Search

Grate boilers are widely applied in biomass combustion for heat and energy generation. However, grate-firing systems often suffer from low efficiency and high emission. Computational fluid dynamic (CFD) becomes increasingly popular as a flexible method to obtain the detailed combustion behaviors in the furnace, and to optimize the performance of existing grate boilers. This paper presents efforts toward a reliable baseline CFD model for an industrial biomass-fired grate boiler, where the structure of the fuel-bed was sufficiently considered. The combustion performances in the furnace were investigated experimentally and numerically by coupling the fuel bed and freeboard, to clarify the impact of bed structure on the simulation accuracy. Results show that when the bed structure has been taken into account, the predictions are in better agreement with the various measurements, as the deviations between them are all within 10 %. Moreover, the flame center drops, the total heat transferred to the water-cooled walls increases, and the residence time of flue gas is greatly prolonged from 2.76 s to 3.90 s, resulting in a distinct difference of the temperature and species patterns in the freeboard. The simulations will provide valuable theoretical guidance for the accurate modeling of grate-fired boiler. [Display omitted] • A baseline model is proposed for accurate modeling of biomass-fired grate boiler. • Effect of fuel-bed structure on the simulation of grate boiler is clarified in detail. • Bed structure greatly affects the predicted temperature and species patterns. • Flue gas residence time and heat transfer are also highly impacted by bed structure. [ABSTRACT FROM AUTHOR]

Numerous metrics and indices have been proposed for the assessment of thermal perception and comfort. Commonly used indices based on the estimate of the heat flow between the human and its environment includes the well-known Fanger's method, also known as the PMV-PPD thermal comfort model. Recently this thermodynamic approach has been expanded with the application of the second law of thermodynamics. In this paper, an exergy-based analysis of the relation between the human and its environment is presented. Heat and mass flow rate due to convection, radiation, evaporation and respiration are calculated and expressed in terms of exergy loss. This value is used in the modified Fanger's thermal comfort model, where the heat balance equation is replaced by the exergy loss. The paper builds upon prior studies on determination of internal and external exergy losses by revised and newly defined formulas taking into account clothing and indoor conditions. The results quantitatively show that thermal energy load in the original Fanger's PMV model could be substituted by exergy loss and it corresponds to a certain level of subjective assessed thermal comfort level. [ABSTRACT FROM AUTHOR]

Abstract The subject of this paper is the steam boiler nonlinear mathematical model. The furnace chamber heat transfer is modelled assuming that the flue gas temperature across the entire furnace chamber is constant. Only radiation heat transfer to the boiler waterwalls is modelled. Distributed parameter mathematical models are developed of the steam superheater individual stages: the superheater first stage, the platen superheater, the second and third stage of the superheater. In the same way, the boiler two-stage economizer is modelled. The flue gas and air temperatures downstream the air heater individual stages are determined using the ε-NTU method (effectiveness ε - the number of transfer units NTU). Due to such arrangement of the boiler heating surfaces and the heat transfer nonlinear character, iterative calculations of the steam, flue gas, and air temperatures are necessary. The developed mathematical model makes it also possible to determine temperatures of the tube outer and inner surfaces and external fouling for all heating surfaces of the boiler. The developed non-linear mathematical model of the entire boiler has high practical importance. The proposed mathematical model can be used to simulate the boiler operation in various conditions, allowing to determine and correct selection the parameters of boiler operation. A non-linear mathematical model of the boiler can also be used to design the individual stages of the superheater properly so as to achieve the desired temperature behind the individual stages. Determination of the local temperature of the tube wall allows selecting an appropriate grade of steel for the individual stages of the superheater. Also, the developed non-linear mathematical model can be used for boiler operation monitoring in on-line mode. Highlights • A nonlinear mathematical model of the entire steam boiler was presented. • Mathematical models of individual heating surfaces in the boiler were presented. • Results of the boiler operation simulations were presented. • Results of the boiler economizer operation were presented. [ABSTRACT FROM AUTHOR]

