It is shown that a “nanofluid” consisting of copper nanometer-sized particles dispersed in ethylene glycol has a much higher effective thermal conductivity than either pure ethylene glycol or ethylene glycol containing the same volume fraction of dispersed oxide nanoparticles. The effective thermal conductivity of ethylene glycol is shown to be increased by up to 40% for a nanofluid consisting of ethylene glycol containing approximately 0.3 vol % Cu nanoparticles of mean diameter <10 nm. The results are anomalous based on previous theoretical calculations that had predicted a strong effect of particle shape on effective nanofluid thermal conductivity, but no effect of either particle size or particle thermal conductivity.

Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles

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

This paper compares theory and experiment for behavior very near critical points. The primary experimental results are the "critical indices" which describe singularities in various thermodynamic derivatives and correlation functions. These indices are tabulated and compared with theory. The basic theoretical ideas are introduced via the molecular field approach, which brings in the concept of an order parameter and suggests that there are close relations among different phase transition problems. Although this theory is qualitatively correct it is quantitatively wrong, it predicts the wrong values of the critical indices. Another theoretical approach, the "scaling law" concept, which predicts relations among these indices, is described. The experimental evidence for and against the scaling laws is assessed. It is suggested that the scaling laws provide a promising approach to understanding phenomena near the critical point, but that they are by no means proved or disproved by the existing experimental data.

Static Phenomena Near Critical Points: Theory and Experiment

https://www.umsl.edu/~chickosj/JSCPUBS/dsccal.pdf

Reference materials for calorimetry and differential thermal analysis

Effective viscosities and thermal conductivities of aqueous nanofluids containing low volume concentrations of Al2O3 nanoparticles

A new apparatus for measuring the thermal conductivity of fluids is described. This is an absolute method utilizing a transient hot wire. Measurements are made with a 12.7 p,m diameter platinum wire at real times of up to 1 second. The data acquisition system includes a minicomputer and a digital voltmeter. The experimental core of the system incorporates a compensating hot wire in a Wheatstone bridge circuit. The cell containing the core of the apparatus is designed to accommodate pressures from 0 to 70 MPa and temperatures from 70 to 320 K. Oxygel1 was measured over a wide range of physical states including the dilute gas, the moderately dense gas, the near critical region, the compressed liquid states, and the vapor ai temperatures below the critical temper· ature. Performance checks of the apparatus were conducted with nitrogen, helium and argon. Measurement of rare gases allows a direct comparison to the kinetic theory of gases through the viscosity. A second check looks at the variation of the measured thermal conductivity as a function of the applied power. The precision (2 a) of the new system is between 0.5 and 0.8 percent for wire temperature transients of 4 to 5 K, while the accuracy is estitnated at around 1.5 percent.

/pdf/a-transient-hot-wire-thermal-conductivity-apparatus-for-58gocv9m51.pdf

A Transient Hot Wire Thermal Conductivity Apparatus for Fluids.

A new apparatus for measuring both the thermal conductivity and thermal diffusivity of fluids at temperatures from 220 to 775 K at pressures to 70 MPa is described. The instrument is based on the step-power-forced transient hot-wire technique. Two hot wires are arranged in different arms of a Wheatstone bridge such that the response of the shorter compensating wire is subtracted from the response of the primary wire. Both hot wires are 12.7 µm diameter platinum wire and are simultaneously used as electrical heat sources and as resistance thermometers. A microcomputer controls bridge nulling, applies the power pulse, monitors the bridge response, and stores the results. Performance of the instrument was verified with measurements on liquid toluene as well as argon and nitrogen gas. In particular, new data for the thermal conductivity of liquid toluene near the saturation line, between 298 and 550 K, are presented. These new data can be used to illustrate the importance of radiative heat transfer in transient hot-wire measurements. Thermal conductivity data for liquid toluene, which are corrected for radiation, are reported. The precision of the thermal conductivity data is ± 0.3% and the accuracy is about ±1%. The accuracy of the thermal diffusivity data is about ± 5%. From the measured thermal conductivity and thermal diffusivity, we can calculate the specific heat, Cp , of the fluid, provided that the density is measured, or available through an equation of state.

/pdf/a-high-temperature-transient-hot-wire-thermal-conductivity-104n4swaxt.pdf

A High-Temperature Transient Hot-Wire Thermal Conductivity Apparatus for Fluids.

A transient hot-wire apparatus was used to measure the thermal conductivity of argon with both steady-state and transient methods. The effects of wire diameter, eccentricity of the wire in the cavity, axial conduction, and natural convection were accounted for in the analysis of the steady-state measurements. Based on measurements on argon, the relative uncertainty at the 95 % level of confidence of the new steady-state measurements is 2 % at low densities. Using the same hot wires, the relative uncertainty of the transient measurements is 1 % at the 95 % level of confidence. This is the first report of thermal conductivity measurements made by two different methods in the same apparatus. The steady-state method is shown to complement normal transient measurements at low densities, particularly for fluids where the thermophysical properties at low densities are not known with high accuracy.

/pdf/absolute-steady-state-thermal-conductivity-measurements-by-2jz2r038zr.pdf

Absolute Steady-State Thermal Conductivity Measurements by Use of a Transient Hot-Wire System.

This paper presents new absolute measurements of the thermal conductivity and the thermal diffusivity of nitrogen made with a transient hot wire instrument. The instrument measures the thermal conductivity with an uncertainty less than ±1% and the thermal diffusivity with an uncertainty of ±5% except at the fluid critical point. The data cover the region from 80 to 300KK pressures to 70 MPa. The data consist of 8 supercritical isotherms, 3 vapor isotherms, and 4 liquid isotherms. A surface fit is developed for our nitrogen thermal conductivity data from 80 to 300 K at pressures to 70 MPa. The data are compared with a recent theory for the first density coefficient of thermal conductivity and a new mode-coupling theory for the thermal conductivity critical enhancement. These data illustrate that it is necessary to study a fluid over a wide range of temperatures and densities in order to characterize the thermal conductivity surface. Isobaric heat capacity results were determined from the simultaneously measured values of thermal conductivity and thermal diffusivity, using the density calculated from an equation of state. The heat capacities obtained by this technique are compared to the heat capacities predicted by a recent equation of state developed specifically for nitrogen.

The thermal conductivity and heat capacity of fluid nitrogen

Experimental data are presented at closely spaced intervals of temperature and density. The range of experimental densities is from 0.064 to 2.8 times the critical density. There are presented, in addition, tables interpolated uniformly in arguments density and temperature, and also in pressure and temperature.

/pdf/pressure-density-temperature-relations-of-fluid-para-15r6nwbo8k.pdf

H. M. Roder

Papers

A Transient Hot Wire Thermal Conductivity Apparatus for Fluids.

A High-Temperature Transient Hot-Wire Thermal Conductivity Apparatus for Fluids.

Absolute Steady-State Thermal Conductivity Measurements by Use of a Transient Hot-Wire System.

The thermal conductivity and heat capacity of fluid nitrogen

Pressure-density-temperature relations of fluid para hydrogen from 15 to 100 deg k at pressures to 350 atmospheres