Armando Gallegos-Muñoz

Surface selection and design of plate-fin heat exchangers

Abstract This paper presents a methodology for the design of compact plate–fin heat exchangers where full pressure drop utilization is taken as a design objective. The methodology is based on the development of a thermo-hydraulic model that represents the relationship between pressure drop, heat transfer coefficient and exchanger volume. A simple approach to surface selection based on the concept of volume performance index (VPI) is also presented. Surfaces that result in the smallest volume will exhibit higher VPI. Surfaces are compared on the basis of VPI and envelopes for best performance are produced. Simultaneous surface selection and design for full pressure drop utilization can be achieved by using envelopes for best surface performance together with the thermo-hydraulic model. Design algorithms for cross-flow and counter-flow arrangements are presented and results tested by comparing them with case studies from the literature.

Thermal integration of heat pumping systems in distillation columns

A methodology for the thermal integration of electrically driven heat pumping systems between intermediate stages in distillation columns using the concept of column grand composite curve is proposed. An optimization procedure based on a four way trade off between energy, capital, temperature lift and electricity to fuel cost ratio is discussed.

Second Law Analysis of a Mobile Air Conditioning System with Internal Heat Exchanger Using Low GWP Refrigerants

This paper investigates the results of a Second Law analysis applied to a mobile air conditioning system (MACs) integrated with an internal heat exchanger (IHX) by considering R152a, R1234yf and R1234ze as low global warming potential (GWP) refrigerants and establishing R134a as baseline. System simulation is performed considering the maximum value of entropy generated in the IHX. The maximum entropy production occurs at an effectiveness of 66% for both R152a and R134a, whereas for the cases of R1234yf and R1234ze occurs at 55%. Sub-cooling and superheating effects are evaluated for each one of the cases. It is also found that the sub-cooling effect shows the greatest impact on the cycle efficiency. The results also show the influence of isentropic efficiency on relative exergy destruction, resulting that the most affected components are the compressor and the condenser for all of the refrigerants studied herein. It is also found that the most efficient operation of the system resulted to be when using the R1234ze refrigerant.

Numerical Study of Erosion due to Solid Particles in Steam Turbine Blades

This article presents a numerical study of solid particle erosion (SPE) on the nozzle of the first stage steam turbine. The analysis consists of the application of Eulerian conservation equations to the continuous phase and the comparison of the DNV-Haugen, Neilson-Gilchrist, Tulsa-Ahlert, and Tabakoff-Wakeman erosion models to the discrete phase. This analysis is developed using computational fluid dynamics software based on the finite volume method. This study permits the selaction of the erosion model which presents the best numerical predictions to know the SPE influence on the blade surface, with variation of parameters such as steam mass flow rate, diameter, impact angle, velocity, and distribution of particles. These parameters were modified within a real operational turbine range. This study shows that the Tabakoff-Wakeman model predicts with greater accuracy the SPE in steam turbine blades and the particle diameter is the most critical parameter.

Thermomechanical Transient Analysis and Conceptual Optimization of a First Stage Bucket

This paper presents a thermomechanical analysis of a first stage bucket during a gas turbine startup. This analysis uses two simulation techniques, computational fluid dynamics (CFD) for the conjugate heat transfer and flow analysis, and finite element analysis (FEA) for the thermostructural analysis. Computational three-dimensional models were developed using two commercial codes, including all elements of the real bucket to avoid geometric simplifications. An interface was developed to transfer the three-dimensional behavior of bucket temperatures during turbine startup from CFD analysis to subsequent FEA analysis, imposing them as a thermal load. This interface virtually integrates the computational models, although they have different grids. The results of this analysis include temperature evolution and related stresses, as well as the thermomechanical stresses and zones where they are present. These stresses are dominated by thermal mechanisms, so a new temperature startup curve is proposed where the maximum calculated stress decline around 100 MPa, and almost all stresses are lower throughout the transient analysis. The results are compared with experimental data reported in the literature obtaining acceptable approximation.

