Effects of Different Ventilation Modes and Outlet Height on Nursery Piggery Environment

Fang Jun-long, Ba Wen-ge, Wu Zhi-dong,2, Wu Shuang, and Tan Ke-zhu*

1 College of Electrical and Information, Northeast Agricultural University, Harbin 150030, China

2 College of Mechanical and Engineering, Qiqihar University, Qiqihar 161006, Heilongjiang, China

Abstract: In order to study the effects of ventilation modes and outlet height on the airflow field of a nursery piggery, computational fluid dynamics (CFD) technology was used to simulate the wind speed and temperature of an experimental pig house in the cold northern region. This study was conducted with simulation and a comparative analysis of transverse ventilation, longitudinal ventilation and roof air intake modes. Furthermore, the effects of the air outlet height of 0.7, 0.6 and 0.5 m with the roof air inlet mode on the environment in the pig house were studied. Field experiments verified the model of roof air intake model. The results showed inadequate ventilation in both the vertical and horizontal ventilation. However, the airfield gradients were less variable and more balanced when using the rooftop air intake mode. The variation of outlet height significantly affected nursery pig houses'airflow velocity and temperature. Roof air inlet mode with an outlet height of 0.7 m was better than the other two. The normalized mean square error (NMSE) of air velocity and temperature was less than 0.01, and the simulation analysis could genuinely reflect the distribution of the airflow field in the nursery.

Key words: nursery piggery, wind velocity, temperature, air outlet height, ventilation mode

Introduction

In the large-scale modern pig breeding industry,precise environmental control in housing is required increasingly, and suitable ecological parameters are essential for promoting pigs' growth (Xieet al.,2014). Temperature, humidity, airflow velocity and content of harmful gases are the leading indicators for evaluating the quality of the internal environment(Gaoet al., 2006). Ventilation and insulation are the most significant parameters in cold northern regions.A suitable ventilation mode needs to be selected to achieve the best temperature, humidity and wind velocity for the growth of pigs, and to expel harmful gases (Liet al., 2020; Wanget al., 2018).

Some studies show that ventilation patterns and the location of vents have a significant impact on the environment in livestock housing. The first numerical simulation technique for predicting airflow field distribution based on ventilation systems was proposed by Chiang in 2003. Studies have confirmed that ventilation patterns and vent locations are the main factors in improving the nursery piggery environment(Chiang and Shou, 2001). The inadequate ventilation characteristic is attributed to the open-slotted undertube ventilation model (Steve and Mylo, 1978).However, different vent positions affect air quality in the house for mechanical ventilation systems(Maghiranget al., 2001). Some scholars have explored the airflow field distribution of natural ventilation in a cow house with three types of ventilating windows. Moreover, studies showed that the openings with ventilate layers perform best for ventilation efficiency and thermal comfort (Nortonet al., 2009).Computational fluid dynamic (CFD) implements a simulation of temperature distribution in the poultry house under low-temperature conditions by improving the layout of the ventilation openings (Seoet al.,2009). Bjerget al. (2014) investigated a numerical simulation of airflow in livestock areas to simulate NH3dispersion in pig houses with partial cavity ventilation systems, showing that different layouts of the vents has a significant impact on air quality.Kwonet al. (2016) proposed an interesting ducted exhaust system to improve the environmental quality of nursery pig houses.

In recent years, the CFD techniques have been used wildly to analyze airflow, temperature, humidity and gas concentrations in various livestock feeding environments. The effects of the position of the deflector at the air inlet and the position of the vent on the surrounding environment of the stack cage chicken house were explored by Chenget al(2019).The results showed that the deflectors improve the uniformity of the airflow field in the air inlet nearby area. Wanget al. (2011) investigated the effects of air inlet angle on the airflow field in pig houses and concluded that the uniformity of temperature field distribution is better at an air inlet angle of 45°.This conclusion is confirmed by Fuet al. (2020),who uses a vertical ventilation system to simulate the distribution of airflow organization in a piggery house. Jinget al. (2016) analyzed the effects of five types of window openings on the airflow field of pig houses under natural ventilation. The results showed that increasing the number of outlets under certain conditions openings is conducive to regulating the airflow environment in the barn. Denget al. (2015;2019) simulated the temperature and humidity field of the low-roof transversely ventilate cattle barn and proposed that the low wall below the wind baffle and neck shackle significantly affects the environment inside cowsheds. Through a comparative simulation study,some scholars have confirmed that the air inlet position and angle significantly affect airflow distribution in the pig house (Denget al., 2014; Caoet al., 2020).

