Numerical investigation of heat transfer enhancement of nanofluids in an inclined lid-driven triangular enclosure

The behavior of nanofluids is investigated numerically in an inclined lid-driven triangular enclosure to gain insight into convective recirculation and flow processes induced by a nanofluid. The present model is developed to examine the behavior of nanofluids taking into account the solid volume fraction δ. Fluid mechanics and conjugate heat transfer, described in terms of continuity, linear momentum and energy equations, were predicted by using the Galerkin finite element method. Comparisons with previously published work on the basis of special cases are performed and found to be in excellent agreement. Numerical results are obtained for a wide range of parameters such as the Richardson number, and solid volume fraction. Copper–water nanofluids are used with Prandtl number, Pr = 6.2 and solid volume fraction δ is varied as 0%, 4%, 8% and 10%. The streamlines, isotherm plots and the variation of the average Nusselt number at the hot surface as well as average fluid temperature in the enclosure are presented and discussed in detailed. It is observed that solid volume fraction strongly influenced the fluid flow and heat transfer in the enclosure at the three convective regimes. Moreover, the variation of the average Nusselt number and average fluid temperature in the cavity is linear with the solid volume fraction.     

Buoyancy-driven heat transfer of water-based Al2O3 nanofluids in a rectangular cavity

In this paper, thermal characteristics of natural convection in a rectangular cavity heated from below with water-based nanofluids containing alumina (Al₂O₃ nanofluids) are theoretically investigated with Jang and Choi’s model for predicting the effective thermal conductivity of nanofluids and various models for the effective viscosity. To validate theoretical results, we compare theoretical results with experimental results presented by Putra et al. It is shown that the experimental results are put between a theoretical line derived from Jang and Choi’s model and Einstein’s model and a theoretical line from Jang and Choi’s model and Pak and Cho’s correlation. In addition, the effects of the volume fraction, the size of nanoparticles, and the average temperature of nanofluids on natural convective instability and heat transfer characteristics of water-based Al₂O₃ nanofluids in a rectangular cavity heated from below are theoretically presented. Based on the results, this paper shows that water-based Al₂O₃ nanofluids is more stable than base fluid in a rectangular cavity heated from below as the volume fraction of nanoparticles increases, the size of nanoparticles decreases, or the average temperature of nanofluids increases. Finally, we theoretically show that the ratio of heat transfer coefficient of nanofluids to that of base fluid is decreased as the size of nanoparticles increases, or the average temperature of nanofluids is decreased.     

Review of convective heat transfer enhancement with nanofluids

Nanofluids are considered to offer important advantages over conventional heat transfer fluids. Over a decade ago, researchers focused on measuring and modeling the effective thermal conductivity and viscosity of nanofluids. Recently important theoretical and experimental research works on convective heat transfer appeared in the open literatures on the enhancement of heat transfer using suspensions of nanometer-sized solid particle materials, metallic or nonmetallic in base heat transfer fluids. The purpose of this review article is to summarize the important published articles on the enhancement of the forced convection heat transfer with nanofluids.     

Investigation of heat transfer enhancement in an enclosure filled with nanofluids using multiple relaxation time lattice Boltzmann modeling

This work investigates the heat transfer performance in an enclosure including nanofluids with a localized heat source. The velocity field is solved by multiple relaxation time lattice Boltzmann modeling (MRT) which has superior numerical advantages to single relaxation time lattice Boltzmann modeling (SRT); however, heat transfer is simulated separately using SRT-lattice Boltzmann modeling. The hydrodynamics and thermal fields are then coupled together using the Boussinesq approximation. The main objective of this study is to investigate the influence of several pertinent parameters such as Rayleigh number, solid particle volume fraction of nanoparticles, and the geometry as well as location of the localized heat source on the heat transfer performance of nanofluids. The results obtained from lattice Boltzmann modeling clearly indicate that heat transfer augmentation is possible using nano-fluids in comparison to conventional fluids, resulting in the compactness of many industrial devices.     

