Shahab Khushnood

CFD analysis of heat transfer within a bottom heated vertical concentric cylindrical enclosure

The CFD analysis of heat transfer within a bottom heated vertical concentric cylindrical enclosure is important with respect to the process in the chemical and process industries. The research work focuses on the CFD analysis of the enclosure with respect to the machines used for the segregation of chemicals in the chemical industries. The CFD simulations are performed to study the effects of inner cylinder material and outer cylinder geometric configurations on the heat transfer mechanism in the enclosure. The CFD simulations are conducted at a bottom disc temperature of 393 K and compared with the published results at a temperature of 433 K. This research study depicts the behavior of bottom heated vertical concentric cylindrical enclosure at different bottom disc temperatures. This study also investigates the heat transfer mechanism of the enclosure using different inner cylinder materials and different configurations of the outer cylinder. In such enclosures a uniform temperature is required for the segregation of chemicals. A more uniform temperature is observed in the enclosure by using aluminum inner cylinder of the bottom disc and using two different diameter outer cylinders as compared to the mild steel and stainless steel.

Cross-Flow-Induced-Vibrations in Heat Exchanger Tube Bundles: A Review

Over the past few decades, the utility industry has suffered enormous financial losses because of vibration related problems in steam generators and heat exchangers. Cross-flow induced vibration due to shell side fluid flow around the tubes bundle of shell and tube heat exchanger results in tube vibration. This is a major concern of designers, process engineers and operators, leading to large amplitude motion or large eccentricities of the tubes in their loose supports, resulting in mechanical damage in the form of tube fretting wear, baffle damage, tube collision damage, tube joint leakage or fatigue and creep etc.

Effect of Temperature on Damping in Shell and Tube Heat Exchangers

Flow-induced vibration in heat exchangers has been a key source of concern in the process, power generation and nuclear industry for several decades. Many incidents of failure of heat exchangers due to apparent flow-induced vibration have been reported. Design of tube bundles with loosely supported tubes in baffles for process shell and tube heat exchanger and steam generator needs estimation of energy dissipation mechanisms or damping for a safer and long term operation. Damping has a major influence on the flow induced vibrations and is dependant on a variety of factors such as mechanical properties of the tube material, geometry and number of intermediate supports, the physical properties of shell-side fluid, type of tube motion, tube frequency, shell-side temperature etc. Various damping mechanisms have been identified and quantified such as Friction damping, Viscous damping, Squeeze film damping, Support damping and Two-Phase damping which affect the performance with respect to flow induced vibration design, including standard design guidelines. But generally the effects of the higher operating temperatures on the various damping mechanisms are neglected in the general design procedure. The operating temperatures play significant role on the contribution of various damping mechanisms. The current paper focuses on the thermal aspects of damping mechanisms subjected to single phase cross-flow in process heat exchangers and formulates the design guidelines for safer design based on experimental and empirical formulations. The research results show that he increase in the temperature results in the increase of the damping. Moreover it found that the natural frequency is higher for lower mass flow rate and lower working pressures and lower temperatures.Copyright

Modeling and simulation of an orbit controller for a communication satellite

It is of critical importance to control the altitude of a satellite especially one used for worldwide communications in a geo-stationary orbit. The objective of this work is to evolve a design based on modeling and simulation of an orbit controller for a satellite orbiting into a circular orbit. This involves a good understanding of the system dynamics. Once a satellite is launched in a desired orbit, it never remains in that ideal orbit. The external forces present in space cause perturbations to this ideal orbit. To bring back the satellite into the desired orbit, on-board thrusters provide in-orbit propulsion. In this work, the altitude of the satellite is controlled by the thrust produced by the on-board thrusters installed in the radial and tangential directions. However, dictated by the controllability requirement, we have analyzed the dynamic system stabilization with the application of two thrusters as well as one thruster. Thus the feedback dynamic control system considers both the two-input and the single-input cases. The model developed is essentially a linearized, normalized and state-space model. The simulation of this model is based on the MATLAB software package. The design thus evolved is used to study the effect of pole-placement on the controlling parameters like settling time, peak time, overshoot, and damping ratio of the closed-loop system. This enables us to make predictions about the stability requirements for any dynamic system of the type considered. The design tool thus developed is applied to a current actual communication satellite design. The design results are compared and recommendations are made.

