Laminar Convection Heat Transfer Research Articles

Numerical and experimental investigation of laminar and turbulent convective heat transfer in a coiled flow reverser with twisted tape insert

This paper presents a comprehensive investigation of heat transfer enhancement and pressure drop in a Coiled Flow Reverser (CFR) with a twisted tape insert. Both laminar (10 < Re < 2000) and turbulent (12,000 < Re < 18,000) regimes have been studied using both experimental and numerical methods. The effects of twist ratio of twisted tape, inlet Reynolds number, and perforating the twisted tapes have been investigated. The study found that the best performance in the laminar regime is achieved with a twisted tape with a twist ratio of 15.73 and no holes on it at a Reynolds number of 2000. In the turbulent regime, the same twisted tape performs the best at a Reynolds number of 17,000. The highest Thermal Performance Ratio (TPR) in laminar regime is 1.55 and in turbulent regime is 0.99. In comparison to the straight pipe, all types of CFRs show better performance in heat transfer enhancement. Also, as the Reynolds number increases, the friction coefficient decreases in all configurations. The study reveals that the twisted tape insert enhances heat transfer performance while increasing pressure drop. This study provides valuable insights into the design and optimization of heat exchangers with CFR. The findings can be used to improve the efficiency and performance of various industrial processes, such as refrigeration systems, air conditioning, and power plants.

An experimental study of the influence of Fe3O4, MWCNT-Fe3O4, and Fe3O4@MWCNT nanoparticles on the thermophysical characteristics and laminar convective heat transfer of nanofluids: A comparative study

In this study, the main objective is to investigate the impact of single, hybrid, and doped magnetic nanoparticles (NPs) on the convective heat transfer capabilities of a water/ethylene glycol (EG)-based nanofluid (NF) under the influence of a magnetic field (MF). The NFs were prepared by incorporating Fe3O4, Fe3O4-multi-walled carbon nanotubes (Fe3O4-MWCNT), and Fe3O4@MWCNT in concentrations of 0.05, 0.1, and 0.2 wt.%. The NPs were characterized using various techniques such as XRD, SEM, VSM, Raman spectroscopy, and FTIR to confirm their properties. The stability of the NFs was assessed through dynamic DLS and zeta potential measurements. The study then focused on evaluating the effects of these nano-additives on thermal conductivity (TC), viscosity at 25 °C, convective heat transfer coefficient (CHTC), and friction factor in a circular tube under laminar flow regime with a constant wall flux, both with and without the application of a MF. The results of present preliminary study with limited samples revealed that the CHTC of NFs containing Fe3O4 and Fe3O4@MWCNT NPs increased with NP concentration and application of a MF, with maximum increases of approximately 19.01 % and 32.84 %, respectively. However, in the case of NFs containing MWCNT-Fe3O4 NPs, the CHTC decreased when the MF was applied at concentrations higher than 0.1 wt.%. Furthermore, the NFs containing 0.2 wt.% of Fe3O4, MWCNT-Fe3O4, and Fe3O4@MWCNT NPs exhibited the highest friction when subjected to a 400 G MF, resulting in increases of 5.2 %, 9.2 %, and 5.9 %, respectively, compared to the base fluid. The performance index, which assesses the effectiveness of using NFs instead of the base fluid, indicated that all samples had a performance index greater than one, indicating their efficacy. The maximum performance index achieved was 1.343, corresponding to the NF containing 0.2 wt.% of Fe3O4@MWCNT NPs at a Reynolds number (Re) of 2200 and a MF strength of 400 G.

