Steady-state Boundary Conditions Research Articles

Moisture Transport in Loose Fibrous Insulations under Steady-State Boundary Conditions.

This research aimed to compare the transport capacity of loose-fill mineral wool, cellulose fibers, and wood wool to transfer moisture under steady-state conditions. The tests were carried out in the heat flow meter apparatus, which created a constant thermal field, limiting samples of sorptive moistened materials. The thermal conductivity, stabilization time, and moisture content of the samples were measured. Based on the variation in the results, the dynamism of moisture transport in the materials was determined. Mineral wool samples showed the lowest sorption. As a result, the moisture transport in this material stopped the fastest. In the case of cellulose and wood fibers, moisture transport continued throughout the whole test procedure. It was noted that the amount of moisture transport is influenced primarily by the structure of the fibers, the moisture content, and the possible presence of air in the pores. The wetter the material, the faster the transport. The dynamism of moisture transport according to trends of thermal conductivity changes over time was analyzed. The greater the slope of the linear regression line, the greater the dynamics of change. The smallest dynamics of change were found for mineral wool, for which the measured slope was between -0.008 and -0.033. For cellulose and wood wool, the range of slope was from -0.141 to -0.210, and from -0.162 to -0.211, respectively. The results of this research may provide the basis for further work on buffering moisture in the adjacent internal layers of the frame walls.

Extended Development of a Fission Gas Release Behavior Model Inside Spherical Fuel Grains for LWR Reactors

Fission gas plays a significant role in fuel rod performance following accidents. The amount of fission gas increases dramatically under accidental conditions. This leads to a subsequent rise in the fuel rod internal pressure and temperature due to aggravated gap conductance between the fuel pellet and cladding. As a result, fuel rod performance degrades. Therefore, studying fission gas behavior is crucial for accident assessment and evaluating fuel rod performance. Minimizing the Impact of fission gas on fuel rods is essential for maintaining their integrity and safety within nuclear reactors. One important aspect of ensuring safety is predicting fission gas release (FGR). In this study, we presented an extended model to be used in light water reactors (LWRs). The FGR can be modeled using COMSOL Multiphysics with the finite element method. This modeling approach considers both normal and abnormal conditions, with the latter categorized as Class-II type incidents. The model assumes that the gas diffusion inside a spherical grain varies over time. By examining perfect sinks with gas production, perfect sinks without gas production, and imperfect sinks under steady-state conditions, different initial and boundary conditions are set. To validate the accuracy and universality of expressions used in the model, input parameters from other models and experiments are utilized. By comparing the model’s results with these inputs, the accuracy and applicability of the expressions can be confirmed. This validation process ensures that the model provides reliable predictions for fission gas behavior in fuel rods under both normal and abnormal operating conditions. Based on our findings, it is evident that the FGR fraction displays an upward trend as diffusion coefficients and temperatures rise. Conversely, larger grain sizes and higher linear heat generation rates are associated with a reduction in the FGR fraction. Notably, enhanced resolution leads to a postponed onset of FGR. Furthermore, the influence of the diffusion coefficient on the FGR fraction primarily stems from the interconnected effects of temperature and linear heat generation rate.

Laplace and State-Space Methods for Calculating the Heat Losses in Case of Heavyweight Building Elements and Short Sampling Times

Reducing heat losses through the building envelope is one of the most important aspects to be met if the targets set by the European Union are to be achieved. In order to obtain a more realistic energy demand, dynamic heat transfer simulations are used to calculate the energy consumption of buildings, since steady-state calculations do not take into account the thermal mass in buildings. These dynamic simulations employ methods based on analytical models since numerical models are unsuitable for longer time periods. The analytical models used herein fall into the category of conduction transfer functions (CTFs). Two methods for computing CTFs that are addressed in this research are the Laplace method and the State-Space method. The objective of this paper is to verify the efficiency of the Laplace and State-Space methods for calculating the energy demand of a building in the case of heavyweight building elements and shorter sampling times, and to provide a means for improving the algorithms used by these methods. The Laplace and State-Space method algorithms were implemented in Mathematica, and the results were compared to EnergyPlus and TRNSYS, which use similar algorithms to calculate energy demand. It was shown in this paper that for the heavyweight wall element and a time step of 0.25 h, the difference between the total energy transferred through the inner surface was about 31% for EnergyPlus and 78% for TRNSYS compared to the reference solution. For the lightweight wall element, the results were stable for the time step of 0.25 h, but for the time step of 0.1 h, the differences were 45.64% and 303% between EnergyPlus, TRNSYS and the reference solution, respectively, compared to the State-Space and Laplace methods for which the maximum difference was 12.03% with a time step of 0.1 h. While dynamic heat transfer simulations are better than calculations based on steady-state boundary conditions, they also have their limitations and could lead to unsatisfactory results for short sampling times and if not applied properly.