The project reported in this paper used CFD as a tool to investigate the effect of multi-pass on the shell side heat transfer of a heat exchanger system. The heat exchanger system is equipped with six vertical thermosyphons transferring thermal energy from a heat source (air) to a heat sink (water). The CFD model has been experimentally validated. The two-phase change processes inside the thermosyphons were not modelled during the simulation. Instead, the thermosyphons were treated as solid rods with a constant thermal conductivity, which was calculated theoretically by applying the thermal resistance analogy with the aid of convection, boiling and condensation correlations found in the literature. The heat source consists of multiple air passes on the evaporator section of the thermosyphons and two water passes on the condenser section. Three different arrangements on the evaporator section were investigated with one, two or three shell passes and the thermal performance compared for the three configurations. The investigation was performed at various inlet conditions: a range of air inlet temperatures (100, 150, 200 and 250 °C) and mass flow rates (0.05, 0.08, 0.11 and 0.14 kg/s). The water inlet conditions were kept constant (a temperature of 15 °C and a mass flow rate of 0.08 kg/s). The overall rate of heat transfer was obtained by both CFD and a theoretical model, and the results lay within 15% of the experimental data. The numerical predictions demonstrated that the k − ε Realizable turbulence model is a reliable tool for predicting heat transfer and fluid flow in such heat exchangers. [ABSTRACT FROM AUTHOR]

The Enhanced Geothermal System (EGS) creates an artificial geothermal reservoir by hydraulic fracturing which allows heat transmission through the fractures by the circulating fluids as they extract heat from Hot Dry Rock (HDR). The technique involves complex thermal–hydraulic–mechanical (THM) coupling process. A numerical approach is presented in this paper to simulate and analyze the heat extraction process in EGS. The reservoir is regarded as fractured porous media consisting of rock matrix blocks and discrete fracture networks. Based on thermal non-equilibrium theory, the mathematical model of THM coupling process in fractured rock mass is used. The proposed model is validated by comparing it with several analytical solutions. An EGS case from Cooper Basin, Australia is simulated with 2D stochastically generated fracture model to study the characteristics of fluid flow, heat transfer and mechanical response in geothermal reservoir. The main parameters controlling the outlet temperature of EGS are also studied by sensitivity analysis. The results shows the significance of taking into account the THM coupling effects when investigating the efficiency and performance of EGS. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the new approach of constructal theory has been employed to design shell and tube heat exchangers. Constructal theory is a new method for optimal design in engineering applications. The purpose of this paper is optimization of shell and tube heat exchangers by reduction of total cost of the exchanger using the constructal theory. The total cost of the heat exchanger is the sum of operational costs and capital costs. The overall heat transfer coefficient of the shell and tube heat exchanger is increased by the use of constructal theory. Therefore, the capital cost required for making the heat transfer surface is reduced. Moreover, the operational energy costs involving pumping in order to overcome frictional pressure loss are minimized in this method. Genetic algorithm is used to optimize the objective function which is a mathematical model for the cost of the shell and tube heat exchanger and is based on constructal theory. The results of this research represent more than 50% reduction in costs of the heat exchanger. [Copyright &y& Elsevier]