Laminar cross-flow convective heat transfer of a finned cylindrical tube in the presence of particle deposition and deposit erosion

Cross-flow finned tube heat exchangers are extensively used in a variety of industrial and commercial applications. Although the fins increase surface area and hence enhance heat transfer rate, the fouling potential also increases due to the more complex geometry leading to performance deterioration. This paper presents an integrated numerical model to study the effect of fouling on convective heat transfer of a finned cylindrical tube. In this model, fluid flow, particle transport, particle deposition, deposit erosion and heat transfer are all accounted for in a fully coupled manner. Moreover, the fluid-deposit interface is captured using a level-set method and the interfacial velocity is derived from the net effect of combined particle deposition and deposit erosion. In such a way, the evolution of deposit layer is implicitly tracked in a fixed mesh, and its influences on fluid flow and heat transfer are considered by updating the thermophysical properties of the deposit region. Based on this integrated model, comprehensive parametric studies are conducted to investigate the effects of tube orientation angle (β), Reynolds number (Re), Damkholer number (Da), erosion number (Er), particle Peclet number (PeC) and thermal Peclet number (PeT) and dimensionless critical shear stress (τ*crit). It is found that the amount of deposits accumulated increases for a lower β within the range of 0°-45°, a higher β in the range of 45°-90°, a higher Re, a higher Da, a lower Er, a lower PeC and a lower τ*crit. Heat transfer deteriorates as the insulating deposit layer grows thicker and at a higher PeT. This study demonstrates the capabilities of the developed model, and provides fundamental insights on the coupled fouling and heat transfer processes of cross-flow finned tube heat exchangers in the trend towards miniaturization.

Analysis of transient laminar forced convective heat transfer in parallel-plate channels with time-varying thermal boundary conditions of the inlet and wall

The problem of transient laminar forced convective heat transfer in parallel-plate channels with time-varying thermal boundary conditions of the inlet and wall is thoroughly investigated in this study. By employing the methodologies of generalized integral transform technique (GITT) and Duhamel's theorem, we successfully derive an analytical solution for this problem. To validate the accuracy of our analytical findings, a comparison is made with the corresponding numerical solution, demonstrating a remarkable level of agreement between the two approaches. In order to obtain the general patterns of heat transfer, we employ a decoupling technique to separate the thermal response in the case of harmonic temperature boundary conditions of the inlet and wall. This approach enables us to uncover the intricate relationship between the weight coefficient of heat input and the two-dimensional spatial location of the channel, as well as the dimensionless angular frequency of heat input. Furthermore, we present the corresponding dimensionless cut-off angular frequency of heat input at various locations within the channel. To gain further insights, we conduct an in-depth analysis of the effects of integration time or distance on the dimensionless optimum angular frequency corresponding to the dimensionless maximum average heat flux.

Improved asymptotic expansion method for laminar fluid flow and heat transfer in conical gaps with disks rotating

PurposeThe purpose of this paper was to study laminar fluid flow and convective heat transfer in a conical gap at small conicity angles up to 4° for the case of disk rotation with a fixed cone.Design/methodology/approachIn this paper, the improved asymptotic expansion method developed by the author was applied to the self-similar Navier–Stokes equations. The characteristic Reynolds number ranged from 0.001 to 2.0, and the Prandtl numbers ranged from 0.71 to 10.FindingsCompared to previous approaches, the improved asymptotic expansion method has an accuracy like the self-similar solution in a significantly wider range of Reynolds and Prandtl numbers. Including radial thermal conductivity in the energy equation at small conicity angle leads to insignificant deviations of the Nusselt number (maximum 1.23%).Practical implicationsThis problem has applications in rheometry to experimentally determine viscosity of liquids, as well as in bioengineering and medicine, where cone-and-disk devices serve as an incubator for nurturing endothelial cells.Social implicationsThe study can help design more effective devices to nurture endothelial cells, which regulate exchanges between the bloodstream and the surrounding tissues.Originality/valueTo the best of the authors’ knowledge, for the first time, novel approximate analytical solutions were obtained for the radial, tangential and axial velocity components, flow swirl angle on the disk, tangential stresses on both surfaces, as well as static pressure, which varies not only with the Reynolds number but also across the gap. These solutions are in excellent agreement with the self-similar solution.

