Sheikh Zahidul Islam

Fouling Reduction Characteristics of a No-Distributor-Fluidized-Bed Heat Exchanger for Flue Gas Heat Recovery

Heat exchangers and heat exchanger networks are extensively used for the purpose of recovering energy. In conventional flue gas heat recovery systems, the fouling by fly ashes and the related problems such as corrosion and cleaning are known to be major drawbacks. To overcome these problems, a single-riser no-distributor-fluidized-bed heat exchanger is devised and studied. Fouling and cleaning tests are performed for a uniquely designed fluidized bed-type heat exchanger to demonstrate the effect of particles on the fouling reduction and heat transfer enhancement. The tested heat exchanger model (1 m high and 54 mm internal diameter) is a gas-to-water type and composed of a main vertical tube and four auxiliary tubes through which particles circulate and transfer heat. Through the present study, the fouling on the heat transfer surface could successfully be simulated by controlling air-to-fuel ratios rather than introducing particles through an external feeder, which produced soft deposit layers with 1 to 1.5 mm thickness on the inside pipe wall. Flue gas temperature at the inlet of heat exchanger was maintained at 450°C at the gas volume rate of 0.738 to 0.768 CMM (0.0123 to 0.0128 m3/sec). From the analyses of the measured data, heat transfer performances of the heat exchanger before and after fouling and with and without particles were evaluated. Results showed that soft deposits were easily removed by introducing glass bead particles, and also heat transfer performance increased two times by the particle circulation. In addition, it was found that this type of heat exchanger had high potential to recover heat of waste gases from furnaces, boilers, and incinerators effectively and to reduce fouling related problems.

Numerical study of the effect of effective diffusivity and permeability of the gas diffusion layer on fuel cell performance

A three-dimensional, single-phase, isothermal, explicit electrochemistry polymer electrolyte membrane fuel cell model has been developed and the developed computational model has been used to compare various effective diffusivity models of the gas diffusion layer. The Bruggeman model has traditionally been used to represent the diffusion of species in the porous gas diffusion layer. In this study, the Bruggeman model has been compared against models based on particle porous media, multi-length scale particles and the percolation-type correlation. The effects of isotropic and anisotropic permeability on flow dynamics and fuel cell performance have also been investigated. This study shows that the modelling of the effective diffusivity has significant effects on the fuel cell performance prediction. The percolation-based anisotropic model provides better accuracy for the fuel cell performance prediction. The effects of permeability have been found to be negligible and the specification of any realistic value for permeability has been found to be sufficient for polymer electrolyte membrane fuel cell modelling.

Numerical investigation of two-phase flow induced local fluctuations and interactions of flow properties through elbow.

The local interactions and fluctuations of multiphase flow properties present in upward slug/churn flow patterns through a 90\(^0\) pipe bend has been investigated. Numerical modelling technique using the Volume of Fluid method (VOF) and Reynolds Averaged Naiver-Stokes equation (RANS) was used in this study. Validation of the modelling approach was carried out using the void fraction signals from the simulation and its PDF result. These signals compared well with reported experimental results for slug and churn flow patterns. Result analysis which focused on velocity and pressure fluctuations at three different cross-sectional planes of the elbow showed a reduction in the fluctuation energy (PSD) of the velocity signal at the downstream locations compared to the upstream. Similar behaviour was seen in the pressure signal. The observation was attributed to the change in multiphase flow patterns from slug to stratified/stratified wavy flow pattern after the bend. The results from this study intend to inform enhanced description of the local fluctuations of slug geometry, density and frequency for the accurate prediction of flow induced fluctuating forces due to slug-churn turbulent flows at pipe bends.

Modelling Multiphase Flow in Vertical Pipe Using CFD Method

Investigations of gas-liquid-solid flows in large diameter vertical pipes are scarce and detailed three phase flow study is still required to understand the flow interactions. Further investigation using high fidelity modelling is thus necessary due to complex flow interactions of the phases.

Fuel cells as an energy source for desalination applications.

Nowadays, there is a renewed interest in fuel cell technology from industry and academia, electrochemistry and catalysis scientists. This interest is due to environmental legislations for CO2 and other greenhouse gases emissions (United Nations Environment Programme and the World Trade Organization, 2009) that demand the use of high efficiency energy production systems. Such systems have great potential in the area of desalination technology (Kenet, 2003, Al-Hallaj et al., 2004, Singh, 2008, Wang et al. 2011, Jones, 2013). Fuel cells are characterised by high operation efficiency, which results in decreased fuel consumption, and low environmental impact. A fuel cell is a device that converts the chemical energy of a fuel directly into electricity through electrochemical reactions, with low waste heat (e.g. SOFC in Fig. 1). The first fuel cell was fabricated back in 1830s, and slow but steady progress has been made toward their commercialization since then.

Parametric Study of Polymer Electrolyte Membrane Fuel Cell Performance Using CFD Modelling

A comprehensive three-dimensional, isothermal, steady-state, straight-channel proton exchange membrane (PEM) fuel cell model was developed to investigate the transport limitations of fresh reactants at high current densities. The model is created based on the existing models in literature to predict the reactant’s transport limitations at higher current densities using three-dimensional framework. A user-defined function (UDF) code was developed considering source terms for porous zones, effective diffusivity models for species transport inside cells and electrochemistry algorithm to predict cell voltage at an average current density. Water transport through membrane was implemented considering electroosmotic drag and back diffusion inside PEM fuel cell. Simulation-predicted cell performances for different average current densities were validated with experimental results, and the effect of design parameters on cell performance is obtained using parametric studies. Parametric studies were performed to determine the best possible operating and geometrical design parameters of PEM fuel cell.

Design Considerations and Heat Transfer Enhancement of CFB Heat Exchangers for Flue Gas Heat Recovery

Now-a-day’s energy recovery process in the industry is a common practice for improving the production process while major concern goes to environment. The performance of the heat exchangers, used for the purpose of recovering energy, decreases continuously with time due to fouling depending on surface temperature, surface condition, construction material, fluid velocity, flow geometry and fluid composition. To overcome the fouling of fly ash on the heat transfer surface and erosion and periodical cleaning which are the major drawbacks in conventional heat exchangers for flue gas heat recovery, a no-distributor-circulating-fluidized-bed (NDCFB) heat exchanger with automatic particle controlling is devised. One of the main advantages of this model is the reduced pressure drop through the entire heat exchanger system, while increasing heat transfer performance. The research started with a single riser system with multiple down comers and multi-riser system is also studied. The heat transfer performance and pressure drop have been evaluated through experiments for these gas-to-water lab scale heat exchanger systems. However, due to the operational complexity, these two models are not readily applicable to real applications. As a derivation of the previous studies regarding the no-distributor CFB heat exchangers, third generation model of the heat exchanger is now under investigation.Copyright