Rupture-disk-less shock-tube with compression tube driven by free piston, CHEMIA I PIROTECHNIKA, Chemia i ...

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Shock Waves (1997) 7: 205{209
Rupture-disk-less shock-tube with compression tube
driven by free piston
T. Abe, E. Ogura, S. Sato, K. Funabiki
Institute of Space and Astronautical Science,Yoshinodai 3-1-1, Sagamihara, Kanagawa 229, Japan
Received 6 June 1996 / Accepted 6 October 1996
Abstract.
A new technique is proposed for a shock tube driven
by a freely moving piston. In a conventional free-piston-driven
shock tube, a rupture disk is employed between the compres-
sion tube and the shock generation tube. In the present method,
however, the conventional rupture disk is replaced by a newly
developed fast action valve which is activated by the com-
pressed gas generated in the compression tube. The present
method enables us to generate highMach number shock waves
of arbitrary strength with good reproducibility. The perfor-
mance of the newmethod is demonstrated experimentally. This
also enables us to be carefree to scattering of fragments of the
rupture disk.
complex factors, it is rather difcult to manufacture the rup-
ture disk with the same specic performance. Hence, in the
conventional Stalker tube, it is rather difcult to make a shock
wave or a hypersonic flow condition with good reproducibil-
ity. Because of the same reason, it is also difcult to design in
advance the rupture disk with a specic performance. Hence
the proper rupture disk must be selected on trial-and-error ba-
sis. Furthermore we must point out another drawback of the
rupture disk; the burst of the rupture disk causes a scattering
of fragments. These fragments may cause damages to the fa-
cility or undesirable effects on the experiment. In the present
paper, we propose a new method in which a conventional rup-
ture disk is replaced by a newly-developed fast action valve.
The performance of the fast action valve is demonstrated by
a shock tube equipped with such a valve. Because of the fea-
ture of the fast-action valve, the present method enables us to
produce shock waves with arbitrary strength in good repro-
ducibility. This also enables us to be carefree to scattering of
the fragments of the rupture disk.
Key words:
Rupture-disk-shock tube, Free piston, Stalker
tube, Fast action valve
1 Introduction
Recently an interest for hypersonic flow research is revital-
ized aiming to develop new type recoverable space vehicles
which conduct hypersonic flight through the atmosphere. For
the development of such vehicle, ground facilities to realize hy-
personic flow conditions are inevitable. To realize hypersonic
flow conditions on ground, the shock tunnel or shock tube is
useful even though only a short running time can be provided
by such facilities. In the shock tunnel or the shock tube, a high
pressure gas source is inevitable to generate a shock wave.
In the free-piston-driven shock tunnel known as Stalker tube,
the high pressure gas source is generated by compressing a
gas in the compression tube by means of the piston moving
freely through the tube (Stalker 1961, 1965). The compres-
sion tube is connected to the shock generation tube, in which
the shock is generated by using a rupture disk between them
which breaks at a proper pressure level in the compression
tube. The strength of the shock wave thus generated depends
on the pressure level at which the rupture disk breaks. Hence
the reproducibility of the shock wave depends on the repro-
ducibility of the burst phenomenon of the rupture disk. Since
the burst phenomenon of the rupture disk is dominated by
2 Experimental set-up and a fast action valve
The schematical view of the experimental set-up is shown in
Fig. 1. The facility is composed of 4 main parts; 1) a high-
pressure vessel to drive the free piston, 2) a compression tube
through which the free piston runs and compresses the gas, 3)
a shock generation tube, and 4) a fast action valve between the
compression tube and the shock generation tube.
The high-pressure vessel to drive the free piston has a vol-
ume of 0.0867 m
3
and is equipped with a fast action valve
actuated by an electromagnetic valve (Abe et al. 1992). Thus
the start of the facility can be initiated by an external electrical
trigger to open the electromagnetic valve. The compression
tube is 5.3 m in length and is 82 mm in diameter. As shown in
Fig. 2, inside the compression tube, is loaded the free piston
which is a circular cylinder made of aluminum and is 3.72 kg
in weight. The weight of the free piston is adjustable by in-
serting a balance weight into an inner cavity of the free piston.