Numerical Predictions of Laminar Free and Mixed Convection Heat Transfer to Supercritical Water from a Non-Isothermal Vertical Plate

Numerical predictions of heat transfer have been made for laminar free and mixed convective heat transfer from non-isothermal vertical plane surface to supercritical water. Variation of all the thermo-physical properties of the supercritical fluid under investigation have been considered. The numerical scheme employed in the present investigation is first validated with respect to experimental data for isothermal wall conditions and good agreement is achieved with maximum deviation not exceeding 15 %. Linear and parabolic variation of wall temperatures are considered for the present investigation. Computations are made for two pressures of 225 Bar and 235 Bar, for wide range of temperature difference from 4K to 20K. The values of temperature difference are chosen. Based on the results obtained, correlations have been proposed to evaluate the Nusselt numbers for both free and mixed convection with pressure ratios of P/Pc = 1.02 (p = 225 bar) and P/Pc = 1.065 (p = 235 bar). The correlations cover a range of Rayleigh numbers, Rax from 106 to 109 for Free Convection and grashoff Number/Reynolds Number2 ranging from 0.4 to 8 for Mixed convection. The proposed correlations for local Nusselt numbers do not exceed maximum deviation of 15% from the predicted values. It has been found that the maximum deviation occurs for those cases for which the pseudo-critical temperature, T* is within the boundary layer. It is further observed that for these cases both the velocity and temperature profiles are distorted at locations where the temperature is close to the pseudo-critical temperature thereby affecting the gradients at the wall, which in turn influences the heat transfer coefficient.

Experimental Study For a Laminar Natural Convection Heat Transfer From an Isothermal Heated Square Plate With and Without Circular Hole

An experimental investigation of natural convection heat transfer from an isothermal horizontal,vertical and inclined heated square flat plates with and without circular hole, were carried out in two cases, perforated plates without an impermeable adiabatic hole "open core" and perforated plates with an impermeable adiabatic hole "closed core" by adiabatic plug. The experiments covered the laminar region with a range of Rayleih number of (1.11x106 ≤RaLo≤4.39x106 ), at Prandtle number (Pr=0.7). Practical experiments have been done with variable inclination angles from horizon (Ф=0o ,45o,90o,135oand 180o),facing upward (0o≤Ф&lt;90o), and downward (90o≤Ф&lt;180o). The results showed that the temperature gradient increases while the thermal boundary layer thickness decreases when Grashof number and perforation ratio (m) increase . The temperature gradient for inclined position facing upward is less than facing downward,while the thermal boundary layer thickness is greater. The temperature gradient decreases while the thermal boundary layer thickness increases for perforated plates with an adiabatic core as compared with perforated plates without an adiabatic core. The value of average Nusselt number increases with increasing perforation ratio, and Grashof number for all specimens with and without an adiabatic core, also increases by increase in inclination of plates approaching the higher value at vertical position (Ф=90o), then decreases with increasing inclination of plates till horizontal position (Ф=180o). The average Nusselt number values for perforated plates with an adiabatic core are lower than for perforated plates without an adiabatic core for all perforation ratios. Maximum heat transfer rate occurs at perforated plate with perforation ratio of (m=0.1) without adiabatic core for vertical position (Ф=90o), at a range of Grashof number (1.576x106≤GrLo≤6.292x106 ), while the rate of heat transfer decreases with increasing perforation ratio for plates with and without adiabatic core for decrease in heat transfer rate area. The rate of heat transfer forperforated plates with circular hole is more than for perforated plates with square hole at the same perforation ratios (m=0.1,0.16,0.24 and 0.36). It found that the lack of core flow decreases the overall heat transfer rate by (6.477%) . There was a good agreement for the experimental present work results compared with other pervious results .

