Thermal Wall Research Articles

Porous aggregate developed with the use of coal-containing clays of the Angren field

Relevance. One of the ways to solve the issues of resource and energy conservation is the production and use of porous aggregates. Porous aggregates allow obtaining effective lightweight concrete for thermal insulation, wall panels, monolithic walls and other load-bearing structures, contribute to the increase of energy efficiency, improvement of thermal insulation, reliability, increase of fire resistance, frost resistance and seismic resistance of buildings, reduction of their weight. Therefore, in the production of porous aggregates, the primary task is the use of industrial waste and products of their processing. Solving these problems leads not only to saving valuable natural resources, but also to solution of environmental problems. Aim. Development of compositions and study of properties of porous aggregate based on bentonite clay of Navbakhar deposit and coal-containing clay of Angren brown coal deposit. Object. Coal-containing clay of Angren brown coal deposit, bentonite clay of Navbakhar deposit and artificial porous aggregate based on them. Methods. Chemical, energy-dispersive X-ray, X-ray phase and infrared spectroscopic analysis, scanning electron microscopy, mathematical modeling, etc. Results. The authors have determined chemical and mineralogical compositions of the clays used. Using the mathematical modeling method they developed the regression equations describing the effects of the amount of bentonite clay in the batch, firing temperature and isothermal holding time on the bulk density and water absorption of the porous aggregates. The resulting porous aggregates with a bulk density of 395 to 690 kg/m3 have a compressive strength in a cylinder of 2.74 to 6.46 MPa, respectively. It was found that the aggregates obtained meet the requirements of regulatory documents.

Dynamic combustion optimization of a pulverized coal boiler considering the wall temperature constraints: A deep reinforcement learning-based framework

Coal-fired power generation boilers are susceptible to low combustion efficiency, pollutant exceedance, and over-temperature tube burst during deep peak shaving due to load fluctuation and mass flow rate variation, which seriously affects unit economics, environmental protection, and safety. In this paper, the issue of boiler combustion optimization under variable load conditions is investigated using a data-driven approach. (1) Based on the published literature, a multi-objective prediction model based on Channel Selection Convolutional Neural Network (CS-CNN) for boiler thermal efficiency, NOx emission and wall temperature was developed, taking into account the safety of the unit under fast variable loads (the range of wall temperature varies widely and is prone to over-temperature). (2) Conventional multi-objective optimization algorithms are time-consuming and hard to satisfy the real-time decision-making of combustion in the furnace. In this research, we developed a combustion decision-making agent based on the Twin Delayed Depth Deterministic Policy Gradient (TD3). By interacting with the predictive model and learning the combustion strategy, the agent has the ability to optimize the unfamiliar operating conditions of the boiler. Simulation experimental tests were carried out on real historical data from a 600 MW down-fired boiler and the results showed that thermal efficiency increased by 0.411 %, and NOx emissions decreased by 17.701 mg/m3, with the safety of the wall temperature maintained. A single decision by the TD3 is almost instantaneous, with an average time of 0.004 s. The decision-making efficiency was much better than the traditional search-based optimization algorithms.

Experimental and numerical investigations on phase change thermal storage foam concrete based on CaCl2∙6H2O/silica aerogel composite phase change material

A new type of phase change thermal storage foam concrete was developed by effectively incorporating a composite phase change material (CPCM) into foam concrete. The CPCM was obtained by using 75 wt% CaCl2∙6H2O as PCM and 25 wt% silica aerogel as carrier, with a phase change temperature of 27.0 °C and an enthalpy value of 110.9 J/g. The mechanical and thermal properties of the obtained phase change thermal storage foam concrete blocks were investigated experimentally, along with a numerical analysis of their heat transfer characteristics. The experimental results demonstrated that after adding CPCM, the thermal performances of foam concrete blocks significantly increased, under the condition of their dry densities and compressive strengths met the specification requirements. Adding 20 wt% CPCM, the enthalpy value, specific heat capacity, thermal conductivity and thermal inertia of foam concrete block respectively reached 37.0 J/g, 1782.0 J/(kg∙K), 0.131 W/(m·K) and 770.9, with appropriate phase change temperature (27.2 °C). Numerical analysis for the heat transfer characteristics of the phase change thermal storage foam concrete wall revealed that optimal thermal insulation and thermal storage performances were achieved when the wall contained 20 wt% CPCM and thickness of 80 mm. In this case, the model exhibited a temperature wave attenuation factor of 6.66, the temperature amplitude of 5.84 K and a decline of 9.87 K for the highest temperature of the wall as well as delay time of 198,000 s, effectively regulating indoor temperature and saving energy. Therefore, the as-prepared phase change thermal storage foam concrete had great potential for application in building energy conservation.

