AFT Blog
With the release of AFT Arrow 6 last month, this month I am going to depart from my usual monthly high level focus and instead get more detailed technically on compressible pipe flow. I am going to revive a topic from 1996 where we built a "Triple Choke" steam flow model in AFT Arrow 1.0. This model is discussed in our AFT Arrow training seminars, so those of you have attended one of these will have seen it. It also has been part of the AFT Arrow Help system documentation since at least 1999.
When I started AFT back in 1994 I had the idea to develop a product for compressible flow in pipe systems. After AFT Fathom 1.0 was released in April 1994 I began research on numerical methods for compressible flow in pipe networks. Books and papers that claimed to offer methods for compressible flow inevitably focused on single pipe applications with adiabatic or isothermal flow - and often with ideal gases. I wanted to develop a real gas software that could model heat transfer and simulate pipe networks.
When dealing with a compressible gas system, heat transfer and thermal effects are very important to account for. When a gas is flowing down a pipeline, it will cool down as the gas expands due to the frictional pressure drop. Many would say that adiabatic or isothermal conditions will bracket the potential flow rates that are possible for a constant pressure drop in a pipe. However, this is not the case. If a gas is cooled or heated as it flows down the pipeline, the flow rates that can result for a given pressure drop can actually be outside the “bounds”...
AFT Arrow solves all of the fundamental controlling equations governing gas dynamics. For more information about which fundamental thermodynamic equations are solved in AFT Arrow, here is an excellent article that discusses gas flow calculations in detail. The "stack effect" is simply one aspect of these equations, and is accounted for by the user input and boundary conditions. Ultimately, flow is driven by pressure difference. The higher you raise a chimney, the lower the atmospheric density and pressure at the discharge. In order to accurately model the "stack effect", the system boundary conditions must be modeled accurately. As the discharge...
Several generations of engineers ago engineers realized that performing calculations on gas flow is a lot more difficult than for liquids. As a result, the bulk of what is gas flow engineering history has revolved around how to treat gases as liquids. Another large portion of history is how to simplify gas flow equations down to simple isothermal relationships. In other words, the majority of gas flow engineering historically is about how to take short cuts with gas flow calculations.
A frequently asked question at our training seminars - usually during a break or over lunch - is how we came up with the name "Arrow". More completely, the product is named AFT Arrow™ and it is considered by many as the world's leading compressible pipe flow modeling software product.