Mixed convection Casson polymeric flow from a nonlinear stretching surface with radiative flux and non‐Fourier thermal relaxation effects: Computation with CSNIS

AbstractThermal non‐Newtonian polymer coating flows is growing as a major area in materials processing. Inspired by new developments in this field which require more sophisticated mathematical models, the current investigation examines the laminar viscoplastic boundary layer flow and mixed convective heat transfer over a power‐law nonlinear stretching surface. To simulate thermal relaxation effects the hyperbolic Cattaneo‐Christov heat flux model is deployed. The non‐Newtonian polymer characteristics are described by employing the Casson flow model. High temperature conditions invoke thermal radiation flux which is analyzed with an algebraic flux model. Via robust similarity transformations, the primitive partial differential conservation equations for momentum and energy equations are rendered into a system of coupled non‐linear ordinary differential equations with associated wall and free stream boundary conditions. The emerging boundary value problem is solved numerically with an efficient Chebyshev Spectral Newton Iterative scheme (CSNIS), in the MATLAB platform. The resulting solutions are discussed for different emerging parameters using graphs and tables. Validation is included with special cases from the literature. With increasing power law stretching index increases, the flow is decelerated, and temperatures are reduced. Increment in mixed convection parameter boosts the velocity but suppresses temperature and thermal boundary layer thickness. Increasing non‐Fourier Deborah number, temperatures are depleted whereas with increasing radiative flux parameter they are increased. With elevation in Casson non‐Newtonian parameter, velocity is decreased whereas temperature is enhanced and Nusselt number is suppressed.

Simulation of conductive fins effect on mixed convection heat transfer in a two-dimensional inclined channel using the lattice Boltzmann method

The simulation of the laminar mixed convection heat transfer inside a 2-D inclined channel was investigated here by employing the lattice Boltzmann method (LBM). The channel was equipped with conductive fins. The effects of inclination angle (φ), geometrical parameters (e and A), and a wide range of the Richardson numbers (Ri = 0.1-10) at fixed Reynolds number (Re = 100) on the heat transfer and flow structure were studied. The numerical model was validated by the published benchmark problems. Results were analyzed in terms of isotherms, streamlines, average Nusselt number, and average temperature. It found that the Nu increases with a decrease in the number, height, and width of fins. Results showed that when the channel equipped with 16 fins the reduction in Nu value reaches 62% compared to the empty channel. In addition, at Ri = 1 and N = 8, the Nusselt number decreased by about 52-77% for different fin widths compared to the channel without fins. However, the Richardson number and inclination angle enhanced the heat transfer rate in terms of the average Nusselt number. It revealed that a higher heat transfer rate belongs to the Ri = 10 and φ = 60°. The finding of this work showed the capability of LBM in simulating the mixed convection heat transfer in an inclined channel equipped with fins. A numerical analysis of the results produced a comprehensive set of results that could be utilized to reduce heat transfer. This paper can provide a novel framework for the design of air solar collectors and heat exchange systems.

Numerical simulation on energy transfer enhancement of a Williamson ferrofluid subjected to thermal radiation and a magnetic field using hybrid ultrafine particles

In this numerical investigation, completely developed laminar convective heat transfer characteristics of a Williamson hybrid ferronanofluid over a cylindrical surface are reported. This new model in 2D is engaged to examine the effects of the magnetic field, thermal radiation factor, volume fraction of ultrafine particles, and Weissenberg number with the help of the Keller box method. The numerical calculations are implemented at a magnetic parameter range of 0.4 to 0.8, volume fraction range of 0.0 to 0.1, and a Weissenberg number range of 0.1 to 0.8. The numerical outcomes concluded that the velocity increases when the thermal radiation parameter and the volume fraction of a nanoparticle are increased, but inverse impacts are obtained for the magnetic parameter and the Weissenberg number. The rate of energy transport increases with increasing thermal radiation and volume fraction, while it declines with increasing the magnetic parameter and Weissenberg number. The drag force shows a positive relationship with the thermal radiation parameter and has an opposite relationship with the Weissenberg number and the magnetic parameter. Furthermore, even when the magnetic field, thermal radiation, volume fraction, and Weissenberg number are all present, the heat transfer rate of Williamson hybrid ferronanofluid is greater than that of mono Williamson ferronanofluid.

