Luben Cabezas-Gómez

Effectiveness-ntu computation with a mathematical model for cross-flow heat exchangers

Abstract - Due to the wide range of design possibilities, simple manufactured, low maintenance and low cost, cross-flow heat exchangers are extensively used in the petroleum, petrochemical, air conditioning, food storage, and others industries. In this paper a mathematical model for cross-flow heat exchangers with complex flow arrangements for determining e-NTU relations is presented. The model is based on the tube element approach, according to which the heat exchanger outlet temperatures are obtained by discretizing the coil along the tube fluid path. In each cross section of the element, tube-side fluid temperature is assumed to be constant because the heat capacity rate ratio C*=C min /C max tends toward zero in the element. Thus temperature is controlled by effectiveness of a local element corresponding to an evaporator or a condenser-type element. The model is validated through comparison with theoretical algebraic relations for single-pass cross-flow arrangements with one or more rows. Very small relative errors are obtained showing the accuracy of the present model. e-NTU curves for several complex circuit arrangements are presented. The model developed represents a useful research tool for theoretical and experimental studies on heat exchangers performance.

Thermal Performance of Multipass Parallel and Counter-Cross-Flow Heat Exchangers

Univ Sao Paulo, Escola Engn Sao Carlos, Dept Engn Mecan, BR-13566590 Sao Carlos, SP, Brazil

Numerical simulation of a radial diffuser turbulent airflow

Abstract In the present work are presented results from numerical simulations performed with the ANSYS-CFX® code. We have studied a radial diffuser flow case, which is the main academic problem used to study the flow behavior on flat plate valves. The radial flow inside the diffuser has important behavior such as the turbulence decay downstream and recirculation regions inside the valve flow channel due to boundary layer detachment. These flow structures are present in compressor reed valve configurations, influencing to a greater extent the compressor efficiency. The main target of the present paper was finding the simulation set-up (computational domain, boundary conditions and turbulence model) that better fits with experimental data published by Tabatabai and Pollard. The local flow turbulence and velocity profiles were investigated using four different turbulence models, two different boundary conditions set-up, two different computational domains and three different flow conditions (Rein – Reynolds number at the diffuser inlet). We used the Reynolds stress (BSL); the k–e; the RNG k–e; and the shear stress transport (SST) k–ω turbulence models. The performed analysis and comparison of the computational results with experimental data show that the choice of the turbulence model, as well as the choice of the other computational conditions, plays an important role in the results physical quality and accuracy.

A generalized alternating-direction implicit scheme for incompressible magnetohydrodynamic viscous flows at low magnetic Reynolds number

Abstract This paper presents numerical simulations of incompressible fluid flows in the presence of a magnetic field at low magnetic Reynolds number. The equations governing the flow are the Navier–Stokes equations of fluid motion coupled with Maxwell’s equations of electromagnetics. The study of fluid flows under the influence of a magnetic field and with no free electric charges or electric fields is known as magnetohydrodynamics. The magnetohydrodynamics approximation is considered for the formulation of the non-dimensional problem and for the characterization of similarity parameters. A finite-difference technique is used to discretize the equations. In particular, an extension of the generalized Peaceman and Rachford alternating-direction implicit (ADI) scheme for simulating two-dimensional fluid flows is presented. The discretized conservation equations are solved in stream function–vorticity formulation. We compare the ADI and generalized ADI schemes, and show that the latter is more efficient in simulating low Reynolds number and magnetic Reynolds number problems. Numerical results demonstrating the applicability of this technique are also presented. The simulation of incompressible magnetohydrodynamic fluid flows is illustrated by numerical solution for two-dimensional cases.

Some modeling and numerical aspects of the two-fluid simulation of the gas-solids flow in a CFB riser

The gas-solids flow in a CFB riser is simulated applying two-fluid modeling. Two different procedures are used for the calculation of the solids phase pressure and stress tensor: the traditional procedure and an algebraic version of the kinetic theory of granular flows. Three different numerical meshes and two different discretization schemes for the advective terms are used. Results are compared to available experimental data from the literature. The effects of the solids phase modeling procedure, advection discretization scheme, and mesh size are discussed.

Wavelet-Galerkin method for one-dimensional elastoplasticity and damage problems: Constitutive modeling and computational aspects

Abstract This work presents an analysis of the wavelet-Galerkin method for one-dimensional elastoplastic-damage problems. Time-stepping algorithm for non-linear dynamics is presented. Numerical treatment of the constitutive models is developed by the use of return-mapping algorithm. For spacial discretization we can use wavelet-Galerkin method instead of standard finite element method. This approach allows to locate singularities. The discrete formulation developed can be applied to the simulation of one-dimensional problems for elastic–plastic-damage models.

Numerical Procedure for LMTD Correction Factor Calculation for One Tube and One Shell Pass Shell-and-Tube Heat Exchangers

This paper outlines the application of a numerical procedure to compute the LMTD correction factor for one tube pass and one shell pass, namely 1-1, shell-and-tube heat exchangers. Although the procedure was applied for this specific arrangement, it can easily be applied to different heat exchanger arrangements. The numerical methodology is based on an association of ε-NTU and LMTD approaches introduced by Kays and London (1998). Unlike other shell-and-tube heat exchanger arrangements, such as one tube pass and two shell pass, no available analytical expression in closed form is available in the literature. Hence, the presented numerical procedure is applied to determine the LMTD correction factor for 1-1 shell-and-tube heat exchangers and numerical results were obtained by increasing the number of baffles (1, 3, 7, 9, 19 and 49), in order to analyze the obtained results. Finally, charts for the LMTD correction factor is presented as a function of two dimensionless parameters, namely P and R.

Continuous Improvements Analysis in Energy Efficiency of Steering Power Systems to Light Vehicles

This study analyses the energy consumption of a vehicle steering system and evaluates potential earnings in using more efficient systems. The energy consumption of the steering system to provide assistance can be until 3% of the vehicle total energy consumption. The technology evolution of the steering system provides a significant reduction in energy consumption. Three types of steering systems were evaluated: Hydraulic Power System, Electro-Hydraulic Power System and Electrical Power System, respectively, considering the consumption and the energy efficiency of their components. Some evaluations of the energy consumed by system components were performed using data from the available sources in the literature and other evaluations were performed via experimental tests on a bench test. A comparative study was set to determine the differences between steering systems regarding the energy consumption and energy efficiency. Consumption analysis of steering system were extended to evaluate the impact in the final vehicle consumption. The Electrical Power System can increase the efficiency in fuel use during operation in the vehicle.

Numerical Determination of the LMTD Correction Factor for Shell-and-Tube 1-2 Heat Exchangers

This paper outlines a novel numerical methodology to compute the LMTD correction factor for a shell-and-tube heat exchangers. Although the presented methodology can be extended to other shell-and-tube heat exchangers, the analysis presented is this paper concerns the one shell pass and two tube pass configuration, namely 1-2 shell-and-tube heat exchanger. The correction factor is compute by means of an association of e-NTU and LMTD approaches proposed by Kays and London (1998). An analysis of the convergence of the solution provided by the numerical methodology is confronted against results from an analytical solution available in the literature to compute the LMTD correction factor for infinite number of baffles. Numerical results shows that, as the number of baffles is increased, the numerical solution approaches the analytical solution available in the literature. The presented numerical methodology allows the direct computation of the LMTD correction factor for a determined arrangement, giving a new perspective on the project and sizing of shell-and-tube heat exchangers, since typically the analytical solution is applied for all arrangements.