Showing papers on "Heat transfer published in 2018"

Review on thermal conductivity enhancement, thermal properties and applications of phase change materials in thermal energy storage

Abstract: In recent years, energy conservation and environmental protection have become most important issues for humanity. Phase change materials (PCMs) for thermal energy storage can solve the issues of energy and environment to a certain extent, as PCMs can increase the efficiency and sustainability of energy. PCMs possess large latent heat, and they store and release energy at a constant temperature during the phase change process. Thereby PCMs have gained a wide range of applications in various fields, such as buildings, solar energy systems, power systems and military industry. However, low thermal conductivity of PCMs leads to low heat transfer rate, thus, numerous studies have been carried out to improve thermal conductivity of PCMs. The main purpose of this paper is to review the methods for enhancing thermal conductivity of PCMs, which include adding additives with high thermal conductivity and encapsulating phase change materials. It is found that addition of thermal conductivity enhancement fillers is a more effective method to improve thermal conductivity of PCMs, where carbon-based material additives possess a more promising application prospect. Finally, the applications of PCMs in solar energy system, buildings, cooling system, textiles and heat recovery system are also analyzed.

Thermal conductivity of concrete – A review

Abstract: The thermal conductivity (k-value) of cement-based materials like concrete is an important factor when considering the amount of heat transfer through conduction. The amount of heat loss through walls and roofs has a direct effect on the energy consumption of buildings. The steady state and transient methods are considered the two main thermal conductivity measurement approaches. The moisture content, temperature, type of aggregate, type of cementitious material and density of concrete are influential factors on the thermal conductivity. The aim of this paper is to review the techniques most commonly used to measure the thermal conductivity of concrete as well as to consider the factors affecting the thermal conductivity of cement-based materials. In addition, a general equation for predicting the thermal conductivity of concrete is proposed in this study based on data reported by researchers. The results of this review indicate that most researchers have measured the k-value of cement-based materials based on transient methods. The reported k-value in saturated conditions is higher than in dry conditions. Moreover, the measured k-value exhibits a declining trend with increasing temperature. It is concluded that using lightweight concrete in structural and non-structural building envelopes is a valuable method of reducing the amount of heat transfer and energy consumption owing to the lower k-value of lightweight concrete compared to normal weight concrete.

Nanoencapsulation of phase change materials for advanced thermal energy storage systems

Abstract: Phase change materials (PCMs) allow the storage of large amounts of latent heat during phase transition. They have the potential to both increase the efficiency of renewable energies such as solar power through storage of excess energy, which can be used at times of peak demand; and to reduce overall energy demand through passive thermal regulation. 198.3 million tons of oil equivalent were used in the EU in 2013 for heating. However, bulk PCMs are not suitable for use without prior encapsulation. Encapsulation in a shell material provides benefits such as protection of the PCM from the external environment and increased specific surface area to improve heat transfer. This review highlights techniques for the encapsulation of both organic and inorganic PCMs, paying particular attention to nanoencapsulation (capsules with sizes <1 μm). We also provide insight on future research, which should focus on (i) the development of multifunctional shell materials to improve lifespan and thermal properties and (ii) advanced mass manufacturing techniques for the economically viable production of PCM capsules, making it possible to utilize waste heat in intelligent passive thermal regulation systems, employing controlled, “on demand” energy release/uptake.

A review of phase change material and performance enhancement method for latent heat storage system

Abstract: Latent heat storage (LHS) is considered as the most promising technique for thermal energy storage, due to its high energy storage density and nearly constant working temperature. However, the lower thermal conductivity of the phase change material (PCM) used in LHS system seriously weakens thermal energy charging and discharging rates. In order to improve the thermal performance of LHS system, a lot of research on performance enhancement have been carried out. This review paper will concern on the development of PCMs and performance enhancement methods for LHS system in the last decade. The available enhancement methods can be classified into three categories: using high thermal conductivity additives and porous media to enhance PCM thermal conductivity, using finned tubes and encapsulated PCMs to extend heat transfer surface, using multistage or cascaded LHS technique and thermodynamic optimization to improving the heat transfer uniformity. The comparative reviews on PCMs, corresponding performance enhancement methods and their characteristics are presented in present paper. That will help in selecting reliable PCMs and matching suitable performance enhancement method to achieve the best thermal performance for PCM based LHS system. In addition, the research gaps in performance enhancement techniques for LHS systems are also discussed and some recommendations for future research are proposed.

