Airflow characteristics at the surface of manure in a storage pit affected by ventilation rate, floor slat opening, and headspace height

To inform the modelling ammonia of emissions from manure surfaces and provide basic knowledge of the characteristics of airflow at the emission surface, laboratory experiments were performed using a scale model of a growing/finishing pig house with water in the pit and a clean slatted floor with various opening areas: 100%, 33.3% and 16.7%. At 100% opening area the headspace formed an integral part of the room airspace and this configuration was used as the reference treatment. The pig house model had two sidewall inlets and exhaust in the middle of the ceiling. The effects of room ventilation rate, slatted floor opening and headspace height in slurry pit on air velocity, turbulence intensity (Ti) and airflow pattern at manure surface were investigated. The mean air velocity at manure surface increased as ventilation rate increased with a constant inlet opening. Increasing the opening ratio of the slatted floor increased the air exchange rate in the slurry pit, resulting in a higher air velocity above manure surface. For 33.3% and 16.7% floor slat openings, the mean air velocity at the manure surface decreased as headspace height increased and this resulted in an increased mean Ti above the manure surface for 50 mm and 82 mm headspace heights, respectively. However, no significant differences were found in the mean Ti between four ventilation rates for 26 mm headspace height. Tests using smoke visualisation showed that the supply air travelled longer distances parallel to the manure surface at lower headspace heights than at higher heights before returning to the room space. Ammonia emission results from previous experiments can be explained by using the results of measurements of air velocity and Ti. A statistical model was developed to calculate the mean air velocity as a function of ventilation rate, opening ratio of the slatted floor and headspace height in the slurry pit (coefficient of determination R2 = 0.97). The mean air velocity was more sensitive to the ventilation rate than to the slatted floor opening ratio and the headspace height in the pit. Moreover, the mean air velocity was much more sensitive to variations in the ventilation rate at higher ventilation rates than at lower ventilation rates. However, the mean air velocity was much more sensitive at lower floor slat opening ratios and headspace heights than at higher ones.