Design Optimization for Dairy Barns Using Computational Fluid Dynamics

摘要

Computational Fluid Dynamics (CFD) models are computer algorithms which approximate fluid dynamics equations to predict the behavior of a fluid within a specified boundary. These models have become a popular tool to aid in the refinement and optimization of design, and to give a visual representation of fluid behavior. The main equations that describe fluid dynamics, the Navier-Stokes Equations, were formulated in the 19th century. To this date, the Navier-Stokes equations have only been solved for very particular and simplified cases. However, with the evolution of computers, approximations to these equations can be obtained by means of using numerical methods. In basic fluid mechanics, the Bernoulli Equation is modified to include a headloss factor which accounts for pressure losses due to turbulence effects. Darcy-Weibasch or Hazen Williams equations can be used to determine headloss factors and use it to predict pressure loss between two points. This approach is often sufficient for most practical ventilation engineering problems when the actual turbulence effects and mechanism do not have to be fully modeled. For many other engineering problems, this approach is insufficient, and a more detailed modelling and understanding of turbulence is required. Detailed turbulence models started being developed in the 20th century. One of these models is the "k-epsilon " turbulence model. The model is based on the Reynolds Averaged Navier-Stokes (RANS) equations which are a statistically averaged modification of the original Navier-Stokes. Barns for dairy cows require proper air movement to maintain animal comfort to produce a better-quality product. In literature, the design air velocity range is typically accepted as 5-7 mph at cow level. A challenge that any mechanical designer faces when engineering the ventilation system is to confirm thorough air distribution within the space, and as such, CFD modelling is a useful tool in solving this challenge. Traditionally, fans are used to supply air, but it is ofien impractical in dairy bams. Moreover, the designer must consider reduced ventilation flows in the Winter and high flows in the Summer due to cost prohibitive refrigeration equipment. This paper covers the actual modeling to design a real ventilation system for a 3,900 m~2 (42,000 ft~2) dairy farm in cold Canadian weather. CFD is analyzed to compare the following design alternatives including increasing size of the ventilation supply and exhaust fans, using exhaust or supply fans only vs both supply and exhaust fans; and provision of baffles only, transfer fans only or a combination of both.