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The role of interzonal airflows is especially pronounced in naturally ventilated buildings. In such buildings, reversed airflows in the ventilation stacks might occur as well. This affects the air exchange rate and contaminant distribution in buildings. A significant increase in carbon dioxide concentration is a characteristic phenomenon for poorly ventilated rooms. This paper demonstrates the application of metabolic carbon dioxide concentration measurements for interzonal airflow estimation in naturally ventilated buildings. The presented method is based on the continuous measurements of CO2 concentration at one point in each zone. These measurements are used to estimate airflow pattern in a multizone building by applying an inverse analysis. The developed methodology employs an iterative Levenberg-Marquardt procedure to maximise the nonlinear likelihood function. The validity of the method was verified against measurements carried out in a single naturally ventilated room. Further, the method was applied to calculate the airflow pattern in two apartments in Poland, containing 4 and 6 zones. The obtained results revealed very poor ventilation in both investigated apartments and reversed airflow in exhaust ducts. The amount of fresh air entering the rooms was insufficient to ensure good indoor air quality. The developed methodology can be effectively used as a diagnostic tool to identify the potential problems with ventilation systems. [ABSTRACT FROM AUTHOR]

In the last few decades vertical axis wind turbines (VAWTs) have gained increased interest among researchers worldwide. This paper aims to present full two-way Fluid Structure Interaction (FSI) approach to model operation of the Savonius-type VAWT. The developed methodology couples the set of Unsteady Reynolds Averaged Navier-Stokes Equations (URANS) with the unsteady linear elasticity equations and dynamic mesh approach to comprehensively describe the deformations of the turbine rotor. The obtained results provide detailed information on the unsteady flow fields around the operating wind turbine as well as deformations of a turbine rotor caused by an interaction between solid and flowing air. This allowed us to determine the influence of the unsteady deformations of a turbine rotor on the instantaneous and average performance of a wind turbine. Computations were carried out for the wind speed equal to 15 m/s and the tip speed ratio equal to 0.8. The simulations were performed using the commercial ANSYS software. The obtained results showed that deformation of the turbine rotor increases the wind turbine power coefficient by around 5%. Moreover, it was shown that the centrifugal force has a significant effect on the rotor deformation and turbine operation, hence it cannot be neglected. • Development of the fully coupled Fluid Structure Interaction model of Savonius wind turbine. • Results of the fully coupled FSI model was compared with validated CFD model of the Savonius wind turbine. • The influence of the centrifugal force on the turbine operation was investigated. [ABSTRACT FROM AUTHOR]

The paper aims to present structural topology optimisation of the structural topology of the H-Rotor wind turbine combined with the one-way Fluid Structure Interaction (FSI) approach. The developed methodology couples the set of Unsteady Reynolds Averaged Navier–Stokes Equations (URANS) with the steady state linear elasticity equations and density based topology optimisation method. This approach allowed us to reduce the mass of the blades with respect to the maximum stress and deformation limits. The results obtained provide detailed information on the unsteady flow fields around the operating wind turbine, as well as optimised topology of the blade interior without affecting the external aerodynamic profile. The load of pressure profile from the CFD simulation was implemented at the point of rotation at which the force values observed on the blade surface were the highest. The centrifugal force was considered in the structural model. Computations were carried out for wind speed equal to 30 m/s and rotational velocity equal to 90 rad/s. The turbine operating parameters were chosen to represent extreme operating conditions. The results obtained showed that the blade mass, made of the same material, can be easily reduced by around 60% with the reduction of the observed stress values and deformation. [Display omitted] • One way FSI model of operating H-rotor wind turbine was developed. • Topological optimisation of blade under realistic loads has been carried out. • Two blade manufacturing processes in the optimisation have been considered. • Significant reduction of deformation and stress values and over 60% mass reduction. [ABSTRACT FROM AUTHOR]

