Computational fluid dynamics evaluation of pig house ventilation systems for improving the internal rearing environment

b i o s y s t e m s e n g i n e e r i n g 1 8 6 ( 2 0 1 9 ) 2 5 9 e2 7 8

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Research Paper

Computational fluid dynamics evaluation of pig house ventilation systems for improving the internal rearing environment Uk-Hyeon Yeo a, In-Bok Lee a,b,*, Rack-Woo Kim a, Sang-Yeon Lee a, Jun-Gyu Kim a a

Department of Rural Systems Engineering, Research Institute for Agriculture and Life Sciences, College of Agriculture and Life Sciences, Seoul National University, Gwanakno 1, Gwanakgu, Seoul, 08826, Republic of Korea b Research Institute of Green Eco Engineering, Institute of Green Bio Science and Technology, Seoul National University, 1447, Pyeongchang-daero, Daehwa-myeon, Pyeongchang-gun, Gangwon-do, 25354, Republic of Korea

article info

Due to the cold stress experienced by pigs, and the increased energy load during winters

Article history:

and the changes from winter to spring and summer to autumn, it is difficult to provide

Received 18 April 2019

sufficient ventilation in pig houses. These factors can result in a poor internal environ-

Received in revised form

ment. Therefore, fundamental measures to increase the ventilation rate and a corre-

15 July 2019

sponding analysis of the effects were needed to improve internal rearing environment. Due

Accepted 8 August 2019

to the characteristics of invisible air, it was difficult to analyse the aerodynamic charac-

Published online 31 August 2019

teristics inside a pig house by field experiment. Computational fluid dynamics (CFD) has been used to overcome such limitations for the last 30 years. In this study, environmental

Keywords:

monitoring (air temperature, humidity, etc.) in a commercial pig house were conducted to

Computational fluid dynamics

identify environmental problems. After this, CFD validated models were designed and

Pig house

evaluated to find effective solutions, by changing the conditions of the pig house air inlets

Rearing environment

and outlets (air buffer space, inlet duct, and exhaust fan). Compared with the conventional

Ventilation

ventilation system of the experimental pig house, adjusting the hole spacing of the inlet duct and installing a roofechimney exhaust fan did not significantly improve the rearing environment. However, when an air buffer space was installed just before the location of the inlet on the sidewall, the air temperature flowing through the air buffer space increased making it possible to supply more than twice the external air to the pig house, while maintaining the air temperature distribution at the height of the animal-occupied zone. © 2019 IAgrE. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

South Korea has four distinct seasons; but in summer air temperatures rise up to 40.0
C and drop to 29.2
C in winter

(Korean Meteorological Administration (KMA), 2018). In addition, a daily temperature difference of 15
C in maximum occurs during the change from winter to spring (Suh, Hong, & Kang, 2009). Pigs can be stressed depending on the internal

* Corresponding author. Gwanakno 1, Gwanakgu, Seoul, 08826, Republic of Korea. Fax: þ82 2 873 2087. E-mail address: [email protected] (I.-B. Lee). https://doi.org/10.1016/j.biosystemseng.2019.08.007 1537-5110/© 2019 IAgrE. Published by Elsevier Ltd. All rights reserved.

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Nomenclature AOZ Y obs Y si ! Ji CFD CMH CMM r R2  DP Keff teff Sh

Animal-occupied zone Average of the measured values Average of the simulation values Component of diffusion flux Computational fluid dynamics Cubic metre per hour Cubic metre per minute Density (kg m3) Determination coefficient Diameter Duct point Effective conductivity (kg m1 s3 K1) Effective stress tensor (kg m1 s2) Enthalpy rise by chemical reaction or radiation (kg m1 s3) ! F External force vector (kg m s2) FDM Finite difference method FEM Finite element method FVM Finite volume method ! g Gravitational acceleration (m s2) Wo and Wi Humidity ratios of the outer and indoor air (kg kg1 [DA]) d Index of agreement ith measured value Y obs i ith simulation value Y si i W Latent animal heat production (J kg1) Latent heat of evaporation (J kg1 head1) We L Length in the longitudinal direction of the duct Mass source term by chemical reaction (kg m2) Sm Maximum ventilation Qmax Max:del T Maximum air temperature deviation Minimum ventilation Qmin MP Moisture production rate (kg [H2 O] s1) PE Polyethylene RMSE Root-mean-square-error P Static pressure (kg m1 s2) t Stress tensor (kg m1 s2) T Temperature (K) E Total energy (kg m2 s2 kg1) Lmax Total length of circular duct ! y Velocity (m s1) Q Ventilation rate at the exhaust fan (m3 s1)

aero-environment of a building, which changes temporally and seasonally according to external weather conditions. In particular, it is likely for the pig to be exposed to cold stress during the changes from winter to spring and summer to autumn. Therefore, for precise environmental control inside a pig house, mechanically ventilated pig houses are required more than naturally ventilated buildings. The mechanical ventilation system and physical structure of pig houses is € llvik & commonly affect the internal rearing environment (Sa Walberg, 1984; Minton, Nichols, Blecha, Westerman, & Phillips, 1988; Harral & Boon, 1997; Aarnink, Schrama, Heetkamp, Stefanowska, & Huynh, 2006; Nardone, Ronchi, Lacetera, Ranieri, & Bernabucci, 2010; Banhazi, Stott, Rutley,

Blanes-Vidal, & Pitchford, 2011; Kuczynski et al., 2011; Carroll, Burdick, Chase, Coleman, & Spiers, 2012), and has a direct influence on pig behaviour, weight gain, health, function of the immune system, and physiological characteristics. This is because the characteristics of air movement (advection and dispersion of the air), are the main mechanisms determining the distribution of internal environmental factors and they can vary depending on the physical structure of the pig house, as well as on the installation and operation conditions of the ventilation system. The general factors that cause problems related to the rearing environment include air temperature, humidity, gas, dust, etc (Banhazi et al., 2011; Choi, Song, Lee, & Albright, 2010; Ni, Heber, Diehl, & Lim, 2000; Seo et al., 2008; Yasuhara, Fuwa, & Jimbu, 1984). The productivity of a pig is particularly sensitive to the air temperature and humidity of the animal-occupied zone (AOZ). This is because these factors can cause respiratory problems or chronic wasting diseases of the pigs, and influence heat dissipation and evaporation from the body surface depending on the surrounding thermal and moisture environment. Moreover, they are related to the energy costs for the pig house because the internal environment of the pig house is controlled by the field-measured air temperature and humidity as a ventilation control index. When the internal environment is controlled through the air temperature, it can be appropriately controlled. However, the distribution of other environmental variables (humidity, gas, dust) can be poor, as it is not possible to supply sufficient external air when it is cold. In particular, during winters and during the changes from winter to spring and summer to autumn, when the external air temperature remains low. Therefore, a new ventilation system that considers the uniformity and stability of the environmental factors is needed in order to solve the problem. Increasing the air temperature of the external inflow air as much as possible and then supplying it to the AOZ and complementing the insulation of the pig house can be a basic solution (Adrion, Threm, Gallmann, Pflanz, & Jungbluth, 2013; Bjerg, 2011; Geers et al., 1985; Harral & Boon, 1997; Jacobsen, 2008; Lee et al., 2004, 2010; Li, Rong, Zong, & Zhang, 2017; Myer & Bucklin, 2007; Seo et al., 2012; Zong, Li, & Zhang, 2015). Typical methods used to improve the rearing environment include the installation of such features as an air buffer space, a inlet duct, a perforated ceiling, roofechimney exhaust fans and the use of heating equipment. However, it has been difficult to determine the optimal design of a ventilation systems for pig houses, because field experiments to analyse the effects of the ventilation systems require expensive instruments, time, and labour. Some field-based studies (Song, Park, Jeon, Choi, & Barroga, 2013; Zong, Feng, Zhang, & Hansen, 2014; Ni et al., 2015, 2016; Hviid & Svendsen, 2013) have been conducted, but the ventilation systems installed in the field were fixed and not able to be moved or changed. In addition, the qualitative and quantitative analyses of the studies were insufficient because of the limited number of measuring points, difficulties in artificially controlling the environmental conditions, and unstable and unpredictable weather conditions. In the case of numerical analyses, a complicated validation procedure has been required to ensure the accuracy and reliability of numerical solutions. However, there is a potential

