Effect Of Thermal Inertia Research Articles

Numerical investigation of conjugate heat transfer and entropy generation of MHD natural convection of nanofluid in an inclined enclosure

PurposeThe purpose of this paper is to investigate conjugate heat transfer of natural convection and entropy generation of nanofluid in the presence of external magnetic field via numerical approach in an inclined square cavity enclosure.Design/methodology/approachControl volume finite volume method with collocated arrangement of grids was used for discretization of continuity, momentum, solid and fluid energy equations. Rhie and Chow interpolation technique was applied to avoid checkerboard problem in pressure field and the well-established SIMPLE algorithm was followed to deal with the pressure and velocity coupling. The cavity is filled with water and nanoparticles of the aluminum oxide (Al2O3). This study has been conducted for the certain pertinent parameters of the volume fraction of nanoparticle (φ = 0–0.08), the angle of inclination (ϴ = 0°–330°), the Ra number (Ra = 103–108), the solid to fluid conductivity ratio (ksf = 1–400), the Ha number (Ha = 0–80) and the wall thickness ratio (δ/L = 0–0.3).FindingsThe results indicate that averaged Nu number increases by approximately 9% by increasing volume fraction from 0.0 to 0.08. Nu increases with an increasing inclination angle to 40° and decreases abruptly in 90° because of the formation of two weaker vorticity with opposite circulation pattern intensifying the density of isotherm curves in a vertical direction. Nu increases sharply with increasing Ra more than 105. Nu also augments almost 67% by increasing ksf = 1 to ksf = 50 and remains constant by increasing ksf more than 50. Nu number reduction is almost 72% with a variation of wall thickness ratio from d/L = 0 to 0.3. Entropy generation because of fluid flow, magnetic field and heat transfer reduces linearly almost 30%, 19% and 16% by increasing volume fraction, respectively. With increasing ksf, entropy generation because of fluid flow, magnetic field and heat transfer increases asymptotically, but Bejan number decreases.Originality/valueA brief review of conducted research studies in nanofluid flow and heat transfer reveals that the effect of wall thermal inertia was not investigated in MHD natural convection of nanofluids in an inclined enclosure. The aim of the present study is to analyze conjugate heat transfer in an inclined cavity filled with water and Al2O3.

The characteristics of a 1 m methanol pool fire

A series of measurements was made to characterize the structure of a 1 m diameter methanol (CH3OH) pool fire steadily burning with a constant lip height in a quiescent environment. Time-averaged local measurements of gas-phase temperature were conducted using 50 μm diameter, Type S, bare wire, thermocouples with a bead that was approximately spherical with a diameter of about 150 μm. The thermocouple signals were corrected for radiative loss and thermal inertia effects. The mass burning rate was measured by monitoring the mass loss in the methanol reservoir feeding the liquid pool. The heat release rate was measured using oxygen consumption calorimetry. The heat flux was measured in the radial and vertical directions and the radiative fraction was estimated, which correspond to previous results.

Improvement of thermal inertia effect in buildings using shape stabilized PCM wallboard based on the enthalpy-temperature function

The use of phase change material as an efficient way to use building energy has recently been discovered as this material occupies 40 % of the total carbon emissions through energy used in the building sector. In order to apply phase change materials to buildings, phase stabilization must first be achieved; some researchers have developed shape-stabilized phase change material (SSPCM). In this study, the enthalpy-temperature function based on the thermal properties of 22 types of SSPCMs were analyzed and applied to a dynamic energy simulation program. The SSPCM was applied to improve the low heat storage performance of wooden buildings along with building energy savings. The SSPCM was applied to the inner side of a 20-mm-thick external wall in a case study concerning the inside and outside of an external wall. An analysis of the annual energy consumption of buildings showed that applying SSPCM resulted in average savings of 5 %. To confirm the improvement in the heat storage performance of buildings, the indoor temperature behavior during the heating and cooling periods was analyzed. Maintaining the thermal inertia of SSPCM was found to have reduced the peak temperature in summer by 4.1 °C.

