Abstract
Low specific speed pumps find applications in a broad range of domains, but suffer from a low efficiency and a risk of head instability close to shut-off. The numerical computations on these pumps performed in the last years have shown discrepancies with experimental results. Recent studies suggest that the use of wall-functions underpredicts the losses of these pumps, especially at overload. The reason has been attributed to a detachment zone downstream the volute tongue, not well captured with the wall-function approach. This paper focuses on the influence of the volute casing on a pump with a specific speed of 8.9 on two issues. First, the influence of the wall modelling approach relatively to the low-Reynolds number method on the performance prediction is discussed. The results are, as expected, an underprediction of the losses when the wall-function approach is used. With a larger volute, the difference between the two wall modeling approaches is smaller. Secondly, the influence of the volute throat area enlargement on the pump performances is discussed. Both the head and efficiency are improved at the design point and at overload with an increased volute throat area. However the part-load head decreases and the head flattens. The study of the flow at part-load, in the region at the outlet of the impeller reveals that with a larger volute, larger flow exchanges are present, contributing to additional mixing losses and head loss.
Highlights
The design of low specific speed (LSS) pumps presents many challenges
The results revealed that the wall function approach (W F ) overestimated the head and the efficiency, especially at overload
It is seen that using wall-function approach leads to large differences in the predicted pump performances as compared to the low-Reynolds number approach
Summary
The design of low specific speed (LSS) pumps presents many challenges. A typical large impeller diameter causes considerable disk friction and volute losses, reducing the optimal efficiency. The volute casing plays a more important role for low specific speed pumps It mainly determines the position of the BEP and largely adds to losses. 65, typical values for the first ratio drops from 0.2 to 0.09 while the second ratio drops from 200 to 20 This second part is directly due to the volute having a large wetted surface Svol to adapt to the large impeller diameter, with a small throat area Avs due to the small flow rate. This explains the large importance of friction at low specific speed. The second objective is to investigate the influence of the volute throat area enlargement on a very low specific speed pump, and evaluate its influence on the main pump performance parameters
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