Mark A. Kedzierski

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.

Effect of CuO Nanoparticle Concentration on R134A/Lubricant Pool Boiling Heat Transfer

This paper quantifies the influence of copper (II) oxide (CuO) nanoparticle concentration on the boiling performance of R134a/polyolester mixtures on a roughened horizontal flat surface. Nanofluids are liquids that contain dispersed nanosize particles. Two lubricant-based nanofluids (nanolubricants) were made with a synthetic polyolester and 30 nm diameter CuO particles to 1% and 0.5% volume fractions, respectively. As reported in a previous study for the 1% volume fraction nanolubricant, a 0.5% nanolubricant mass fraction with R134a resulted in a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) between 50% and 275%. The same study had shown that increasing the mass fraction of the 1% volume fraction nanolubricant resulted in smaller, but significant, boiling heat transfer enhancements. The present study shows that the use of a nanolubricant with half the concentration of CuO nanoparticles (0.5% by volume) resulted in either no improvement or boiling heat transfer degradations with respect to the R134a/polyolester mixtures without nanoparticles. Consequently, significant refrigerant/lubricant boiling heat transfer enhancements are possible with nanoparticles; however, the nanoparticle concentration is an important determining factor. Further research with nanolubricants and refrigerants is required to establish a fundamental understanding of the mechanisms that control nanofluid heat transfer.

The effect of lubricant concentration, miscibility, and viscosity on R134a pool boiling ☆

Abstract This paper presents pool boiling heat transfer data for 12 different R134a/lubricant mixtures and pure R134a on a Turbo-BII™-HP surface. The mixtures were designed to examine the effects of lubricant mass fraction, viscosity, and miscibility on the heat transfer performance of R134a. The magnitude of the effect of each parameter on the heat transfer was quantified with a regression analysis. The mechanistic cause of each effect was given based on new theoretical interpretation and/or one from the literature. The model illustrates that large improvements over pure R134a heat transfer can be obtained for R134a/lubricant mixtures with small lubricant mass fraction, high lubricant viscosity, and a large critical solution temperature (CST). The ratio of the heat flux of the R134a/lubricant mixture to that of the pure R134a for fixed wall superheat was given as a function of pure R134a heat flux for all 12 mixtures. The lubricant that had the largest CST with R134a exhibited the greatest heat transfer: 100%±20% greater than that of pure R134a. By contrast, the heat transfer of the mixture with the lubricant that had the smallest viscosity and the smallest CST with R134a was 55%±9% less than that of pure R134a. High-speed films of the pure and mixture pool boiling were taken to observe the effect of the lubricant on the nucleate boiling.

Use of fluorescence to measure the lubricant excess surface density during pool boiling

This paper presents what are believed to be the first measurements of the non-adiabatic lubricant excess surface density on a roughened, flat, plain horizontal pool-boiling surface. Pool boiling heat transfer data is given for pure R123 and a R123/lubricant mixture. Lubricant excess surface density data are given for the boiling R123/lubricant mixture. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of a R123/lubricant mixture on a test surface. The fluorescent measurement technique was used to confirm the existence of the lubricant excess layer during refrigerant/lubricant mixture boiling. The refrigerant preferentially boils, thus, concentrating and accumulating the lubricant on the surface in excess of the bulk concentration. The excess lubricant resides in a very thin layer on the surface and influences the boiling performance. Accordingly, the ability to measure the lubricant excess density on the heat transfer surface would lead to a fundamental understanding of the mechanism by which lubricants can degrade or improve boiling performance. In support of this effort, heat transfer data are provided for both pure R123 and an R123/lubricant (1.8% lubricant mass fraction) mixture at 277.6 K. The heat transfer data shows that the lubricant excess causes an average degradation of 12% in the heat flux for a given superheat.

Pressure Drop During Refrigerant Condensation Inside Horizontal Smooth, Helical Microfin, and Herringbone Microfin Tubes

We present a study of pressure drops during condensation inside a smooth, an 18-deg helical microfin, and a herringbone microfin tube. Measurements were conducted with refrigerant flowing through the tube of a concentric heat exchanger, with water flowing in a counterflow direction in the annulus. Each tube was part of a condenser consisting of eight subcondensers with instrumentation preceding each subcondenser. Three refrigerants were used, namely R-22, R-407C, and R-134a, all operating at a saturation temperature of 40 °C with moss fluxes ranging from 400 to 800 kg/m 2 s

