Assessment of a common nonlinear eddy-viscosity turbulence model in capturing laminarization in mixed convection flows

Laminarization is an important topic in heat transfer and turbulence modeling. Recent studies have demonstrated that several well-known turbulence models failed to provide accurate prediction when applied to mixed convection flows with significant re-laminarization effects. One of those models, a well-validated cubic nonlinear eddy-viscosity model, was observed to miss this feature entirely. This paper studies the reasons behind this failure by providing a detailed comparison with the baseline Launder–Sharma model. The difference is attributed to the method of near-wall damping. A range of tests have been conducted and two noteworthy findings are reported for the case of flow re-laminarization

Sensitivity Analysis of the Heat Exchanger Design in Net Power Oxy-Combustion Cycle for Carbon Capture

The global warming and its impact on climate change is one of main challenges for current century. Global warming is mainly due to the emission of greenhouse gases (GHG) and carbon dioxide (CO2) is known to be the major contributor to the GHG emission profile. Whilst the energy sector is the primary source for CO2 emission, Carbon Capture and Storage (CCS) are believed to be the solution for controlling this emission. Oxyfuel combustion (Oxy-combustion) is one of the major technologies for capturing CO2 from power plants. For gas turbines, several Oxy-combustion power cycles (Oxyturbine cycles) have been investigated by means of thermodynamic analysis. NetPower cycle is one of the leading oxyturbine power cycles with almost full carbon capture capability from a natural gas fired power plant. In this manuscript, sensitivity analysis of the heat exchanger design in NetPower cycle is completed by means of process modelling. The heat capacity variation and supercritical CO2 with gaseous admixtures are considered for multi-zone analysis with Aspen Plus software. It is found that the heat exchanger design has a major role to increase the efficiency of NetPower cycle. The pinch-point analysis is done to extract the composite and grand composite curve for the heat exchanger. In this paper, relationship between the cycle efficiency and the minimum approach temperature (∆Tmin) of the heat exchanger has also been evaluated. Increase in ∆Tmin causes a decrease in the temperature of the recycle flue gases (RFG) and an overall decrease in the required power for the recycled gas compressor. The main challenge in the design of heat exchangers in power plants is a tradeoff between the capital and operational costs. To achieve lower ∆Tmin, larger size of heat exchanger is required. This means a higher capital cost but leading to a better heat recovery and lower operational cost. To achieve this, ∆Tmin is selected from the minimum point in the diagrams of capital and operational costs. This study provides an insight into the NetPower Oxy-combustion cycle’s performance analysis and operational condition based on its heat exchanger design

Design, Manufacture and Test of a Micro-Turbine Renewable Energy Combustor

The ever-increasing demand on highly efficient decentralized power generation with low CO2 emission has made microturbines for power generation in micro-combined heat and power (mCHP) generation systems popular when running on biofuels as a renewable source of energy. This document presents a state-of-the-art design, and optimization (in terms of design, performance and emission control) of a micro-turbine renewable energy combustor that fits into the existing Bladon 12kWe recuperated microturbine plenum while running on a range of biofuels as it can successfully provide the required power of the mCHP. Governing equations for in-depth analysis of the combustor consist of manufacturer empirical data to simulate system-level operation with respect to replacement of the fossil with biofuels. The Model developed and validated at company’s ISO conditions confirms the output power of the new combustor fits the conventional system with slight eco-energy improvements. The modeling of the combustor in a complete microturbine assembly system is performed, then was utilized to further analysis of the microturbine with the designed combustor. The experimental results gave on average 46.7% electrical efficiency, 83.2% system efficiency, 12 kWe electrical power, and 90% recuperator effectiveness at nominal operating conditions of microturbine (MT). Sensitivity analyses evaluate changes in performance with respect to fuel phase (e.g., liquid or gaseous) and design variables (e.g., orientation, shape, and dimensions of combustor), leading to energy optimization of the unit. Experimental findings demonstrate that the combustor in microturbine can meet the target performance specifications of a company conventional diesel microturbine with significant savings. An objective function including both combustor and recuperator technical energy data is defined for finding the best ratio of fuel and air and their flow rates to find the most effective operating points for the operation of MT. Annual time series simulations completed for Coventry, West Midlands, United Kingdom indicate a new combustor can reduce operational costs of diesel fuel combustor by 8%, 2%, 36%, and 25% when supplying bioethanol, DME, biogas, and NG, respectively. Annual operating time of the renewable microturbine combustor at rated capacity included an 11% reduction in exergy loss with biogas fuel relative to diesel fuel

Design and numerical analysis of a 3 kWe flameless microturbine combustor for hydrogen fuel

In this work, a new 3 kWe flameless combustor for hydrogen fuel is designed and analyzed using CFD simulation. The strategy of the design is to provide a large volumetric combustion for hydrogen fuel without significant rise of the temperature. The combustor initial dimensions and specification were obtained from practical design procedures initially, and then optimized from CFD simulations. To this end, a three-dimensional model for the designed combustor is constructed to further analysis of flameless hydrogen combustion and consideration that leads to disappearance of flame-front and flameless combustion. The key design parameters including aerodynamic, temperature at walls and flame, NOX, pressure drop, combustion efficiency for the hydrogen flame is analyzed in the designed combustor. To well demonstrate the combustor, the NOX and entropy destruction and finally energy conversion efficiency, and overall operability in the microturbine cycle of hydrogen flameless combustor is compared with a 3 kWe design counterpart for natural gas. The findings demonstrate that hydrogen flameless combustion is superior to derive the microturbines with significantly lower NOX, and improvements in energy efficiency, and cycle overall efficiency with low wall temperatures guaranteeing the long-term operation of combustor and microturbine parts. Keywords: Hydrogen, microturbine, flameless combustor, low NOX, low carbon