Heat transfer enhancement in microchannel heat sinks using nanofluids

Heat transfer enhancement in a 3-D microchannel heat sink (MCHS) using nanofluids is investigated by a numerical study. The addition of nanoparticles to the coolant fluid changes its thermophysical properties in ways that are closely related to the type of nanoparticle, base fluid, particle volume fraction, particle size, and pumping power. The calculations in this work suggest that the best heat transfer enhancement can be obtained by using a system with an Al₂O₃–water nanofluid-cooled MCHS. Moreover, using base fluids with lower dynamic viscosity (such as water) and substrate materials with high thermal conductivity enhance the thermal performance of the MCHS. The results also show that as the particle volume fraction of the nanofluid increases, the thermal resistance first decreases and then increases. The lowest thermal resistance can be obtained by properly adjusting the volume fraction and pumping power under given geometric conditions. For a moderate range of particle sizes, the MCHS yields better performance when nanofluids with smaller nanoparticles are used. Furthermore, the overall thermal resistance of the MCHS is reduced significantly by increasing the pumping power. The heat transfer performance of Al₂O₃–water and diamond–water nanofluids was 21.6% better than that of pure water. The results reported here may facilitate improvements in the thermal performance of MCHSs.     

Effect of rotating cylinder on heat transfer in a square enclosure filled with nanofluids

Convective heat transfer in a differentially heated square enclosure with an inner rotating cylinder is studied theoretically. The free space between the cylinder and the enclosure walls is filled with water–Ag, water–Cu, water–Al₂O₃ or water–TiO₂ nanofluids. The governing equations are formulated for velocity, pressure and temperature formulation and are modeled in COMSOL, a partial differential equation (PDE) solver based on the Galerkin finite element method (GFEM). The governing parameters considered are the solid volume fraction, 0.0 ? ? ? 0.05, the cylinder radius, 0 ? R ? 0.3 and the angular rotational velocity, ?1000 ? Ω ? 1000. The results are presented to show the effect of these parameters on the heat transfer and fluid flow characteristics. It is found that the strength of the flow circulation is much stronger for a higher nanoparticle concentration, a better thermal conductivity value and a smaller cylinder with a faster, negative rotation. The maximum heat transfer are obtained at a high nanoparticle concentration with a good conductivity value, a slow positive rotation and a moderate cylinder size located in the center of the enclosure.     

Thermal wave effects on heat transfer enhancement in nanofluids suspensions

The heat transfer enhancement revealed experimentally in nanofluids suspensions is being investigated theoretically at the macro-scale level aiming at explaining the possible mechanisms that lead to such impressive experimental results. In particular, while the possibility that thermal wave effects via hyperbolic heat conduction could have explain the excessively improved effective thermal conductivity of the suspension the comparison with experimental results rules-out this explanation.     

Natural convection heat transfer enhancement in horizontal concentric annuli using nanofluids

Heat transfer enhancement in horizontal annuli using nanofluids is investigated. Water-based nanofluid containing various volume fractions of Cu, Ag, Al₂O₃ and TiO₂ nanoparticles is used. The addition of the different types and different volume fractions of nanoparticles were found to have adverse effects on heat transfer characteristics. For high values of Rayleigh number and high L/D ratio, nanoparticles with high thermal conductivity cause significant enhancement of heat transfer characteristics. On the other hand, for intermediate values of Rayleigh number, nanoparticles with low thermal conductivity cause a reduction in heat transfer. For Ra = 10³ and Ra = 10⁵ the addition of Al₂O₃ nanoparticles improves heat transfer. However, for Ra = 10⁴, the addition of nanoparticles has a very minor effect on heat transfer characteristics.     

Heat transfer enhancement using hybrid nanofluids in spiral plate heat exchangers

A possible way to enhance the rate of heat transfer in the spiral plate heat exchanger (SPHE) is by employing hybrid nanofluids as its working medium. Hence, in the present work, effects of hybrid nanofluids on the thermal performance of SPHE has been investigated numerically. First, a countercurrent SPHE is designed and modeled. Later, simulation of SPHE has been carried out by employing conventional fluid , nanofluids , and hybrid nanofluids to investigate the heat transfer rates. Finally, the performance of SPHE using hybrid nanofluid is compared with that of using water and nanofluids. The heat transfer augmentation of approximately 16%‐27% with hybrid nanofluids of overall 4% nanoparticles volume concentration and 10%‐16% with 2% nanoparticles volume concentration is observed when compared with that of pure water. Therefore, it can be inferred that the application of hybrid nanofluids in SPHE seems to be one of the promising solutions for augmentation of its thermal performance.     