Enhancement of R123 pool boiling by the addition of hydrocarbons

Abstract This paper presents pool boiling heat transfer data for 10 different R123/hydrocarbon mixtures. The data consisted of pool boiling performance of a GEWA-T ™ surface for pure R123 and for 10 dilute solutions of five different hydrocarbons: (1) pentane, (2) isopentane, (3) hexane, (4) cyclohexane, and (5) heptane with R123. The heat flux and the wall superheat were measured for each fluid at 277.6 K. A maximum (19±3.5)% increase over the pure R123 heat flux was achieved with the addition of 0.5% mass isopentane to R123. Other mixtures of isopentane, pentane, hexane, and cyclohexane with R123 exhibited smaller maximums than that of the R123/isopentane (99.5/0.5) mixture. Presumably, a layer enriched in hydrocarbon at the heat transfer surface caused the heat transfer enhancement. Conversely, an R123/heptane (99.5/0.5) mixture and an R123/cylcohexane (99.5/0.5) mixture exhibited only degradations with respect to the pure component performance for all test conditions. Several characteristics of the hydrocarbons were examined to determine their influence on the boiling heat transfer performance: molecular weight, molecular structure, composition, surface tension, and vapor pressure.

A semi-theoretical model for predicting refrigerant/lubricant mixture pool boiling heat transfer

Abstract This paper outlines the framework of a semi-theoretical model for predicting the pool boiling heat transfer of refrigerant/lubricant mixtures on a roughened, horizontal, flat pool-boiling surface. The predictive model is based on the mechanisms involved in the formation of the lubricant excess layer that exists on the heat transfer surface. The lubricant accumulates on the surface in excess of the bulk concentration via preferential evaporation of the refrigerant from the bulk refrigerant/lubricant mixture. As a result, excess lubricant resides in a thin layer on the surface and influences the boiling performance, giving either an enhancement or degradation in heat transfer. A dimensionless excess layer parameter and a thermal boundary layer constant were derived and fitted to data in an attempt to generalize the model to other refrigerant/lubricant mixtures. The model inputs include transport and thermodynamic refrigerant properties and the lubricant composition, viscosity, and critical solution temperature with the refrigerant. The model predicts the boiling heat transfer coefficient of three different mixtures of R123 and lubricant to within ±10%. Comparisons of heat transfer predictions to measurements for 13 different refrigerant/lubricant mixtures were made, including two different refrigerants and three different lubricants.

DESIGN AND MACHINING OF COPPER SPECIMENS WITH MICRO HOLES FOR ACCURATE HEAT TRANSFER MEASUREMENTS

This article presents a technique that has been developed specifically for drilling 0.5334-mm-diameter, 19-mm-deep holes in copper for use in temperature measurement. The holes accept thermocouples, which are used for the measurement of the temperature gradient and the wall temperature of the specimen. Errors due to the intrusion of the probe, ami the finite size and mass of the probe, are reduced as the diameter of the probes is reduced. A machining procedure for drilling deep micro holes in copper cannot be found in conventional texts; this article advocates holes that are deeper than those traditionally recommended. This article is written for both machinists and experimentalists. Both heat transfer and machining criteria are considered. The necessary equipment and their specifications are discussed. Special attention is given to specifying drilling speeds, feed rates, and lubricants. Step-by-step drilling instructions are given. An analysis is performed to reveal the important parameters for reducing ...

Viscosity, density, and thermal conductivity of aluminum oxide and zinc oxide nanolubricants

This paper presents liquid kinematic viscosity, density, and thermal conductivity measurements of eleven different synthetic polyolester-based nanoparticle nanolubricants (dispersions) at atmospheric pressure over the temperature range 288 K to 318 K. Aluminum oxide (Al2O3) and zinc oxide (ZnO) nanoparticles with nominal diameters of 127 nm and 135 nm, respectively, were investigated. A good dispersion of the spherical and non-spherical nanoparticles in the lubricant was maintained with a surfactant. Viscosity, density, and thermal conductivity measurements were made for the neat lubricant along with eleven nanolubricants with differing nanoparticle and surfactant mass fractions. Existing models were used to predict kinematic viscosity (±20%), thermal conductivity (±1%), and specific volume (±6%) of the nanolubricant as a function of temperature, nanoparticle mass fraction, surfactant mass fraction, and nanoparticle diameter. The liquid viscosity, density and thermal conductivity were shown to increase with respect to increasing nanoparticle mass fraction.

Practical Fin Shapes for Surface-Tension-Drained Condensation

This paper introduces a new family of high-performance fin profiles for surface-tension-drained condensation. Previously described profiles for this situation have been defined in terms of the fin curvature and arc length. The existing profiles are generally not suitable for commercial manufacture. The fin profiles presented in this paper are conveniently defined by the fin tip radius, the fin height, and the fin base thickness. Consequently, the designer may easily specify a fin shape with parameters that are compatible with those used by the manufacturing industry. The heat transfer performance of the new profiles provides an improvement over existing, commercial fin shapes. An analysis is presented to show the R-11 condensation performance of the new profiles as a function of the geometric variables. A recommended design practice for fins for surface-tension-drained condensation is given also.