This paper addresses the optimal arrangement and design of a dual pressure heat recovery steam generator coupled to two steam turbines. A superstructure that embeds various alternative configurations is optimized considering the following two single objective functions: (a) the maximization of the total net power generation for a given total heat transfer area and (b) the minimization of the total heat transfer area for a given total net power. The optimal number of heat exchangers and pumps and how they should be connected are the discrete decisions. The dimensions and operating conditions are the continuous decisions. A discrete and continuous mathematical model is developed and logical propositions are used for discrete decisions. The results are compared with a reference case reported by other authors. The results indicated that the optimization of the proposed superstructure allowed to find a more efficient HRSG configuration. The obtained configurations differ from the configuration of the reference case in how the heat exchangers and pumps are connected. A considerable increase in about 8% of the total net power generation in (a) and a significant reduction in about 24% of the total heat transfer area in (b) are achieved when compared to the reference case. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a physical model is built to study the eutectic and progressive melting of salt solution in DSC (Differential Scanning Calorimetry) cells. The finite volume method is utilized to solve the heat transfer problem, and the general implicit source-based enthalpy method is employed to handle the absorbed latent heat during the phase change process. The resulting numerical solution is implicit in nature and although iteration on the time step is needed at each time level, the tridiagonal matrix algorithm can be utilized within the iterations leading to efficient and accurate solutions. The effects of the heating rate and the initial salt concentration on the kinetics of the melting process are examined. [Copyright &y& Elsevier]

Abstract: Combustion of pulverized coal in oxy-combustion technology is one of the effective ways to reduce the emission of greenhouse gases into the atmosphere. The process of transition from conventional combustion in air to the oxy-combustion technology, however, requires a thorough investigations of the phenomena occurring during the combustion process, that can be greatly supported by numerical modeling. The paper presents the results of numerical simulations of pulverized coal combustion process in swirl burner using RANS (Reynolds-averaged Navier–Stokes equations) and LES (large Eddy simulation) methods for turbulent flow. Numerical simulations have been performed for the oxyfuel test facility located at the Institute of Heat and Mass Transfer at RWTH Aachen University. Detailed analysis of the flow field inside the combustion chamber for cold flow and for the flow with combustion using different numerical methods for turbulent flows have been done. Comparison of the air and oxy-coal combustion process for pulverized coal shows significant differences in temperature, especially close to the burner exit. Additionally the influence of the combustion model on the results has been shown for oxy-combustion test case. [Copyright &y& Elsevier]

Abstract: This paper deals with the development of a phenomenological combustion single zone model of Diesel engine. The aim of this work is to build a model suitable for HiL (Hardware in the Loop), and thus to be able to run in Real-Time applications. The combustion sub-model estimates the heat release rate by the sum of two algebraic expressions related to premixed and diffusive burning process. The Wiebe correlation has been used to model each algebraic expression of the combustion sub-model. The ignition delay time required by the combustion sub-model has been modeled through an Arrhenius correlation and an algebraic simple correlation. An extensive identification analysis has been performed to express their dependence with respect to engine operating conditions. The global model has been built with the constraints of computational CPU load that characterize the real time simulations. The accuracy of the model in predicting the in cylinder pressure has been tested over a large set of measurements at different engine operating conditions. The comparison of in-cylinder pressure profiles with experimental traces has shown that the Arrhenius expression is less accurate than the simple correlation. However, these two approaches may be used in real time modeling. [Copyright &y& Elsevier]

Abstract: This paper presents a novel superstructure and optimization model for the simultaneous synthesis of process water and heat exchanger networks. This superstructure combines the water network and heat exchanger network using interconnecting hot and cold streams. The water network has been extended for both direct and indirect heat exchanges. In addition, opportunities for heat integration between hot and cold streams, splitting and mixing of the freshwater and wastewater streams are incorporated within the superstructure. The proposed model is formulated as a non-convex MINLP (mixed-integer non-linear program), where the objective is to minimize the total annual costs of the network. A new convex hull formulation is presented for identifying the streams' roles within the network. Three examples involving single and multiple contaminant problems are presented in order to illustrate the applicability and capabilities of the proposed superstructure and model. In all cases the resultant networks exhibit lower total annual costs, whilst the freshwater and utilities consumption are the same as reported in the literature. In addition, novel designs for heat-integrated process water networks with smaller or same number of heat exchangers are presented. [Copyright &y& Elsevier]

