Modeling the power requirements of a rotary feeding and cutting system
Veikle, Eric Emerson
The purpose of this study was to develop an analytical model that could be used by the designers of a rotary feeding and cutting system (RFCS) to identify the power demand of the RFCS with limited or no required field or laboratory data. Two separate RFCS were investigated, incorporated with either a low-speed cutting process (LSCP) or a high-speed cutting process (HSCP). The results from the laboratory and field trials were used to create and validate the analytical model. Laboratory tests were completed with the LSCP RFCS and these concluded that counter-knife sharpness, serrations and bevel angle all had significant effects on the specific energy required by the LSCP RFCS when processing cereal straw and alfalfa. The specific energy required by the LSCP RFCS, while processing cereal straw, increased by 0.35 kW∙h/tonne (or 96%) when the sharpness of the counter-knives decreased from 0.13 to 0.63 mm (where the sharpness was recorded by the leading-edge-width of the counter-knives). With the same decrease in sharpness, the specific energy required by the LSCP RFCS while processing alfalfa increased by 0.04 kW∙h/tonne (or 32%). The specific energy required by the LSCP RFCS while processing cereal straw with sharp counter-knives (counter-knives with a leading edge width of 0.13 mm) increased by 0.11 kW∙h/tonne (or 51%) when serrated counter-knives were used instead of un-serrated counter-knives. However, counter-knife serrations did not have a significant effect on the specific energy demand of the LSCP RFCS when sharp counter-knives were used to process alfalfa. The increase in bevel angle from 15 to 90° caused the specific energy required to process cereal straw and alfalfa to approximately triple. The moisture content of alfalfa also had a significant effect on the specific energy required to process alfalfa with the LSCP RFCS. The specific energy demand of the LSCP RFCS was at a maximum when alfalfa at a moisture content of 53% on a wet basis (w.b.) was processed and decreased slightly (approximately 0.04 kW∙h/tonne or 10%) when dryer and wetter alfalfa was processed. Field tests were completed with the HSCP RFCS and it was concluded that in general, there was a direct relationship between the specific energy required by the HSCP RFCS and the moisture content of the straw, counter-knife engagement and throughput. Further, it was also concluded that the specific energy requirements of the HSCP RFCS were more sensitive to counter-knife engagement when higher moisture content straw was processed. Depending on the type of chopper used, the specific energy required by the HSCP RFCS increased anywhere from 0.15 to 0.77 kW∙h/tonne (or 22 to 61%) when the counter-knife engagement was increased from 0 to 100% (or fully removed to fully engaged). Again, depending on the type of chopper used, when the moisture content of the straw processed by the chopper increased from approximately 7 to 25% w.b. the specific energy required by the chopper increased by 0.14 to 0.96 kW∙h/tonne (or 28 to 84%). The effect of throughput on the specific energy demand of the HSCP RFCS was dependent on the type of chopper used. For one of the choppers, an increase in throughput from 10.5 to 13.5 tonne/h caused the specific energy required by the HSCP RFCS to increase by 0.24 kW∙h/tonne (or 35%); however for a different chopper, an increase in throughput from 12 to 13 tonne/h caused the specific energy demand of the HSCP RFCS to decrease by 0.16 kW∙h/tonne (or 19%). The analytical model was validated using a subset of the data that were collected while employing each cutting device under field conditions and the data collected with the use of a custom-designed material properties test stand. The output of the analytical model fell within the 95% confidence interval of the measured power demand for each of the rotary feeding and cutting systems, and the analytical model was therefore deemed sufficiently accurate. Based on the analytical model, the total power demand of both the LSCP and HSCP rotary feeding and cutting systems was largely attributed to the power required to transport plant material. Further, the power required to transport the plant material along the sides of the counter-knives was much greater than the power required to transport the plant material along the rotor bed and along the leading edge of the tines. Because of the excessive power required to transport plant material along the sides of the counter-knives, three techniques were identified as potential strategies to decrease the power demand of the RFCS. The first technique involved removing half of the tines from the RFCS, and modifying the remaining tines to decrease the amount of plant material that is entrapped between sides of the counter-knives and the tines. The second technique involved coating the inside surface of the tines with a baked Teflon, to decrease the coefficient of friction between the plant material and the RFCS. The third technique involved reshaping the counter-knives, to decrease the surface area over which plant material was transported along the side of the counter-knives. According to the analytical model, employing any of the three techniques would result in the total power demand of the RFCS to decrease by 15 to 26%. For the HSCP RFCS, a stochastic model was developed to identify which of the four choppers tested during field trials would have the best performance when subjected to the same operating conditions. The chopper with the best performance was the WR chopper as its use resulted in the minimum geometric mean length of material exiting the combine harvester while also consuming the least amount of specific energy.
specific energy