Nucleate pool boiling heat transfer of TiO2–R141b nanofluids

Nucleate pool boiling heat transfer of a refrigerant-based-nanofluid was investigated at different nanoparticle concentrations and pressures. TiO₂ nanoparticles were mixed with the refrigerant HCFC 141b at 0.01, 0.03 and 0.05 vol%. The experiment was performed using a cylindrical copper tube as a boiling surface. Pool boiling experiments of nanofluid were conducted and compared with that of the base refrigerant. The results indicate that the nucleate pool boiling heat transfer deteriorated with increasing particle concentrations, especially at high heat fluxes. At 0.05 vol%, the boiling heat transfer curves were suppressed. At high pressures of 400 and 500 kPa, the boiling heat transfer coefficient at a specific excess temperature was almost the same.     

A dispersion model for predicting the heat transfer performance of TiO2–water nanofluids under a laminar flow regime

Nanofluids are a suspension of particles with ultrafine size in a conventional base fluid that increases the heat transfer performance of the original base fluid. They show higher thermal performance than base fluids especially in terms of the thermal conductivity and heat transfer coefficient. During the last decade, many studies have been carried out on the heat transfer and flow characteristics of nanofluids, both experimentally and theoretically. The purpose of this article is to propose a dispersion model for predicting the heat transfer coefficient of nanofluids under laminar flow conditions. TiO₂ nanoparticles dispersed in water with various volume fractions and flowing in a horizontal straight tube under constant wall heat flux were used. In addition, the predicted values were compared with the experimental data from He et al. [14]. In the present study, the results show that the proposed model can be used to predict the heat transfer behaviour of nanofluids with reasonable accuracy. Moreover, the results also indicate that the predicted values of the heat transfer coefficient obtained from the present model differ from those obtained by using the Li and Xuan equation by about 3.5% at a particle volume fraction of 2.0%.     

Studies on heat transfer and friction factor characteristics of turbulent flow through a micro-finned tube fitted with left–right inserts

Experimental investigation on heat transfer and friction factor characteristics of micro-finned tube fitted with full-length twisted tape and left–right twisted tape inserts of twist ratio 7.44, 8.27 and 11.17 has been presented. The experimental data obtained were compared with those obtained from plain tube published data. The heat transfer coefficient enhancement for left–right twisted inserts is higher than that for full-length twisted tape inserts for a given twist ratio. The empirical correlation for Nusselt number, friction relating Reynolds number and twist ratio was formed for both twisted tape inserts and left–right inserts and found to fit the experimental data within ±3.6% and ±2.5% for Nusselt number and ±7% and ±8.5% friction factor, respectively. Performance evaluation analysis was made and the maximum performance ratio was obtained for left–right twisted tape inserts.     

Experimental investigation of heat transfer and friction factor with water–propylene glycol based CuO nanofluid in a tube with twisted tape inserts

Convective heat transfer and friction factor characteristics of water/propylene glycol (70:30% by volume) based CuO nanofluids flowing in a plain tube are investigated experimentally under constant heat flux boundary condition. Glycols are normally used as an anti-freezing heat transfer fluids in cold climatic regions. Nanofluids are prepared by dispersing 50 nm diameter of CuO nanoparticles in the base fluid. Experiments are conducted using CuO nanofluids with 0.025%, 0.1% and 0.5% volume concentration in the Reynolds numbers ranging from 1000 < Re < 10000 and considerable heat transfer enhancement in CuO nanofluids is observed. The effect of twisted tape inserts with twist ratios in the range of 0 < H/D < 15 on nanofluids is studied and further heat transfer augmentation is noticed. The increment in the pressure drop in the CuO nanofluids over the base fluid is negligible but the experimental results have shown a significant increment in the convective heat transfer coefficient of CuO nanofluids. The convective heat transfer coefficient increased up to 27.95% in the 0.5% CuO nanofluid in plain tube and with a twisted tape insert of H/D = 5 it is further increased to 76.06% over the base fluid at a particular Reynolds number. The friction factor enhancement of 10.08% is noticed and increased to 26.57% with the same twisted tape, when compared with the base fluid friction factor at the same Reynolds number. Based on the experimental data obtained, generalized regression equations are developed to predict Nusselt number and friction factor.     

