The Research On Internal Flow Mechanism And Structural Coupling Characteristics Of Roto-jet Pump

Roto-jet pump（also known as pitot pump or rotary shell pump）is a low specific speed pump with high head and small flow.Its performance is better than that of some reciprocating pumps and high-speed pumps.Due to its special working form and structural components,which is widely applied in petrochemical,aerospace,food industry,mining and transportation,carbon black manufacturing and other fields.The main reason why the performance of the roto-jet pump is better than that of the small flow and high head centrifugal pump is that the synchronous rotation of the impeller and the rotary shell is beneficial to reduce the friction loss of the disc.However,how the impeller structure and the rotating casing effect jointly increase the liquid energy and the deep level influence mechanism are still unclear.The existence of the collecting pipe leads to the asymmetric flow characteristics of the liquid inside the rotary shell,and the periodic liquid pressure pulsation under the action of the bluff body can easily cause vibration,deformation,and even fatigue damage of the structural components,which becomes a stumbling block for the structural design and performance optimization of the roto-jet pump.In view of the above reasons,a closed test bench for the roto-jet pump was designed in this paper,combining experiments and numerical calculations,the external characteristics of the roto-jet pump,the internal pressure test of the rotary shell,and the transient flow characteristics were studied by using the test pump as the object,the influence of the swirl shell effect and the flow around the collecting pipe on the liquid flow characteristics in the cavity was carried out,and the impeller structure,the swirl shell effect and the structure coupling mechanism were revealed.The main research contents and conclusions are as follows:（1）Through the external characteristics of the roto-jet pump and the internal field test experiment,it’s found that the head of the roto-jet pump decreases with the increase of the flow rate,and the efficiency curve increases at the beginning and then decreases,the shaft power curve is relatively flat.the rotating casing effect can increase the radial liquid pressure gradient.The comparative analysis of the rotary casing rotation and fixed data shows that the rotary casing effect causes an increase of about 12.5m in the head at the inlet of the collecting pipe.The flow rate,head and shaft power of the pump satisfies similar conversions in the variable speed range,and the optimal efficiency basically remains unchanged and tends to be a constant.If the inlet diameter of the header is too small or too large,the head and efficiency of the roto-jet pump will decrease.A reasonable inlet diameter of the header will help to improve the inflow conditions of the header and improve the performance of the roto-jet pump.（2）Using numerical methods to analyze the time-averaged and transient distribution characteristics of the internal flow field on the roto-jet pump,the following conclusions are obtained:the distribution of the liquid isobars inside the rotary casing is approximately concentric circles,there is a slight height difference near the collecting pipe,the pressure in the upstream region is higher than that in the wake region.The velocity inside the spiral shell is linearly distributed along the radial direction,and there is a sudden change in the flow velocity at the inlet of the header;the inner wall of the spiral shell and the inside of the header have high pressure,velocity and turbulent kinetic energy pulsation intensity;there are different scales in the spiral shell.The velocity distribution inside the rotary casing is linearly distributed along the radial direction,and there is a sudden change in velocity at the inlet of the collecting pipe,the inner wall of the rotary casing and the inside of the collecting pipe have high pressure,velocity and turbulent kinetic energy pulsation intensity;there are different scales in the spiral shell.There are vortices of different sizes in the rotary casing,the front wing vortex that obviously exists on the upstream surface is the main feature of the around vortex in the rotary casing,and there are phenomena such as backflow and secondary flow inside the impeller and the collecting pipe.The closer the monitoring point of the impeller inlet is to the impeller inlet,the lower the pressure.The pulsation characteristics of the medium at each monitoring point in the intermediate streamline of the impeller are affected by the blade frequency,and the circumferential pressure pulsation of the liquid medium in the rotary casing is mainly affected by the axial frequency,the pressure value of each monitoring point inside the collecting pipe varies greatly,and the diffusion effect is obvious.