Laminar gas inlet – Part 2: Wind tunnel chemical transmission measurement and modelling

<p>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 for gas-phase measurements of H<span class="inline-formula">₂</span>SO<span class="inline-formula">₄</span> 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 H<span class="inline-formula">₂</span>SO<span class="inline-formula">₄</span> 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 sticking coefficient, <span class="inline-formula"><i>α</i>_<i>i</i></span> <span class="inline-formula">=</span> 0.70 <span class="inline-formula">±</span> 0.05 for H<span class="inline-formula">₂</span>SO<span class="inline-formula">₄</span> on inlet lines. The experimental data and simulation results show consistently that gas transmission efficiency increases with an increased sampling flow rate. At <span class="inline-formula"><i>Re</i></span> <span class="inline-formula">∼</span> 2300, the overall inlet transmission is 16 <span class="inline-formula">±</span> 6 % for H<span class="inline-formula">₂</span>SO<span class="inline-formula">₄</span> at ground and high altitude. 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 <span class="inline-formula">∼</span> 25 % transmission at <span class="inline-formula"><i>Re</i></span> <span class="inline-formula">∼</span> 6000. A decrease in transmission is predicted only for higher <span class="inline-formula"><i>Re</i></span> numbers. These results challenge the widely held assumption that laminar flow core sampling is the best strategy for sampling condensable vapors. The gas-phase H<span class="inline-formula">₂</span>SO<span class="inline-formula">₄</span> transmission efficiency can be optimized (increased by a factor <span class="inline-formula">∼</span> 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 (<span class="inline-formula"><i>α</i>_<i>i</i></span> <span class="inline-formula">&gt;</span> 0.25), the laminar diffusivity is important to predict the transmission efficiency via the aircraft inlet section, while for less sticky species (<span class="inline-formula"><i>α</i>_<i>i</i></span> <span class="inline-formula">&lt;</span> 0.25) the gas-phase diffusivity plays a minor role in predicting the gas transmission efficiency in the sampling line.</p>