Convective Heat Transfer Flow Research Articles

Steady natural convection heat and mass transfer flow through a vertical porous channel with variable viscosity and thermal conductivity

AbstractThe present article studies the effects of suction/injection and variable physical properties on steady natural convection flow through a vertical channel. The investigation was done analytically using Differential Transformation Method (DTM). The variability in viscosity and thermal conductivity are considered linear function of temperature. The governing equations are transformed into a set of coupled nonlinear ordinary differential equations. Results obtained were compared with exact solution when some of the flow conditions were relaxed and results from DTM show an excellent agreement with the exact solution which was obtained analytically. The influence of the flow parameters on fluid temperature, concentration, and velocity are presented graphically and discussed for variations of the governing parameters. From the course of investigation, it was found that an increase in suction (S > 0) through the heated plate causes fluid temperature, velocity as well as concentration to decrease. However, an increase in injection causes heat transfer rate as well as skin friction at the heated plate to decrease.

Heat transfer and pressure drop in a sinus blowing of copper oxide- water non-Newtonian nanofluid in a sudden expansion process in the presence of variable magnetic field: a numerical solution

ABSTRACT In the current article, the effect of nanofluid blast on the forced convection heat transfer in the intra-channel turbulent flow with a sudden expansion is investigated. The different cases of Newtonian, non-Newtonian, and incompressible fluids have been studied. The effects of non-Newtonian fluid index, magnetic field, nanoparticle volume fraction and nanofluid blowing frequency parameters on the pressure drop and heat transfer in the intra-channel flow have been reviewed. The simulation was performed using finite volume method as the solution strategy. The results revealed that higher values of Re and nanoparticles volume fraction resulted in the Nusselt number increasement. Also, increasing the magnetic field and decreasing the non-Newtonian fluid index increased the forced heat transfer coefficient and subsequently the Nusselt number. The results showed that the sinus velocity caused to increase the rate of the heat transfer and slightly lower friction coefficient. However, higher frequencies caused increasing of heat transfer and friction coefficient. Finally, it was founded that higher values of the Re had the most influence on the performance of the system and the magnetic field and sinus velocity were in the next step.

Flow structure and convective heat transfer in a bladed structure under wind conditions

Bladed receivers have been proposed as an alternative to conventional external receivers for high-temperature concentrating solar power systems. Instead of flat banks of tubes arranged on the receiver surface, the tube-banks are oriented into the form of blades protruding from the back-wall of the receiver. This modified arrangement improves light-trapping, without the size and cost implications of a large cavity receiver, and also acts to trap hot air between the blades, hence reducing convective transfer per-tube-area. The present paper investigates the convective heat transfer from a scaled model of a bladed receiver and its correlation with the flow behavior between the blades. Wind tunnel measurements were undertaken to investigate the effects of the wind speed, the blade length and the receiver orientation on the convective heat transfer. It was observed that increasing the pitch angle from 26° to 70°, measured downwards from the vertical in a headwind configuration, leads to a significant reduction in the convective heat transfer coefficient of up to 60%, for the investigated blade lengths. To identify the effect of flow structure on the rate of the convective heat transfer at low Richardson umbers, a second experimental study with isothermal conditions was conducted in a water channel matching the Reynolds number of the wind tunnel experiment. The mean velocity fields and the associate streamline topologies within an open cavity with different pitch angles and blade length to spacing ratios (RBS) were also investigated through Particle Image Velocimetry (PIV) measurements. The streamlines revealed that the shear layer at a pitch angle of 70° for the receiver with the longest blade,RBS = 3, bridges the cavity opening without a strong interaction with the counter-rotating vortices inside the cavity, which resulted in an enlargement of the stagnation zone close to the cavity back-wall. This stagnation zone was identified to be the cause of the convective heat transfer reduction observed in the earlier wind tunnel experiments. It was also found that the Reynolds number has a strong impact on the heat transfer rate, as increasing the wind speed from 3 m/s to 6 m/s enhanced the heat transfer coefficient by up to 50%. As the corresponding Reynolds number in the water channel increased from 1.4 × 104 to 2.8 × 104, PIV data showed that the shear layer was drawn into the cavity and consequently, the velocity magnitude near the back-wall of the cavity reached up to 90% of the freestream velocity. The results provide a good understanding of flow behavior in the vicinity of the blades and its impacts on the convective heat transfer from the receiver.

Investigation of Overlapped Twisted Tapes Inserted in a Double-Pipe Heat Exchanger Using Two-Phase Nanofluid.

