Ewa Piotrowska

An improved model for forces on narrow soil cutting tines

The existing models for predicting forces on narrow soil cutting tines are based on different assumptions and give different results of calculated values. A new mathematical model for predicting forces on narrow soil cutting tines has been developed including a more realistic geometry of soil wedge, compared to previous mechanical models. The new model takes into account variability of the inclination angles of bottom failure surface in the side segments. Also included is the calculation of inertial forces for differential side segments based on the spatial position of the velocity triangle, which gives different absolute velocity for particular segments. The forward rupture distance depends on critical depth and the side rupture distance is given by a regression equation. Draft prediction is based on the complex set of forces, including the critical depth and two-dimensional failure zone together with inertial forces in the three-dimensional zone. The unknown values of the critical depth together with the rupture angle have been calculated by the minimisation of the total force acting on the tool. In order to check the extended model, experiments were performed in a soil bin and the results were compared with those calculated from the new model as well as from existing models. Absolute values of difference between calculated and observed values, related to measured values, taken as an error of different theoretical models, amounted to 11.7% for our model, and 17.1–34.9% for existing models. Statistical analysis of measured and calculated forces showed that for 58 degrees of freedom and a confidence level α=0.05, the t-test value amounted to 0.795 and the significance level was 0.430. It means that the H0 hypothesis cannot be rejected and the developed model predicts the forces satisfactorily. The main result of our model is the possibility of more exact prediction of the resistance of narrow tines, by introducing a variability of the inclination angles of bottom failure surface in side segments of different deflections from the direction of a tine motion, which was not taken into consideration in the existing models.

The Character of Heat Exchange in the Element Powered by Renewable Energy

Among other renewable resources of energy, solar energy has a small but dynamically increasing share. Solar irradiance is characterized by large variability, especially in the 24-hour cycle. Therefore, machines that use solar energy work in transient states. The character of heat exchange for the plate solar collector and the heat exchanger working in the hybrid system was investigated. The character of heat exchange turned out to be oscillatory for both machines mentioned above. The investigations of the heat exchanger model in laboratory conditions have been carried out to analyse the observed phenomenon in detail. The investigations confirmed the previously observed oscillatory character of heat exchange. As this phenomenon is unfavourable, working out methods of process control to stabilize the operation of these machines is highly recommended.