Wall Thermal Conductivity Research Articles

Thermal Inertia Performance via TRNSYS Software of Recycled Wastewater Treatment Plant Sludge as a Construction Material Additive to Ecological Lightweight Earth Bricks

This research investigates the thermal performance of earth bricks made with different percentages of wastewater sludge additive (0%, 1%, 3%, 7%, 15%, 20%) in terms of cooling and heating loads, time lag and decrement factor. The simulation of a reference house (2.5m,10m,6m) using TRNSYS software allows for the evaluation of these parameters, external wall thicknesses, bulk density, thermal conductivity, and specific heat capacity are employed as inputs in dynamic thermal inertia model. The results showed that the use of bricks with higher sludge additive percentages resulted in a drop in cooling and heating loads, the lowest cooling and heating loads of 1720 KWH and 1534 KWH respectively were recorded with the highest percentage of wastewater sludge additive of 20% and the biggest wall thickness of 30cm, it was also noted that the use of higher wastewater sludge additive percentages and bigger wall thicknesses led to higher time lags and lower decrement factor, the highest time lag of 15 hours and the lowest decrement factor of 0.019 were as well recorded with the highest wastewater sludge additive of 20%, and the biggest wall thickness of 30cm. These results were attributed to the higher specific heat capacity, and lower thermal conductivity of the bricks with higher wastewater sludge additive percentages.

Modeling the thermal and hydrodynamic performance of grooved wick flat heat pipes

A compact model is developed to predict the thermal and hydrodynamic performance of a flat heat pipe with a rectangular grooved wick. The present model relies on the analytical solution to the energy equation in the wall and an equivalent heat transfer coefficient predicted using a computationally efficient iterative method. This efficient iterative method can also provide a framework for modeling other grooved or porous wick heat pipes for which analytical or semi-analytical solutions to the wall conduction, fluid flow, and film equations exist. Compared to prior numerical tools, the present modeling approach is computationally efficient, making it compelling for use in parametric and optimization studies. Instead of numerically solving a set of coupled differential equations, the present model considers only analytical and semi-analytical solutions for evaporation and condensation heat transfer rates. The non-discretized nature of the present model allows computations on a typical workstation to be completed within seconds as opposed to the hours required for prevalent numerical tools. The present model accounts for varying liquid fill volumes, geometry, and interfacial properties, such as surface tension and contact angle. The present model closely agrees with published numerical and experimental results for wall temperatures and maximum heat transfer rates. Parametric studies, which vary wall thermal conductivity, water contact angle, and groove dimensions are conducted on a previously experimentally investigated heat pipe to demonstrate the present model’s capabilities. The present model found that the maximum heat transfer rate of the heat pipe can be enhanced by about 15 and 20% by varying its wetting angle and groove dimensions, respectively.

Thermal conductivity of an external wall with simulated smart Aerogel insulation system

Modern technologies are more than ever adopting intelligent designs that respond to environmental variants to produce more desirable outcomes. In the building and housing industries, intelligent designs aim to conserve energy through temperature controls and energy storage and distribution systems. In this context, an intelligent Aerogel insulation system is modelled and studied for the exterior building walls. Aerogel with a thermal conductivity of about 0.012 W/m-K has a superior insulating property, and an intelligent design built around it would introduce additional energy storage and delivery enhancements to the system. In this study, a test room and a control room in an open urban environment were built, and the intended intelligence, first considered by choosing Aerogel that has the potential to be engineered with smart properties according to researchers in composite material science, and second was considered by automation which was applied manually to the test room. The calculated thermal conductivity of an external wall with intelligent Aerogel insulation is -(1.83 ± 0.06) × 10−1 W/m. The negative sign is an attribute of a system function rather than of a material. The external wall with intelligent Aerogel insulation, in comparison to an identical non-insulated wall, exhibited 2.7 times better energy conservation.

A comprehensive heat transfer prediction model for tubular moving bed heat exchangers using CFD-DEM: Validation and sensitivity analysis

