Jet Pump (Ejector) Simulator

The core principle is the conservation of momentum between the high-speed motive jet and the mixed flow in the diffuser. The momentum from the fast motive fluid is transferred to the combined stream.

Here, $\rho$ is fluid density, $Q_m$ is motive flow rate, $V_m$ is motive fluid velocity, $Q_d$ is entrained (secondary) flow rate, and $V_{mix}$ is the velocity of the combined stream after mixing. This equation governs how much secondary fluid can be dragged along.

From this momentum balance, we derive the key performance metrics that define the pump's operation and efficiency.

$P_m$ is motive pressure, $P_s$ is suction pressure, and $P_b$ is discharge (back) pressure. $C_r$ shows the pressure boost capability, while $\eta$ combines both flow and pressure performance into a single figure of merit.

Marine Bilge Pumping: Jet pumps are perfect for moving water containing debris or sand in ship bilges, as they have no impellers to clog or wear out. They use seawater from a primary pump as the motive fluid to entrain and discharge the dirty bilge water.

Steam Jet Vacuum Systems: In chemical plants and power stations, high-pressure steam is used as the motive fluid to entrain and compress gases, creating a vacuum for processes like distillation or dehydration. Multiple ejectors can be staged for very high vacuum levels.

Aquarium & Pond Filtration: Small jet pumps use water from a main return pump to entrain and draw water through a sand or bead filter, providing efficient filtration without needing a separate, sealed suction pump that could get clogged.

Fuel Systems in Aircraft: In some aircraft, fuel from a boost pump is used as the motive fluid to entrain and transfer fuel from remote wing tanks to the main collector tank, ensuring a reliable fuel supply to the engines with minimal moving parts.

When you start using this simulator, there are a few points that are easy to misunderstand. First, you might tend to think "if I just increase the motive fluid pressure, everything will be solved," but that's a pitfall. While increasing the pressure does temporarily raise the entrainment ratio, this assumes the mixed flow outlet pressure (discharge pressure) remains unchanged. In a real system, back pressure increases due to factors like downstream piping resistance, and efficiency quickly plateaus. For example, even if you raise the motive fluid pressure to 5 MPa, if the outlet pressure is pinned at 4 MPa, you'll see almost no increase in flow; it might even cause surging (unstable vibration).

Next, are you unconsciously assuming all fluids are water (with the same properties)? This tool assumes the same density and viscosity. But in practice, you might have steam as the motive fluid and water as the secondary fluid, or oil and air. When properties differ, the momentum exchange efficiency during mixing changes drastically. It's a common story: a design that seems to "work well" in the simulator might not entrain at all in a real test.

Finally, don't confuse the "Entrainment Ratio" with "Efficiency η". A high entrainment ratio can sometimes come with extremely low efficiency. For instance, if the entrained flow is high but the energy consumption of the motive fluid required to push it is enormous, that's "inefficient." When adjusting parameters in the tool, get into the habit of always looking at these two metrics together. The point where efficiency plummets can be considered the practical limit for that system.