Heat transfer characteristics of nanofluids: a review

Nanofluids are potential heat transfer fluids with enhanced thermophysical properties and heat transfer performance can be applied in many devices for better performances (i.e. energy, heat transfer and other performances). In this paper, a comprehensive literature on the applications and challenges of nanofluids have been compiled and reviewed. Latest up to date literatures on the applications and challenges in terms of PhD and Master thesis, journal articles, conference proceedings, reports and web materials have been reviewed and reported. Recent researches have indicated that substitution of conventional coolants by nanofluids appears promising. Specific application of nanofluids in engine cooling, solar water heating, cooling of electronics, cooling of transformer oil, improving diesel generator efficiency, cooling of heat exchanging devices, improving heat transfer efficiency of chillers, domestic refrigerator-freezers, cooling in machining, in nuclear reactor and defense and space have been reviewed and presented. Authors also critically analyzed some of the applications and identified research gaps for further research. Moreover, challenges and future directions of applications of nanofluids have been reviewed and presented in this paper. Based on results available in the literatures, it has been found nanofluids have a much higher and strongly temperature-dependent thermal conductivity at very low particle concentrations than conventional fluids. This can be considered as one of the key parameters for enhanced performances for many of the applications of nanofluids. Because of its superior thermal performances, latest up to date literatures on this property have been summarized and presented in this paper as well. However, few barriers and challenges that have been identified in this review must be addressed carefully before it can be fully implemented in the industrial applications.

/pdf/a-review-on-applications-and-challenges-of-nanofluids-ibdekzmmqr.pdf

A review on applications and challenges of nanofluids

Suspended nanoparticles in conventional fluids, called nanofluids, have been the subject of intensive study worldwide since pioneering researchers recently discovered the anomalous thermal behavior of these fluids. The enhanced thermal conductivity of these fluids with small-particle concentration was surprising and could not be explained by existing theories. Micrometer-sized particle-fluid suspensions exhibit no such dramatic enhancement. This difference has led to studies of other modes of heat transfer and efforts to develop a comprehensive theory. This article presents an exhaustive review of these studies and suggests a direction for future developments. The review and suggestions could be useful because the literature in this area is spread over a wide range of disciplines, including heat transfer, material science, physics, chemical engineering and synthetic chemistry.

https://www.tandfonline.com/doi/pdf/10.1080/01457630600904593

Heat Transfer in Nanofluids—A Review

This study provides a detailed literature review and an assessment of results of the research and development work forming the current status of nanofluid technology for heat transfer applications. Nanofluid technology is a relatively new field, and as such, the supporting studies are not extensive. Specifically, experimental results were reviewed in this study regarding the enhancement of the thermal conductivity and convective heat transfer of nanofluids relative to conventional heat transfer fluids, and assessments were made as to the state-of-the-art of verified parametric trends and magnitudes. Pertinent parameters of particle volume concentration, particle material, particle size, particle shape, base fluid material, temperature, additive, and acidity were considered individually, and experimental results from multiple research groups were used together when assessing results. To this end, published research results from many studies were recast using a common parameter to facilitate comparisons of data among research groups and to identify thermal property and heat transfer trends. The current state of knowledge is presented as well as areas where the data are presently inconclusive or conflicting. Heat transfer enhancement for available nanofluids is shown to be in the 15-40% range, with a few situations resulting in orders of magnitude enhancement.

Review and Comparison of Nanofluid Thermal Conductivity and Heat Transfer Enhancements

A review of nanofluid stability properties and characterization in stationary conditions

The enhancement of the thermal conductivity of water in the presence of carbon-multiwall nanotubes (C-MWNT) was investigated. Sodium dodecyl sulfate (SDS) was employed as the dispersant, and a 0.6 vol% suspension of C-MWNT in water was used in all measurements. The thermal conductivity was measured with a transient hot-wire instrument built for this purpose, and operated with a standard uncertainty better than 2 The maximum thermal conductivity enhancement obtained was 38 %. In an attempt to explain the experimental observations, a number of micro-structural investigations have been carried out and those results are presented here along with the analysis.