Estimation of Two- and Three-dimensional Spatial Magnet Temperature Distributions for Interior PMSMs Based on Hybrid Analytical and Lumped-parameter Thermal Model

A hybrid analytical and lumped-parameter thermal model is proposed in this paper to estimate the transient and steady-state two- and three-dimensional (2/3-D) spatial magnet temperature distributions for an interior permanent magnet synchronous machine (IPMSM). On the basis of a lumped-parameter thermal model (LPTM), the magnet analytical thermal models (ATMs) are synergized to establish the hybrid thermal model. By utilizing the transient or steady-state boundary conditions calculated by the LPTM, the ATMs are obtained by solving the heat conduction equations to estimate the spatial magnet temperature distributions in the 2-D horizontal-vertical and axial-vertical planes, which can also be extended to the 3-D temperature distribution by utilizing the multi-slice method. Meanwhile, the non-uniform magnet eddy current loss is also considered. Finally, both electromagnetic-thermal coupled finite element analysis method and experiments are used for verification based on a totally enclosed IPMSM.

Effect of steady-state and unstable-state inlet boundary on the thermal performance of packed-bed latent heat storage system integrated with concentrating solar collectors

The real-time change of solar radiation intensity leads to the instability of the outlet temperature of the collector, which has an important influence on the thermal performance of the thermal energy storage system. Therefore, the dynamic thermal performance of a packed-bed latent heat storage system integrated with a solar parabolic trough collector is studied in this paper. Moreover, the effects of different mass flow rates on the total charging capacity, total exergy input, total exergy storage and exergy efficiency of the system are investigated. The result indicates that the steady-state and unstable-state inlet boundary conditions show significant differences in thermal performance. Under the unstable-state boundary conditions, when the mass flow rate increases from 0.1 to 0.2 kg/s and 0.1–0.4 kg/s, the maximum temperature difference between the PCM capsules and the air decreases to 24.11% and 47.39%, respectively. The larger the mass flow rates, the smaller the temperature difference, which is the opposite of the steady-state situation. Under steady-state inlet temperature conditions, the exergy efficiency gradually decreases with the increase of mass flow rate. Under unstable-state inlet temperature conditions, the mass flow rate has little effect on the exergy efficiency, and which is about 41.00%.

A Computational Analysis of Heat Transfer in an Air-Cooled Proton Exchange Membrane Fuel Cell for Unsteady Flow Conditions Using a Reynolds Stress Turbulence Model

Within telecom applications, the air-cooled fuel cell, as a backup power system, has recently been gaining popularity. These fuel cells are cost effective, simple, and additionally they do not require a secondary coolant loop. However, since airflow is used as coolant, the heat generated by the fuel cell limits the current density and thus the power density. A turbulence inducing grid is placed in front of the cathode channel to make the airflow turbulent and hence increase the convective cooling effect from the air. The turbulence inducing grid is placed 2.5mm from the cathode channel since previous studies have shown this distance to be most effective.In this study, a three dimensional CFD analysis on the effect of a transient boundary conditions on the cooling effect of the turbulence grid has been conducted. Special emphasis has been on using the comprehensive Reynolds Stress Model as turbulence model. Additional analysis has been made on the cooling effect of pulsating flow with a period of 10 seconds excluding the turbulence inducing grid. At an electrode heat flux of 2600 W/m2, the results indicate that the convective cooling effect increases when using transient boundary conditions compared to steady state boundary conditions. Furthermore, results show that the start-up time is seven minutes, and the time-averaged peak temperature in the transient case is two degrees Celsius below the peak temperature of the steady state case. The CFD analysis show an additional cooling effect due to accelerated-decelerated flow conditions. Figure 1

