Wall Heat Capacity Research Articles

Impact of Uninsulated Slab-on-Grade and Masonry Walls on Residential Building Overheating

Studies of the effects of removing underfloor insulation and increasing the thermal capacity of building walls are currently found almost exclusively in existing vernacular architecture and rammed-earth buildings, mostly in countries with warm climates. This paper proposes the combined use of these two measures to reduce the risk of overheating in a detached single-family house in a temperate climate during the summer. Experimental studies conducted during the largest heat wave on record in the summer of 2019 showed that peak daytime temperatures decreased by 5.2 °C to 7.1 °C, and peak nighttime temperatures decreased by 4.7 °C to 6.8 °C. Simulation studies taking into account occupant heat showed that the proposed passive methods could, under the IPCC 8.5 scenario, eliminate the need for mechanical cooling in a detached single-family house in the temperate climate of Central and Eastern Europe by 2100. The actual heating energy consumption for the building with an uninsulated floor and increased wall heat capacity was 5.5 kWh/m2 higher than for the reference building, indicating that it can be a near-zero energy building. The proposed concept is in line with the new approach to the energy design of buildings, which should not be limited to reducing thermal energy demand, but should also respond to the needs arising from global warming.

Analytical model for the simulation of Trombe wall operation with heat storage

Passive solar systems, such as the Trombe wall, are cost-effective ways to reduce the energy consumption of buildings for heating, cooling, and ventilation. The operation of these systems can be simulated either with Building Energy Simulation Tools—BES like TRNSYS, EnergyPlus, etc either with Computational Fluid Dynamics—CFD. In both cases, the purchase of special software and/or special programming skills are required. In parallel analytical calculating tools are being developed, which also require some programming to solve an implicit system of non-linear equations but with fewer software requirements. The majority of analytical models concerns energy balance models for steady-state conditions with the result that heat storage is not taken into account, which in the case of a Trombe wall has a significant effect on the developed transport phenomena. In the present work, an analytical energy balance implicit model was developed for the simulation of the transient operation of a Trombe wall taking into account the heat storage. Using this model, the operation of a Trombe wall for 7 typical days of the year was simulated. The results are presented in terms of the daily evolution of the temperature with which the air enters the room served by the passive system, of the temperature of the Trombe wall surface adjacent to the served room, and of the airflow rate inside the air gap. These results are compared with the results that a system without heat storage would give. Both systems are assessed based on annual performance as calculated by a quasi-steady explicit model. The developed model can be used to calculate the operation of a Trombe wall as well as to supply explicit quasi-steady models with values for airflow rate inside the air gap for Trombe wall operation without mechanical ventilation. Feeding these values to a quasi-steady model developed by authors it was found that the increase of storage wall heat capacity, either changing the storage wall material, either using phase change materials, can offer better utilization of Trombe wall heat gains up to 35% yearly. Background: The present work aims to develop an analytical model for simulating the operation of a Trombe wall in a transient state taking into account the heat storage in the wall. Methods: A closed system of equations is developed, based on 5 energy balances and a series of assumptions and auxiliary relations, to calculate the operation of a wall Trombe with heat storage with an hourly time step. Results: Characteristics Trombe wall temperatures and mass flow rate through the air gap are calculated for typical days of 7 winter months. These are used for the calculation of utilizable heat gains from Trombe wall. Conclusions: The model that does not take into account heat storage predicts higher temperatures and air mass flow rate in the gap than the present model by 10%. However heat storage increase the utilizable heat gains by 35% compared with a system without heat storage.

Thermal design method of building wall considering solar energy utilization and thermal insulation

This study makes a breakthrough out of the original thermal design idea which is mainly aim to improve thermal insulation performance of building wall, putting forward thermal design method of building wall to consider solar energy utilization and insulation simultaneously. Firstly, a day is divided into daytime and nighttime, and the expressions of indoor heat gain through the wall during such two periods are derived respectively according to the harmonic response method. The weighted sum of daytime and nighttime heat gain, then, could be used as the evaluation index of wall thermal performance. Taking this index as objective function and wall thermal resistance as well as heat capacity per unit wall area having the corresponding thickness of the thermal resistance as variables, a method for determining the optimum wall thermal resistance and heat capacity is proposed. Finally, for the room without heating measures in Lhasa area, using the typical weather data in winter as input conditions, the optimal matching curve of wall thermal resistance and heat capacity taking into account both the effect of solar energy utilization and insulation is calculated, so are the optimal value of them under different indoor temperatures (0–12 °C, interval of 2 °C). The results show that, in Lhasa area, the south wall has great potential to improve indoor thermal environment by using solar energy. From the perspectives of wall’s “carrier” role of utilizable climatic resources, the higher thermal insulation performance doesn’t always bring better results.

