Abstract
In graded structure aerogels, change of pores diameter through the thickness affects the effective thermal conductivity as the most important parameter. As the diameter of the pores is reversely correlated to the density, the effective thermal conductivity (\(\lambda_{\text{eff}}\)) of aerogel is often normalized to the apparent density (\(\rho\)) and it is expressed as the B (\(\frac{{\lambda_{\text{eff}} }}{\rho }\)) parameter. Lower values of B would be the optimum conditions for the insulation performance of resulting aerogel. The objective of this work is to simulate the heat transfer of the optimum structure and to compare it with functionally graded structures that pore diameter varies through the thickness. For this purpose, the structural characteristics and properties of silica aerogel along with the effect of coupling thermal conductivity have to be taken into consideration. The heat transfer and time–temperature history diagram were modeled for an optimum structure (OPT) having a minimum value of the B parameter. The results were compared to the structurally graded aerogels in which the density was varied in two fashions, from higher to lower values (HtL) density and from lower to higher values (LtH) density. The change of temperature with time was tracked for all the cases. Results indicated that the minimum value of heat transfer was obtained for the structurally graded aerogel of the type of LtH (2% increase in efficiency for LtH compared to OPT). Therefore, this structure introduces as the best candidate for producing a thermal insulator.
Similar content being viewed by others
Abbreviations
- C p :
-
Specific heat capacity, J kg−1 K−1
- d p :
-
Aerogel particle diameter, m
- D :
-
Mean pore size of aerogel, m
- E s/ρ s :
-
Specific extinction coefficient of aerogel, m2 kg−1
- K n :
-
Kundsen number
- l :
-
Height of sample, m
- n :
-
Refractive index
- Q n :
-
Normal heat flux, W m−2
- R :
-
Radius of sample, m
- S ext :
-
Specific surface area, m2 kg−1
- t :
-
Time, s
- T :
-
Temperature, K
- T H :
-
Temperature in hot surface, K
- V pore :
-
Pore volume, m3 kg−1
- β :
-
Coefficient in Eq. 3
- λ :
-
Thermal conductivity, W m−1 K−1
- ρ :
-
Density of aerogel, kg m−3
- ρ s :
-
Density of aerogel solid backbone, kg m−3
- σ B :
-
Stefan–Boltzmann constant in Eq. 4
- \(\upsilon\) :
-
Sound velocity in aerogel, m s−1
- \(\upsilon_{\text{s}}\) :
-
Sound velocity in solid backbone, m s−1
- ° :
-
Aerogel solid backbone
- c:
-
Coupling
- eff:
-
Effective
- g:
-
Gas
- g,° :
-
Gas in free space
- g − s:
-
Between solid and gas phase
- p:
-
Solid particle
- r:
-
Radiation
- s:
-
Solid
References
Nojoomizadeh M, D’Orazio A, Karimipour A, Afrand M, Goodarzi M. Investigation of permeability effect on slip velocity and temperature jump boundary conditions for FMWNT/water nanofluid flow and heat transfer inside a microchannel filled by a porous media. Phys E Low Dimens Syst Nanostruct. 2018;97:226–38.
Hooman K, Tamayol A, Dahari M, Safaei MR, Togun H, Sadri R. A theoretical model to predict gas permeability for slip flow through a porous medium. Appl Therm Eng. 2014;70:71–6.
Sadeghi R, Shadloo MS, Hopp-Hirschler M, Hadjadj A, Nieken U. Three-dimensional lattice Boltzmann simulations of high density ratio two-phase flows in porous media. Comput Math Appl. 2018. https://doi.org/10.1016/j.camwa.2017.12.028.
Nasiri H, Abdollahzadeh Jamalabadi MY, Sadeghi R, Safaei MR, Nguyen TK, Safdari Shadloo M. A smoothed particle hydrodynamics approach for numerical simulation of nano-fluid flows: application to forced convection heat transfer over a horizontal cylinder. J Therm Anal Calorim. 2018;4:1–9.
Heydari A, Akbari OA, Safaei MR, Derakhshani M, Alrashed AAAA, Mashayekhi R. The effect of attack angle of triangular ribs on heat transfer of nanofluids in a microchannel. J Therm Anal Calorim. 2018;131:2892–912.
Pierre AC, Pajonk GM. Chemistry of aerogels and their applications. Chem Rev. 2002;102:4243–65.
