The Number of Increments and Parallel Pipe Discretisation Features
What does the number of increments and number in parallel inputs mean on a Pipe element?
This article describes the functionality of the discretisation inputs that can be specified for Pipe elements in Flownex®, such as “Number of increments” and “Number in parallel”.
Discretisation: Number of Increments
The pipe incrementation feature gives the user control over simulation detail as the “Number of increments” input discretises the pipe into smaller sections. Each smaller section will have a length equal to the total pipe length divided by the number of specified increments. The following figure illustrates how to apply pipe incrementation:
The default input is 1 increment, implying that the conservation equations are performed over the entire length of the pipe. This value should be selected as an integer greater than or equal to 1. The following figure illustrates the functionality of the feature.
The figure above illustrates three separate flow networks, each modelling the same process of heating two-phase water flowing through a pipe. The top left flow network performs the simulation with a “Number of increments” discretisation input value specified as 1 increment whereas the bottom left flow network as 2 increments. Comparing the generated results, pipe incrementation has a significant impact on pressure drop, with the pressure drop results changing from 6.4002 [kPa] to 6.2075 [kPa] when using a “Number of increments” discretisation input value specified as 2 as opposed to 1.
The bottom right flow network models the pipe as two Pipe elements, with a single increment to illustrate what the flow network on the bottom left achieves with incrementation. The pressure drop in each Pipe element in the bottom right flow network summates to the total pressure drop obtained in the bottom left flow network.
Increasing the number of increments is essential to ensure grid independence and improves the accuracy of the results, especially when modelling two-phase flow or water hammer simulations.
Discretisation: Number in Parallel
The “Number in parallel” input allows the user to model a single pipe section and indicate how many pipes should be modelled in parallel. This saves the user time, canvas space and simulation time. The following figure illustrates how to apply the parallel pipes feature.
The default input for the number in parallel pipes is 1, implying only a single pipe is modelled to convey the fluid. This value should be specified as an integer greater than or equal to 1. The following figure illustrates the functionality of the feature.
The figure above illustrates three separate flow networks, modelling the process of conveying the same amount of water flowing through a single pipe or parallel circuits. The top left flow network models the water flowing through a single pipe, whereas the two bottom flow networks through two parallel pipes.
The “Number in parallel” discretisation input value is set to 1 for the top left flow network’s Pipe element and the bottom left flow network to 2. Comparing the results, the pressure drop in the bottom left flow network is less than the top flow network since the water flowing through the pipes will now be distributed evenly through the number of pipe elements modelled in parallel, as the available cross-sectional flow area has doubled.
The flow network on the bottom right contains two identical Pipe components connected in parallel, with both having a “Number in parallel” discretisation input value set to 1. The flow network in the bottom left and bottom right model identical processes.