The effect of PCM layer on the natural air flow movement in the façade cavity of BiPV system

The incorporation of a phase change material (PCM) in building integrated photovoltaic (BiPV) façade can be applied to improve its thermal performance including more effective electrical conversion, however their real interaction needs to be investigated at adequate level. The aim of the presented paper is focused on the application of a material with high latent heat capacity based on PCM in the structure of double-skin BiPV façade. The key aspect concerns on an analysis of natural air flow movement affected by the storing/releasing heat energy from the PCM during daytime/night-time of days. The experimental campaign was performed using an experimental outdoor test cell where two full-scale façade samples (reference BiPV and experimental BiPV/PCM) were investigated. The strong effect of thermal inertia of the PCM identified by experimental measurements was observed on the façade air cavity temperature and air flow movement. Experimental results revealed that natural air flow regimes in façade cavities are nearly equal in the daytime, but in the night-time are totally different. The thermal emission from PCM increase the air temperature in the façade cavity and increase the velocity of air flow movement (PCM started solidified), mainly in the night-time.

Soil Thermal Inertia Effect on Shallow Basement Energy Performance in Different Morocco Climates

Abstract This study deals with the energy performance of a shallow basement in six climates ranging from cold to desert climate. A case study of a partially buried house located in the hot semi-arid climate of Marrakech (Morocco) is first monitored during 2 months in summer and 2 months in winter. A trnsys model of this house was built and validated against the monitoring results. Simulation results show that the annual thermal load of the partially buried house is always lower than that of the corresponding above ground building house except for the Atlantic climate. Indeed, burying the house at 1.2 m reduces its annual thermal load by 19% at least in the Mediterranean climate and by about 39% in the cold climate.

Dynamic behavior of supercritical organic Rankine cycle using zeotropic mixture working fluids

As a prominent technology for recovering low-grade waste heat, supercritical organic Rankine cycle (ORC) exhibits a better performance due to the higher endothermic temperature and better thermal match with the heat source. The irreversibility in system can be reduced greatly when the zeotropic mixtures are used as working fluid to match the heat source and sink profiles. Affected by the fluctuations in waste heat sources, there is a challenge for ORC to recovery waste heat. An improved dynamic model of supercritical ORC using zeotropic mixture R134a/R32 as working fluid is developed and dynamic behaviors of supercritical ORC are analyzed. It is found that an abnormal fluctuation may occur in some parameters due to the effects around pseudo-critical point and thermal inertia of the heater. A fitting correlation to predict the response time based on different heat transfer coefficients of heater is found. Besides, three dynamic regimes are defined to investigate the effects of heat source frequencies and system thermal inertia on the dynamic response. As the heat source frequency increases or the heat exchange in the heater is enhanced, the fluctuation amplitude of the pressure decreases in the heater.

How much is the accuracy of activation energy affected by ignoring thermal inertia?

AbstractThis work estimates the magnitude of the effect of thermal inertia on the value of the activation energy determined from heat‐flux differential scanning calorimetry (DSC) data. The estimates are obtained via analysis of the literature data on crystallization of copper and thermal degradation of isotactic polystyrene (iPS). The copper crystallization data have been obtained for very large masses (200 mg) and fast heating rates up to 80 K min−1. The iPS degradation data have been collected on small masses (3 mg) and at the heating rates up to 20 K min−1. For crystallization of copper, the Kissinger activation energy obtained from the DSC data corrected for thermal inertia is 34% larger than the value estimated from uncorrected data. This difference drops to 8% and becomes statistically insignificant when the fastest heating rate used is decreased to 10 K min−1. For iPS degradation, the difference in the isoconversional activation energies estimated, respectively, from corrected and uncorrected DSC data is less than 3% and is not statistically significant. Overall, the effect of thermal inertia on the activation energy appears negligible provided that DSC measurements are conducted on smaller samples and at slower heating rates, that is, as advised by the International Confederation for Thermal Analysis and Calorimetry (ICTAC) recommendations. It is suggested that the difference in the activation energies should generally be within the typical 5‐10% uncertainty as long as the product of the time constant and the maximum heating rate does not exceed 2‐3 K.

Model reduction in computational homogenization for transient heat conduction

This paper presents a computationally efficient homogenization method for transient heat conduction problems. The notion of relaxed separation of scales is introduced and the homogenization framework is derived. Under the assumptions of linearity and relaxed separation of scales, the microscopic solution is decomposed into a steady-state and a transient part. Static condensation is performed to obtain the global basis for the steady-state response and an eigenvalue problem is solved to obtain a global basis for the transient response. The macroscopic quantities are then extracted by averaging and expressed in terms of the coefficients of the reduced basis. Proof-of-principle simulations are conducted with materials exhibiting high contrast material properties. The proposed homogenization method is compared with the conventional steady-state homogenization and transient computational homogenization methods. Within its applicability limits, the proposed homogenization method is able to accurately capture the microscopic thermal inertial effects with significant computational efficiency.