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advantage in that quantitative and qualitative analyses based on various physical and chemical conditions can be conducted by artificially changing the structure of the facility and environmental conditions, according to the intention of the researcher (Yeo, Lee, Seo, & Kim, 2018). Computational fluid dynamics (CFD) can assess invisible aerodynamic problems related to air flow, heat, humidity, gas, and the dust environment for full-scale models, and it has been actively applied for the past 30 years in agricultural (Bjerg and Zhang, 2012; Hong et al., 2017; Kwon, Lee, Zhang, & Ha, 2015; Lee et al., 2013; Li et al., 2017; Norton, Sun, Grant, Fallon, & Dodd, 2007; Seo, Lee, Hong, Noh, & Park, 2015, 2008; Yeo et al., 2015). Most studies related to the pig houses have focussed on environmental control, and were mainly aimed at reducing gas and dust concentrations, rather than controlling air temperature and humidity (Bjerg et al., 2013; Calvet et al., 2013; De Paepe, 2014; Kwon, Lee, & Ha, 2016; Rong, Bjerg, & Zhang, 2015; Saha, Zhang, & Kai, 2012; Ye, Zhang, Li, Strom, & Dahl, 2008). Lee et al. (2004), Morsing, Strøm, Zhang, and Kai (2008), Kwon et al. (2010), Choi, Han, Albright, and Chang (2011), Seo et al. (2012), Ni et al. (2016), and Zhang, Bjerg, and Zong (2017) have analysed internal environments according to the location and number of mechanical equipment in pig houses using the CFD technique. However, there has been no field application or evaluation for the effective solutions that were determined from the CFD-computed results. Most of the studies have focused on the housing of piglets, or growing pigs, because it is assumed that they are more sensitive to changes in the internal environment than finishing pigs. However, even with finishing pigs, during the final growth stage prior to sale, environmental control is worthy of consideration. It is also necessary because in recent years allin-all-out (AIAO) production systems rather than conventional continuous-flow production systems have been adopted. Under AIAO only pigs of similar age and weight are kept together to prevent the spread of disease. Therefore, the objective of this study was to find efficient solutions by installing environemtnal control systems to improve the internal rearing environment of a finishing pig house in the winters and during the change of seasons from winter to spring and summer to autumn.

2.

Materials and methods

The aim of the study was to investigate the problems associated with internal environmental factors (air temperature, humidity, ammonia, and dust) of a finishing pig house through field experiments, and to evaluate the efficiency of improvement solutions for solving the identified problems in the field in winter and during the changes from winter to spring and summer to autumn. The CFD technique was used to effectively evaluate various solutions, and to qualitatively and quantitatively visualise the internal air temperature and humidity distributions, as well as the internal air flow. The field-measured and CFD-computed results were compared with each other to validate the CFD model. Using the validated CFD model, various solutions were designed, and the efficiency of the solutions were evaluated. Lastly, the most effective solution, as determined from the CFD-computed

261

results, was applied to the field, and its effectiveness was validated. Figure 1 shows the overall research flow of this study.

2.1.

Experimental pig house

The experimental pig house (latitude: 35
40 1200 , longitude: 127
470 4000 , elevation: 40.5 m) was 17.2 m wide, 60.0 m long, and 5.3 m high, as shown in Fig. 2(a). It has asymmetric structural characteristics with different heights of sidewalls where the inlet and outlet are located. From Fig. 2(b), the pig house accommodated 600e960 finishing pigs (85e110 kg) over thirteen weeks. A negative-pressure mechanical ventilation system was applied to accurately control the internal air temperature. The external air of the pig house was supplied in winters and during the changes from winter to spring and summet to autmn through 20 circular inlet ducts with diameters of 500 mm (0.5 m) and length of 13.5 m each. These ducts were installed at a height of 3.15 m from the ground surface, with one duct arranged in two rows at 0.3 m intervals. The perforated holes of the duct had a diameter of 0.05 m, and there were 100 holes in total. The exhaust fans on the sidewall were installed at intervals of 2.5 m. The pig house was made of 17 exhaust fans with Ø 500 mm (8500 m3 h1) and 6 exhaust fans with Ø 1000 mm (19,550 m3 h1) (Fig. 2(c)). Only seven exhaust fans with Ø 500 mm were operated in winter and during change of seasons from winter to spring and summer to autumn. The air flow rates of the exhaust fan with Ø 500 mm were determined as follows: the minimum ventilation (Qmin) was supplied when the air temperature inside the pig house was below the designed air temperature of the ventilation controller, and the maximum ventilation (Qmax) was supplied when the internal air temperatures were higher than the sum of the designed air temperature and maximum air temperature deviation (Max.DT). The air flow rate under the condition that the internal air temperature was higher than the designed air temperature and lower than the sum of the designed air temperature and Max.DT (designed internal air temperature < X < designed internal air tempe rature þ Max.DT) was set to be proportional to the air temperature deviation based on the minimum ventilation rate (Qmin þ (Qmax e Qmin)/DT). That is, the air flow rate of the variable exhaust fans was controlled by the current air temperature inside the pig house, the designed air temperature of the ventilation controller, and the air temperature deviation. The control logic used for controlling the air flow rate of the ventilation controller is shown in Fig. 3. The designed air temperature of the controller was 25e26
C, which was unusually high, and the Max.DT was set to 5
C. This was because the ventilation control of the experimental pig house was irregularly conducted using the personal rearing experience of a farm owner, with the aim of preventing cold and reducing the corresponding respiratory diseases caused by cold stress.

2.2.

Computational fluid dynamics (CFD)

CFD was used to understand the problems related to the internal rearing environment of the experimental pig house and to suggest ventilation methods to improve the rearing

262

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Fig. 1 e Research process for evaluating improvement methods for the environmental problems associated with the experimental pig house.

environment. CFD is a numerical analyses method for predicting phenomena such as fluid flow, heat and mass transfer, and chemical reactions. The governing fluid-flow NaviereStokes equations, which are a set of nonlinear partial differential equation, were formulated in a discretised form through the finite volume method (FVM). The spatial domain is then transformed into algebraic equations for a small volume mesh, and a numerical algorithm is applied to analyse the fluid flow phenomenon qualitatively and quantitatively. Numerical analyses for fluid and energy flow are based on mass, momentum, and energy conservation laws. The conservation equations (Eqs. (1)e(3)) for each of these physical variables are as follows (ANSYSInc, 2013). Mass conservation equation: vr þ V$ðr! y Þ ¼ Sm vt

(1)

Energy conservation equation:
X !   v ðrEÞ þ Vð! y ðrE þ PÞÞ ¼ V keff VT  hJiþ ! t eff ! y þ Sh vt

(2)

Momentum conservation equation: v ! ! ðr y Þ þ V$ðr! y! y Þ ¼ VP þ Vð! t Þ þ r! g þ F (3) vt where r is the density (kg m3), ! y is the velocity (m s1), Sm is a mass source term based on the chemical reaction (kg m2), E is the total energy (kg m2 s2 kg1), P is the static pressure (kg m1 s2), keff is the effective conductivity (kg m1 s3 K1), T is the temperature (K), t is the stress tensor (kg m1 s2), teff is ! the effective stress tensor (kg m1 s2), J i is the component of diffusion flux, Sh is the enthalpy rise based on the chemical

reaction or radiation (kg m1 s3), ! g is the gravitational ac! celeration (m s2), and F is the external force vector (kg m s2).