The studies above have less analyses on the location of air outlets for nursery piggery in cold northern regions. Therefore, the CFD techniques was used in this paperto compare and simulate three ventilation methods: horizontal ventilation, longitudinal ventilation, roof air intake and the effects on the internal flow field for different outlet heights of the pig house.A theoretical basis for the structure and ventilation design of pig barns in cold northern regions by comparing and verifying the data measured in the field was provided in this paper.

Materials and Methods

Experimental nursery pig house

The nursery pig house used in this paper is located in Jianhua District, Qiqihar City, Heilongjiang Province(124.0°E, 47.3°N). Pig house was a north-south orientation, with a length of 17 m, a width of 10 m and a height of 2.5 m. An artificial passage was located in the middle of the sheds. Six fences were installed on either side of the corridor, and the size of the individual pig fences was 3.5 m*2.5 m*0.75 m. The width of each piggery door was 1 m, and the gaps in the rails were 7 cm. The leaky floor was 0.5 m above the floor, and a 150 W heat lamp was set in the center of the fence. Each section was equipped with 10-12 piglets with an average of (16±3.5) kg. Two ventilation windows stood on both the north and south walls of the pig house, and sized at 1.75 m*1.35 m. The distance between the two ventilation windows was 2.5 m,and the lower edge of the ventilation windows was 0.7 m from the ground, with a fixed speed fan in each of the windows on the south wall. A simplified model of the original nursery pig house is shown in Fig. 1.

Fig. 1 Simplified model of original nursery piggery

Experimental methods

Usually, two height planes were selected for the piggery airflow velocity field and temperature field.One was the breathing area of the pigs at 0.6 m from the ground, and the other was the average breathing area of the feeders at 1.6 m. The temperature and wind speed data inside the nursery pig house were collected from the 5th to the 11th of January 2020. Twelve test points (A1-A6 and D1-D6) were installed in the pigs'breathing zone, located in straight lines atX=1 m andX=9 m, respectively, with a vertical height of 0.6 m. Another 12 test measurement points (B1-B6 and C1-C6)were placed in the breeders' breathing area, evenly distributed above the fence near the artificial channel.The distribution of measuring points is shown in Fig. 2. More details about the measurements were given by Huanet al(2016).

Fig. 2 Distribution diagram of measuring points inside nursery piggery

Mathematics modeling

Fluid flows were subject to basic control equations(e.g., conservation of mass, conservation of momentum,and conservation of energy). The control equations were given by:

CFD Simulation

Fig. 3 Schematic diagram of ventilation in nursery piggery

Nursery pig house model

A full-scale model was created using the Design Modeler software according to the measured nursery piggery; the width was 10 m inXdirection, the length was 17 m inYdirection, and the height was 2.5 m inZdirection. In this step, some simplifications were made to reduce the difficulty of meshing and improve the solutions' computational efficiency. The small internal elements were excluded or simplified, such as fences, pipes, gutters and leaky floors. Because pigs lived in groups, each cell in the simulation could be equated to a rectangular body of 1.5 m*1.2 m*0.4 m. Additionally, the three ventilation methods were used in the experiment. The ventilation schematics are shown in Fig. 3. The pig houses' ventilation pattern and outlet parameters settings are shown in Table 1.

Table 1 Air outlet height setting in different cases

Meshing

The pig house model meshed with a tetrahedral mesh for the fluid region and a square hexahedral mesh for the solid part. The volume mesh size was 0.2 m in the model of the pig house, and the skewed mesh parameter was used as a criterion to evaluate the quality of the mesh. The skewness of all the models was below 0.85 and satisfied the computational requirements.

