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The utilization of phase change materials (PCMs) in buildings leads to the reduction of energy consumption and maintaining the indoor temperature within the comfort range. The PCM performance strongly depends on the climatic conditions, causing a major challenge. To overcome this issue, the employment of a double-layer PCM system is introduced. The energy and economic assessments of various double PCM systems in the building located in different climatic conditions of Iran have been conducted for the first time. In this study, two PCM wallboards with different enthalpy and melting temperatures were embedded in the external walls of a residential building located in various climatic regions of Iran. The performance of the double-layer PCM system in five cities of Iran including Tehran, Isfahan, Shiraz, Tabriz, and Bandar Abbas during hot and cold months of the year was evaluated from energy and economic viewpoints. It was inferred that embedding a double-layer PCM system reduces the heating/cooling energy consumption compared to single-layer ones. It was concluded that the RT18/RT28 double PCM system was the best candidate to be employed in the external walls of the building located in Tehran and Shiraz, which reduced the total energy consumption of the building up to 6.26% and 5.17%. The RT18/RT22 double PCM system was the most efficient system for the building placed in Tabriz and Isfahan reducing the total energy consumption up to 4.41% and 3.87%, and the RT22/RT28 double PCM system had the best performance in the building located in Bandar Abbas, which could save 6.58% of total energy. Moreover, it was concluded that the use of a double PCM system reduced temperature fluctuations in the external walls. In addition, the economic analysis was performed using the dynamic payback period and the results revealed that the employment of a double-layer PCM system reduced the investment return period by up to 50%. [ABSTRACT FROM AUTHOR]

Irreversibility factors for the TiO2–water nanofluid flow are evaluated through entropy generation rates caused by friction and heat transfer by taking into account nanoparticle migration. Effects of three factors of viscosity gradient, non-uniform shear rate and Brownian diffusion are considered. The investigations are performed in both conventional channels and minichannels since the channel size has the considerable effects of on entropy generation. Influence of particle migration on the irreversibility factors is more noticeable in smaller channels such that with considering particle migration, the maximum thermal entropy generation rate increases about 51% for 0.2 mm channel, while this increment is almost 16% for 1 mm channel. Unlike results are obtained for conventional channels and minichannels, such that total entropy generation intensifies in minichannels and reduces in conventional channels by particle concentration increment. Friction is main factor in entropy generation for minichannels, while heat transfer demonstrates a higher contribution in conventional channels. The optimal channel size where minimum entropy generation occurs is obtained which is dependent on Reynolds number. Therefore, for lower Reynolds number, optimal case occurs at smaller channel size. Furthermore, diagrams of total entropy generation show minimum (optimal) point for 3 mm channel, while they demonstrate the constant trends for other channels. [ABSTRACT FROM AUTHOR]

This paper assesses the contribution of TiO2 nanoparticles on thermal performance of a 0.5 mass% aqueous solution of carboxymethyl cellulose (CMC) in a square channel. In this regard, a neural network model is firstly developed for modeling of power law index, consistency index, and thermal conductivity of the aqueous solution of TiO2/CMC-water non-Newtonian nanofluid in terms of the nanoparticle concentration and temperature. Then, an attempt is made to evaluate the friction factor and heat transfer coefficient relative values. According to the results, it is found that the friction factor ratio is directly proportional to the temperature and nanoparticle content, while it inversely varies relative to the shear rate. Moreover, heat transfer coefficient ratio is improved at elevated nanoparticle content, and this improvement is much more profound at higher temperature conditions. For practical purposes, the nanofluid hydrothermal performance index is examined since the addition of nanoparticles increases both heat transfer and friction factor. The corresponding data disclose that the performance index is directly proportional to the nanoparticle content, especially at decreased shear rate and elevated temperature conditions. The application of TiO2/CMC-water nanofluid is found to be more favorable for applications with elevated shear rate conditions. [ABSTRACT FROM AUTHOR]