2.3.

Experimental procedures

2.3.1. Measurement of rearing environment of the experimental pig house To identify problems in the pig rearing environment in winter and during the change of seasons, and to design and validate the CFD model, the time-dependently changed air temperature, humidity, ammonia, and dust distribution at the height of the AOZ inside the pig house were measured during the following experimental periods (winter: December 1, 2016, to February 28, 2017, and two change of seasons: March 1, 2017, to May 31, 2017 (winter to spring), and September 1, 2017, to November 30, 2017 (summer to autmn)). The internal climatic conditions such as air temperature and humidity of the pig house were continually monitored using a temperature and humidity transmitter (HTX 75 series, Dutech Inc., Korea). The sensors were installed at a height of 1.5 m above the slurry pit, considering the size and behaviour habits of the finishing pigs and the ease of maintenance. The sensors were not installed uniformly throughout the experimental pig house because all internal equipment and fences were symmetrically located, and the intervals of inlets and outlets were uniform. It was considered that the direction of the air flow from the inlet to the outlet was constant, as shown in Fig. 4. Thus, the sensors were installed in three rows and in three columns over a section with a length of 10.0 m. Sensors P1 to P3 measured the air temperature and relative humidity around the corridor (inlet), sensors P7 to P9 measured the values around the centre, and sensors P10 to P12 measured the values around

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263

Fig. 2 e Geographical location and internal and external views of the experimental pig house (a) satellite photograph of the pig house, (b) internal view of the pig house, and (c) a full-scale pig house.

the exhaust fan (outlet). To measure the change of the air temperature and humidity into the exhaust direction, sensor P4 was installed between sensors P2 and P8. To measure the internal environment at each height inside the pig house, sensors P5 to P6 were also installed at heights of 3.0 m and 4.3 m, respectively. In addition, because the distribution of the air temperature and humidity inside the pig house varies with the external weather environment and the air flow rate of the

Fig. 3 e Control logic applied to the internal air temperature-based ventilation controller.

exhaust fan, the external air temperature and humidity were measured at sensor P13, and the changes of the air flow rate of the exhaust fan were monitored in real time using a manometer (TSI5815, TSI, Shoreview, MN, USA) and an air flow meter (a self-fabricated device). In addition, feed intake and water consumption were monitored, because the pigs showed a change in feed intake and water consumption according to the ambient rearing environment. To further investigate the environment, ammonia and dust concentrations inside the pig house were additionally measured. The measurement points were at the same locations as the temperature and humidity sensors. In order to analyse the local ammonia distribution, a MuitiRAE IR device (RAE Systems Inc., USA.), was used which can measure an ammonia concentration in real time. Dust concentrations were measured using an aerosol spectrometer (Model 1.109, Grimm Inc., Germany) which collect the dust particulates through an embedded pump with a flow rate of 1.2 l min1 and use optical light scattering method for the measurement of particulate size and mass in the chamber of the device. The suitability of the rearing environment inside the pig house was evaluated using the recommended levels of air temperature, humidity, feed intake, water consumption, ammonia, and dust (Table 1).

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Fig. 4 e Sensor placement for air temperature and humidity measurements inside the pig house: blue dotted lines indicate the area around the exhaust fans (outlet), red dotted lines indicate the centre part, and green dotted lines indicate the corridor (inlet).

2.3.2.

Design of the CFD simulation model

Three-dimensional CFD simulation models were designed to evaluate the effects of the various conditions on the air inlets and outlets of the pig house (Fig. 5). Objects and meshes for the numerical analysis were designed using the Design Modeller and ANSYS Meshing software tools (ANSYS Inc., Canonsburg, PA, USA). ANSYS Fluent CFD code (Version 18.1, ANSYS Inc. Canonsburg, PA, USA) was used to solve the numerical solutions. As shown in Fig. 2, the length of the experimental pig house was 60.0 m. As stated earlier, because all internal equipment and fences were symmetrically placed, and the intervals of air inlets and air outlets were uniform it was considered that the direction of the air flow from the inlet to the outlet was constant. Therefore, a oneesixth scale model of the experimental pig house was designed within the

calculation domain. Both sidewalls were set to a symmetric boundary condition in which there was no friction and no energy flux at the wall. In addition, although the holes of the circular inlet duct were very small compared to the size of the pig house, the mesh of the duct hole (Ø: 0.05 m) was designed to be able to contain at least six cells. One hundred and sixty simplified pig models (Fig. 6), as designed by Seo et al. (2012), were placed to model the turbulent dissipation caused by the distribution of the pigs inside the pig house. So, it was hard to perfectly match the y þ criteria because of the number of polygons and heads of the pig model. However, in order to take the Yþ criteria into account as much as possible, a tiny mesh in the CFD model was designed. The AOZ was fixed with a mesh size of 0.01 m to ensure the quality of the mesh in the AOZ because many pigs are found within this limited area.

Table 1 e Recommended environmental levels used to raise finishing pigs. Rearing environment Air temperature (
C) Relative humidity (%) Feed intake (kg day1) Water consumption (L day1) Gas (ppm) Ammonia (NH3) Respirable Dust (mg m3)

Recommended level

References

21 50e80 2.3e3.4 4.5e7.3 <25 <230

Rural Development Administration (RDA) (2016) Rural Development Administration (RDA) (2016) National Pork Board (NPB), 2003 Quiniou, Dubois, and Noblet (2000) Koerkamp (1998) Cigr, 1994

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Fig. 5 e Drawing of the three-dimensional mesh structure of the experimental finishing pig house.

The growth rate of the mesh in the pig house was set to 1.1 to prevent sudden increases in mesh size, with the exception of the AOZ and the duct. The simulation was performed using a pressure-based solver, which is widely applicable in flow analysis, and the gravitational acceleration was set to 9.81 m s1. An standard operating pressure of 101,325 Pa was applied. The wall characteristics of the pig house in the simulation were obtained from field-measured results, and since pig are homoeothermic animals, a surface temperature of the pigs of 38
C was used. Because the slurry pit consisted of concrete pits with very low porosity, moisture and heat were assumed to be generated from the slurry pits. To quantify the moisture production, the total moisture production in the pig house was calculated from the equilibrium equation (Eqs. (4) and (5)). Moisture production from the finishing pigs was calculated using Eqs. (6)e(9) (Aarnink, 2018; CIGR, 2002). The moisture production was classified as the amount generated by manure, urine, spilled water, and the amount of pig respiration (Stinn & Xin, 2014). However, it is difficult to know how much moisture is generated from each source. But, the amount of moisture generated by the pigs can be calculated, and the remaining three factors (manure, urine, and negative water) could be thought of as the difference in the amount of moisture produced in pigs from the total amount of moisture produced in the pig house. The amount of generation for the three factors was assumed to be generated from floor due to the low porosity of the concrete pit. This moisture production

was then applied to the CFD simulation model. The initial boundary condition for solving the numerical solution is listed in Table 2. The solution was assumed to have converged when the sum of residuals for all of the cells in the computational domain was less than 1  106 for continuity and energy, and 1  103 for other variables such as X, Y, Z-velocity, k, and ε. Total MP ¼ rQðWi  We Þ

