Unsteady friction in transient vertical-pipe flow with trapped air

ABSTRACT

Existing pipe-filling models have primarily considered steady friction alone, tending to underestimate pressure attenuation. Various popular unsteady friction models are considered here to simulate pressures within a rapidly filling vertical pipe. Numerical predictions are compared to each other and to experimental results. The models considering unsteady friction better reproduce measured pressure oscillations. The greatest errors in predicted pressures neglecting unsteady friction occur after the first period, but numerical errors are markedly greater for cases with smaller initial air volumes and higher inlet pressures. Significantly, the largest errors in the prediction of maximum air pressure when ignoring unsteady friction effects occur near the critical value of initial air volume (here around 2–3% of pipe volume). Compared to accurate but computationally demanding convolution models, the Trikha-Vardy–Brown simplified model is shown to be indistinguishable in terms of accuracy; however, the improved Brunone model is even more computationally efficient and retains excellent numerical accuracy.

Keywords:

• Transient flow
• pipe flow
• entrapped air
• friction factor
• unsteady friction

Additional information

Funding

This work is partly supported by the National Natural Science Foundation of China [grant numbers 51679066 and 51839008], the Fundamental Research Funds for the Central Universities [grant number 2018B43114], Fok Ying Tong Education Foundation [grant number 161068], and the China Scholar Council [grant number 201806715024].