Spatial and Temporal Changes of Groundwater Storage in the Quaternary Aquifer, UAE

To study the temporal and spatial variations of the groundwater quantity and quality in response to intensive groundwater exploitation from the Quaternary aquifer in UAE, a water budget model with a cell size of one km2 was developed. The available historical records of groundwater levels and salinity have been used to develop the water table and salinity maps of UAE for the years 1969, 2005, 2010, and 2015. The available water resources and soil information system was used to facilitate validity, cogency, and consistency of the groundwater analysis. The spatial analysis module of GIS was used to define the aquifer setting, saturated thickness, aquifer base elevation, effective porosity, and groundwater salinity at each grid cell. The obtained results indicated that the volume of fresh groundwater resources in the Quaternary aquifer in UAE has decreased from 238 km3 in 1969 to around 10 km3 in 2015. A major part of these depleted fresh groundwater resources was replaced by brackish water, and, therefore, the total groundwater storage in this aquifer has only decreased from 977 in 1969 to 922 km3 in 2015, respectively. If the same groundwater exploitation continues, the freshwater storage in the surficial aquifer might be totally depleted in agricultural areas. Most probably, the brackish groundwater resources will be exploited. In such areas, more attention should be devoted to the management of brackish water resources to avoid the exacerbation of the saltwater intrusion problem. Despite the fact that the obtained results indicate the negative impacts of the improper water resources management in a small part of the arid area, the learned lessons are valid for other arid countries, in particular, using the proper steady state boundary conditions for the initial conditions in modeling the available future management alternatives.

Development of a one-dimensional model of a closed thermosiphon for cooling a spent-fuel pool

Using closed thermosiphons for cooling the spent-fuel pools of nuclear power plants requires calculation tools for simulating the thermal performance of long thermosiphons. The ability of the computer program THERESA to simulate the thermal performance of a vertical thermosiphon with water as the working fluid has been investigated. THERESA was used for simulating a cylindrical thermosiphon with a one-meter-long evaporator and a 32-mm inside diameter. The condenser section had the same inside diameter and a length of 0.5 m. Modelling the long thermosiphons of the type required to span the distance from the spent-fuel pool to the atmosphere requires that the effects of hydrostatic pressure in the evaporator and the pressure drop in the vapor between the evaporator and condenser sections be correctly simulated.The input data to THERESA are the outside wall temperatures of the evaporator and condenser. The thermal resistances between these two surfaces consist of conduction through the walls, the thermal resistance due to vapor-pressure drop in the adiabatic section between the evaporator and the condenser, and the various modes of heat transfer in the liquid phase of the working fluid. The heat transfer in the liquid of the condenser was simulated by a mixture of drop-wise and film-wise condensation. The heat transfer in the evaporator was simulated by a mixture of natural convection, nucleate boiling, and film boiling. After simulating the thermal resistances, THERESA solves for the heat-transfer rate of the thermosiphon.The simulation results were compared with data from experiments with steady-state boundary conditions. The trends in the experimentally derived heat-transfer coefficients as a function of heat-transfer rate were used to identify the type of phenomena that dominate the processes in the evaporator and the condenser. Additional insight into the instabilities occurring during these experiments led to new methods for calculating the heat-transfer coefficients in THERESA. The number of various heat-transfer phenomena simulated in THERESA was reduced to those with the trends that matched the trends in the experimental data. The heat-transfer coefficients for those dominant phenomena were modified in order to represent a time-averaged value over an entire instability cycle. Empirical constants for the modifications were obtained from the experimentally derived values.

Performance and analysis of battery thermal management system used in electric vehicles (EVS)

The depletion of non-renewable energy resources is compelling us to search and innovate for more reliable options. The reduction in the use of petroleum in the automobiles can significantly affect the use of non-renewable energy resources. In the long run, electric vehicles are the best alternatives to the conventional IC engine automobiles. In electric vehicles, lithium-ion batteries are most commonly used as an energy. During discharging, excessive heat generation inside the battery will lead to thermal runaway. To operate efficiently, these batteries should be maintained within the desired temperature range of 25 °C to 40 °C. Among the various cooling methods, in this studyair cooling is adopted. The battery pack is discharged at various constant C ratings. The effect of airflow rate on the battery surface temperature is studied with experimentation. The heat transfer coefficient is calculated by considering the average surface temperature of the battery as the wall surface temperature. The CFD analysis is done by considering the steady-state condition and boundary conditions are taken with experimental observations. Experimental results are compared with CFD results with appropriate approximations.

