Characteristic curve of pumping suspended solids
CHANGES IN PERFORMANCE OF PUMPING SUSPENDED MATERIALS
When the velocity of suspended solids flow in the channel exceeds that assumed Reynolds number Re with a fixed value, the flow becomes self-mode flow. In this case, the hydraulic friction loss is the same when pumping suspended solids and clean water.
When the suspension flow moves, the change of hydraulic friction loss in impeller passage will affect the total hydraulic loss in the pump. In a pressurized water chamber with strong mixing fluid flow, i.e. mixing loss, the hydraulic loss is self-model loss.
The velocity of suspended solids in impeller passage is mainly related to the circumferential velocity, which is quite large in slurry pump.
Consideration should be given to the transition from pseudo-laminar state to modelling state, which occurs in the hypothetical Reynolds number wide variation paradigm. If the pseudolaminar flow begins at Re'< 3000, the self-model state is stable at Re=11000-12000. In most cases when the Reynolds number is greater than the corresponding self-model state value, the flow state with large flow rate may be significantly different from the self-model state on some sections of the pump in the condition of small flow location.
In this way, most of the state of the pump will be self-model state. Therefore, the loss in the passage of the flow passage of the pump components will be the same whether it is pumping clean water or suspended solids.
When the flow rate is very small, the flow velocity decreases, so that the flow state in the impeller passage changes from a model to a turbulent or laminar flow state. In this case, the hydraulic loss is much greater than that in the self-model state at the same speed (see Figure 2-3-4).
The increase of hydraulic friction loss in flow passage of flow passage components will lead to the "depression" phenomenon of pump head characteristic curve at small flow rate (Figure 3-6-2). At very small flow rate, a large amount of fluid is exchanged intensively between impeller and pressure chamber, which leads to turbulence or self-modelling, and the Yang-Cheng increases slightly, approaching pumping as liquid mass. Lift of body time.
The Reynolds number Re at the beginning of self-model state increases with the increase of suspended matter density (at the same suspended material flow rate). Therefore, when the suspended matter density increases, the head characteristic curve decreases obviously under the condition of small flow rate.
Because the theoretical lift of pumping clear water and suspended solids is the same, their hydraulic power is also related to the density of suspended solids and suspended solids. The friction loss power of bearings and stuffing box accounts for a small percentage of hydraulic power. The ratio of disc loss power to disc loss power in pumping suspended solids is much larger than that in pumping clear water. The reasons are as follows. In the first approximation, the angular velocity of most liquid in the impeller chamber is equal to half of the angular velocity of the impeller. The circumferential velocity decreases as the distance between the liquid in the cavity and the pump shaft decreases. When the circumferential velocity of suspended solids in the cavity decreases significantly, the flow state is no longer self-modeled. This phenomenon is related to the increase of friction in the range of the two cavities studied. In this way, when the suspended matter is pumped by the pump, it is observed that the friction loss of the disc increases. This effect is particularly significant in lateral liquid suction pumps, where the impeller back cover is integral (without perforation), i.e. there is a large surface with a small circumferential velocity. Therefore, the suspended matter rotates at a very small speed in this area.
According to IO. H. Mogilevski's high liquid test data of magnet powder and ferrosilicon suspension, in order to consider the additional loss of circumferential friction, the power of the pump must be increased by 10%~12% compared with that of pumping homogeneous liquid with the same density. Therefore, if the power of pumping clear water is known to be No, then the power of pumping suspended solids is zero.
Now we study the change of cavitation residual R in pumping suspended solids relative to that in pumping clean water.
Because the pump head characteristic curve in a wide flow range (except for very small flow state) is the same as when pumping, so all kinds of losses, especially the impeller inlet loss, are the same as when pumping clean water.
The available test data prove that the cavitation characteristic curve of pumping suspended solids pump is worse than that of pumping clean water pump (Figure 8-6-3). The allowable cavitation allowance Oh of pumping suspended solids is higher than that of pumping clean water. The reasons are as follows.
The deceleration of flow can be observed at a certain distance when an object flows around it. For example, when a cylinder flows around it, the uniform velocity field begins to destroy at a distance of 5 to 6 times the radius in front of the cylinder. Singularities of flow diversion around objects often occur, and their velocities are equal to zero.
We study the flow around the blade inlet in relative motion. Because the relative thickness of impeller blade of pumping abrasive solid-liquid mixture pump is very large, the flow around the blade inlet can be regarded as the flow around the cylinder surface. Turbulence of suspended solids can be observed at the inlet of impeller. Because the liquid flow decelerates before the blade inlet, the velocity of the liquid flow along some streamlines decreases to the critical velocity U. At this velocity, the liquid flow begins to have an incomplete destruction structure. Because the shunt point (where the relative velocity is equal to zero) of the flow around the blade inlet is in front of the blade, viscoplastic fluid is formed in this area (Fig. 3-6-4), which is no longer Newtonian fluid. This fluid is similar to the plastic body and moves at a speed close to the critical velocity v.
Because of the squeezing of the whole flow at the blade entrance, the velocity in the rest of the flow, which is not occupied by the plastic body, increases, including the velocity on the back of the blade (Fig. 3-5-4, point a). Therefore, the velocity is the highest at this point and the pressure is the lowest. Because the local velocity of suspended solids flow increases faster than that of clear water (at the same flow rate), the cavitation characteristic curve deteriorates when suspended solids are pumped.
In front of the blade