This study investigated the laminar convective heat transfer and fluid flow of Al2O3 nanofluid in a counter flow double-pipe heat exchanger equipped with overlapped twisted tape inserts in both inner and outer tubes. Two models of the same (co-swirling twisted tapes) and opposite (counter-swirling twisted tapes) angular directions for the stationary twisted tapes were considered. The computational fluid dynamic simulations were conducted through varying the design parameters, including the angular direction of twisted tape inserts, nanofluid volume concentration, and Reynolds number. It was found that inserting the overlapped twisted tapes in the heat exchanger significantly increases the thermal performance as well as the friction factor compared with the plain heat exchanger. The results indicate that models of co-swirling twisted tapes and counter-swirling twisted tapes increase the average Nusselt number by almost 35.2–66.2% and 42.1–68.7% over the Reynolds number ranging 250–1000, respectively. To assess the interplay between heat transfer enhancement and pressure loss penalty, the dimensionless number of performance evaluation criterion was calculated for all the captured configurations. Ultimately, the highest value of performance evaluation criterion is equal to 1.40 and 1.26 at inner and outer tubes at the Reynolds number of 1000 and the volume fraction of 3% in the case of counter-swirling twisted tapes model.

Geometry modification of solar collector to improve performance of solar chimneys

Abstract Besides meeting the ever increasing energy demand, renewable energy source can play a critical role in reducing global carbon emissions and the pace of investment on such renewable energy has greatly increased as the cost of technologies fall and efficiency continues to rise. However, the benefits of renewables go beyond reducing carbon emissions and mostly relates to crises in fossil energy. The solar chimney is a novel technology which has proved capable of generating electrical energy from the sun and atmospheric air. However, it’s implementation has been involved with many challenges. In the present study, thermal performances of solar chimneys are assessed experimentally for some different experimental setups. Experimental setups that are employed in this study have two distinct features in comparison with other setups that have been used in the prior arts: 1. Setups are indoor, in contrary to conventional outdoor setups, the environmental parameters can be entirely controlled in this study and steady state condition can be reached. 2. Both setups are capable of measuring thermal performance of the collector in addition to performance of solar chimney. Setups differ in geometry to investigate effects of geometry on overall performance of solar chimneys. It is demonstrated in this study that the thermal and hydraulic performances of common form of solar chimneys are not adequate and low thermal performance of collectors is one of the important reasons of poor power generation of solar chimneys. It is found that by changing shape of the collector from circular to an innovative square shape, Nusselt number (Nu), air flow velocity and convective heat transfer coefficient (h) increases 1225%, 245% and 603% respectively. Finally, the reasons for poor performance of common solar chimneys are comprehensively discussed.

Role of movement of the walls with time-dependent velocity on flow and mixed convection in vertical cylindrical annulus with suction / injection

The unsteady, viscous flow and mixed convection heat transfer of an incompressible fluid within a vertical concentric cylindrical annulus with time-dependent moving walls are investigated. Fluid is suctioned/injected through the cylinders' walls. Role of movement of the walls on flow and heat transfer inside the vertical annulus is sought. An exact solution of the Navier-Stokes and energy equations is obtained in this problem, for the first time. Here, the transfer of heat is from the hot cylinder walls with constant temperature to the cooler moving fluid. It is interesting to note that the results indicate that the time-dependency of the cylinder walls movement has no effect on the temperature profile. The results also indicate that, compared to other time functions, usage of inverse of time for velocity movement causes significant increase in velocity, stress tensor, and Nusselt number in the vicinity of the moving wall. Also, increasing the mixed convection and suction/injection parameters would increase the Nusselt number and decrease the stress tensor on the inner and outer cylinders no matter what the velocity function is chosen. Therefore, the flow and heat transfer is controllable using the velocity function of the wall movement and change of the other non-dimensional parameters.

Convective heat transfer and laminar flow characteristics of flow over a circular cylinder in presence of control plates

The location and orientation effect of control plates (CP) in and around the flow separation region of a circular cylinder has been numerically investigated in the laminar flow regime at Reynolds number 100. The vortex shedding and convective heat transfer characteristics have been analysed for both upstream and downstream locations of the control plates using Ansys Fluent 18.0. The drag and lift coefficient (CD, CL), the dimensionless vortex shedding frequency (Sr) and dimensionless convective heat transfer coefficient (Nu) are reported for various CP orientation and locations. The CP positioned along the main flow direction and located near the cylinder induce more shear on the cylinder surface, stabilities the wake vortices. In addition to shear generation, the CP orientation interacts effectively with the cylinder wake. The vortex shedding suppression with reduced drag on the cylinder is identified at many locations of CP. Convective heat transfer from the cylinder is suppressed at most of the locations of CP, at very few locations the cylinder heat transfer enhanced.