Stress Redistribution Analysis of Piezomagnetic Rotating Thick-Walled Cylinder with Temperature-and Moisture-Dependent Material Properties

In this article, the problem of time-dependent stress redistribution of a piezomagnetic rotating thick-walled cylinder under an axisymmetric hygro-thermo-magneto-electro-mechanical loading is analyzed analytically for the condition of plane strain. Using the constitutive equations, a differential equation is found in which there are creep strains. Primarily, eliminating creep strains, an analytical solution for the primitive electric and magnetic potential in addition to stresses is obtained. Then, creep strains are kept and creep stress rates are found by utilizing Norton’s law and Prandtl-Reuss equations for steady-state hygrothermal boundary condition. Lastly, the history of stresses and radial displacement as well as magnetic and potential fields during the time is obtained using an iterative method. In the numerical examples, the effect of angular velocity, hygrothermal loading and thermal and moisture concentration dependency of elastic constants is investigated comprehensively.

Measurements of thermal Transmittance of an External massive timber wall in-situ and in the Laboratory

The thermal transmittance of an exterior massive timber wall was measured in situ in Appenzell, Switzerland according to the standard ISO 9869-1. The measurements were performed with two different measurement sets in parallel. The measurements started in February and stopped at end of April. The measuring data were analyzed using mean values of the thermal transmittance coefficient and of the thermal resistance following the procedure of ISO 9869-1. In order to clarify if the in-situ measurement results show significant deviations from the measurement results of the thermal transmittance obtained in the laboratory, the thermal transmittance of the identical wall construction was measured in the laboratory of Bern University of Applied Sciences in Biel according to the standard EN ISO 8990 for steady state boundary conditions in a guarded and calibrated hot box. The test results will be presented and the measurement setup will be described. The calculation value of the thermal transmittance coefficient of the massive timber wall according to EN ISO 6946 is U = 0.53 W/(m2K). The test results of the thermal transmittance coefficient, U-value of the wall, measured in the hot box, agreed well within a confidence level of 95 % with the calculated value. The in-situ measurement results of the thermal transmittance coefficient of the two measurement sets differ significantly in the order of 8 % referred to the calculated U-value of the wall as the basic amount. Furthermore, both in situ test results of the U-value of the wall show significant deviations from the calculated U-value up to 27 %.

Comparison of regularized ensemble Kalman filter and tempered ensemble transform particle filter for an elliptic inverse problem with uncertain boundary conditions

In this paper, we focus on parameter estimation for an elliptic inverse problem. We consider a 2D steady-state single-phase Darcy flow model, where permeability and boundary conditions are uncertain. Permeability is parameterized by the Karhunen-Loeve expansion and thus assumed to be Gaussian distributed. We employ two ensemble-based data assimilation methods: ensemble Kalman filter and ensemble transform particle filter. The formal one approximates mean and variance of a Gaussian probability function by means of an ensemble. The latter one transforms ensemble members to approximate any posterior probability function. Ensemble Kalman filter considered here is employed with regularization and localization—R(L)EnKF. Ensemble transform particle filter is also employed with a form of regularization called tempering and localization—T(L)ETPF. Regularization is required for highly non-linear problems, where prior is updated to posterior via a sequence of intermediate probability measures. Localization is required for small ensemble sizes to remove spurious correlations. We have shown that REnKF outperforms TETPF. We have shown that localization improves estimations of both REnKF and TETPF. In numerical experiments when uncertainty is only in permeability, TLETPF outperforms RLEnKF. When uncertainty is both in permeability and in boundary conditions, TLETPF outperforms RLEnKF only for a large ensemble size 1000. Furthermore, when uncertainty is both in permeability and in boundary conditions but we do not account for error in boundary conditions in data assimilation, RLEnKF outperforms TLETPF.

