An elbow is part of the piping that changes the direction of a fluid by virtue of its curved shape (typically bent). The boundary layer flows prevailing within the elbow undergo an increase in pressure in the following areas: at the outer elbow radius at the elbow inlet and at the inner elbow radius at the elbow outlet. The boundary layer swells at these points which results in flow separation (see boundary layer) if the elbow radius is too narrow. In addition, the different static pressures in the elbow cross-section generate a secondary flow which interferes with the primary flow (see flow through a section) in such a way that a fluid particle follows a longer helical stream line (see stream line) through the elbow instead of following the bend.
The secondary flow generated in the elbow takes the form of two vortices rotating in opposite directions with axes in the direction of flow. Even if the elbow is followed by a straight piping section, these vortices take some time to disappear by way of friction as they move further and further away from the elbow.
When travelling along curved channels, compared with straight ones (pipe flow), the flow is subject to additional losses (see pressure loss), which include the higher losses caused by the secondary flow in a subsequent straight piping section. These elbow losses can be reduced by means of various measures.
Measures for reducing elbow losses.
- Where possible, arrange guiding surfaces on the inside of the elbow (in practice, these often take the form of a vane cascade). See head loss, Fig. 11
- Increase the elbow radius (particularly effective on the inside of the elbow).
- Reduce the flow velocity (arrange diffuser upstream of the elbow instead of combining the two).
- Reduce the deflection angle.
In the case of elbows with cross-sections that are not rotationally symmetric, the longest cross-section axis should be perpendicular to the elbow plane in order to minimise pressure losses. In centrifugal pumps, and tubular casing pumps in particular, elbows can be used as an intake or a discharge elbow.