On Finding the Right Sampling Line Height through a Parametric Study of Gas Dispersion in a NVB

Abstract

The tracer gas method is one of the common ways to evaluate the air exchange rate in a naturally ventilated barn. One crucial condition for the accuracy of the method is that both considered gases (pollutant and tracer) are perfectly mixed at the points where the measurements are done. In the present study, by means of computational fluids dynamics (CFD), the mixing ratio NH3/CO2 is evaluated inside a barn in order to assess under which flow conditions the common height recommendation guidelines for sampling points (sampling line and sampling net) of the tracer gas method are most valuable. Our CFD model considered a barn with a rectangular layout and four animal-occupied zones modeled as a porous medium representing pressure drop and heat entry from lying and standing cows. We studied three inflow angles and six combinations of air inlet wind speed and temperatures gradients covering the three types of convection, i.e., natural, mixed, and forced. Our results showed that few cases corresponded to a nearly perfect gas mixing ratio at the currently common recommendation of at least a 3 m measurement height, while the best height in fact lied between 1.5 m and 2.5 m for most cases.

Concentration Gradients of Ammonia, Methane, and Carbon Dioxide at the Outlet of a Naturally Ventilated Dairy Building

“…Improper mixing ratios in NVDB have also been reported in computational fluid dynamics modeling studies, for example Doumbia et al [25], which suggest that mixed convection flows may be a potential reason for this phenomenon. Based on our test results, the ratios mostly deviated at lower sampling height (H ≤ 0.9), which corresponds to the animal zones, i.e., close to the emission sources such as the animal's mouth for c CH 4 and the barn floor for c NH 3 .…”

“…However, this presumption faces challenges due to variables such as animal activity, pregnancy, or milk yield, which impact the modeled heat output and subsequently affect the release rate of c CO 2 , leading to Q estimation uncertainty and potential bias in emission outcomes [20,21]. Other studies have also underscored the impact of factors like temperature, relative humidity, and gas sampling positions/strategies on indirect emission estimation in NVDBs [11,19,[22][23][24][25]. These factors result in temporally and spatially non-uniform gas release patterns, which, when combined with airflow dynamics, contribute to significant variability in c P [10].…”

“…These factors result in temporally and spatially non-uniform gas release patterns, which, when combined with airflow dynamics, contribute to significant variability in c P [10]. Moreover, studies based on VERA [15] demonstrate that gas sampling locations should be 3 m high from the ground and 2 m away from the opening walls [24,25]. Notably, while these considerations have been explored in the context of the indirect method, their relevance to the direct method of emission estimation in NVDBs remains largely unexplored to date.…”

Comparison of Methane Emission Patterns from Dairy Housings with Solid and Slatted Floors at Two Locations

“…This might be related to the fact that gas sampling setups are typically optimized for the flow pattern under the dominating mixed convection, from which the pattern under natural convection strongly deviates. In consequence, larger inaccuracies in the measured concentration/emission values can be expected under natural convection (cf., e.g., [55]). In addition to the low significance of mutual information under natural convection, we found no clear trend among the different convection regimes.…”