T. Sundararajan

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: Manifestation of anomalous enhancement and chemical effects

Thermal conductivities of two kinds of Au nanoparticles were measured in water and toluene media. The water soluble particles, 10–20 nm in mean diameter, made with citrate stabilization showed thermal conductivity enhancement of 5%–21% in the temperature range of 30–60 °C at a loading of 0.000 26 (by volume). The effect was 7%–14% for Au particles stabilized with a monolayer of octadecanethiol even for a loading of 0.011%. Comparatively lower thermal conductivity enhancement was observed for larger diameter Ag particles for significantly higher loading. Effective enhancement of 9%, even at vanishing concentrations, points to additional factors in the thermal conductivity mechanism in nanofluids. Results also point to important chemical factors such as the need for direct contact of the metal surface with the solvent medium to improve enhancement.

Natural convective heat transfer in a fluid saturated variable porosity medium

Abstract A generalised non-Darcian porous medium model for natural convective flow has been developed taking into account linear and non-linear matrix drag components as well as the inertial and viscous forces within the fluid. The results of the general model have been validated with the help of experimental data and compared with the various non-Darcy porous media model predictions reported in literature. It has been observed that the wall Nusselt number is significantly affected by the combination of dimensionless parameters such as Rayleigh number, Darcy number and porosity in the non-Darcy flow regime. A detailed parametric study has been presented for natural convective flow inside a rectangular enclosure filled with saturated porous medium of constant or variable porosity. It is observed that the thickness of the porous layer and the nature of variation in porosity significantly affect the natural convective flow pattern as well as the heat transfer features. The present model is also able to predict the channeling effect and associated heat transfer in forced flow through packed beds.

A micro-convection model for thermal conductivity of nanofluids

Increase in the specific surface area as well as Brownian motion are supposed to be the most significant reasons for the anomalous enhancement in thermal conductivity of nanofluids. This work presents a semi-empirical approach for the same by emphasizing the above two effects through micro-convection. A new way of modeling thermal conductivity of nanofluids has been explored which is found to agree excellently with a wide range of experimental data obtained by the present authors as well as the data published in literature

Rheological and flow characteristics of nanofluids: Influence of electroviscous effects and particle agglomeration

Nanofluids have shown remarkable attraction in heat transfer community due to its reported enhanced thermal properties. One factor which can restrict nanofluids in heat transfer application is the increased viscosity value (compared to classical predictions). Particle aggregation occurring was the major reason for this observation. Even though majority of the aqueous nanofluids prepared in literature were stabilized electrostatically by adjusting the pH, studies on the effect of the electrical double layer thus created and its influence on viscosity increase has not been investigated for these nanofluids so far. Thus, in the present paper, rheological properties of alumina-water nanofluids, which are electrostatically stabilized, are measured and the increase in suspension viscosity due to presence of this electrical double layer causing additional electroviscous effects is brought out. Based on dynamic light scattering studies, particle agglomeration and its subsequent effect in increasing the viscosity ...

DOUBLE-DIFFUSIVE NATURAL CONVECTION IN AN ENCLOSURE FILLED WITH FLUID-SATURATED POROUS MEDIUM: A GENERALIZED NON-DARCY APPROACH

The double-diffusive natural convective flow within a rectangular enclosure has been studied using a generalized porous medium approach. The results have been validated with the help of theoretical heat transfer results available for various porous medium flow models and also with the experimental results for double-diffusive convection in a fluid-filled rectangular cavity. The present generalized model covers the entire range from Darcy flow to free fluid flow. Numerical predictions by the model indicate that the flow pattern as well as the heat and mass transfer are profoundly influenced by the buoyancy ratio. Also non-Darcy effects on flow, heat, and mass transfer become significant when the Rayleigh or Darcy numbers are large. The Sherwood and Nusselt numbers become sensitive to bed porosity variation in the non-Darcy regime.

Predicting the effective thermal conductivity of carbon nanotube based nanofluids

Adding a small volume fraction of carbon nanotubes (CNTs) to a liquid enhances the thermal conductivity significantly. Recent experimental findings report an anomalously wide range of enhancement values that continue to perplex the research community and remain unexplained. In this paper we present a theoretical model based on three-dimensional CNT chain formation (percolation) in the base liquid and the corresponding thermal resistance network. The model considers random CNT orientation and CNT-CNT interaction forming the percolating chain. Predictions are in good agreement with almost all available experimental data. Results show that the enhancement critically depends on the CNT geometry (length), volume fraction, thermal conductivity of the base liquid and the nanofluid (CNT-liquid suspension) preparation technique. Based on the physical mechanism of heat conduction in the nanofluid, we introduce a new dimensionless parameter that alone characterizes the nanofluid thermal conductivity with reasonable accuracy (∼ ± 5%).

ANALYSIS OF SPARK PROFILES DURING EDM PROCESS

Abstract A fundamental study of electro discharge machining (EDM) based on the physics of an arc and heat transfer theory has been carried out. The field equations for electric potential and temperature in the spark region are simultaneously solved by employing the finite element method. Using the criterion of constant current at any cross section of a spark, the arc radii at different cross sections are corrected until convergence. The final spark shape obtained is noncylindrical, and has different radii at different cross sections. Also, the percent of heat input absorbed by cathode, anode, and dielectric has been calculated. The computed relative electrode wear has been compared with experimental results.

Interaction effects between surface radiation and turbulent natural convection in square and rectangular enclosures

The interaction effects of surface radiation with turbulent natural convection of a transparent medium in rectangular enclosures have been numerically analyzed, covering a wide range of Rayleigh number from 10 9 to 10 12 and aspect ratio from 1 to 200. The vertical walls of the enclosure are isothermal and maintained at different temperatures. The adiabatic top and bottom walls of the enclosure have been modelled for the limiting cases of negligible or perfect conduction along their lengths. The interaction with surface radiation results in larger velocity magnitudes and turbulence levels in the vertical as well as horizontal boundary layers, leading to an increase in the convective heat transfer by ∼25 percent. Due to the asymmetrical coupling of radiation, the augmentation of convective Nusselt number of the cold wall is larger than that of the hot wall. In tall enclosures, the convective Nusselt number exhibits three distinct regimes with respect to aspect ratio, viz. the slow growth regime, the accelerated growth regime and the invariant (or saturated) regime

A theoretical model of brick drying as a conjugate problem

Abstract The evaporative drying of a two-dimensional rectangular brick is studied numerically as a conjugate problem. The conservation equations for the solid are obtained using the continuum approach. The Navier–Stokes equations have been employed for obtaining the flow field and the corresponding flow solutions are used for predicting the drying behavior of the brick. The predictions of temperature and moisture content show that the leading edge dries faster compared to other sides of the solid. The full two-dimensional solutions differ considerably from the solutions based on heat and mass transfer through the boundary layers over the top surface. Average heat and mass transfer coefficients appropriate to the conjugate problem have been defined, based on constant temperature and moisture differentials between the solid and the ambient. The corresponding Nusselt and Sherwood number values indicate that analogy does not exist between heat and mass transfer, until the entire brick reaches wet bulb conditions. Free convection effects on drying are also studied for some initial period for low Reynolds number. Due to the influence of buoyant forces imparted by gravity, the overall drying rate has improved.