(4)

W ¼ mass of water vapour=mass of dry air

(5)

MP of the finishing pig ¼ LHP=LHe

(6)

LHP ¼ THP  SHP

(7)

9 8 > = < 5:09  m0:75 þ ½1  ð0:47 þ 0:003  mÞ >   THP ¼ 0:001  0:75 0:75 > >  5:09  m ; :  n  5:09  m

(8)

 ½1000 þ 12  ð20  TÞ 8 9 > < 5:09  m0:75 þ ½1  ð0:47 þ 0:003  mÞ > =   SHP ¼ 0:001  0:75 0:75 > >  5:09  m  n  5:09  m : ;  7  0:62  ½1000 þ 12  ð20  TÞ  1:15  10  T6

where MP is the moisture production rate (kg s1), Wi and We are the humidity ratios of the internal and external air (kg (DA)1) respectively, Q is the ventilation rate at the exhaust fan (m3 s1), r is the air density (kg m3), LHP is the latent animal heat production (W), LHe is the latent heat of evaporation (J kg1 head1), SHP is the sensible heat production (W), m is body mass (kg), and n is the feed intake efficiency.

2.3.3.

Fig. 6 e Simplification process of a three-dimensional pig model (Seo et al., 2012).

(9)

Validation of the CFD simulation model

To obtain the accurate and reliable CFD-computed results, the efficiency of computation and a grid independence test, which is an optimisation process for mesh size, was conducted depending on the turbulence models. The CFD-computed

266

Input values 1.225 kg m3 Pressure inlet (11.4
C) Velocity inlet (2.18 m s1) 26.0
C, 28.4
C and 38.0
C Symmetry Coupled thermal condition 0.0284 (mass fraction) 0.0036 (mass fraction) Selected from grid independence test (0.2, 0.5, 0.7 and 1.0 m) Selected from validation test (Standard keε, RNG keε, Realisable keε, SST keu, Standard keu) 1  106

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results from four different mesh sizes inside the pig house (0.2, 0.5, 0.7, and 1.0 m), and five different turbulence models including keε models (Standard keε, RNG keε, Realisable keε model) and the keu model (Standard keu and SST keu model) were tested. A reasonable mesh size and turbulence model that computed predictions similar to the experimental results were selected. Validation between the field-measured and the CFD-computed results was conducted using R2 (coefficients of determination). The R2 value can be applied as a measure of the closeness between trends of the field-measured and CFDcomputed results. However, because there was a limit to the power of explanation for the error between the fieldmeasured and CFD-computed results, the index of agreement (IoA) was used for further analysis (Eq. (10)). The IoA value indicates agreement when the value is close to 1.0 but it is generally accepted that agreement is suitable at values > 0.5. 2 Pn
obs si i¼1 Y i Y i IoAðindex of agreementÞ¼1 P




2

si n obs obs Y obs

þ Yi

i¼1 Y i Y

Convergence criteria

2.3.4.

Maximum cell size Turbulence models

Moisture production inside pig house

Y obs

is the ith measured value, is the average of the where measured values, Y sii is the ith simulation value, and Y si is the average of the simulation values.

Floor, roofs and pigs Sidewall Duct Floor (water and manure) Pig Contents Density of air Inlets Outlet Surface temperature of walls Boundary conditions of CFD model

Table 2 e Boundary conditions for the computational fluid dynamics (CFD) simulation model.

(10) Y obs i

CFD-based studies for solving environmental problems

In order to find effective ventilation systems for the pig house, the CFD-based studies were conducted with designs including control of hole spacing on the inlet duct, installation of roofchimney exhaust fans and air buffer space. Table 3 described specific conditions for each case. When the external air temperature is very low, it is recommended that the external air entering the building is mixed with the relatively warm air located in the upper part of the pig house rather than being directly supplied to the AOZ. This is because relatively warm air is generally located in the upper part of the pig house due to buoyancy effects, and it can reduce the cold stress of the pigs. The higher the discharge velocity from the inlet ducts the greater the possibility of thermal exchange between incoming cold air from the outside of the building to the inside and the warm air located in the upper part of the pig house. By contrast, if the discharge velocity is low, cold air can drop directly into the AOZ causing unfavourable thermal conditions. Therefore, the distribution of the air temperature supplied to the pig house according to the hole spacing conditions of the inlet duct was analysed. Figure 7 showed the hole spacing conditions of the circular inlet duct. The ducts of the experimental pig house were installed at a height of 3.15 m from the pit surface. The perforated holes of the duct had a diameter of 0.05 m, and total 100 holes were arranged in two rows at 0.3 m intervals as shown in Fig. 7(a). The cases where the hole spacing on the surface of the inlet duct increased by 1.1 times in the direction of the inflow air (Fig. 7(b)) and decreased by 0.9 times in the direction of the inflow air (Fig. 7(c)) were evaluated while the exhaust fan was operated at 25 m3 min1. A roof-chimney exhaust fan can help exhaust heated and warm air out of the pig house and change the airflow, having a

Table 3 e CFD-designed cases to improve rearing environment inside the pig house. Cases

8

9

10

7e1 7e2 7e3 7e4 7e5 7e6 8e1 8e2 8e3 8e4 8e5 8e6 9e1 9e2 9e3 9e4 9e5 9e6 10e1 10e2 10e3 10e4 10e5 10e6

Cross-sectional area of air buffer space

Number of roof-chimney exhaust fan

Increase ratio of hole spacing on the duct

25 25 25 25 25 25 10 25 35 50 65 70 10 25 35 50 65 70 10 25 35 50 65 70 10 25 35 50 65 70

e e e e e e 0.4

e e e 1 EA (centre) 2 EA (both sides) 3 EA (all) e

Uniform 1.1 times 0.9 times Uniform Uniform Uniform Uniform

Experimental Experimental Experimental Experimental Experimental Experimental Experimental

0.8

e

Uniform

Experimental pig house with 0.8 m2 of cross-sectional area of air buffer space on the x-y plane

1.2

e

Uniform

Experimental pig house with 1.2 m2 of cross-sectional area of air buffer space on the x-y plane

2.0

e

Uniform

Experimental pig house with 2.0 m2 of cross-sectional area of air buffer space on the x-y plane

Description

pig house pig house pig house pig house pig house pig house pig house

with a duct increasing hole spacing in the direction of inflow air with a duct decreasing hole spacing in the direction of inflow air with 1 EA of roof-chimney fan with 2 EA of roof-chimney fan with 3 EA of roof-chimney fan with 0.4 m2 of cross-sectional area of air buffer space on the x-y plane

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1 2 3 4 5 6 7

Ventilation rate (m3 min1)

267

268

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Fig. 7 e Cross-sectional area of the inlet duct and hole spacing of inlet duct: (a) Case 1: a duct with a uniform hole spacing in the direction of the inflow air, (b) Case 2: a duct with increasing hole spacing in the direction of the inflow air, (c) Case 3: a duct with decreasing hole spacing in the direction of the inflow air, and (d).

direct effect on the pigs. Therefore, the effect of the chimneyroof exhaust system operating at 25 m3 min1 was also analysed. The roofechimney exhaust fans were uniformly installed along the longitudinal direction at a position 13.5 m from the wall that contained the winch curtain. The roofechimney exhaust duct was positioned at a height of 1.5 m above the slurry pit. Figure 8 showed installation conditions of the roof-chimney exhaust fans, depending on the number and location of the roof-chimney exhaust fans. Moreover, the air buffer space has a design advantage, in that it can increase the air temperature of the inflow air supplied to the pig house. In addition, energy loss caused by heat conduction from the inside of the pig house to the outside can be minimised. Therefore, analysis on the effect of the air buffer space according to its cross-sectional areas was also carried out as shown in Fig. 9.

