[PDF] Numerical Simulation Of Kraft Recovery Boiler Sootblower Jets eBook

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Page : pages
File Size : 31,26 MB
Release : 2002
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PhD.

Author : Babak Emami
Publisher :
Page : 0 pages
File Size : 34,14 MB
Release : 2009
Category :
ISBN : 9780494609460

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The fouling of heat transfer surfaces in kraft recovery boilers is a significant concern for the pulp and paper industry. The usual approach to controlling fouling is the use of so-called "sootblowers," that utilize boiler steam to generate supersonic steam jets that are literally used to knock deposits off of the boiler tubes. About 3 to 10% of the total steam produced in a recovery boiler is used for sootblowing. This high energy cost demands that they be operated as efficiently as possible. It is thus essential to devise improved strategies for maximizing sootblower efficiency and minimizing steam consumption. To achieve this, the behaviour of sootblower jets, and the effects of various parameters on sootblowing, must be well understood. This thesis documents a study of the performance of sootblower jets using numerical simulation; CFDLib 3.02, a CFD code from the Los Alamos National Laboratory, was used for the simulations. This work had two main parts. In the first part, sootblower jets that perform at the design condition (fully-expanded jets) were studied; in the second part, the study was extended to off-design (under/over-expanded) sootblower jets. In the second part, the study was extended to sootblower jets not operating at the design condition. The compressibility-corrected code failed to properly simulate these under-over/expanded supersonic jets. A wide series of tests was carried out to determine that the problem was due to the turbulence model. The model was then modified to account for turbulence/shock wave interaction, by adding corrections to take into account shock unsteadiness and a realizability constraint. The new model yielded good agreement with some available measurements. The new model was then used to successfully predict some actual sootblower measurements, and to study the interaction of a sootblower jet with geometries similar to tube banks in recovery boilers. A parametric study was carried out to examine the effect of the offset between a sootblower jet and a tube bank, and of deposit size on a sootblower jet. The results indicate that the shock cell structure of a jet is only slightly affected by the offset, but that the size of a deposit strongly affects the pressure exerted by the impinging sootblower jet, which depends both on the jet shock cell structure, and on the location where the interaction occurs. In the first part, a compressibility-corrected version of CFDLib was validated against a wide range of available experimental data, of subsonic and fully-expanded supersonic free and impinging jets; simulations successfully predicted all of the cases. This compressibility-corrected model was then deemed suitable for modeling the fluid mechanics of fully-expanded sootblower jets, and so was used to study the effects of two parameters on sootblower jets: the lance pressure, and the rate of rotation of a sootblower. To study the effect of the lance pressure, numerical simulation was used to model fully-expanded sootblower jets corresponding to a range of lance pressures. To study the effect of rotation, the equations of motion were modified by adding the Coriolis and centrifugal terms, so that computations could be performed in a rotating frame of reference. Simulations were then run to study a fully-expanded sootblower jet operating at different rotation rates. The results indicate that sootblowers operate more efficiently at lower lance pressures, and that the rate of rotation does not significantly affect the structure of a sootblower jet.

Author : Kayhan Kermani
Publisher :
Page : pages
File Size : 29,40 MB
Release : 2001
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Sootblowers are used to control fireside deposit accumulation in kraft recovery boilers. A sootblower produces high-velocity steam jets which impinge on deposits and remove them from heat transfer tube surfaces. How the jet interacts with deposits and tubes is not well understood. Since direct measurement of sootblower jet flow characteristics in a kraft recovery boiler is difficult, a numerical model has been developed to simulate a sootblower jet as it propagates between two platens and interacts with deposits. The numerical model was first used to simulate a scaled laboratory air jet exerted on a deposit. The numerical results were validated by the experimental data. The model was subsequently used to simulate a full-scale sootblower jet under recovery boiler conditions. It shows that the deposit removal efficiency of a sootblower is a strong function of the distance between the nozzle and deposit, pressure, and deposit height.

Author : Seyed Abdolreza Ebrahimi-Sabet
Publisher :
Page : pages
File Size : 42,67 MB
Release : 2001
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ISBN :

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The accumulation of fireside deposits on heat exchange surfaces in a kraft recovery boiler drastically reduces the boiler thermal performance, restricts the flue gas flow, and in severe cases plugs the flue gas passages leading to unscheduled boiler shutdowns for cleaning. High-pressure steam sootblower jets are used to remove these deposits. Deposits may be brittle or "plastic", depending on their temperature, and may be removed by sootblowers by two main mechanisms: brittle fracture and debonding. Brittle fracture has been previously studied and applies only to brittle and low-strength deposits. Debonding is likely to be the main mechanism by which strong deposits are removed by breaking the weak bond between the deposit and the tube surface. Deposit removal by a debonding mechanism is the main subject studied in this thesis. To study the effects of aerodynamic drag and lift forces and flow induced vibrations on deposit debonding, the instantaneous drag and lift forces generated by a supersonic jet impinging on artificial deposits were measured in laboratory experiments. By numerical analysis, the fluctuation components of the drag and lift forces were decoupled from the flow-induced vibrations. Results showed that the lift force fluctuations and flow-induced vibrations acting on deposits increased and reached maxima at a distance of about 45-50 nozzle diameters downstream. The flow-induced vibrations were found to be much larger in the lateral direction than in the flow direction. These vibrations increase the lift force fluctuations by a maximum factor of '0.872 '/'ß0.5', where 'ß ' is the damping coefficient of the tube-deposit assembly. This is in good agreement with the results of the experiments, where the fluctuating lift forces were observed to be as effective as the mean drag force in removing artificial deposits. An analysis of the dimensionless power spectra of lift forces exerted on cylindrical deposits showed a high degree of similarity under a broad range of operating conditions. The information can be used to estimate the power spectra of lift forces exerted on deposits in full-scale boilers. The results would provide a basis for estimating the role of vibration in deposit removal in full-scale boilers.

Author : Thomas B. Gatski
Publisher : Academic Press
Page : 343 pages
File Size : 48,64 MB
Release : 2013-03-05
Category : Science
ISBN : 012397318X

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Compressibility, Turbulence and High Speed Flow introduces the reader to the field of compressible turbulence and compressible turbulent flows across a broad speed range, through a unique complimentary treatment of both the theoretical foundations and the measurement and analysis tools currently used. The book provides the reader with the necessary background and current trends in the theoretical and experimental aspects of compressible turbulent flows and compressible turbulence. Detailed derivations of the pertinent equations describing the motion of such turbulent flows is provided and an extensive discussion of the various approaches used in predicting both free shear and wall bounded flows is presented. Experimental measurement techniques common to the compressible flow regime are introduced with particular emphasis on the unique challenges presented by high speed flows. Both experimental and numerical simulation work is supplied throughout to provide the reader with an overall perspective of current trends. An introduction to current techniques in compressible turbulent flow analysis An approach that enables engineers to identify and solve complex compressible flow challenges Prediction methodologies, including the Reynolds-averaged Navier Stokes (RANS) method, scale filtered methods and direct numerical simulation (DNS) Current strategies focusing on compressible flow control