Numerical investigation of heat transfer and pressure drop characteristics of tube–fin heat exchangers in ice slurry HVAC system

This paper analyzes the heat transfer and pressure drop characteristics of a tube–fin heat exchanger in ice slurry HVAC system. Ice slurry is a suspension of crystallized water based - ice solution with a freezing point depressant like ethylene glycol. The ice- slurry is pumpable, hence it is also called pumpable ice. The composition of ice slurry considered for analysis is 14% ice fraction, 16% ethylene glycol, and 70% water by volume. It is deduced that the ice slurry HVAC system results in 7.4% increase in temperature drop over the conventional chilled water system The latent heat absorbed by ice slurry on melting makes it an attractive choice for achieving high degree of cooling. The numerical analysis was conducted by simulating the ice slurry tube flow region and air flow region of tube–fin heat exchanger in the air-handling unit of HVAC system. For the simulation six different louver patterns with 10 to 55 louver angle were considered. The design of the tube–fin heat exchanger for optimal heat transfer and pressure drop characteristics was also determined with the optimization parameter like louver angle, fin pitch, and ice slurry flow velocity.     

Enhancement of heat transfer using nanofluids—An overview

A colloidal mixture of nano-sized particles in a base fluid, called nanofluids, tremendously enhances the heat transfer characteristics of the original fluid, and is ideally suited for practical applications due to its marvelous characteristics. This article addresses the unique features of nanofluids, such as enhancement of heat transfer, improvement in thermal conductivity, increase in surface volume ratio, Brownian motion, thermophoresis, etc. In addition, the article summarizes the recent research in experimental and theoretical studies on forced and free convective heat transfer in nanofluids, their thermo-physical properties and their applications, and identifies the challenges and opportunities for future research.     

Numerical investigation of heat and mass transfer within different configurations of LaNi5–H2 reactor using the unstructured Lattice Boltzmann Method

In this study, the Control-Volume Lattice Boltzmann Method (CVLBM) based on the unstructured grids is proposed as a numerical solver for transient heat and mass transfer in a Metal Hydrogen Reactor (MHR) during the absorption process. To check the validity of the numerical approach, computational results were compared with those of the literature and a good agreement was obtained. The obtained results were also compared with those of the unstructured Control Volume Finite Element Method (CVFEM). We found that the new approach correctly predicts hydrogen absorption phenomena and has less CPU time compared with the CVFEM (about 8 times faster). In addition, various tank geometries were numerically studied and a new geometric configuration is proposed. The dynamic performances of these layouts were compared based on the numerical simulation. We found that the geometrical modification improves the hydriding performance. For the new configuration, we found that the storage time can be reduced by 87% compared to the basic configuration.     

Numerical investigation of heat and mass transfer during the desorption process of an Mg2Ni–H2 reactor

In this paper, a numerical study of coupled heat and mass transfer during the desorption process of metal–hydrogen reactor (Mg₂Ni–H₂), is presented. Analytical expressions describing, the reaction kinetic and the equilibrium pressure of the Mg₂Ni-H₂ system have been determined and integrated into a theoretical model that describes the dynamic behavior of the reactor. This model, which takes into account radiative heat transfer, is solved by the control volume finite element method (CVFEM). The numerical simulation is used to present the time–space evolutions of the temperature and the hydride density within the reactor and to evaluate the effect of radiative heat transfer and the governing operating parameters (outlet pressure, temperature of heating fluid, heat exchange coefficient) on the dynamic behavior of the reactor. In addition, a new geometric configuration of the reactor is proposed and simulated.     

Laminar convective heat transfer and viscous pressure loss of alumina–water and zirconia–water nanofluids

Laminar convective heat transfer and viscous pressure loss were investigated for alumina–water and zirconia–water nanofluids in a flow loop with a vertical heated tube. The heat transfer coefficients in the entrance region and in the fully developed region are found to increase by 17% and 27%, respectively, for alumina–water nanofluid at 6 vol % with respect to pure water. The zirconia–water nanofluid heat transfer coefficient increases by approximately 2% in the entrance region and 3% in the fully developed region at 1.32 vol %. The measured pressure loss for the nanofluids is in general much higher than for pure water. However, both the measured nanofluid heat transfer coefficient and pressure loss are in good agreement with the traditional model predictions for laminar flow, provided that the loading- and temperature-dependent thermophysical properties of the nanofluids are utilized in the evaluation of the dimensionless numbers. In other words, no abnormal heat transfer enhancement or pressure loss was observed within measurement errors.     