Simulation of heat transfer enhancement in nanofluids using dissipative particle dynamics

The current paper applied dissipative particle dynamics (DPD) approach to investigate heat transfer within nanofluids. The DPD approach was applied to study natural convection in a differential heated enclosure by considering the viscosity and the thermal conductivity of the nanofluid to be dual function of temperature and volume fraction of nanoparticles. Experimental data for viscosity and thermal conductivity are incorporated in the current DPD model to mimic energy transport within nanofluids. This incorporation is done through the modification of the dissipative weighting function that appears in the dissipative force vector and the dissipative heat flux. For the entire range of Rayleigh number considered in this study, it was found that the DPD results show a deterioration in heat transfer in the enclosure due to the presence of nanoparticles for φ > 4%. However, some slight enhancement is shown to take place for small volume fraction of nanoparticles, φ ≤ 4%. The DPD results experienced some degree of compressibility at high values of Rayleigh number Ra ≥ 10⁵.     

Numerical study of transient buoyancy-driven convective heat transfer of water-based nanofluids in a bottom-heated isosceles triangular enclosure

A numerical study of transient buoyancy-driven convective heat transfer of water-based nanofluids inside a bottom-heated horizontal isosceles triangular cylinder is presented. Nano-sized copper oxide (CuO) particles suspended in water with two different volume fractions are considered. The thermophysical properties of water in the presence of nanoparticles are predicted using existing models, in which the effects of the Brownian motion of nanoparticles are taken into account. It is shown that pitchfork bifurcation appears for relatively high Grashof numbers and the critical Grashof number is found to be 5.60 × 10⁴. The predicted development of convective flow of nanofluids is presented by means of the average Nusselt number over the bottom. Additionally, the flow development time towards a steady/quasi-steady state and the time-averaged Nusselt number are scaled with Grashof number. It is also shown that at constant Grashof numbers the time-averaged Nusselt number is lowered as more nanoparticles are added to the base liquid and will be overestimated if the Brownian motion effects are not considered.     

Numerical investigation of heat transfer enhancement using carbon nanotube-based non-Newtonian nanofluids

In this study convective heat transfer of multi-wall carbon nanotube (MWCNT)-based nanofluids in a straight tube under constant wall heat flux condition is numerically investigated. To achieve this goal Navier–Stokes equations are solved using the finite volume technique with considering CNT-based nanofluids as non-Newtonian fluids of shear-thinning character using the non-Newtonian power law model. The objectives of this research are to provide detailed information of non-Newtonian behavior of CNT nanofluids, comparison of the numerical simulation predictions to the experimental measurements and investigation of non-Newtonian effects on the local heat transfer of the CNT nanofluid and compare the thermal performance of the CNT nanofluids and conventional fluids. As a result the heat transfer coefficient is dominated by the wall region due to non-Newtonian behavior of CNT nanofluid. The results reported in this paper illustrate that the numerical simulation can be one of the most powerful and beneficial tools for the CNT nanofluids optimization and performance analysis.     

Heat transfer enhancement of nanofluids flow in microtube with constant heat flux

In this paper, laminar convective heat transfer in a two-dimensional microtube (MT) with 50 μm diameter and 250 μm length with constant heat flux is numerically investigated. The governing (continuity, momentum and energy) equations were solved using the finite volume method (FVM) with the aid of SIMPLE algorithm. Different types of nanofluids Al₂O₃, CuO, SiO₂ and ZnO, with different nanoparticle size 25, 45, 65 and 80 nm, and different volume fractions ranged from 1% to 4% using ethylene glycol as a base fluid were used. This investigation covers Reynolds number in the range of 10 to 1500. The results have shown that SiO₂–EG nanofluid has the highest Nusselt number, followed by ZnO–EG, CuO–EG, Al₂O₃–EG, and lastly pure EG. The Nusselt number for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nusselt number increases with Reynolds number.     

