Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE_INVENTION
This invention relates to flow sensors, and relates
more particularly ~o a true mass flow sensor
Prior art fluid flow measurement is most commonly
performed by positive displacement meters, turbine meters, or
restriction-type Elow meters. Positive displacement flow
meters, turbine flow meters, anemometers, and blu~f body-type
flow m~ters measure only volumetric flow rate. Measurement
of mass ~low requires correction of such volumetric flow sig-
nals for ~luid density changes which in a liquid ~low is afunction o the temperature of the liquid, while a gas flow
density correction must be made as a function of both pressure
and temperature of the fluid~
Restrictive type of prior art meters operate on a
principle that a restric~ion in a fluid stream creates a pres-
sure drop which i~ a function of mass flow rate for a given
set of fluid parameters. For accurate measurement of flow,
parameters such as pressure ratio, Reynold~s number, orifice
con~iguration, and fluid compressibility must be measured
acc~rately. Methods to automate such computations require use
of eithex transducers and electronic computation or complex
mechanical mechanisms which are undesirable for most industrial,
military, and commercial applications.
True ma~s flow sen~ors presently available operate
on the principle of 1uid inertia measurement, Such devices
utilize measurement or coriolis Eorce~ gyroscopic e~fect, or
angular momentum. ~owever, such prior art true mass flow sen-
sors are both expensive and relatively unreliableO
Prior art volumetric flow meters of the blu~f body
type are based upon the phenomenon of vortex-shedding behind a
`~ '~'''';
bluff body disposed in the flow stream. ~or a wide range of
Reynold's number, a regular patter~ of vortices is generated
by such a bluff body in the flow stream~ The frequ~cy of
these vortices is directly proportional to the stream ~elocity
past the body. When installed in a moving stream~ a wide
variety of bluff body shapes generate a wake consisting of a
series of vortices. It is believed these vortices form in
the boundary layer around the body and grow until they sepa~
rate and are shed into the flow stream. A regular pattern of
alternating clockwise and counterclockwise vorti¢es are gener-
ated from Reynold's number~ from about 60 to over 200,000. The
frequency o~ the periodic vortices is directly proportional to
the flow velocity past the body and inversely proportional to
the characteristic dimension of the bluff body in a direction
substantially perpendicular to the direction of fluid flow,
More specificallyJ the volumetric flow rate multiplied by the
Strouhal number and divided by the characteristic dimension is
equal to the frequency of vortex shedding. For a particular
shape of bluf~ body, such as the cylindrical body~ the Strouhal
number is constant for Reynold's numbers greater than 600.
Accordingly within the appropriate Reynold's number range~ the
frequency is determined by the stream velocity divided by the
characteristic dimension and multiplied by a constant~ For
determining mass flow in using such volumetric flow meters,
separate sensing o the fluid temperature and/or presqure must
also be accomplished to correct for density changes in the mass
flow. Then, appropriate computation must be made of thsse sen-
sed parameters in order to generate a mass flow sensor.
SUMMARY OF THE I~VENTION
,
It is a primary object of the preeent invention to
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provide a true mass flow sensox based upon the blu ff body,
vortex~shedding principle. More particularly, it is an impor-
tant object o~ ~he present inven~ion to provide a vortex-
shedding type flow sensor which generates vor~ices at a ~re-
quency which is a ~unction of mass flow rather than volumetric
flow rate.
Another important object o~ the pre ent invention is
to provide apparatus and method for accomplishing the object
set orth in preceding paragraph, which includes the intrinsic
compensation of the vortex shedding in relation to the density
of the mass El~w being measured.
Accordingly, the present invention contemplates an
improved mass flow sensor and method of extreme simplicity,
economy and greater reliability of operation in comparison to
previous mass flow sensors. ~ore particularly, the present
invention contemplates a bluff body type of ~low me~er operat-
ing on the relationships set forth previously~ Consid0ring
that mass flow i5 determined by density times volumetric ~low,
and that volumetric flow in turn is a function o~ the cross-
sectional flow area times the stream velocity, it c~n be seen
that the freque~cy of the generated periodic vortices is in-
versely proportional to the product quantity of the cross-
sectional area times the charactexistic dimension of the bluf~
body. The present invention contemplates structure and method
which varies the product quantity of cross-sectional area A
times the characteristic dimension d, i.e. A x d, in inverse
proportion to the density o~ the mass flow. The fre~uency o~
the vortices then become a direct function of the mass flow
rather than the stream ~elocity past the bluf~ body.
