Note: Descriptions are shown in the official language in which they were submitted.
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ENERGY-EFFICIENT TUNNEL VENTILATION DEVICE
BACKGROUND OF THE INVENTION
[0001] Longitudinal ventilation via jetfans is generally acknowledged as being
a
cost-effective solution for tunnels, where the length and risk profile of the
tunnel
allows such an installation. However, jetfans are not particularly energy
efficient,
with typical installations wasting over half the supplied electrical power.
[0002] 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. A significant proportion
of the
aerodynamic thrust, typically 20% to 30%, is thereby wasted through the
friction
between the jet and the surrounding tunnel surfaces.
[0003] A previous patent GB2465261 granted to the present Applicant describes
convergent nozzles that can be installed on one or both sides of jetfans, in
order to
accelerate the tunnel air and turn it away from the tunnel surfaces. In
practice, this
invention has been implemented by fitting conical nozzles onto jetfans.
[0004] The fitting of convergent nozzles onto jetfans does however come with
an
energy performance penalty where such nozzles are fitted to the inlet side of
a
reversible jetfan. The reason for this is that the power absorbed due to the
inlet-side
pressure drop cannot be recovered. This is contrary to the exit side where the
kinetic
energy of the discharged air serves to accelerate the tunnel air.
[0005] In order to reduce the inlet pressure losses to jetfans, circular
bellmouths
are typically fitted to the inlet side, in order to ensure a smooth flow. For
reversible
flow jetfans, such bellmouths are typically fitted to both sides of the
jetfan. Due to
manufacturing reasons, belhnouths are generally spun from sheet metal into a
circular shape. The circularity of the bellmouths introduces a significant
constraint
on the shape of a jetfan nozzle. In particular, it has not previously been
possible to
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combine the advantages relating to a reduction of the Coanda effect through
the
fitting of convergent nozzles with low inlet flow losses into a jetfan.
[0006] JP-A-H1-237400 discloses a jetfan with an undercut on the lower side of
the cylindrical casing, to encourage the discharged air to turn away from the
tunnel
soffit.
[0007] JP01130099A discloses an arrangement with multiple fans connected in
parallel, delivering flow to a common plenum which in turn supplies air to a
nozzle
fitted with turning vanes. This complex arrangement is not suitable for most
tunnels,
which are ventilated using individual jetfans.
[0008] Neither JP-A-H1-237400 nor JP01130099A discloses a system that is
practical or efficient. The Applicant believes that there remains scope to
improve
the energy efficiency of longitudinal tunnel ventilation systems.
SUMMARY OF THE INVENTION
[0009] According to one aspect of the invention, there is provided a fan
assembly
for installation in a tunnel to provide ventilation in the tunnel, the fan
assembly
comprising:
a fan rotor for generating a ventilating flow; and
the inflow into the fan rotor being substantially parallel to the outflow from
the fan rotor;
a nozzle coupled to the fan, the nozzle having a trailing edge at the distal
end
from the fan; and
wherein the assembly is arranged or arrangeable such that a ventilating flow
generated by the fan will pass through the nozzle before exiting the assembly
to
enter a tunnel to be ventilated; and
the nozzle being arranged to turn the flow away from the surrounding tunnel
surfaces, in that at least one edge of the nozzle throughbore is at an angle
to the fan
centreline; and
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wherein the angle made between the nozzle trailing edge and the centreline
of the nozzle is within the range of 45 degrees to 85 degrees.
[0010] The flow through a jetfans is driven by an axial fan, which gives an
impulse to the tunnel airflow. The invention provides a solution to the
technical
issue of how to turn the flow from a jetfan away from the surrounding tunnel
surfaces and hence achieve greater in-tunnel aerodynamic thrust, without
choking
the flow through the jetfan through increased pressure losses.
[0011] According to a further aspect of the invention, there is provided a fan
assembly for tunnel ventilation, the assembly comprising:
a fan for generating a ventilating flow in a first direction; and
a nozzle adjacent to the fan in the first direction so that the ventilating
flow
will pass through the nozzle before exiting into a tunnel to be ventilated;
wherein the nozzle has a first end proximal to the fan and a second end distal
from the fan having a trailing edge, the angle between the trailing edge and
the nozzle centre line is substantially within the range of 45 to 85 degrees
and the nozzle is arranged to direct the ventilation away from surrounding
tunnel surfaces.