This study established a complete heat transfer prediction model that integrates four heat transfer sub-models, including contact heat conduction, gas film heat conduction, heat radiation, and heat convection, along with an artificial softening correction based on combined approach of computational fluid dynamics and discrete element method (CFD-DEM). The model’s precision was confirmed by experimental results with relative errors below 5 %. The discussion on the heat flux contributions of the above four heat transfer mechanisms shows that with an initial particle temperature of 200℃, tube wall temperature of 30℃, and particle descent velocity of 1.3 mm/s, gas film conduction heat flux is the primary contributor to the total tube wall heat flux at 60.5 %, followed by radiation heat flux (19.5 %), contact conduction heat flux (12.1 %), and convection heat flux (7.9 %). Besides, the presentation of physical fields related to particles and the fluid showcases the model’s capability in elucidating the heat and mass transfer characteristics of the particle–fluid-wall system within tubular MBHEs. Furthermore, a local nominal range sensitivity analysis, along with a study of factors’ influence rules, was performed to quantify the effects of the 8 key model parameters on heat transfer results. The analysis indicates that fluid thermal conductivity is the predominant factor with its nondimensionalized first-order sensitivity coefficient of 54.9 %, succeeded by particle-to-wall angle factor (15.4 %), wall emissivity (14.4 %), gas film thickness (9.6 %), wall thermal conductivity (2.3 %), wall roughness (-1.6 %), particle Young’s modulus (-1.4 %), and particle specific heat capacity (0.4 %). Finally, their impacts on the heat prediction results were presented in detail.

The influence of geometry on the heat exchange potential of embedded planar retaining walls with seasonally-balanced thermal load

A number of operational cases exist where embedded retaining walls, used in the construction of underground spaces such as basements and shallow tunnels, have also been utilised as ground-heat exchangers in shallow geothermal energy systems. These are complex structures in terms of their geometry, the surrounding temperature field and boundary conditions, and there are currently no methods to assess their heat exchange capacity in a simple and expedient manner. This contribution uses the finite element method to validate the use of the method in predicting heat flow for this application and then, to assess the influence of wall and excavation geometry in the heat exchange process. The influence of the soil and wall thermal conductivity is shown to be quasi-linear with the latter showing the greatest influence on peak heat exchange. The work identifies a geometric parameter - the ratio of excavation depth to total wall panel depth, H/L which in combination with the wall thickness (D), provide a consistent and simple means by which the heat exchange potential can be estimated, for a given set of wall and soil thermal properties and boundary conditions.

Influence of technological parameters on the air heater exergy efficiency of the heat recovery system boiler plant

The analysis results of the technological parameters influence on the air heater exergetic efficiency of the heat recovery system a boiler plant with a heating capacity of 2 MW are presented. A comprehensive methodology based on the balance methods of exergetic analysis and the selection of the necessary evaluation criteria was used to evaluate the efficiency. Exergetic losses and heat-exergetic efficiency criterion, which characterizes exergetic losses per unit of heat output of the air heater, were chosen as these criteria. Using the developed methodology, the nature and regularities of the heating capacity the plate air heater influence on its exergetic efficiency were established. The local exergetic losses associated with heat transfer between heat-transfer agents, wall thermal conductivity, and heat-transfer agents motion, as well as their relative contribution to the total local exergetic losses, are investigated. It was established that the largest exergetic losses and the heat- exergetic efficiency criterion in the air heater for all values of its heat output are related with heat transfer from the wall to the air. The values of exergetic losses and the heat-exergy efficiency criterion for heat transfer from flue gases to the wall are 1.4-1.5 times less than the exergetic losses for heat transfer from the wall to the air. The values of exergetic losses and the heat-exergy efficiency criterion related with wall thermal conductivity and heat-transfer agents movement are, on average, an order of magnitude lower. It was established that to increase the air heater exergetic efficiency, it is advisable to reduce local exergetic losses by increasing the heat transfer coefficients, mainly by increasing the heat transfer coefficient from the wall to the air. The optimal values heating capacity of the air heater studied, which corresponds to its high exergetic efficiency, does not exceed 55 kW.

Surface-gas chemistry coupling and stability limits of hydrogen/air combustion in catalytic microchannels

The catalytic (heterogeneous) and coupled catalytic-gaseous (hetero-/homogeneous) combustion of fuel-lean hydrogen/air mixtures (equivalence ratio φ = 0.4) in palladium- and rhodium-coated catalytic microchannels was numerically investigated in planar microchannels having a canonical geometry of 10 mm length and 1 mm height. Steady, extinction-induced combustion stability limits were demarcated as a function of inlet velocity and external heat loss at pressures of 1 and 5 bar, with wall thermal conductivities of 1 and 16 W/mK. In each case, interplays between the catalytic and gas-phase chemical reaction pathways, and their impact on the stability limits were identified. The stability results were further compared with literature data for platinum. The simulations indicated that Pd was more resilient against extinction than Rh and had a stronger surface reactivity when competing with gas-phase chemistry in the channel. Similar to Pt, the strong H2 surface reactivity on Pd resulted in wide stability limits purely determined by surface reactions and independent of gas-phase chemistry. In stark contrast, the presence of gas-phase combustion significantly expanded the stability limits of the Rh channel. The stability limits of Rh at 5 bar were consistently broader than those at 1 bar under all investigated conditions, which was also a behavior different to that of Pd and Pt channels, whose stability limit curves had crossover points between the two pressures. Additional simulations were performed in a Surface Perfectly Stirred Reactor (SPSR), providing comprehensive chemistry information, including sensitivity analyses of key reactions and surface coverages. When approaching extinction, OH(s) was a major surface species on Pd, while the Rh surface was primarily blocked by O(s).

