Ejector (v 1) Performance Test

Date: conducted 2/6/09

Purpose: To determine the performance of the initial ejector implementation.

The testing aparatus: right: high P inlet manifold - air flow rotameter, propane flow rotameter, and pressure gauge. Center: ejector, swirl burner, and low P inlet air valve (1.5"). Left: Manometer on 2" inlet pipe (inlet vacuum)

Results:

When open to the atmosphere, the ejector produced a total outflow volume of ~4.2 times the inflowing air/propane mix (originally computed as ~5 when not accounting for propane in flow). This was determined finding the dilution of the outgoing gas (dividing the slope of the open best fit by the closed best fit). Closed best fit is the stociometric mix (4.68%) (no air coming in from the low pressure inlet), open best fits includes the incoming air from the low pressure inlet, showing the amount of dilution occuring.

This version 1 "design" was thrown together with available parts and not optimized. Modeling is being done to find better optimized dimensions (which will have dependancies on pump/compressor flow rate and pressure for optimization).

Data:

Ejector Test.xls

Analysis, Modelling, Validation:

(sorry, work in progress... I'll remove this "disclaimer when I finish here!  - DanielC)

In this section an attempt is made to understand the parameters that this ejector is operating under, apply these to the theoretical and engineering models that are available and produce some predicted performance, and then compare these theoretical predictions to the actual measured conditions (validation of the model).  If we can get the model agreeing with the measurements, we can then use the model to explore improvements, and provide some guidance for the next round of metal cutting.

Sometime a spreadsheet doesn't provide quite enough power and flexibility for what you would like to model; I've recently discovered "octave", and have been using it for my models and calculations that don't easily and naturally work in a spreadsheet.  It is a free (GNU) program www.gnu.org/software/octave/download.html, and has been quite useful to me so far.

An ejector consists of several parts:

1. the motive fluid jet
2. the pumped,or "suction" inlet fluid port
3. a mixing section (usually straight-walled)
4. an optional diffuser (not used on this ejector)

In this ejector, the motive fluid jet issues from a .172" orifice, which is the i.d. of a piece of standard 1/4" tubing.  Bear has told me that there isn't any special treatment on either end, it's just cut off squarely and cleanly, so I'll SWAG nozzle coefficients of 0.90 to account for this (with a rounded inlet, and an outside-chamfered outlet, we could perhaps use 0.95).  What these nozzle coefficients account for is the contraction in flow that a nonindeal entry or exit might induce.

The first thing we'll figure out is the flow rate through the jet; we are interested in both the mass flow rate, as well as the jet velocity.  Bear indicated that 9.5 psi of pressure across the tube caused 100 litres per minute of airflow. This is enough pressure such that this can't be considered "incompressible flow", so we'll have to use slightly-more-complicated equations than if we could make the simplifying assumption of incompressible flow (pressure difference less than 4psi, or flow speeds less than 150 m/s, or pressure ratios less than 1.3).