Fluid_Aspect_Course_5_5
Pumps & fans typically make flow in a ducting arrangement or loop, which we call the “system”. Once the system’s conductances have been tuned, you know it will always flow the correct amount at a given delta-pressure. During that tuning effort, you had replaced the GunnsGasFan or GunnsLiquidCentrifugalPump links with the simpler GunnsFluidPotential to make tuning to that flow rate vs. delta-pressure design point easier. Now it’s time to change them back to the real pump/fan links and tune them.
The GunnsGasFan and GunnsLiquidCentrifugalPump links extend GunnsFluidPotential, which extends GunnsFluidConductor. Thus these pump/fans are also conductors, and have the same mMaxConductivity term that governs the pressure drop due to flow through the fan itself. This makes sense: if the fan isn’t spinning, it must still resist flow through it somewhat.
When the fan is spinning it’s creating a pressure rise. Some of this is lost due to the internal conductance, so the total pressure rise across the link is the impeller’s pressure source minus the loss through the conductor.
You should already have tuned this conductance in the previous step of tuning the entire flow system. When you replace the GunnsFluidPotential with the new pump/fan link, simply keep this tuned conductance value.
In GUNNS, these pump/fan links create a delta-pressure rise, from which flow results through the system. The delta-pressure rise created by the fan is matched by an equal pressure drop across the system (which also includes the fan link).
Pump/fans force a relationship between the volume flow through them, Q (m3/s), and the pressure rise they create, P (kPa). This relationship is modeled by a curve fit. This is called the “reference performance curve” or “P-Q” curve of the fan. It is often given in vendor documentation for the pump/fan. The fan performance is also a function of fluid density, ρ (kg/m3), and how fast the fan is spinning, N (rev/min). The vendor curve defines the P-Q curve at a given reference density and speed.
The link uses a polynomial curve fit. So the first thing you need to do is fit a polynomial to the vendor-supplied curve data. Make sure you convert the vendor data to units of (kPa) and (m3/s) before you fit. Spreadsheet applications like MS Excel or Open Office Calc can help. The model supports up to 5th-order polynomial.
There is pump/fan theory called Affinity Laws that the pump/fan model uses in run-time to scale the reference P-Q curve to the current N and ρ. This models the performance of the fan as a function of speed and density. This shows how the curve scales with speed and density:
- P scales with rho and N 2
- Q scales with N and is independent of rho
The important thing to remember is that just as the pressure and flow through a conductor must follow the flow model curve (Bernoulli’s, etc.), the pressure and flow through the fan must lie on the adjusted P-Q curve. Since the same flow and pressure is going through both the fan and the system, then the pressure and flow must be at the intersection of the fan and system curves:
In run-time, the model works by predicting where this intersection will be and manipulating its [A] and {w} so that the network solution will approach this point.
Let’s talk about the min & max flow limit conditions:
- Dead-head:
- Occurs at Q = 0.
- Happens when the system is completely blocked.
- System has zero conductance, the system curve is a vertical line at Q = 0, and the intersection with the fan curve happens at Q = 0.
- The resulting pressure is called the dead-head pressure: it’s the standing pressure difference sitting across the fan when the flow can’t go anywhere.
- Max flow:
- Occurs at P = 0.
- Happens when the system has infinite conductance.
- System curve is a horizontal line at P = 0, and the intersection with the fan curve happens at P = 0.
- This is the maximum flow that can go through the fan.
The model has a stability filter that controls how fast it attempts to approach the predicted intersection point of the P-Q and system curves. We recommend starting with a value of 0.5. This is good for most applications. You may run into stability issues when you have multiple fans interacting with each other, and lowering this gain can help. See the link help page for GunnsGasFan for more info.
Not covered here:
- Axial/Mixed/Radial flow types & curve shapes
- Stall
- Efficiency, torque & power
- Integrating with motor