On the fore and aft part of the piston, respectively, a pair of
the Teflon O-rings is mounted for the purpose of sealing the
Correspondence to:T.Abe
206
Fig. 1.
Experimental set-up
gap between the piston and the inner surface of the compres-
sion tube. The free piston is driven by the high pressure gas in
the high-pressure vessel, runs towards the end of the compres-
sion tube and compresses the gas ahead of the free piston. The
motion of the free piston inside the compression tube is moni-
tored by the optical measurement system the main component
of which is the paired optical bers (see Fig. 2). A set of the op-
tical bers is mounted in the wall of the compression tube at a
location of 160mmapart from the end of the compression tube.
One optical ber of the pair is used for transmission of light
from an external light source into the compression tube and the
other one is used for transmission of the scattered light from
the piston surface to an external sensor, a photo-multiplier. To
measure the temporal variation of the piston velocity during
its passage at the optical measurement system, a circular stripe
pattern is carved on the piston surface (Komuro et al. 1995).
The stripe pattern modulates the scattered light, which enables
us to identify the stripe at the location of the optical measure-
ment system. This enable us to measure both the location and
the instant velocity of the free piston during its passage along
the optical measurement system. At the end of the compression
tube, a ring-shaped obstacle made of Nylon is mounted inside
the tube in order to reduce the damage on the compression
tube in case of an emergency in which the free piston hits the
end of the compression tube. A pressure monitor is mounted
at the end of the compression tube.
Between the compression tube and the shock generation
tube, the newly developed fast action valve is inserted instead
of a conventional rupture disk. A conventional rupture disk can
also replace the newly developed fast action valve for compar-
ison of the performance. The sketch of the fast action valve is
presented in Fig. 3. The fast action valve is composed of a pair
of cylinders and the casing for them. The mass of a cylinder is
0.300 kg. The pair of cylinders are pressed against each other
by pressurized gas charged into the respective cavities behind
the cylinders inside the casing. A through-hole is drilled at
the center of the casing to directly connect the compression
tube and the shock generation tube when the pair of cylinders
are separated and the fast action valve opens. The pressur-
ized gas in the compression tube is able to generate a force
to move the pair of cylinders against the adverse force due to
the pressure in the cavities. When the pressure at the compres-
sion tube rises and reaches a certain value, its opening force
acting on the cylinders overcomes the pressing force supplied
by the pressure in the cavities, and the cylinders are moved
aside; the fast action valve opens. The moving cylinders open
the through-hole in the casing to connect the compression tube
and the shock generation tube, and enables the compressed gas
to rush into the shock generation tube. Eventually the highly
compressed gas entering into the shock generation tube gen-
erates the shock wave in the tube.
Since the cavity behind the cylinder in the casing reduces
its volume due to the backward motion of the cylinder, the
pressure in the cavity increases with the opening motion of the
fast action valve. Even though the pressure behind the cylinder
rises by this backward motion of the cylinders and it strength-
ens the force to push back the cylinder, the fast action valve
does not open instantaneously but only sufciently after the
highly compressed gas enters into the shock generation tube,
because 1) the pressure in the compression tube keeps rising
even after the initial opening of the fast action valve because
of the inertial motion of the free piston and 2) there is a mo-
tion delay of the cylinders due to the mass of the cylinders. The
rapid opening motion of the fast action valve, however, still re-
quires cylinders of light weight. Hence, in the actual cylinders
for the fast action valve, cavities are drilled in the cylinders
from behind in order to reduce the mass of the cylinders as
shown in Fig. 3. The additional cavities in the cylinders can
also reduce the pressure rise in the cavity in the casing behind
the cylinders because of the increase of the cavity volume. To
monitor the motion of the cylinders in the casing, the optical
measurement system similar to the system for the free piston in
the compression tube is equipped in the fast action valve. Also
the pressure gauges are equipped to monitor the pressure rise
in the cavity behind the cylinders. Between the compression
tube and the fast action valve, a thin aluminum foil is inserted
for the purpose to tighten a seal between the compression tube
and the shock generation tube before the operation of the facil-
ity. As for the shock generation tube, we employed an exiting
tube of 82 mm in diameter. Since the diameter is larger than
the through-hole in the fast action valve, we cannot expect an
efcient generation of the shock wave. But the tube is suf-
ciently useful for the purpose only to monitor the shock wave
generated by the highly compressed gas through the fast ac-
tion valve. The speed of the shock wave is measured by using
a pair of pressure gauges located at 2.5 m downstream of the
fast action valve with a separation distance of 100mmbetween
them along the tube.
