Valery Ponyavin

The Parametric Study of an Innovative Offset Strip-Fin Heat Exchanger

Offset strip-fin heat exchangers have numerous applications throughout various industries because they can provide a large amount of heat transfer area in a small volume. The widespread use of the offset strip-fin design has ensured that there are numerous dimensional variations and shown that changes in dimensional parameters affect performance. It is then important to understand how the geometry of an offset strip-fin heat exchanger can affect its performance. Therefore, an investigation into the parametric effects on the global performance of an innovative high-temperature offset strip-fin heat exchanger was numerically performed in this study, where the numerical solution was obtained through a finite-volume method. Computations were carried out for each of the heat exchangers geometrical parameters: fin thickness (t), fin length (1), channel height (H), spanwise pitch (p x ), and the newly introduced gap parameter (g). Also, the effects of rounding the fins leading and trailing edges were investigated, while the heat exchangers volume, mass flow rates, and inlet temperatures were kept constant. The results are presented in the form of pressure drops and heat transfer rates, and the coefficient of performance parameter shows that fins with rounded leading and trailing edges outperform fins with rectangular edges.

Design of a Compact Ceramic High-Temperature Heat Exchanger and Chemical Decomposer for Hydrogen Production

This article describes a compact silicon carbide ceramic, high-temperature heat exchanger for hydrogen production in the sulfur iodine thermochemical cycle, and in particular, to be used as the sulfuric acid decomposer. In this cycle, hot helium from a nuclear reactor is used to heat the SI (sulfuric acid) feed components (H2O, H2SO4, SO3) to obtain appropriate conditions for the SI decomposition reaction. The inner walls of the SI decomposer channels are coated with platinum to catalytically decompose sulfur trioxide into sulfur dioxide and oxygen. Hydrodynamic, thermal, and the sulfur trioxide decomposition reaction were coupled for numerical modeling. Thermal results of this analysis are exported to perform a probabilistic mechanical failure analysis. This article presents the approach used in modeling the chemical decomposition of sulfur trioxide. Stress analysis of the design is also presented. The second part of the article shows the results of parametric study of the baseline design (linear channels). Several alternate designs of the chemical decomposer channels are also explored. The current study summarizes the results of the parametric calculations whose objective is to maximize the sulfur trioxide decomposition by using various channel geometries within the decomposer. Based on these results, a discussion of the possibilities for improving the productivity of the design is also given.

Calculation of Fluid Flow Distribution Inside a Compact Ceramic High Temperature Heat Exchanger and Chemical Decomposer

Numerical analysis of flow distribution inside a compact ceramic high temperature heat exchanger and chemical decomposer (thereafter, heat exchanger), which will be used for hydrogen production, wherein the sulfur iodine thermochemical cycle is performed. To validate the numerical model, experimental investigation of the heat exchanger is accomplished. The study of the flow distribution in the base line design heat exchanger shows that the design has large-flow maldistribution and the reverse flow may occur at poor inlet and outlet manifold configurations. To enhance uniformity of the flow rate distribution among the heat exchanger internal channels, several improved designs of the heat exchanger manifolds and supply channels are proposed. The proposed designs have a sufficiently uniform flow rate distribution among the internal channels, with an appropriate pressure drop.

Modeling and Parametric Study of a Ceramic High Temperature Heat Exchanger and Chemical Decomposer

It is proposed to use ceramic high temperature heat exchanger as a sulfuric acid decomposer for hydrogen production within the sulfur iodine thermo-chemical cycle. The decomposer is manufactured using fused ceramic layers that allow creation of channels with dimensions below one millimeter. A three-dimensional computational model is developed to investigate the fluid flow, heat transfer, stresses and chemical reactions in the decomposer. Fluid, thermal and chemical reaction analyses are performed using FLUENT software. Temperature distribution in the solid is imported to ANSYS software and used together with pressure as the load for stress analysis. Results of this research can be used as a basis for investigation optimal design of the decomposer that can provide maximum chemical decomposition performance while maintaining stresses within design limits.Copyright

Numerical Modeling of Bayonet-Type Heat Exchanger and Decomposer for the Decomposition of Sulfuric Acid to Sulfur Dioxide

The growth of global energy demand during the 21st century, combined with the necessity to master greenhouse gas emissions, has led to the introduction of a new and universal energy carrier: hydrogen. The Department of Energy (DOE) proposed using a bayonet-type heat exchanger as a silicon carbide integrated decomposer (SID) to produce the sulfuric acid decomposition product sulfur dioxide, which can be used for hydrogen production within a sulfur–iodine thermochemical cycle. A two-dimensional computational model of SID having a boiler, superheater and decomposer was developed using GAMBIT and fluid. The thermal and chemical reaction analyses were carried out in FLUENT. The main purpose of this study is to obtain the decomposition percentage of sulfur trioxide for the integrated unit. Sulfuric acid (H2SO4), sulfur trioxide (SO3), sulfur dioxide (SO2), oxygen (O2), and water vapor (H2O) are the working fluids used in the model. Concentrated sulfuric acid liquid of 40 mol% was pumped into the inlet of the boiler and the mass fraction of concentrated sulfuric acid vapor obtained was then fed into the superheater to obtain sulfur trioxide. The decomposer region, which houses the pellets, placed on the top of the bayonet heat exchanger acts as the porous medium. As the decomposition takes place, the mass fraction of SO3 is reduced and mass fractions of SO2 and O2 are increased. The percentage of SO3 obtained from the integrated decomposer was compared with the experimental results obtained from Sandia National Laboratories (SNL). Further, effects of various pressures, flow rates, and acid concentrations on the decomposition percentage of sulfur trioxide were studied.