2.3.5.

Field application of air buffer space

The most effective solution for improving the internal rearing environment, as determined through the CFD-computed

results, was applied to the experimental pig house (Fig. 10(a)), and then was validated. The air buffer space was installed below the eaves using a waterproof material made of polyethylene (PE) film (Fig. 10(b)). The inlet of the air buffer space was designed to have the same area as that of the inlet duct in the lower part of the air buffer space. The size of the inlets were 0.6 m by 0.15 m and they were installed at 1.5 intervals. The effect of the air temperature supplied to the circular inlet duct was analysed by comparing the air temperature inside the duct of the existing experimental pig house with the air temperature inside the duct of the improved pig house by the air buffer space. The total length of the circular duct (Lmax) was 13.5 m. Four thermocouples (Thermal couple T type, Omega Engineering Inc., Stanford, CT, USA) were installed at points (L) at 0.5 m (L/Lmax ¼ 0.04), 4.5 m (L/Lmax ¼ 0.33), 9.0 m (L/Lmax ¼ 0.67), and 13.0 m (L/ Lmax ¼ 0.96) to measure the temperature of the air supplied to the duct with and without the air buffer space. The data were collected using a multi-channel data logger (GL-820, Graphtec Inc., Jessup, MD, USA).

Fig. 8 e Installation conditions of roof-chimney exhaust fans to improve the rearing environment: Case 4: one roofechimney exhaust fan (centre), Case 5: two roofechimney exhaust fans (both sides without centre), and Case 6: three roofechimney exhaust fans (centre and both sides).

b i o s y s t e m s e n g i n e e r i n g 1 8 6 ( 2 0 1 9 ) 2 5 9 e2 7 8

269

Fig. 9 e Installation conditions of air buffer space to increase air temperature of inflow air: (a) Case 0: the cross-sectional area of the air buffer space on the xey plane is 0 m2, Case 7: 0.4 m2, Case 8: 0.8 m2, Case 9: 1.2 m2, and (b) Case 10: 2.0 m2.

Fig. 10 e Design of the air buffer space outside the pig house: (a) an experimental pig house and (b) an improved pig house with an air buffer space.

3.

Results and discussion

3.1. Analysis of field-measured internal rearing environment of the experimental pig house The average external air temperature of the pig house during the experiment period (winters and change of seasons) was 10.6
C. The lowest air temperature in the same period was 11.1
C, observed in mid-December (winter), and the daily temperature differences were 14.8 ± 3.4
C and 11.4 ± 2.9
C during change of seasons and winter, respectively. The difference was 3.4
C larger during change of seasons than in the winter. Regardless of the changes of the external air temperature, the internal air temperature of the pig house was 26.2
C on average, and it was kept close to the designed air temperature of the ventilation controller (25.0e26.0
C). When compared to the recommended air temperature of the AOZ (21
C), the thermal environment exceeded it by 5.2
C on average. The average internal relative humidity was 87.3%, which was 7.3% greater than the maximum recommended relative humidity of 80%. In addition, concentrations of ammonia and dust that were measured regularly at farms have an average value of 86.7 ppm and 191 ug m3, respectively. Although the allowable exposure levels for the ammonia is 25 ppm and respirable dust is 230 ug m3, the experimental pig house showed the highest concentrations of 113 ppm of ammonia and 272.8 ug m3 of respirable dust. The concentration exceeded the allowable exposure level for the worker and

pigs. This was because the operator of the experimental pig house empirically minimised the ventilation rate of the pig house by setting the designed air temperature of the exhaust fan to 26
C, which was unusually high, to reduce the cold stress of the pigs and to reduce thermal energy costs. The average internal air temperatures of the experimental pig house were 26.1 ± 1.2
C, 26.4 ± 1.3
C, and 26.0 ± 1.3
C for winter, spring, and autumn, respectively. The highest internal air temperatures of the experimental pig house were shown 29.4
C, 34.0
C, and 31.3
C at 2 P.M. regardless of seasons. In contrast, the lowest internal air temperatures of the experimental pig house in each season were 17.4
C (winter), 20.9
C (spring), and 21.9
C (autumn), due to the temporary opening of the winch curtain at the shipping date of the finishing pigs. In the case of relative humidity, the internal relative humidity of the experimental pig house was close to 100%, because of the minimal ventilation of the pig house and the moisture production from the pig, manure, and water. However, the average relative humidity during change of seasons was 81.6%, since a stable climatic condition (relatively high external air temperature) was formed during daytime (Table 4). When analysing the internal distribution of the environmental factors in the pig house by classifying them according to their locations (around corridor, centre, and exhaust fan), the average temperature values for each location during the winters and during the change of seasons were 25.9
C, 26.1
C, and 26.5
C, respectively. Additionally, the air temperature increased from the corridor towards the exhaust direction. However, the regional air temperature difference was

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Table 4 e Seasonal air temperature and relative humidity distribution inside and outside the experimental pig house during the experimental period. Locations

Internal air

External air

Period

Maximum Minimum Avg. ± S.D Maximum Minimum Average.±S.D

Winter

Change of the seasons

DecembereFebruary

MarcheMay

29.4 17.4 26.1 ± 1.2 15.2 11.1 2.3 ± 4.8

100 71.2 99.5 ± 1.5 100.0 54.7 61.4 ± 19.7

insignificant among the regions around the centre and around the exhaust fans (Table 5). Moreover, there was no regional distribution difference of air temperature according to the season. When pigs are exposed to unfavourable environmental conditions this may lead to an increase in the possibility of a metabolic imbalance occurring and there may be incidences of viral infection. Lopez, Jesse, Becker, and Ellersieck (1991) investigated the relationship between the air temperature and the feed intake in a finishing pig house. The feed intake of the finishing pigs that experienced a daily change in air temperature from 20 to 35
C was approximately 11% lower than that for pigs exposed to a constant air temperature of 20
C. In the case of finishing pigs that were exposed to a high air temperature, they found that the body mass gain of the finishing pigs was 17.6 g d1 less, and the pigs consumed 43.5 g d1 less feed. The experimental finishing pigs consumed 8.3 l d1 of water, whereas the feed intake was 2.4 kg d1. Considering the recommended water requirement (4.5e7.3 l) and feed intake (2.5e3.0 kg) per day (NPB, 2003; Quiniou et al., 2000), the water consumption was 40% higher because of the inadequate air temperature and humidity conditions, whereas the feed intake was 17% lower because of weight loss and increased water consumption. In summary, the designed air temperature of the ventilation controller showed that the internal air temperature was well-controlled with regard to the reference value. However, because the thermal environment was subjectively controlled through the experience of the farmer, the ventilation rate of the experimental pig house was, at maximum, 46% lower than the required ventilation rate. In addition, the ventilation control system did not meet the recommended humidity, ammonia, and dust conditions, because the ventilation controller was operated while considering only the air temperature. From these results, it was found that the feed intake

34.0 20.9 26.4 ± 1.3 32.7 0.1 16.2 ± 6.6

SeptembereNovember

100.0 36.3 79.7 ± 10.7 99.7 52.7 65.3 ± 24.8

31.3 21.9 26.0 ± 1.3 30.6 2.6 13.6 ± 7.2

100.0 41.3 83.4 ± 7.5 100 56.9 78.2 ± 22.1

of pigs was less than the recommended value, and that the water consumption of pigs was more than the recommended value. Therefore, it should be noted that the air temperature was maintained at the designed air temperature level of the ventilation controller, but the other environmental variables (humidity, gas, dust) exceeded allowable exposure levels. It was therefore considered that a new solution was needed to enable the farmer to stably increase the ventilation rate.

