Modeling Frost Growth for Subcooled Tube-Array Configurations

2009 ◽  
Vol 131 (6) ◽  
Author(s):  
V. Yadav ◽  
C. G. Moon ◽  
K. Kant
Keyword(s):  
Thermal Conductivity ◽  
Temperature Field ◽  
Relative Humidity ◽  
Local Level ◽  
Airflow Velocity ◽  
Array Configuration ◽  
Novel Technique ◽  
Conventional Analysis ◽  
Good Agreement ◽  
Frost Growth

Methodology for predicting frost growth trends on a subcooled cylindrical surface is developed and implemented for multitube array configuration. Extension of conventional analysis and a novel technique for understanding frost formation phenomenon on the cylindrical surfaces is proposed; later one takes into account the nonsteady temperature field, which affects the density and thermal conductivity at a local level in the growing frost mass, for more accurate prediction of thermal resistance. The influence of migration of liquid water due to tortuosity effect is also considered. The results due to new model are found to be in good agreement with the data in the open literature. Data for frost thickness ratio (FTR) versus time for a section of array with four (tube) rows in the airstream are presented and thoroughly analyzed. The trends of FTR noted are complex and considerably dependent on the tube location, temperature of subcooled surface (Ts), airflow velocity (Ua), and the relative humidity (RHa) values. Approximate ranges for important parameters are −30≤Ts≤−5.0°C, 1.0≤Ua≤5.0 m/s, and 0.20≤RHa≤0.80. Presented analysis and the results are valuable in order to predict probable locations and precursors to partial or complete choking of airflow passages due to frost deposition in the evaporator coils.

Simulation and Comparative Analysis on Environmental Control in Subway

2012 ◽  
Vol 209-211 ◽  
pp. 1068-1072
Author(s):  
Ming Liu ◽  
Bao Gang Zhang ◽  
Liu Wen ◽  
Zhong Zhi Huang
Keyword(s):  
Temperature Field ◽  
Energy Consumption ◽  
Comparative Analysis ◽  
Environmental Control ◽  
Thermal Environment ◽  
Airflow Velocity ◽  
Waiting Room ◽  
Local Climate ◽  
Average Temperature ◽  
Environment Control

To ensure passengers can have a comfortable thermal environment in the subway waiting room is one of the main targets in subway environment control. By using the CFD software, this paper takes an island platform with double-layer of Shenyang as the object to simulate the thermal environment of platform screen door and ventilation in the open system, then detailed analysis on the variation of the temperature field, airflow velocity field in the typical position. The results indicated that average temperature of no-PSD system platform is 2.5 ~3 °C higher than that of PSD system, standing room is 1.5 °C higher than that of PSD system , but these also meet the requirement of the standards of the environmental control and passengers’ thermal comfort. Meanwhile, through the comparative analysis to the energy consumption of the two systems, we find ventilating condition has superior to PSD system in energy saving, and it is more suitable for the local climate in the northeast.

The Estimation of Thermal Conductivity for Alumina-Epoxy Composite Material with High Filling Volume Fraction

2014 ◽  
Vol 918 ◽  
pp. 21-26
Author(s):  
Chen Kang Huang ◽  
Yun Ching Leong
Keyword(s):  
Thermal Conductivity ◽  
Composite Material ◽  
Composite Materials ◽  
Physical Model ◽  
Electron Scattering ◽  
Volume Fraction ◽  
Epoxy Composite ◽  
The Matrix ◽  
Transport Theorem ◽  
Good Agreement

In this study, the transport theorem of phonons and electrons is utilized to create a model to predict the thermal conductivity of composite materials. By observing or assuming the dopant displacement in the matrix, a physical model between dopant and matrix can be built, and the composite material can be divided into several regions. In each region, the phonon or electron scattering caused by boundaries, impurities, or U-processes was taken into account to calculate the thermal conductivity. The model is then used to predict the composite thermal conductivity for several composite materials. It shows a pretty good agreement with previous studies in literatures. Based on the model, some discussions about dopant size and volume fraction are also made.

scholarly journals Thermal conductivity of unsaturated clay-rocks

2010 ◽  
Vol 14 (1) ◽  
pp. 91-98 ◽  
Author(s):  
D. Jougnot ◽  
A. Revil
Keyword(s):  
Thermal Conductivity ◽  
Porous Material ◽  
Solid Phase ◽  
Model Parameters ◽  
Electrical Parameters ◽  
New Model ◽  
Unsaturated Clay ◽  
Unsaturated Porous Material ◽  
Clay Rocks ◽  
Good Agreement

Abstract. The parameters used to describe the electrical conductivity of a porous material can be used to describe also its thermal conductivity. A new relationship is developed to connect the thermal conductivity of an unsaturated porous material to the thermal conductivity of the different phases of the composite, and two electrical parameters called the first and second Archie's exponents. A good agreement is obtained between the new model and thermal conductivity measurements performed using packs of glass beads and core samples of the Callovo-Oxfordian clay-rocks at different saturations of the water phase. We showed that the three model parameters optimised to fit the new model against experimental data (namely the thermal conductivity of the solid phase and the two Archie's exponents) are consistent with independent estimates. We also observed that the anisotropy of the effective thermal conductivity of the Callovo-Oxfordian clay-rock was mainly due to the anisotropy of the thermal conductivity of the solid phase.