Influence of electric discharge machining on fatigue limit of high strength aluminum alloy under finish machining

Abstract The aim of this study was to investigate the fatigue limit of the electric discharge machined aluminum alloy 2024 T6. Machining was performed at 3, 6, 9, and 12 A discharge current values while all other parameters were kept constant. The fatigue tests were performed on a four-point rotating bending machine at the frequency of 50 Hz and at ambient temperature. Fatigue limits at 107 cycles were determined using staircase (up-and-down) method and the obtained data was analyzed statistically. For reference purposes, fatigue strength of the conventionally turned specimens was also found by the same technique. The EDM surface is characterized by its morphology, roughness, hardness, and thickness of the resolidified layer. The effects of discharge current values on these surface characterizing parameters and subsequent influence on fatigue limit have been discussed.

Numerical study of conjugate heat transfer within a bottom heated cylindrical enclosure

The conjugate heat transfer within a bottom-heated non-conventional cylindrical enclosure is important with respect to the processes in chemical and nuclear industries. In this research work numerical analysis of the enclosure with respect to the centrifuge machines used in the process industry is presented. Various CFD simulations of the experimental work of [1] are performed to study the effects of materials of the disc and inner cylinder and geometries of outer cylinder on the enclosure. Generally, a uniform temperature is required in such enclosures. A more uniform temperature is observed in the enclosure by using aluminum inner cylinder at a temperature of 433K of the bottom disc and using two different diameter outer cylinders as compared to the mild steel and stainless steel.

Hydraulic Autofrettage Technology: A Review

Thick-walled cylinders play a unique role in military, automobiles, aircraft and oil fields. The autofrettage method is used to increase the fatigue life of thick-walled components like gun barrel, titanium alloy tubes, nuclear reactor vessels, high pressure oil field pipes and LDPE. There are three types of autofrettage processes and hydraulic is the one applied mostly. The autofrettage process uses a pressure that is high enough to plastically deform the bore of the part but not so high that it bursts the component, is applied to the inside of tube. The result is that after the pressure is removed, the elastic recovery of the outer wall put the inner wall into compression, providing a residual compressive stress. In this research paper a study has been carried out to investigate the progress made in different fields by the hydraulic technique and proposals are sought for future work.Copyright

Modeling and Analysis of Thermal Damping in Heat Exchanger Tube Bundles

Most structures and equipment used in nuclear power plant and process plant, such as reactor internals, fuel rods, steam generator tubes bundles, and process heat exchanger tube bundles etc., are subjected to Flow Induced Vibrations (FIV). Costly plant shutdowns have been the source of motivation for continuing studies on cross-flow induced vibration in these structures. Damping has been the target of various research attempts related to FIV in tube bundles. A recent research attempt has shown the usefulness of a phenomenon termed as “thermal damping”. The current paper focuses on the modeling and analysis of thermal damping in tube bundles subjected to cross-flow. It is expected that the present attempt will help in establishing improved design guidelines with respect to damping in tube bundles.Copyright

Economic Dispatch Using Genetic Algorithm Based Hybrid Approach

Power Economic Dispatch (ED) is vital and essential daily optimization procedure in the system operation. Present day large power generating units with multi-valves steam turbines exhibit a large variation in the input-output characteristic functions, thus non-convexity appears in the characteristic curves. Various mathematical and optimization techniques have been developed, applied to solve economic dispatch (ED) problem. Most of these are calculus-based optimization algorithms that are based on successive linearization and use the first and second order differentiations of objective function and its constraint equations as the search direction. They usually require heat input, power output characteristics of generators to be of monotonically increasing nature or of piecewise linearity. These simplifying assumptions result in an inaccurate dispatch. Genetic algorithms have used to solve the economic dispatch problem independently and in conjunction with other AI tools and mathematical programming approaches. Genetic algorithms have inherent ability to reach the global minimum region of search space in a short time, but then take longer time to converge the solution. GA based hybrid approaches get around this problem and produce encouraging results. This paper presents brief survey on hybrid approaches for economic dispatch, an architecture of extensible computational framework as common environment for conventional, genetic algorithm & hybrid approaches based solution for power economic dispatch, the implementation of three algorithms in the developed framework. The framework tested on standard test systems for its performance evaluation.Copyright

A Comparative Study of Cross-Flow Induced Vibrations in Heat Exchanger Tube Bundles Using Bond Graph Approach

Flow-induced vibration (FIV) has been a major concern in the nuclear and process industries involving steam generator and heat exchanger tube bundle design. Various techniques and models have been developed and used for the analysis of cross-flow induced vibration of tube bundles. Bond Graph approach has been applied to existing FIV excitation models, followed by a comparative study. Results have been obtained using 20-SIM software. It is expected that the current approach will give a new dimension to the FIV analysis of tube bundles.© 2005 ASME