Local bubble slip velocity in a downward laminar tube flow
- PDF / 637,186 Bytes
- 10 Pages / 595.32 x 842.04 pts (A4) Page_size
- 105 Downloads / 245 Views
DOI: 10.1134/S0869864320020080
Local bubble slip velocity in a downward laminar tube flow L.S. Timkin and R.S. Gorelik Kutateladze Institute of Thermophysics SB RAS, Novosibirsk, Russia Е-mail: [email protected] (Received September 25, 2019; revised October 23, 2019; accepted for publication November 6, 2019) The paper presents a study of the slip velocity for almost spherical bubbles in a downward laminar tube flow. The local slip velocity is defined as a difference between the mean liquid velocity (measured with electrodiffusional method) and the mean bubble velocity (measured with a modified LDA tool with a small size of measurement volume). The law for slip velocity for downward flow is significantly different from a similar law for upward flow. This reveals that the bubble flow motion is rather random than structured in the upward flow. Keywords: laminar downward flow, electrodiffusional technique, LDA, local bubble slip velocity.
Introduction The bubble slip velocity is one of important factors for bubbly flow prediction. It determines the momentum transfer between the gas and the liquid flow phases. Different experimental techniques have been applied for measuring the bubble slip velocity in upward flows [1−3]. The data demonstrate that the slip velocity significantly depends on a local void fraction. The experimental slip velocity for polydispersed bubbles averaged over the channel width reveals a difference between upward and downward flows [4]. Although the downward bubbly flow has been studied for many years, the problem of slip velocity is understudied yet. Few calculations for slip velocity in a downward laminar flow through a rectangular channel are presented in [5]. To obtain the local slip velocity of bubbles in stationary conditions, ordinarily experimentations are taken for measuring the mean liquid and bubble velocity profiles, with following subtraction of the bubble velocity from the liquid velocity. According to this definition of bubble slip velocity, the result would depend on the local gradient of liquid flow and the bubble size distribution. To mitigate the contributions from these factors, the local-oriented experimental techniques are required or the monosize bubbly mixtures should be used for those experiments. The laser Doppler anemometry (LDA) technique is one of preferable methods for the bubble velocity measurements due to its nonintrusive character. Usually the size of the LDA measurement volume is about several millimeters. For local measurements, it is necessary to dwarf this size down to the size of liquid velocity gradient at the wall (which is usually much smaller). Previously, the LDA Doppler signals from large transparent particles were L.S. Timkin and R.S. Gorelik, 2020
259
L.S. Timkin and R.S. Gorelik
studied in [6] and discussed in [1−4]. The calculations of light-scattering signals from such particles were performed in [7]. Meanwhile the actual position of the bubble center and the real size of the LDA measurement volume (m.v.) usually were out of scope in this d
Data Loading...