A Review of the Building Heating System Integrated with the Heat Pipe

The heat pipe (HP) is widely applied in the thermal management field at present. In order to make use of the low-grade and renewable energies to maintain building thermal comfort in the heating season, more and more studies with respect to improving the thermal performance of the building heating system integrated with the HP (BHSIHP), such as the floor heating system integrated with the HP (FHSIHP), the thermal storage wall heating system integrated with the HP (TSWIHP), conventional wall integrated with the HP (WIHP) and radiator heating system integrated with the HP (RHSIHP), are conducted. This paper aims to summarize different types of HPs applied in the building heating system and offers an overview of the thermal performance improvement for the BHSIHP. The thermal response, thermal conductivity, thermal resistance, heat capacity, heat transfer coefficient, temperature distribution, thermal storage and heat release capacity are always selected to investigate characteristics of the BHSIHP. Results show that the thermal performance of the FHSIHP, the TSWIHP, the WIHP and the RHSIHP is more outstanding than that of the conventional heating system. The thermal performance of the BHSIHP is affected by heat source temperature, installation tilt angle, working fluid, and filling ratio of the HP. The heat source temperature, which positively affects the performance of the BHSIHP, is crucial for the selection of the working fluid and filling ratio. However, the performance of the BHSIHP is increased first and then decreased with the increase of the installation tilt angle. The optimal filling ratio of the working fluid has been proven not to be a fixed value.

Development and tests of the novel configuration of the solar chimney with sensible heat storage

Heating, ventilation, and air conditioning systems account for a significant part of the energy usage in buildings and are primarily responsible for greenhouse gas emissions and operating costs. These can be lowered by the adoption of solar energy. This paper presents the developing and testing of a novel solar chimney with sensible heat storage. The thermal performance and potential contribution of the developed thermal chimney to decreasing residential buildings energy consumption in Polish climate conditions is investigated. The developed solar air chimney is composed of a novel accumulation material that allows for sensible heat storage. Solar radiation can heat the accumulation layer during the day and the stored energy can be used to preheat the ventilation air. The prototype of the solar chimney was tested in laboratory conditions using a dedicated experimental rig. Furthermore, numerical calculations were performed using TRNSYS software, where a validated dynamic numerical model was developed to simulate the performance of the device. The results show that airflow direction, volume, and the number of heated walls significantly impact the power transferred from the solar chimney walls to the air. The maximum power transferred from the solar chimney walls to the air per unit of surface varied from 360.2 W/m2 (when all walls were heated) to 672.4 W/m2 (when only one wall was heated). The validated model underestimates the outlet air temperature of the solar chimney by a maximum of 0.39 °C on average basis and shows that the increase of air temperature between −10 and 20 °C at 1000 W/m2 determines a decrease of 12.5 % of the thermal output. The obtained results allow one to highlight that the proposed solution can be considered as a replacement for typical brick chimneys located on roofs or as a thermal storage wall.

Thermally Conductive Polydimethylsiloxane-Based Composite with a Three-Dimensional Vertically Aligned Thermal Network Incorporating Hexagonal Boron Nitride Nanosheets and Nanodiamonds.

Thermal interface materials play a pivotal role in efficiently transferring heat from heating devices to thermal management components, thereby reducing the risk of component degradation due to overheating. In this study, we propose a strategy for enhancing the out-of-plane thermal conductivity (TC) of composite materials by fabricating a three-dimensional (3D) thermal network within a polydimethylsiloxane (PDMS) matrix. Specifically, the composite material was designed to incorporate a dense thermal network comprising hexagonal boron nitride nanosheets (BNNSs) and nanodiamonds (NDs). The fabrication process commenced with the preparation of BNNSs through liquid-phase exfoliation, followed by the creation of a 3D BNNSs-NDs/polyimide aerogel thermal framework using a unidirectional solidification ice templating method and subsequent heat treatment. Vacuum impregnation and curing were then employed to finalize the production of the 3D BNNSs-NDs/PDMS composite material. Characterization analyses indicated that the addition of NDs filled the voids between BNNSs, leading to the densification of the thermal framework pore walls and the establishment of additional thermal pathways. Impressively, with concentrations of BNNSs and NDs of 17.99 and 7.71 wt %, respectively, the out-of-plane TC of the 3D BNNSs-NDs/PDMS composite material reached 1.623 W m-1 K-1, marking notable enhancements of 754.21% and 256.70% compared to those of pure PDMS and composites prepared via direct blending with randomly distributed BNNSs and NDs, respectively. Furthermore, the 3D BNNSs-NDs thermal framework improved the compressive strength and the dimensional stability of the composite material. Finite element simulations additionally confirmed the synergistic improvement of the TC achieved through the combination of BNNSs and NDs, demonstrating that the 3D BNNSs-NDs/PDMS composite material displayed superior heat conduction and a greater density of thermal pathways compared to those of its counterparts, including 3D BNNSs/PDMS and 3D NDs/PDMS composite materials. In summary, this work presents a strategy for enhancing the out-of-plane TC of polymer-based composite materials by incorporating vertically aligned thermal networks.

Thermal effect on the flow induced by a single-dielectric-barrier-discharge plasma actuator under steady actuation

The thermal effect of a single-dielectric-barrier-discharge plasma actuator under steady actuation is numerically investigated. A new actuator model is proposed and validated using experimental data. A discrete Galerkin method based on high-order flux reconstruction schemes is employed to solve the flow governing equations and the actuator model equations on unstructured quadrilateral grids. By comparing the induced heated and cold flow fields of the actuator with and without a plasma thermal source, its thermal effect is revealed. The actuator generates a thermal wall jet with rich vorticity, forming a monopolar starting vortex with a high-temperature and low-density core. Over time, the starting vortex becomes unstable and transforms into a dipole. Actuator heating enhances jet velocity and width, as well as vortex stability, while slowing down vorticity generation. The relative change in density and temperature fields due to actuator heating is four orders of magnitude greater than that without actuator heating. Additionally, the actuator heating causes the background thermodynamic fields to increase approximately linearly with time. Two stages in the actuator's thermal effect are distinguished due to time accumulation. Initially, the actuator heating minimally affects the monopolar starting vortex motion, and the temperature and density fields are treated as passive variables driven by the velocity field. During this stage, the momentum and thermal effects of the actuator can be studied separately. However, after the starting vortex becomes unstable, the actuator heating significantly impacts its motion and morphology, and these two effects are coupled with each other.

Solid and Homogeneous Thermal Wall of Cored Lightweight Concrete Blocks Solidarized in situ

Abstract: The exterior walls of buildings play a major role in the road for a near-zero energy building. Nowadays, in Portugal, the ETICS system is the most used technology to ensure the thermal insulation of exterior walls of buildings. However, the ETICS system has low resistance to impacts and requires a self-supporting sheet of wall, among other disadvantages. Here, the concept of new technology for exterior walls of residential and commercial buildings is presented. It consists of a hybrid technology for the easy and quick execution of a single sheet wall, which may provide high thermal efficiency for buildings. This hybrid technology relates to parallelepipedshaped cored lightweight concrete blocks comprising two parallel sides separated by one or more solid connectors in which the connectors allow the filling of the block core with lightweight concrete. No bed or head joints are used to connect the blocks because the wall made of cored blocks gets solid and homogeneous when filled with lightweight concrete. When the block thickness is 30 cm and the thermal conductivity of the lightweight concrete is lower than 0.09 W/m.ºC, in Portugal, this type of exterior wall does not require the application of extra thermal insulation materials.

