the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Sep 2024
Laminar gas inlet – Part 2: Wind tunnel chemical transmission measurement and modelling
Abstract. Aircraft-based measurements of gas-phase species and aerosols provide crucial knowledge about the composition and vertical structure of the atmosphere, enhancing the study of atmospheric physics and chemistry. Unlike aircraft-based aerosol particle sampling systems, the gas loss mechanisms and transmission efficiency of aircraft-based gas sampling systems are rarely discussed. In particular, the gas transmission of condensable vapors through these sampling systems requires systematic study to clarify the key factors of gas loss and to predict and improve gas sampling efficiency quantitatively. An aircraft gas inlet for aircraft-based laminar sampling of condensable vapors is described in part 1 (Yang et al., 2024), which describes the inlet dimensions, flow analysis and modelling, along with initial gas transmission estimates. Here we test and characterize the complete inflight sampling system using for gas-phase measurements of π»2ππ4 in a high-speed wind tunnel, and conduct detailed computer fluid dynamics (CFD) simulations to assess inlet performance under a range of flight conditions. The gas transmission efficiency of π»2ππ4 through different sampling lines was measured using Chemical Ionization Mass Spectrometry (CIMS), and the experimental results are reproduced by the CFD simulations of flow and mass diffusion using a mass accommodation coefficient, πΌπ = 0.70 ± 0.05 for π»2ππ4 on inlet lines. The experimental data and simulation results show consistently that gas transmission efficiency increases with an increased sampling flow rate. The simulation results further indicate that sampling efficiency can continue to improve to a certain level after the sampling flow enters the turbulent flow regime, up to Reynolds numbers, Re ~ 6000. A decrease in transmission is predicted only for higher Re numbers. These results challenge the widely held assumption that laminar flow core sampling is the best strategy for sampling condensable vapors. The gas-phase π»2ππ4 transmission efficiency can be optimized (increased by a factor ~2) by minimizing residence time, rather than maintaining laminar flow; this benefit extends to other condensable vapors and applies over the full range of operating conditions of the aircraft inlet system. For a sticky species (πΌπ > 0.25), the laminar diffusivity is important to predict the transmission efficiency via the aircraft inlet section, while for less sticky species (πΌπ < 0.25) the gas-phase diffusivity plays a minor role in predicting the gas transmission efficiency in the sampling line.
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RC1: 'Comment on egusphere-2024-2390', Anonymous Referee #1, 08 Nov 2024
This is a short review as all aspects of the paper were not evaluated: it is too long, thus the Fair rating on Presentation.Β I gave it Fair on Scientific quality for reasons detailed below.Β I gave it Good for Significance as the loss of sticky species on inlets is important to get correct.Β I think with a rewrite and a careful paring down of the text along with addressing the science quality issues below, it could be a pretty good report.Β
1) Wall loss is a tricky thing to model.Β How is it incorporated in Fluent?Β OF course no loss (set species to have no flux at the surface) and diffusion limited loss (set species to have zero concentration at the surface) are conceptually easy to understand and to implement in Fluent.Β How does one address mass acc. coefficients other than 0 and 1 in Fluent?Β
2) Related to that issue,Β the mass acc. results do not make sense in the laminar realm.Β At 298 K and using 98 for molar mass:Β (i) At 1 atm (or 0.85 atm? as in Colorado Springs), the diffusion limited loss rate in a 1cm ID tube is about 1.5 s-1.Β (ii) The kinetic limit (for a mass acc. = 1) is about 2.3x10^4 s-1.Β What this means is that mass acc. values greater than about 0.001 are at the diffusion limit and there should be very little to no dependence on mass acc. for the throughput of the sampling tube.Β
3) Another related issue is the diffusion coefficient.Β It looks like the authors are using the pressure independent value for all pressures: it actually has units of atm.cm2/s.Β Diffusivity at altitude will be some 6 or 7 times that at sea level due to the pressure change.Β Yet it is colder so apply a typical T^1.75 factor.Β What temperature is the sampling tube at altitude?Β
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Citation: https://doi.org/10.5194/egusphere-2024-2390-RC1 -
RC2: 'Comment on egusphere-2024-2390', Anonymous Referee #2, 11 Nov 2024
General CommentsΒ
Overall a good paper but some sections could be decreased as there is a long lead up to the main results on the gas transmission efficiency of the inlet. Saying that, I would encourage the authors to expand the methods section to include more information on the operation and setup of the CIMS as these measurements are fundamental to the paper. Furthermore, it is not clear to me how widely applicable the results are to other aircrafts/instruments based on the experimental conditions and assumptions used throughout the study. For example, some CIMS instruments would sub-sample a smaller flow from the sample line or some sample lines may experience a temperature gradient due to differences between the ambient and cabin temperature. It would be useful to clarify the broader applicability of the findings. Β
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Specific Comments
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Introduction paragraph one β some statements are repeated multiple times and disrupts the flow of the paragraph (e.g. importance of condensable vapours for aerosol growth and hence health). This could be rewritten so that it is clearer.
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Line 43 β The sentence on the relevance of trace gases currently reads as this is an exhaustive list. Should be made clear that these are examples.
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Line 45 β Understanding the formation and growth of short-lived reactive gases. Suggest change word growth, this feels more appropriate to describe aerosols.
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Line 110 β What is the material of the sampling tube?
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Line 114 β What is the range of flow rates sampled by the CIMS?
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Fig1d β What does the dashed line represent?
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Line 135-136 β What concentrations are used for each of the reagents? And what is the resulting concentration of H2SO4?
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Line 174 β Later on in the paper you mention the different humidity conditions in the wind tunnel across the experiment period. Is it correct that H2SO4 is diffusing in dry air?
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Line 176 - In addition, as the temperature gradient in the transmission line is insignificant, we neglect thermal diffusion loss. Does this remain true for ambient sampling where there can be larges differences between the cabin and ambient temperatures?
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Line 229 - These ion concentrations were recorded under different operating conditions by CIMS. Different inlet or CIMS operating conditions? If CIMS what are these different conditions and what is the rationale for this?
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Line 314 β Can you include a schematic of the NO3 CIMS in the methods that highlights the IMR region you are describing here.
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Line 315 β I would be explicit here that the lower signal response at 16 SLM is specific to the instrument used in this study and you cannot be certain that this holds true for other CIMS instruments that are operated under different conditions.
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Fig 6 β it would be helpful to the reader to define Q in the caption as this is defined later in the paper.
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Technical Comments
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Line 17 β remove using
Line 40 β composition-dependent, . remove comma
Line 56 β replace aboard with onboard
Line 109 β replace aboard with onboard
Line 161 - sampling tube designs use the commercial code β needs rewording
Line 174 β replace refer with referred Β
Line 237 - (π»2ππ4 ππΆππ,) β remove comma
Line 338 β Hanson et all., remove et al as Hanson only author
Fig5 caption β description of chapter 2.4. Change to section 2.4
Line 397 - This is due to sample flow is more turbulent. Typo
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Citation: https://doi.org/10.5194/egusphere-2024-2390-RC2
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