Investigation on Laminar Flow and Heat Transfer of Helium–Xenon Gas Mixtures with Variable Properties

The space Brayton nuclear reactor system usually adopts the helium–xenon gas mixture (He–Xe) as the working fluid. The flow of He–Xe in the micro channel regenerator of the system is generally laminar. Since the properties of He–Xe are significantly different from those of common pure gases, the impact of this difference on the laminar flow and heat transfer needs to be evaluated. In present study, the numerical simulations of laminar convective heat transfer for helium, nitrogen and He–Xe are conducted by Ansys Fluent. Compared with simulation results, the applicability of existing laminar friction factor (f) and Nusselt number correlations is evaluated. By establishing the functions of property ratios with the temperature ratio and the mixing ratio, a new laminar f correlation for property-variable He–Xe is proposed. Results show that the calculation error of existing f correlations for He–Xe is obviously large, exceeding 13%. With the new f correlation, the predictions of laminar f for He–Xe are in good agreement with the simulation results in the fully developed region, and the calculation error is reduced to 3%.

Effect of non-uniform wall corrugations on laminar convective heat transfer through rectangular corrugated tube by using graphene nanoplatelets/MWCN hybrid nanofluid

Hybrid nanofluids and non-uniform wall corrugations have been reported to enhance convective heat transfer, which is not yet fully explored. In this study, the heat transfer efficacy of non-uniform wall corrugations on laminar convective heat transfer through a rectangular corrugated tube by using a hybrid nanofluid is investigated. Three different non-uniform wall corrugations; gradually increasing wall corrugation (GIWC), gradually decreasing wall corrugation (GDWC) and periodic wall corrugation (PWC) are investigated and compared with uniform wall corrugation (UWC). The applied hybrid nanofluid is a mixture of graphene nanoplatelets (GnP) and multi-walled carbon nanotubes (MWCN) with water, wherein the volume fractions of nanoparticles are 0.1%, 0.15%, and 0.2%. The computation is conducted under 2000 kW/m2 heat flux conditions on the heated surface of the corrugated wall with the Reynolds number in the range of 500–2000. The heat transfer efficacy and required pumping power are investigated for different wall corrugations and Reynolds numbers. The developed numerical model was validated with experimental studies from the published literature on different corrugated conduits. The computation results indicate that non-uniform wall corrugations GIWC, GDWC, and PWC provided better performance compared to uniform corrugation UWC. The average Nusselt number is the highest for PWC compared to others. At a constant Reynolds number 1000, the heat transfer was enhanced by an overall 15.61% for GDWC, 18.70% for GIWC, and 21.16% for PWC compared to UWC. In addition, PWC provided the highest increase in the pumping power requirement compared to others. At a constant Reynolds number 1000, the pumping power ratio is 1.37 for GDWC, 1.47 for GIWC, and 1.54 for PWC compared to UWC. GnP/MWCN-water hybrid nanofluid with a 0.2% volume fraction provided the highest enhancement in heat transfer compared to pure water. For PWC configuration, mixing nanoparticle volume fractions from 0.1% to 0.2%, the Nusselt number increases from 6.66% to 23.75%,while the pumping power ratio increases from 1.24 to 1.39 compared to water. It can be concluded that further experimental investigation is critical to validate the implementation of non-uniform wall corrugation for enhancing the thermal performance of rectangular corrugated tubes.

Numerical investigation, environmental consideration, and the use of machine learning in optimizing the dimensions of a rectangular blade between two blades in the presence of a magnetic field (Two-phase method)

Human life is being impacted by nanotechnology in many ways, including economics, production, and the environment. The present study has numerically investigated the laminar flow and natural convection heat transfer (NACHT) in a square enclosure filled with nanofluids (NFs), considering the entropy generation (ETG) and Be. The 2D enclosure includes two rectangular fins attached to the left hot wall and a single fin of very high temperature positioned in the middle of the enclosure. A constant magnetic field (MCF) has been applied to the enclosure. The average Nu, maximum value of stream function (MXSF), generated entropy, and Be have been examined by changing the length and thickness of the middle fin and the distance between heated fins (fin spacing). Finally, these parameters have been optimized through machine learning methods to determine the best case with the highest HT rate and lowest ETG rate. A two-phase model based on control volume has been used for NFs flow simulation. Results indicated that increasing fin length (w) increases Be and decreases the MXSF. Also, by increasing the w, the Nu and generated entropy initially decrease and then increase. Increasing the fin thickness (b) and fin spacing lead to a decrease in the MXSF. Increasing thickness for the fin increases the Nu but decreases it for larger fin spacing.Furthermore, the studied model is less harmful to the environment than existing models.

