Parametric Study of a Centrifugal Pump using Solidworks

Aim:- Parametric Study of a Centrifugal Pump using Solidworks

Objective:-

1. To obtain the relationship between Pressure ratio and Mass Flow Rate
2. To obtain the Pressure Ratio (Pressure ratio = Outlet total pressure / Inlet total pressure)

Introduction:-

A centrifugal pump is a mechanical device designed to move fluid by means of the transfer of rotational energy from one or more driven rotors, called impellers. Fluid enters the rapidly rotating impeller along its axis and is cast out by centrifugal force along its circumference through the impeller’s vane tips. The action of the impeller increases the fluid’s velocity and pressure and also directs it towards the pump outlet. The pump casing is specially designed to constrict the fluid from the pump inlet, direct it into the impeller, and then slow and control the fluid before discharge.

Working Principle:-

The impeller is the key component of a centrifugal pump. It consists of a series of curved vanes. These are normally sandwiched between two discs (an enclosed impeller). For fluids with entrained solids, an open or semi-open impeller (backed by a single disc) is preferred.

Fluid enters the impeller at its axis (the ‘eye’) and exits along the circumference between the vanes. The impeller, on the opposite side to the eye, is connected through a drive shaft to a motor and rotated at high speed (typically 500-5000rpm). The rotational motion of the impeller accelerates the fluid out through the impeller vanes into the pump casing.

Types:-

There are two basic designs of pump casing: volute and diffuser. The purpose of both designs is to translate the fluid flow into a controlled discharge at pressure.

In a volute casing, the impeller is offset, effectively creating a curved funnel with an increasing cross-sectional area towards the pump outlet. This design causes the fluid pressure to increase towards the outlet.

The same basic principle applies to diffuser designs. In this case, the fluid pressure increases as fluid are expelled between a set of stationary vanes surrounding the impeller.

Diffuser designs can be tailored for specific applications and can, therefore, be more efficient. Volute cases are better suited to applications involving entrained solids or high viscosity fluids when it is advantageous to avoid the added constrictions of diffuser vanes. The asymmetry of the volute design can result in greater wear on the impeller and driveshaft.

Main Features of Centrifugal Pump:-

There are two main families of pumps: centrifugal and positive displacement pumps. In comparison to the latter, centrifugal pumps are usually specified for higher flows and for pumping lower viscosity liquids, down to 0.1 cP. In some chemical plants, 90% of the pumps in use will be centrifugal pumps. However, there are a number of applications for which positive displacement pumps are preferred.

Limitations:-

The efficient operation of a centrifugal pump relies on the constant, high-speed rotation of its impeller. With high viscosity feeds, centrifugal pumps become increasingly inefficient: there is greater resistance and higher pressure is needed to maintain a specific flow rate. In general, centrifugal pumps are therefore suited to low pressure, high capacity, pumping applications of liquids with viscosities between 0.1 and 200 cP.

Slurries such as mud, or high viscosity oils can cause excessive wear and overheat leading to damage and premature failures. Positive displacement pumps often operate at considerably lower speeds and are less prone to these problems.

Any pumped medium that is sensitive to shearing (the separation of emulsions, slurries, or biological liquids) can also be damaged by the high speed of a centrifugal pump’s impeller. In such cases, the lower speed of a positive displacement pump is preferred.

A further limitation is that, unlike a positive displacement pump, a centrifugal pump cannot provide suction when dry: it must initially be primed with the pumped fluid. Centrifugal pumps are therefore not suited to any application where the supply is intermittent. Additionally, if the feed pressure is variable, a centrifugal pump produces a variable flow; a positive displacement pump is insensitive to change pressures and will provide a constant output. So, in applications where accurate dosing is required, a positive displacement pump is preferred.

