The rules on energy consumption and pollutant emissions impose increasingly restrictive limitations in today's industrial sectors. The glass production sector is one of the highest sources of energy consumption in Europe, but also in Italy. For this reason it is very important to develop strategies for consumption reduction in a glass production plant. A glass furnace is in fact conceived with systems to recover heat from the combustion gases through regenerative and recuperative systems in order to increase the efficiency of the plant. Several systems to reduce nitrogen oxides emissions have been also developed and designed. A further method to exploit the residual heat from the exhausted gases is to pre-heat the recycled glass raw material to be introduced into the furnace so as to require a smaller amount of energy for its melting. This paper shows various numerical strategies for the design of a pre-heating system for the recycled glass raw material through CFD techniques. In this regard, numerical models have been developed for systems with direct (hot gases come directly into contact with the raw material) and indirect heat exchange (the raw material is heated through the diffusion of heat from a tube bundle).
The design features of a centrifugal compressor must guarantee high performance and a wide operating range. The ported shroud was developed specifically to extend the operating limit. It is a passive flow control device based on a cavity for flow recirculation to avoid blade passage blocking in near surge conditions. A CFD simulation campaign using a simplified model identified the differences in the performance of the centrifugal compressor with ported shroud, compared to the baseline case. The use of a stability criterion to determine the limit mass flow rate, developed in a previous study by the authors, highlighted and quantified the extension of the surge margin in the case with ported shroud for different rotational speeds. An increase in the surge margin of 11% was detected at design speed, but with a lower trend at higher speeds. An in-depth flow analysis showed the main physical mechanisms in the compressor that occur for different operating conditions: at near surge conditions the cavity recirculates the low momentum flow located in the inducer region; it re-energizes the mainstream decreasing the circumferential velocity component; an improvement of up to 7% of the pressure ratio was obtained. Instead, at best efficiency conditions the flow recirculation worsens the performance by reducing the flow incidence at the rotor leading edge. Finally, using unsteady simulations with a complete 3D model and with the application of the stability criterion it was possible to confirm that the ported shroud can effectively extend the operating range.
The glass production industry has one of the highest energy consumption rates and environmental emission impact with respect to the existing industrial sectors. Moreover, the glass production sector is important in Italy (with production plants spread all over the country from North to South) from both production rates and engineering design and development competencies point of view. The glass furnaces are nowadays conceived with regenerative or recuperative systems to take advantage of the residual heat from the combustion exhausts in order to increase the thermal efficiency of the system. The exhaust gases are also used in innovative systems to reduce the NOx emissions in specifically designed gas recirculation systems tailored to the glass furnace. A remaining portion of the heat content in the exhaust gases could be used to pre-heat the raw material from recycled glass that, in some applications, forms a significant percentage of the glass recipe. In order to develop pre-heating systems for recycled glass as compact as possible, a detailed analysis of the heat transfer from the gases to the glass need to be developed. In the paper different CFD models for the heating process of recycled glass are presented. A numerical model for the recycled glass loose particles has been developed and used in the CFD models for both direct (the exhaust gases flow through the recycled glass matrix) and indirect (the recycled glass is heated through heat diffusion from the hot gases that flow into pipes) systems.
An important aspect in the glass production industry is related to the heat recovery of the combustion gases. It is usually obtained throughout the use of well-tested technologies, such as regenerative towers with refractory material. For an effective heat recovery, a good distribution of the flow rate at the entrance of the chambers is crucial. The use of Computational Fluid Dynamics (CFD) allows the detailed analysis of the gas evolution during the process; the same would be impractical with experimental measurements, due to prohibitive ambient local conditions. The CFD approach during the design phase typically considers CAD geometries without the level of details related to technological features of the actual installed configuration (i.e. sharp edges vs rounded edges). A brand new built furnace has blunt edges in every connection between 3D walls of refractory blocks. The above edges will be rounded by the erosion-corrosion process due to the harsh chemical/mechanical/thermal environmental conditions inside the plant components (i.e. regenerative chambers, connecting ducts, furnace). The purpose of this work is to evaluate the influence of the geometrical details of the CAD (with focus on the edges connecting adjacent walls), due to technological or erosion aspects, on the flow structure in the furnace components.
The use of CFD has proven to be a very effective tool for the design of regenerative systems in glass production plants. Different CFD approaches have been set up by the authors to simulate the regenerative chambers to include heat transfer and pressure losses effects from the internal refractory structure. The above models have been used to conceive a gas recirculation strategy for the NOx containment from natural gas combustion and are currently used in industry to design such systems. On the other hand, it can be observed that the actual geometry found in a new regenerative chamber or after years of continuous operation is different from the CAD geometry used in the design process. In fact, the CAD geometry has sharp edges at the junction of solid surfaces while the actual geometry of a refractory block is with blunt edges and the above edges are rounded off by corrosion and erosion processes during operation. The effects of the above geometrical details on the flow structure inside the furnace components have been highlighted in a previous paper. The dramatic change that can occur in the flow topology with the introduction of those geometrical details can jeopardize the use of the standard CFD approaches for design purposes. In the present paper the attention is focused on the regenerative chamber with the gas recirculation systems. This NOx containment strategy is effective if the recirculated gases, in the regenerative chamber with combustion air, are correctly distributed above the flame structure in order to reduce the maximum temperature that will cause NOx formation. The effects of the geometrical details on the edges in the regenerative chamber are investigated in order to develop a more effective and consistent design approach based on CFD and to setup a proper simulation tool to predict the effects of erosion (rounded edges) on the gas recirculation strategy. The simulation approach is applied to actual regenerative chambers equipped with the gas recirculation strategy to understand the above effects on an existing glass production plant.
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