207
Fig. 2.
Sketch of the free piston and the optical measurement system
to monitor the free piston motion in the compression tube
Fig. 3.
Sketch of the fast action valve
Fig. 4.
Typical signals to monitor the operation
3 Results
we can see that the free piston, reaching a speed of 83.3 m/sec
at the measuring point, is decelerated continuously and halts
at a location of around 45 mm from the end of the compression
tube. The pressure of the compressed gas reaches a value of
17.6 MPa at the peak. Just before the compressed gas pressure
reaches its peak value, the fast action valve is initiated to open.
The pressure of the compressed gas keeps rising even after the
initial opening of the fast action valve. Parallel to this, the fast
action valve keeps increasing its opening width. Since, once
the fast action valve opens widely, the highly compressed gas
in the compression tube rushes into the shock generation tube,
the pressure of the compressed gas goes down sharply after it
reaches the peak value.
As described above, the cylinders inside the fast action
valve begin to move just before the compressed gas reaches
its peak pressure. After 1.79 ms from the initial valve opening
motion, the valve reaches an almost full-open status. After al-
Air was used as working gas for the facility. To demonstrate
a performance of the present facility, we select the following
variables as free parameters of the operation. One is the pres-
sure for the high-pressure vessel and another is the pressure
behind the cylinders in the fast action valve. The pressures of
the compression tube and the shock generation tube are xed
at 0.101 MPa and 8.0 kPa, respectively. The monitor signals
are displayed in Fig. 4 for a typical run of the facility. In this
run (the experiment number 21507 in Table 1), the pressure of
the high-pressure vessel is 1.5 MPa and the pressure behind
the cylinders in the fast action valve is 3.0 MPa. In the mon-
itor signals of the free piston and the cylinder motions in the
fast action valve, we can see modulated signals composed of
pulses of various duration which is generated by the scattering
light modulated due to the circular stripe patterns on the free
piston or the cylinders in the fast action valve. From the signal,
208
most all the compressed gas escapes into the shock generation
tube, the valve starts to close again. The opening duration of
the valve is sustained during about 3.20 ms. The opening mo-
tion of both the cylinders are well synchronized to each other
since the cavities are connected by a pipe so that the pres-
sures in both the cavities keep balance. The pressure behind
the cylinders rises simultaneously with the opening motion of
the cylinders. The peak value for the back pressure behind the
cylinder is 8.31 MPa which is expected from the volume ra-
tio between the cavity volumes in the casing allocated for the
completely closed and opened conguration. When the valve
closes again after opening, both heads of the cylinders collide
with each other and the cylinders stick to each other as seen
from the monitor signals for the back pressure and the motion
of the cylinders. The visual inspection of the valve, however,
exhibits no damage on it. Also the shutting of the valve does
not have any impact on the shock generation performance be-
cause it occurs only after almost all the highly compressed
gas has escaped into the shock generation tube. Nevertheless
we can see that some gas still remains in the compression
tube as can be seen from the slight pressure rise in the com-
pression tube after the valve closed. The highly compressed
gas generated by the free piston rushes into the shock gener-
ation tube when the fast action valve is open, and generates
the shock wave there. As shown in Fig. 4, the pressure mea-
surement shows that the shock wave is generated properly and
there is an arrival time difference at the pressure ports sepa-
rated by 100 mm along the tube. This gives a shock speed of
1.31 km/sec for this run, which coincides with the shock speed
estimated from the pressure rise behind the shock wave. The
summary of the experimental results is collected in Table 1.
