UWE Bristol Engineering showcase 2015

Toby Renton
Mechanical Engineering
Project Supervisor
Dr Aruna Palipana
CFD and Experimental Analysis of Air Jets and Air Sheets and Design
Enhancements to Reduce Turbulent Separation
Introduction
Linear nozzles are widely used in various areas in the modern engineering
world; they are extensively useful in industry primarily in deposition devises,
e.g. printers, film application, dusters. This technology has recently been
applied to the personal hygiene sector, applications including highly effective
hand driers. In these devices the flows are described as air blades or air
sheets, suggesting that the flow is a continuous thin filament of air. Over
short distances the flow is expected to remain compact, though
experimental data provides proof that over large distances the jet expands .
This investigation aims to use theoretical and CFD modelling techniques to
design an optimized nozzle structure which will reduce jet expansion over
distance.
Procedure
The design procedure was broken up into two distinct stages:
Phase 1) all aspects of the nozzle geometry will be studied to model their effects
upon the flow emitted, these will continuously be compared back to a base case
to provide evidence of their positive or negative flow construction
characteristics. The aspects investigated in Phase 1 include; the external shape,
internal structure shape, internal structure size and internal structure location.
Once this is completed a basic nozzle design will be acquired.
Phase 2) the nozzle design derived from Phase 1 will be taken as the Phase 2
base case minor tweaks to the general geometry will be made to model the
impact and those tweaks which are effective will be implemented in the final
design.
Practical Analysis
Two experimental procedures were performed to understand the
air sheet structure emitted from a real linear nozzle .
- Pitot tube analysis gave defined for the flow centreline velocity
decay with relation to the axial distance away from the nozzle
outlet.
- Tuft analysis is a flow visualisation technique which means it
does not provide real flow values, it is a method of visualising
the flow direction across the jet profile.
The results of these two experiments could then be used in
conjunction to produce an experimental jet profile plot which
would be comparable to CFD vector plots.
Project summary
A computational fluid dynamics investigation was
undertaken to study the effects of nozzle design upon
the turbulent separation of nozzle emitted air jets
and to assess whether there were better potential
designs to increase the focus. Computerised Fluid
Dynamics (CFD) simulation models results, which
were deemed more reliable than the experimental
results, demonstrated that based upon exit
velocities some distance from the nozzle the flow
shape could be focussed by the insertion of a
hemispherical shape into the nozzle itself with the
best focusing performance generated from the use of
a tear-drop shape. Whilst more work is required to
properly validate the conclusions the results suggest
a more effective nozzle design may be achievable
which could have industrial significance.
Project Conclusion
The study concluded that:
1) CFD simulation predicted flow characteristics
reasonably well, but that there were divergences
between experimental and simulated CFD results.
There were shortcomings in both methods and
overall more faith was put in the simulated CFD
results.
2) From the simulated CFD results the insertion of a
tear drop shape in the nozzle throat had the effect
of focussing the flow, increasing velocity and
reducing air sheet separation. An optimal nozzle
design for reduced turbulent separation is
proposed.
3) More work is required to better match the
experimental results to CFD simulated results and
only when there is a convergence between the
methodologies can confidence be placed on the
conclusion that the proposed optimal nozzle design
will have better flow focusing characteristics.
4) If subsequent work proves the assertion that flow
can be focussed under the proposed nozzle design
then this could have significant benefits in the
industrial sectors which utilise nozzles that benefit
from accurate focussed emission jets.