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air flowing through the elbow, a centrifugal force is created and increases the pressure
at the outside radius of the elbow thus increases the bias flow and makes the discharge
coefficient greater [45, 63]. On the other hand, when pressure setting becomes greater,
the discharge coefficient decreases to 0.97. This is because when the pressure increases,
the flow rate and flow velocity through the holes also increases. When the velocity is
low, the Vena Contracta [45] may be close to the narrowest point of the holes near the
shrinkage at the edge of the inner surface. However, when velocity increases, the higher
velocity may make the Vena Contracta phenomenon gradually more significant (Figure
5.8). Overall the constant discharge coefficient 0.985 matches well with the
experimental result over the CPAP working range.
Figure 5.8 High velocity vena contracta phenomena at bias vent holes
5.2.2 Fluid dynamic model under breathing added fluctuating flow
The fluid dynamic outputs should include pressure fluctuation in the mask and the
dynamic fluctuating airflow velocity in HADT. All these outputs should be tested over
the CPAP pressure setting range. However, due to instruments availability, the
validations have not been fully conducted. The setups for future validation are shown in
Appendix XXI. The already conducted experiments are described and compared with
the model outputs below.
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