Experimental investigation of a distributed photovoltaic heating system based on building envelope thermal storage

Low-carbon heating in farmhouses can be achieved using photovoltaic (PV) heating. However, the severe mismatch between the PV energy supply and the building thermal load requires expensive energy storage devices and control systems. In this study, a controller-less PV heating system utilizing the building envelope for thermal storage was evaluated on a farmhouse in northern China. The building envelope simultaneously functions as the heating terminal and is composed of insulation boards, electric heating wires, and the thermal storage wall. PV electricity is fed to be absorbed by the electric heating wire to produce heat. The thermal storage wall absorbs the heat generated by the heating wire before releasing it into the room through convection and radiation. Long-term monitoring reveals that the black bulb temperature of the farmhouse was above 16 °C for more than 60 % of the time throughout the heating season. The building envelope acting as a thermal reserve improved system supply and demand matching by 77 % and the indoor temperature fluctuation was reduced by about 47 %. The thermal storage wall can provide 58 % of the heating energy when there is no PV power supply. The payback period of the heating system is only 6.5 years, verifying the good rate of return of the system. This study proposes a lower cost energy storage solution for PV heating than previous studies, and engages the building envelope into the building energy system.

Joint Effects of Velocity and Thermal Wall Slips on Steady, Laminar, MHD Jeffrey-Nano Fluid Flow Over a Non-Linearly Stretching of a Flat surface

Joint Effects of Velocity and Thermal Wall Slips on Steady, Laminar, MHD Jeffrey-Nano Fluid Flow Over a Non-Linearly Stretching of a Flat surface

Weathering resistance of novel sustainable prefabricated thermal insulation wall

External walls, serving as the primary medium for heat exchange between the building and the external environment, has its thermal loss comprising the largest proportion of building energy consumption. Therefore, enhancing the thermal insulation capacity of the wall is of great significance in reducing building energy consumption. In this paper, a novel sustainable prefabricated expanded polystyrene (EPS) thermal insulation wall panel with irregular column frame structures was developed. And weathering tests combined with finite element simulations were conducted to investigate its weathering performance and degradation patterns. The results revealed that In the weathering test, the panel surfaces did not exhibit apparent water seepage cracks, powdering, hollowing, peeling, etc. There was no occurrence of facing brick detachment or damage. The outer surface concrete of the wall panel experienced resistance during normal thermal expansion and contraction, generating compressive stress during expansion and tensile stress when contracted. In addition, the bond strength of the specimens decreased by 8.1% after the thermal-rain cycles, 5.1% after the thermal-cold cycles, and 12.1% after the freeze-thaw cycles. In the numerical simulations, the temperature stress at various positions on the concrete wall had a noticeable mutual restraining effect on the force deformation of the nearby concrete. There was a significant risk of cracking in the middle and around the opening, particularly in the lower part of the wall panel. This study serves as a basis for the degradation analyses and optimization design of the sandwich insulation wall panels for sustainability.

Dimensionless resolutions for heat flux decrement factor and time lag of the wall during cyclic variations in outdoor air temperature

The heat flux decrement factor (DF) and time lag (TL) are two vital indicators for assessing wall dynamic thermal performance. However, computing these indicators involves complex variables such as material thermal properties, wall thickness, and others. Therefore, exploring the inherent relation between these two indicators and the influencing parameters is an urgent problem for reducing building energy consumption. To this end, an analytical method based on the auxiliary function was employed to calculate the heat flux DF and TL of a single-layer wall. Then, the impact of the influencing factors on heat flux DF and TL were evaluated. The results showed that heat flux DF and TL are only determined by two dimensionless parameters: diffusion length and thermal efficiency coefficient. The heat flux DF decreases by at least 5 times as the diffusion length varies from 0 to 2. Besides, it is also affected by the diffusion length when the thermal efficiency coefficient (ε) is less than 5, and the smaller the ε, the smaller the heat flux DF. On the other hand, the heat flux TL increases linearly with the diffusion length initially but declines precipitously as the diffusion length approaches 6–7. When the diffusion length is the same, the heat flux TL slightly decreases as the thermal efficiency coefficient increases.

