Experimental study of enhanced heat transfer by addition of CuO nanoparticle

An energy storage system has been designed to study the thermal characteristics of paraffin wax with an embedded nano size copper oxide (CuO) particle. This paper presents studies conducted on phase transition times, heat fraction as well as heat transfer characteristics of paraffin wax as phase change material (PCM) embedded with CuO nanoparticles. 40?nm mean size CuO particles of 2, 5 and 10% by weight were dispersed in PCM for this study. Experiments were performed on a heat exchanger with 1.5–10?l/min of heat transfer fluid (HTF) flow. Time-based variations of the temperature distributions are revealed from the results of observations of melting and solidification curves. The results strongly suggested that the thermal conductivity enhances 6, 6.7 and 7.8% in liquid state and in dynamic viscosity it enhances by 5, 14 and 30% with increasing mass fraction of the CNEPs. The thermal conductivity ratio of the composites can be augmented by a factor up to 1.3. The heat transfer coefficient during solidification increased about 78% for the maximum flow rate. The analysis of experimental results reveals that the addition of copper oxide nanoparticles to the paraffin wax enhances both the conduction and natural convection very effectively in composites and in paraffin wax. The paraffin wax-based composites have great potential for energy storage applications like industrial waste heat recovery, solar thermal applications and solar based dynamic space power generation with optimal fraction of copper oxide nanoparticles.     

Heat transfer characteristics of multi-walled carbon nanotubes suspension in a developing channel flow

The present study experimentally investigates the effect of multi wall carbon nanotubes (MWCNT) suspensions on the convective heat transfer coefficients. The MWCNT suspensions used in this study were prepared by dispersing MWCNTs in deionized water 0.25 wt% arab gum solution. The heat transfer characteristics were measured for thermally developing laminar flow in a finite length horizontal circular pipe under isothermal wall conditions. The study was conducted over a range of Reynolds number of 300–2,300, based on 0.8 mm tube diameter. Results indicate enhancements of the convective heat transfer coefficient as a function of Reynolds number and volume fractions. An average enhancement of heat transfer coefficient of 50 % was observed over the base fluid. An overall increase of pumping force varying from 20 to 30 % over the flowing range is observed. The results suggest an optimum MWCNT volume fraction point of 0.1 % which gives the best heat transfer enhancement.     

Analytical approximation for solidification processes in PCM storage with internal fins: imposed heat flux

Uses of thermal energy storage systems have expanded notably in recent decades. In thermal energy systems, internal heat transfer enhancement techniques such as fins are often used because of the low thermal conductivity of the phase change materials (PCMs). In this paper, solidification of a PCM is studied in a rectangular storage with horizontal internal plate fins and an imposed constant heat flux on the vertical walls. A simplified analytical solution is presented and its results are compared to those for a numerical approach based on an enthalpy method. The fraction of solidified PCM in storage is calculated with the derived analytical model which determines how much of the storage is solidified after a certain time. The results show that the analytical model satisfactorily estimates the solid–liquid interface and the temperature distribution for the fin, which are useful in the design of PCM-based thermal energy storage or cooling systems.     

Numerical study of solidification of a nano-enhanced phase change material (NEPCM) in a thermal storage system

The effects of nanoparticle dispersion on solidification of a Cu-n-hexadecane nanofluid inside a vertical enclosure are investigated numerically for different temperatures of the left vertical wall. An enthalpy porosity technique is used to trace the solid-liquid interface. The resulting nanoparticle-enhanced phase change materials (NEPCMs) exhibit enhanced thermal conductivity in comparison to the base material. The effect of the wall temperature and nanoparticle volume fraction are studied in terms of the solid fraction and the shape of the solid-liquid phase front. It has been found that a lower wall temperature and a higher nanoparticle volume fraction result in a larger solid fraction. The increase in the heat release rate of the NEPCM shows its great potential for diverse thermal energy storage applications.     

Experimental characterization of convective heat transfer with MWCNT based nanofluids under laminar flow conditions

This study describes an investigation on the convective heat transfer performance of aqueous suspensions of multiwalled carbon nanotubes. The results suggested an increase on heat transfer coefficient of 47 % for 0.5 % volume fraction. Moreover, the enhancement observed during thermal conductivity assessment, cannot fully explain the heat transfer intensification. This could be associated to the random movements among the particles through a fluid, caused by the impact of the base fluid molecules.     

Experimental investigation on thermal conductivity and viscosity of aluminum nitride nanofluid

Aluminum nitride nanoparticles (AlNs) have been found to be a good additive for enhancing the thermal conductivity of traditional heat exchange fluids.At a volume fraction of 0.1,the thermal conductivity enhancement ratios are 38.71% and 40.2%,respectively,for ethylene glycol and propylene glycol as the base fluids.Temperature does not have much influence on the enhanced thermal conductivity ratios of the nanofluids,though a volume fraction of 5.0% appears to signify a critical concentration for rheology:fo...     