[0012] This aspect of the invention is achieved by tilting the trailing edge
of the
nozzle, so that one side of the nozzle (the 'pressure side') is longer than
the opposite
side (the 'suction side'). 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.
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[0013] In case a convergent nozzle is used as described in patent GB2465261,
tilting the trailing edge of the nozzle has the effect of increasing the
aerodynamic
throat of the nozzle, and hence reducing the pressure drop through the nozzle
throughbore. The power consumption of the jetfan is thus significantly
reduced.
[0014] The range of angles between the trailing edge and the nozzle centre
line
has been selected on the basis of experimental evidence with the design,
manufacture and testing of such jetfans. For a typical overall nozzle length
to fan
diameter ratio of unity and a circular trailing edge of the same diameter as
the fan,
the lower value of 45 degrees for the angle between the trailing edge and the
nozzle
centre line corresponds to a throughbore to fan area ratio of approximately
1.4,
which would significantly choke most jetfan impellers. The higher value of 85
degrees for the angle between the trailing edge and the nozzle centre line
corresponds to the minimum change from a conventional jetfan nozzle
arrangement
that our experience indicates would be commercially beneficial to produce.
[0015] In practice, manufacturers stock a standard range of bellmouths. The
present invention permits the selection of a standard bellmouth size which can
be
installed at a tilt to the nozzle centre-line. In particular, a bellmouth with
the same
nominal diameter as the fan on which the nozzle is to be installed can be
used. This
option to use standard jetfan parts is a key advantage of the present
invention.
[0016] The nozzle can typically be used for acoustic silencing, as well as for
turning the discharged flow away from the tunnel surrounding surfaces. From
previous laboratory measurements, it has been established that the performance
of
the silencer is dependent upon the solid angle subtended by the silencer
surface onto
the fan outlet. Through judicious choice of nozzle geometry, adequate acoustic
silencing can be achieved, given the occlusion of the fan outlet by the nozzle
'pressure side'.
[0017] The arrangement of the circular fan outlet connected to a tilted
bellmouth
typically leads to a non-conical shape for the nozzle, and a complex developed
shape
for the nozzles skins is required for sheet metal cutting. The shape of the
proposed
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nozzle is thus different from the shapes envisaged in GB2465261 and JP-A-H1-
237400. In the case of the latter reference, since the nozzle trailing edge is
shaped
as an ellipse, it is not feasible to attach bellmouths on the nozzle trailing
edges,
which in turn implies significant pressure losses. In addition, the nozzle is
straight
and hence there is no effective turning of the discharged air. That prior art
design
therefore does not provide a practical or efficient solution for tunnel
ventilation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] A number of preferred embodiments of the present invention will now be
described by way of example only, and with reference to the accompanying
drawings, in which:
[0019] Like reference numerals are used for like components throughout the
figures;
[0020] Fig.1 shows an embodiment of a ventilation apparatus with nozzles as
described in this invention installed on both sides of a fan;
[0021] Fig. 2 shows an end view of a ventilation apparatus with a nozzle as
described in this invention;
[0022] Fig. 3 shows an embodiment of a ventilation apparatus with a nozzle as
described in this invention installed on one side of a fan.
[0023] Fig. 4 shows a typical flat developed pattern for a nozzle skin which
is to
be cut from sheet metal.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0024] Referring to Figure 1, this shows a side view of an embodiment of the
present invention within a bidirectional ventilation apparatus, which is
designed to
operate in a fully reversible manner.
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[0025] In this embodiment, a fan assembly comprising a fan rotor (3) driven by
a
motor (4) is installed within a fan housing (15). Airflow (5) enters the fan
rotor (3)
through a bellmouth (1) and an inlet nozzle throughbore (10A), before being
discharged thorough an outlet nozzle throughbore (10B).
[0026] As can be seen in Figure 1, the nozzle has a centreline (8), defined as
the
geometric mean between the pressure side (11) and suction side (12) lines. An
angle
(13) is defined between the fan centreline (7) and the nozzle centreline (8).
The
pressure side of the nozzle (11) is arranged to turn the flow direction, so
that in use,
the discharged air flows away from the surrounding tunnel surfaces.