The goal of the work is to determine unknown parameters of reactor heating in the process of biomass solar pyrolysis. The lab-scale reactor is heated by a xenon lamp, but the fraction of heat absorbed by the reactor is unknown, as well as the heat transfer coefficient from the reactor walls to the nitrogen flowing through internal reactor channels, and the overall heat transfer coefficient through the external reactor walls to the surroundings. The missing parameters are impossible to measure, therefore they need to be determined by solving the inverse problem. As the problem is strongly ill-conditioned, since the quantities are directly dependent on each other, two different inverse algorithms were used to retrieve them, namely, the Levenberg-Marquardt method and the Metropolis-Hastings method. The effectiveness of both approaches was assessed and then they were applied to the real data. The inverse problem was implemented in the MatLab software, while validation of the mathematical model and optimisation procedure was carried out with a CFD model built in Ansys Fluent 19.2. Calculations showed that 14.4% of the lamp power penetrates inside the reactor and heat transfer coefficients to the flowing nitrogen equal 8.74 W/m2K and 0.965 W/m2K to the surroundings. • Two inverse algorithms were used: Levenberg-Marquardt and Metropolis-Hastings. • The validation was carried out with a 3D CFD model. • Calculations showed that only 14.4% of the lamp power penetrates inside the reactor. [ABSTRACT FROM AUTHOR]

The article presents a review of works on mathematical modelling of biomass pyrolysis process. Models types, number of dimensions, captured phenomena, type and dimensions of the reactors and biomass particles as well as models validation methods were analysed. Particular attention was paid to descriptions of chemical reactions, which are the essence of biomass thermal decomposition simulation. The article briefly describes the most important issues related to biomass pyrolysis and provides a review of commonly used methods of modelling this process with emphasis on solutions "ready to employ" in CFD modelling. A summary of other review papers on this topic is also provided. The problem of mathematical modelling in the context of "model - an engineering tool" was also discussed. Mathematical model for an engineering applications must be prepared with consideration of necessary consensus between the calculations accuracy, calculation time, preparation efforts and analysis requirements. Therefore, in pyrolysis modelling context, the inclusion of chemical reactions and changes of heat transport conditions were considered as priority, while taking into account moisture evaporation and bed shrinkage should be conditioned by their actual contribution in the calculations accuracy and by their influence on the calculations time. • A review of research on mathematical modelling of biomass pyrolysis is presented. • Captured phenomena, kinetic models, properties change, reactor types are summarized. • Compromise between model precision and calculations costs is discussed. [ABSTRACT FROM AUTHOR]

The paper presents a Computational Fluid Dynamics (CFD) model of the cold alpha-type Stirling engine. The developed mathematical model comprises of Unsteady Reynolds Averaged Navier-Stokes (URANS) set of equations, i.e. continuity, momentum and energy equations. Moreover, the developed mathematical model comprises a non-equilibrium description of the regenerator, by the introduction of the energy conservation equation for the regenerator matrix. It was implemented in the framework of commercial CFD software ANSYS Fluent. The work presents an in-depth analysis of the influence of the numerical mesh and applied turbulence model on the obtained results. The model was used to analyse the influence of the regenerator on the performance of the cold Stirling engine at different operating conditions. It appeared that on an average application of the regenerator increased the engine specific work by 10 J/kg and this increase is significantly higher at a high rotational speed of the engine. At the same time, the engine efficiency averagely was increased by 10 percent points and again this increase was significantly higher at higher rotational speeds of the engine. Especially, the influence of the heat source and sink temperatures on the engine operation were analysed. It appears that the lower are temperatures of a heat source and sink the more pronounced is the presence of the regenerator in the engine. Finally, the influence of the elementary parameters: particle diameter and porosity of the solid matrix which constitute the regenerator filling was investigated. It was shown that for the analysed engine the optimal value of the regenerator packing porosity equals about 0.72 it results in the specific area of the regenerator equal around 1675 m2/m3. Obtained results can be used to analyse the possibility of application of Stirling engines to recover low-temperature exergy of regasified Liquified Natural Gas (LNG), which for small regasification units can be alternative for commonly utilised Ambient Air Vaporizers (AAV) and it aside for regasification of LNG it could produce electric energy. [ABSTRACT FROM AUTHOR]

