Comparisons of two numerical approaches to simulate slatted floor of a slurry pit model - Large eddy simulations

Authors:
Wentao Wu;Chao Zong;Guoqiang Zhang
Affiliations:
Department of Engineering, Aarhus University, Blichers Alle 20, P.O. Box 50, 8830 Tjele, Denmark;Department of Engineering, Aarhus University, Blichers Alle 20, P.O. Box 50, 8830 Tjele, Denmark;Department of Engineering, Aarhus University, Blichers Alle 20, P.O. Box 50, 8830 Tjele, Denmark
Venue:
Computers and Electronics in Agriculture
Year:
2013

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Abstract

For dairy cattle buildings with slatted floor systems, about 40% of the ammonia emission originates from the slurry pit. In order to find a solution to abate this part of emission, a better understanding of the ammonia transportation from the pit to the room space is crucial. Large eddy simulation (LES) was adopted to investigate the transportation of airflow and ammonia under slatted floor. To tackle the involvement of the slatted floor, two approaches were proposed: modelling slatted floors directly with geometrical details (LESD) and treating them as porous media (LESP). The main purpose of this work was to study the potential of using porous media to model the slatted floor. The LES results were validated by the air velocities measured using a LDA (Laser Doppler Anemometer) in a 1:8 scale pit model placed in a wind tunnel. The results showed that LESP was able to estimate the mean air velocities and turbulence kinetic energy in the core of the pit headspace; but it cannot well predict the mean air velocities and turbulence kinetic energy in the space next to the upwind wall. Clear vertical air motion in the top surface of the slot was observed for LESD results. There was not such trend found for LESP results. Both the air velocity and NH"3 fraction fluctuated weaker for LESP results. By spectral analysis, LESP was able to capture the entire power spectrum compared with LESD. A dominant Strouhal number 0.23 was found for LESD results but no dominant Strouhal number was found for LESP results. The emission rate and total mass of NH"3 in the pit headspace calculated by LESD was double of those calculated by LESP. Pollutants were confined in the headspace for longer time by means of using LESP than using LESD. For both LESD and LESP, turbulence transportation was the dominant removal mechanism to transport pollutants from the headspace to the free stream.