Investigation of effective thermal conductivity of SiC foam ceramics with various pore densities

1 Introduction

Foam ceramics are widely used in many engineering fields due to their large specific surface area, high heat transfer efficiency, and uniform mixing of fluids. The parameter, effective thermal conductivity, is a major concern in predicting the heat transfer efficiency of systems, such as porous media combustion system [1], heat exchangers [2], and solar collectors [3].

Many researchers have paid attention to heat transfer in SiC foam ceramics from different perspectives. Based on experimental tests [4], analytical analysis [5], and numerical simulations [6,7], researchers mostly focused on effects of temperature, pore parameters, and solid thermal conductivity on effective thermal conductivity of SiC foam ceramics. Nemoto et al. [8] investigated thermal conductivity of foam ceramics at low temperature. Results pointed out that the thermal conductivity of SiC foam ceramics increases gradually with the increase in temperature during a temperature rise from 4 to 300 K. Zhou et al. [9] investigated the effective thermal conductivity of foam ceramics at 800°C. Experimental results showed that foamed mullite-SiC ceramics have an excellent potential as thermal-insulating materials for cement kiln preheaters. Jiang et al. [10] reported the effect of SiC foam ceramic particle size on an effective thermal conductivity of form-stable NaNO3. It was found that form-stable NaNO3 (56.6%) that had a skeleton modified by 10% SiC foam ceramics with particle size of 50 nm possessed an effective thermal conductivity of 2.06 W (m K)−1 at 25°C, which was 265% higher than that of pure NaNO3. Qiu et al. [11] experimentally studied effects of specific surface area and pore size on thermal conductivity from mesoporous to macroporous SiC ceramics. Results showed that the thermal conductivity of both gas and solid phases increased with the pore size.

Pore structure directly affects the heat transfer performance of foam ceramics and plays a leading role in the control of temperature distribution under high temperature heat source [12]. Many researchers have studied the effect of porosity and pore density on the effective thermal conductivity of foam ceramics. Mendes et al. [13] experimentally investigated the effect of 10 pores per inch (PPI) foam ceramics porosity on the effective thermal conductivity by the transient surface heat source method. Their results showed that the effective thermal conductivity increased by 22% when porosity increased from 0.57 to 0.74 at 1,500 K. Dietrich et al. [14] reported that the effective thermal conductivity increased with the increase of porosity, and effective thermal conductivity of foam ceramics with low PPI number was higher than that with high PPI number. Kang et al. [15] added nano-sized SiO2 to a nano-sized SiC and nano-sized carbon template mixture to reduce porosity, resulting in an extremely low effective thermal conductivity in porous SiC–SiO2 foam ceramics reaching as low as 0.066 W (m K)−1. Liu and Zhao [16] numerically investigated the influence of geometric (e.g., pore size, pore type, and porosity) on effective thermal conductivity of porous structures with open and closed pores. They found that using nanoscale pores is an effective strategy to achieve an ultralow effective thermal conductivity for the highly porous structures with open and closed pores.

Most of earlier studies focused on measurements of the effective thermal conductivity with single homogeneous materials, especially at high temperature environment for test conditions. However, modular nonuniform foam ceramic composed of foam ceramic sheets with different pore densities are mostly used in porous media burners. Xie et al. [17] revealed that the use of high-porosity ceramic foam in combustion zone and low-porosity ceramic foam in preheating zone can achieve efficient combustion. Song et al. [18] conducted an experimental study on the premixed combustion of low calorific gas an axial and radial gradually varied porous media burner. They found that the gradually varied porous media burner can burn ultra-low calorific gas of 1.4 MJ m−3. Al-attab et al. [19] designed a two-layer porous medium combustion regenerative device for burning low heating value. Results showed that the maximum heat recovery heat exchanger effectiveness was about 93% with an overall system efficiency of 54%. The arrangement of pore structure affects combustion temperature by affecting heat flow transfer characteristics [20,21]. The effective thermal conductivity of modular nonuniform ceramic foam with different pore densities is probably not a simple superposition relationship. Moreover, there are very few studies on the effect of pore mutation and solid skeleton discontinuity on the effective thermal conductivity of foam ceramics. This research aims to investigate effective thermal conductivity of variable pore densities SiC foam ceramics formed by stacking heterogeneous or homogeneous porous media. Three nonuniform pore structures will be designed by superposition of porous media with different pore densities. Effects of heat source temperature, pore density, thickness of porous media, and pore arrangement structure on the effective thermal conductivity will be investigated using a steady plane heat source method. Results have a great significance to the research of high-temperature heat transfer in porous media with variable porosity. This study provides insight into the understanding of effective thermal conductivity of modular SiC foam ceramics and has reference value for the design of thermal insulation materials and energy storage materials.