Passive systems in traditional houses in Middle East areas: solutions and effects of natural ventilation

The Middle East area, has a specific culture and habits, enhanced by adaptation to the environment and local climate through the traditional building constructions where typical design, structures and materials represent the basis for passive strategies. In these hot-dry regions, many traditional houses consist of compact design structures with internal courtyard where the effects of thermal inertia and natural ventilation can be well exploited. This paper focuses on the analysis of passive techniques used in some houses located in Damascus old city, which are renowned for their distinctive adaptability. These traditional houses have a unique structure consisting of two levels: the first floor built with heavy mass stones, while the second has light mass (timber and mud). The presence of an internal courtyard on which several rooms of different size overlook, give rise to an appreciable natural ventilation (single side or cross ventilation) through a proper opening of windows. Based on experimental data (air temperature, air velocity) acquired during a summer period in a traditional house, dynamic simulation for different type of structures and opening are performed in order to investigate the influence of natural ventilation on reducing internal temperature and evaluate comfort conditions using ASHRAE standard. Results also suggest strategies to optimize passive techniques to reduce energy needs.

River-Induced Anomalies in Seasonal Variation of Traffic-Emitted CO Distribution over the City of Krasnoyarsk

Seasonal variation of air quality in a city with a large river was investigated by means of numerical simulations of air movement and pollutant dispersion over inversion-capped diurnal cycles using a Reynolds-averaged Navier–Stokes (RANS) approach with algebraic turbulent flux model. The study accounts for the effects of urban heat island (UHI), terrain orography and high thermal inertia of the river body. The case mimics the real environment of the Krasnoyarsk region with the river Yenisei (Russia). Two scenarios were considered typical of the winter and summer seasons. The study is focused on the dynamics of dispersion of CO emanating mainly from road traffic, which remains fairly uniform throughout the year. The simulation starts from a mild low-altitude inversion with penetrative convection gradually developing over the daytime and attenuating during the night. The main difference between the two cases is in the temperature of the river surface relative to the ambient air. In winter, the non-freezing river acts as a source of positive thermal buoyancy, while in summer the cool river at the daytime acts in the opposite way, as a heat sink. The effect of the river-induced air circulation appears significant enough to account for the observed winter accumulation of the pollutant in the city center.

A Thermal Inertia Map of Titan

AbstractLike Earth, the atmosphere of Titan, Saturn's largest moon, is affected by the ability of the surface to store and release heat. Modeling efforts have shown that global temperature distributions and wind profiles differ between simulations describing Titan's surface with a uniform thermal inertia. Cassini‐Huygens demonstrated, however, that a variety of morphologies and compositions make up the Titanian landscape. Using data from Cassini RADAR and the Visible and Infrared Mapping Spectrometer, we classified the surface into five terrain types: dune, lake, hummocky, plains, and labyrinth. We estimated the thermal and physical properties (conductivity, specific heat, and density) for each type, creating a 1° × 1° global map of thermal inertia values. For the lakes, as the depth of convection is not yet known, we considered both still and convective bodies. Four simulations of the Titan Atmospheric Model were run with different surface thermal properties: a low homogeneous thermal inertia, a moderate homogeneous, the heterogeneous map with still lakes, and the heterogeneous map with convective lakes. In dry regimes (i.e., without the hydrological cycle), the differences between the four cases were generally minimal, suggesting that general circulation models can use a single (moderate) value for surface thermal inertia value for large‐scale investigations that do not consider diurnal variations. Given the importance of hydrological processes and regional spatial diversity on Titan, future work should consider the effects of nonuniform thermal inertia on Titan's climate on local and regional scales.

Measurement of heat flow through cast slabs of calcium sulfate dihydrate

SummarySlabs of cast calcium sulfate dihydrate have been used for decades as a heat transfer barrier in commercial and residential construction and are assessed in accordance with the parameters defined by ASTM C1396. The study described herein hypothesizes that minor impurities have significant effects on the high‐temperature performance of these casts. Five thermocouples are cast into a 20‐mm‐thick slab at approximately 4‐mm intervals are measured between ambient and approximately 1000°C in a furnace. Thermal diffusivity and inertia are estimated from this temperature profile. This study compares natural, flue gas desulfurized (FGD) and reagent grade hemihydrates that have been converted into the dihydrate form with distilled water; the common additives kaolin and borax are used to modify the cast. The thermocouple data allow an effective thermal diffusivity (α′) and an effective thermal inertia (I′) to be calculated. The hemihydrate source and kaolin content are found to affect the high‐temperature performance; the FGD source increases the thermal inertia, and kaolin inhibits the formation of other borate compounds through intercalation.

