1. Introduction

Controlling the temperature and/or heat transfer is of crucial importance in numerous technical processes and natural occurrences, including cooling of electronics and telecommunications

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equipment in aerospace applications. Phase change materials (PCMs), which store excess heat generated from the work components and release it reversibly when needed, provide a smart approach for more efficient temperature management and thermal control utilization. Latent heat absorption phenomena associated with melting of a suitable PCM may be effectively used to delay or modify the temperature rise of the surface subjected to high heat flux. Thermal management utilizing solid-liquid PCMs is one of the most interesting passive thermal management techniques due to its simplicity and reliability. Solid-liquid PCMs including salt hydrates, paraffin waxes, certain hydrocarbons, and metal alloys are often used for thermal protection applications.

also not degraded the system performance thermally. To design PCM thermal protection system properly, the thermodynamic conservation relations and heat transfer must be considerate.

Porous Ceramic Matrix Phase Change Composites for Thermal Control Purposes of Hypersonic Vehicle

http://dx.doi.org/10.5772/intechopen.70863

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In this chapter, the combined thermodynamic/heat transfer relations of the PCM in silica nanoporous materials are developed to obtain mass, thickness, and temperature excursion as functions of percentage area of PCM under given maximum energy and thermal flux. And then, the preparation and characterization of form-stable PCM/porous silica ceramic composite are reported. The paraffin acts as thermal absorb material, and porous silica serves as the supporting material, which provides structural strength and prevents the leakage of melted PCM. To adjust the pore structure of porous silica matrices with different molar ratios of EtOH and TEOS for PCM infiltration is mainly discussed. At last, numerical and experimental studies are proposed to predict and investigate the thermal absorption characteristics of porous silica infiltrated with PCM for thermal control applications. The numerical simulation was performed using a volume-averaging technique, and a finite volume modeling (FVM) was used to discretize the heat diffusion equation. The phase change process was modeled using

2. Theory: modeling of combined thermodynamic/heat transfer

To develop the combined thermodynamic/heat mass transfer relations for the PCM/silica nanocomposite, two additional assumptions are made in the development of the relations: firstly, contact resistance between the PCM and the silica nano-materials is assumed negligible, and, secondly, three-dimensional heat transfer effects are neglected, based on the assumption that the skeleton of silica nano-materials is so closely spaced that these effects are negligible. When the component temperature rise for a particular application exceeds the maximum operational temperature of component, the supporting material raises the equivalent thermal conductivity of the PCM by providing low thermal-resistance paths through the PCM and reduces the temperature gradient necessary to dissipate the imposed cold-plate heat load. The reduction in temperature gradient reduces the temperature excursion of the component. Consider the thermal protection system shown in Figure 1. An electrical component is thermally protected by the PCM/silica nanocomposite system. The component internally generates energy cyclically. For cyclical operation, three equations can be derived for the system based on the model.

After the maximum energy that must be stored by the PCM/silica nanocomposite, EMax, is

where ρP, CP, and AP are density, specific heat, and area of PCM, respectively; t is the thickness of the nanocomposite; ρS, AS, and CS are the density, specific heat, and area of silica, respectively;

�

t

<sup>2</sup> � ð Þ TMax � <sup>T</sup> (1)

EMax ¼ ρ<sup>P</sup> � AP � t � hf þ ρ<sup>S</sup> � AS � CS þ ρ<sup>P</sup> � AP � CP

the enthalpy-porosity method.

2.1. Conservation of energy

determined, the following heat balance will hold.

Most practical applications for hypersonic vehicles require PCMs to have high density of latent heat and appropriate phase change point. In this criterion, certain hydrocarbons are very promising for temperature controlling applications for its comparative high fusion latent heats, suitable melting temperature, and chemical stability [1]. However, the applications of the kinds of solid-liquid PCMs are largely limited, as leaking of the liquid phase occurs above the phase change point. Thus, accommodation of the PCMs in appropriate host materials is necessary to prevent the leakage of the liquid phase for temperature controlling applications using the solid-liquid phase change. The leakage is usually circumvented by introducing shape stabilization support [2, 3]. Devoting to develop composites with a high PCM load and heat storage density, researchers have been recently interested in silica nanoporous ceramics with high specific area as a shape stabilization matrix or a supporting structure materials as PCMs' skeleton for avoiding leakage of solid-liquid transition [4, 5]. Silica aerogels have been used extensively in many engineering applications. They are sol-gel-derived porous inorganic materials recognized for their low density (as low as 0.01–0.02 g/cm<sup>3</sup> ) [6], high porosity (>90%) [7], and high specific surface area (600–1000 m<sup>2</sup> /g) [8]. These structural properties result in a few thermal conductivities and high optical transparencies [9]. However, monolithic silica aerogels are very fragile and transparent to thermal radiation at high temperature, and these two defects restraint the application range of silica aerogels. Fortunately, researchers have improved both the mechanic strength property and high transparency using reinforced fibers [10] and pacifiers doped in the aerogels [11]. The composite silica aerogels are called silica nanoporous materials, and they have the similar space structure and have superior properties such as higher mechanic strength properties at high temperature [12, 13]. Thus, they have better applicability than the monolithic aerogels. The silica nanoporous materials have broad applications or application prospect in the fields which have strict limit of space or weight, for example, in aeronautics and aerospace [14].

The concept of thermal design using PCM has been well established through various studies in recent decades and is widely used in space applications [15]. To apply PCMs as alternating heat storage and discharge in space applications, the material should accommodate the special needs on the thermophysical properties that include conformance of phase change temperature to the design limits, high latent heat, and low density difference between liquid and solid phases. Structurally and thermally, the synthesis of the support materials for a PCM must be also considered. The supporting materials assure the seepage-proof for the molten PCMs and can withstand some imposed mechanical loads structurally. The supporting materials must be also not degraded the system performance thermally. To design PCM thermal protection system properly, the thermodynamic conservation relations and heat transfer must be considerate.

In this chapter, the combined thermodynamic/heat transfer relations of the PCM in silica nanoporous materials are developed to obtain mass, thickness, and temperature excursion as functions of percentage area of PCM under given maximum energy and thermal flux. And then, the preparation and characterization of form-stable PCM/porous silica ceramic composite are reported. The paraffin acts as thermal absorb material, and porous silica serves as the supporting material, which provides structural strength and prevents the leakage of melted PCM. To adjust the pore structure of porous silica matrices with different molar ratios of EtOH and TEOS for PCM infiltration is mainly discussed. At last, numerical and experimental studies are proposed to predict and investigate the thermal absorption characteristics of porous silica infiltrated with PCM for thermal control applications. The numerical simulation was performed using a volume-averaging technique, and a finite volume modeling (FVM) was used to discretize the heat diffusion equation. The phase change process was modeled using the enthalpy-porosity method.
