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Introduction:
A Regenerative pump is a type of rotodynamic machine whose working is based on the principle of energy transfer from a moving rotor to gas in the channel of the pump. As the gas passes multiple numbers of times through rotor blades, along with being accelerated each time, energy is being transferred from the rotor to the gas in the channel. Due to this phenomenon of “multi-staging,” high compression ratios are developed in regenerative pumps within a single stage. Regenerative pumps are cost-effective in generating high heads at low flow rates. They can function at low NPSH with stable performance characteristics.

Working Principle:
Regenerative pump’s working is based on the principle of circulatory flow of gas between the rotor blades and the channel. During this motion, there is an exchange of momentum in the gas between the blades of the rotor and the channel. The circulatory flow in a regenerative pump has been observed by Burton and also investigated via particle-image-velocimetry by Kossek.

Theoretical Model:
The flow in regenerative pumps can be described by two theoretical models, and each model is associated with a set of assumptions. that have a dominant effect on predicting performance. These models can be classified based on their flow mechanisms. It can be either turbulence or exchange of angular momentum.

To predict the pumps characteristics, Engels developed a one-dimensional model by examining a regenerative pump with semi-circular blades. The helical flow pattern in regenerative pumps was found to be responsible for its unique head rise. This model is valid only for a specific geometry of the pump because the coefficients are calculated experimentally. He derived an inverse relationship between flow circulation and flow rate from the results. Turbulence in the stream is the main driving force in turbulence mixing theory. In some cases, the turbulent stresses were found to be transmitted based on mixing length theory, while in few other cases these stresses were confined to impeller-fluid and fluid-casing. Senoo modeled the internal flow, assuming that turbulent friction force to be responsible for the pumping mechanism under the condition of the adverse pressure gradient of a radial blade impeller.

In contradiction to Engels and Senoo, a viscous model for performance prediction of a radial blade impeller was proposed by Iversen which was based on the theory that shear stresses are imparted to the fluid by the impeller. It was determined experimentally that this model included two shear coefficients and an average impeller velocity.

Prevention of Cavitation:
In a centrifugal impeller pump, the fluid only passes through the centrifugal impeller once, so it only gains energy in that single pass. So it has a flat discharge head curve. Whereas similar flow rates are provided from the smaller regenerative turbine pump. The increase in pressure in the regenerative turbine is also responsible for the increase in power required. There is Continuous energy building in these pumps due to subsequent passes which manifest itself as pressure. As the discharge flow is reduced, the time the fluid takes to move from inlet to outlet increases, and consequently, the fluid has more energy imparted to it. This additional buildup of pressure provides the ramped pressure curve. the fluid arriving at the discharge of a regenerative turbine impeller will have undergone a gradual pressure increase. By design, any entrained vapor bubbles occurring at the inlet will move to the center of the vortex (where the pressure is the lowest); here, they will collapse gently, over a relatively extended period, and generally away from the metal surfaces. This gentle collapse of the vapor bubbles occurs in a process similar to that in a condenser. By contrast, in the centrifugal impeller, the bubbles move rapidly from low to high pressure and subsequently collapse violently. In the inlet of a regenerative turbine pump, the presence of vapor bubbles reduces flow but does not deteriorate the pump parts. This characteristic is reflected in the reduced flow that results when the Net Positive Suction Head Available (NPSHa) is reduced below the NPSH required (NPSHr). The regenerative turbine pump's ability to continue operation without damage in the presence of vapor bubbles is particularly advantageous when transient events occur that cause inlet pressures to fall below normal levels.

References:

 * M.G.Wasel “Performance of Steam Regenerative Turbine Compressor,” Mansoura Engineering Journal (MEJ), Faculty of Engineering, vol.22, No.3, September 1997.


 * M.A.Rayan “Text Book of Hydraulic machines,”


 * V.M.Cherkassy “Pumps Fans Compressors,” Mir Publisher-Mosco,1985.


 * Robert.A.Nasca “Testing fluid power components,” Industrial Press Inc, 1990.
 * R.s.Khurmi, I.K, Gupta “ Text Book of Machine Design,” Urasia Publishing House, 1999.


 * Ernest O.Doebelin “Measurement System Application and Design,” Mc.Graw-Hill International Book Company, 1982.
 * http://www.rothpump.com/index.html