Pump Comparison: Centrifugal vs Positive Displacement

Property	Centrifugal	Positive Displacement
Effective Viscosity Range	Efficiency decreases with increasing viscosity (max. 200 Cp)	Efficiency increases with increasing viscosity
Pressure tolerance	Flow varies with changing pressure	Flow insensitive to changing pressure
	Efficiency decreases at both higher and lower pressures	Efficiency increases with increasing pressure
Priming	Required	Not required
Flow (at constant pressure)	Constant	Pulsing
Shearing (separation of emulsions, slurries, biological fluids, foodstuffs)	High-speed damages shear-sensitive mediums	Low internal velocity. Ideal for pumping shear-sensitive fluids

Applications:-

Centrifugal pumps are commonly used for pumping water, solvents, organics, oils, acids, bases, and any ‘thin’ liquids in both industrial, agricultural and domestic applications. In fact, there is a design of a centrifugal pump suitable for virtually any application involving low viscosity fluids.

Type of centrifugal pump	Application	Features
Canned motor pump	Hydrocarbons, chemicals where any leakage is not permitted	Sealless; impeller directly attached to the motor rotor; wetted parts contained in a can
Magnetic drive pump		Sealless; impeller is driven by close-coupled magnets
Chopper/grinder pump	Wastewater in industrial, chemical and food processing/ sewage	Impeller fitted with grinding teeth to chop solids
Circulator pump	Heating, ventilation and air conditioning	Inline compact design
Multistage pump	High-pressure applications	Multiple impellers for increased discharge pressures
Cryogenic pump	Liquid natural gas, coolants	Special construction materials to tolerate low temperatures
Trash pump	Draining mines, pits, construction sites	Designed to pump water containing solid debris
Slurry pump	Mining, mineral processing, industrial slurries	Designed to handle and withstand highly abrasive slurries

Procedure:-

1. Design the model in the MKS system and save it.
2. Go to flow simulation and create the Lids at the two faces because it is an internal flow system.
3. We will go for wizard and then internal flow problem and we will switch on the rotating region as local averaging.
4. Now we will click on the water as the fluid and now we will click on ok.
5. After that define the Boundary conditions at the inlet as 101325 Pa (Environment pressure) and outlet velocity of 10 m/s at the outlet face for the baseline study.
6. Now we will define the rotating region and then we will select the propeller domain which is the moving reference frame and then rpm of -1000 rad/s (clockwise direction).
7. Now we will be adding the Global and Surface goals at the inlet face and at the outlet face and to calculate velocity and pressure and mass flow rate.
8. Now we will do the parametric study using the input parameter as the outlet velocity with discrete values ranging from 10 m/s to 20 m/s with 6 intervals and output parameter as the goals and results so that we can obtain performance data.

3D Models:-

Results:-

Forward impeller _ -1000rad/sec__clockwise rotation

Cutplots of Velocity for each Design Point:-

Pressure Ratio Plot:-

As the outlet velocity increases, the pressure ratio decreases with some fluctuations may be due to the cavitation. The pressure ratio is the ratio of Outlet pressure to the inlet pressure.

The maximum pressure ratio is 0.642 Pa and the minimum value is -8.5687 Pa.

Mass Flow Rate Plot:-

In the goal plot table, the mass flow rate is in negative values which means that the fluid is going out of the computational domain. So for plotting, we have taken their positive values to represent the actual trend that is the mass flow rate increases with the increase in outlet velocity.

The maximum mass flow rate is 0.4993 kg/s and the minimum value is 0.9989 kg/s.

Pressure Ratio vs Mass Flow Rate:-

As the mass flow rate from the outlet increases, the pressure ratio decreases. As the purpose of the pump is to increase the pressure at the outlet point. From the plot, it is clear that when the velocity is low the pressure is high.

Velocity Flow Trajectory:-

Pressure Flow Trajectory:-

Conclusion:-

1. It can be observed that with the increase in Velocity, the Mass Flow Rate increases, and hence the Pressure Ratio decreases.
2. With an increase in Outlet Velocity, the Average Mass Flow Rate increased linearly.
3. As the purpose of the pump is to increase the pressure at the outlet point. From the plot, it is clear that when the velocity is low the pressure is high.
4. When we increase the outlet velocity there will be drop-in pressure at the outlet.

Google drive link - https://drive.google.com/drive/folders/1wpHOarvDVt5gECsWxNqm8BtNG1p4p7i8?usp=sharing

Reference:-

https://en.wikipedia.org/wiki/Centrifugal_pump

https://www.michael-smith-engineers.co.uk/resources/useful-info/centrifugal-pumps