Thermodynamic characterization of the power loss factor in steam turbines

Abstract Erosion, roughness, steam path damage etc., are factors that reduce the power capacity in a steam turbine (ST). Any power loss occurring locally in intermediate stages of a ST results in more available energy in the downstream stages. This effect is well known as the loss factor (LF) [Steam Turbines and Their Cycles, Krieger, NY, USA, 1974; Steam and Gas Turbines, McGraw-Hill, NY, USA, 1927; Steam Turbines Theory and Design, McGraw-Hill, NY, USA, 1984]. Currently, it is calculated by graphical methods [Evaluting and Improving Steam Turbine Performance, Gilson, NY, USA, 1993]. In this work, a new thermodynamic expression for the LF is introduced in order to improve applications to evaluate malfunctions in the first and intermediate stages of STs. The proposed thermodynamic expression for the LF is based on second law analysis and concepts like the internal parameter θ , and the dissipation temperature T d [Las Ecuaciones Caracteristicas, Doctoral Thesis, University of Zaragoza, 1992]. To show the main features and easiness of application of the proposed method, a 158 MW conventional power plant is analyzed, comparing the classical graphical method [Evaluating and Improving Steam Turbine performance, Gilson, NY, USA, 1993; Simplified Performance Test of Steam Turbines, ASME, NY, USA, 1970] and the proposed expression of the LF. Special emphasis is made on the thermoeconomical deviations that could arise by an imprecise application of the LF Method during an energy audit of the steam turbine internal parts.

Numerical Modeling of Vertical Axis Wind Turbines

This paper aims to predict the performance of Vertical Axis Wind Turbine (VAWT), hence the modeling of kinetic energy extraction from wind and its conversion to mechanical energy at the rotor axis, is carried out. The H-type Darrieus turbine consists of three straight blades with shape of aerofoil attached to a rotating vertical shaft. The criterion on the selection of this kind of turbines, despite its reduced efficiency, is the easy manufacture in workshops.A parametric study has been carried out to analyze the camber effect on the non dimensional curves of power coefficient so that the self starting features as well as the range of tip speed ratio of operation could be predicted.Copyright

Analysis of the Manifold Area Change in a Flat Solar Collector

A numerical analysis of the characterization of the water flow through a flat solar collector is presented. The manifold area change for minimizing the water flow variation in the solar collector is analyzed. The area ratio in the inlet and outlet of the manifolds were modified in a range of Am/Ao = 1 to 4, where Am and Ao are the cross-sectional area modified and original of the manifolds, respectively. The solar collector investigated is equipped with six riser tubes, which are attached to the manifolds pipe. The numerical study was developed in a commercial Computational Fluid Dynamics (CFD) using FLUENT®. This code allows to solve the Reynolds averaged Navier-Stokes equations and the transport equations of the turbulence quantities. The results shown that increasing the inlet and outlet area of the manifolds allow a more uniform flow distribution compared to the original configuration of the solar collector. It also shows that the overall pressure drop in the solar collector is reduced.Copyright

Numerical Analysis of the Internal Fuel Processing in Solid Oxide Fuel Cells

The high operating temperature of a SOFC (solid oxide fuel cell) has several consequences, from which the most important one is the possibility to feed the cell directly with unprocessed fuels. This eliminates the need for expensive external fuel reformers that hinder the cell from achieving a greater overall efficiency when coupled into a power generation system. Direct internal reforming (DIR) takes place directly on the anode of a SOFC by harnessing the available Nickel catalyst on its surface to process the incoming fuel. In this study a three dimensional steady state computational fluid dynamics model is implemented in a planar DIR SOFC to compare the overall cell performance operating on biogas, and coal syngas. Since chemical kinetics plays a significant role in the model accuracy, the present work also focuses on comparing three different chemical reaction mechanisms for the internal reforming process. These include a detailed heterogeneous mechanism consisting of 42 elementary reactions, a global homogeneous catalyzed mechanism, and a Langmuir-Hinshelwood based mechanism. The former includes autothermal reforming, steam reforming and water gas shift reaction effects, the latter two include steam reforming, and water gas shift reaction effects. The analysis yields detailed information about the cell, including polarization curves that help to assess the cell performance for each fuel. Meanwhile the chemical kinetics comparison amongst the analyzed mechanisms helps in establishing the best compromise between the accuracy of the model, and the computational resources devoted for the calculation.Copyright

Numerical Analysis of Airfoils Used for Vertical Axis Wind Turbine

A numerical analysis of a three-bladed straight vertical axis wind turbine with NACA0015 airfoils-shaped is presented. The effect generated on the moment coefficient and power coefficient of the wind turbine rotor by the twist angle variation at the chord ends was analyzed. The configurations included the variation of blade twist angle of 15° and 30° located at 70%, 80% and 90% of chord length from leading end of the straight blade. The numerical study was developed in a commercial Computational Fluid Dynamics (CFD) using FLUENT®. This code allows to solve the Reynolds averaged Navier-Stokes equations and the transport equations of the turbulence quantities. The results show the aerodynamic performance for each configuration of the blade twist angle in the wind turbine, and are compared with data obtained from straight blade without twist angle. The wind turbine performance decrease about 67% as the blade twist angle increases, due to an increment in the drag force causing a negative moment against the rotation of vertical axis wind turbine. Also, the surface pressure distribution in a VAWT’s is presented.Copyright