Six species of wood samples, namely, pine, beech, cherry, oak, maple, and ash, were investigated by autoignition in a cone calorimeter to identify the influence of moisture on autoignition. It was observed that (1) for autoignition, as different from piloted ignition, there is no obvious trend in ignition temperature when moisture content increases from 0% to 11%; (2) ignition temperature decreases with a higher external heat flux, and the influence of specimen thickness to the ignition temperature can be ignored; (3) ignition time correlates linearly with [Formula: see text], and the coefficient rises with the increase of moisture content; and (4) the influence of moisture to the average mass loss rate and time at 50% mass loss can be ignored if the moisture content of wood sample is lower than 11%.
A solar chimney is a renewable energy system used to enhance the natural ventilation in a building based on solar and wind energy. It is one of the most representative solar-assisted passive ventilation systems attached to the building envelope. It performs exceptionally in enhancing natural ventilation and improving thermal comfort under certain climate conditions. The ventilation enhancement of solar chimneys has been widely studied numerically and experimentally. The assessment of solar chimney systems based on buoyancy ventilation relies heavily on the natural environment, experimental environment, and performance prediction methods, bringing great difficulties to quantitative analysis and parameterization research. With the increase in volume and complexity of modern building structures, current studies of solar chimneys have not yet obtained a unified design strategy and corresponding guidance. Meanwhile, combining a solar chimney with other passive ventilation systems has attracted much attention. The solar chimney-based integrated passive-assisted ventilation systems prolong the service life of an independent system and strengthen the ventilation ability for indoor cooling and heating. However, the progress is still slow regarding expanded applications and related research of solar chimneys in large volume and multi-layer buildings, and contradictory conclusions appear due to the inherent complexity of the system.
Curved double-skin façades (CDSFs) have been widely used as building envelopes that combine the advantages of aesthetic and energy-saving. However, because of its complex structure and airflow, it is difficult to be precisely reduced in experiments and has been barely studied in the existing literature. Hence, a thermal similarity mathematical model for the CDSF is developed and validated by using methods combining CFD and experiments. The validation results show a good similarity between the reduced-scale model and the full-scale model. On this basis, the influence of dimensionless numbers, boundary conditions, size and appearance on the similarity are analyzed. The results show that the similarity improves with the increment of the Reynolds number until it reaches the self-similarity region. And the Grashof number shows a significant impact on the similarity when the Richardson number >1. So, it is necessary to ensure that the Richardson number <1 when the similarity of the Grashof number cannot be guaranteed. Moreover, solar radiation intensities between 200 W/m2 and 400 W/m2 and solar incident angles between 45° and 60° show the best similarity between the reduced-scale model and full-scale model. The scale of the model has a significant effect on the similarity and is recommended between 1/20 and 1/50 for large buildings with CDSFs. Besides, a smaller ellipse aspect ratio shows a better similarity between the two models. Therefore, a scaled CDSF model with ellipse aspect ratios less than 1 exhibits better validity compared to models with the ratio larger than 1.
Horizontal spiral-coil type ground heat exchanger (HSGHE) has been increasingly used in ground source heat pump systems owing to its high heat transfer performance and low cost. This study proposes a method to optimize the design and operation parameters based on the combination of response surface methodology (RSM) and multi-objective genetic algorithm (MOGA). First, a 3D numerical model simulating the thermal–hydraulic characteristics of the HSGHE was built and the reliability of the model was validated through an indoor test rig. Subsequently, some parametric studies were performed to analyze the degree of influences that each input parameter on the outputs. On this basis, the Box-Behnken design method and RSM were used to establish response surface models between the input parameters (spiral diameter, pitch, and fluid velocity) and objective functions (net heat exchange rate and thermal performance capability) and determined the interactions between them. The results showed that the spiral diameter had the most significant influence, followed by the pitch, and fluid velocity. Finally, based on the developed models, the MOGA employing the Pareto optimum solutions were used to optimize the design and operation parameters and determine their optimal combination: spiral diameter of 0.4 m, pitch of 0.1 m, and fluid velocity of 0.4 m/s. The findings of this study may assist designers in the development of high efficiency ground heat exchangers.
No abstract is provided for this article.
No abstract is provided for this article.
The oxidation behavior of K38 alloy with 0, 0.05, 0.1, 0.5wt% yttrium concentrations has been investigated during exposures in air at 1173K for 100 hours. The results indicated that Cr2O3 and TiO2 scale mainly formed on the surface of the alloy without yttrium. Yttrium addition promoted the selective oxidation of aluminum and reduced the internal oxidation. The alloy with 0.1 wt.% yttrium addition exhibits excellent oxidation behavior among the four types of the alloys for its decreasing the oxidation rate and forming more continuous and compact Al2O3 scales. Yttrium-rich phase formed in the alloy with 0.5wt.% yttrium, result in a negative effect on the oxidation resistance of cast alloys.
Current research efforts on solar chimney are much on those configurations with single space or single vent, hampering its applications in complex buildings. To expand its implementation, the viability of solar chimney in multi-storey atrium buildings was explored numerically and theoretically. It was known that its natural ventilation performance can fulfil the WHO requirements (i.e., 6 air change per hour, ACH) even under low solar radiation of 200 W/m2 if it is appropriately designed, such as cavity gap (d), cavity height (Hc ), solar radiation (Q), and window width (W). Its natural ventilation rate was found to be positively linear to the exponential form of the analyzed factors in this study, including cavity gap (d 0.63∼0.69), cavity height (Hc 0.59∼0.60), solar radiation (Q 0.42∼0.43), and window width (W 0.14∼0.20). The impacts of cavity gap and solar radiation are relatively weaker for big space such as atrium than those scenarios with single-space ventilation in the literature. The volume of the connected space was found to affect its natural ventilation performance, where the obtained ventilation rate of 4-storey is averagely 5.1 % lower than those of 3-storey buildings. A theoretical model was developed and experimentally validated, for the first time, to predict the natural ventilation rate of solar chimney when it is applied to the atrium with multiple air inlets. It was also noticed that the same theoretical model can be applied to the atrium buildings with odd and even storeys, but the area coefficients are different. The research outcomes of this study confirm the viability of solar chimney in complex buildings and offer a theoretical foundation for its implementation.
Silicon carbide (SiC) aerogels with high porosity and excellent thermal stability are a promising thermal insulation and adsorption material. However, the poor mechanical properties of SiC aerogels greatly limit its practical application. Herein, inspired by flexible palm bark, for the first time, we report a novel method called “bioinspired link assembly” (BLA) to fabricate bioinspired SiC aerogels (BSA) with significant layered structure. The layered structure and elastic links are in situ constructed in bioinspired link assembly process. The BLA method optimizes structure of the BSA by regulate monolayer dimensions. Owing to the elastic links between the layered structures, the BSA can be bent 180°. Moreover, the BSA shows super-elasticity (elastic deformation up to 70 %) and fatigue resistance (100000 compressions). Benefiting from layered structure, the BSA exhibits an ultralow thermal conductivity (0.027 W m−1 K−1). In addition, BSA shows excellent absorption capacities (weight gain about 44 ∼ 72 times) for organic solvents. Therefore, the excellent comprehensive properties make BSA have broad application prospects in the fields of high temperature thermal insulation and oil pollution treatment.