Non Newtonian annular alloy solidification in mould

The annular solidification of an aluminium–silicon alloy in a graphite mould with a geometry consisting of horizontal concentric cylinders is studied numerically. The analysis incorporates the behavior of non-Newtonian, pseudoplastic (n?=?0.2), Newtonian (n?=?1), and dilatant (n?=?1.5) fluids. The fluid mechanics and heat transfer coupled with a transient model of convection diffusion are solved using the finite volume method and the SIMPLE algorithm. Solidification is described in terms of a liquid fraction of a phase change that varies linearly with temperature. The final results make it possible to infer that the fluid dynamics and heat transfer of solidification in an annular geometry are affected by the non-Newtonian nature of the fluid, speeding up the process when the fluid is pseudoplastic.     

Carbon nanotube glycol nanofluids: Photo-thermal properties,thermal conductivities and rheological behavior

The efficiency and effectiveness of solar energy capture and storage are to a large extent functions of the heat transfer and storage capacity of the medium used. This paper investigates the potential of using carbon nanotube (CNT)-glycol nanosuspension as such a medium, prepared by freeze drying-ultrasonic dispersing after oxidation treatment with HNO₃. The influences of the mass fraction of CNTs glycol nanofluids and temperatures on photo-thermal properties, thermal conductivities and rheological behavior were investigated. The results show that CNTs with oxidation treatment exhibited good dispersing performance. Strong optical absorption of the CNTs glycol nanofluids was detected in the range of 200–2500 nm. At room temperature, 18% enhancement was found in the photo-thermal conversion efficiency of the 0.5% mass fraction CNTs glycol nanofluids in comparison to the basic fluids, without significant increase in viscosity. At 55 °C, CNTs glycol nanofluids with 4.0% mass fraction exhibited much lower viscosity and 25.4% higher thermal conductivity in comparison to that of pure glycol at room temperature.     

Determination of thermal conductivities of Sn–Zn lead-free solder alloys with radial heat flow and Bridgman-type apparatus

The variations of thermal conductivities of solid phases versus temperature for pure Sn, pure Zn and Sn–9 wt.% Zn, Sn–14 wt.% Zn, Sn–50 wt.% Zn, Sn–80 wt.% Zn binary alloys were measured with a radial heat flow apparatus. The thermal conductivity ratios of liquid phase to solid phase for the pure Sn, pure Zn and eutectic Sn–9 wt.% Zn alloy at their melting temperature are found with a Bridgman-type directional solidification apparatus. Thus, the thermal conductivities of liquid phases for pure Sn, pure Zn and eutectic Sn–9 wt.% Zn binary alloy at their melting temperature were evaluated by using the values of solid phase thermal conductivities and the thermal conductivity ratios of liquid phase to solid phase.     

Planar solidification of a warm flowing-liquid under different boundary conditions

Planar solidification of a warm flowing liquid with the convective heat transfer to the growing solid layer, has been analysed for the boundary conditions of constant temperature, constant heat flux and convective heat flux at the surface respectively. The mathematical formulation of the problem resulted in a coupled set of two differential equations in temperature and solid thickness as function of position, time and the problem parameters. Analytical expressions for the temperature distribution within the growing solid layer, the rate of solidification and the solidification time are obtained. The perturbation techniques employed here is simple and straight forward in contrast with the earlier techniques. Good agreement between the experimental results and the present solutions is obtained for the convective heat flux boundary condition. The results of this analysis are useful in the design and analysis of experiments dealing with freezing/melting in one dimension. The role of the parameter Stefan number which is small for phase change materials, is discussed in context with the storage of thermal energy.     

Numerical modelling of a melting-solidification cycle of a phase-change material with complete or partial melting

A high accuracy numerical model is used to simulate an alternate melting and solidification cycle of a phase change material (PCM). We use a second order (in time and space) finite-element method with mesh adaptivity to solve a single-domain model based on the Navier-Stokes-Boussinesq equations. An enthalpy method is applied to the energy equation. A Carman-Kozeny type penalty term is introduced in the momentum equation to bring the velocity to zero inside the solid region. The mesh is dynamically adapted at each time step to accurately capture the interface between solid and liquid phases, the boundary-layer structure at the walls and the multi-cellular unsteady convection in the liquid. We consider the basic configuration of a differentially heated square cavity filled with an octadecane paraffin and use experimental and numerical results from the literature to validate our numerical system. The first study case considers the complete melting of the PCM (liquid fraction of 95%), followed by a complete solidification. For the second case, the solidification is triggered after a partial melting (liquid fraction of 50%). Both cases are analysed in detail by providing temporal evolution of the solid-liquid interface, liquid fraction, Nusselt number and accumulated heat input. Different regimes are identified during the melting-solidification process and explained using scaling correlation analysis. Practical consequences of these two operating modes are finally discussed.     