Convective Heat Transfer in a Rotating Hollow Cylinder with an End Wall

A method and calculation results are presented for the flow field and convective heat transfer in a hollow cylinder having an end wall and rotating about the axis of symmetry under variation of the angular velocity of rotation and cylinder height.

Wake and thermal characteristics for cross-buoyancy mixed convection around and through a porous cylinder

The influence of cross buoyancy on the steady flow and mixed convective heat transfer around and through a porous cylinder with internal heat generation is investigated numerically. Based on the Darcy–Brinkman–Forchheimer extended porous medium model, the finite volume method is applied to investigate the wake structure and thermal characteristics in terms of the streamlines, asymmetry of recirculating wakes, temperature distribution, and average Nusselt number. The ranges chosen for the Reynolds number (Re), Darcy number (Da), and Richardson number (Ri) are 5 ≤ Re ≤ 40, 10−6 ≤ Da ≤ 10−2, and 0 ≤ Ri ≤ 1, respectively. For certain ranges above, a pair of asymmetric recirculating wakes is observed, with the upper recirculating wake detached from and the lower one partially penetrating or also detached from the cylinder. The asymmetry of the recirculating wake increases with Ri but decreases with Re. Two or three regimes with the distinct asymmetric characteristics are identified over the range of Da investigated, depending on Re. For the heat transfer performance, cross buoyancy is found to have a certain impeditive impact on the average Nusselt number.

Thermokinetic transport of dilatant/pseudoplastic fluids in a hydrophobic patterned micro-slit

The flow enhancement and convective heat transfer along with entropy generation analysis are studied numerically in a micro-slit with alternating hydrodynamic slip patches. The advances in molecular simulations and micro-scale experiments confirmed that the slip of fluid on the solid surfaces occurred at small scale flows and the traditional no-slip boundary conditions cannot be applicable for the flow simulation at the micro- and nano-scale. The coupled Poisson–Boltzmann–Navier–Stokes equations dealing with an external electric potential are involved for the flow enhancement and entropy generation analysis of non-Newtonian fluids in a micro-slit with periodic slips. From the finite volume simulation, it is observed that the drag force effect is very strong along the wall for the transportation and mixing of fluids. This effect is found to be minimized by imposing periodic hydrophobic slippage along the boundary. An additional pressure gradient is generated by imposing electrokinetic pumping, resulting in a higher velocity gradient in the flow direction in the presence of viscous dissipation and Joule heating effects. The results are predicted in terms of the flow enhancement factor (Ef) (which provides maximum species transport), the average heat transfer rate (Nu), and the average entropy generation due to fluid friction, heat transfer, and Joule heating effects. The advantages and disadvantages of utilizing slip conditions are discussed, which has large scale applications on drug delivery and DNA analysis and sequencing, since cell damage due to pumping will be minimized.

A New Thermodynamic and Heat Transfer Model for Nanolubricants and Refrigerant Heat Transfer Processes in Smooth Copper Tubes

Abstract In air conditioning systems, lubricating oil leaves the compressor and circulates through the other system components. This lubricant acts as a contaminant affecting heat transfer in heat exchangers. The literature indicated that mixtures of refrigerants and nanolubricants, that is, nanoparticles dispersed in the lubricant oils, have potentials to augment heat transfer exchange effectiveness. However, the nanoparticle mechanisms leading to such heat transfer changes are still unclear and not well included in the models. In this work, an existing single-phase forced flow convective heat transfer model, originally developed for water-based nanofluids, was modified to include the effects of diffusion and mass balance of different shape nanoparticles within the laminar sublayer and turbulent layer of the flow. A new physics-based superposition heat transfer model for saturated two-phase flow boiling of refrigerant and nanolubricants was also developed by integrating the modified forced flow convective heat transfer model and a semi-empirical pool boiling model for nanolubricants. The new model included the several physical effects that influenced heat transfer, such as slip mechanisms at the nanoparticles and base fluid interface and its influence on the laminar sublayer thickness, momentum transfer from the nanoparticles to the growing bubbles, and formation of lubricant excess concentration at the tube surface and its influence on bubble growth and tube wetting. The new model was validated for single-phase convective heat transfer and two-phase flow boiling of refrigerant R410A with two nanolubricants, having nonspherical ZnO nanoparticles and spherical Al2O3 nanoparticles.