Experimental study of heat transfer enhancement in a drag-reducing two-dimensional channel flow

In this paper, drag reduction and heat transfer enhancement were studied in a fully developed two-dimensional water channel flow. Surfactant solutions at different concentrations, namely 30, 70, 80 and 90 ppm, were used to examine the influence of surfactant additives on the skin friction drag and heat transfer coefficient. The magnitudes of the maximum achievable drag reduction at the above four different surfactant concentrations are about 7%, 30%, 50% and 55%, respectively. The present results show that there is no heat transfer reduction when 30 ppm of surfactant is added to the flow. With the increase of surfactant concentration to 90 ppm, heat transfer rate was reduced by about 55%. The critical Reynolds number for loss of heat transfer reduction increases with the increase of surfactant concentration. The effect of the low-profile vortex generators on heat transfer rate was examined for the surfactant concentration of 90 ppm. The results show that the averaged Nusselt number is enhanced by 180%, 160% and 150% for the Reynolds numbers of 7000, 12,000 and 16,200, respectively, as compared with that obtained in the surfactant solution without the use of vortex generators and yet the pressure drop penalty for heat transfer enhancement is rather small.     

Heat transfer enhancement of nanofluids in a double pipe heat exchanger with louvered strip inserts

The effect of using louvered strip inserts placed in a circular double pipe heat exchanger on the thermal and flow fields utilizing various types of nanofluids is studied numerically. The continuity, momentum and energy equations are solved by means of a finite volume method (FVM). The top and the bottom walls of the pipe are heated with a uniform heat flux boundary condition. Two different louvered strip insert arrangements (forward and backward) are used in this study with a Reynolds number range of 10,000 to 50,000. The effects of various louvered strip slant angles and pitches are also investigated. Four different types of nanoparticles, Al₂O₃, CuO, SiO₂, and ZnO with different volume fractions in the range of 1% to 4% and different nanoparticle diameters in the range of 20 nm to 50 nm, dispersed in a base fluid (water) are used. The numerical results indicate that the forward louvered strip arrangement can promote the heat transfer by approximately 367% to 411% at the highest slant angle of α = 30° and lowest pitch of S = 30 mm. The maximal skin friction coefficient of the enhanced tube is around 10 times than that of the smooth tube and the value of performance evaluation criterion (PEC) lies in the range of 1.28–1.56. It is found that SiO₂ nanofluid has the highest Nusselt number value, followed by Al₂O₃, ZnO, and CuO while pure water has the lowest Nusselt number. The results show that the Nusselt number increases with decreasing the nanoparticle diameter and it increases slightly with increasing the volume fraction of nanoparticles. The results reveal that there is a slight change in the skin friction coefficient when nanoparticle diameters of SiO₂ nanofluid are varied.     

Natural convection heat transfer in a circular enclosure

Natural convection heat transfer in a circular enclosure, one half of which was heated and the other half of which was cooled, was investigated experimentally, focusing on the effect of the inclination angle. The experiments were carried out with water. Flow and temperature field were visualized by using the aluminum and liquid-crystal suspension method. The results show that with downward heating the heat transfer coefficient increased as the inclination angle of the boundary between the heating wall and the cooling wall approached the vertical. But with upward heating, the heat transfer coefficient showed minimal change, exhibiting a small peak value when the inclination angle was γ ˜ –45°. The heat transfer coefficient of a flat circular enclosure was estimated from the circular enclosure's heat transfer coefficient. These results can be explained by the obtained flow and temperature fields. © 1999 Scripta Technica, Heat Trans Asian Res, 28(2): 152–163, 1999     

Numerical simulation on enhancement of natural convection heat transfer by acoustic cavitation in a square enclosure

