An Experimental Analysis of Water–Air Two-Phase Flow Pattern and Air Entrainment Rate in Self-Entrainment Venturi Nozzles

Round 1

Reviewer 1 Report

General comments:
The authors present an experiment to measure the air entrainment rate, observe flow pattern, and measure friction loss in a self-entrainment venturi nozzle.  The experiment is described in detail and the results are presented well.  There are some areas where sentences are awkwardly worded and may contain typograhpic errors, although they do not prevent the reader from understanding the research; the paper should be carefully reviewed for this.  The authors could do a better job analyzing the meaning of their results, there are some concerns regarding the analysis of their results.

Specific Comments:
1. In the introduction, it would be useful if the authors gave examples of some important uses for this type of venturi nozzle.
2. In fig. 1, why are such sharp angles (up to 80 degrees) on the diverging side of the Venturi nozzle considered when the converging side angle is 20 degrees?  Most venturi nozzles have sharper angles on the converging side and shallower angles on the diverging side, in order to prevent boundary layer separation and reduce loss through the ventui nozzle.
3. The authors list the exposure time as 2 micrometers, which is not a measurement of time.
4. Why was vertical downward two-phase flow chosen?  In diverging nozzles, vertical downward two-phase flow can become unstable - leading to kinematic shocks and other phenomena.  Why was vertical upward flow or horizontal flow not used?
5. Why was this study performed at much smaller scale than previous studies?  The authors should address how that change in scale affects their results.  This likely explains why annular flow is reported here but not in previous studies (larger-wavelength instabilities that break up the gas core into slugs cannot occur in such a small channel).
6. The test results from the 40 degree and 80 degree diverging angle sections are somewhat inconsistent with the others.  Have the authors verified that this is not due to a manufacturing difference/defect in those sections?
7. The author's two-phase multiplier in Figure 9 does not match those of other researchers for several reasons that were not addressed by the authors.  First, two-phase flow only exists for part of the test - from the throat to the exit pressure measurement - while the pressure drop is measured from the inlet to the outlet.  Therefore the two phase multiplier should only by applied for a relatively small section.  Further, these multipliers are designed (and best used) for constant-area pipes rather than fittings or blockages like that represented by the venturi meter.
8. At the end of section 3.3 the authors state that future study is needed to understand why the flow pattern changes and the relationship between the air entrainment rate and flow transition.  This is relatively easy to understand.  Flow pattern changes and transitions have been heavily studied by authors such as Ishii, Mishima and Ishii, Taitel and Dukler, and others.  The authors should perform further literature review on this subject.  As for the relationship between flow pattern and entrainment rate: when the flow pattern changes from annular to bubbly flow, the frictional pressure drop decreases (the observed drop in friction multiplier).  Since exit pressure is held constant, this means that the pressure at the throat will drop.  Lower throat pressure means higher pressure difference between the air and throat, and therefore higher entrainment rate.  

Author Response

Please see the attachment.

Reviewer 2 Report

Issues to address:

1. Figure 1: This is a useful figure but the labeling can be made clearer. It is currently a bit cluttered with information.
2. The figure 2, appears to be centered around the flow tank as opposed to the Venturi nozzles. I would ask to make the picture of the Venturi tube larger and more explicit.
3. In line 108 of page 3, the Reynolds number is defined based on the hydraulic diameter. A clearer explanation of the hydraulic diameter must be provided in the text and probably can also be shown in the figure 2.
4. On page 5, line 138 authors mention oscillatory behavior. It would be good have more details about this behavior. Can it be quantified or characterized behavior?
5. In the case of annular flows, when steady patterns form, how long does it take to reach such a state? At what stage in the experiments (i.e after how long) are the images, taken in the figures being taken?
6. Figure 6 on page 6 should be reorganized. In fact, I am not sure if all these panels are necessary. Perhaps just a few sample figures would do. Please also make sure to focus on the appearance of the figures. Once again, there is far too much information in the panels and it all looks cluttered.
7. The figure 8 is a very useful one but once again, it suffers from poor presentation. The images are not easy to see. Here again, I might ask to selectively present some useful representative figures only. I am not sure if showing all of these add any value to the paper. If you must, some of the figures can be moved to supplementary information.
8. Figure 9: Indicate explicitly that 'Bubbly and Annular flow' data corresponds to the 'Current study'. The square data points can be made darker for visual clarity.
9. It would be good for authors to discuss, in the final section, how these results close the gap and advance this fied.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf