Medhat A. Nemitallah

Heat Transfer Characteristics in a Double-Pipe Heat Exchanger Equipped with Coiled Circular Wires

An experimental investigation has been carried out to study the enhancement in heat transfer coefficient by inserting coiled wire around the outer surface of the inner tube of the double-pipe heat exchanger. Insulated wires, with a circular cross-section of 2 mm diameter, forming a coil of different pitches (p = 6, 12, and 20 mm), were used as turbulators. The investigation is performed for turbulent water flow in a double-pipe heat exchanger with cold water in the annulus space for both parallel and counter flows. The experiments were performed for Reynolds numbers ranging from 4,000 to 14,000. The experimental results reveal that the use of coiled circular wires leads to a considerable increase in heat transfer coefficients compared with a smooth wall tube for both parallel and counter water flows. The mean Nusselt number increases with Reynolds number and pitch. The convective heat transfer coefficient for a turbulent water flow increases for all coiled wire pitches, with the highest enhancement of about 450% for counter flow and 400% for the parallel flow. New correlations for mean relative Nusselt numbers at different coiled wire pitches are provided.

Solid Particle Erosion Downstream of an Orifice

The paper deals with solid particle erosion downstream of a sharp-edged orifice commonly found in many chemical processing industries. The orifice is installed in a pipe that is long enough to ensure fully developed turbulent flow in both upstream and downstream directions. Both the k-e model and the Lagrangian particle-tracking technique were used for predicting solid particle trajectories. Gambit 2.2 was used to construct the computational grid and the commercial Fluent 12.1 code was used to perform the calculations. The available erosion correlations were used for determination of erosion characteristics considering carbon steel and aluminum pipes. The investigation was carried out for a flow restricting orifice of fixed geometry and pipe flow velocities in the range 1–4 m/s using solid particle of diameters 50–500 μm. The results indicated two critical erosion regions downstream of the orifice: the first is in the immediate neighborhood of the orifice plate and the second is in the flow reattachment zone. The results showed also a strong dependence of erosion on both particle size and flow velocity.

Investigation of liquid ethanol evaporation and combustion in air and oxygen environments inside a 25 kW vertical reactor

The combustion characteristics of liquid ethanol in air and oxygen–carbon dioxide environments are investigated numerically inside a vertical reactor. Gambit 2.2 was used to construct the mesh and Fluent 12.1 was used to perform the calculations. Different oxidizer environments were considered including pure air, oxygen-enriched air, in addition to the cases of OF21 (21% O2 and 79% CO2) and OF29 (29% O2 and 71% CO2). Comparisons were performed between the different cases and the results were validated against wide range of the experimental data. Nonpremixed combustion model which utilizes probability density function to predict the scalar quantities was incorporated to simulate the combustion process. Two turbulence models, realizable k–ɛ (RKE) model and Reynolds stress model (RSM), were applied and their results were compared. The Euler–Lagrange approach was utilized to solve the discrete phase model. The results showed the ability of the RKE model to predict much closer data to the experimental data than the RSM which over predicts the temperature. For the case of OF21, the flame was lifted and the combustion temperature was reduced as compared to the air combustion case. However, the OF29 combustion case resulted in a very close performance to the case of air combustion.

Investigations of an Ion Transport Membrane Reactor Specially Designed for a Power Cycle

This work is aiming to investigate the dependence of the performance of a 3-D ITM reactor depends on the operating conditions and flow configuration. This work is a design problem, where the oxygen separation requirements of the ITM reactor are specified, and then the reactor is designed to meet them. The effect of subdividing the total reactor length into a number of parallel subunits rather than only one unit on the flow characteristics and membrane stability is studied. The results indicate that the average wall temperature is higher in the case of counter current flow than in the case of co-current flow; this is attributed to the effective heat transfer in the case of counter-current flow, and as a result, the average partial pressure driving force was found to be much lower in the case of counter current flow in order to get the same average flux for both flow configuration. The present results indicate that the use of parallel design instead of series design will result in shorten the channel length, reduce pressure drop through the system and will result in more stable operation of the membrane. Also, this design takes the benefit of high oxygen permeation flux at channel inlet which will reduce the total size of the reactor.

Boilers Optimal Control for Maximum Load Change Rate

In many cogeneration systems, one or more boilers are used in hot standby to meet the plant demand of steam in case of failure or upset in the cogeneration unit. Such boilers need to quickly respond to sudden and large steam load changes. However, fast changes in the firing rate cause transient changes in both the drum-boiler steam pressure and drum level, in addition to the potential of developing of thermal stresses in the walls of steam risers. A genetic algorithm (GA) based optimization scheme is proposed for tuning the conventional boiler control loops to maximize the ability of the boiler to respond to large steam demand while keeping the fluctuations in pressure, drum level, and feed rate within acceptable operation limits. A nonlinear model for an actual boiler is first built, validated, and then, it is used to demonstrate the performance of the boiler with the proposed control loop optimization.