Molecular dynamics simulation of heat transfer with effects of fluid–lattice interactions

The nonequilibrium molecular dynamics simulation is employed to investigate the thermal properties of fluid confined in different FCC nanochannels. The results show that fluid in different lattice channels appears diverse wetting characteristics at low temperature. Based on wall parameters, a ratio is defined to describe the fluid–lattice interaction. Wall attraction, number of absorbed particles and thermal conductivity are increased as the increase of this ratio as well as the location of particles get closer to the wall. Thermal resistance exists along with the fluid–wall interface and loses the dominant of heat transport as the system temperature gets raised. At the same time, the thermal conductivity of nanoscale experiences unconventional increase. The fluid thermal properties are influenced both by wall–fluid interaction and temperature.     

Solutions of Falkner–Skan equation with heat transfer by Fourier series

In this paper, we solve the Falkner–Skan equation with heat transfer through an expansion in Fourier series, results are improved by incorporating an asymptotic analysis for the Fourier series coefficients. The results show that the classical expansion in Fourier series together with the incorporation of an asymptotic analysis for coefficients of the series delivers a solution with very good accuracy and rapid convergence when it is compared with other methods of solution found in the literature as finite difference and shooting method.     

Pressure corrections for the potential flow analysis of Kelvin–Helmholtz instability with heat and mass transfer

Pressure corrections for the viscous potential flow analysis of Kelvin–Helmholtz instability at the interface of two viscous fluids have been carried out when there is heat and mass transfer across the interface. Both fluids are taken as incompressible and viscous with different kinematic viscosities. In viscous potential flow theory, viscosity enters through normal stress balance and effect of shearing stresses is completely neglected. We include the viscous pressure in the normal stress balance along with irrotational pressure and it is assumed that this viscous pressure will resolve the discontinuity of the tangential stresses at the interface for two fluids. It has been observed that heat and mass transfer has destabilizing effect on the stability of the system. A comparison between viscous potential flow (VPF) solution and viscous contribution to the pressure for potential flow (VCVPF) solution has been made and it is found that the effect of irrotational shearing stresses stabilizes the system.     

Second law analysis of coupled conduction–radiation heat transfer with phase change

This work considers an exergy-based analysis of two-dimensional solid-liquid phase change processes in a square cavity enclosure. The phase change material (PCM) concerns a semi-transparent absorbing, emitting and anisotropically scattering medium with constant thermodynamic properties. The enthalpy-based energy equation is solved numerically using computational fluid dynamics. Once the energy equation is solved, local exergy loss due to heat conduction and radiative heat transfer during the phase change process is calculated by post processing procedures. In this work, the radiation exergy loss in the medium and at the enclosure boundary is taken into consideration. It is found that radiation exergy loss is significant in the high-temperature phase change process. Parametric investigation is also carried out to study the effects of Stefan number, Biot number, Planck number, single scattering albedo and wall emissivity on exergy loss. The results show that the total exergy loss increases with Biot number, single scattering albedo and wall emissivity. The second law effects of the conduction–radiation coupling in the energy equation are also shown in this work.     

Numerical study of nanofluid heat transfer for different tube geometries – A comprehensive review on performance

The heat transfer performance of a system can be improved using a combination of passive methods, namely nanofluids and various types of tube geometries. These methods can help enhance the heat transfer coefficient and consequently reduce the weight of the system. In this paper, the effect of tube geometry and nanofluids towards the heat transfer performance in the numerical system is reviewed. The forced convective heat transfer performance, friction factor and wall shear stress are studied for nanofluid flow in different tube geometries. The thermo-physical properties such as density, specific heat, viscosity and thermal conductivity are reviewed for the determination of nanofluid heat transfer numerically. Various researchers had measured and modelled for the determination of thermal conductivity and viscosity of nanofluids. However, the density and specific heat of nanofluids can be estimated with the mixture relations. The different tube geometries in simulation work are analyzed namely circular tube, circular tube with insert, flat tube and horizontal tube. It was observed that the circular tube with insert provides the highest heat transfer coefficient and wall shear stress. Meanwhile, the flat tube has greater heat transfer coefficient with a higher friction factor compared to the circular tube.     