The effect of the wall heat capacity on the temperature stratification and pressure rise during natural convection of hydrogen vapor in a vertical cylindrical vessel

The effect of the wall heat capacity on the temperature stratification and pressure rise during natural convection of hydrogen vapor in a vertical cylindrical vessel

Activation of catalysts in commercial scale fixed-bed reactors: Dynamic modelling and guidelines for avoiding undesired temperature excursions

During the exothermic gas phase activation (e.g. reduction) or passivation (oxidation) of a catalyst in a commercial scale fixed bed, the interaction of the heat wave propagation phenomenon and the (desired) exothermic gas-solid reaction(s) may give rise to local temperature excursions way beyond the adiabatic temperature rise of the reactions. Inspired by a decoking study by Westerterp et al. (1988), a full dynamic model for generic gas-phase catalyst activation and deactivation (reduction/oxidation) in an adiabatic fixed-bed reactor was developed. Counter intuitive effects were elucidated, e.g. for cases where a lowering of the reactant concentration or the presence of a significant reactor wall heat capacity can lead to a further increase of the maximum catalyst temperature. An easy-to-use expression was derived, initially using simplifying assumptions of full rate control by external mass transfer and neglecting heat losses and reactor wall effects. This was subsequently tested on its adequacy for more general cases accounting for (1) slower reaction kinetics, (2) significant heat losses and (3) the reactor wall heat capacity. Only for the latter effect the original expression had to be adjusted. The modified expression proved to be remarkably robust and remained relevant as a simple tool to prevent local temperature excursions also for slower reaction kinetics and significant heat losses.

In-situ rapid determination of walls’ thermal conductivity, volumetric heat capacity, and thermal resistance, using response factors

Accurate determination of walls' thermo-physical characteristics is a necessity for execution of energy conservation strategies in existing buildings. In practice, such data is not available because the current determination methods are time expensive and therefore rarely used. Based on the theory of Response Factors, a rapid transient in-situ method, Excitation Pulse Method, EPM, was introduced as proof of concept in a former article. In the present article, detailed conditions for accurate application of the method in heavy and multi-layered walls are further studied. Theory, simulations, and experiments are combined to determine the method’s performance in different types of walls, with specific attention to the effects of the walls’ thermal response time and the response factors’ time interval, leading to the accuracy of Rc-value determination. It is demonstrated that the two main thermo-physical properties of a wall, thermal conductivity and volumetric heat capacity, as well as the wall’s thickness can be determined using inverse modelling of the Response Factors. The ratios of the response factors have shown to determine wall’s minimum thermal response time and to give an indication of the wall’s composition. The use of longer time intervals has shown to be advantageous in terms of the accuracy and the performance of the method. Longer experiment times as a result of long time intervals are still considerably shorter than the time required for making measurements according to the current standards and other conventional methods.

Reduction the Effect of Heat Transmission for the Heat Capacity of Building Wall in Summer

For the wall model of a building affected by solar radiation, a one-dimensional transient thermal conduction analysis was conducted. The purpose of the analysis was to examine the effect of wall thickness and heat capacity on heat transfer. On the westward wall in summer, the temperature distribution indoor the wall became parabolic. Even after the evening, the heat flux direction was outdoor from the wall and indoor from the wall, even in the conditions where the sol-air temperature was higher than the indoor temperature. The re-emit of the outside surface continued from evening till the morning of the next day. In the daytime, the heat quantity that entered from the outdoor air into the wall body did not all flow through the room, but a part was re-emitted to the outdoor. Particularly in the case of materials with low thermal conductivity and high volumetric specific heat, the effect of re-emit was remarkable. Regarding the amount of re-emit, the woody material with a large volumetric specific heat and the glass wool with a small volumetric specific heat were compared. It was suggested that the heat capacity could reduce the heat flux.

