Influence of displacement ventilation on the distribution of pollutant concentrations in livestock housing

The object of this study is the large space room model; the room is 20 m long, 7 m wide and 3.5 m high, with supply and exhaust air outlets on the east and west sides; the size of the air outlets is 3 m × 2 m, the pollutant CO is distributed in the centre of the room, the mass flow rate of the pollutant is set to 0.72 Kg/(m²/s), the surface size of the pollutant is 1 m×2 m, seven measurement points are set inside the room, and the measurement points are 1 m, 2 m and 3 m in the height direction, one at each of the supply and exhaust air outlets. The height direction is 1 m, 2 m and 3 m, respectively, where one measurement point is set at each of the air supply and exhaust outlets.

Fig. 1

In the equation, Φ denotes a generic variable that can represent fractional velocity, temperature, constants, etc., and Γ and S denote the generalised diffusion coefficient and the generalised source term, respectively. The conservation of mass equation Φ takes 1, Γ takes 0 and S takes 0. For the conservation of momentum equation, Φ takes ui, Γ takes u and S takes - D0 + ; and for the conservation of energy equation, Φ takes T, Γ takes k/c and S takes ST. (2) ∂(ρk)∂t+∂(ρkui)∂xi=∂∂xj[(μ+μiσk)∂k∂xj]+GK+Gb−ρε+Sk {{\partial (\rho k)} \over {\partial t}} + {{\partial (\rho k{u_i})} \over {\partial {x_i}}} = {\partial \over {\partial {x_j}}}\left[ {\left( {\mu + {{{\mu _i}} \over {{\sigma _k}}}} \right){{\partial k} \over {\partial {x_j}}}} \right] + {G_K} + {G_b} - \rho \varepsilon + {S_k} In the equation, GK is the kinetic energy of the turbulent flow generated by the laminar velocity gradient; and Gb is the turbulent kinetic energy generated by buoyancy. (3) ∂∂x(uxC)+∂∂y(uyC)+∂∂z(uzC)=Γ(∂2C∂x2+∂2C∂y2+∂2C∂z2)+S(x,y,z) {\partial \over {\partial x}}\left( {{u_x}C} \right) + {\partial \over {\partial y}}\left( {{u_y}C} \right) + {\partial \over {\partial z}}\left( {{u_z}C} \right) = \Gamma \left( {{{{\partial ^2}C} \over {\partial {x^2}}} + {{{\partial ^2}C} \over {\partial {y^2}}} + {{{\partial ^2}C} \over {\partial {z^2}}}} \right) + S(x,y,z) In the equation, Γ is the diffusion coefficient at any point; s(x, y, z) is the diffusion intensity at any point; and C is the pollutant concentration.

As the room volume is calculated according to different ventilation times, different air supply and exhaust volumes are obtained. When the concentration of pollutants in the room is high, it follows that the ventilation times should be appropriately increased. This simulation assumes a general feeding farm, and thus the number of air changes is increased from 30 times/h to 50 times/h; the specific operating mode set parameters used are shown in Table 1.

From the simulated data, it can be known that the concentration of CO in the room does not decrease with the increase of ventilation quantity. When the frequency of ventilation in the room is 30 times/h, the concentration of pollutants in the room is relatively high. At the same time, it will keep the value of CO concentration around 0.3–0.4 kg/m³ for a period of time. With the increase of ventilation quantity, when the frequency of ventilation is 45 times/h, the lowest concentration of pollutants in the room is 0.2 kg/m³ on average. However, continuing to increase the number of air changes in the room, when the frequency of ventilation is 50 times/h, the concentration value of pollutants in the room increased, which is not conducive to the timely discharge of pollutants. The concentration of pollutants at the air supply outlet apparently reduces as the frequency of ventilation increases. When the frequency of ventilation is up to 45–50 times/h, the concentration value of pollutants near the air supply outlet changes very little, and the concentration value of pollutants is basically reduced to zero. The change rule of the pollutant concentration value at the exhaust outlet is similar to the one inside the room, and the pollutant concentration value at the exhaust outlet is the lowest when the number of air changes is 45 times/h. After 750 min, the pollutant concentration value decreases to different degrees under different operating conditions.

The specific numerical results of the simulation are shown in Figures 2–8.

Fig. 2

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Fig. 8

The increase in the amount of room ventilation makes the speed of air supply and exhaust speed to increase, and the vortex effect of airflow organisation is formed above the source of pollution, which enhances the mixing and diffusion of airflow and surrounding air and affects the effect of replacement ventilation.

Figures 2 to 8 show the measured values in different operating modes.

As it can be seen from Figures 9 to 13 of the simulation results, the airflow velocity field in the room increases significantly with the increase of ventilation. When the ventilation rate is 30 times/h and 35 times/h, the airflow rate is relatively small in the room, and the concentration of pollutants is comparatively high. When the ventilation rate is 40 times/h and 45 times/h, the concentration of pollutants is low and the wind speed is moderate, which is conducive to the discharge of pollutants. When the ventilation rate is increased to 50 times/h, the airflow in the room produces the vortex phenomenon and the air mixing degree is enhanced; with the increase of room ventilation, the speed of air supply and exhaust is increased and the vortex effect of airflow is formed above the pollution source. As a result, the airflow and the surrounding air mixing diffusion is enhanced, which affects the effect of replacement ventilation.

Fig. 9

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Fig. 13

The velocity vector of the airflow organisation inside the room is shown in Figures 14 to 18, and we see that it is constantly changing. When the number of air changes increases from 30 times/h to 45 times/h, the airflow organisation above the pollutant source gradually moves in the direction of the exhaust air outlet, and when the number of air changes increases to 50 times/h, the exhaust air velocity is relatively large, and a larger swirling airflow is generated above the pollutant source, which greatly reduces the effect of replacement ventilation. Following the changes of time, the airflow of pollutants near the source gradually and uniformly moved in the direction of the exhaust air outlet.

Fig. 14

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Fig. 18