Justification of the design and technological parameters of the methane tank stirrer

Abstract

The article presents the results of the initial experimental studies of the methane tank stirrer for the biogas plant system with the hydraulic system of transportation and mixing of the substrate. At the previous stages of scientific researches, the composition and parameters of the hydraulic system, as well as the hydromechanical stirrer based on the Segner wheel type, were theoretically determined. Taking into account the real material and technical possibilities and the ban on conducting experiments with real manure in the conditions of an educational institution, research experiments were conducted with a model of a stirrer on a model liquid. Since the dimensions of the model are significantly different from the real sample, the Froude similarity criterion was applied, which is essentially a measure of the ratio of the energy of the mass forces to the inertial forces of the flow. The radius of the blade is taken as the determining size. Then, for a real sample, l1 = 1.5 m; for the model l2 = 0.15 m. According to Froude's criterion, to maintain similarity, the speed of the blade should be 0.158 m/s, the angular speed of rotation of the model 1.05 rad/s; a rotation frequency of 10 min-1 is required.

To evaluate the quality of mixing, the coefficient of variation is taken, and the concentration of dry matter is the key component. Therefore, during the experiments, it was necessary to determine the moisture content in 6 samples taken at three points of the container at the upper and lower level.  The obtained data analysis showed that in almost all options for installing the blade, the concentration of dry matter is higher in samples from the upper layer of the mixture. That is, the stirrer will definitely prevent segregation (stratification) of the mixture. However, the coefficient of variation shows that the best quality of mixing (no more than 10) will be in the case when the blade deviation angle is in the range β1 = -150...+300. At an angle of more than 450, the quality becomes unsatisfactory because the coefficient of variation is Vc>20.

As the obtained results show, the change in the β2 angle has a stronger effect on the process’s quality. The acceptable mixing quality is ensured in the range β2 = -15...+200, but the best indicator is at β2 =0...100. When the blade is tilted at an angle of more than -200 (that is, with the upper edge in the direction of movement), the mixing quality becomes unsatisfactory.

From the experiments, it was established that when the blade is turned against the direction of rotation, the power decreases, obviously, due to a decrease of the midsection area. But at the same time, the mixing quality deteriorates rapidly. The minimum point corresponds to the vertical position of the blade. In all other options, the power gradually increases, obviously due to the formation of zones of increased pressure.

In general, the blade installation angles β1=29° and β2=12°, determined by us theoretically, are close to those that ensure the proper quality of mixing with minimum power consumption.