Physical properties of chalcopyrite-type Cu1−xAgxGaTe2 by first-principles study

2016 ◽  
Vol 30 (30) ◽  
pp. 1650373 ◽  
Author(s):  
Li Xue ◽  
Yi-Ming Ren ◽  
Zheng-Long Hu
Keyword(s):  
Thermal Conductivity ◽  
Elastic Constants ◽  
Density Functional ◽  
Lattice Thermal Conductivity ◽  
Lattice Parameter ◽  
Functional Theory ◽  
Lattice Volume ◽  
Te Material ◽  
Ag Substitution ◽  
Good Agreement

[Formula: see text] is a promising thermoelectric (TE) material for high temperature TE applications. This work systematically investigated the structural, elastic and thermodynamic properties of [Formula: see text] ([Formula: see text] = 0, 0.25, 0.5, 0.75 and 1) by density functional theory. The calculated lattice volume is expanded with the increase of Ag content, but this expansion is anisotropic. The lattice parameter along [Formula: see text]-axis is linear expansion, and along [Formula: see text]-axis is parabolic expansion, which is in good agreement with available experimental data. The phase stability of [Formula: see text] alloy is studied by analyzing the formation energy, cohesive energy and elastic constants. Shear modulus, Young’s modulus, sound velocities, Debye temperature and the minimum thermal conductivity are obtained from the calculated elastic constants. The results show that Ag substitution could reduce the lattice thermal conductivity, which is helpful for improving the TE properties of [Formula: see text].

scholarly journals Effect of microclimate on the airborne dust and endotoxin concentration in a broiler house

2008 ◽  
Vol 53 (No. 2) ◽  
pp. 83-89 ◽  
Author(s):  
M. Vučemilo ◽  
K. Matković ◽  
B. Vinković ◽  
J. Macan ◽  
V.M. Varnai ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Measured Temperature ◽  
Air Concentration ◽  
Airflow Velocity ◽  
Poultry House ◽  
Airborne Dust ◽  
Endotoxin Concentration ◽  
Fattening Period ◽  
Broiler House ◽  
Poultry Farming

Poultry farming is considered to be a notable source of bioaerosols. They can be a risk factor from the aspect of some diseases and for the environment. A study was conducted to assess the effect of microclimate on the level of airborne dust and endotoxins in an intensive broiler fattening facility. The content of airborne dust, endotoxins, air temperature, relative humidity, airflow velocity, ammonia and carbon dioxide were determined. The study was conducted in a poultry house accommodating 22 000 broilers of Ross-308 breed. The measured temperature in the broiler house ranged from 22.02°C to 31.05°C, relative humidity from 49.55% to 65.45%, and airflow velocity from 0.07 m/s to 0.09 m/s. The air concentration of dust ranged from 2.0 mg/m<sup>3</sup> at the end of fattening period to 4.9 mg/m<sup>3</sup> in the mid-fattening period, and endotoxins from 6.21 EU/m<sup>3</sup> in the second study week to 99.40 EU/m<sup>3</sup> at the end of fattening period. The air concentration of ammonia ranged from 5.17 ppm at the beginning to 25.49 ppm at the end of fattening period. Air concentrations of dust and endotoxins recorded in this poultry house varied during the fattening period and depended on relative humidity and temperature as demonstrated by multiple regression at the level of <I>P</I> ≤ 0.05.

Method of Lines to Solve 2-D Steady Temperature Field of FGM

2007 ◽  
Vol 353-358 ◽  
pp. 2003-2006 ◽  
Author(s):  
Wei Tan ◽  
Chang Qing Sun ◽  
Chun Fang Xue ◽  
Yao Dai
Keyword(s):  
Thermal Conductivity ◽  
Temperature Field ◽  
Transfer Problem ◽  
Method Of Lines ◽  
Main Idea ◽  
Functionally Graded ◽  
Temperature Fields ◽  
Steady Temperature ◽  
Thermal Transfer ◽  
Steady Temperature Field

Method of Lines (MOLs) is introduced to solve 2-Dimension steady temperature field of functionally graded materials (FGMs). The main idea of the method is to semi–discretized the governing equation of thermal transfer problem into a system of ordinary differential equations (ODEs) defined on discrete lines by means of the finite difference method. The temperature field of FGM can be obtained by solving the ODEs with functions of thermal properties. As numerical examples, six kinds of material thermal conductivity functions, i.e. three kinds of polynomial functions, an exponent function, a logarithmic function, and a sine function are selected to simulate spatial thermal conductivity profile in FGMs respectively. The steady-state temperature fields of 2-D thermal transfer problem are analyzed by the MOLs. Numerical results show that different material thermal conductivity function has obvious different effect on the temperature field.

