This paper presents a detailed description and experimental validation of a modified collocated unit cell model for the estimation of effective thermal conductivity of packed beds. The effect of primary and secondary parameters influencing the effective thermal conductivity viz. conductivity ratio (α)and concentration (ν)of the spherical particles (fraction of total volume occupied by the particles)of the packed bed, inter-particle gap, thermal contact conductance between the surfaces in contact, temperature, pressure and Knudsen number respectively are investigated in detail. Analytical expressions for effective thermal conductivity are derived for spatially periodic 2D square cylinder and in-line touching geometries based on the unit cell based thermal resistance approach. Using the developed model, the effective thermal conductivity of various packed materials in the conductivity ratio (k s /k f )range between 1–1000 and concentration (ν)0 to 1 are predicted for varied temperature and pressure regimes. Predictions obtained from the developed model are compared against existing models available in literature in the pressure range 10 −2 to 10 3 kPa at constant temperature. Furthermore, effective thermal conductivities of packed beds comprising of glass and ceramic beads are predicted using the developed model and compared with measured values obtained using the standard square guarded hot plate (SGHP)apparatus in the temperature range of 323–673 K. Both values are found to agree within ±12.24% and ±14.54% for glass and ceramic beads respectively. © 2018 Elsevier Ltd

Arul Jayachandran S

Department Of Civil Engineering

Srinivas Reddy K.

Department of Mechanical Engineering

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Thermal Science and Engineering Progress

Composite materials are novel inventions of material science which are often used as viable solutions in many technologically challenging situations like aero-space and satellite applications, because of their well known superior mechanical properties. The challenge which these materials always pose to the thermal designers, is knowledge of their thermal properties, effective directional thermal conductivity being one of them as this is critical in the design of systems employing these materials. The present work aims at developing a novel experimental technique for the simultaneous estimation of principal thermal conductivities of a layered honeycomb composite widely used in aerospace structures. A new standard test material exhibiting structural anisotropy with respect to thermal transport is first conceptualised, designed and fabricated. A full scale direct numerical simulation is performed on the standard test material to understand the thermal transport process in it and determine its principal thermal conductivities. Following this, carefully designed experiments are performed on the standard test material in simulated space environment (inside a vacuum chamber) and its principal thermal conductivities are estimated using the developed inverse methodology based on a synergistic combination of artificial neural network (ANN) and genetic algorithm (GA). Experimentally obtained thermal conductivities are compared with those obtained from full scale numerical simulations. A close agreement between the experimentally and numerically estimated thermal conductivities is observed, validating the technique and establishing the possibility of use of a full scale numerical model as an alternate and standalone approach for estimation of thermal conductivities of structured integral composites. Finally, a layered honeycomb composite having carbon fiber reinforced plastic (CFRP) facesheet and aluminium core actually used in satellite applications is tested and its principal thermal conductivities are estimated. Problems related to interface thermal contact conductance in case of honeycomb composites are also brought out and addressed. © 2016 Elsevier Masson SAS

International Journal of Thermal Sciences

Estimation of principal thermal conductivities of layered honeycomb composites using ANN–GA based inverse technique

This paper presents a detailed description of a double-sided square guarded hot plate (SGHP) apparatus designed specifically for testing low to moderate thermal conductivity materials having thermal conductivities in the range of 0.02–3.0 Wm−1K−1. Measurements are conducted in the temperature range of 323–573 K with a temperature difference of 20 K imposed across the test specimen. An analysis of measurement data obtained from tests performed on standard reference materials (SRM) yielded a combined measurement uncertainty of ±6.22%, thereby giving an insight into the precision of the designed SGHP apparatus presented in the study. Experimental investigations on several low thermal conductivity insulation materials having thermal conductivities in the above stated range have been performed to validate the measurement system as a whole and the results are presented. © 2017 Elsevier Masson SAS

Investigations on design and construction of a square guarded hot plate (SGHP) apparatus for thermal conductivity measurement of insulation materials

In this article, the collocated parameter models are used to estimate the effective thermal conductivity of the two-phase materials. The algebraic equations are derived using a unit-cell-based isotherm approach for a two- and three-dimensional spatially periodic medium. The geometry of the medium is considered as matrix of touching and nontouching in-line square and circular cylinders as well as touching and nontouching in-line solid and hollow cubes. The models are used to predict the thermal conductivity of numerous two-phase materials (maximum conductivity ratio of 1000 and concentration ranging between 0 and 1). The estimated thermal conductivity data is in good agreement with experimental data within 16.67% deviation for various two-phase systems. The results are compared with the standard models and experimental data for different types of two-phase systems; it is shown that the touching in-line solid cube model predicts better as compared to other geometrical configurations.

Heat Transfer Engineering

Estimation of effective thermal conductivity of two-phase materials using collocated parameter model

In this paper, correlations are proposed to estimate the effective thermal conductivity of two-phase materials. For any α, Maxwell equation for 0.0 < c ≤ 0.10 and phase inverted Maxwell for 0.9 ≤ c ≤ 1 are considered. For concentrations between 10% and 90%, and low α (<20), an equation based on the unit-cell approach (constant isotherms) is proposed. For α > 20, three correlations are proposed based on field solution approach which includes three α ranges viz. medium (20 ≤ α ≤ 100), high (100 ≤ α ≤ 1000) and very high (1000 < α). The predicted effective thermal conductivity of two-phase system is compared with well-established models. Comparison of the predicted values of the correlations with experimental results is also made. The predictions of effective thermal conductivity of two-phase materials match well with the experimental values. © 2006 Elsevier Ltd. All rights reserved.

Journal	Data powered by TypesetThermal Science and Engineering Progress
Publisher	Data powered by TypesetElsevier Ltd
ISSN	24519049
Open Access	No