Numerical modeling for thermal behavior of nanomaterial laminar flow and convective heat transfer in appearance of magnetic field

The new computational approach has been incorporated for modeling the nanofluid flow through a cavity involving a wavy inner wall which experiences uniform flux. The fluid was created by loading alumina powders with various shapes within the water. The stream function formulation was applied after employing Darcy law and final equations have been generated. The container is under the impact of Lorentz force in addition to buoyancy forces. With loading additives, Nu enhances about 17.07% and 41.57% depending on the amount of Ha. The greater value of Ha makes the effect of adding nanoparticles become more sensible. With increasing the value of shape factor, Nu increases about 11.76%. As Ha grows, the rate of cooling decreases about 41.31%. With augment of Ra, Nu enhances about 50.41% in absence of Ha.

Laminar convective heat transfer, entropy generation, and exergy efficiency studies on ethylene glycol based nanofluid containing nanodiamond nanoparticles

Laminar convective heat transfer, entropy generation, and exergy efficiency studies on ethylene glycol based nanofluid containing nanodiamond nanoparticles

NUMERICAL INVESTIGATION OF LAMINAR NATURAL CONVECTION HEAT TRANSFER IN SQUARE ENCLOSURE WITH TWO HOT INNER CYLINDERS: THE EFFECT OF ONE ELLIPTICAL CYLINDER WITH VARIOUS ASPECT RATIOS

This study conducted two-dimensional numerical simulations of the natural convection in a square enclosure with two inner hot cylinders: one is circular and the other is elliptical at Rayleigh numbers (Ra) of 10&lt;sup&gt;4&lt;/sup&gt; &amp;le; Ra &amp;le; 10&lt;sup&gt;6&lt;/sup&gt; and a Prandtl number (Pr) of Pr &amp;#61; 0.7 (air). The aspect ratio (AR) of the elliptical cylinder ranging from 0.25 &amp;le; AR &amp;le; 4. The flow structures and heat transfer mechanism within a square enclosure were investigated with respect to the transition of the flow regime from steady to unsteady state. The thermal and flow fields reached steady state in the Rayleigh number range of 10&lt;sup&gt;4&lt;/sup&gt; &amp;le; Ra &amp;le; 10&lt;sup&gt;5&lt;/sup&gt;, regardless of the variation in aspect ratio. However, at Ra &amp;#61; 10&lt;sup&gt;6&lt;/sup&gt;, the numerical solutions show time-dependent characteristics in the case of the lower and upper elliptical cylinders except at AR &amp;#61; 4. Thus, the steady or unsteady flow regime is strongly dependent on the Rayleigh number and aspect ratio of the elliptical cylinder. Thus, the variation of the aspect ratio has a significant effect on the heat transfer characteristics at the walls of the enclosure and the surface cylinders.

A natural convection conjugate heat transfer of Nano-Encapsulated Phase Change Materials (NEPCMs) in an inclined blocked square enclosure

The current investigation deals numerically with a two-dimensional steady laminar flow natural convection heat transfer of a dilute suspension of a base fluid and suspension of Nano Encapsulated Phase Change Materials (NEPCMs) nanoparticles saturated inside an inclined square enclosure with a centered internal conducting square block with a ratio of 0.5. The enclosure inclination angle varies from 0° to 90° with increments of 15°, and block thermal conductivity to the host fluid has been chosen to be 0.2 and 5. The obtained results are in excellent agreement with those available in the literature for validation. The findings reveal that the heat transfer depends considerably on the inclination angle, block thermal conductivity ratio, and Stefan number for all considered ranges. The present results suggest that the average Nusselt number rate is notably enhanced as the inclination angle rises from 0° to 90° for thermal conductivity KR = 0.2 and KR = 5; this enhancement is more evident as the Rayleigh number is increased. The average Nusselt number improves by 5%, 153%, 558%, and 1120% for Rayleigh numbers of 103, 104, 105, and 106, respectively, when the inclination angle varies from 0° to 90° for a thermal conductivity ratio of 0.2. This improvement is somewhat reduced in the case of a thermal conductivity of 5, where the average Nusselt number grows by around 0.4%, 27%, 213%, and 537% for Rayleigh numbers 103, 104, 105, and 106, respectively. The higher value of the latent heat capacity of encapsulated particles enhances the heat transfer rate; the decreased Stefan number induces a notable improvement in the average Nusselt number. This enhancement is more evident as the inclination angle increases. The findings clearly show a considerable influence on the isotherms and heat capacity ratio distributions for all considered block thermal conductivities and Rayleigh numbers.