Rayleigh-Bénard convection heat transfer in nanoparticle suspensions

Natural convection heat transfer of nanofluids in horizontal enclosures heated from below is investigated theoretically. The main idea upon which the present work is based is that nanofluids behave more like a single-phase fluid rather than like a conventional solid-liquid mixture, which implies that all the convective heat transfer correlations available for single-phase flows can be extended to nanoparticle suspensions, provided that the thermophysical properties appearing in them are the nanofluid effective properties calculated at the reference temperature. In this connection, two empirical equations, based on a wide variety of experimental data reported in the literature, are developed for the evaluation of the nanofluid effective thermal conductivity and dynamic viscosity, whereas the other effective properties are evaluated by the traditional mixing theory. The heat transfer enhancement that derives from the dispersion of nano-sized solid particles into the base liquid is calculated for different operating conditions, nanoparticle diameters, and combinations of solid and liquid phases. One of the fundamental results is the existence of an optimal particle loading for maximum heat transfer across the bottom-heated enclosure. In particular, for any assigned combination of suspended nanoparticles and base liquid, it is found that the optimal volume fraction increases as the nanofluid average temperature increases, and may either increase or decrease with increasing the nanoparticle size according as the flow is laminar or turbulent. Moreover, the optimal volume fraction has a peak at a definite value of the Rayleigh number of the base fluid, that depends on both the average temperature of the nanofluid and the diameter of the suspended nanoparticles.     

Mathematical Models for Metal Matrix Composites Manufacturing

Infiltration by a pure liquid metal into a rigid preform in the presence of partial solidification is analyzed. The mathematical model which is developed takes into account the coupling between the flow through the porous medium and the thermal effects. The model has a two-scale formulation: phase change occurs at the microscopic scale, while macroscopic flow and heat transfer involve large scale quantities.     

The effect of real viscosity on the heat transfer of water based Al2O3 nanofluids in a two-sided lid-driven differentially heated rectangular cavity

The heat transfer and fluid flow behavior of water based Al₂O₃ nanofluids are numerically investigated inside a two-sided lid-driven differentially heated rectangular cavity. Physical properties which have major effects on the heat transfer of nanofluids such as viscosity and thermal conductivity are experimentally investigated and correlated and subsequently used as input data in the numerical simulation. Transport equations are numerically solved with finite volume approach using SIMPLEC algorithm. It was found that not only the thermal conductivity but also the viscosity of nanofluids has a key role in the heat transfer of nanofluids. The results show that at low Reynolds number, increasing the volume fraction of nanoparticles increases the viscosity and has a deteriorating effect on the heat transfer of nanofluids. At high Reynolds number, the increase in the viscosity is compensated by force convection and the increase in the volume fraction of nanoparticles which results in an increase in heat transfer is in coincidence with experimental results.     

Laminar flow and heat transfer characteristics of nanoparticle colloidal dispersions in water

Numerical analysis is performed to examine axisymmetric laminar flow and heat transfer characteristics of colloidal dispersions of nanoparticles in water (nanofluids). Effect of volume fraction on flow and heat transfer characteristics is investigated. Eight different materials, alumina, copper, copper oxide, diamond, gold, graphite, silver, and zirconia are considered. Heat transfer and property measurements were conducted previously for Alumina nanofluid and the results have shown that nanofluids behave as homogeneous mixtures. It is found that oxide-based nanofluids offer the least heat transfer enhancement compared to elements-based nanofluids. For a given volume flow rate, all nanofluids exhibited linear increase in heat transfer enhancement with increasing colloids volume fraction, up to 0.05. Furthermore, it is found that in the thermal entrance region, a hydrodynamically developing flow exhibits significantly higher heat transfer enhancement than fully-developed conditions.     

Performance analysis and optimization of radiating fins with a step change in thickness and variable thermal conductivity by homotopy perturbation method

Although tapered fins transfer more rate of heat per unit volume, they are not found in every practical application because of the difficulty in manufacturing and fabrications. Therefore, there is a scope to modify the geometry of a constant thickness fin in view of the less difficulty in manufacturing and fabrication as well as betterment of heat transfer rate per unit volume of the fin material. For the better utilization of fin material, it is proposed a modified geometry of new fin with a step change in thickness (SF) in the literature. In the present paper, the homotopy perturbation method has been used to evaluate the temperature distribution within the straight radiating fins with a step change in thickness and variable thermal conductivity. The temperature profile has an abrupt change in the temperature gradient where the step change in thickness occurs and thermal conductivity parameter describing the variation of thermal conductivity has an important role on the temperature profile and the heat transfer rate. The optimum geometry which maximizes the heat transfer rate for a given fin volume has been found. The derived condition of optimality gives an open choice to the designer.     