The impact of thermal mass on cold and hot climate zones of Portugal

The aim of this paper is focused on the energy performance of buildings containing massive wall alternatives. The analysis comprised the comparison of the heating and cooling loads of seven characteristic wall configurations of one sample building with different dynamic internal heat capacity (ISO 13790:2008) in spite of the equal thermal resistance. The equal thermal resistance, as derived from simple steady-state condition, was imposed in order to allow research of effects solely attributed to the wall heat capacity on the building performance. A detached one floor dwelling exposed to different climate conditions in Portugal was analysed to illustrate the effect of the same wall in terms of energy demand during cold and hot weather conditions. A whole building dynamic modelling using EnergyPlus was employed for the energy analysis. The best thermal performance was obtained with massive walls that were located at the inner side, for a very heavy weight building and high building time constant.

Heat transfer of pulsating laminar flow in pipes with wall thermal inertia

The effects of wall thermal inertia on heat transfer of pulsating laminar flow with constant power density within the pipe wall are investigated theoretically. The energy equation of the fully developed flow and heat transfer is solved by separation of variables and Green's function. The effects of the pulsation amplitude and frequency, the Prandtl number and the wall heat capacity on heat transfer features characterized by temperature, heat flux and Nusselt number are analyzed. The results show that the oscillation of wall heat flux increases along with the wall thermal inertia, while the oscillation of temperature and Nusselt number is suppressed by the wall thermal inertia. The influence of pulsation on the average Nusselt number is also obtained. The pulsating laminar flow can reduce the average Nusselt number. The Nusselt number reduction of pipe flow are a little more remarkable than that of flow between parallel plates, which is mainly caused by differences in hydraulic and thermal performances of the channels.

Theoretical analysis of wall thermal inertial effects on heat transfer of pulsating laminar flow in a channel

The effects of wall heat capacity on heat transfer of pulsating laminar flow with constant heat input between two parallel plates are theoretically investigated. The analytical solution of the fully developed thermal profile under constant wall heat flux is obtained. The results show that both the fluid temperature and Nusselt number fluctuate periodically as the driven pressure pulsates. The effects of the wall thermal inertia and Prandtl number, as well as pulsation amplitude and frequency on the fluctuations are investigated. Compared with pulsating flow without considering wall thermal inertia, the fluctuation amplitude of the fluid temperature and Nusselt number is smaller while the mean temperature difference is larger. The average Nusselt number, for both considering and neglecting wall thermal inertia, is reduced by pulsating flow. The reduction is up to 10% and cannot be neglected.

A3606LS – Improved-Lumped Formulations for Heat and Mass Transfer in Adsorbed Gas Storage

The purpose of this study is to provide an improved-lumped formulation for simulating adsorbed gas storage operations. The approach is shown to reduce the original spatial-dependent PDEs to two time-dependent ODEs, as similar to the ones obtained using classical lumped-system analyses (CLSA), the main differences being an additional equation that is required for calculating the reservoir wall temperature, and modified Biot numbers. The average reservoir temperature is then calculated with the improved lumped formulation, using two different approximation schemes, and compared with the results of the CLSA and the one-dimensional formulation that originated all approximate lumped models. A case of slow discharge with negligible wall heat capacity is analyzed, and the results show that, even for a large Biot number, one of the proposed approximation schemes can reproduce the 1D results with reasonable accuracy, while the CLSA yields very different results.

Numerical Investigation on the Hydrogen-Assisted Start-Up of Methane-Fueled, Catalytic Microreactors

The hydrogen-assisted start-up of methane-fueled, catalytic microreactors has been investigated numerically in a plane-channel configuration. Transient 2-D simulations have been performed in a platinum-coated microchannel made of either ceramic or metallic walls. Axial heat conduction in the solid wall and surface radiation heat transfer were accounted for. Simulations were performed by varying the inlet pressure, the solid wall thermal conductivity and heat capacity, and comparisons were made between fuel mixtures comprising 100% CH4 and 90% CH4–10% H2 by volume. A significant reduction in the ignition (tig) and steady-state (tst) times was evident for microreactors fed with hydrogen-containing mixtures in comparison to pure methane-fueled ones, for all pressures and reactor materials investigated, with hydrogen having a direct thermal rather than chemical impact on catalytic microreactor ignition. The positive impact of H2 addition was attenuated as the pressure (and the associated CH4 catalytic reactivity) increased. Reactors with low wall thermal conductivity (cordierite material) benefited more from hydrogen addition in the fuel stream and exhibited shorter ignition times compared to higher thermal conductivity ones (FeCr alloy) due to the creation of spatially localized hot spots that promoted catalytic ignition. At the same time, the cordierite material required shorter times to reach steady state. Microreactor emissions were impacted positively by the addition of hydrogen in the fuel stream, with a significant reduction in the cumulative methane emissions and no hydrogen breakthrough. Finally, gas-phase chemistry was found to elongate the steady-state times for both ceramic and metallic materials.

