Efficiency of tube bundle cleaning by low-frequency flow pulsations with solid particles

Scale buildup on the tube surface in the intertubular space of the shell-and-tube heat exchangers reduce their efficiency. The topical issue is the search for clean-in-place methods. The tube bundle cleaning by low-frequency nonsymmetrical pulsations is understudied. The aim of the paper is numerical analysis of the influence of pulsations on the key cleaning factors (wall shear stress, erosion rate). For the numerical experiment the symmetrical element of a staggered tube bundle with a crossflow of turbine oil (T22) (Re = 100; Pr = 273) and the quartz sand as a cleaning agent is used. The model of incompressible fluid flow comprises the system of Navier-Stokes and continuity equations, the turbulent model Spallart-Allmaras. The motion of solid particles is calculated by the discrete element method, and the erosion rate is calculated by the Campos-Amezcua method. In unsteady conditions with time step 0,001 sec, numerical simulations are performed in Ansys Fluent. Pulsations are generated on entry boundary condition. To estimate the flow pulsation efficiency, the wall shear stresses on the central tube of bundle and erosion rates are compared under the same average rate in steady and nonsteady flow. It is found that asymmetrical flow pulsations (duty cycle 0,25) increase of wall shear stress in all the modes under consideration (amplitude 25 ≤ A/d ≤ 35, frequency 0,3125 ≤ f ≤ 0,5 Гц), but an increase in erosion rate takes place only at maximal frequency. The amplitude variation displaces the localization of the reinforcing effect of flow pulsations on the tube surface. However, it is found that flow pulsations increase the wall shear stress and erosion rate in the front and rear sides of the tubes that are most susceptible to scale buildup. The conducted analysis confirms the significant influence of asymmetrical pulsations on cleaning factors and the perspective of their application for intensification of tube bundle cleaning. The detected effects can be the base to develop new technologies of cleaning intertubular space of heat exchangers.

Ensuring Dynamic Accuracy of Aircraft’s Air Data System with Motionless Flush-Mounted Receiver of Flow

The article views, that draw-backs of aircraft’s traditional air data systems (ADS), built based installed in incoming air flow and installed outside the fuselage the pitot tube booms, temperature braking receivers, vane sensors of incidence angle and gliding angle are eliminated in original ADS with motionless flush-mounted receiver of flow. The functional scheme of aircraft’s air data system with motionless flush-mounted receiver of flow, built based on the original ion-mark sensor of aerodynamic angle and true airspeed, on receiving board of which the hole-receiver is installed to perceive the static pressure of incoming air flow. Models of operator sensitivity and dynamic errors of instrumentation channels due to random stationary atmospheric turbulence and random flow pulsations at location of the ion-mark sensor on fuselage of the aircraft are presented. Recommended to use the optimal linear Wiener filter, the synthesis method of which is revealed on example of the true airspeed instrumentation channel to reduce the stationary dynamic errors of instrumentation channels of air data system with motionless flush-mounted receiver due to atmospheric turbulence. Recommended to use the principle of integration to reduce the stationary random dynamic errors of instrumentation channels of air data system with motionless flush-mounted receiver due to flow pulsations near fuselage at location of ion-mark sensor. Proposed to use aeromechanical measuring and computing system built based VIMI method with Luenberger observer as an additional component of integrated air data system. Integrated system simulates the movement of aircraft in this flight mode and by flight parameters measured with high accuracy using flush-mounted receivers "restores" air signals included in equations of movement of aircraft. The structure, method and algorithms for determining air signals in channels of aeromechanical measuring and computing system with a Luenberger observer are presented. Using the example of true airspeed measurement, the analysis and quantitative assessment of residual dynamic error of integrating channel of integrated aircraft’s air data system with motionless flush-mounted receiver of flow is carried out.

Hydrodynamics of Pulsed Fluidized Bed of Ultrafine Powder: Fully Collapsing Fluidized Bed

The processing of fine and ultrafine particles using a fluidized bed is challenging in view of their unpredictable hydrodynamic behavior due to interparticle forces. The use of assisted fluidization techniques in such cases can be effective in improving the bed hydrodynamics. This work investigates the dynamics of pulsed fluidized bed of ultrafine nanosilica subjected to square-wave flow pulsations. The pulse duration used in this study is sufficient to allow the complete collapse of the pulsed fluidized bed between two consecutive flow pulsations. The proposed pulsation strategy is carefully implemented using electronic mass flow controllers with the help of analog output signals from data acquisition system. Given that the different regions of the fluidized bed exhibit varying dynamics, which together contribute to overall bed dynamics, the bed transients in the upper, central, and lower regions of the fluidized bed are monitored using several sensitive pressure transducers located along the height of the bed. The effect of the flow pulsation on the hydrodynamics of the fluidized bed is rigorously characterized. A significant reduction in the minimum fluidization velocity was obtained and an increase in the bed homogeneity was observed due to flow pulsations. The frequency domain analysis of the signals clearly delineated the frequency of the various events occurring during the fluidization.

Deagglomeration of Ultrafine Hydrophilic Nanopowder Using Low-Frequency Pulsed Fluidization

Low-frequency flow pulsations were utilized to improve the hydrodynamics of the fluidized bed of hydrophilic ultrafine nanosilica powder with strong agglomeration behavior. A gradual fluidization of unassisted fluidized bed through stepwise velocity change was carried out over a wide range of velocities followed by a gradual defluidization process. Bed dynamics in different regions of the fluidized bed were carefully monitored using fast and sensitive pressure transducers. Next, 0.05-Hz square-wave flow pulsation was introduced, and the fluidization behavior of the pulsed fluidized bed was rigorously characterized to delineate its effect on the bed hydrodynamics by comparing it with one of the unassisted fluidized bed. Flow pulsations caused a substantial decrease in minimum fluidization velocity and effective agglomerate diameter. The frequencies and amplitudes of various events in different fluidized bed regions were determined by performing frequency domain analysis on real-time bed transient data. The pulsations and their effects promoted deagglomeration and improved homogeneity of the pulsed fluidized bed.