3.2.

Validation of the CFD simulation model

In order to confirm the accuracy and reliability of the CFDcomputed results and to consider computational efficiency, the CFD simulation model was validated by comparing with field-measured results at the same locations where the sensors were installed. First, a grid independence test was conducted, considering different mesh sizes (0.2, 0.5, 0.7, and 1.0 m). The R2 between the field-measured and CFD-computed results for each mesh size are shown in Fig. 11 (i.e., 0.2 (Fig. 11(a)), 0.5 (Fig. 11(b)), 0.7 (Fig. 11(c)), and 1.0 m (Fig. 11(d))). When the mesh size was 1.0 m, the R2 values of the CFDcomputed air temperature and humidity values were 0.83 and 0.87, whereas the R2 values of the CFD-computed air temperature and humidity values for the mesh sizes of 0.2 m were 0.93 and 0.95, respectively. As the mesh size decreased, the accuracy of the CFD-computed results gradually increased. For the mesh size of 0.2 m, the IoA values between the field-measured results and CFD-computed results for air temperature and humidity were 0.86 and 0.84, respectively. In the case of a turbulence model, a Realisable keε turbulence model showed the highest R2 values among the keε models for the air temperature and humidity values (0.93 and 0.95, respectively). Additionally, the IoA values for the air temperature and humidity were 0.84 and 0.86, respectively (Table 6). When applying the Standard keε turbulence model (the most

Table 5 e Seasonal and regional air temperature and relative humidity distribution of the experimental pig house. Period

Location Around corridor

DecembereFebruary MarcheMay SeptembereNovember

Around centre

Around exhaust fan

External air

Temp.

Relative humidity (R.H)

Temp.

R.H

Temp.

R.H

Temp.

R.H

25.9
C 26.1
C 25.6
C

99.9% 81.9% 84.9%

26.1
C 26.5
C 25.8
C

99.5% 79.5% 83.2%

26.2
C 26.7
C 26.6
C

98.8% 78.5% 81.5%

2.3
C 16.2
C 13.6
C

61.4% 65.3% 78.2%

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Fig. 11 e Correlation analysis between the field-measured and CFD-computed result with a Realisable keε model by validation of grid independence: (a) mesh size 0.2 m, (b) mesh size 0.5 m, (c) mesh size 0.7 m, and (d) mesh size 1.0 m.

commonly-used method in numerical analysis), the R2 values for air temperature and humidity were 0.85 and 0.80, respectively. The IoA values were 0.72 and 0.62 for the air temperature and humidity values, respectively. It was considered that the Realisable keε turbulence model could show more accurate results because it was commonly suitable for analysing rotation flow, separation, recirculation, and large pressure

gradients by solving different forms of turbulent dissipation and viscosity equations, unlike the Standard keε turbulence model (Hong et al., 2017; Shih, Liou, Shabbir, Yang, & Zhu, 1995). For these reasons, a mesh size of 0.2 m within the computational domain and the Realisable keε turbulence model were chosen for simulating the internal rearing environment of the experimental pig house.

Table 6 e Analysis of statistical validity through the comparison of the field-measured and CFD-computed results. Measurement points P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 R2 d

Absolute humidity (g kg (DA)1)

Air temperature (
C)

Field-measured results CFD-computed results Error Field-measured results CFD-computed results Error 26.8 26.7 26.8 27.1 27.2 27.4 27.1 27.3 27.2 27.7 27.9 27.8 0.93 0.86

26.3 26.3 26.2 26.4 26.8 27.2 27.0 26.8 26.7 27.9 27.6 27.5

0.5 0.4 0.4 0.7 0.4 0.2 0.1 0.5 0.5 0.2 0.3 0.3

16.4 16.2 16.3 16.3 16.4 16.8 16.0 16.8 15.9 18.1 18.1 17.4 0.95 0.84

16.5 16.3 16.3 16.5 16.3 16.6 16.4 16.5 16.3 17.6 17.1 16.9

0.1 0.1 0 0.2 0.1 0.2 0.4 0.3 0.4 0.5 1.0 0.5

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The air temperature and humidity values at the 12 points inside the simulated pig house, using the selected mesh size and turbulence model, are shown in Table 6. The average air temperature at sensors P1 to P3 (around the corridor) was 26.8
C. The field-measured results showed a þ0.5
C error as compared with the CFD-computed results, and this error was relatively high compared to other points, because of the technical limitations in realising air infiltration from the winch curtain in the CFD simulation model. At sensors P4 to P9 (around the centre), the average air temperature was 27.2
C at a distance of 4.5 m from the sidewall. The CFDcomputed air temperature value was 26.8
C on average, and the air temperature increased, owing to the buoyancy effect of air as the height increased. In addition, as the amount of saturated water vapour increased with increasing air temperature, the absolute humidity also increased. The average air temperatures at sensors P7 to P9 and sensors P10 to P12 corresponding to the locations at 9.5 m and 13.5 m from the sidewalls, were 27.2
C and 27.8
C, respectively. The average humidity values were 16.4 g kg1 for sensor P7 to P9 and 17.2 g kg1 for sensor P10 to P12. As the distance increased from the sidewall where the winch curtain was installed, the air temperature and the absolute humidity also increased. The error of the absolute humidity at sensors P7 to P12 was relatively high compared to other points. It was assumed that there were technical limitations in fully realising the water phase change and moisture accumulation in the CFD simulation model.

3.3. Analysis of improvement methods of rearing environment 3.3.1.

Effect of the hole spacing for the inlet duct

To maintain a uniform and appropriate rearing environment inside the pig house, various CFD simulations were conducted, by varying the hole spacing of the inlet duct and the locations and number of the roofechimney exhaust fans. Table 7 listed the average and standard deviation of the air temperature at the height of the AOZ (1.0 m), and the average and deviation of the air temperature inside the duct according to the hole spacing of the duct when the ventilation rate is 25 m3 min1. For the existing experimental pig house with the uniformly-perforated duct (Case 1), it was confirmed that the average internal air temperature at the height of the AOZ was 26.9
C, and the air temperature deviation was 0.56
C. The air temperature deviation was smaller than in Case 2, because the external air entering from the holes having the uniform spacing evenly influenced the internal environment of the pig house. Because the insulation performance of the winch curtain was lower than those of the other walls, a relatively

low air temperature was found around the corridor, and the air temperature increased into the exhaust direction, as shown in Fig. 12(a). In addition, the average air temperature and standard deviation inside the duct were 15.9
C and 2.0
C, respectively. When the hole spacing on the duct was gradually increased in the direction of the exhaust (Case 2), most of the cold air with high density entering from the duct was supplied to the corridor side. This was because the air flow rate of the exhaust fan was too low to form a jet stream (the ventilation rate was only 54% of the required ventilation rate). As a result, the average air temperature inside the duct decreased by 0.7
C due to the shorter residence time of the external air in the duct as compared with the air temperature inside the duct of Case 1. However, the average air temperature at the height of the AOZ increased by 0.4
C because the air moved into the direction of the exhaust fan and was warmed, the thermal environment around the exhaust fan was relatively worse (Fig. 12(b)). When the duct hole spacing was decreased, the average air temperature inside the duct was 16.8
C, and the standard deviation of the air temperature was 1.1
C. The air temperature of 0.5
C increased at the height of the AOZ, and the standard deviation of the air temperature distribution was the lowest. This was because the air temperature difference between the inside the pig house and inside the duct was smaller. The supplied air temperature through the duct in Case 3 was increased by 1.6
C as compared with the air temperature supplied through the duct of Case 2, and the standard deviation of the air inside the duct decreased by 2.3
C. However, as shown in Fig. 12(c), a reverse air flow occurred, because air flow was intensively supplied to the vicinity of the exhaust fan in spite of the low ventilation rate. The thermal environment around the winch curtain, where the air temperature was low because of the low performance of the heat insulation, was improved by the increase in the residence time of the air owing to the reverse flow. From these results, it was concluded that the adjustment of the duct hole spacing could improve the thermal conditions of the pig house. However, the relative humidity was maintained at 79.3% in Case 1, 88.2% in Case 2, and 90.7% in Case 3. All of the simulation cases were unsuitable for field application because they exceeded the recommended humidity range.

