published data for permanent gases (CO, CO2, H2, CH4) and tars produced by pyrolysing 15 wildland fuels were reanalysed using compositional data analysis techniques. Mass and mole fractions were compositionally equivalent. Plant species, moisture status and heating mode effects on compositional balances formed from subsets of pyrolysates were tested. Plant species affected the amount of phenol, primary and secondary/tertiary tars relative to permanent gases and relative amounts of single- and multi-ring compounds. Plant moisture status affected the amount of CO relative to other permanent gases, of H2 to CH4 and tars to phenol. Heating mode and rate strongly influenced pyrolysate composition. Slow heating produced more primary tars relative to multi-ring tars than fast heating convective and combined radiant and convective heating modes. Slow heating produced relatively more compounds with fewer rings and fast heating produced relatively more multi-ring compounds. Compositional data analysis is a well-developed statistical methodology, providing models and methods equivalent to traditional ones, that accounts for the special constraining features of relative data. Future analysis of the compositional data related to wildland fire using compositional techniques is recommended.

Performance analysis of a tubular solar dryer for drying Mexican mint (Plectranthus amboinicus) — An experimental approach

This work reports the use of tubular solar dryer for drying the Mexican mint (Plectranthus amboinicus) under various mass of leaves. Experiments were carried out in a tubular solar dryer under natural convection mode with direct solar heating. Parameters such as the temperature of dryer tray, relative humidity, and moisture content were recorded for 0.25, 0.5, 0.75 and 1 kg of P. amboinicus leaves placed in the dryer tray. Results showed that under an increased mass of P. amboinicus leaves in the tray the parameters such as dryer tray temperature are reduced, and the moisture content available with 0.75 kg of P. amboinicus leaves is higher as compared to 0.25 kg of P. amboinicus leaves. Also, it is found that the relative humidity inside the drying chamber is higher with increased mass of P. amboinicus leaves in the drying chamber.

Analysis of the entropy due to radiative flow of nano–encapsulated phase change materials within inclined porous prismatic enclosures: Finite element simulation

The heat transfer and fluid flow due to the interaction between the radiation and convective modes is examined. Also, the irreversibility processes within the considered system is calculated based on the second law of the thermodynamic. The Nano–Encapsulated Phase Change Materials (NEPCM) in which their core is nonadecane and their shell is polyurethane are used to formulate the worked suspension. The flow domains are prismatic enclosures and two designs (D1 and D2) based on the aspect ratio of the enclosures walls are carried out. The geometry is filled by glass balls as an isotropic porous medium. The solution methodology is based on the finite element method and a semi implicit technique is applied for the velocities. The main outcomes disclosed that the alteration of the radiation parameter reduces the melting/solidification processes. Also, using NEPCM's gives a clear improvement in the heat transfer.

Triple diffusive convection in a viscoelastic Oldroyd-B fluid layer

The stability of a triply diffusive viscoelastic fluid layer in which the fluid density depends on three stratifying agencies possessing different diffusivities is investigated. The viscoelastic fluid is modeled by means of the Oldroyd-B constitutive equation. Analytical expressions are obtained for steady and oscillatory onset by carrying out the linear instability analysis and the crossover boundary between them is demarcated by identifying a codimension-two point in the viscoelastic parameters plane. The occurrence of disconnected closed oscillatory neutral curve lying well below the stationary neutral curve is established for some values of governing parameters indicating the requirement of three critical values of thermal Rayleigh number to specify the linear instability criteria. However, the possibility of quasiperiodic bifurcation from the motionless basic state is not perceived and this is in contradiction to the case of inelastic couple stress and Newtonian fluids. The corresponding weakly nonlinear stability of stationary and oscillatory modes has been carried out using a perturbation method. The cubic Landau equations are derived and the stability of bifurcating solution is discussed. The viscoelastic parameters influence the stability of stationary bifurcation despite their effect is not felt on the stationary onset. The stationary and oscillatory finite amplitude solution is found to bifurcate either subcritical or supercritical depending on the choice of governing parameters. The effect of Prandtl number and viscoelastic parameters on stationary and oscillatory convection modes of heat and mass transfer is analyzed.

Effect of thermal conductivity of substrate board for temperature control of electronic components: A numerical study

The current study deals with the numerical investigation of the substrate board characteristics using different materials (FR4, silicon cladding and copper cladding) on which nine nonidentical electronic components (ECs) are mounted, which are supplied with nonuniform heat flux. The study is conducted for natural and forced convection (3[Formula: see text]m/s and 5[Formula: see text]m/s) modes of heat transfer. It is observed that for 5[Formula: see text]m/s the temperature of the ECs is dropped by 30–47∘C using copper cladding board in comparison to FR4 and silicon cladding board. It is also noted that with the use of copper cladding board the velocity of air required for cooling the ECs is substantially reduced by 2[Formula: see text]m/s and the temperature of the IC chip is reduced by 1.50–11.12∘C.