Multi-Aspect Shaping of the Building’s Heat Balance

In the European Union, buildings account for 42% of the energy consumption and 36% of the direct and indirect energy-related greenhouse gas emissions. Reducing thermal power for heating purposes is crucial to achieve climate neutrality. The main purpose of this article is to identify the places in the building where it is possible to significantly improve energy efficiency through the use of appropriate construction and material solutions. This article contains a multi-aspect approach to the heat balance of a building. Solutions that have a direct impact on building energy consumption were analysed, taking into account architectural, technological, and material aspects. Particular attention was paid to energy-efficient design and material solutions for non-transparent and transparent external walls and thermal storage walls (Trombe walls). An analysis of heat transfer through building elements was carried out, along with the optimisation of energy-efficient solutions for non-transparent and transparent barriers. Two methods for determining the equivalent heat transfer coefficient Ue for solar active partitions are presented. The analysis presented in the work using the original method of the balanced heat transfer coefficient Ue is a testing ground for identifying unfavourable features of the building structure, as well as the most energy-efficient solutions that can be used in establishing standards for the construction and modernisation of buildings. The value of the Ue coefficient illustrates the actual heat transfer through the partition. Having Ue values for various structural solutions of building envelopes, the designer can easily select the most effective ones. The use of the presented methodology will allow for the optimisation of technical solutions for building elements to improve its energy efficiency.

Planning and design of an architectural energy supply system based on photovoltaic-thermal technology

China’s building energy consumption is growing and the energy consumption of the whole process of buildings accounts for more than 46.5% of the country’s total energy consumption, of which the proportion of energy consumption used for heating, ventilation and air conditioning is as high as 50%. Solar energy has the greatest potential to achieve energy conservation in buildings. Buildings are the best carrier for solar energy utilization because of the advantages of local collection and local application. The combination of solar energy and buildings can meet a variety of energy and health needs in buildings, among which solar photovoltaic and CSP building integration technology is an important way to reduce building energy consumption and improve indoor air quality. This article presents a PV/T-based thermal storage wall system that efficiently utilizes solar energy. It outlines the PV/T technology’s fundamental principles and provides a detailed description of the system’s construction and operation, highlighting critical components like the solid thermal storage medium and heat transfer via a heat pump. The analysis indicates improved energy utilization and favorable economic feasibility. This technology offers a practical solution for the construction industry, promoting sustainable energy practices and environmental conservation.

Analysis of the thermal performances of uninsulated and bio-based insulated compressed earth blocks walls: from the material to the wall scale

The lively international debate on the future of the built environment has placed emphasis on the possibilities offered by bio and geo-based building materials. Among these, raw earth-based materials offer several advantages associated to their reusability and low embodied energy.Nowadays, several companies are basing their production lines on prefabricated raw earth products, as is the case of compressed earth blocks (from now on CEBs). CEBs are commercialized for the construction of massive vertical envelopes, characterized by a high thermal inertia. Nevertheless, in order to compete with conventional building materials, it is also necessary to guarantee a high thermal resistance.In this work, this issue was solved by the design and testing of full-scale uninsulated and bio-based thermal insulated CEB walls. In this way, the thermal performance of CEB walls can be increased to meet the high energy requirements currently adopted in European Countries. Furthermore, the choice of bio-based insulations represents the main novelty of the study, aimed at finding hygrothermal compatible solutions at a low environmental cost.More in detail, this work reports the results of the thermal and physical material characterization of CEBs and of two innovative bio-based thermal insulations (lime hemp and sugarcane bagasse panels), and compare them with measurements made on full-scale uninsulated and insulated CEB walls. For this purpose, the walls are tested inside a double-room climatic chamber where they are subjected to variable temperatures on their two faces, reproducing typical indoor and outdoor conditions during summer and winter conditions in a continental climate.Results show the enhancement of thermal performances of compressed earth blocks walls when thin layers of bio-based thermal insulations are added. The thermal resistance of weakly bio-based insulated CEB walls is found to be nine times (for the sugarcane bagasse insulated CEB wall) and four times (for the lime hemp insulated CEB wall) higher than that of uninsulated CEB walls. Moreover, the addition of the insulation layers enhances the time lag and the decrement factor values of CEBs walls.