The Effect of Fin Geometry on Design of Compact Off-Set Strip Fin High Temperature Heat Exchanger

This paper deals with the development of a three-dimensional numerical model to predict the overall performance of an advanced high temperature heat exchanger design, up to 1000°C, for the production of hydrogen by the sulfur iodine thermo-chemical cycle used in advanced nuclear reactor concepts. The design is an offset strip-fin, hybrid plate compact heat exchanger made from a liquid silicon impregnated carbon composite material. The two working fluids are helium gas and molten salt (Flinak). The offset strip-fin is chosen as a method of heat transfer enhancement due to the boundary layer restart mechanism between the fins that has a direct effect on heat transfer enhancement. The effects of the fin geometry on the flow field and heat transfer are studied in three-dimensions using Computational Fluid Dynamics (CFD) techniques. The pre-processor GAMBIT is used to create a computational mesh, and the CFD software package FLUENT that is based on the finite volume method is used to produce the numerical results. Fin dimensions need to be chosen that optimize heat transfer and minimize pressure drop. Comparison of the overall performance between two fin shapes (rectangular versus curved edges) is performed using computational fluid dynamics techniques. Fin and channel dimensions need to be chosen such as to optimize heat transfer performance and minimize pressure drop. The study is conducted with helium gas and liquid salt as the working fluids with a variety of Reynolds number values and fin dimensions. Both laminar and turbulent modeling is performed for the helium side fluid flow. The effect of the fin geometry is performed computational fluid dynamics techniques and optimization studies are performed. The model developed in this paper is used to investigate the heat exchanger design parameters in order to find an optimal design.Copyright

Fluid/Thermal Analysis of High Temperature Heat Exchanger and Chemical Decomposer for Hydrogen Production

This paper presents fluid flow and heat transfer study of a high temperature heat exchanger and chemical decomposer. The decomposer will be used as a part of the plant for hydrogen production. The decomposer is manufactured using fused ceramic layers that allow creation of channels with dimensions below one millimeter. The main purpose for this study is to increase thermal performance of the decomposer which can help to intensify sulfuric acid decomposition rate. Effects of using various channel geometries of the decomposer on the pressure drop are studied as well. A three-dimensional computational model is developed for the investigation of fluid flow and heat transfer in the decomposer. Several different geometries of the decomposer channels such as straight channels, ribbed ground channels, hexagonal channels, and diamond-shaped channels are examined. Based on results of the calculation, the recommendations for the improved design of the decomposer are obtained.Copyright

CFD Modeling of Bayonet Type High Temperature Heat Exchanger and Chemical Decomposer With Different Packed Bed Designs

The present work is concerned with use of bayonet type high temperature heat exchanger as silicon carbide integrated decomposer (SID) which produces sulfuric acid decomposition product - sulfur dioxide. The product can be used within the sulfur iodine thermo-chemical cycle portion of the hydrogen production process. The chemical decomposition occurs in packed bed area of the decomposer. The engineering design of the packed bed is very much influenced by the structure of the packing matrix, which is governed by the shape, dimensions and the loading of the constituent particles. Optimum design of catalyst pellet in terms of shape configuration, packing method and available surface area can promote catalytic activity and the prevailing transport properties of the system. Knowledge of the underlying factors should enable designers to engineer the optimum design for a given system with prescribed conditions. The investigations of fluid flow and the arrangement of cylindrical and spherical pellets in packed bed are presented in the paper.Copyright

Stress Analysis of a High Temperature Heat Exchanger Used in an Advanced Nuclear Reactor

This paper presents the stress analysis of the offset strip-fin type compact high temperature heat exchanger for use in the cooling cycle of an advanced nuclear reactor for hydrogen production by the sulfur iodine thermo-chemical cycle. Three different geometry types of heat exchangers were considered: geometry with rectangular fins; geometry with rounded fins and geometry with rounded fins which include manufacturing geometrical effects (fins with roundings on their bases). The material of the heat exchanger is liquid silicon impregnated carbon composite. The two working fluids for the heat exchanger are helium gas and molten salt with maximum temperature about 1000°C. The finite element code ANSYS 9.0 was used for the simulations. The boundary conditions for temperature and pressure were obtained as results of CFD and heat transfer calculations of the heat exchanger using the finite volume code FLUENT 6.1.18 The obtained results will be used for further optimization of the high temperature heat exchanger geometry.Copyright

Numerical Modeling of the Turbulent Supersonic Flow over a Backward Facing Step

The flow over a backward facing step is a classic problem in applied aerodynamics. Among many other applications, backward facing steps are often used for ignition and stabilization of the flame in a scramjet engine. In this study, the steady two-dimensional viscous supersonic turbulent flow over a backward facing step was calculated using FLUENT. The one-equation Spalart and Allmaras turbulence model was employed for the turbulent flow simulation. The free stream Mach number was 2. The simulated flow field is in good qualitative agreement with flow visualizations, pressure and temperature measurements and theoretical predictions. The boundary layer ahead of the step turns through the right angle over the corner. Then a separation occurs below the corner on the step wall. The numerical results indicate that the separation point is positioned on the step face, below the corner. The overexpansion at the corner is balanced by a lip-shock. This phenomenon was observed in experimental measurements, in flow visualizations and is also obtained in the present numerical calculation.© 2004 ASME