Heat Transfer Mechanism of Heat–Cold Alternate Extraction in a Shallow Geothermal Buried Pipe System under Multiple Heat Exchanger Groups

Shallow geothermal energy usually uses underground buried pipes to achieve the purpose of extracting heat while storing cold in winter and extracting cold while storing heat in summer. However, the heat transfer mechanism under the alternate operation of heat–cold extraction in winter and summer under multiple heat exchanger groups is still worth studying. Based on the constructed flow and heat transfer model in pipelines and reservoirs, this study first analyzes the temperature field evolution of a shallow buried pipe system (SBPS) under the alternate operation of heat–cold extraction, and then discusses the heat transfer performance under different pipeline flow rates, pipeline wall thermal conductivity, heat injection durations, numbers of heat exchanger groups, and flows of underground fluid. The results show that the continuous alternating process of heat–cold extraction has a promoting effect on the temperature increase or decrease in the next operating cycle due to the low- or high-temperature zone produced in the previous operating cycle. As the number of multiple heat exchanger groups increases, the heat transfer efficiency of the SBPS significantly improves. With a rise in the groundwater flow velocity, the heat transfer efficiency first decreases and then increases.

Vial Wall Effect on Freeze-Drying Speed

The vial wall thermal conductivity and thickness effect on freeze-drying speed is simulated. A 2D axisymmetric numerical simulation of Mannitol freeze-drying is employed using the boundary element method. The originality of the presented approach lies in the simulation of heat transfer in the vial walls as an additional computational domain in contrast to the typical methodology without a vial wall. The numerical model was validated using our measurements and the measurements from the literature. Increasing the glass vial thickness from 1 mm to 2 mm has been found as the major factor in primary drying time, increasing the gravimetrical Kv up to 20 % for all the simulated chamber pressures. The effect of thermal conductivity was simulated using a polymer and aluminium vial replacing the standard glass vial of the same thickness. The polymer vial‘s decreased Kv value is 5.6 % at a low chamber pressure of 50 mTorr, and 12.2 % at 400 mTorr, which is in excellent agreement with the experiment. Using higher conductivity materials, for example, aluminium, only 3.7 % and 2.3 % Kv increase were computed for low and high chamber pressures respectively.

Thermal conductivity of the cell wall of wood predicted by inverse analysis of 3D homogenization

This work proposes an accurate prediction of the directional thermal conductivity of wood at the cell wall scale. To that purpose, the strategy combines the physical characterization at the macroscopic scale, the description of the cellular morphology, homogenization calculations and an inverse analysis. The first step consisted in measuring the macroscopic thermal conductivity using the Transient Plane Source (TPS) method in the three material directions of spruce and poplar. The values range from 0.10 to 0.31 Wm−1K−1 depending on direction and species. The 3D morphology was acquired by micro-tomography and subsequently processed to generate a digital representation of the cellular structure. This representation is the input geometry in an homogenization procedure. Based on the results of this algorithm, the measurements of the solid fraction and the macroscopic values, we are able to determine the microscopic thermal conductivity of the cell wall λ0s, which is impossible to measure directly. λ0s was also found to be anisotropic with respective values of 0.6 and 1.0 Wm−1K−1 in the transverse plane and the longitudinal direction.

A numerical investigation on combustion characteristics in a thermally orthotropic narrow channel with hydrocarbon fuels

A novel approach of using thermally orthotropic wall materials to enhance stabilization of premixed methane/air and propane/air flames in a narrow channel between two parallel plates is numerically demonstrated. The streamwise and spanwise wall thermal conductivities kxx and kyy are respectively adjusted from 2 to 128 W/m⋅K by a step of eight times, to investigate the influence of directional heat transfer in solid walls on the combustor’s performance in terms of the flame stability limits and heat loss resistance. Results show that the lower velocity limit (LVL) is nearly insensitive to the wall thermal conductivities while the upper velocity limit (UVL), on the contrast, can be significantly affected. The thermally orthotropic combustor with a higher level of kxx but lower level of kyy, is observed to show a unique kind of wall temperature profile in the pre-flame region, which leads to a larger extent of preheatings to the unburnt mixture and less heat losses to the ambient, and consequently an elevated flame propagation speed. The resistance to external heat losses can be enhanced by decreasing the heat conductivity in either the streamwise or spanwise direction. However, since the latter has exhibited much more noticeable effects, and in order to avoid sacrificing the flame stability limit, the thermally orthotropic combustor thereby takes the superiority in overall performance.