Natural convection heat transfer of liquid in a square enclosure with and without acoustic cavitation is numerically investigated. In order to model the effect of sound field, a prescribed periodic change pressure is imposed on liquid in the region with ultrasonic beam. The cavitation model which takes into consideration such effects as phase change, bubble dynamics, and noncondensable gases is established. In order to capture the cavitation phenomenon and obtain a realistic pressure field and cavitation zones, the time step is set to quarter period of sound pressure and a relatively small relaxation factor for momentum equation and near zero relaxation factors for vaporization mass and density are used. The simulation results show that both liquid temperature uniformity and heat transfer coefficient of heating surface are greatly improved by means of acoustic cavitation. This phenomenon can be explained by jet-flow bundle formed by the asymmetry collapse of cavitation bubbles on the heating surface. The jet-flow bundle cannot only enhance fluid mixing around the heating surface and thus improve the uniformity of liquid temperature in the square enclosure, but also directly impact the heating surface and thus make thermal boundary layer become thin. The field synergy principle is introduced to analyze the heat transfer enhancement by acoustic cavitation.     

Natural convection of water-based nanofluids in an inclined enclosure with a heat source

This study investigates natural convection heat transfer of water-based nanofluids in an inclined square enclosure where the left vertical side is heated with a constant heat flux, the right side is cooled, and the other sides are kept adiabatic. The governing equations are solved using polynomial differential quadrature (PDQ) method. Calculations were performed for inclination angles from 0° to 90°, solid volume fractions ranging from 0% to 20%, constant heat flux heaters of lengths 0.25, 0.50 and 1.0, and a Rayleigh number varying from 10⁴ to 10⁶. The ratio of the nanolayer thickness to the original particle radius is kept at a constant value of 0.1. The heat source is placed at the center of the left wall. Five types of nanoparticles are taken into consideration: Cu, Ag, CuO, Al₂O₃, and TiO₂. The results show that the average heat transfer rate increases significantly as particle volume fraction and Rayleigh number increase. The results also show that the length of the heater is also an important parameter affecting the flow and temperature fields. The average heat transfer decreases with an increase in the length of the heater. As the heater length is increased, the average heat transfer rate starts to decrease for a smaller inclination angle (it starts to decrease with inclination at 90° for ? = 0.25, 60° for ? = 0.50, 45° for ? = 1.0, respectively).     

Numerical research of nature convective heat transfer enhancement filled with nanofluids in rectangular enclosures

Heat transfer enhancement utilizing nanofluids in a two-dimensional enclosure is investigated for various pertinent parameters. The Khanafer's model is used to analyze heat transfer performance of nanofluids inside an enclosure taking into account the solid particle dispersion. Transport equations are model by a stream function-vorticity formulation and are solved numerically by finite-difference approach. Based upon the numerical predictions, the effects of Rayleigh number (Ra) and aspect ratio (AR) on the flow pattern and energy transport within the thermal boundary layer are presented. The diameter of the nanoparticle d_p is taken as 10 nm in nanofluids. The buoyancy parameter is 10³ ≤ Ra ≤ 10⁶ and aspect ratios (AR) of two-dimensional enclosure are 1/2, 1, 2. Results show that increasing the buoyancy parameter and volume fraction of nanofluids cause an increase in the average heat transfer coefficient. Finally, the empirical equation was built between average Nusselt number and volume fraction.     

Natural convection heat transfer in a nanofluid-filled trapezoidal enclosure

Heat transfer enhancement utilizing nanofluids in a trapezoidal enclosure is investigated for various pertinent parameters. Transport equations are modelled by a stream-vorticity formulation and solved numerically by finite difference approach. The inclined sloping boundaries is treated by adopting staircase-like zigzag lines. Based upon the numerical predictions, the effects of Grashof number, inclination angle of the sloping wall, volume fraction of nanoparticles on flow and temperature patterns as well as the heat transfer rate within the enclosure are presented. Water–Cu and water–Al₂O₃ nanofluids were tested. We found that acute sloping wall and Cu nanoparticles with high concentration are effective to enhance the rate of heat transfer. We also developed a new correlation for the average Nusselt number as a function of the angle of the sloping wall, effective thermal conductivity and viscosity as well as Grashof number.