CFD allows engineers to look into complex flow fields

Did you ever wish you could “see” into a complex flow field, such as a pump station wet well or a storage reservoir? Traditionally, some type of experimenting or physical modelling was required to develop an insight into complex flows. In recent years, computation fluid dynamics (CFD), or numerical modelling multi-dimensional flow fields, has become more widespread. The results of a CFD analysis allow detailed examination of complete two-dimensional and three-dimensional flow fields.

Associated Engineering has developed CFD analysis capability and is using this tool on a number of projects. We are using FLUENT, a versatile multi-physics CFD software package, to provide a higher level of analytical support for our water, wastewater, and industrial projects. In one case, potential pump sump configurations for a proposed raw water pump station have been examined using three-dimensional CFD modelling.

The objective of pump station design is to promote low velocity, uniform flow toward the pump inlet bell. The consequences of an unsuitable sump design could be premature wear or pump failure. However, flow is usually delivered to the pump station through pipes of relatively small diameter with high flow velocities. The challenge in pump sump design is to convert this rapidly moving, highly non-uniform flow to a relatively slow moving, uniform flow before the pump inlets. Furthermore, a shorter transition zone that still promotes uniform pump approach flow can reduce the pump station footprint area, and construction costs.

The City of Saskatoon intends to construct a new raw water intake in the South Saskatchewan River to supply its water treatment plant. A new pump station is also required. While the precise locations of the intakes and pump station have not been finalized, the capacity requirements have been determined.

The ultimate capacity of the pump station will be approximately 430 million litres per day (5.0 cubic metres per second), but the current average winter flow rate is approximately 100 million litres per day (1.2 cubic metres per second). The pumping head required is approximately 25 metres. The preliminary pump station design calls for four identical vertical turbine pumps. Based on the number of pumps required and the design flow rates, the basic layout of the pump station wet well has been established based on Hydraulic Institute Standards.

The pump station will be connected to an in-river intake structure by two 1350 millimetre diameter pipelines. A twinned pipeline has been proposed so that one line can be isolated during low flow periods to maintain a reasonable scour velocity and to maintain redundancy. The basic configuration of the four-pump model is shown. The intake pipelines connect to a forebay, where some of the inflow kinetic energy dissipates. A 1 metre high wall extends above the pump bay floor at the end of the forebay. The transition area that separates the forebay and the four pump bays is intended to promote uniform flow into the pump bays. The inside dimensions of the pump station model shown are 13.3 metres in length and 10.7 metres in width. The pump columns extend above the model to minimize the effect of the outflow boundary on the flow near the pump inlets and to allow velocity measurement in the pump column at approximately the same locations that will be used in physical model tests.

The focus of this analysis is the transition region between the pump station intake piping and the pump inlets. The objective is to configure this transition region to promote uniform flow in each pump bay under a variety of flow conditions while minimizing the footprint.

Since the results of CFD modelling allow insight into the entire flow field, flow through the transition area can be examined in detail. In this case, conditions that lead to non-uniform pump approach flows can be evaluated by examining flow velocity contour plots and velocity vector plots.

In this application, CFD modelling is an excellent prototyping tool to try different pump sump configurations. The primary advantage of this approach is that the character of the approach flow can be quantitatively examined. Also, the acceptance criteria for suitable pump sump design used in physical modelling studies can be “measured” from the CFD results: acceptable level of pump column swirl velocity and velocity distributions at the pump inlet bell. The results of the analysis show that further improvements in the sump configuration are required when the intake and pump station locations have been confirmed. When a suitable configuration has been developed, a physical model can be constructed to confirm the design and to test for vortex formation. At the predesign stage, the reduction in construction cost due to the smaller station footprint is estimated to be in the order of $300,000.

FLUENT is capable of modelling a wide range of flow phenomena including multi-phase flow, cavitation, and multi-species mixing. This modelling tool provides better insight into complex flow fields, which leads to better understanding and better design. This insight is particularly valuable in identifying root causes to undesirable flow conditions in existing facilities.

For more information on CFD, contact Bob Hawboldt Ph.D., P.Eng. at hawboldtb@ae.ca.