Modelling of bubble dynamics on vertical rough wall with conjugate heat transfer

The wall temperature distribution and wettability greatly influence the bubble dynamics and heat transfer process. In this work, lattice Boltzmann (LB) method is implemented to investigate the bubble dynamics on the vertical rough heating wall, where the conjugate heat transfer process is taken into consideration. The interaction between bubble dynamics and boiling heat transfer characteristics is revealed. The results reveal that the bubble motion and the gas–liquid interface are dependent on the wall thermal conductivity. A high wall thermal conductivity will cause the gas film to cover the wall surface and hinder the heat transfer process. Moreover, the influence of the wall mixed wettability on the conjugate heat transfer behaviors is also analyzed. The findings demonstrate that the step wall wettability can promote the nucleation efficiency so as to achieve the enhancement of the wall heat transfer.

Optimal Experimental Design for the Assessment of Thermophysical Properties in Existing Building Walls

The estimation of wall thermal properties through an inverse problem procedure enables to increase the reliability of the model predictions for building energy efficiency. Nevertheless, it requires defining an experimental campaign to obtain in situ observations for existing buildings. The quality of the estimated parameter strongly depends on the quality of the experimental data used for the parameter identification. In other words, there is a close relation between the experiment design and the precision of the retrieved parameters. The design of experiments enables to search for the optimal measurement plan. It ensures the highest precision of the parameter to be estimated. For in situ measurement in buildings, the design of experiments seeks to answer the following questions: How many sensors do we need? What is the sensor position in the wall? The optimal experiment design methodology enables us to answer those questions. The unknown parameter is the thermal conductivity of wall façade model considering two-dimensional heat transfer induced by time and space varying boundary conditions.

Performance assessment of active insulation systems in residential buildings for energy savings and peak demand reduction

Active insulation systems (AISs) in buildings are envelopes that integrate thermal insulation, thermal energy storage, and controls. Although different designs for AISs have been proposed in the literature, a comprehensive analysis of feasible AISs is lacking. This paper discusses the energy performance, peak demand reduction potential, and performance characteristics of an AIS that uses a concrete wall as thermal mass sandwiched between two solid-state thermal switches (STSs). These STSs change their thermal conductivity using an on/off metal switch to create or break a thermal bridge across the STS. This paper first describes the experimental setup, used to determine the ratio of thermal resistance during R-high (low thermal conductivity) and R-low (high thermal conductivity) states of the STSs. This ratio was then used in whole-building energy simulations to evaluate the performance of AIS walls across different climate zones with/without a freeze timer of 60 min. The timer was added to reduce the number of switches of STSs from one state to another, and hence the energy needed for these switches. Analysis of the switching frequency and interval of STSs, thermal conductivity of walls, impact of wall orientation, and heat transfer through the wall from the use of AIS at different climate zones/locations were performed. The simulation results show that the AIS can achieve energy savings ranging from ∼ 980 to 2,290 kWh in a single-family home with a floor area of ∼ 220 m2 compared with an IECC 2018 baseline. The energy savings was higher in dry climate zones which represent 17% of residential buildings in the United States, compared to humid or marine climate.

Evaluating the accuracy of in-situ methods for measuring wall thermal conductance: A comparative numerical study

Evaluating the accuracy of in-situ methods for measuring wall thermal conductance: A comparative numerical study

Numerical analysis of combined heat transfers through concrete hollow bricks in a hot climate of Morocco

The aim of this study is to determine the thermal conductance of concrete hollow bricks, which is crucial for assessing the energy efficiency of buildings. The study focuses on three types of concrete hollow bricks that are frequently used to build walls in Morocco. The three modes of heat transfer by convection, conduction and radiation are taken into account. The thermal behavior of the three types was simulated using a computational model created with the finite volume method. The effect of internal surfaces emissivity and solid wall thermal conductivity on overall heat transfer is investigated using practical values of the thermal excitations. Simulation results of the three types were compared and showed that the concrete hollow brick of Type (C) is the best performing configuration. Indeed, it reduces the thermal conductance ( U-value) by 40% compared to hollow brick of Type (B), which will contribute to improve the thermal resistance of the building walls.