Abstract: Thermoelectric devices (TEs) can achieve direct conversion of heat and electricity by semiconductor materials, coupling of heat transfer and electric conduction is important to accurately predict the performance of TEs. This paper develops a general, three-dimensional numerical model of TEs with consideration of coupling of temperature field and electric potential field. The model is used to figure out the performance of thermoelectric coolers (TECs) with the temperature-dependent thermal conductivity, electric conductivity, and Seebeck coefficient of semiconductor materials. A miniature TEC is considered and Bi2(Te0.94Se0.06)3 and (Bi0.25Sb0.75)Te3 are selected as the n-type and p-type thermoelectric materials, respectively. The effect of parameters such as the temperature difference and the current is investigated under conditions of variable material properties as well as radiation and convection heat transfer occurred between the TEC and the ambient gas. The results show that the variable properties and the heat losses to the ambient gas have significant effects on the cooling capacity and the coefficient of performance (COP) of the TEC. Three-dimensional temperature distributions within the semiconductors is observed under convective boundary condition and it becomes remarkable at large temperature differences and high currents. [Copyright &y& Elsevier]

A Stirling engine configuration consisting of two cylinders, a regenerator and a sliding disc actuating mechanism (“swashplate”) is considered in this paper. A mathematical model, which combines fundamental and empirical correlations, and principles of classical thermodynamics, mass and heat transfer accounting for variable heat transfer coefficients, is developed. The proposed model is then utilized to simulate numerically the system transient and steady state response under different operating and design conditions. A system global optimization for maximum performance in the search for optimal parameters that lead to maximum cycle efficiency is performed with low computational time. Appropriate dimensionless groups are identified and the results presented in normalized charts for general application. The numerical results show that the two-way maximized system efficiency, , occurs when two system characteristic parameters, the ratio between the total swept volume during the expansion, and the total swept volume, , and the ratio between the heat transfer area of the hot side heat exchanger and the total heat exchange area, y, are optimally selected, i.e., . The two-way maximized cycle efficiency found with respect to the optimized parameters is sharp, in the sense that a 225% variation of the calculated efficiency values was observed within the range of tested configurations in this study, and “robust” (i.e., relatively insensitive) to the variation of several parameters, thus stressing the importance to be considered in actual design. It is also found that the twice-maximized cycle efficiency and the total engine work output increase monotonically with the temperature of the hot source, T h. As a result, the model is expected to be a useful tool for simulation, design, and optimization of Stirling engines. [ABSTRACT FROM AUTHOR]

Abstract: This work describes the application and the performance of a new radiation model in CFD calculations for the simulation of thermal radiation transfer effects on a fire scenario. A 3D Cartesian coordinates radiative heat transfer procedure based on coupling of the FTn finite volume method (FTnFVM) with the bounded high-order resolution CLAM scheme is developed. The narrow-band based weighted-sum-of-gray-gases (NB-WSGG) model is applied to take account of nongray effects by CO2, H2O and soot. To treat irregular boundaries, the present model used the blocked-off-region procedure. This radiation code is implemented in the Fire Dynamics Simulator (FDS), a Computational-Fluid-Dynamics-based fire model, where a the combustion is represented by means of the mixture fraction with a single step chemical reaction model and the Large Eddy Simulation (LES) is used to model the dissipative processes. Computational results with and without radiation effects are compared against available experimental data and quasi-steady state law correlations of in-rack storage fire, which consists a complex configuration of double tri-wall corrugated paper cartons placed onto a wood pallet. Sensibility analyses of spatial and angular grids demonstrate the improvements due to the FTnFVM and to the CLAM scheme in the configuration studied. Results show that the simulations of the flame height, the gas temperature and the gas velocity are strongly influenced by thermal radiation. Overall, simulations predicted closer profiles to the experimental results only when the nongray-sooting radiation model was incorporated and an over-prediction of the gas temperature and the flame height is found when radiation is neglected. A sensibility analysis has shown that the flame characteristics are strongly affected by the soot yield. [Copyright &y& Elsevier]