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

Under replacement ventilation conditions, a simplified physical model of a tall space (livestock house) was established using ProSim software, and the relevant boundary conditions were set. The airflow size and direction distribution under different air supply and exhaust airflow conditions were obtained by slicing, and the simulation results showed that appropriately increasing the air supply and exhaust airflow helps to increase the pollutant emission rate so that pollutants can be discharged in time, but when the air supply and exhaust airflow were increased to a certain extent, these will cause the airflow vortex phenomenon near the pollutant source, which will enhance the mixing effect of pollutants and surrounding air, making up the pollutant concentration also. The study found that when the indoor air supply and exhaust air volume is increased by 45 times/h, the replacement ventilation effect is optimal and the indoor pollutant concentration is the lowest.


Introduction
With the improvement of people's pursuit of indoor air quality, the control and research of indoor environment such as temperature and humidity, pollutant concentration, etc., are becoming more and more extensive.Generally speaking, people often open windows and other natural ventilation to discharge pollutants [1] and improve the quality of indoor air, but as indoor pollutants are more concentrated, the natural ventilation effect is not ideal and requires mechanical ventilation to attain proper ventilation.The concentration of indoor pollutants is often controlled by mechanical ventilation [2][3][4].
At present, many scholars have conducted research on the distribution of indoor pollutant concentrations.Bo and Xiao [5] and others analysed the effect of air supply speed on the distribution of velocity field and pollutant concentration field in space by numerical simulation; the pollutant concentration distribution increases with air supply speed, but the mixing will also be enhanced, and the air supply speed should be 0.25 m/s; Zhiping and Liang [6] analysed the effect of airflow in student dormitories under different ventilation methods by numerical simulation.The numerical simulation study of the velocity field in student dormitories under different ventilation methods by Zhiping and Liang [6] concluded that when the window-to-ground ratio was above 0.114, the ventilation effect increased very slowly if the window area continued to be increased, and when the window-to-ground ratio was below 0.114, the ventilation effect increased significantly by increasing the window area; Fei et al. [7] analysed the effects of different barn spacing and different pollutant release locations on the air flow patterns and pollutant distribution of natural ventilation in barn and concluded that the pollutants released from the downwind barn might reach the upwind barn.It was found that the pollutants released from the downwind barn might reach the upwind barn, but the amount was very small and could be reasonably ignored; it was effective and economical to choose two times the ridge height as the barn isolation distance for epidemic prevention when pollutants were released from the upwind barn.As the amount of ventilation has been less studied, the effect of different air supply and exhaust air volumes on the distribution of indoor pollutants is obtained through different air exchange numbers, and the air supply and exhaust air volumes under the optimal air exchange numbers are modelled [8][9][10].
2 Physical and mathematical models

Physical modelling overview
The object of this study is a 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/(m 2 /s), the surface size of the pollutant is 1 m × 2 m, seven measurement points are set inside the room, the measurement points are 1 m, 2 m and 3 m in 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.
In the formula, Φ denotes a generic variable that can represent fractional velocity, temperature, and 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;− ∂ p ∂ x i + S i for the conservation of energy equation, Φ takes T, Γ takes k/c and S takes ST.
In the formula, G K is the kinetic energy of the turbulent flow generated by the laminar velocity gradient; G b is the turbulent kinetic energy generated by buoyancy.
In the formula, Γ is the diffusion coefficient at any point; s(x, y, z) is the diffusion intensity at any point; C is the pollutant concentration.

Simulation of operating mode settings
From the room volume in accordance with the different number of air changed calculated from different air supply and exhaust volume, when the room pollutant concentration is high, should be appropriate to increase the number of air changed, the simulation assumes a general feeding ground, the number of air changes from 30 times/h increased to 50 times/h, the specific operating mode set parameters as shown in Table 1. 3 Analysis of numerical simulation results