Effects of Stack Structure and Heat Loss on the Stability and Efficiency of Steam-Methanol Reforming Microreactors for Hydrogen Production

The linear scale-out approximation ignores the influence of exterior heat loss and thus cannot address extinction issues. The present study is focused primarily upon the roles of stack structure and heat loss in the stability and efficiency of microreactors for hydrogen production by steam-methanol reforming. Computer simulations are performed to model the complex physicochemical processes in a variety of different heat loss situations. The effects of channel number, wall thermal conductivity, and surface-area-to-volume ratio are investigated to assess the importance of stack structure and heat loss in microreactor design. The results indicate that the total heat loss becomes increasingly significant for designing reactors with small stacks of channels. About half of heat released is lost if a global-type extinction occurs. The critical heat loss coefficient depends heavily upon the number of channels. The design with a large stack of channels more closely beneficially improves heat utilization and reduces heat loss. There is a significant linear relationship between energy efficiency and heat loss coefficient. Significant heat loss may cause extinction in edge channels while still permitting very high conversion in center channels. The wall thermal conductivity is of vital importance to ensure stability operation. Higher wall thermal conductivity provides stability benefits, but walls with very low thermal conductivity can be employed to reduce heat loss by sacrificing the thermal coupling capability.

Computational study of the segmental catalysis of steam-methanol reforming in heat integrated microchannel reactors for hydrogen production

The present study focuses upon segmentally catalyzed steam-methanol reforming processes for producing hydrogen in heat integrated microchannel reactors. Computational simulations are carried out to understand the causes of the fundamental phenomena involved in segmental catalysis and the autothermal operation. The benefits of segmental catalysis are discussed, and some aspects of heat phenomena are explained. The effects of wall thermal conductivity, flow velocity, and catalyst segment thickness are evaluated to determine how to design heat integrated reactors for specific operations by tailoring segmental catalysis approaches. To understand the importance of thermal management, the phenomena of heat in the catalyst structure are discussed in terms of anisotropic thermal conductivity and reaction heat flux. The results indicate that the benefits of segmental catalysis are surprisingly great. Segmental catalysis can be applied to greatly extend the operation regime, and the segmented oxidation design offers distinct advantages. The segmented design may offer improvements in heat and mass transport but presents thermal management challenges caused by the rapidly varying concentrations and temperatures. The wall thermal conductivity is vital in achieving heat balance and high process efficiency. Transverse wall heat conduction is of especially critical importance in thermal management. The flow velocity and catalyst segment thickness are of great importance to the improvement of conversion and productivity. An optimum flow velocity for highest overall performance exists.

Combustion optimization in consolidated porous media for thermo-photovoltaic system application Using Response Surface Methodology

Motivated by the need to optimize the operation of an MTPV system, three consolidated porous media (PM) geometries namely spherical, conical and cylindrical and a non-PM combustor are numerically simulated. The geometry with the best methane conversion rate is identified and optimized with the right porous media and combustor diameter ratio. Using Response Surface Methodology, a design of experiment (DOE) approach is adopted to examine the effects of three independent parameters namely: wall thermal conductivity, chamber radius and O2 mass fraction on output parameters critical to the operation of the MTPV system specifically radiation heat transfer rate and emitter efficiency. Results show that PM combustion produces higher and uniform wall temperatures compared to the non-PM combustor. The spherical shape consolidated PM produces the highest average methane conversion rate of 96.8% and highest mean wall temperature of 1032 K even though it produces the least flame temperature. The highest radiation heat transfer rate predicted by DOE and CFD simulations are 3.25 W and 3.17 W respectively. The best conditions to realize the optimal MTPV performance is by employing the spherical PM combustor with diameter ratio of 0.7, porosity of 0.5, equivalence ratio of 0.8, wall thermal conductivity of 22.99 W/(m∙K).

Effects of circumferential heat conduction on heat transfer characteristics of supercritical R134a in horizontal tubes

In this paper, simulations of supercritical heat transfer of R134a in horizontal tubes are performed to study the effects of circumferential heat conduction on the heat transfer deterioration, and the abnormal phenomena of higher temperature distribution of non-gravity supercritical flow is explained. The results show the heat transfer deterioration is caused by the impairment of specific heat and heat conduction in the boundary layer while the following recovery of heat transfer is due to the enhancement of thermal conduction and turbulent convection. A dimensionless parameter of Biot number is defined to characterize the thermal resistance ratio of the circumferential conduction to the convective heat transfer. The wall temperature redistribution due to the circumferential conduction affects the supercritical convection in horizontal tubes, and non-gravity supercritical flow may have a higher wall temperature when the Bi number is small. The deterioration of the top surface can be significantly alleviated in a tube with a larger wall thickness or thermal conductivity.