Boundary conditions and parameter settings

The CFD simulations were carried out in the steadystate turbulence RNG k-epsilon model, where the acceleration of gravity was set to -9.8 m ? s-2along the negativez-axis. The air properties of the conservatory were assumed to be incompressible ideal gasses. Brick walls with negligible thickness were set up as no-slip walls, while standard wall function was used in the near-wall area. The pigs' body surface temperature was set at 38℃, while the density was 1 100 kg ? m-3,the thermal conductivity was 0.464 W ? (m ? K)-1, and the specific heat was 3 500 J ? (kg ? K)-1. The air outlet was set as a pressure outlet. The pressure and velocity coupling were solved by the SIMPLE-C algorithm with the second-order upwind scheme (Chia, 1981).Three types of ventilation in nursery pig houses were analyzed by controlling the ventilation time to ensure the same amount of ventilation. The airflow inlet and outlet parameters are shown in Table 2.

Table 2 Air inlet and outlet parameter setting

Results

Effects of different ventilation patterns on environment of nursery pig houses

Effects of different ventilation patterns on air velocity in nursery pig houses

A wide stream flowed through the garden, and on it floated richly ornamented12 barges13 and gondolas14 filled with people dressed in the most elegant manner and covered with jewels

The velocity clouds for longitudinal ventilation (case 1),lateral ventilation (case 2) and vertical ventilation (case 3)atZ=0.6 m are shown in Fig. 4 a, b and c, respectively.It could be seen from Fig. 4 that in case 1, the airflows entered the pig house from the air inlet. Gradually the airflow velocity decreased when blocked by the pigs, while the primary airflow spread weakly towards the east and west side walls, reaching the maximum wind speed of 0.9 m ? s-1or more. Additionally, there was an ample ventilation weak zone. The airflow across the distance was reduced in case 2. The three primary airflows formed a giant vortex under the effect of the wall; the maximum wind speed was 0.69 m ? s-1. However, two weak ventilation areas still existed nearY=6 m andY=11 m. In case 3, the airflow entered vertically from the top of the pig house with a relatively even distribution throughout the area, where the maximum wind speed reached 0.37 m ? s-1. The flow trace simulation is shown in Fig. 5.

Fig. 4 Wind speed distribution at different ventilation at Z=0.6 m

Fig. 5 Airflow trajectory diagram

In case 1, one of the reasons for the significant difference in wind speed was the long longitudinal span, where the pig was the equivalent of a rectangular body in the piggery, which widened exposure to the airflow and impeded its movement. There was relatively little difference in wind speed for case 2.The main reason was the short span of the lateral ventilation airflow. In addition, the position of the windows was distributed among the pig models,which was a less obstructive effect on the airflow. The air velocity in case 3 was more evenly distributed,attributable to the increased number of air inlets during the same amount of ventilation.

Effects of different ventilation patterns on temperature of nursery piggery

Fig. 6a, b and c showed the temperature clouds at Z=0.6 m for cases 1, 2 and 3 ventilation modes.As shown in Fig. 6, cases 1 and 2 presented lower airflow temperatures in the ventilated weak zone; the main reason was not diffused of the airflow. The heat generated by the pigs did not follow the airflow and spread throughout the pig house, resulting in high temperatures in the areas where the pigs were active and low temperatures in the weakly ventilated areas.The temperatures in the vicinity of the air outlets were both more extraordinary than the temperatures in the inlet area. In case 3, there was no significant difference in temperature changes, explained mainly by the increased numbers of air inlets. Therefore, the overall temperature distribution was very uniform.The overall average temperature of the pig house was 20.2℃, which met the requirements of the growing environment of the nursery pigs.