As can be seen from the results for the same operational con-
ditions, the reproducibility of the shock speed is reasonable.
As demonstrated in the successful operation conditions listed
in Table 1, the various shock speeds can be attained by vary-
ing the pressure of the high-pressure vessel and the pressure
behind the cylinders in the fast action valve.
In Table 1
requirement for the high pressure behind the cylinders, how-
ever, violates the second requirement. That is, due to the high
pressure behind the cylinders, the opening width and open-
ing duration of the fast action valve is reduced. In the results
listed in Table 1, we can see that the higher back pressure
gives rise to the higher compression because of the fulllment
of the rst requirement. Within the present experimental con-
ditions, the higher back pressure gives rise to a faster shock
speed, even though it causes a narrower opening width and a
shorter opening duration. This is because, within the present
experimental conditions, the higher compression attained in
the compression tube compensates the drawback due to the
narrower opening width. However, it is expected that a back
pressure higher than in the present experimental conditions
will cause, sooner or later, a slower shock speed because of
the narrower opening width and the shorter opening duration.
In this sense, there must be an optimal pressure behind the
cylinder in the fast action valve corresponding to a given pres-
sure value in the high-pressure vessel. The higher pressure in
the high-pressure vessel can give rise to a stronger compres-
sion of the gas in the compression tube. Hence, when we select
an appropriate pressure behind the cylinders in the fast action
valve against the pressure in the high-pressure vessel, this en-
ables us to obtain a faster shock speed. As can be seen from
Table 1, the appropriate pressure behind the cylinders in the
fast action valve shifts to higher values with higher pressures
in the high-pressure vessel. This is because the higher back
pressure is required in order to keep the valve shut until the
higher compression is attained in the compression tube due to
the higher pressure in the high-pressure vessel. It is expected
that the higher pressure in the high-pressure vessel gives rise to
the more rapid motion of the cylinders in the fast action valve,
which may cause damage to them and may shorten their life
time. Under the present experimental conditions, however, no
signicant damage on the fast action valve can be observed.
Therefore, it can be expected that the harder operational con-
ditions may be applicable to the present fast action valve. The
results obtained from the facility equipped with a conventional
rupture disk are collected in Table 2. An iron plate of 1.5 mm
in width is used as rupture disk with a cross groove on its
surface. The performance of the rupture disk depends on the
depth of the groove. Within the experimental conditions, the
rupture disk with the shallower groove gives rise to higher
compression and faster shock speed. The shock speed avail-
able, under corresponding experimental conditions, is rather
faster than the value obtained from the facility equipped with
the present fast action valve. This suggests that, in the present
fast action valve, there still exists a slight loss in the procedure
of generating the compressed gas and its release into the shock
generation tube, in comparison with the facility equipped with
the conventional rupture disk.
In Table 2
P
max
: Peak pressure in the compression
P
i
: Pressure at which the fast action valve initiates its
motion,
t
p
: Mean half duration of the pressure development
in the compression tube,
t
v
: Duration of the open status of the
t
D
: Delay
time from the initial motion of the valve to the peak pressure
in the compression tube,
W
: Opening width of the valve,
V
s
: Speed of the shock wave.
There are two major requirements on the fast action valve
in order to effectively generate a shock wave in the shock
generation tube; 1) to be closed until a compression as high
as possible is attained in the compression tube, 2) to open as
widely, speedily, and long as possible once the necessary high
compression is attained. The former is necessary to generate a
shock wave as strong as possible while the latter is necessary
to make use of the highly compressed gas as effectively as pos-
sible. Both requirements, however, are contradictory; in order
to attain the rst requirement, the pressure behind the cylin-
ders in the fast action valve must be as high as possible. The
P
D
is Drive pressure in the high-pressure vessel,
W
t
: Thickness of the rupture disk,
W
s
: Groove depth on the
P
max
: Peak pressure in the compression tube,
P
R
: Pressure at which the rupture disk breaks,
t
P
: mean half
duration of the pressure development in the compression tube,
V
s
: Speed of the shock wave.
P
D
is Drive pressure in the high-pressure vessel,
P
V
: Valve back pressure (the pressure behind the cylinder in
the fast action valve),
tube,
fast action valve,
rupture disk,
209
Table 1.