Mixed Convection in a Ventilated Enclosure by Considering Both Geometrical Parameters and Thermo-Physical Properties of Water/Cu Nanofluid

In this study, mixed convection heat transfer and fluid flow is numerically investigated in an open system enclosure containing a heated obstacle. Effects of the positions of both obstacles and fluid flow outlet on the flow field and heat transfer within the enclosure were studied. Richardson number ranging from 0.1 to 10 was examined for a constant Grashof number at different Reynolds numbers in both base fluid and fluid mixed with Cu nanoparticles (0 to 4% of volume fractions). It is found that the obstacle and outlet position as well as the Richardson number greatly affect the flow field and heat transfer rate. The maximum heat transfer rate occurs for Richardson number 0.1, when the outlet and obstacle position are located at the bottom of the right side of the enclosure (P3) and near right side of the enclosure (8H), respectively. Also, the heat transfer rate is higher when the obstacle is placed close to the inlet (2H) and outlet (8H) compared to the center of the enclosure (5H). It is shown that although the change of volume fraction of nanofluid does not have significant effect on the flow pattern, however, it enhances the heat transfer rate through improving the effective thermo-physical properties of the base fluid.

Heat Transfer Augmentation

This article presents a brief review of various methodologies applied for heat transfer enhancement in laminar flow convection regime. Experimental setup for laminar flow convection heat transfer enhancement using insertions has been explained along with the associated results. Nusselt’s number is found to be a key parameter for investigatigation in order to perceive the enhancement in heat transfer. Similarly, the magnetohydrodynamic mixed convection heat transfer enhancement technique has also been explored. The results of isotherms and fluid flow parameters are discussed which directly affect the heat transfer coefficient. This review article complements the literature in related field and thus will be helpful in order to carry out further experiments in heat transfer enhancement in future.

Heat Transfer Augmentation

This article presents a brief review of various methodologies applied for heat transfer enhancement in laminar flow convection regime. Experimental setup for laminar flow convection heat transfer enhancement using insertions has been explained along with the associated results. Nusselt’s number is found to be a key parameter for investigatigation in order to perceive the enhancement in heat transfer. Similarly, the magnetohydrodynamic mixed convection heat transfer enhancement technique has also been explored. The results of isotherms and fluid flow parameters are discussed which directly affect the heat transfer coefficient. This review article complements the literature in related field and thus will be helpful in order to carry out further experiments in heat transfer enhancement in future.

Effect of thermal buoyancy on flow pattern from a pair of side-by-side confined triangular cylinders

The effects of ax ial and radial thermal buoyancy on fluid flow and mixed convection heat transfer from a pair of identical triangular cylinders in side-by-side arrangement confined within a straight channel. The numerical simulations are carried out by solving continuity, momentum and energy equations using the commercial code ANSYS-CFX. The obtained results are presented and discussed within the range of following conditions: Richardson number Ri = 0 to 2, Reynolds Re = 20, and Prandtl number Pr = 1 at fixed value of blockage ratio β = 0.2. The main results are depicted in terms of streamline and isotherm contours to analyze the fluidic and energetic behaviors. The total drag coefficient and average Nusselt number are also computed. Moreover, a simple correlation indicating the variations of drag coefficient and average Nusselt number versus Richardson number are also provided. It was found that for axial effect of thermal buoyancy, increase in buoyancy strength enhances the heat transfer rate for both cylinders. In other hand, for radial effect, increase in buoyancy strength increases the heat transfer rate of down cylinder and it is reduced for the upper cylinder.
 Bangladesh J. Sci. Ind. Res.55(1), 9-14, 2020

MHD effect on mixed convection of annulus circular enclosure filled with Non-Newtonian nanofluid

