Centrifugal pump design and analysis

OBJECTIVE:

To design a simple centrifugal pump and perform flow analysis of the model. Further, obtain the relationship between the pressure ratio and mass flow rate from the flow analysis.

SOLUTION:

Centrifugal pumps are devices used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy is usually obtained from an electric motor or an engine. It is used to induce flow of fluid from a region of low level to high level. It mainly consists of two parts, the impeller and the volute casing. The impeller is the rotating part of the pump and the casing provides an airtight passage around the impeller. The impeller is always immersed in the fluid and it consists of a series of vanes to transfer the rotational energy into the fluid. When the pump is operated, the fluid moves radially outwards from the centre of the impeller leaving a region of low pressure at the centre, also called the eye of the impeller. The eye of the impeller is connected to the sump by a suction pipe. Due to the low pressure at the centre of the impeller fluid is sucked from the sump to the centre of the impeller through the suction pipe. This fluid then moves out tangentially due to the rotary motion, thus gaining high kinetic energy and pressure energy. The fluid that exits the impeller enters the casing of the pump which directs it to a common discharge point. Finally, the fluid with the high energy is delivered through the discharge point. Usually, water is the fluid that is transported using the pump.

Priming is the process of filling the pump with the fluid so that the low pressure can be generated during the working. The impeller should always be immersed in the fluid for proper working of the pump. If the fluid is not present, then during the working, sufficient low pressure cannot be generated at the centre so as to facilitate the fluid from the sump. Hence priming is very important for the working of a pump.

The figure above shows the theoretical relation between pressure ratio and mass flow. The performance of the pump is limited by two parameters, surging and choking. At point B, the pressure ratio and efficiency is maximum for that particular speed. This point signifies the design mass flow of the pump. Beyond B, the pressure ratio decreases and at a particular point C, the mass flow is maximum and the pressure ratio is zero. Between A and B the flow is said to be unstable because of surging.

Surging is a momentary back flow through the machine from the discharge to the suction. This can occur when the mass flow below a critical level with a high pressure difference across the machine. Consider a point D between A and B, any mass flow decrease at this point will be accompanied by a pressure decrease. This will make the discharge fluid to flow backwards through the machine towards the suction side. This back flow will tend to decrease the speed of rotation while the speed controller will try to maintain the correct speed. The fluid flowing backward provides more volume to the suction side and the machine picks up and begins to push gas forward again. The machine speed will tend to increase and the governor will again try to maintain the speed. This rapid forward and backward fluid flow together with the speed control action cause rapid fluctuation in the flow and pressure in the system.

DESIGN AND FLOW ANALYSIS:

Step 1: Design of the pump

The pump has two main parts, the impeller and the casing. The part was modelled as shown in the below figures. The impeller is modelled with 5 vanes and it is placed centrally inside the casing. In Solidworks, the meshing does not deform when a part rotates. Hence to simulate such a rotational behaviour, without actually rotating the impeller, the Moving Reference of Frame (MRF) method is used. In MRF approach, a volume around the rotating member is created and the condition is simulated such that the solver automatically adds some terms in the governing equation, so as to simulate the rotating behaviour. To create an airtight model for internal flow simulation, lids are created at the suction and discharge end.

Step 2: Flow Analysis

The flow analysis is implemented using the wizard option. A steady state rotational flow with water as working fluid is defined for the analysis. Since the volume around the impeller is not a part of the actual body, it is removed from the analysis. This can be done by in the component control option in input data tab. In the rotating region tab, the volume covering the impeller is selected and an angular rotation of 1000 rad/s is defined.

Further, an environmental pressure of 101325 Pa is defined at the suction end and an output velocity of 10 m/s is defined at the discharge end. The average total pressure at the inlet and outlet was defined as the goals to be achieved. The system is then run for 200 iterations.

Step 3: Parametric Study

Once the base simulation is completed, a parametric study is carried out to analyse the relation between the pressure ratio and the outlet mass flow rate. The velocity was varied from 10m/s to 50m/s and the mass flow rate and pressure ratio at 5 different velocities were analysed.

RESULTS:

Velocity profiles

Pressure Profiles

Parameter	Design Point 1	Design Point 2	Design Point 3	Design Point 4	Design Point 5
Outlet Velocity [m/s]	10	20	30	40	50
Inlet Pressure [Pa]	374817.5519	230174.961	401220.2239	289269.651	238414.529
Outlet Pressure [Pa]	2627202.498	1748226.717	4127283.593	1566296.204	72477.52673
Pressure Ratio	7.009283542	7.59520805	10.2868284	5.41465791	0.303997944
Mass flow rate [kg/s]	0.495072043	0.992271641	1.482318598	1.983429475	2.478053233

Performance Curve

From the results, it can be observed that as the outlet velocity increase, the mass flow rate also increases. Further, as mass flow increases, the pressure ratio initially increases to reach a maximum value and then falls to a minimum. The maximum pressure ratio is attained at a mass flow rate of 1.5kg/s. This point is the surging limit for the current pump design.