Multiphysical time-dependent creep response of FGMEE hollow cylinder in thermal and humid environment

In this paper, we investigate the history of radial displacement, stresses, electric potential, and magnetic potential of a functionally graded magneto-electro-elastic (FGMEE) hollow cylinder subjected to an axisymmetric hygro-thermo-magneto-electro-mechanical loading for the plane strain condition. The material properties are taken as a power-law function of radius. Using stress-displacement relations, equations of equilibrium, electrostatic and magnetostatic equations, we find a differential equation including creep strains. Initially, eliminating creep strains, we obtain an analytical solution for the primitive stresses and electric and magnetic potential. In the next step, considering creep strains, we find the creep stress rates by applying the Norton law and Prandtl–Reuss equations for steady-state hygrothermal boundary condition. Finally, using an iterative method, we find the time-dependent creep stresses, radial displacement, and magnetic and potential field redistributions at any time. In numerical section, are comprehensively investigate the effects of grading index, hygrothermal environmental conditions, rotating speed, and temperature- and moisture-dependency of elastic constant of FGMEE.

A New Triple-Switch-Triple-Mode High Step-Up Converter With Wide Range of Duty Cycle for DC Microgrid Applications

DC microgrid is gaining attraction and a recent trend in distribution power generation system due to penetration of renewables (especially photovoltaic or fuel cell). In this paper, a new triple-switch-triple-mode high step-up converter (TSTM-HS converter) is presented for dc microgrid applications. In the proposed converter, voltage lift technique is employed and range of duty cycle is extended by incorporating an additional switch in converter circuitry. By doing this, high voltage conversion ratio is achieved without using a transformer, coupled inductor, and multiple stages of switched capacitors. Moreover, the TSTM-HS converter operated in three modes with two types of the duty cycles to achieve low to high voltage conversion without using high duty cycle for each switch. The effects of difference in the inductance values on the regulation and operating behavior of the TSTM-HS converter are discussed. The continuous conduction mode and discontinuous conduction mode characteristics of the TSTM-HS converter are discussed in detail with steady-state analysis and boundary condition. The comparison is provided to highlight the benefits of the TSTM-HS converter. The selection of semiconductor devices and the design of reactive components are discussed for the TSTM-HS converter. The experimental results of the proposed converter are provided which validate the theoretical approach, performance, and feasibility of converter.

Simulation Study of Dry Floor Heating Systems

This case study is aimed at simulation of dry floor heating system. Heating pipes are inserted in system boards made of thermal insulation. These boards should be supplemented with spreader plates which are installed under the heating pipes. Impact of different thermal conductivity of spreader plates on useful heat flux and uniformity of temperature field is examined. Heat losses are also investigated. These simulations are performed using software CalA with time steady-state boundary conditions. The results show that the dry floor heating system without spreader plates has very low useful heat flux caused by positioning of heating pipes in the insulation material. On the other hand, use of spreader plates causes significant increase of useful heat flux of this system. The higher heat conductivity of spreader plates is, the higher useful heat flux is. The floor surface temperature is also more uniform and the thermal comfort is better. The minimal thickness of additional heat insulation is determined in order that heat losses are lower than ten percent of total heat flux.

Investigation on Steady-State and Transient Boundary Condition Scenarios for Optimizing the Creation of Multiport Surrogate Thermal Models

Board-level simulation has to consider, at the earliest stage of the conception, the impact of the vicinity of numerous high- and medium-powered devices. In 1996, the European consortium DELPHI defined the concept of a surrogate, or compact thermal model (CTM), to minimize the computation times from days to minutes. A DELPHI-style CTM resumes an electronic component as a simple cuboid form and a network of resistors that links a single temperature-sensitive node to the major surfaces of heat extraction. That steady-state network is derived from a detailed numerical model, which is subjected to a set of boundary conditions (BCs) applied on its external surfaces. At first, this paper investigates the combinations of various sets of scenarios to generate and to test a multiport surrogate model. The objective is to define the lowest number of numerical simulations required while keeping the highest accuracy level of the derived BC-independent (BCI) thermal network. Then, this paper demonstrates the need of a smart selection of the isothermal surfaces to create a most relevant surrogate model. Finally, using that calibrated steady-state model, an extension of the DELPHI method to create multiport dynamic surrogate model is proposed as well as a set of BCs dedicated to transient network fitting.