Numerical investigation on the start-up of methane-fueled catalytic microreactors

Transient simulations have been performed in a plane-channel, methane-fueled microreactor made of either cordierite or FeCr alloy walls and coated with a platinum catalyst. A two-dimensional model for the flow domain was used, which included detailed catalytic and gas-phase chemical reaction mechanisms. In the solid wall, axial heat conduction and surface radiation heat transfer were accounted for. Simulations were performed by varying the inlet pressure, the solid wall thermal conductivity and heat capacity, the inlet velocity, and the equivalence ratio at fuel-lean stoichiometries. The effect of solid material properties as well as the impact of gas-phase chemistry and surface radiation on the ignition ( t ig ) and steady-state ( t st ) microreactor times has been assessed. An increase in inlet pressure from 1 to 5 bar induced a ∼50% reduction in both t ig and t st owing to the enhancement of the catalytic reactivity with rising pressure. A similar behavior was also attested when increasing the equivalence ratio from 0.4 to 0.6. Reactors with low wall thermal conductivity (cordierite material) exhibited shorter ignition times compared to higher thermal conductivity ones (FeCr alloy) due to the creation of spatially localized hot spots that promoted catalytic ignition. At the same time, the ceramic material required shorter times to reach steady-state. Higher inlet velocities reduced the time required for steady-state, however, at the cost of increased cumulative reactor emissions. Surface radiation heat transfer played a key dual role in the start-up process of low thermal conductivity channels. Radiation increased t ig by removing heat away from the initial hot spot, but from the other side it decreased t st due to a very efficient transfer of heat from the rear to the front of the reactor. Gas-phase chemistry elongated the steady-state times for both ceramic and metallic materials and impacted the emissions of catalytic microreactors.

Effect of wall properties on the behavior of bench‐scale reverse flow reactors

AbstractAlthough in big‐sized industrial fixed bed combustors the effect of the reactor wall in heat transfer phenomena can often be neglected, this is not so for smaller reactors. This article focuses on the study of the wall influence (especially the effect of the wall physical properties, such as thickness, heat capacity, and thermal conductivity) in the performance of bench scale reverse flow reactors. The reaction considered is the combustion of methane lean mixtures, very relevant for environmental applications, and useful as a model for VOC combustion. The study was carried out both experimentally (in a bench‐scale apparatus, equipped with a temperature control system that minimizes the heat exchange between the reactor and the surroundings) and by simulations (using a heterogeneous one‐dimensional model). The results confirm the importance of the wall effects on the performance of reverse flow reactors, and the influence of the physical properties of the wall is reported. So, higher wall thermal conductivity leads to a decrease in the reactor stability range. High wall heat capacity is also detrimental for the reactor performance, although its effect depends strongly on the range of the wall thermal conductivity considered. Finally, the stability range of reverse flow reactors is less affected by wall effects as the surface velocity increases. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Parametric space distribution effects of wall heat capacity and thermal resistance on the dynamic thermal behavior of walls and structures

The present work aims at the investigation of the combined space distribution effects of heat capacity and thermal resistance on the transient thermal behavior of a wall, seen as a continuum of distributed parameters. The physical system is compared with the idealized wall lumped parameter model and its thermal time constant is related to its effective wall heat capacity, defined as the fraction of the wall heat capacity which participates in a transient thermal process. The effect of the space distribution of heat capacity and thermal resistance on the effective wall heat capacity is investigated for a wide range of homogeneous and multilayer thermally insulated walls. It is derived that the decrease of thermal resistance in homogeneous walls leads to an increase of their effective heat capacity. However, the effects are remarkably stronger on the effective heat capacity of thermally insulated multilayer walls, in which when the thermal insulation layer is at the ambient side, it leads to a significant increase of effective heat capacity, although when it is installed at the room side it leads to very low effective heat capacity, irrespective of the wall thermal resistance. Based on the first order results from a simplified room model, it was subsequently found that the influence of these parameters on the effective heat capacity of the building envelope leads to significant effects on the transient thermal behavior, thermal time constant and stability of structures.