Direct numerical simulations of compressible turbulent channel flows with asymmetric thermal walls

This paper extends the work of Tamano & Morinishi (J. Fluid Mech., vol. 548, 2006, pp. 361–373) by simulating supersonic turbulent channel flow with asymmetric thermal walls using a larger computational domain and a finer mesh. Direct numerical simulation is carried out for four cases with different thermal wall boundaries at the top wall at fixed $Ma=1.5$ , $Re=6000$ and $Pr=0.72$ , while the bottom wall is maintained at a constant temperature of $T_L$ equal to the reference temperature. These cases are referred to as the adiabatic case TAd, where the top wall is adiabatic; the pseudo-adiabatic case T32, where the top wall is isothermal with temperature $T_{w,t}=T_A$ ; the sub-adiabatic case T25, with $T_{w,t}=0.77T_A$ ; and the super-adiabatic case T40, with $T_{w,t}=1.24T_A$ . Here, $T_A=3.234$ is the mean temperature at the adiabatic wall in the TAd case. The objective of this study is to compare and contrast the TAd case with its corresponding T32 case, and to investigate the effect of the wall temperature difference between the two isothermal walls. Comparisons of the basic turbulent statistics, the heat transfer between the Favre-averaged mean-flow kinetic energy, the Favre-averaged turbulent kinetic energy and the Favre-averaged mean internal energy, as well as the wall heat transfer properties, indicate that the TAd case and its corresponding T32 case are generally equivalent. The only discernible difference is in the region very close to the top wall for the temperature-fluctuation-related quantities. The analysis reveals that the asymmetry of the thermal walls causes asymmetry in the flow and thermal fields. In addition, the transfer of the heat generated by the pressure dilatation and the viscous stress is facilitated by the turbulent heat flux term and the mean molecular heat flux term.

The Thermal Properties of an Active–Passive Heat Storage Wall System Incorporating Phase Change Materials in a Chinese Solar Greenhouse

The use of renewable energy for food and vegetable production is a potential sustainable method to reduce fossil energy consumption. Chinese solar greenhouses (CSGs) are horticultural facility buildings in the northern hemisphere that use solar energy to produce off-season vegetables in winter. The north wall heat storage and release capacity of CSG has a significant impact on the indoor thermal–humidity environment. However, common traditional solar greenhouses commonly have problems such as insufficient heat storage and release, thick temperature stability zones inside the walls, and inability to dynamically regulate the entire greenhouse environment. Therefore, a novel active–passive heat storage wall system (APHSWS) incorporating phase change materials has been developed to promote the thermal performance of the CSG and its internal temperature of the thermal storage wall in this paper. Through experimental and simulation methods, the heat storage and release of the APHSWS and its impact on the greenhouse environment are investigated. The findings indicate that the APHSWS has increased the wall heat storage and release capacity, compared to the ordinary greenhouse without the APHSWS, in three typical weather conditions in winter (i.e., sunny, overcast, and cloudy); the average temperature of greenhouse with the APHSWS has increased in indoor air temperature, wall surface temperature, and soil surface temperatures of 1.58–6.06 °C, 2.71–6.58 °C, 0.91–6.39 °C, respectively; and during the experiment, the greenhouse with the APHSWS has a monthly average daily effective accumulated temperature of 1.39 times, 1.18 times, 0.60 times, and 0.20 times that of the ordinary greenhouse without the APHSWS from December to March of the next year, respectively. Under typical sunny conditions, the greenhouse wall heat storage capacity increased by 1.59–2.44 MJ/m2 and the heat release capacity increased by 0.97–1.17 MJ/m2. At the direction of wall thickness, the temperature at each point inside the wall with the APHSWS is always higher than that of ordinary wall without the APHSWS. In addition, the operating cost of the APHSWS in winter is analyzed.

