Turbulent friction and heat transfer behaviors of dispersed fluids (i.e., uttrafine metallic oxide particles suspended in water) in a circular pipe were investigated experimentally. Viscosity measurements were also conducted using a Brookfield rotating viscometer. Two different metallic oxide particles, γ-alumina (Al2O3) and titanium dioxide (TiO2), with mean diameters of 13 and 27 nm, respectively, were used as suspended particles. The Reynolds and Prandtl numbers varied in the ranges l04-I05 and 6.5-12.3, respectively. The viscosities of the dispersed fluids with γ-Al2O3 and TiO2 particles at a 10% volume concentration were approximately 200 and 3 times greater than that of water, respectively. These viscosity results were significantly larger than the predictions from the classical theory of suspension rheology. Darcy friction factors for the dispersed fluids of the volume concentration ranging from 1% to 3% coincided well with Kays' correlation for turbulent flow of a single-phase fluid. The Nusselt n...

Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles

Oxide nanofluids were produced and their thermal conductivities were measured by a transient hot-wire method. The experimental results show that these nanofluids, containing a small amount of nanoparticles, have substantially higher thermal conductivities than the same liquids without nanoparticles. Comparisons between experiments and the Hamilton and Crosser model show that the model can predict the thermal conductivity of nanofluids containing large agglomerated Al{sub 2}O{sub 3} particles. However, the model appears to be inadequate for nanofluids containing CuO particles. This suggests that not only particle shape but size is considered to be dominant in enhancing the thermal conductivity of nanofluids.

Measuring Thermal Conductivity of Fluids Containing Oxide Nanoparticles

Conceptions for heat transfer correlation of nanofluids

Usual heat transfer fluids with suspended ultra fine particles of nanometer size are named as nanofluids, which have opened a new dimension in heat transfer processes. The recent investigations confirm the potential of nanofluids in enhancing heat transfer required for present age technology. The present investigation goes detailed into investigating the increase of thermal conductivity with temperature for nano fluids with water as base fluid and particles of Al 2 O 3 or CuO as suspension material. A temperature oscillation technique is utilized for the measurement of thermal diffusivity and thermal conductivity is calculated from it

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Temperature dependence of thermal conductivity enhancement for nanofluids

Detailed knowledge of solids circulation, bubble motion, and frequencies of porosity oscillations is needed for a better understanding of tube erosion in fluidized bed combustors. A predictive two-phase flow model was derived starting with the Boltzman equation for velocity distribution of particles. The model is a generalization of the Navier-Stokes equations of the type proposed by R. Jackson, except that the solids viscosities and stresses are computed by simultaneously solving a fluctuating energy equation for the particulate phase. The model predictions agree with time-averaged and instantaneous porosities measured in two-dimensional fluidized beds. Observed flow patterns and bubbles were also predicted.

A bubbling fluidization model using kinetic theory of granular flow

Many mathematical formulations of the heat transfer in living tissues'-' have been for the purposes of studying thermal regulation, comfort, or other phenomenon where significant localized (as opposed to whole-body or regional) variations in temperature and heat flux were of little interest. The advent of intensified interest in hyperthermia as a cancer therapy and the safety associated with ultrasound and microwave radiation, as well as attempts at a quantitative interpretation of thermographic measurements, however, have made it highly desirable to have formulations that are valid also for small-scale temperature variations. Living tissues differ from nonbiological materials primarily because of the presence of the vasculature. The large number and the architectural and dimensional variety of blood vessels clearly make it impractical to account for their individual contribution to heat transfer processes in the tissue with the exception, of course, of the larger arteries and veins. In the fields of heat transfer and fluid flow, when one encounters problems with a large number of structures whose individual dimensions are small relative to the macroscopic phenomenon under study, a common practice is to adopt the so-called continuum description. In this description, only the collective behavior of the small structures is taken into consideration in a certain statistical manner. Usually the influence of the small structures are ultimately expressed in terms of continuum properties of the medium, in our case, the thermal conductivity, specific heat, and blood perfusion rate of the tissue. It is the purpose of this report to explore the theoretical basis for the relationship between these properties and the architecture and function of the vasculature. It will be shown that because of the vasculature, and the large rate of blood perfusion, living biological tissues are fundamentally different from inert materials. Consequently, the familiar thermal properties can no longer be assumed to be independent of the parameters of the temperature field. In other words, these properties may vary, depending on the nature of the application. In view of the fact that existing formulations of the bio-heat transfer problem have been found to be more or less satisfactory for the description of heat transfer involving large-scale tempera-

Microvascular contributions in tissue heat transfer.