The fluid flow and mixed convection heat transfer of a non-Newtonian (Cu–water) nanofluid-filled circular annulus enclosure in a magnetic field are investigated numerically for a two-dimensional, steady-state, incompressible, laminar flow using the Galerkin finite element method (GFEM). The Prandtl number (Pr = 6.2) and Grashof number (Gr = 100) are assumed to be constants, whereas the Richardson number varies within a range of 0 ≤ Ri ≤ 1, the Hartman number within a range of 0 ≤ Ha ≤60, the Power law index within a range of 0.2 ≤ n ≤ 1.4, and the volume fraction within a range of 0 ≤ φ ≤ 1. The enclosure consists of an outer rotating cylinder that is kept at a cold temperature (Tc) and an inner non-rotating cylinder kept at a hot temperature (Th). The ratio of the inner circular diameter to the annulus space length is kept constant at 2. The results depict that the stream function increases with increasing power law index, even up to n = 1, which causes the fluid to behave as a Newtonian fluid. The magnetic field has a critical impact on the fluid flow pattern. The average Nusselt number increases with decreasing Richardson number, owing to the improved heat transfer by forced convection.

English

English

Effect of Lattice Setting Density on Fluid Flow and Convective Heat Transfer Characteristics of Bricks in Tunnel Kilns

Abstract This paper reports the effect of setting density on flow uniformity, pressure drop, pumping power, and convective heat transfer coefficients (CHTCs). High-density setting (HDS) comprises 768 bricks, and low-density setting (LDS) contains 512 bricks are tested for different inlet air velocities using both local and average approaches. The investigation is carried out using a 3D-computational fluid dynamics (CFD) model with k–ω turbulence model. Both settings are validated against experimental data reported in the literature with errors less than 1.9% for pressure drop and −1.0% for brick surface temperature. The reported results indicated that the LDS has distinct benefits over the HDS as it enhances the flow uniformity, particularly in the stack channels. Also, LDS attains lower pressure drop, pumping power, and firing time than HDS by 45.93%, 50%, and 35%, respectively. In addition, LDS produces larger CHTCs, rates of heat transfer for individual bricks, and the ratio of heat transfer to pumping power than HDS by 24.53%, 35%, and 34%, respectively. Moreover, LDS produces more homogenous heating of the setting bricks than HDS as the maximum difference of CHTCs between bricks is about 4.39% for LDS and 19.62% for HDS. The best performance of the firing process is accomplished at low inlet air velocity (3 m/s), whereas the highest productivity is achieved at high inlet air velocity (9 m/s).

Natural convection heat transfer and fluid flow around a thick hollow vertical cylinder suspended in air: A numerical approach

The present study reports numerical investigation on natural convection heat transfer and fluid flow from a thick hollow vertical cylinder suspended in air within laminar regime for a Rayleigh number (Ra) of 104 to 108. Numerical simulations have been performed by changing the length to outer diameter ratio (L/Do) of the cylinder from 0.2 to 20 and thickness ratio (Di/Do) from 0.5 to 0.9. Variation of average Nusselt number (Nu) and local Nusselt number with the above parameters has been elucidated graphically. Subsequently, the influences of Ra, L/Do, Di/Do on mass-flow rate and velocity profile inside the cylinder has also been investigated. Temperature contour and velocity vector plot around the cylinder have been represented pictorially to describe the thermal and velocity field around the vertical thick hollow cylinder. It has been observed that the average Nu of a thick hollow cylinder increases with L/Do, attains a peak and then decreases to a minimum and then again increases with L/Do. This phenomenon is very well observed at high Ra of 106 compared to lower Ra of 104. The Nu of the inner surface is found to be always lower compared to the outer surface and as L/Do increases beyond 8 the inner surface Nu falls drastically at high Ra compared to lower Ra. It has also been observed that the non-dimensional mass-flow rate due to buoyancy driven flow inside the thick hollow cylinder increases with L/Do, attains a peak and then decreases for all Ra. Cooling curve and a correlation for Nusselt number as a function of geometrical and thermal parameter have been presented which are useful for industrial purposes.

Experimental analysis of flow and convective heat transfer in the water-cooled packed pebble bed nuclear reactor core

A randomly packed pebble bed test facility is set up to study the flow and convective heat transfer in the water-cooled pebble-bed type nuclear reactor core. Water is used as the test fluid. The electrical induction heating system provides internal heat source for the packed pebble bed. The effects of input power, porosity, fluid velocity and inlet temperature are studied on the flow resistance and convective heat transfer coefficients. A new correlation is proposed to predict the pressure drop in the random packed pebble bed. The correlations given by Wakao et al. and KTA agree well with the experimental convective heat transfer coefficients in the high-velocity and low-velocity conditions respectively. A new correlation is presented based on the trend similarity between the Colburn j factor and friction factor, to predict the pebble bed convective heat transfer characteristics, with a discrepancy no more than 10%.