A double distribution lattice Boltzmann scheme for unsteady Conjugate Heat Transfer: The DD-CHT LB method

In this paper, we propose an innovative formulation of the “well-known” thermal lattice Boltzmann method for solving transient conduction heat transfer problems in heterogeneous media: the DD-CHT LB method for Double Distribution – Conjugate Heat Transfer. The main idea of this new formulation is to bring out a second distribution function that represent the effect of thermal inertia and its variation in a heterogeneous media. By adopting this original formulation we achieved several goals, mainly conservation of local properties of the studied media and ensuring the stability of the proposed formulation by avoiding any alteration of the “well-known” TLBM collision step. Validation of the formulation is fulfilled through three different benchmark conduction heat transfer problems in heterogeneous media. All results are compared with those obtained by the Control Volume Method (CVM) and excellent agreements were found.

A Two-Dimensional Numerical Analysis for Thermal Performance of an Intermittently Operated Radiant Floor Heating System in a Transient External Climatic Condition

Building thermal inertia and operation control strategies have impacted on the thermal performance of a radiant floor heating system. This study conducts a two-dimensional numerical analysis of an intermittently operated radiant floor heating system using the Re-Normalization Group model with Discrete Ordinates Radiation model. A detailed numerical simulation setups and various analyses are provided, including grid independency analysis, initial condition, time step sizes and external boundary conditions. Three different weekend day intermittent operation strategies are investigated. The results showed that Case 3 designed with pre-heating of 20 h has better performance compared to Case 1 designed with pre-heating of 8 h and Case 2 designed with pre-heating of 14 h. The average indoor air temperature differences of approximate 2.1, 1.6 and 1.2 K are observed for Case 1, Case 2 and Case 3, respectively, when comparing two-time slot at 8:00am on Friday morning and Monday morning. This significantly highlights the effect of thermal inertia and the potential of energy saving due to the utilization of intermittent operation. Therefore, the current study presents numerical simulation potential in evaluating the radiant floor heating effects on indoor thermal environment, taking into account building thermal inertia and transient external climatic conditions.

Experimental investigation on heat transfer of different phase in variable polarity plasma arc welding

The heat-transfer properties of variable polarity plasma arc (VPPA) during the electrode negative (EN) phase and electrode positive (EP) phase in the real-time process are the key factors for the stability control and optimization of the welding process. In this paper, the heat transfer of different phase in the variable polarity plasma arc was investigated by measuring the real-time temperature of the heat capacity medium in the workpiece. Based on the heat transfer mechanism of VPPA in the phase of EN and EP, the influence rule of current on heat transfer characteristics and the correlation mechanism of heat transfer during EN and EP phases were studied. The experimental results show that with the same current increase in both phases, the heat transfer of arc has a more increase in EN phase. However, the heat transfer efficiency of the arc has a more decrease in EP phase due to its special arc behavior. Furthermore, the effect of thermal inertia between positive and negative polarity on the next phase of heat transfer could be ignored. That is to say, the positive and negative polarity currents have great influence on the heat transfer characteristics of each polarity, and this influence will not save to the next phase.

Influence of building envelopes, climates, and occupancy patterns on residential HVAC demand

Heating, ventilation and air-conditioning (HVAC) demand accounts for 40% of household energy use. While the impacts of local climates, building characteristics, and occupancy patterns on HVAC demand are all well-known, the interplay between these factors has not been systematically studied. This paper provides a quantitative analysis of the complex interplay between these parameters on the annual and peak HVAC demand of residential buildings. Australia is used as a case study because the building construction practices are similar to many other countries, particularly in Europe and the USA, and the Australian climates span the majority of climates found in the world. The wall types considered are cement render, timber clad, brick veneer, reverse brick veneer, and cavity brick. The climates range from tropical in the north of the country, to cold temperate in the south of the country. The occupancy patterns are modeled using six common Occupancy Scenarios. The results show the highest energy reductions are observed in the climates with high diurnal temperature variation, such as Melbourne (51%) and Hobart (54%). The effect of thermal inertia is less in the climates with low diurnal temperature variation such as Darwin (19%). Occupancy scenarios that include unoccupied periods result in lower annual HVAC demand, yet they increase the peak cooling and heating demand. Increasing the indoor thermal mass by using brick veneer internal walls is shown to reduce the demand by approximately 10–15%.

