Sreenivas Jayanti

Abstract: An assessment is made of the various viewpoints on the slug-to-churn flow transition in vertical upward flow in the light of recent experimental results obtained at Harwell Laboratory. It is found that the flooding model of McQuillan & Whalley and the bubble entrainment model of Barnea & Brauner give satisfactory results at low and high liquid flow rates, respectively. An improved model for flooding, which takes account of the effect of the falling film, has been proposed. It is shown that this new model is in good agreement with experimental results at both low and high liquid flow rates.

Abstract: The pressure losses in the flow distributor plate of the fuel cell depend on the Reynolds number and geometric parameters of the small flow channels. Very little information has been published on the loss coefficients for laminar flow. This study reports a numerical simulation of laminar flow though single sharp and curved bends, 180° bends and serpentine channels of typical fuel cell configurations. The effect of the geometric parameters and Reynolds number on the flow pattern and the pressure loss characteristics is investigated. A three-regime correlation is developed for the excess bend loss coefficient as a function of Reynolds number, aspect ratios, curvature ratios and spacer lengths between the channels. These have been applied to calculate the pressure drop in typical proton-exchange membrane fuel cell configurations to bring out the interplay among the important geometric parameters.

Abstract: Parallel-channel configurations for gas-distributor plates of planar fuel cells reduce the pressure drop, but give rise to the problem of severe flow maldistribution wherein some of the channels may be starved of the reactants. This study presents an analysis of the flow distribution through parallel-channel configurations. One-dimensional models based on mass and momentum balance equations in the inlet and exhaust gas headers are developed for Z- and U-type parallel-channel configurations. The resulting coupled ordinary differential equations are solved analytically to obtain closed-form solutions for the flow distribution in the individual channels and for the pressure drop over the entire distributor plate. The models have been validated by comparing the results with those obtained from three-dimensional computational fluid dynamics (CFD) simulations. Application of the models to typical fuel-cell distributor plates shows that severe maldistribution of flow may arise in certain cases and that this can be avoided by careful choice of the dimensions of the headers and the channels.

Abstract: The objective of the present paper is to propose a new variant of the oxy-fuel combustion for carbondioxide (CO2) sequestration in which steam is used to moderate the flame temperature. In this process, pure oxygen is mixed with steam and the resulting oxidant mixture is sent to the boiler for combustion with a fossil fuel. The advantage of this method is that flue gas recirculation is avoided and the volumetric flow rates through the boiler and auxiliary components is reduced by about 39% when compared to the conventional air-fired coal combustion power plant leading to a reduction in the size of the boiler. The flue gas, after condensation of steam, consists primarily of CO2 and can be sent directly for compression and sequestration. Flame structure analysis has been carried out using a 325-step reaction mechanism of methane-oxidant combustion to determine the concentration of oxygen required to ensure a stable flame. Thermodynamic exergy analysis has also been carried out on SMOC-operated CO2 sequestration power plant and air-fired power plant, which shows that though the gross efficiency increases the absolute power penalty of ∼8% for CO2 sequestration when compared to air-fired power plant.

Abstract: Mixing of reactants, catalysts, etc. in a chemical reactor may be achieved using jets which offer the advantage of having no moving parts inside the reactor. While there have been many experimental studies and thumb rules for the design of jets, the detailed hydrodynamics of the mixing process is not properly understood. In the present paper, computational fluid dynamics (CFD) techniques are used to simulate jet mixing in a cylindrical vessel. The flow circulation patterns within the reactor and their effect on mixing of a soluble salt are studied. By simulating various configurations of jets, it is shown that the key factor in reducing mixing time is minimizing or eliminating dead zones in the reactor. This can be achieved, for example, by having a conical bottom.

