【Mechanical Equipment Technology Exchange Edition】Mechanical Equipment 【Daily Question】20191114
Thread Content
Question: What are the two components that make up the frictional loss of the impeller? There are no answers provided for this series of posts; fellow netizens are free to share their own opinions – just reply with what you understand. Replies earn rewards ranging from 5 to 15 points; all forum members are welcome to participate actively and support the development of the forum! ! ! Chemical Equipment and Machinery
When the impeller rotates at high speed, friction occurs between its surface and the steam on either side of it; to overcome this frictional resistance, a portion of the work done by the impeller is lost. Furthermore, because steam is viscous, the steam in close contact with the impeller will rotate together with the impeller; under the effect of centrifugal force, it moves outward in a radial direction. The surrounding steam then flows in to fill the resulting voids, thereby creating vortex movements on both sides of the impeller. To overcome the energy consumed by frictional resistance and eddy currents
(1) Friction losses caused by the roughness on both sides of the impeller and on the surface of the shroud; (2) Losses caused by vortex movements in the meridional plane
Impeller friction loss: When the impeller rotates at high speed, friction occurs between its surface and the steam on either side of it; to overcome this frictional resistance, a portion of the work done by the impeller is lost. Furthermore, due to the viscosity of steam, the steam in close contact with the impeller will rotate together with the impeller, and under the effect of centrifugal force it will flow outward radially. The surrounding steam will then flow in to fill the resulting voids, thereby creating vortex movements on both sides of the impeller. The energy consumed to overcome frictional resistance and eddy currents is known as impeller friction loss.
The impeller friction loss refers to the portion of useful work that is consumed as a result of friction between the rotating impeller and the steam surrounding it, which in turn causes that steam to move. Impeller friction loss and blade height loss.
When the impeller rotates at high speed, friction occurs between its surface and the steam on either side of it; to overcome this frictional resistance, a portion of the work done by the impeller is lost. Furthermore, because steam is viscous, the steam in close contact with the impeller will rotate together with the impeller; under the effect of centrifugal force, it moves outward in a radial direction. The surrounding steam then flows in to fill the resulting voids, thereby creating vortex movements on both sides of the impeller. To overcome the energy consumed by frictional resistance and eddy currents
The impeller friction loss, also referred to simply as friction loss, consists of two components: (1) the friction loss caused by the roughness of the surfaces on both sides of the impeller and on its peripheral edge. When the impeller rotates within a chamber filled with steam, due to the viscosity of the steam and the roughness of the rotating surface, the vapor particles attached to the sides and outer edges of the impeller are carried along as it rotates; their circumferential velocity is roughly equal to that of the corresponding points on the impeller’s surface. Meanwhile, the circumferential velocity of the vapor particles in close contact with the cylinder walls or partition surfaces is zero (see Figure 1.5.2). The circumferential velocity of the steam parcels varies across the distance from the impeller surface to the cylinder wall; that is, a velocity gradient exists, which leads to friction between the steam parcels as well as between the steam and the wall surface. To overcome friction and drive the motion of steam particles, a portion of the peripheral work must be consumed. (2) Losses caused by vortex motion within the meridional plane: The vapor parcels located right near the surface of the impeller rotate together with the impeller; under the effect of centrifugal force, they experience an outward radial flow. The vapor parcels near the cylinder wall or the surface of the partition have lower speeds, and therefore experience less centrifugal force; they move naturally toward the center to fill the space occupied by the vapor flowing radially outward at the impeller. As a result, vortex motion of vapor is formed within the meridional planes on both sides of the impeller (Figure 1.5.2). Eddy currents themselves consume a portion of the peripheral work, and they also increase frictional resistance.
(1) Friction loss caused by the roughness on both sides of the impeller and on the surface of the shroud. (2) Losses caused by vortex motion within the meridional plane.
①Friction losses caused by the roughness on both sides of the impeller and on the surface of the shroud; ② Losses resulting from vortex motion within the meridional plane.