Review of thermo-physical properties, wetting and heat transfer characteristics of nanofluids and their applicability in industrial quench heat treatment

The success of quenching process during industrial heat treatment mainly depends on the heat transfer characteristics of the quenching medium. In the case of quenching, the scope for redesigning the system or operational parameters for enhancing the heat transfer is very much limited and the emphasis should be on designing quench media with enhanced heat transfer characteristics. Recent studies on nanofluids have shown that these fluids offer improved wetting and heat transfer characteristics. Further water-based nanofluids are environment friendly as compared to mineral oil quench media. These potential advantages have led to the development of nanofluid-based quench media for heat treatment practices. In this article, thermo-physical properties, wetting and boiling heat transfer characteristics of nanofluids are reviewed and discussed. The unique thermal and heat transfer characteristics of nanofluids would be extremely useful for exploiting them as quench media for industrial heat treatment

Ideal and mixture permeation selectivity of flexible prototypical zeolitic imidazolate framework - 8 membranes

Permeance, ideal and mixture permeation selectivity have been evaluated based on molecular simulations of gas adsorptions by the grand canonical Monte Carlo simulation and diffusion by molecular dynamics simulation under a flexible framework. Additional transport phenomena are also obtained using transition state theory under rigid framework assumptions. A number of gas systems in the simulation involving H, CO, CH, N, O and Ar penetrating through a thin layer of ZIF-8 membrane are analyzed and compared with permeance, ideal and mixture permeation selectivity experiments. Three contributions have been achieved in this work. Firstly, it is demonstrated that the assumption of a rigid framework may be sufficient for simulating adsorption isotherms while diffusivity requires a flexible framework, especially when a diffusing molecule is bigger than the pore window of the MOFs, with an appropriate force-field and charges. Secondly, simulations of the ideal permeation selectivity are higher than those of mixture permeation selectivity. However, both calculations with flexible framework accuracy may be acceptable for the purpose of screening membrane materials when compared with experiments, especially the permeation selectivity of an equimolar mixture. Finally, even though simulated isotherms, diffusivity and mixture selectivity are consistent with available experiments and the same data have been used for estimating permeance of the membrane, the difference between the estimation and measurement of the permeation at low pressure is considerable. Therefore, it is confirmed that the effects of defections and multi-scale (macroscopic) diffusion cannot be ignored

A multi-scale approach to the physical adsorption in slit pores

Adsorption isotherms are the foundation of gas storage and separation operations. The isotherm models are classified into three scale levels with empiricisms in macroscopic level, requirements of long computing time and idealized conditions in microscopic level, as well as gaps in knowledge between these two levels. A multi-scale modeling methodology is developed in this paper in order to reduce the identified limitations. Microscopic molecular simulations (MS) based on the grand canonical Monte Carlo (GCMC) method are carried out followed by the development of the localized adsorption isotherms defined as the intermediate level models. They are represented by the Boltzmann factor and the local Langmuir equations. The macroscopic models are then formulated through the integration of small scale models. The following three contributions are achieved in the paper. First of all, guidelines for the validity of the Boltzmann factor are established, showing its practical significance, and the local Langmuir isotherm is justified as a good approximation to the results from microscopic simulations. Secondly, it is demonstrated that the pore size distributions can be determined using GCMC simulations coupled with the measured adsorption isotherms as exemplified by a case study on a coal specimen. Finally, using the measurement data reported by Bae and Bhatia (2006) for carbon dioxide adsorption on coal, we show that the overall adsorption isotherms can be determined from the multi-scale approach through the integration of smaller scale models with pore size distributions without the empiricism, indicating the success of the methodology. Further work is needed to improve the prediction accuracy for methane adsorption on coal specimens. (C) 2011 Elsevier Ltd. All rights reserved

Multi-component adsorption in heterogeneous carbonaceous porous media through the integration of small-scale, homogenous models

In this work, a simplified multi-scale modeling approach to multi-component adsorption is developed. A number of binary gas systems involving CO, CH and N adsorbed on carbon pores are analyzed in three scales. Comprehensive molecular simulations at the microscopic level using the GCMC method are the basis for the validation of the simplified (mesoscopic-level) models for isotherms of homogenous adsorbent. The macroscopic models are then formulated through the integration of the smaller scale models. Three contributions are made in this work. Firstly, it is demonstrated that the form of extended Langmuir equation can represent adsorption isotherms, developed by GCMC simulation in homogeneous micro-pores under mono-layer adsorption. Secondly, the methodology for the estimation of transferable pore-size distributions using optimization algorithms is successfully provided that more than one single component adsorption isotherm is used in computations. Finally, the extended dual-site Langmuir equation is justified as a suitable macroscopic model for multi-component adsorption on heterogeneous porous media with parameters estimated from small-scale models

An experimental and simulation study of binary adsorption in metal-organic frameworks

Large surface area, high gas adsorption capacity and convenient synthesis methods make microporous metal–organic frameworks (MOFs) a promising adsorbent for gas separation of CO2/N2 and CO2/CH4. This study examines the selective adsorption of CO2 on MOFs through the experimental measurement of equilibrium adsorption capacities from pure fluids (CO2, CH4 and N2) and mixtures of CO2/N2 and CO2/CH4. The derived adsorption selectivity from binary adsorption measurements is higher than the ideal selectivity. Comparing with direct binary adsorption experiments, the Ideal Adsorbed Solution Theory (IAST) model using best-fit parameters for Langmuir isotherms of each pure fluid provides satisfactory predictions for the binary mixtures of CO2/N2 and CO2/CH4. This combined experimental and modeling approach can provide criteria to screen metal–organic frameworks for the separation of gas mixtures at industrially relevant compositions, temperatures and pressures

Analysis Of Convective Heat Transfer Enhancement By Nanofluids: Single-Phase And Two-Phase Treatments

Nanofluids have been investigated regarding their advantages and potentialities for the purpose of increasing convective heat transfer rates inside thermal systems where they are used as working fluids. Researchers in thermophysics have investigated these fluids experimentally and numerically. This review provides extensive theoretical information concerning nanofluids in the single-phase and two-phase treatments. Important published works on nanofluid properties and correlations are summarized and reviewed in detail. Heat transfer enhancement by nanofluids is a challenging problem due to the difficulties inherent in the model of the physical mechanism of interaction between the paricles. Here the interaction between the phases is modeled by several two-phase models, and the results are given in graphical and tabular forms. Despite the advantages of the mixture model, such as imlementation of physical properties and less computational power requirements, some studies showed that the results of the single-phase and two-phase models are very similar. The main difference consists in the effect of the drift velocities of the phases relative to each other