The paper considers the influence of low-frequency pulsed force of an electromagnetic field on the formation of a solidifying metal structure, investigated experimentally. Pulses in the range from 0.1 to 10 Hz were applied to the melt, during the ingots growth in the cylindrical volume, to create the conditions of the forced convection. The final metal structure and deformation of the samples free surface, solidified in different conditions was analyzed. The reduction of porosity in metal ingots was stated for the cases of forced convection under the pulsed frequencies from 0.1 to 1 Hz, in comparison to the structure, formed by permanent electromagnetic stirring. The specimens, grown under the pulsed forcing with equal time of the pulses and the pauses have concave free surface, as well as in the cases of natural convection and permanent stirring. A flatter form of the free surface was obtained during the solidification process under the short pulses of electromagnetic force. Intorduction
Abstract. External heat transfer coefficient changing from corridor tube bundle to the reverse cross fluid flow with low-frequency asymmetrical pulsations was analyzed experimentally. The experiments were performed for Reynolds numbers in the tube bundle in the range of 100 ≤ Re ≤ 500 and changing of flow impulses frequency was limited with values 0.125 ≤ f ≤ 0.5 Hz, fluid pulsations amplitude in the tube bundle corresponded to 0.5 d ≤ A ≤ 1.2 d mm. A comparison of steady and non-steady conditions was made. The most efficient regime of pulsations in the studied range was found.
The methane-hydrogen fraction is a gaseous hydrocarbon by-product during oil processing for obtaining petroleum products. Until recently, the methane-hydrogen fraction was used as furnace oil in internal technological processes at a refinery. Some of the low-calorie methane-hydrogen fraction was burned in flares. Driven by the prospect of the methane-hydrogen fraction use as a fuel alternative to natural gas for burning in thermal power plants boilers, it became necessary to study the methane-hydrogen fraction combustion processes in large volumes. The conversion of ON-1000/1 and ON-1000/2 furnaces from the combustion of the methane- hydrogen fraction with combustion heat of 25.45 MJ/m3 to the combustion of the composition with combustion heat of 18.8 MJ/m3 leads to a decrease in temperature in the flame core for 100 °C as an average. The intensity of flame radiation on the radiant tubes decreases. Therefore, the operation of furnaces during combustion of methane-hydrogen fraction with a low heat of combustion at the gas oil hydro-treating unit is carried out only with a fresh catalyst, which allows lower flame temperatures in the burner.The experiments to determine the concentration of nitrogen oxides NOx and the burning rate w of the methane-hydrogen fraction in the ON-1000/1 furnace and natural gas in the TGM-84A boiler, depending upon the heat of combustion Qnr were carried out. The obtained results showed that the increase in the hydrogen content Н2 from 10.05 % to 18.36% (by mass) results in an increase in the burning rate w by 45%. The burning rate of natural gas with methane CH4 content of 98.89% in the TGM-84A boiler is 0.84 m/s, i.e. it is 2.5 times lower than the burning rate of the methane- hydrogen fraction with H2 content of 10.05%. The distributions of heat flux from the flame qf over the burner height h in the TGM-84A boiler were obtained in case of natural gas burning and calculation of burning of the methane-hydrogen fraction with a hydrogen content of 10.05% and methane of 28.27%. The comparison of the obtained data shows that burning of methane- hydrogen fraction causes an increase in the incident heat flux qf at the outlet of the burner.
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.
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