This paper deals with the Bayesian estimation of flow parameters in a porous material based on the measurements of local temperature at selected points inside the packed-bed. The unknown probability distribution function of estimated parameters was sampled using a Markov Chain Monte Carlo (MCMC) method. The MCMC method is efficient in sampling unknown distributions, but it is extremely time-consuming. The Proper Orthogonal Decomposition was used to construct off-line, low order approximation of the porous media flow problem. This model was further incorporated into the Metropolis-Hastings algorithm to retrieve the posterior distribution of the unknown parameters. This approach allowed us to decrease the computational time necessary to generate an ergodic Markov Chain. The efficiency of the proposed approach was tested on the simulated data generated with commercial CFD tools, and finally, it was applied to the real data acquired on the dedicated experimental stand. • Research highlight are summarized as follows: • Development of the mathematical model of biomass drying process. • Development of the coupled POD-MCMC inverse methodology estimate unknown parameters of the model. • Analysis of the accuracy of the developed approach. • Application of the inverse method to estimate model parameters for a real measurement data. [ABSTRACT FROM AUTHOR]

This paper presents the computational fluid dynamics (CFD) model of small-scale a-type Stirling engine. The developed mathematical model comprises of unsteady Reynolds averaged Navier-Stokes set of equations, i.e., continuity, momentum, and energy equations; turbulence was modeled using standard Κ-ω model. Moreover, presented numerical model covers all modes of heat transfer inside the engine: conduction, convection, and radiation. The model was built in the framework of the commercial CFD software ANSYS FLUENT. Piston movements were modeled using dynamic mesh capability in ANSYS FLUENT; their movement kinematics was described based on the crankshaft geometry and it was implemented in the model using user-defined functions written in C programming language and compiled with a core of the ANSYS FLUENT software. The developed numerical model was used to assess the performance of the analyzed Stirling engine. For this purpose, different performance measures were defined, including coefficient of performance (COP), exergy efficiency, and irreversibility factor. The proposed measures were applied to evaluate the influence of different heating strategies of the small-scale α-type Stirling engine. [ABSTRACT FROM AUTHOR]

In the paper, a second order thermodynamic analysis of a small scale alpha-type Stirling engine is presented. The developed mathematical model is based on the Finite Time Thermodynamics (FFT) approach and it is derived from the differential time dependent equations of energy and mass conservation. The engine model consists of three spaces: compression, expansion and regenerator. In contrast to available models, the model presented in this paper assumes polytropic processes in the compression and expansion spaces, which corresponds to the non-ideal heat transfer in these spaces. The results, obtained using the developed second order model, were compared with the results obtained using the Computational Fluid Dynamics (CFD) analysis of the same engine. A very good agreement between the second order model and the CFD model was achieved. Finally, the developed model was applied to analyse potential to recover cryogenic exergy of the Liquefied Natural Gas (LNG). The paper presents the results of the dimensional upscaling of the engine and the influence of the average working pressure. It was revealed that increasing the size of the engine or the average working pressure moves the engine to practically unachievable working conditions. [ABSTRACT FROM AUTHOR]

The main purpose of this article is to verify and validate the mathematical description of the airflow around a wind turbine with vertical axis of rotation, which could be considered as representative for this type of devices. Mathematical modeling of the airflow around wind turbines in particular those with the vertical axis is a problematic matter due to the complex nature of this highly swirled flow. Moreover, it is turbulent flow accompanied by a rotation of the rotor and the dynamic boundary layer separation. In such conditions, the key aspects of the mathematical model are accurate turbulence description, definition of circular motion as well as accompanying effects like centrifugal force or the Coriolis force and parameters of spatial and temporal discretization. The paper presents the impact of the different simulation parameters on the obtained results of the wind turbine simulation. Analysed models have been validated against experimental data published in the literature. [ABSTRACT FROM AUTHOR]