Effects of air outlet height on environment of nursery pig houses

Effects of air outlet height on air velocity in nursery pig houses

A simulated cloud of airflow velocities for outlet heights of 0.7, 0.6 and 0.5 m are shown in Fig. 7.There was no significant difference in the distribution of atmospheric velocity fields compared to case 3 with cases 4 and 5. In case 4 pigs' area, the average wind speed was 0.18 m ? s-1, and the maximum local wind speed was 0.27 m ? s-1. Compared to case 3, the average wind speed decreased by 0.01 m ? s-1, and the local maximum wind speed decreased by 0.1 m ? s-1.However, in case 5, the average wind speed was 0.13 m ? s-1in the pig activity area, and the maximum local wind speed was 0.25 m ? s-1. Compared to cases 3 and 4, the average wind speed decreased by 0.06 and 0.02 m ? s-1, respectively, and the maximum local wind speed decreased by 0.12 and 0.02 m ? s-1. From the above, it was obvious that different heights of the air outlets significantly affected the wind speed in the nursery. Since it was kept at the same position and the air inlet angle, there were no significant changes in the airfield in the house. However, the airflow trace in the pig house changed with the height of the air outlet.

Fig. 6 Temperature distribution at different ventilations at Z=0.6 m

Fig. 7 Wind speed distribution at different air outlet heights at Z=0.6 m

Fig. 8 Temperature distribution at different air outlet heights at Z=0.6 m

Effects of air outlet height on temperature of nursery piggery

Fig. 8 showed simulated temperature clouds with 0.7, 0.6 and 0.5 m. Since the position of the air outlet affected the values of the air velocity in the pig house, the distribution of the airflow field was not a significant change. Therefore, the temperature field distribution was also similar. In cases 4 and 5, the average temperature was 20.9℃ and 21.5℃ in the pig activity area. Compared to case 3, the average temperature increased by 0.7℃ and 1.3℃, respectively.With the outlet height gradually decreasing, the airflow speed was reduced, and there was relatively little heat exchange with the outside world, causing the temperature to rise. It was apparent that slightly decurrent the outlet height was beneficial to the heat preservation of the nursery pig house, but not to the emission of harmful gases.

Experimental verification

Where,Evwas the relative error between the simulated and measured values in the nursery,Cswas the simulated value, andCmwas the measured value.Csiwas the simulated value at the measurement pointi,Csmwas the average of the simulated values,andComwas the average of the measured values.

Wind velocity field verification

In Fig. 9, the nursery piggery was compared to simulate and measure wind speeds with 0.7, 0.6 and 0.5 m air outlets. The A and D represented the measurement points atZ=0.6 m, which was the pig's breathing level. The B and C indicated the human respiratory level measurement points atZ=1.6 m. The relative error ranged from 0.68% to 15.6% for the simulated and measured airflow velocity in case 3. In case 4, the error coverage was from 1.05% to 16.7%.For case 5, the error was between 1.67% and 18.5%.The measurement points with notable errors were located in the A and D (1, 6). The reasons for the large errors in the measurement points were as the followings.Firstly, the measurement points were close to the windows so that airflow entered the piggery through gaps. Secondly, the troughs, enclosures and leaky floors were ignored in the nursery. More importantly, the turbulence model selection and the iterative calculation of the flow field were subject to some errors.

The equations (4) and (5) showed that the NMSE of airflow velocity for cases 3, 4 and 5 were 0.0074,0.0081 and 0.008, respectively, which were less than 0.01. It suggested that the simulated results of the air velocity field well corresponded to the actual measured data in the piggery.

Temperature field verification

The comparison of simulated and measured temperatures are shown in Fig. 10, with air outlets respectively of 0.7, 0.6 and 0.5 m air outlets. The relative error between temperature simulated and measured values ranged from 0.13% to 9.4% in case 3. Only one measurement point was more than 5% error, and 62.5% was less than 3%. For case 4, the relative errors ranged from 0.48% to 10.88%. The relative errors of case 5 varied between 0.61% and 11.24%. The maximum temperature change was 2.36℃ at each measurement point in the actual measurement.