Darcy-Forchheimer flow with variable thermal conductivity and Cattaneo-Christov heat flux

2016 ◽  
Vol 26 (8) ◽  
pp. 2355-2369 ◽  
Author(s):  
T. Hayat ◽  
Taseer Muhammad ◽  
Saleh Al-Mezal ◽  
S.J. Liao
Keyword(s):  
Thermal Conductivity ◽  
Temperature Field ◽  
Velocity Field ◽  
Thermal Relaxation ◽  
Heat Transfer Process ◽  
Relaxation Parameter ◽  
Content Type ◽  
Opposite Behavior ◽  
Non Linear System ◽  
Forchheimer Flow

Purpose The objectives of present communication are threefolds. First is to model and analyze the two-dimensional Darcy-Forchheimer flow of Maxwell fluid induced by a stretching surface. Temperature-dependent thermal conductivity is taken into account. Second is to examine the heat transfer process through non-classical flux by Cattaneo-Christov theory. Third is to derive convergent homotopic solutions for velocity and temperature distributions. The paper aims to discuss these issues. Design/methodology/approach The resulting non-linear system is solved through the homotopy analysis method. Findings An increment in Deborah number β causes a reduction in velocity field f′(η) while opposite behavior is observed for temperature field θ(η). Velocity field f′(η) and thickness of momentum boundary layer are decreased when the authors enhance the values of porosity parameter λ while opposite behavior is noticed for temperature profile θ(η). Temperature field θ(η) is inversely proportional to the thermal relaxation parameter γ. The numerical values of temperature gradient at the sheet − θ′(0) are higher for larger values of thermal relaxation parameter γ. Originality/value To the best of author’s knowledge, no such consideration has been given in the literature yet.

Anomalous Heat Conduction, Diffusion and Heat Rectification in Nanoscale Structures

2009 ◽  
Author(s):  
Gang Zhang ◽  
Nuo Yang ◽  
Gang Wu ◽  
Baowen Li
Keyword(s):  
Thermal Conductivity ◽  
Heat Conduction ◽  
Heat Transport ◽  
Nano Materials ◽  
Fourier Law ◽  
Nanoscale Structures ◽  
Recent Developments ◽  
Theoretical Results ◽  
Good Agreement ◽  
Anomalous Heat

In this paper, we report the recent developments in the study of heat transport in nano materials. First of all, we show that phonon transports in nanotube super-diffusively which leads to a length dependence thermal conductivity, thus breaks down the Fourier law. Then we discuss how the introduction of isotope doping can reduce the thermal conductivity efficiently. The theoretical results are in good agreement with experimental ones. Finally, we will demonstrate that nanoscale structures are promising candidates for heat rectification.

Brownian-Motion-Based Convective-Conductive Model for the Effective Thermal Conductivity of Nanofluids

2005 ◽  
Vol 128 (6) ◽  
pp. 588-595 ◽  
Author(s):  
Ravi Prasher ◽  
Prajesh Bhattacharya ◽  
Patrick E. Phelan
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Particle Sizes ◽  
Brownian Movement ◽  
Liquid Temperature ◽  
Interfacial Resistance ◽  
Conduction Model ◽  
Order Of Magnitude ◽  
Base Liquid ◽  
Good Agreement

Here we show through an order-of-magnitude analysis that the enhancement in the effective thermal conductivity of nanofluids is due mainly to the localized convection caused by the Brownian movement of the nanoparticles. We also introduce a convective-conductive model which accurately captures the effects of particle size, choice of base liquid, thermal interfacial resistance between the particles and liquid, temperature, etc. This model is a combination of the Maxwell-Garnett (MG) conduction model and the convection caused by the Brownian movement of the nanoparticles, and reduces to the MG model for large particle sizes. The model is in good agreement with data on water, ethylene glycol, and oil-based nanofluids, and shows that the lighter the nanoparticles, the greater the convection effect in the liquid, regardless of the thermal conductivity of the nanoparticles.

scholarly journals A General Expression for the Stagnant Thermal Conductivity of Stochastic and Periodic Structures

2018 ◽  
Vol 140 (5) ◽  
Author(s):  
X. Bai ◽  
C. Hasan ◽  
M. Mobedi ◽  
A. Nakayama
Keyword(s):  
Thermal Conductivity ◽  
General Expression ◽  
Metal Foam ◽  
Solid Angle ◽  
Periodic Structures ◽  
Three Dimensional ◽  
High Thermal Conductivity ◽  
Numerical Calculations ◽  
The One ◽  
Good Agreement

A general expression has been obtained to estimate thermal conductivities of both stochastic and periodic structures with high-solid thermal conductivity. An air layer partially occupied by slanted circular rods of high-thermal conductivity was considered to derive the general expression. The thermal conductivity based on this general expression was compared against that obtained from detailed three-dimensional numerical calculations. A good agreement between two sets of results substantiates the validity of the general expression for evaluating the stagnant thermal conductivity of the periodic structures. Subsequently, this expression was averaged over a hemispherical solid angle to estimate the stagnant thermal conductivity for stochastic structures such as a metal foam. The resulting expression was found identical to the one obtained by Hsu et al., Krishnan et al., and Yang and Nakayama. Thus, the general expression can be used for both stochastic and periodic structures.

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