In many engineering applications one can find periodically variable gas temperatures. Possible examples include the temperature inside the cylinders of Stirling Engines and other reciprocating machines. New methods are proposed and evaluated for the measurement of periodically varying temperature by thermocouples. A Computational Fluid Dynamics model is created to assess the method viability and efficacy in a realistic setting while avoiding measurement errors. The first method is based on matrix analysis, the two others on the particle filter algorithm used to estimate either the heat transfer coefficients with the additional use of other flow information, or to estimate the thermocouple constants without said information. It is shown that the matrix method is sufficient for cases with constant velocities. For variable velocities, the PF algorithm shows promising results as based on measurements generated from the numerical simulation. • Periodic gas temperature measurement is treated as an inverse heat transfer problem. • Two-thermocouple mechanistic solutions are generalized into an overdetermined system. • Application of 'Particle filter', utilizing other heat transfer information. • Methods are tested using CFD generated measurement data. • Particle Filter provides a high quality reconstruction of the actual temperature. [ABSTRACT FROM AUTHOR]

A 2D transient model of coupled heat and mass transfer phenomena occurring in a single coke chamber is developed. The interior of the chamber is partially filled by the coal charge, which is assumed to be a porous media. The standard equations of continuity, species transport, momentum and energy describing the flow of the single-phase fluid are applied. The model considers a range of aspects associated with the carbonization process, such as the evaporation and condensation of the water, the evolution of the volatile components, the change in the density and the flow resistance during the coking process. Conductive, convective and radiative types of heat transfer are included in the model. The model also provides for the creation of the lateral gap near the walls. Water evaporation and condensation are modeled with the modified Hertz–Knudsen equation linked to the sorption isotherm. The model of the specific process is implemented in the commercial CFD software ANSYS Fluent. Results of the CFD simulation are compared with thermocouple measurements performed at the Centre de Pyrolyse de Marienau test unit. Commonly known information regarding raw gas evolution and energy consumption are also used for validation purposes. The model agrees well with both experimental data and data from the literature. [ABSTRACT FROM AUTHOR]

It has previously been shown heuristically that the topology of the Universe affects gravity, in the sense that a test particle near a massive object in a multiply connected universe is subject to a topologically induced acceleration that opposes the local attraction to the massive object. It is necessary to check if this effect occurs in a fully relativistic solution of the Einstein equations that has a multiply connected spatial section. A Schwarzschild-like exact solution that is multiply connected in one spatial direction is checked for analytical and numerical consistency with the heuristic result. The T1 (slabspace) heuristic result is found to be relativistically correct. For a fundamental domain size of L, a slow-moving, negligible-mass test particle lying at distance x along the axis from the object of mass M to its nearest multiple image, where GM/c2 << x << L/2, has a residual acceleration away from the massive object of 4ζ (3)G(M/L3) x, where ζ (3) is Apéry's constant. For M∼1014M⊙ and L∼10-20h-1 Gpc, this linear expression is accurate to ±10% over 3 h-1 Mpc ⪅ x ⪅ 2 h-1 Gpc. Thus, at least in a simple example of a multiply connected universe, the topological acceleration effect is not an artefact of Newtonian-like reasoning, and its linear derivation is accurate over about three orders of magnitude in x. [ABSTRACT FROM AUTHOR]

The disclosing of new diffusion frontiers for wind energy, like deep-water offshore applications or installations in urban environments, is putting new focus on Darrieus vertical-axis wind turbines (VAWTs). To partially fill the efficiency gap of these turbines, aerodynamic developments are still needed. This work in particular focuses on the development of a mathematical model that allows predicting the possible performance improvements enabled in a VAWT by application of the Gurney flaps (GFs) as a function of the blade thickness, the rotor solidity and geometry of the Gurney flap itself. The performance of airfoil with GFs was evaluated by means of detailed simulations making use of computational fluid dynamics (CFD). The accuracy of the CFD model was assessed against the results of a dedicated experimental study. In the simulations, a dedicated method to simulate cycles of variation of the angle of attack similar to those taking place in a cycloidal motion (rather than purely sinusoidal ones) was also developed. Based on the results from CFD, a multidimensional interpolation based on the radial basis functions was conducted in order to find the GF design solution that provides the highest efficiency for a given turbine in terms of airfoil and solidity. The results showed that, for the selected study cases based on symmetric airfoils, the GF positioned facing outwards from the turbine, which provides the upwind part of the revolution, can lead to power increments ranging from approximately 30% for the lower-solidity turbine up to 90% for the higher-solidity turbine. It was also shown that the introduction of a GF should be coupled with a re-optimization of the airfoil thickness to maximize the performance. [ABSTRACT FROM AUTHOR]