Experimental study on the application of paraffin slurry to high density electronic package cooling

Experiments were performed by using water and paraffin slurry to investigate thermal characteristics from a test multichip module. The parameters were the mass fraction of paraffin slurry (0, 2.5, 5, 7.5%), heat flux (10, 20, 30, 40 W/cm²) and channel Reynolds numbers. The size of paraffin slurry particles was within 10–40 μm. The local heat transfer coefficients for the paraffin slurry were larger than those for water. Thermally fully developed conditions were observed after the third or fourth row. The paraffin slurry with a mass fraction of 5% showed the most efficient cooling performance when the heat transfer and the pressure drop in the test section were considered simultaneously. A new correlation for the water and the paraffin slurry with a mass fraction of 5% was obtained for a channel Reynolds number over 5300. Received on 25 January 1999     

Theoretical analysis of convective heat transfer enhancement of microencapsulated phase change material slurries

This paper analyzes the convective heat transfer enhancement mechanism of microencapsulated phase change material slurries based on the analogy between convective heat transfer and thermal conduction with thermal sources. The influence of each factor affecting the heat transfer enhancement for laminar flow in a circular tube with constant wall temperature is analyzed using an effective specific heat capacity model. The model is validated with results available in the literature. The analysis and the results clarify the heat transfer enhancement mechanism and the main factors influencing the heat transfer. In addition, the conventional Nusselt number definition of phase change slurries for internal flow is modified to describe the degree of heat transfer enhancement of microencapsulated phase change material slurries. The modification is also consistent evaluation of the convective heat transfer of internal and external flows.c volumetric concentration of microcapsules - c_m mass concentration of microcapsules - c_p specific heat, kJ kg^–1 K^–1 - h_fs phase change material heat of fusion, kJ kg^–1 - h_m* modified convective heat transfer coefficient, W m^–2 K^–1 - k thermal conductivity, W m^–1 K^–1 - k_e effective thermal conductivity of slurry, W m^–1 K^–1 - k_b slurry bulk thermal conductivity, W m^–1 K^–1 - ML dimensionless initial subcooling - Mr dimensionless phase change temperature range - Nu conventional Nusselt number - Nu* improved Nusselt number - q_wⁿ wall heat flux, Wm^–2 - Pe Peclet number - Pr Prandtl number - Re Reynolds number - r radial coordinate, m - r₀ duct radius, m - r₁ dimensionless radial coordinate - Ste Stefan number - T temperature, K - T₁ lower phase change temperature limit, K - T₂ upper phase change temperature limit, K - T_i slurry inlet temperature, K - u axial velocity, m/s - v radial velocity, m/s - x axial coordinate, m - x₁ dimensionless axial coordinate - thermal diffusivity, m²/s - dimensionless temperature - dynamic viscosity, N·s/m² - kinematic viscosity, m²/s - _t width of thermal boundary, m - degree of heat transfer enhancement, = h_m*/(h_m*)_single - b bulk fluid (slurry) - b0 slurry without phase change - l liquid - m mean - s solid - f suspending fluid - p microcapsule particles - w wall - single single-phase fluid     

CFD studies on natural convection heat transfer of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-water nanofluids

This work is focused on numerical simulations of natural convection heat transfer in Al₂O₃-water nanofluids using computational fluid dynamics approach. Fluent v6.3 is used to simulate water based nanofluid considering it as a single phase. Thermo-physical properties of the nanofluids are considered in terms of volume fraction and size of nanoparticles, size of base fluid molecule and temperature. The numerical values of effective thermal conductivity have also been compared with the experimental values available in the literature. The numerical result simulated shows decrease in heat transfer with increase in particle volume fraction. Computed result shows similar trend in increase of Nusselt number with Relayigh number as depicted by experimental results. Streamlines and temperature profiles are plotted to demonstrate the effect.     

The influence of the physical properties ratios on the conjugate heat transfer from a drop

This paper analyses the influence of the conductivity ratio, Φ_λ, and the volume heat capacity ratio, Φ_h, on the conjugate heat transfer from a liquid particle in a liquid environment. Both creeping flow and moderate Re number domain are considered. Special attention is given to the phenomenon of thermal wake. The occurrence and the evolution of the thermal wake depend on the values of volume heat capacity ratio and conductivity ratio respectively. The influence of the Pe numbers on the thermal inversion phenomenon and the interaction (at moderate Re number values) between thermal wake and flow separation are analysed. The results obtained show that Φ_λ and Φ_h influence strongly the conjugate heat transfer. Received on 11 January 1999