Ranganathan Kumar

Pool boiling heat transfer experiments in silica–water nano-fluids

Heat transfer measurements taken at atmospheric pressure in silica nano-solutions are compared to similar measurements taken in pure water and silica micro-solutions. The data include heat flux vs. superheat of a 0.4 mm diameter NiCr wire submerged in each solution. The data show a marked increase in critical heat flux (CHF) for both nano- and micro-solutions compared to water, but no appreciable differences in heat transfer for powers less than CHF. The data also show that stable film boiling at temperatures close to the wire melting point are achievable with the nano-solutions but not with the micro-solutions.

Role of ions in pool boiling heat transfer of pure and silica nanofluids

Heat transfer in silica nanofluids at different acidity and base is measured for various ionic concentrations in a pool boiling experiment. Nanosilica suspension increases the critical heat flux 3 times compared to conventional fluids. The 10-nm particles possess a thicker double diffuse layer compared to 20-nm particles. The catalytic properties of nanofluids decrease in the presence of salts, allowing the particles to cluster and minimize the potential increase in heat transfer. Nanofluids in a strong electrolyte, i.e., in high ionic concentration, allow a higher critical heat flux than in buffer solutions because of the difference in surface area. The formation and surface structure of the deposition affect the thermal properties of the liquid.

Droplet formation and stability of flows in a microfluidic T-junction

Flow regimes obtained as a consequence of two immiscible fluids interacting at a T-junction are presented for high Capillary numbers and different flow rates of the continuous and dispersed phases. Through lattice Boltzmann based simulations, a regime map is created that distinguishes parallel flows from droplet flows. Simulations show the dependence of flow rates and viscosity ratio on the volume of droplets formed, which is compared with existing experimental data. At high Capillary numbers, the transition zone which separates parallel and droplet flows shrinks, and is influenced by the viscosity ratio as well.

Study of natural convection in horizontal annuli

Abstract Natural convection of gases in a horizontal annulus, where the inner cylinder is heated by the application of a constant heat flux and the outer cylinder is isothermally cooled, is studied numerically. Detailed results of temperature, velocity and heat transfer are presented for a wide range of Rayleigh numbers extending from conduction to the convection-dominated steady flow regime, and diameter ratios of 1.2–10. A crescent-shaped eddy dominates for the small diameter ratio and a kidney-shaped flow pattern appears for the large diameter ratio as observed by previous investigators in their flow visualization studies. The inner wall temperature is a function of diameter ratio and Rayleigh number. An increase in Rayleigh number based on the same temperature difference for the inner wall boundary condition of constant heat flux or constant temperature increases the heat transfer rate; however, the increase is larger for the constant heat flux case. At large diameter ratios ( κ ≥ 10), the heat transfer rates are the same for both types of heating, and a single cylinder in an infinite atmosphere gives nearly the same results.

Heat Transfer Behavior of Silica Nanoparticles in Pool Boiling Experiment

The heat transfer characteristics of silica (SiO 2 ) nanofluids at 0.5 vol % concentration and particle sizes of 10 nm and 20 nm in pool boiling with a suspended heating Nichrome wire have been analyzed. The influence of acidity on heat transfer has been studied. The pH value of the nanosuspensions is important from the point of view that it determines the stability of the particles and their mutual interactions toward the suspended heated wire. When there is no particle deposition on the wire, the nanofluid increases critical heat flux (CHF) by about 50% within the uncertainty limits regardless of pH of the base fluid or particle size. The extent of oxidation on the wire impacts CHF, and is influenced by the chemical composition of nanofluids in buffer solutions. The boiling regime is further extended to higher heat flux when there is agglomeration on the wire. This agglomeration allows high heat transfer through interagglomerate pores, resulting in a nearly threefold increase in burnout heat flux. This deposition occurs for the charged 10 nm silica particle. The chemical composition, oxidation, and packing of the particles within the deposition on the wire are shown to be the reasons for the extension of the boiling regime and the net enhancement of the burnout heat flux.

Laminar thermal convection between vertical coaxial isothermal cylinders

A numerical investigation of natural convective heat transfer of a fluid in vertical cylindrical annuli with the inner wall maintained at a higher temperature than the outer wall has been earned out. The top and the bottom plates are insulated for the majority of the cases Perfectly conducting horizontal walls are also considered for comparison of heat transfer results. Numerical results of heat transfer rates and flow fields for 10 ⩽ RaL ⩽ 106, 1 ⩽ k⩽ 15 and 0.3 ⩽ A ⩽ 10 are reported. Discrepancies in the existing numerical and experimental data are noted and discussed New correlations for heat transfer rate are given.

Flow regime transition at high capillary numbers in a microfluidic T-junction: Viscosity contrast and geometry effect

Flow regimes obtained as a consequence of two immiscible fluids interacting at a T-junction are presented for transitional to high capillary numbers and different ratios of the continuous and dispersed phase flow rates and viscosities. Results are presented for the formation of micron-sized droplets using simulations performed based on a three-dimensional lattice Boltzmann method. The influence of viscosity and geometry of the device on the frequency and volume of droplets formed has been examined and the nondimensional drop size correlated with the capillary number and flow rate ratio. This work reveals two important and new physical features: (a) the transition zone separating droplet and jet flows narrows for high capillary numbers and (b) the critical flow rate ratio separating droplet and parallel flows increases as the dispersed to continuous channel width ratio increases, aspects which have been correlated using a simple relation for both transitions from the droplet-at-T-junction to droplet-in-cha...

Effect of surface tension on nanotube nanofluids

This letter presents heat transfer results that single-walled carbon nanotube (CNT) suspensions in a boiling environment can extend the saturated boiling regime and postpone catastrophic failure of the material even further than previously reported if the surface tension of the nanofluid is carefully controlled. The maximum enhancement in the critical heat flux is nearly four times for a surfactant to CNT concentration ratio of 1:5. The experimental results show that the material burnout is a strong function of the relaxation of the nanofluid surface tension with the base fluid.

Role of Brownian motion on the thermal conductivity enhancement of nanofluids

This study involves Brownian dynamics simulations of a real nanofluid system in which the interparticle potential is determined based on Debye length and surface interaction of the fluid and the solid. This paper shows that Brownian motion can increase the thermal conductivity of the nanofluid by 6% primarily due to “random walk” motion and not only through diffusion. This increase is limited by the maximum concentration for each particle size and is below that predicted by the effective medium theory. Beyond the maximum limit, particle aggregates begin to form. Brownian motion contribution stays as a constant beyond a certain particle diameter.

Structural morphology of acoustically levitated and heated nanosilica droplet

We study the vaporization and precipitation dynamics of a nanosilica encapsulated water droplet by levitating it acoustically and heating it with a CO2 laser. For all concentrations, we observe three phases: solvent evaporation, surface agglomeration, and precipitation leading to bowl or ring shaped structures. At higher concentrations, ring reorientation and rotation are seen consistently. The surface temperature from an infrared camera is seen to be dependent on the final geometrical shape of the droplet and its rotation induced by the acoustic field of the levitator. With nonuniform particle distribution, these structures can experience rupture which modifies the droplet rotational speed