Sampling chamber
CJ Ackley | CFD | Project brief

STATUS
Transport brief; probe geometry to define. Detailed design and analysis pending.

THE PROBLEM
An inline sensor sits in a small chamber. When the incoming liquid changes, regions with little flow can retain the previous sample and delay the measured response. This study examines the chamber around a defined probe envelope.

DESIGN QUESTION
Which chamber replaces the old sample with the least delay and acceptable pressure loss?

SCOPE
- Design a fully flooded, rotationally symmetric chamber with a fixed probe envelope.
- Solve slow internal flow and track a tracer representing the previous sample.
- Compare chamber volume, pressure drop, and time to 99% tracer replacement.

CONSTRAINTS
Arrangement: Fully flooded, axisymmetric chamber
Probe: Fixed sensor envelope
Comparison metric: Time and liquid volume to replace 99% of the simulated old sample
Model scope: Flow and tracer transport with specified fluid properties

ENGINEERING DECISIONS (PROPOSED)
Tracer concentration and sample replacement
Track both total old-sample mass and tracer concentration around the probe. Define the 99% replacement measure before comparing chamber geometries.

ALTERNATIVE
Using the outlet concentration alone is easier, but a fast bypass path can mask a slowly clearing region around the sensor.

TRADEOFF
A smaller chamber may clear faster while disturbing probe access or increasing pressure loss. Volume alone is not an adequate ranking metric.

PROPOSED TOOLS AND METHODS
Flow and tracer CFD, Mass balance, Flush-test plan

VERIFICATION SCENARIO (NOT YET RUN)
The outlet clears before the probe region
Apply a step change in inlet tracer and compare outlet, probe-region, and whole-chamber response curves for each geometry.

PASS CONDITION
Report the time to 99% replacement for the declared metric and any lag near the probe. Account for tracer leaving and remaining in the chamber.

EVIDENCE TO RETAIN
Mass-conserving transport results, spatial and time-step studies, response curves, and a physical flush-test sampling plan.

MEASUREMENT AND LIMITS
Separate numerical smearing from physical mixing by refining space and time and documenting the transport scheme. The axisymmetric model excludes asymmetric fittings and trapped gas.

COMPLETION CRITERIA (PLANNED)
- The replacement metric and initial conditions are explicit.
- The response curves are stable under the declared refinement checks.
- The proposed experiment measures the same response used to rank designs.

VERIFICATION PLAN
Transport balance
Account for tracer entering, leaving, and remaining in the chamber.

Spatial and time refinement
Check that the response curve is stable as the mesh and time step are refined.

Physical comparison
Define a flushing experiment that measures the same sample-replacement response as the model.

PLANNED DELIVERABLES
- Chamber CAD and section drawings
- Tracer animations, response curves, and pressure-drop comparisons
- Reproducible model and a physical flushing-test plan

SCOPE NOTE
The study will evaluate fluid replacement. Sensor chemistry, adsorption, and bubbles are outside this defined model.

NEXT STEP
Fix the probe geometry, fluid properties, flow range, and tracer behavior.