Abstract: Oxy-fuel combustion has generated significant interest since it was proposed as a carbon capture technology for newly built and retrofitted coal-fired power plants. Research, development and demonstration of oxy-fuel combustion technologies has been advancing in recent years; however, there are still fundamental issues and technological challenges that must be addressed before this technology can reach its full potential, especially in the areas of combustion in oxygen-carbon dioxide environments and potentially at elevated pressures. This paper presents a technical review of oxy-coal combustion covering the most recent experimental and simulation studies, and numerical models for sub-processes are also used to examine the differences between combustion in an oxidizing stream diluted by nitrogen and carbon dioxide. The evolution of this technology from its original inception for high temperature processes to its current form for carbon capture is introduced, followed by a discussion of various oxy-fuel systems proposed for carbon capture. Of all these oxy-fuel systems, recent research has primarily focused on atmospheric air-like oxy-fuel combustion in a CO2-rich environment. Distinct heat and mass transfer, as well as reaction kinetics, have been reported in this environment because of the difference between the physical and chemical properties of CO2 and N2, which in turn changes the flame characteristics. By tracing the physical and chemical processes that coal particles experience during combustion, the characteristics of oxy-fuel combustion are reviewed in the context of heat and mass transfer, fuel delivery and injection, coal particle heating and moisture evaporation, devolatilization and ignition, char oxidation and gasification, as well as pollutants formation. Operation under elevated pressures has also been proposed for oxy-coal combustion systems in order to improve the overall energy efficiency. The potential impact of elevated pressures on oxy-fuel combustion is discussed when applicable. Narrower flammable regimes and lower laminar burning velocity under oxy-fuel combustion conditions may lead to new stability challenges in operating oxy-coal burners. Recent research on stabilization of oxy-fuel combustion is reviewed, and some guiding principles for retrofit are summarized. Distinct characteristics in oxy-coal combustion necessitate modifications of CFD sub-models because the approximations and assumptions for air-fuel combustion may no longer be valid. Advances in sub-models for turbulent flow, heat transfer and reactions in oxy-coal combustion simulations, and the results obtained using CFD are reviewed. Based on the review, research needs in this combustion technology are suggested.

Abstract: The performance of heat exchangers can be improved to perform a certain heat-transfer duty by heat transfer enhancement techniques. In general, these techniques can be divided into two groups: active and passive techniques. The active techniques require external forces, e.g. electric field, acoustic or surface vibration, etc. The passive techniques require fluid additives or special surface geometries. Curved tubes have been used as one of the passive heat transfer enhancement techniques and are the most widely used tubes in several heat transfer applications. This article provides a literature review on heat transfer and flow characteristics of single-phase and two-phase flow in curved tubes. Three main categories of curved tubes; helically coiled tubes, spirally coiled tubes, and other coiled tubes, are described. A review of published relevant correlations of single-phase heat transfer coefficients and single-phase, two-phase friction factors are presented.

Abstract: Direct ethanol fuel cells have attracted much attention recently in the search for alternative energy resources. As an emerging technology, direct ethanol fuel cells have many challenges that need to be addressed. Many improvements have been made to increase the performance of direct ethanol fuel cells, and there are great expectations for their potential. However, many improvements need to be made in order to enhance the potential of direct ethanol fuel cells in the future. This paper addresses the challenges and the developments of direct ethanol fuel cells at present. It also presents the applications of DEFC.

Abstract: The possible role of ammonia in a future energy infrastructure is discussed. The review is focused on the catalytic decomposition of ammonia as a key step. Other aspects, such as the catalytic removal of ammonia from gasification product gas or direct ammonia fuel cells, are highlighted as well. The more general question of the integration of ammonia in an infrastructure is also covered.

Abstract: This paper focuses on reviewing the main factors affecting the life time of fuel cells on vehicles. The main difference between fuel cells used in vehicles and fuel cells used as fixed power is the load cycling. Load cycling sometimes leads to water management and gas transport problems, which further leads to degradation of fuel cell performance and attenuation of internal parts. This article is written from the perspective of the fuel cell dynamic cycling as well as its resulting problems of water management and gas starvation, and also analyses the reasons for the degradation of fuel cell life time, and present some mitigation measures during the fuel cell operation.