10027] A further angle (16) is defined between the nozzle centreline (8) and a
trailing edge (6) of the nozzle. Preferably, the angle (16) is between 45
degrees and
85 degrees. Preferably still, the angle (16) is approximately 65 degrees.
[0028] The embodiment of Figure 1 shows a nozzle pressure side angle (17) of 7
degrees. A larger geometric throat (14) can be arranged at both the inlet and
discharge sides of the nozzle, by tilting the nozzle trailing edge (6) by the
angle ( I6)
between the nozzle centreline (8) and the trailing edge (6). This leads to
reduced
pressure losses and improved energy efficiency.
[0029] It is possible to arrange the length of the suction side to be
approximately
equal to one fan diameter, and selecting the pressure side angle (17) to be 6
degrees.
This preferred embodiment provides an enhanced level of acoustic attenuation
compared to the embodiment described in Figure 1.
[0030] Figure 1 shows a preferred embodiment where the suction side of the
nozzle throughbore (12) is arranged to be parallel to the fan centreline (7).
[0031] Referring now to Figure 2, which shows an end view of an embodiment of
this invention, the nozzle shape is arranged to turn to flow in a prescribed
direction,
preferably away from the surrounding tunnel surfaces.
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[0032] Figure 2 shows that the nozzle trailing edge is circular in shape, to
allow
attachment to a circular bellmouth. Such a circular bellmouth significantly
reduces
the inlet pressure drop.
[0033] We refer now to Figure 3, which shows a side view of a particular
embodiment of this invention which would normally (but not exclusively) be
operated in a unidirectional manner.
[0034] In this embodiment, the indicated airflow direction is from left to
right, i.e.
the airflow enters into a straight nozzle via a bellmouth (1) first, prior to
being
accelerated by the fan rotor (3) into a shaped nozzle with a throughbore (10).
The
discharged flow is turned by a pressure side (11) which is longer than the
suction
side (12), such that in use, the discharged air flows away from the
surrounding
tunnel surfaces. Since a straight inlet nozzle is selected in this embodiment,
the inlet
pressure drop to the fan is less than that for the embodiment depicted in
Figure 1.
The aerodynamic thrust can therefore be expected to be higher for the
embodiment
described in Figure 3 compared to that in Figure 1.
[0035] In Figure 3, the flow direction can if necessary be reversed by running
the
fan rotor in the opposite direction. Due to the increased Coanda effect and
additional
inlet pressure drop, a reduction of the in-tunnel aerodynamic thrust can be
expected
in the reverse flow direction (i.e. from right to left) in the embodiment
described in
Fig. 3.
[0036] It would be possible to modify an existing fan assembly in order to fit
nozzles as described in this invention to one or more sides of a fan, and
hence reap
the benefits of improved performance.
[0037] There are no restrictions on the degree of divergence or convergence of
the
throughbore area with this invention. In particular, the throughbore areas at
the inlet
and discharge can be arranged to be equal to, or greater than, the fan area.
Depending on the fan flow characteristics, this flexibility can increase the
efficiency
of the fan assembly. The present invention relieves the 'choking' of the inlet
flow
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which can be present in GB2465261, and thus delivers a significant improvement
in
fan perfonnance.
[0038] Fig. 4 shows the flat developed pattern for a nozzle skin which is to
be cut
from sheet metal, for the jetfan depicted in Figs. 1 and 2. The present
invention
requires a single direction of curvature for the nozzle skins, and the nozzle
skins can
therefore be developed from a flat sheet without the need for stretching. The
topology of the nozzle skins in this invention is therefore particularly
suitable for
sheet-metal manufacture.
[0039] The manufacturability and cost-efficiency of the nozzles in this
invention
have been proven through production trials. It has been found that the nozzle
skin
can be rolled from a single flat sheet of metal for small fan diameters
(around
500mm), while separate sections of nozzle skin, each rolled from a flat sheet,
are
required for larger fan diameters of up to 2m. Both the inner and outer nozzle
skins
can be rolled into the requisite shapes, with acoustic material inserted
between them
for sound attenuation during fan operation.
[0040] It will be appreciated that the foregoing are merely an examples of
embodiments and just some examples of their use. The skilled reader will
readily
understand that modifications can be made thereto without departing from the
true
scope of the inventions.