Overstable convective modes in rotating early-type stars

ABSTRACT We calculate overstable convective (OsC) modes of 2-, 4-, and $20\hbox{-}{\rm M}_\odot$ main-sequence stars. To compute non-adiabatic OsC modes in the core, we assume $(\nabla \cdot \rm{\boldsymbol {F}}_{\rm C})^\prime =0$ as a prescription for the approximation called frozen-in convection in pulsating stars, where $\rm{\boldsymbol {F}}_{\rm C}$ is the convective energy flux and the prime ′ indicates Eulerian perturbation. We find that the general properties of the OsC modes are roughly the same as those obtained by Lee & Saio, who assumed $\delta (\nabla \cdot \rm{\boldsymbol {F}}_{\rm C})=0$, except that no OsC modes behave like inertial modes when they tend towards complete stabilization with increasing rotation frequency, where δ indicates the Lagrangian perturbation. As the rotation frequency of the stars increases, the OsC modes are stabilized to resonantly excite g modes in the envelope when the core rotates slightly faster than the envelope. The frequency of the OsC modes that excite envelope g modes is approximately given by σ ∼ |mΩc| in the inertial frame and hence σm = −2 ≈ 2σm = −1, where m is the azimuthal wavenumber of the modes and Ωc is the rotation frequency of the core. We find that the modal properties of OsC modes do not strongly depend on the mass of the stars. We discuss angular momentum transport by OsC modes in resonance with envelope g modes in the main-sequence stars. We suggest that angular momentum transfer takes place from the core to the envelope and that the OsC modes may help the stars rotate uniformly and keep the rotation frequency of the core low during their evolution as main-sequence stars.

ABOUT SPECIFIC FEATURES OF CONVECTION IN COMPRESSED GAS

Stability characteristics of static Rayleigh - Benard convection regime in a compressible, viscous and heat-conducting gas are investigated by means of numerical simulation. It is shown that, depending on the height of convection region and the magnitude of temperature gradient, various convection modes are realized - isobaric, adiabatic, and superadiabatic. The conditions under which convection develops at stable stratification are determined. A diagram of convection modes is plotted depending on the height of the region and the magnitude of temperature gradient.

A Hysteresis of Supercritical Water Convection in an Open Elongated Cavity at a Fixed Vertical Heat Flux

The supercritical modes of water convection at room temperature in an elongated horizontal cavity, with a ratio of width to height from 1:1 to 5:1 are investigated. The Prandtl number is assumed to be seven. A constant heat flux is set at the upper free and lower solid boundaries, and the side solid walls are assumed to be thermally insulated. Calculations carried out by the finite-difference method for values of the Grashof number exceeding the critical one by up to fifty times have shown that in the considered range of Grashof numbers in moderately elongated cavities, with the aspect ratios of up to 4:1, a single-vortex stationary regime is realized in the supercritical region. In a cavity with the aspect ratio of 5:1 for a wide range of Grashof numbers, a multiplicity of stationary regimes is possible, from onevortex to three-vortex. Hysteresis transitions between them, taking place at a smooth and stepwise change in the Grashof number, have been discovered and investigated.

Influence of realistic rheological properties on the style of mantle convection: roles of dynamic friction and depth-dependence of rheological properties

SUMMARY In order to generate plate tectonics, the near surface layer should not be too strong, but the causes for not-so-strong near surface layer remains unclear. We conduct mantle convection modelling in the spherical geometry to investigate the influence of the strength of the near surface layer. We explore a range of friction coefficients including the static high friction coefficient (∼0.6) as well as the reduced friction coefficients by fast fault motion in earthquakes. When the friction coefficient is low enough (<0.03), the surface layer is yielded by the convective stress, and the style of mantle convection appears the mobile-lid mode (plate tectonics style of convection). This style is relevant for the Earth where fault motion is unstable because of the low surface temperature. In contrast, for a high friction coefficient, the surface layer is too strong, generating the stagnant-lid mode. This case corresponds to Venus where fault motion is stable because of high surface temperature. Our calculations show that, in plate tectonic style of convection, the mantle convection is likely to be more vigorous, inducing the high convective stress that helps the operation of plate tectonics. In contrast, when stagnant-lid mode of convection appears, the convective vigor is likely to be low, inducing the low convective stress. Therefore, in each case, the interplay between the surface strength and convective stress tends to maintain the same mode of convection in a self-consistent way. We also investigate the relationship between mantle temperature and heat flux for two different modes of convection upon a change in friction coefficient. We found that the heat flow associated with mobile lid convection caused by low friction is less sensitive to the mantle temperature compared to a conventional mantle convection model, where the heat flow is highly sensitive to mantle temperature. This provides a possible mechanism to solve the thermal runaway paradox.