Assessment of a common nonlinear eddy-viscosity turbulence model in capturing laminarization in mixed convection flows

Laminarization is an important topic in heat transfer and turbulence modelling. Recent studies have demonstrated that several well-known turbulence models, failed to provide accurate prediction when applied to mixed convection flows with significant re-laminarization effects. One of those models, a well-validated cubic non-linear eddy-viscosity model, was observed to miss entirely this feature. This paper studies the reasons behind this failure by providing a detailed comparison with the baseline Launder-Sharma model. The difference is attributed to the method of near-wall damping. A range of tests have been conducted and two noteworthy findings are reported for the case of flow re-laminarization

Kinetic simulation of a 100kWth oxy-combustor using Aspen Plus

Oxy-fuel combustion is a clean coal technology based on firing fuel in an enriched oxygen atmosphere to obtain high CO2 concentrations in the exhaust gas. Experimental tests were performed at Cranfield University using a 100kWth retrofitted oxy-combustor. In parallel, a kinetic simulation model using Aspen Plus was designed and validated to serve as a computer tool to predict the behaviour of the oxy-combustion process for a wide range of fuels and conditions. The main input parameters varied in the simulation study were: fuel type (El Cerrejon coal, Daw Millcoal, Cereal Co-product biomass, and coal/biomass blends); percentage of recycled flue gas (55, 60,and 65%); type of recycled flue gas (wet or dry); percentage of excess oxygen (0 and 5%), and the amount of air ingress into the process (0, 2, 10, and 18% of the total flue gas fed to the oxycombustor).The last input condition, percentage of air ingress, is of greater importance as a result of the unit being a retrofitted oxy-combustor; for which air ingress is more probable and this represents a situation likely to be an issue for any boiler retrofitted for oxyfuel firing. Results from the simulations as well as the definition of the operating conditions that best represents the behaviour of the rig are presented

Kinetic simulation of a 100kWth oxy-combustor using Aspen Plus

Oxy-fuel combustion is a clean coal technology based on firing fuel in an enriched oxygen atmosphere to obtain high CO2 concentrations in the exhaust gas. Experimental tests were performed at Cranfield University using a 100kWth retrofitted oxy-combustor. In parallel, a kinetic simulation model using Aspen Plus was designed and validated to serve as a computer tool to predict the behaviour of the oxy-combustion process for a wide range of fuels and conditions. The main input parameters varied in the simulation study were: fuel type (El Cerrejon coal, Daw Millcoal, Cereal Co-product biomass, and coal/biomass blends); percentage of recycled flue gas (55, 60,and 65%); type of recycled flue gas (wet or dry); percentage of excess oxygen (0 and 5%), and the amount of air ingress into the process (0, 2, 10, and 18% of the total flue gas fed to the oxycombustor).The last input condition, percentage of air ingress, is of greater importance as a result of the unit being a retrofitted oxy-combustor; for which air ingress is more probable and this represents a situation likely to be an issue for any boiler retrofitted for oxyfuel firing. Results from the simulations as well as the definition of the operating conditions that best represents the behaviour of the rig are presented

The Bio Steel Cycle: 7 Steps to Net-Zero CO2 Emissions Steel Production

CO2 emissions have been identified as the main driver for climate change, with devastating consequences for the global natural environment. The steel industry is responsible for ~7–11% of global CO2 emissions, due to high fossil-fuel and energy consumption. The onus is therefore on industry to remedy the environmental damage caused and to decarbonise production. This desk research report explores the Bio Steel Cycle (BiSC) and proposes a seven-step-strategy to overcome the emission challenges within the iron and steel industry. The true levels of combined CO2 emissions from the blast-furnace and basic-oxygen-furnace operation, at 4.61 t of CO2 emissions/t of steel produced, are calculated in detail. The BiSC includes CO2 capture, implementing renewable energy sources (solar, wind, green H2) and plantation for CO2 absorption and provision of biomass. The 7-step-implementation-strategy starts with replacing energy sources, develops over process improvement and installation of flue gas carbon capture, and concludes with utilising biogas-derived hydrogen, as a product from anaerobic digestion of the grown agrifood in the cycle. In the past, CO2 emissions have been seemingly underreported and underestimated in the heavy industries, and implementing the BiSC, using the provided seven-steps-strategy will potentially result in achieving net-zero CO2 emissions in steel manufacturing by 2030

Modelling of Wax Deposition by Perturbed Hard Sphere Chain Equation of State

This article presents a model to predict the wax appearance temperature (WAT) and the quantity of wax deposition in eight different n-alkane mixtures using a correlative technique. The perturbed hard sphere chain equation of state (PHSC EoS) was employed in conjunction with the multi-solid model to describe the liquid-liquid and solid-liquid equilibria. The results are compared with experimental data. The results showed that PHSC EoS for some mixture of n-alkanes can perceptibly outperform the sole solid solution theory, improving the modelling of wax deposition quantities and wax appearance temperature by giving predictions closer to experimental values