3.3.2.

Effect of roofechimney exhaust fan

The cases were analysed according to the numbers and positions of the roofechimney exhaust fans (Table 8). When the internal air was exhausted by means of the sidewall exhaust fan (Case 1), the average air temperature at a height of the AOZ was 26.8
C. The air temperature around the exhaust fan (9.5e17.2 m) was 27.2
C, because the pig house had an

Table 7 e Average air temperature and humidity according to the duct hole spacing.


Average air temperature at pig height ( C) Relative humidity (%) Standard deviation for air temperature at pig height (
C) Supply air temperature through duct (
C) Standard deviation of supply air temperature through duct (
C)

Case 1

Case 2

Case 3

26.9 79.3 0.56 15.9 2.0

27.3 88.2 0.87 15.2 3.4

27.4 90.7 0.55 16.8 1.1

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Fig. 12 e Air velocity and temperature distribution at a height of 1.0 m from the slurry pit (XeZ sectional view, top view) inside the pig house according to the duct hole spacing: (a) Case 1, a duct with uniform hole spacing in the exhaust direction, (b) Case 2, a duct with increasing hole spacing in the exhaust direction, and (c) Case 3, a duct with decreasing hole spacing in the exhaust direction.

asymmetrical structure, and because the air was warmed when it moved in the direction of the exhaust. Installation of the roofechimney exhaust fans showed an average air temperature of 27.3
C at the height of the AOZ, regardless of the

installation location of the roof-chimney exhaust fans. The CFD-computed average air temperatures were 0.5
C higher than those when using only the sidewall exhaust fan. There was no large improvement in the thermal environment when

Table 8 e Spatial air temperature distribution according to the roofechimney exhaust conditions.

Case Case Case Case

1 4 5 6

Average air temperature at a height of the animal-occupied zone (AOZ) (
C)

Average air temperature (section 0 e 9.5 m,
C)

Average air temperature (section 9.5 e 17.2 m,
C)

26.8 27.3 27.3 27.3

26.5 27.0 27.0 27.0

27.2 27.6 27.7 27.5

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using the roof-chimney exhaust fan as compared to the use of sidewall exhaust fans. This was because the roof-chimney exhaust fans and the sidewall exhaust fans were both installed at the height of 1.5 m. Thus, the improvements that can be achieved on the distribution of the thermal environment inside the pig house by changing the number and location of the roofechimney exhaust fans was found to be insignificant. It was concluded that more studies were needed to estimate an appropriate length of the duct for the roofechimney exhaust.

3.3.3.

Effects of the air buffer space

To prevent a direct inflow of cold external air into the pig house, and to increase the temperature of the air supplied, the air buffer space was designed below the eaves of the experimental pig house according to the cross-sectional areas of the air buffer space. Table 9 listed the CFD-computed results according to the cross-sectional areas of the air buffer space and the air flow rate of the exhaust fan. The air flow rate of the exhaust fan can be increased while maintaining the existing thermal environment, by increasing the cross-sectional area of the air buffer space. If the air buffer space was not installed, an exhaust rate of 50 CMM should be set, to satisfy the recommended level for rearing pigs (21
C). However, the practical exhaust rate of the experimental pig house was operated at 25 m3 min1, due to maintaining the designed air temperature (25
C) of the ventilation controller set by the farm owner. This mean that the ventilation rate was set to be abnormally lower (46% lower than the required ventilation rate) in the experimental pig house to reduce the cold stress of the pigs. On the other hand, when an air buffer space with a cross-sectional area of 2 m2 was installed (Case 10), an air temperature of 25
C could be maintained, even if the exhaust rate was set to 65 m3 min1. As compared with the average air temperature at the height of the AOZ of the existing experimental pig house, the pig house with the air buffer space (Cases 7e10) indicated that the average air temperature at the height of the AOZ was increased by 2.0e5.3
C, according to the air flow rates of the exhaust fan and crosssectional areas of the air buffer space. This was because the convective heat exchange was increased, and the conductive heat loss from the winch curtain was reduced. The temperature of the air supplied into the inlet duct increased, because the cold external air was not directly supplied to the inlet ducts. Especially, in Cases 7 to 9, there was no difference in air temperature at the height of the AOZ according to the crosssectional areas (Fig. 9(a)), even though the ventilation rate was increased. As shown in Fig. 9(b), when the winch curtain

Table 9 e Average air temperature at the height of AOZ for ventilation rate and the air buffer space. CMM 10 25 35 50 65 70

Case 1

Case 7

Case 8

Case 9

Case 10

26.7 24.2 22.5 20.8 19.7 18.6

28.7 27.0 25.4 24.2 23.2 22.7

28.7 27.1 25.5 24.4 23.4 22.9

28.7 27.2 25.6 24.4 23.6 23.1

30.0 28.4 27.0 25.9 25.0 23.9

was covered with the air buffer space to reduce the conductive heat loss from the winch curtain (Case 10; cross-sectional area is 2.0 m2), the air temperature at the height of the AOZ increased by 1.5
C.In the winters and during the change of seasons, the ventilation rate was generally low, indicating that the increased rate of the air temperature inside the duct was high because the velocity of air movement was slow (Fig. 13(a)). When the exhaust fans were operated at 25 m3 min1, the average air temperatures inside the duct for each cross-sectional area were 15.7
C (Case 7), 16.7
C (Case 8), 17.4
C (Case 9), and 18.9
C (Case 10). As compared to average air temperature inside the duct for Case 1 (14.4
C), the air temperature improvements were 9.3%, 16.1%, 21.2%, and 28.8%, respectively. Case 10 had an air temperature of 17.1
C at the beginning of the duct, which was 5.7
C higher than the air temperature at the beginning of the inlet duct of Case 1. When the internal air was exhausted at 35 m3 min1, the air temperature inside the duct increased by 1.5
C on average (Case 7) as compared to the air temperature inside the duct of Case 1 (13.1
C). The average air temperature supplied to the duct increased by 3.7
C in Case 10. This was because the air buffer space prevented heat loss from conduction and increased the air temperature of the external inflow air in the air buffer space through convection mixing (Fig. 13(b)). When the internal air was exhausted at 70 m3 min1 (Fig. 13(d)), the average air temperature inside the duct was 13.6e15.6
C depending on the cross-sectional areas of the air buffer space. As the exhaust rate increased, the air velocity inside the duct increases, so the air temperature difference inside the duct for each case did not show much difference (Fig. 13). In conclusion, when installing an air buffer space, the ventilation rate can be set to a value that is twice that of the ventilation rate of the existing pig house, while maintaining the desired rearing environment. Case 10 was considered to be a good solution to improve the existing ventilation rate of the experimental pig house.

3.4.