Measurements of solids motion in a fluidized bed have been made in a computer-aided particle tracking facility. A radioactive tracer particle, dynamically identical to the solid particles to be studied, was mixed with the solids in thebed. The gamma radiation from the tracer was continuously monitored by a large number of scintillation detectors located around the bed, providing information on the tracer's instantaneous location. Prudent use was made of the purposely introduced redundant distance data to achieve improved accuracy. The recognition of the existence of secondary emission due to the interaction of the primary radiation and other system materials and the subsequent devising of a scheme to mitigate its effect contribute much to the success. Results for a bed with a uniform air distributor plate show the existence of two counter-rotating toroidal vortices whose relative sizes and strengths vary with the fluidizing velocity. Fluctuating motions at low frequencies ranging from 1.6 to 16 Hz have been observed at various locations in the bed.

A novel radioactive particle tracking facility for measurement of solids motion in gas fluidized beds

Eddy transport associated with microscopic flow fields in shearing two-phase flows was investigated. Although such microconvective effects are expected to be present in all disperse two-phase flows, usually they are masked by other collateral mechanisms and could not be studied critically. In the present study, effective thermal conductivities of neutrally buoyant solid-fluid mixtures were measured in a rotating Couette flow apparatus. Low Reynolds numbers were used to avoid the effects of turbulence. Significant enhancement in effective thermal conductivity was observed when the Pe/sub d/ = ed/sup 2//..cap alpha../sub f/ where e is the local mean shear rate, d is the particle diameter, and ..cap alpha../sub f/ is the thermal diffusivity of the fluid. Volume fractions employed were phi = 0.15 and 0.30 for polyethylene beads (2.9 mm in diameter) in a mixture of silicone oil and kerosene, and phi = 0.15 for polystyrene particles (0.3 mm in diameter) in a mixture of silicone oil and Freon-113. Single-phase liquid mixtures were also measured in various shear rates to show that the thermal conductivity was independent of shear rate and hence the observed phenomenon was not an instrumental artifact. The dependence of conductivity on particle Peclet number appeared to approach a powermore » law relationship k/sub e/proportionalPe/sub d//sup 1/2/ for high Peclet numbers (300« less

Microconvective Thermal Conductivity in Disperse Two-Phase Mixtures as Observed in a Low Velocity Couette Flow Experiment

A theoretical analysis of heat transfer due to particle impact

The present communication presents a single microprobe technique for measuring tissue thermal properties based on the dissipation of a measured amount of energy and the observation of the resulting temperature rise a given time later. An advantage of this method is that the effective sampling volume can be varied by varying the measurement time. Using a measurement time of a few seconds, the sampling volume was estimated to be several orders of magnitude greater than the probe volume. Hence artifacts due to probe-induced trauma or stress would be insignificant. Additional advantages of the technique are: the results were independent of the probe shape, size and properties, and hence represents absolute measurements without the need for calibration; the required electronics and computations are simple; the determination of thermal conductivity requires only a single measurement; and comparison of data at different measurement times yields a clear and unequivocal indication of nonconductive contributions of heat transfer, if present.

Michael M. Chen

Papers

Microvascular contributions in tissue heat transfer.

A novel radioactive particle tracking facility for measurement of solids motion in gas fluidized beds

Microconvective Thermal Conductivity in Disperse Two-Phase Mixtures as Observed in a Low Velocity Couette Flow Experiment

A theoretical analysis of heat transfer due to particle impact

Pulse-Decay Method for Measuring the Thermal Conductivity of Living Tissues