Characteristics of Oxyfuel and Air–Fuel Combustion in an Industrial Water Tube Boiler

The characteristics of oxyfuel combustion and air–fuel combustion in the furnace of a typical industrial water tube boiler using methane as the operating fuel are investigated. Two oxyfuel cases are considered. The analysis is conducted for two oxyfuel cases that correspond to 21% O2 and 29% O2 in the oxidizer mixture (O2 + CO2). A renormalized group (RNG) turbulence model and the eddy dissipation model are utilized in the present work to provide the turbulence characteristics and the production rate of species. The solution of the radiative transfer equation was obtained using the discrete ordinates radiation model. The set of governing equations and the boundary conditions are solved numerically using Fluent computational fluid dynamics code considering a single-step reaction kinetics model for methane–oxyfuel combustion. Comparison of both oxyfuel combustion and air–fuel combustion indicates that the temperature levels are reduced in oxyfuel combustion. The results show that the temperature levels are greatly reduced as the percentage of recirculated CO2 is increased. It is concluded that the flame propagation speed in the CO2 environment is lower than that in N2. It is found that the natural gas and oxygen consumption rates are slower in oxyfuel combustion relative to air–fuel combustion. Heat transfer from the burnt gases to the water jacket along the different surfaces of the furnace is calculated. It is shown that the energy absorbed is much lower in the case of oxyfuel combustion along all surfaces except for the end part of the furnace close to the furnace rear wall. However, the same performance of the methane-oxy-flames is expected by increasing the oxygen concentration slightly above 29%.

Boiler dynamic control with optimized nitric oxides and efficiency

Boiler is a steam generating device that is used to generate electricity and provide heat in process industry and buildings. The generation of steam is carried out by harnessing thermal energy generated via combustion process. The key challenges that are posed in this process are harmful nitric oxide emissions and the energy losses from the total energy contained in the fuel. It is highly required to reduce these losses to improve boiler efficiency; however, when the operational parameters are adjusted to maximize boiler efficiency, the nitric oxide formation is adversely affected, that is, nitric oxide formation also goes up. Moreover, a little change in demand of steam may cause disturbance in all the dynamics of boiler which may go unstable if not controlled properly. All these issues necessitate measures to be taken to optimize boiler efficiency and nitric oxide as well as to regulate operational parameters like drum pressure and drum level all at the same time. In this work, a detailed study has been carried out to investigate how thermal nitric oxide emissions, combustion process and dynamics of boiler interact with each other. In this respect, dynamic models of nitric oxides, efficiency and other operational variables of boiler have been investigated, and these models have been combined to form a joint model of whole boiler system. This model is then utilized to form an efficient control of boiler variables along with trade-off-based optimization between efficiency and thermal nitric oxide emission. The results have been formed using an experimental input data from a typical package boiler to ensure the practicability of the proposed technique.

CFD analysis of CO 2 adsorption in different adsorbents including activated carbon, zeolite and Mg-MOF-74

The present study focuses on modelling of a CO2 adsorption system in different adsorbents including activated carbon, zeolite and metal organic frameworks (MOFs) as one of the most promising methods for post-combustion carbon capture. Heat and mass transfer features of the numerical model are validated against the available experimental in the literature. The adsorption characteristics of CO2 in activated carbon are studied in detail under fixed pressure of 2 bar. This is followed by comparisons between different adsorbents including activated carbon, zeolite and MOFs. The effects of storage pressure on adsorption are studied for different adsorbents for a range of pressure from 20 kPa to 100 kPa. For all adsorbents, the results showed high adsorption at the entrance and near wall regions. The adsorption capacity of all adsorbents has been increased by increasing the storage pressure. Mg-MOF-74 adsorption material resulted in the highest adsorption capacity as compared to other materials.

Fluid to Fluid Modeling for Post Dry Out Using Dimensional Analysis and Energy Scaling

In this work, both dimensional analyses using Buckingham Pi-theorem and scaling of the energy equation have been applied successfully in fluid to fluid modeling for post dry out to model the Freon (R-134a) data available in the literature and convert it to water equivalent data. Also the results are compared with the available data in the literature for water. Experimental data sets in two fluids are assumed to be equivalent if the values of the dimensionless groups are equal for both fluids. Both methods are used and the results are compared with the experimental data at different operating conditions. The Katto and the Ahmad modeling dimensionless parameters coming from the analysis using Pi-theorem predicted successfully the equivalent data of water at moderate mass fluxes. However, at too high or low mass fluxes, this method deviated from the experimental data. However, the fluid to fluid modeling using the scaling of energy equation is applicable at any operating conditions and the results are too close to the experimental data.

Investigations of Oxy-Fuel Combustion Characteristics and Oxygen Permeation Process in a Stagnation Flow ITM Reactor

In this work, a modified two-step reaction kinetics model for methane-oxygen combustion is used to predict oxy-combustion characteristics and permeation rate characteristics inside a stagnation flow simple symmetric ITM reactor. New coefficients oxygen permeation equation model is introduced herein this work by fitting the experimental data available in the literature for a LSCF-1991 ion transport membrane. Using CH4 as fuel plus CO2 as sweep gas, the effects of reactivity is analyzed using the same sweep gases (CH4 plus CO2) is investigated here by comparing the same cases with and without considering reactions in the permeate side. Also the influences of the percentage of CH4 in the sweep gases mixture on the permeation and combustion processes are included in this work. It was found that the oxygen permeation flux increases with activating chemical reactions in the permeate side of the membrane and this is due to increased partial pressure driving force across the membrane surface as a result of disappearance of oxygen molecules in the permeate side because of combustion. Another reason for this is the increase in the membrane temperature which affects the activation energy of the membrane and so the permeation rate. More details about the models used and the oxy-fuel combustion characteristics in the permeate side of the membrane are included in the present study.