Variation pattern of CO concentration in the room under different ventilation amounts
From the simulation data, it can be seen that the concentration of carbon monoxide in the room does not decrease with the increase in ventilation; when the number of air changes in the room is 30 times/h, the concentration of pollutants in the room is relatively high at this time, and the value of CO concentration is around 0.3-0.4kg/m 3 for a period of time, while with the increase of ventilation, when the number of air changes is 45 times/h, the lowest concentration of pollutants in the room is 0.2 kg/m 3 on average.kg/m 3 below, continue to increase the number of air changes in the room, when the number of air changes for 50 times/h, the concentration value of pollutants in the room but increased, not conducive to the timely discharge of pollutants; air supply at the concentration of pollutants with the increase in the number of air changes concentration significantly reduced, when the number of air changes to 45 times/h ∼ 50 times/h, the air supply near the concentration value of pollution are very small changes in the value of pollutants, the concentration value of pollutants basically.When the number of air changes reaches from 45 times/h to 50 times/h, the pollutant concentration value near the air supply outlet changes very little, and the pollutant concentration value is basically zero; the pollutant concentration value at the exhaust outlet is similar to the change pattern 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 s, the pollutant concentration value under each operating mode decreases to different degrees.The specific numerical results of the simulation are shown in Figures 2-8.The increase in the amount of room ventilation makes the speed of air supply and exhaust 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.
The following Figures 2-8 show the measured values under different operating mode in Figures.I to V.

Airflow rate distribution in the room under different ventilation volume
As can be seen from the simulation results in Figures 9-13, the airflow velocity field in the room increases significantly with the increase in ventilation; when the number of air changes is 30 times/h and 35 times/h, the airflow rate in the room is relatively small, and the concentration of pollutants is relatively high; when the number of air changes is 40 times/h and 45 times/h, the concentration of pollutants is low, and the wind speed is moderate at this time, which is conducive to the discharge of pollutants; when the number of air changes is increased to 50 times/h, the airflow in the room produces vortex phenomenon, the air mixing degree is enhanced, the increase of room ventilation makes the speed of air supply and exhaust increase, the vortex effect of airflow organisation is formed above the pollution source so that the airflow and the surrounding air mixing diffusion is enhanced, which affects the effect of replacement ventilation.The velocity vector of the airflow organisation inside the room is shown in Figures 14-18, which 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, and with the change of time, the airflow of pollutants near the source gradually and uniformly move in the direction of the exhaust air outlet.

Conclusion
For a large space livestock and poultry shed for pollutant discharge, the air volume of replacement ventilation should be reasonably controlled, and the replacement ventilation volume is calculated by the number of air changes, when the number of air changes is 45 times/h, the pollutant concentration value at the exhaust port is the lowest, the pollutant concentration in the room has the lowest value, and the pollutant is easier to discharge; when the number of air changes is less than 45 times/h, the pollutant discharge rate is relatively slow, and the pollutant concentration in the room is higher at this time When the number of air changes is less than 50 times/h, the vortex phenomenon of airflow above the pollutants is not conducive to the discharge of pollutants, therefore maintaining the number of air changes at 45 times/h can make the concentration of pollutants in the room smaller and more evenly distributed, which is more conducive to the discharge of pollutants.

Fig. 1
Fig. 1 Physical model and distribution of measurement points

Fig. 2 Fig. 3 Fig. 4
Fig. 2 Variation of CO concentration values under different operating mode at one measurement site

Fig. 5 Fig. 6
Fig. 5 Variation of CO concentration values at four measurement points under different operating modes

Fig. 8
Fig. 8 Variation of CO concentration values at seven measurement points under different operating modes

Fig. 9
Fig. 9 Airflow rate distribution in the room for operating mode 1

Fig. 10 Fig. 11 Fig. 12
Fig. 10 Airflow rate distribution in the room for operating mode 2

Fig. 13
Fig. 13 Airflow rate distribution in the room for operating mode 5

Fig. 14 Fig. 15
Fig. 14 Vector diagram of the airflow velocity in the room for operating mode 1

Fig. 16 Fig. 17
Fig. 16 Vector diagram of airflow velocities in the room for operating mode 3

Fig. 18
Fig. 18 Vector diagram of airflow velocities in the room for operating mode 5

Table 1
Setting of operating mode parameters