As seen in cases 3, 4 and 5, the temperature NMSEs were 0.0012, 0.0034 and 0.0023, respectively, each less than 0.01. The results showed that the temperature simulations also kept with the measured data.

Above all, the simulation results for the air velocity and temperature fields were relatively compatible with the measured data in the roof air intake method.The analysis showed that the simulation realistically demonstrated the distribution of the airflow field in the piggery. Meanwhile, it was confirmed that the ventilation efficiency was better than the other two by using the roof air intake method in the cold northern region, with an outlet height of 0.7 m.

Fig. 9 Velocity comparison between simulated and measured values

Fig. 10 Temperature comparison between simulated and measured values

Discussion

Environmental parameters have become an essential factor of production in leading commercial breeding.Hence, it is crucial to identify ventilation strategies to induce good ecological factors to increase pig production and avoid unnecessary economic losses.

In this paper, the choice of ventilation pattern and outlet height was closely related to the quality of the nursery environment. First of all, the ventilation mode affected the wind speed in the nursery pig house. In different ventilation modes, the airflow movement distances were dissimilar. The longer the airflow moved, the more severe the air resistance and the less the airflow velocity. Similarly, the airflow speed gradually declined with the decrease of outlet height.Interestingly, the temperature changes were opposite to the wind speed, which was related to the trajectory of airflow in the pig house.

In cold northern areas, longitudinal and transverse ventilation was inflected airflow field distribution in pig houses. Transverse ventilation showed more ventilation dead zones than those of longitudinal ventilation modes. Still, the temperature was more even and suitable for winter insulation in the north (Wanget al., 2018). Although the temperature distribution of the pig house with horizontal ventilation in this experiment did not differ significantly from the results of the above study, this ventilation pattern was not the most suitable method for pigs housing in cold northern regions. Previous studies had focused more on natural ventilation with different window openings.There was a lack of research on using roof air intake as the primary way to remove polluting gases from pig buildings. In addition, the outlet position as an influential factor in wind-induced airflow had not been addressed. The roof air intake mode could increase the system's efficiency in various wind directions (Matouret al., 2021). In this experiment, the temperature and velocity fields of the roof intake air were uniformly distributed and met the needs of healthy pig growth,which was consistent with the above study results.

In recent years, the roof ventilation model had been a promising technology to address internal air quality and thermal comfort requirements in contemporary livestock houses (Fuet al., 2018; Alessandroet al.,2020). Usually, the roof air intake mode improved airflow uniformity in the pig house compared to horizontal and vertical ventilation, avoiding weak ventilation areas and ventilation dead angles. A roof air intake model was proposed that significantly improved the environment of the nursery piggery, and this ventilation mode was suitable for cold northern regions (Wuet al., 2021). In addition, it was confirmed that the exhaust duct diameter was influential on the environment of the nursery pig house.

In this study, the CFD techniques were used to simulate not only the three modes of ventilation:horizontal, vertical and roof inlet, but also to analyze the location of the air outlets in a comparative simulation, which was verified through field experiments.The result showed that the roof ventilation method occupied an essential role in improving the environment of the nursery piggery.

Conclusions

The ventilation modes and outlet height played a role in the qualities of the nursery piggery environment.In both the longitudinal and horizontal ventilation modes, the temperature was within the standard limits in the nursery pig house. However, neither the weakly ventilated areas nor the local wind speeds were too high for nursery pigs. In contrast, the air velocity and temperature met the requirements of healthy pig growth in the ceiling air intake mode, and the stepchange in velocity and temperature field was not too large. Interestingly, the airflow speed and temperature in the pig house were changed depending on the position of the air outlet.

In addition, it was verified through field measurements that the simulated values of the nursery pig house were in good agreement with the measured data.After several comparison experiments, the roof air intake mode was more suitable for nursery piggery in cold northern regions, when the outlet height was 0.7 m.

This study provided a comprehensive evaluation of the ventilation pattern of nursery pig houses by the CFD technique, which also could provide theoretical basis and technical support for the complete evaluation system of ventilation strategy of other livestock and poultry houses.