Wall heat loss from intermittently conditioned spaces—The dynamic influence of structural and operational parameters

The intermittent daily operation of the HVAC equipment is currently a method usually adopted for reasons of proper building energy management in geographical regions with temperate climatic conditions. Under these conditions, the influence of the building envelope and the specific operational schedule of the mechanical equipment under the effect of time-varying ambient temperature and incident solar radiation is responsible for the development of complex transient heat transfer phenomena, which leads to continuous heat exchange with the environment. The heat exchange through the building envelope, which determines its energy efficiency, is strongly influenced by several parameters along with the thermal characteristics of the building elements like the wall thermal resistance and heat capacity as well as the thermal time constant and operational conditions of mechanical equipment. In the present investigation, an analysis is developed for the evaluation of the daily quasi steady-state energy losses from composite walls under the effect of time-varying meteorological driving forces, when heating at the room side is intermittent. The effect of various significant design and system operational parameters like wall heat capacity, thermal time constant and the insulation layer configuration, daily heating cycle duration and phase, as well as room heat capacity and net heating power input are investigated on the quasi steady-state energy loss and daily room temperature swing under typical winter conditions.

The influence of heat capacity and its spatial distribution on the transient wall thermal behavior under the effect of harmonically time-varying driving forces

An analysis was developed for the theoretical investigation of heat capacity and of the effects of its spatial distribution on the thermal behavior of structural walls of uniform thermal resistance. The numerical solution of the governing differential equations under the appropriate initial and boundary conditions, allows the prediction of the time-varying heat flux during the transient heating of walls until quasi steady-state conditions are developed asymptotically. It is derived that as far as the fundamental physical behavior of a clear wall section is concerned, although the time depended heat flux is effectively damped and phased out as the wall heat capacity increases, the average quasi steady-state heat flux is constant, irrespectively of wall heat capacity, incident solar radiation intensity, radiation absorption at the exterior wall surface and particular weather conditions defining wall heat loss or gain. However, even though the average quasi steady-state heat flux is constant irrespectively of the wall heat capacity, its time depended value during the development of transient is strongly influenced by the spatial distribution of heat capacity at a direction perpendicular to the wall plane, something which is determined by the wall time constant.

Scaling parameter of CHF under oscillatory flow conditions

Critical heat flux (CHF) is reduced by flow oscillations. The reduction of CHF is significantly influenced by flow oscillation period and amplitude, heat capacity of test tube, and mean inlet mass flux. A scaling parameter of the temperature response of the tube wall was derived based on a lumped-parameter model of the tube wall's heat capacity. When this scaling parameter was applied to CHF under flow oscillations, the experimental data were successfully correlated. © 1999 Scripta Technica, Heat Trans Asian Res, 28(6): 541–550, 1999

Scaling Parameter of CHF under Oscillatory Flow Condition.

Critical heat flux (CHF) was reduced by flow oscillations, and the reduction of CHF was significantly influenced by the flow oscillation period, amplitude, heat capacity of test tube and mean inlet mass flux. A scaling parameter of temperature response of tube wall was derived based on a lumped-parameter model of tube wall heat capacity. Applying this scaling parameter to the CHF data under flow oscillation, the experimental data of CHF was successfully correlated.

Warm-up behavior of monolithic reactors under non-reacting conditions

The transient heat transfer characteristics of monolithic reactors are of major importance because the time for the reactor to reach the required reaction temperatures should be minimized. A common application is the automotive catalytic converter. A simple numerical model for the heat transfer in an adiabatic thin wall channel is presented, taking account of the gas-solid convection, axial wall conduction and heat capacity effects in the solid phase. With the aid of the numerical model and the dimensional analysis, the thermal response of the channel after a step change of the inlet feed gas temperature is studied. For the case of negligible axial conduction, the results can be expressed in a single curve using suitably modified coordinates. The critical design and operating parameters are recognized and discussed. The results can be also expressed using common engineering variables to provide reliable assessment of several design and operating parameters of a monolithic reactor, such as void fraction, cell density, space velocity, etc.