Selection principles and thermophysical properties of high temperature phase change materials for thermal energy storage: A review

Abstract: Phase change thermal energy storage (TES) is a promising technology due to the large heat capacity of phase change materials (PCM) during the phase change process and their potential thermal energy storage at nearly constant temperature. Although a considerable amount of research has been conducted on medium and low temperature PCMs in recent years, there has been a lack of a similar systematic and integrated study on high temperature PCMs and high temperature thermal energy storage processes. Analyzing the available literature, this review evaluates the selection principles of PCMs and introduces and compares the available popular material selection software options. The thermophysical property data of high temperature PCMs is comprehensively summarized, including high temperature molten salts and metal alloys. Several heat transfer and performance enhancement techniques are summarized and discussed as potential alternative methods to overcome poor thermal conductivity when using high temperature molten salt as the PCM. The common thermophysical property measurement methods used in literature are also summarized and compared. This review gives a broad overview of material selection, innovation and investigation of thermophysical properties for high temperature PCM development, and will be a helpful reference for the design of high temperature phase change TES systems.

A comprehensive review on single phase heat transfer enhancement techniques in heat exchanger applications

Abstract: The objective of this paper is to review the different techniques, which have been used to enhance the heat transfer rate in heat exchanger devices such as solar air heater, cooling blades of turbine and so on using single phase heat transfer fluids. The results of recent published articles with the development of new technologies such as Electrohydrodynamic (EHD) and Magnetohydrodynamics (MHD) are also included. Enhancement of heat transfer in heat exchanger can achieved by means of several techniques. These techniques are grouped into the active and passive method. In the active methods, system need some external power, however, passive method utilize surface modification either on heated surface or insertion of swirl devices in the flow field. Active methods are very complex because of external power supply, although these methods have great potential and can control thermally. Passive methods include artificial roughness, extended surface, winglets, insertion of swirl devices in the flow which alters the flow pattern causes to disturb the thermal boundary layer, and consequently high heat transfer. Passive methods are dominant over active methods because its can easily employed in existing heat exchangers. In this paper, an effort has been made to categorize the active and passive methods and review the various heat transfer techniques applied in heat exchangers. Important results have been listed for ready reference. It has been concluded that either active or passive methods have been employed alone. Based on literature, a combined method have also been recommended which include both active and passive methods.

Present and future thermal interface materials for electronic devices

Abstract: Packaging electronic devices is a growing challenge as device performance and power levels escalate. As device feature sizes decrease, ensuring reliable operation becomes a challenge. Ensuring effective heat transfer from an integrated circuit and its heat spreader to a heat sink is a vital step in meeting this challenge. The projected power density and junction-to-ambient thermal resistance for high-performance chips at the 14 nm generation are >100 Wcm−2 and <0.2 °CW−1, respectively. The main bottleneck in reducing the net thermal resistance are the thermal resistances of the thermal interface material (TIM). This review evaluates the current state of the art of TIMs. Here, the theory of thermal surface interaction will be addressed and the practicalities of the measurement techniques and the reliability of TIMs will be discussed. Furthermore, the next generation of TIMs will be discussed in terms of potential thermal solutions in the realisation of Internet of Things.

A comprehensive review of preparation, characterization, properties and stability of hybrid nanofluids

Abstract: Nanofluids are emerging as promising thermo fluids for heat transfer application. Nano fluids are two phase fluids of solid liquid mixture. The presence of solid nanoparticles in the base fluid increases significantly the effective thermal conductivity of the fluid and consequently enhances the heat transfer characteristics. Addition of single nanoparticle in base fluid, to improve the flow and heat transfer characteristics of the base fluid is a proven technology. In recent years, attention is focused on research studies involving impregnation of two or more nanoparticles in base fluids, called hybrid or composite nanofluids. Research studies on nanofluid containing composite nanoparticle showed better enhancement in thermal and rheological characteristics of base heat transfer fluid compared to mono nanoparticle based nanofluids. The research studies carried out on preparation, characterization, properties and stability of hybrid nanofluids are comprehensively reviewed in this paper. The models for properties such as thermal conductivity, viscosity, density, specific heat, friction factor and heat transfer coefficient of hybrid nanofluids are presented. The potential application and the challenges including stability methods and measures for hybrid nanofluids are discussed.