Pan Y, He S, Gong L, Cheng X, Li C, Li Z. Low thermal-conductivity and high thermal stable silica aerogel based on MTMS/water-glass co-precursor prepared by freeze drying. Mater Des. 2017;113:246–53.
Fang WZ, Zhang H, Chen L, Tao WQ. Numerical predictions of thermal conductivities for the silica aerogel and its composites. Appl Therm Eng. 2017;115:1277–86.
Antonietti M, Fechler N, Fellinger TP. Carbon aerogels and monoliths: control of porosity and nanoarchitecture via sol–gel routes. Chem Mater. 2014;26:196–210.
Aegerter M, Leventis N, Koebel M. In: Aparicio M, Jitianu A, Klein LC, editors. Aerogels handbook. Boston: Springer; 2012.
Williams JC, Nguyen BN, McCorkle L, Scheiman D, Griffin JS, Steiner SA. Highly porous, rigid-rod polyamide aerogels with superior mechanical properties and unusually high thermal conductivity. ACS Appl Mater Interfaces. 2017;9:1801–9.
Maleki H, Durães L, Portugal A. An overview on silica aerogels synthesis and different mechanical reinforcing strategies. J Non Cryst Solids. 2014;385:55–74.
Shahistha ACPM, Binol V, Anjali B. A review on functionally graded materials. Int J Aeronaut Mech Eng. 2016;4:8–14.
Schachtschneider A, Wessig M, Spitzbarth M, Donner A, Fischer C, Drescher M. Directional materials-nanoporous organosilica monoliths with multiple gradients prepared using click chemistry. Angew Chem Int Ed. 2015;54:10465–9.
Tang GH, Bi C, Zhao Y, Tao WQ. Thermal transport in nano-porous insulation of aerogel: factors, models and outlook. Energy. 2015;90:701–21.
Bi C, Tang GH, Tao WQ. Prediction of the gaseous thermal conductivity in aerogels with non-uniform pore-size distribution. J Non Cryst Solids. 2012;358:3124–8.
Bi C, Tang GH. Effective thermal conductivity of the solid backbone of aerogel. Int J Heat Mass Transf. 2013;64:452–6.
Khodabandeh E, Safaei MR, Akbari S, Akbari OA, Alrashed A. Application of nanofluid to improve the thermal performance of horizontal spiral coil utilized in solar ponds: geometric study. Renew Energy. 2018;122:1–16.
Bi C, Tang GH, Hu ZJ, Yang HL, Li JN. Coupling model for heat transfer between solid and gas phases in aerogel and experimental investigation. Int J Heat Mass Transf. 2014;79:126–36.
Hemberger F, Weis S, Reichenauer G, Ebert H-P. Thermal transport properties of functionally graded carbon aerogels. Int J Thermophys. 2009;30:1357–71.
Wei G, Liu Y, Zhang X, Yu F, Du X. Thermal conductivities study on silica aerogel and its composite insulation materials. Int J Heat Mass Transf. 2011;54:2355–66.
Bi C, Tang GH, Hu ZJ. Heat conduction modeling in 3-D ordered structures for prediction of aerogel thermal conductivity. Int J Heat Mass Transf. 2014;73:103–9.
Lee SC, Cunnington GR. Conduction and radiation heat transfer in high-porosity fiber thermal insulation. J Thermophys Heat Transf. 2000;14:121–36.
Liu H, Xia X, Ai Q, Xie X, Sun C. Experimental investigations on temperature-dependent effective thermal conductivity of nanoporous silica aerogel composite. Exp Therm Fluid Sci. 2017;84:67–77.
Xie T, He YL, Tong ZX, Yan WX, Xie XQ. Transient heat transfer characteristic of silica aerogel insulating material considering its endothermic reaction. Int J Heat Mass Transf. 2014;64:633–40.
Hajizadeh A, Bahramian AR, Seifi A, Naseri I. Effect of initial sol concentration on the microstructure and morphology of carbon aerogels. J Sol–Gel Sci Technol. 2015;73(1):220–6.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Noroozi, M., Panahi-Sarmad, M., Bahramian, A.R. et al. Theoretical investigation of heat transfer in structurally graded silica aerogels with pores diameter changing. J Therm Anal Calorim 135, 1713–1721 (2019). https://doi.org/10.1007/s10973-018-7315-7
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s10973-018-7315-7