Thermophysical properties of fly ash–Cu hybrid nanofluid for heat transfer applications

The effect of temperature and concentration on the thermophysical properties of fly ash–copper (80% fly ash and 20% Cu by volume) water-based stable hybrid nanofluid is studied. The experiments are conducted for the volume concentration range of 0 to 0.5% in the temperature range of 30 to 60°C. The nanoparticles have been characterized by transmission electron microscopy and dynamic light scattering to determine an average nanoparticle diameter of 15 nm. The stability of nanofluid in the presence of surfactant Triton X-100 is examined with the help of zeta potential. The maximum enhancement in thermal conductivity and viscosity is 19% and 22%, respectively. The outcome of the present study showed that density, thermal conductivity, and viscosity of the hybrid nanofluid increased, whereas specific heat decreased with an increase in the nanofluid concentration. In addition, the specific heat and thermal conductivity increase, there is a decrease in density and viscosity of the hybrid nanofluid with an increase in temperature.     

A lattice Boltzmann method for simulation of liquid–vapor phase-change heat transfer

A lattice Boltzmann model for the liquid–vapor phase change heat transfer is proposed in this paper. Two particle distribution functions, namely the density distribution function and the temperature distribution function, are used in this model. A new form of the source term in the energy equation is derived and the modified pseudo-potential model is used in the proposed model to improve its numerical stability. The commonly used Peng–Robinson equation of state is incorporated into the proposed model. The problem of bubble growth and departure from a horizontal surface is solved numerically based on the proposed model.     

Numerical investigation on heat transfer characteristics of high-pressure syngas in the membrane helical-coil cooler of a 2,000 t/d gasifier

In this article, the heat transfer performance of a syngas cooler with membrane helical-coil heat exchanger was numerically studied. A method of combining piecewise simulation and full-scale simulation was proposed, and the influence of fly ash was considered. The models and the proposed method were validated by comparing simulation results with data from industrial test. The simulation results show that radiation accounts for 10–20% of the total heat transfer in the syngas cooler. The surface of inner channel is characterized with high convective heat-transfer coefficient and heat flux. In addition, the quality of produced steam could be significantly enhanced as the heat exchanger of upper group was changed from evaporating surface to superheating surface, and the cooling performance for syngas was hardly affected.     

Thermal performance of an oscillating heat pipe with Al2O3–water nanofluids

An experimental investigation was performed on the thermal performance of an oscillating heat pipe (OHP) charged with base water and spherical Al₂O₃ particles of 56 nm in diameter. The effects of filling ratios, mass fractions of alumina particles, and power inputs on the total thermal resistance of the OHP were investigated. Experimental results showed that the alumina nanofluids significantly improved the thermal performance of the OHP, with an optimal mass fraction of 0.9 wt.% for maximal heat transfer enhancement. Compared with pure water, the maximal thermal resistance was decreased by 0.14 °C/W (or 32.5%) when the power input was 58.8 W at 70% filling ratio and 0.9% mass fraction. By examining the inner wall samples, it was found that the nanoparticle settlement mainly took place at the evaporator. The change of surface condition at the evaporator due to nanoparticle settlement was found to be the major reason for the enhanced thermal performance of the alumina nanofluid-charged OHP.     

Numerical simulations of a coupled radiative–conductive heat transfer model using a modified Monte Carlo method

Radiative–conductive heat transfer in a medium bounded by two reflecting and radiating plane surfaces is considered. This process is described by a nonlinear system of two differential equations: an equation of the radiative heat transfer and an equation of the conductive heat exchange. The problem is characterized by anisotropic scattering of the medium and by specularly and diffusely reflecting boundaries. For the computation of solutions of this problem, two approaches based on iterative techniques are considered. First, a recursive algorithm based on some modification of the Monte Carlo method is proposed. Second, the diffusion approximation of the radiative transfer equation is utilized. Numerical comparisons of the approaches proposed are given in the case of isotropic scattering.