• A zero dimensional model of an α type Stirling engine is created and explained. • Heat transfer models for internal convection in the engine are compared. • Those correlations are implemented in the model and the results are shown. • It is found the models produce starkly different results. In the paper the authors present a zero-dimensional model of an alpha type Stirling engine and special consideration is paid to the problems of the evaluation of internal heat transfer between the heat exchange surfaces and the working fluid. Furthermore, a thermodynamic model of the engine, developed by the authors is shown. The model can be classified as zero-dimensional. Energy and mass conservation equations are written in a differential form, and solved for finite time steps for control volumes, that is the expansion cylinder, the compression cylinder, the regenerator, the regenerator matrix and the engine as a whole, which allows to transform the differential equations into a solvable system of algebraic equations. Unlike most models shown in literature, the one created by the authors evaluates the gas properties based on a real gas model. The impl ementation of a real gas model further helps to evaluate parameters such as viscosity and thermal conductivity of the gas, which is necessary for calculating the heat transfer coefficients. Those are calculated for each volume from known correlations, for a given moment in time. That is, the gas parameters needed are not evaluated as an average for the stroke, rather, they are calculated from instantaneous gas parameters and implemented as variables in the energy equations. The engine working fluid is atmospheric air. The influence of different heat transfer models on the predicted engine performance is shown in the form of engine characteristics for different models, based on variables such as pressure, and rotational. It can be shown, that further research into in-cylinder heat transfer is necessary, as the established models, when implemented into the Stirling engine, predict starkly different engine performance. Furthermore, non-physical results based on those models are shown. [ABSTRACT FROM AUTHOR]

The paper presents a thermo-ecological analysis of a liquefied natural gas regasification system equipped with a Stirling engine. The proposed system includes an alpha configuration machine, working as a cold engine with the heat sink acting as the evaporator for the LNG and the upper heat source being at ambient temperature (sea water) or moderately higher. The engines are modelled using two approaches developed by the authors, that is a CFD model and a second-order one-dimensional model. First a small engine is modelled by both CFD and the second order approach to compare results and after confirming their similarity, due to a lower computational cost a larger machine is modelled with the second order model. Due to the heat transfer conditions being crucial for a Stirling cycle machine's performance, those are investigated. It is shown that the internal, convective heat transfer coefficients are much higher in cryogenic conditions than in typical ones. The results of engine assessment are used for the Thermo-ecological cost (TEC) analysis. Within the thermo-ecological analysis, the influence of selected parameters is demonstrated. Additionally the TEC of electricity generated in cold power plant is presented for different configurations. In the case of analysed Stirling systems savings of natural gas reach the level of only 0.5%. However, when TEC of electricity is calculated, the savings are at the level of 72%. The proposed theoretical investigations are the base for the acquisition of new knowledge in the field of possible ways of converting cryogenic exergy of LNG into electricity and optimization of this cycle with strong focus on ecological effects, including the TEC. [ABSTRACT FROM AUTHOR]

Abstract: A CFD-based numerical analysis of the flow irreversibility in R744 ejectors is presented. A validated CFD tool was used to investigate three cases that were differentiated by the mass flow rate per unit area (mass flux) that passed through the mixer, which represented three dissimilar flow patterns. The mixer mass flux was found to significantly affect the ejector performance both locally and globally. An original approach was introduced to assess the contribution of the local irreversibilities to the overall entropy increase. A new factor was proposed to evaluate the ejector performance based on the reference entropy increase in a classic expansion valve. In addition, the influence of the mixer diameter and length on the ejector performance was numerically analysed, which showed that the effects of both geometric parameters may be significant. Namely, in the conditions considered, both enlargement of the mixer cross section area by 17.4% as well as shortening the mixer length by 33.3% resulted in the increase of the overall entropy growth rate by 8.9% and 5.4%, respectively. [Copyright &y& Elsevier]