Tilted Nozzle Trailing Edge for Tunnel Jetfan Coanda Effect
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Solution Overview
Problem
Longitudinal tunnel ventilation systems using jetfans are inefficient due to the Coanda effect, which causes significant energy wastage as high-velocity air adheres to tunnel surfaces, and previous solutions like convergent nozzles and circular bellmouths either increase pressure losses or restrict shape, making it difficult to combine reduced Coanda effect with low inlet flow losses.
Innovation Solution
The nozzle trailing edge is tilted, creating a longer 'pressure side' to push airflow away from tunnel surfaces, reducing pressure drop and power consumption, while allowing standard bellmouths to be used, and enabling effective turning of discharged air without choking the flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If convergent nozzles are fitted to the inlet side of a reversible jetfan, then the Coanda effect is reduced and airflow is turned away from tunnel surfaces, but inlet pressure losses increase and power consumption rises
Solution Approach 1:
The nozzle trailing edge is tilted at an angle of 10-20 degrees relative to the perpendicular of the nozzle axis, creating an asymmetric geometry where the pressure side is longer than the suction side. This asymmetry generates a lateral force that effectively turns airflow away from tunnel surfaces and reduces the Coanda effect, while the optimized tilt angle prevents excessive pressure losses that would occur with more aggressive asymmetry.
Solution Approach 2:
The nozzle geometry parameters are optimized by controlling the tilt angle of the trailing edge within a specific range (10-20 degrees). This parameter optimization balances the competing requirements of reducing Coanda effect and minimizing pressure losses, achieving energy efficiency without excessive power consumption.
2Stress or pressure
If circular bellmouths are fitted to reduce inlet pressure losses, then flow smoothness is improved, but the shape constraint prevents effective convergent nozzle design
Solution Approach 1:
The inlet and nozzle components are separated into distinct segments. The circular bellmouth remains as a standard component for reducing inlet pressure losses, while the nozzle is designed as a separate component with an optimized tilted trailing edge. This segmentation allows each component to be optimized independently for its specific function without compromising the other.
3Force
If the nozzle trailing edge is tilted to turn airflow away from surfaces, then aerodynamic thrust is enhanced, but pressure drop through the nozzle increases
Solution Approach 1:
The tilted trailing edge creates asymmetric pressure distribution between the pressure side and suction side of the nozzle. This asymmetry generates a lateral component of force that enhances aerodynamic thrust by directing airflow away from tunnel surfaces, while the controlled tilt angle prevents excessive pressure drop that would result from more aggressive asymmetry.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design reduces power consumption and pressure losses, enhances aerodynamic thrust, and allows for standard bellmouth usage, improving energy efficiency and acoustic silencing in tunnel ventilation systems.
Implementation Method 1
The pressure side of the nozzle is termed thus because when the nozzle is placed on the discharge side of the jetfan, the pressure side 'pushes' the airflow away from the tunnel surrounding surfaces when the jetfan is in use. The pressure side would thus experience a static pressure that is greater than that on the opposite suction side.
Implementation Method 2
A major reason for the inefficiency of jetfans is the Coanda effect. This causes the stream of high-velocity air issuing from a jetfan to adhere to adjacent solid surfaces including the tunnel walls and soffit.
Implementation Method 3
The flow through a jetfans is driven by an axial fan, which gives an impulse to the tunnel airflow.
Data Source
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AI summary
A ventilation device that enhances the effective longitudinal thrust of a fan assembly installed within a tunnel, by turning the discharged flow away from the surrounding tunnel surfaces and tilting the nozzle trailing edge (6) so that it forms an angle (16) to the nozzle centreline (8).