Measuring thermal performance in steady-state conditions at each stage of a full fabric retrofit to a solid wall dwelling

The methodology used for measuring the thermal performance of fabric retrofit systems which were applied to a solid wall UK Victorian house situated within an environmental chamber is explored in detail. The work describes how steady-state boundary conditions were approximated, then repeated at the Salford Energy House test facility. How established methods of measuring the fabric thermal performance of buildings in situ were adapted to test the effectiveness of retrofit measures within a steady-state environment. The results presented show that steady-state boundary conditions enable the change in fabric heat loss resulting from the retrofit of a whole house or individual element to be measured to a level of accuracy and precision that is unlikely to be achieved in the field. The test environment enabled identification of heat loss phenomena difficult to detect in the field. However, undertaking tests in an environment devoid of wind underestimates the potential reduction in ventilation heat loss resulting from an improvement in airtightness, and hides the susceptibility of retrofit measures to various heat loss mechanisms, such as wind washing. The strengths and weaknesses of the methods employed, the Energy House test facility, and a steady-state environment, for characterising retrofit building fabric thermal performance are demonstrated.

Computational method for the dynamics of railway tracks on a non-uniform viscoelastic foundation

The foundation of a railway track may be non-uniform due to a number of reasons and this area is currently the subject of significant research. In this article, a new method has been developed for computing the responses of railway tracks based on a non-uniform foundation subjected to moving forces. This method is a coupling of an analytical model for the rail together with the sleepers and a finite element method for the foundation. In steady-state, it is supposed that the responses are unchanged when the moving forces come and go away from a larger interval of the railway track which contains a non-uniform zone. The dynamical stiffness matrix (DSM) of the foundation is computed by the finite element method and it is transformed to meet the steady state boundary condition. On the other hand, the rail together with the sleepers and rail pads are modelled by a periodically supported beam subjected to moving forces. This analytical model leads to a relation between the reactions forces and the displacements of the sleepers. This relation describes also the degrees of freedom (DOFs) of the nodes of the foundation at the contact surfaces with the sleepers. Then, a transformation technique has been developed in order to substitute the analytical relation into the DSM. Finally, the responses have been computed by using the transformed DSM. This method is a coupling of the analytical and numerical methods. Therefore, it has reduced all DOFs of the track components (sleepers, rail, and rail pads) which gives a significant advantage in computational time.

Seasonal thermochemical energy storage: Comparison of the experimental results with the modelling of the falling film tube bundle heat and mass exchanger unit

This paper focuses on the assessment of a heat and mass exchanger dedicated to an absorption/desorption seasonal thermal energy storage system. The closed sorption heat storage based on water vapour absorption in aqueous sodium hydroxide (NaOH-H2O) solution theoretically achieves a significantly higher volumetric energy density compared to conventional hot water storage systems. To this purpose, a prototype designed for a power output of 10 kW was built and operated in both absorption and desorption mode under steady state boundary conditions. In this work, the influence of two main parameters on the exchanged power is evidenced. Furthermore, a comparison with the results of the initial numerical model used to design the heat and mass exchanger is carried out. On one hand it is found that, for the absorption process, the measured exchanged power is much lower than the numerically predicted value. On the other hand, for the desorption process the numerical and experimental results have the same order of magnitude. Physical explications of the strongly diverging results encountered during the absorption process as well as an improved heat transfer coefficient model for the desorption process are proposed.

Probability adjoint identification of airborne pollutant sources depending on one sensor in a ventilated enclosure with conjugate heat and species transports

The definition and governing equation of probability density function (PDF) have been implemented in the present work. The procedure is applied to the backward time identification of the pollutant probability density function history and finally the maximal probability density of airborne pollutant source location in a two-dimensional slot ventilated building enclosure contained with two solid blocks. Steady-state airflow field, sensor location, boundary conditions, thermo-physical properties and geometric characteristics should be known in prior. Spatial probability density function history has been computed under four ventilation modes, i.e., mixed ventilation (MV), displacement ventilation (DV), mixed ventilation with top ceiling outlet (MVS), and displacement ventilation with left side outlet (DVS). Effects of pollutant source location and alarming sensor on the accuracy of the probability density function have been disclosed. The pollutant sources will be more easily identified for the concentrated pollutant strips, where pollutant diffusion is efficient compared with the pollutant source at outlet. Particularly, the good agreement of the probability density function identified source location with the true situation fully shows that adjoint probability density function method is more competitive in the engineering applications involving with complicated convective fluid flows.