Thermal Conductivity of Suspensions of Carbon Nanotubes in Water

Carbon multi-walled nanotubes (C-MWNTs) and alternatively carbon double-walled nanotubes (C-DWNTs) were added in water, following our previous work, in order to enhance the thermal conductivity of this traditional heat transfer fluid. Hexadecyltrimethyl ammonium bromide (CTAB) and Nanosperse AQ were employed as dispersants. The transient hot-wire technique was used for the measurement of the thermal conductivity with an instrument built for this purpose. The absolute uncertainty is better than 2%. The maximum thermal conductivity enhancement obtained was 34% for a 0.6% volume C-MWNT suspension in water with CTAB. All measurements were made at ambient temperature. In an attempt to evaluate and explain the experimental results, information about the microstructure of the suspensions is needed. The findings of these investigations are presented here along with the analysis.

Thermal Conductivity Enhancement in Aqueous Suspensions of Carbon Multi-Walled and Double-Walled Nanotubes in the Presence of Two Different Dispersants

The thermal conductivity of polymethyl methacrylate (PMMA) and borosilicate crown glass BK7 has been studied. The transient hot-wire technique has been employed, and measurements cover a temperature range from room temperature up to 350 K for PMMA and up to 500 K for BK7. The technique is applied here in a novel way that minimizes all remaining thermal-contact resistances. This allows the apparatus to operate in an absolute way and with very low uncertainty. The method makes use of a soft silicone paste material between the hot wires and the solid under test. Measurements of the transient temperature rise of the wires in response to an electrical heating step over a period of 20 μs up to 5 s allow an absolute determination of the thermal conductivity of the solid, as well as of the silicone paste. The method is based on a full theoretical model with equations solved by a two-dimensional finite-element method applied to the exact geometry. At the 95% confidence level, the standard deviations of the thermal conductivity measurements are 0.09% for PMMA and 0.16% for BK7, whereas the standard uncertainty of the technique is less than 1.5%.

Thermal conductivity of polymethyl methacrylate (PMMA) and borosilicate crown glass BK7

The thermal conductivity of nanofluids has been studied experimentally using the transient hot-wire method, and it is shown that a significant increase can be obtained. Existing methods for the prediction and correlation of the thermal conductivity are discussed. It is shown that a lot of work still needs to be done in this area.

/pdf/thermal-conductivity-of-nanofluids-experimental-and-4braayt3i9.pdf

Thermal Conductivity of Nanofluids – Experimental and Theoretical

The thermal conductivity of three thermal-conductivity reference materials, Pyrex 7740, Pyroceram 9606, and stainless steel AISI 304L, has been studied. The technique employed is the transient hot-wire technique, and measurements cover a temperature range from room temperature up to 570 K. The technique is applied here in a novel way that eliminates all remaining contact resistances. This allows the apparatus to operate in an absolute way. The method makes use of a soft silicone paste material between the hot wires of the technique and the solid of interest. Measurements of the transient temperature rise of the wires in response to an electrical heating step in the wires over a period of 20 μs up to 20 s allow an absolute determination of the thermal conductivity of the solid, as well as of the silicone paste. The method is based on a full theoretical model with equations solved by a two-dimensional finite-element method applied to the exact geometry. At the 95% confidence level, the standard deviation of the thermal conductivity measurements is 0.1% for Pyrex 7740, 0.4% for Pyroceram 9606, and 0.2% for stainless steel AISI 304L, while the standard uncertainty of the technique is less than 1.5%.

I. N. Metaxa

Papers

Thermal Conductivity of Suspensions of Carbon Nanotubes in Water

Thermal Conductivity Enhancement in Aqueous Suspensions of Carbon Multi-Walled and Double-Walled Nanotubes in the Presence of Two Different Dispersants

Thermal conductivity of polymethyl methacrylate (PMMA) and borosilicate crown glass BK7

Thermal Conductivity of Nanofluids – Experimental and Theoretical

Thermal Conductivity of Reference Solid Materials