Superhydrophobicity Can Enhance Convective Heat Transfer in Pressure-Driven Pipe Flow

Summary Theoretical evidence is given that it is possible for superhydrophobicity to enhance steady laminar convective heat transfer in pressure-driven flow along a circular pipe or tube with constant heat flux. Superhydrophobicity here refers to the presence of adiabatic no-shear zones in an otherwise solid no-slip boundary. Adding such adiabatic no-shear zones reduces not only hydrodynamic friction, leading to greater fluid volume fluxes for a given pressure gradient, but also reduces the solid surface area through which heat enters the fluid. This leads to a delicate trade-off between competing mechanisms so that the net effect on convective heat transfer along the pipe, as typically measured by a Nusselt number, is not obvious. Existing evidence in the literature suggests that superhydrophobicity always decreases the Nusselt number, and therefore compromises the net heat transfer. In this theoretical study, we confirm this to be generally true but, significantly, we identify a situation where the opposite occurs and the Nusselt number increases thereby enhancing convective heat transfer along the pipe.

Study of mixed convection in a horizontal cylindrical pipe partially filled with a porous medium saturated with a single fluid

The three-dimensional numerical simulation of laminar mixed convection heat transfer with and without porous medium in a cylindrical duct was carried out in this study. The cylinder is divided into three parts. The first and the third parts are adiabatic, while the central part is subjected to a constant heat flux. The effects of Darcy number, porosity, permeability and Nusselt number on heat transfer were considered. This problem is modeled by the Navier-stokes equations in the fluid region and by the Darcy-Brinkman-Forchheimer equation in the porous region, and for the thermal field the energy equation is used. The resolution of these equations was performed by the computational fluid dynamics code Ansys15.2 software using finite volume approach. The obtained results showed an accumulation of the hot fluid with stratification in the upper part of the pipe while the lower region of the pipe remained at practically constant temperature. The effect of the porous matrix is very noticeable on the velocity profile. The insertion of a porous high permeability medium significantly improves heat transfer.

Numerical study of laminar convective heat transfer from a corrugated pipe into an Al2O3–AlN/H2O hybrid nanofluid

Heat transfer in corrugated pipes carrying hybrid nanofluid is investigated. A saw-tooth corrugated pipe is considered. The nanofluid comprises of colloidal mixture of Aluminum Oxide and Aluminum Nitride nanoparticles, mixed as suspension in water. The heat transfer from the hot pipe surface (at 2000 W/m2) into the nanofluid is computed at different flow Reynolds numbers for laminar flow, at varying volumetric nanoparticle concentrations (1%–6%). The effects on the heat transfer and pumping power requirement with increasing Reynolds number and nanoparticle concentrations are studied. The effects of changing the corrugation configurations, such as the corrugation height and the placement eccentricity of the individual corrugation elements, on the heat transfer and pumping power are also examined. Results indicate that the pipe corrugations considerably enhance the heat transfer compared to the corresponding straight pipe case. With increasing Reynolds number and nanoparticle concentration, the heat transmission enhances, and the pumping power is correspondingly increased. The corrugation height and placement eccentricity significantly affect the heat transfer as well as the requirement of the pumping power. The average Nusselt number and pumping power both increase linearly with increasing corrugation height and the corrugation placement eccentricity.