Field application and validation of air buffer space

From the CFD-computed results, it can be concluded that the installation of air buffer space below the eaves of the pig house showed the most positive effects on the improvement of the rearing environment. Therefore, the air buffer space was installed at the experimental pig house and this effect was validated by comparing the air temperatures inside the inlet duct. In the morning of the experiment day (Dec 10, 2017), the variability of the air temperature inside the duct was larger than in the afternoon. This was because the difference between the internal air temperature and the external air temperature was larger. Also, in accordance to thermodynamic characteristics, when the ventilation was low the warm air inside the pig house moved into the duct where the relatively cool air is formed. If there was no air buffer space at Duct Point-1 (DP-1), the internal air temperature at the beginning of the duct was 7.6
C. The daily average air temperatures for each point inside the inlet duct were 11.5
C at 4.5 m (L/ Lmax ¼ 0.33, DP-2), 13.8
C at 9.0 m (L/Lmax ¼ 0.67, DP-3), and 16.3
C at 13.0 m (L/Lmax ¼ 0.96, DP-4), respectively. This was because the air temperature was increased by the heat

275

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Fig. 13 e Distribution of air temperature inside the duct according to the volumes of the air buffer space and flow rate for the exhaust for the air flow rates of (a) 25 m3 min¡1, (b) 35 m3 min¡1, (c) 50 m3 min¡1, and (d) 70 m3 min¡1 (L: measurement point, Lmax: total length of the duct). exchange with the air along the longitudinal direction of the duct. The air temperature fluctuations inside the duct were large at the entrance to the duct where the air was supplied, and it decreased towards the exhaust. However, when the air buffer space was installed, the average air temperature at the beginning of the duct was 11.5
C (DP-1-A). The air temperatures of 12.4
C (L/Lmax ¼ 0.33, DP-2-A), 15.3
C (L/Lmax ¼ 0.67, DP-3-A), and 17.1
C (L/Lmax ¼ 0.96, DP-4-A) at each point in the longitudinal direction of the duct (4.5 m, 9.0 m and 13.0 m, respectively) are listed in Table 10. This showed where an air buffer space was installed, the difference at the end of the duct was 5.6
C higher than that at the beginning of the duct. Compared to the existing experimental pig house without the air buffer space, the average internal air temperature of the duct was 4.0
C higher at the beginning of the duct when the air buffer space was installed. When air was supplied directly to the duct without an air buffer space, the increase in the rate of the air temperature in the longitudinal direction of the duct was relatively high. Air supplied to the ducts caused more heat exchange when passing through the air buffer space

(Fig. 14). However, because the air was supplied at a relatively high temperature when the air buffer space was installed, there was a lower increase in the rate of the internal air temperature in the longitudinal direction of the duct (Table 10). Table 11 showed the CFD-computed and field-measured results for air temperature in the duct with and without the air buffer space in the pig house. When air buffer space was not installed, the air temperature changes at each point in the longitudinal direction of the duct were 11.4
C, 13.4
C, 15.7
C, and 18.6
C. This was because heat exchange via convection and conduction between the air and the inlet duct increased along the direction of movement inside the circular duct. The air temperature distribution at the same points in the duct of the experimental pig house were 11.2
C, 12.9
C, 15.3
C, and 17.9
C. The coefficients of determination between the fieldmeasured and the CFD-computed results for the internal air temperature of the inlet duct were found to be as high as 0.99. The IoA value was also high, and the mean error rate between the points was 2.9%. Additionally, when the air buffer space

Table 10 e Average air temperature and standard deviation by location inside the circular inlet duct with and without the air buffer space. Condition Without air buffer space

DPe1

DPe3

DPe4

External air temperature

7.6 2.4

11.5 1.6

13.8 1.4

16.3 1.2

4.2 3.1

DPe1eA

DPe2eA

DPe3eA

DPe4eA

External air temperature

Average air temperature (
C) Standard deviation (
C)

11.5 3.2

12.4 3.2

15.3 2.4

17.1 2.1

4.2 3.1

Condition With air buffer space

DPe2

Average air temperature (
C) Standard deviation (
C)

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Fig. 14 e Monitoring the internal air temperature of the duct according to the conditions of the air buffer space: without air buffer space (DPe1) and with air buffer space (DPe1eA).

Table 11 e Comparison of air temperature distribution at each point inside the inlet duct with and without the air buffer space. Condition Without air buffer space

DPe1


CFD-computed results ( C) Field-measured result (
C) R2 d Root mean square error (RMSE)

Condition With air buffer space




CFD-computed results ( C) Field-measured result (
C) R2 d RMSE

was installed, the CFD-computed results of the air temperature inside the duct for each point were 14.3
C, 14.7
C, 16.5
C, and 18.4
C. The field-measured average air temperature values inside the duct were 13.8
C, 15.1
C, 17.1
C, and 19.0
C. The mean error rate between the field-measured and CFDcomputed results was only 3.1%. It can be concluded that the accuracy of the CFD simulation models was reasonable, and the use of the air buffer space can increase the air flow rate of the exhaust fan while maintaining the designed air temperature. From these results, the internal air quality (ammonia and dust concentration, and humidity) was expected to be improved when the air buffer space was installed compared to the existing experimental pig house.

4.

Conclusions

The problems associated with internal environmental factors of a finishing pig house were investigated through field experiments. Various solutions for solving the problems identified during the winter and during change of seasons and their efficiency were evaluated using a CFD technique.

DPe2

DPe3

DPe4

11.4 11.2 0.99 0.99 0.55

13.4 12.9

15.7 15.3

18.6 17.9

DPe1eA

DPe2eA

DPe3eA

DPe4eA

14.3 13.8 0.97 0.99 0.54

14.7 15.1

16.5 17.1

18.4 19.0

Considering the designed air temperature of the ventilation controller of the experimental pig house, the internal air temperature was well-controlled with regard to the reference value. However, the ventilation rate of the experimental pig house was at a maximum 46% lower than the required ventilation rate for the purpose of reducing the cold stress of the pigs and lessening the thermal energy cost. Therefore, a new solution was needed to enable the operator to stably increase the ventilation rate while maintaining the existing thermal environment. In order to find effective ventilation systems for the pig house, CFD-based studies were conducted regarding the location of the inlet duct, the installation of the roof-chimney exhaust fans, and the design of an air buffer space. There was no large improvements in the thermal environment in using a roof-chimney exhaust fan compared with the existing sidewall exhaust fans. However, when the cross-sectional area of the air buffer space was increased, the air flow rate of the exhaust fan could be increased while maintaining the existing thermal environment. The installation of an air buffer space with a cross-sectional area of 2.0 m2 was considered as a good solution to improve the existing ventilation rate of the

b i o s y s t e m s e n g i n e e r i n g 1 8 6 ( 2 0 1 9 ) 2 5 9 e2 7 8

experimental pig house. When installing an air buffer space, the ventilation rate can be set to a value that is twice the ventilation rate of the existing experimental pig house, while maintaining the operator's desired rearing environment. When the air buffer space was installed at the experimental pig house, this effect was validated by comparing air temperatures inside the inlet duct. It can be concluded that the accuracy of the CFD simulation models was reasonable. In this study, an efficient way was found to improve the environment at low cost. However, the phase changes and the infiltration of the air inside the pig house could not be realised owing to the technical limitations of CFD. For accurate environmental analysis, it is planned to consider phase changes and infiltration of the air. In addition, although the effect of the roof chimney exhaust fan was shown to be insignificant, it was considered that additional analyses was needed to determine the air flow characteristics and the internal distribution of climate in the pig house according to the length of the roof exhaust duct.

Acknowledgments This work was supported by Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry and Fisheries, through Advanced Production Technology Development Program, funded by Ministry of Agriculture, Food and Rural Affairs (316078e03).

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