Performance analysis on a wood-color thermal catalytic Trombe wall in wood buildings

The wood buildings had huge advantages such as high thermal insulation performance and carbon fixation. While the indoor air pollution caused by the release from volatile organic compounds (VOCs) from the artificial boards in wood buildings creates huge threat on human health. Considering architectural aesthetics and solar building-integration principles, this article proposed a novel thermal catalytic Trombe wall with wood-color thermal catalysts that realized heating and air purification. Firstly, the color difference between thermal catalysts and wood board was analyzed. Secondly, the heat and mass transfer model of the wood building with Trombe wall was developed and verified. Thirdly, the thermal and purification performance under different occupancy ratios of Trombe wall to whole wall (RT) and wood species were discussed. The main results were: (1) The color difference between thermal catalysts and wood was 0.97, which was similar and acceptable to meet architectural aesthetics; (2) Compared to ordinary wood building, the indoor temperature increased by 18 ℃ when combined with Trombe wall. The average thermal efficiency, purification efficiency and generated clean air were 26.91 %, 94.34 % and 385.58 m3, respectively. (3) With the increase of RT, the formaldehyde degradation performance and indoor air temperature increased from 73.52 % to 94.34 %, 25 to 38 ℃, respectively. (4) When considering natural running in daytime and forced running in nighttime, more than 90 % of indoor formaldehyde was degraded for five wood boards with different formaldehyde emission rates. The indoor formaldehyde concentration would be below 80 ppb for the release rate of 0.98 and 1.40 mg/L.

Numerical study on methane heterogeneous reaction characteristics in micro catalytic combustors with orthogonal thermal anisotropic walls

This numerical study investigates the combustion characteristics of premixed methane/air in micro catalytic combustors, specifically focusing on orthogonal anisotropic walls. A two-dimensional computational fluid dynamics (CFD) model incorporates detailed methane heterogeneous (catalytic) chemical reactions over platinum catalyst, validated against experimental data with maximum deviations of 4.9 % at an equivalence ratio of 0.6. The impact of wall materials, such as pyrolytic graphite and stainless steel 316, on heterogeneous reactions (HTR) performance is explored. In addition, the effect of different wall thermal conductivities on HTR stability is also studied. The results show that pyrolytic graphite has better combustion performance and a wider range of stable combustion limits. This advantage is mainly attributed to the thermal anisotropy of pyrolytic graphite, which enhances fuel preheating and facilitates the stable combustion of methane. The reduction of transverse thermal conductivity reduces heat losses from external walls and diminishes the preheating effect on unburnt gases. Conversely, an increase in transverse thermal conductivity enhances the heat transfer performance of external walls, reduces temperature gradients, diminishes convective heat contributions from external walls, and ultimately lowers the Biot number of external walls. In summary, this research yields vital insights for designing and optimizing micro-combustors, particularly in the practical application of selecting wall materials and adjusting thermal conductivity parameters.

On the stabilization mechanism of high-speed deflagrations in narrow channels with heat loss

Statistically steady supersonic deflagrations are numerically investigated in narrow channels with strong thermal expansion and heat loss. Four modes of flame propagation are observed, namely, extinction, low-speed deflagration, high-speed deflagration, and DDT. It is determined that larger thermal expansion facilitates initiation of high-speed deflagrations while the heat loss can suppress the transition to detonation. The high-speed deflagration mode is shown to be the result of the dynamic balance between thermal expansion and wall heat loss. The limits of high-speed deflagration in terms of the thermal expansion and heat loss coefficients are determined. The statistically steady oscillatory high-speed deflagrations propagate at average velocities close to half of the CJ detonation velocity. The dynamics of the flame front and shock waves are visualized using numerical schlieren. Periodic acceleration and deceleration of the leading shock are identified, and the mechanism of DDT suppression is elucidated.