Operational conditions and test results in case of the fast action valve
P
D
P
V
P
max
P
i
t
p
t
v
Wt
D
V
s
Experiment No.
[MPa] [MPa] [MPa] [MPa] [ms] [ms] [mm]
[ms]
[km/sec]
1.7
8.0
35.3 29.0 1.98 2.14 8
10 0.598 1.62
22602
1.7
8.0
36.9 29.4 1.86 2.14 8
10 0.652 1.60
22604
1.7
7.0
33.3 22.3 1.90 2.42 10
12 0.628 1.57
22601
1.7
7.0
33.7 23.9 1.97 2.36 10
12 0.630 1.54
22603
1.7
6.0
33.7 24.3 2.01 2.46 10
12 0.646 1.58
22701
1.7
6.0
32.9 23.1 2.00 2.52 10
12 0.646 1.59
22702
1.5
5.0
25.3 18.0 2.31 2.86 10
12 0.818 1.56
21503
1.5
5.0
24.7 16.5 2.35 3.01 10
12 0.732 1.45
21505
1.5
4.0
21.6 12.4 2.63 3.54 10
12 0.818 1.42
21502
1.5
4.0
20.8 12.4 2.63 3.68 10
12 0.778 1.38
21506
1.5
3.0
18.2 9.80 2.56 3.92 14
15 0.906 1.31
21507
1.5
3.0
17.6 9.21 2.64 4.02 14
15 0.858 1.29
21504
1.3
4.0
17.2 12.2 3.31 3.81 8
10 0.942 1.32
21601
1.3
4.0
17.6 15.1 3.31 3.61 10
12 0.9 1.38
21603
1.3
3.0
14.5 11.6 3.49
10
12 1.02 1.21
21605
1.3
3.0
14.9 11.8 3.48
10
12 1.05 1.21
21606
1.3
2.0
11.2 7.64 3.98
14
15 1.12 1.20
21602
1.3
2.0
10.3 6.08 4.19
4
15 0.906 1.12
21604
Table 2.
Operational conditions and test results in case of the rupture disk
P
D
W
t
W
s
P
max
P
R
t
p
V
s
Experiment no.
[MPa] [mm] [mm] [MPa] [MPa] [ms] [km/s]
1.5
1.5
0.6
31.8 31.8 1.86 1.89 22703
1.5
1.5
0.8
22.3 22.0 2.27 1.73 22801
1.5
1.5
1.0
17.7 16.7 2.55 1.70 22802
1.5
1.5
1.1
14.9 12.7 3.66 1.56 22803
1.5
1.5
1.1
15.2 14.9 3.87 1.51 22901
1.5
1.5
1.1
14.9 14.5 3.68 1.53 22902
1.5
1.5
1.1
14.9 13.7 3.68 1.54 22903
4 Conclusion
References
In the present paper, we proposed a new method utilizing a
fast action valve instead of a conventional rupture disk. The
performance of the fast action valve is demonstrated in the
free-piston-driven shock tube. The present method enables us
to produce a shock wave with good reproducibility and also to
easily produce a shock wave with arbitrary strength. This also
enables us to be carefree regarding scattering of fragments of
the rupture disk. The shock speed available from the present
method is slightly slower in comparisonwith the case using the
rupture disk instead of the present fast action valve. However,
the drawback in the present method can be compensated by
its merits, such as good reproducibility and easy generation of
shock waves of arbitrary strength.
Abe T, Funabiki K, Oguchi H (1992) A combined facility of ballistic
range and shock tunnel using a fast action valve. in: Takayama
(ed) Shock Waves 1025{1030
Komuro T, Sato K, Tanno H, Ueda S and Itoh K (1995) Pilot free
piston shock tunnel. Proc 27th Fluid Dynamics Conference, at
Kagamihara, Japan.
Stalker RJ (1961) An investigation of free piston compression of
shock tube driver gas. National Research Council of Canada,
MT-44
Stalker RJ (1965) The free-piston shock tube, Aeron Quart 17:351
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