Revealing the thermal environmental quality of the high-density residential tall building from the Brazilian bioclimatic modernism: The case-study of Copan building

Looking back to the glorious years of Brazilian modernism between 1930 and 1964, the country´s architectural heritage reveals an environmental design approach still relevant to buildings of today. In this context, the COPAN building (1966), located in São Paulo (latitude 23°S), is one of the most iconic buildings of its time with 1.160 residential units encompassing various types of studio flats (called in Portuguese “kitnets”) as well as one, two and three bedroom apartments distributed across six blocks of independent vertical circulation accesses. This naturally ventilated tall building is recognized by its massive concrete curvilinear structure of 140 m high, shaded towards the north and northwest orientations by horizontal concrete brise-soleis and opened with a single glazed curtain wall on the south and southeast orientations. The thermal performance of the Copan building was investigated by means of fieldwork and analytical procedures. The fieldwork included measurements in loco of environmental variables in four residential units, during warm and cooler periods of the year (2016) and interviews with a sample of 100 occupants, this was followed by thermal dynamic computer simulations performed with the Thermal Analysis Simulation Software (TAS). The adaptive comfort model defined in ASHRAE 55-2013 was adopted as the performance criteria for analytical studies. One of the main conclusions of these technical studies was about the combined effect of thermal inertia, external shading and controlled natural ventilation resulting in steady thermal conditions, with air temperatures oscillating between 24 °C and 26 °C when external temperatures reach as high as 32 °C in a typical warm afternoon.

Thermal inertia effects of the structural elements in heat losses during the charcoal production in brick kilns

Brazil is the largest producer of charcoal from planted forests with 5.5 million tons in 2016. The Brazilian steel industry consumes 85% of the national production of charcoal from eucalyptus. The walls and floor of industrial brick kilns are built using isolation materials that minimize heat losses during the wood carbonization stage. However, the thermal inertia of these components represents additional heat that must be removed during the charcoal cooling stage, as reflected in the extended process time. This study aims to evaluate the effect of the thermal inertia of the kiln structural elements for the charcoal production. A CFD (Computational Fluid Dynamics) analysis was performed to simulate the heating and cooling of the system composed of wood, carbonization gases, brick walls and floor. A typical industrial kiln with capacity of 700 m3 was modeled and validated using a set of experimental measurements of temperatures during a 4-day carbonization stage with final temperature of 400 °C and an 8 day cooling stage. The temperature profile in the walls was linear, corresponding to a pseudo-steady state, where the thermal load increases with the pyrolysis time. The heat transfer at the floor is extensive; therefore, the adiabatic boundary condition cannot be imposed at the wood bed–floor interface. Our findings provide important information for the improvements in the kiln operation and allow establishment of consistent initial conditions of temperature and heat flux for kinetics models for charcoal cooling in kilns.

Transient analysis and optimization of a recuperative sCO2 Brayton cycle assisted by heat and mass storage systems

This article discusses the transient behavior of a supercritical CO2 recuperative Brayton cycle, which, in addition to standard components, includes heat and mass storage systems. The cycle's time dependent behavior is promoted by enforcing a temperature profile on the heat source, which mimics that of a hybrid power plant using an idealized solar radiation flux on top of a baseline energy input. The finite volume code developed allows the spatial and temporal discretization of key components of the cycle. The simulations show that the thermal inertial effects are critical and capable of drastically changing the cycles' power delivery. More specifically, the analysis shows that the overall system might need several days to reach a periodic representative operational pattern, suggesting that the commonly used steady state and quasi-steady state hypotheses must be carefully considered if the cycle is assisted by a heat storage. Also, the results indicate that there is a compromise between the dimensions of the heat storage and its medium's thermophysical properties. Finally, the calculations show that, given a proper control strategy for the heat storage's charging and discharging, its use can significantly extend the cycle's power output delivery, allowing a better fitting to the demand profile.

Collisionless magnetic reconnection in curved spacetime and the effect of black hole rotation

Magnetic reconnection in curved spacetime is studied by adopting a general relativistic magnetohydrodynamic model that retains collisionless effects for both electron-ion and pair plasmas. A simple generalization of the standard Sweet-Parker model allows us to obtain the first order effects of the gravitational field of a rotating black hole. It is shown that the black hole rotation acts as to increase the length of azimuthal reconnection layers, per se leading to a decrease of the reconnection rate. However, when coupled to collisionless thermal-inertial effects, the net reconnection rate is enhanced with respect to what would happen in a purely collisional plasma due to a broadening of the reconnection layer. These findings identify an underlying interaction between gravity and collisionless magnetic reconnection in the vicinity of compact objects.