（3）Comparison of the internal flow field information of the roto-jet pump in the rotating and fixed states of the rotary casing,and the influence mechanism of the rotary shell rotation speed on the liquid flow characteristics in the rotary shell was studied.Finally,a calculation model of the internal pressure field of the roto-jet pump was established based on the rotary shell effect.The following conclusions are drawn:no matter whether the rotary casing rotating or not,the tangential velocity at the same measuring point inside the rotary casing is dozens of radial speeds and hundreds times of the axial velocity,the circulation characteristics are very obvious,and the liquid conforms to the hypothesis of axisymmetric plane circulation.The speed of the impeller is constant,and the increase of the rotating speed of the rotary casing can increase the pressure gradient of the liquid in the rotary casing.The relative radius s<1area is inversely proportional to the liquid pressure and the rotating speed of the rotary casing,and s>1 area is directly proportional.The reduction of the rotary casing speed within a certain range can reduce the total disc friction loss and improve the pump efficiency.The rotary casing rotation coefficient i=0.8 position,the roto-jet pump efficiency has the maximum value,which is 1.3%higher than that of the rotary casing in the synchronous rotation mode.Combining the effect of the rotary casing,the mathematical model of the internal pressure of the rotary casing is deduced,and the test results of a dual-impeller roto-jet pump are used for verification,which proves that the mathematical model of the internal liquid pressure of the roto-jet pump proposed in this paper has a high degree of credibility.（4）The dynamic characteristics of the flow around the collecting pipe are analyzed,and the concept of static pressure recovery coefficient is proposed to evaluate the static pressure recovery ability of the collecting pipe.The results show that the inlet diameter of the collecting pipe has a significant impact on the external characteristics of the roto-jet pump.The efficiency of the collecting pipe with the inlet diameter of 10 and 20mm at the rated flow point is lower efficiency by 7.8%and 3.3%than that of the 15mm collecting pipe,respectively,this is basically consistent with the experimental conclusions in Chapter 2.Model pumps with wing-shaped and elliptical collecting pipe installed at rated flow points have higher efficiency by 3.1%and 1.7%than those with circular collecting pipe.The wing-shaped collecting pipe has a positive effect on improving the pressure distribution of the rotary casing and improving the performance of the pump.Within the range of relative radius 0.5<s<1.0 of the wake region of all models,Reynolds stress-u_x’u_y’≈0,momentum transfer between fluid layers is relatively weak,and the rotating casing effect is obviously affected in the range of s<0.5 and s>1.0.The momentum transfer between the high-speed layer and the low-speed layer is intense,and the difference of the Reynolds stress values of different collecting pipe models at the same position is generally on the order of 10^-4.The calculation formula for the static pressure recovery coefficient inside the collecting pipe is deduced,the circular collecting pipe with an inlet diameter of 15mm matches the rated flow point reasonably,the static pressure recovery capacity is the strongest,and the recovery coefficient can reach up to 0.405.Other models The static pressure recovery ability is slightly weaker.（5）A research on the coupling mechanism between the impeller structure and the rotary casing effect was carried out.The results showed that the pump head difference caused by the blade exit angle within the experimental range was the largest 1.7m,the maximum efficiency difference was 0.4%,and the maximum difference in the head caused by the blade wrap angle was 1.6 m,the maximum difference in efficiency is 1%,the maximum difference in head caused by the number of blades is 16.4m,and the maximum difference in efficiency is 3%.The number of blades has the most significant impact on the performance of the roto-jet pump,and the blade exit angle and blade wrap angle have a weak effect on which.The interaction of various factors shows that from the perspective of high head,high efficiency,and low power of the roto-jet pump,the combination of small blade exit angle,large blade wrap angle,and high number of blades can be selected.When the impeller and the rotary casing rotate synchronously,the structure of the impeller has tiny effect on the liquid pressure and tangential velocity at the same coordinate position.The head of the straight blade with the same closed structure impeller at rated flow point is 1.6m higher than the backward curved blade,and the efficiency is 2.9%lower.The closed impeller with the same straight blade has a head higher than the semi-open impeller by 8.3m and the efficiency is 0.5%lower.Although the head of the semi-open impeller is reduced to a certain extent,the efficiency is close to that of the closed impeller.It is reasonable to use the semi-open straight blade impeller for roto-jet pumps.（6）Using the Workbench to analyze the structural dynamic characteristics of the roto-jet pump,the following conclusions are obtained:the maximum deformation of each collecting pipe is located in the inlet area,Although the wing-shaped collecting pipe have the effect of reducing flow,the vibration displacement is significantly higher than that of the circular collecting pipe under the same conditions,structural deformation is more likely to occur,and he stability of the circular collecting pipe is relatively high.During the deformation process of all collecting pipes,the inlet area is reduced,and the header of the collecting pipe is shifted toward the impeller side in the axial direction.All collecting pipes have stress concentration at the second turn position.The fluctuation difference of equivalent stress at each monitoring point on the blade within a rotation period is small,and have no obvious regularity,the numerical difference is on the order of 10^-2.The greater the flow rate,the greater the radial and axial force of the impeller,and the greater the peak-to-valley difference.