To accomplish this, the present invention contemplates
method and apparatus for varying either the characteristic
dimension d, or the corss-sectional area A, or both, such that
the product quantity A x d is in inverse proportion to the den-
sity of the mass flow. In preferred arrangem2nts, this is
accomplished by expanding the bluEf body characteristic dimen-
sion d through US8 of a bellows which expands and contracts both
in response to changes in pressure and/or temperature of ~he
mass Elow, or by use of a bimetallic Plement that expands and
contracts in response to changes in temperature of the mass
flowO In another arrangement the bellows or bimetallic element
acts as a driving mechanism for adjusting the ~ross-sectional
area A in response to changes of temperature and/or pr0~sure~
Thesa and other more particular objects and advantages
in the present invention are specifically set forth in or will
become apparent from the following detailed description of
preferred forms of the invention when read in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a front elevational view of the mass ~low
sensor constructed in accordance with the principles of the
present invention;
Fig. 2 is a cross-sectional elevational view taken
along the lines of 2-2 of Fig. l;
Fig. 3 is a cross-sectional view in enlarged Eorm
taXen along the lines of 3-3 of Fig. 2 and showing the expanding
bluff body and driving element associated therewith;
Fig. 4 is an enlarged, plan cros~-sectional view along
the lines of 4-4 of th~ expandable bluff body;
~ ig. 5 is a view similar to Fig. 4 but showing the
bluff body in a more expanded state;
~aS_
Fig. 6 is a cross-sectional elevational view of
another form ~f the invention;
Fig. 7 is an enlarged view of a portion of the bluff
body of Fig. 6 with portions broken away -to reveal internal
detail~ o~ constructionl
Fig. 8 is a cross-sectional view o the bluEf body
of Fig. 6 tak~n along lines 8-8 of Fig. 6;
Fig. 9 is a partial cross-sectional-.elevational view
o~ another modif~ed form of the invention; and
Fig. 10 is a partial cr~ss-sectional view of yet
another form o th~ invention.
DETAILEI) DESC~IPTIO~ OF TEIE PRE~ERRED EMBODIMENTS
Re~erring now more particularly to the Figs. 1-5 of
the drawings, there is illustrated a mass flow sensor as con~
templated by the present invention generally denoted by the
~umeral 20. The sensor includes a housing 22 which may be com
prised of several different components as illustrated in Fig. 2,
which housing defines an internal conduit opening 24 therewithin
having appropriate fittings 25 at opposite ends connactable with
an existing mass flow transmitting conduit. The ¢onduit 24 has
a reduce diameter, rectangular cr~ss-sectional æection 26.
Disposed within section 26 is a bluff body 28 having
a pair of spaced sidewalls 30, 32 de~ining a distance d there~
between in a direction substantially perpendicular to the direc-
tion of fluid flow through section 26~ The bluff body 28 coop-
erates with section 26 to define a fluid flow transmitting area
A between the sidewalls 30, 32 and the periphery o~ the section
26. Pxeferably, the bluff body i~ arranged such that the width
of section 26 in the direction of the distance d, is at least
three times the length o~ distance d, At the rearward end of
bluff body 28 relative to th~ direction of mass flow, the
bluEf body includes a pair of vertical sensor tubes 34 inter-
secured such as by weld joint 38. Each of the walls 30, 32
are attachad to the associated sensor tubes 36 in cantilever
arrangement.
The sensor tubes 34 have openings 36 therein for
receiving vorticas shed from the bluf~ body, and as illustratad
in Fig. 2 the sensor tubes 36 extend downwardly through housing
22 to a transducer arrangement which may include a ~luidic
stack 40 that is operable ~o amplify the pxessure fluctuations
sensed by tubes 34. The sensed pre sure fluctuations are trans-
mitted through a passage 42 to a piezo ceramic transducex 44
that is operable to generate an electrical voltage o-ltput whose
frequency is responsive to the frequ~ncy o the pressure fluc-
tuations sensed by tubes 34. Through an appropriate electrical
outlet 46 the sensed electrical ~requency signal may be trans-
mitt~d to a desired readout, control, or other utilization
device. The fluidic stack 40 and piezo ceramic transducer 44
are shown only in outline form, it being understood that such
transducers are generally widely available and are known to
those skilled in the art.
The invention further contemplates movable means in
the form of a density compensatox element which includes a
bellow~ 50 di~posed within a chamber ~8 of housing 22~ The
flexible walls of bellows 50 coopera~e with end plates 54, 56
thereof to define an enclosed interior 58 of the -b~llows. If
desired, a spring or other biasing mechanism 62 may also be
incorporatPd within the bellows. Upper plate 54 rests again~t
or is secured to a stop 60 which is adjustable by rotation of
an adjusting nut 61 disposed eæteriorly of the housing 22~
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Secured to lower plate 56 are a pair of parallel compensating
struts 68 and 70 which extend downwardly between s.idewalls 30,
32 of bluff body 28. The low0r ends of struts 68 and 70 are
held rigidly by housing 22. Accordingly, in response to expan-
sion and contraction of bellows 5U, the c~mpensator struts 70
and 68 shift vertically as illus~rated in Fig. 3 so that their
central bowed portions between walls 30, 32 will shift in a
aorresponding hori~ontal directi~n and thereby expand and con-
tract the cantilevered sidewalls 30, 32 to alter the character-
istic dimension d of the bluff body. The interior 58 of bellows50 is sealed and filled with fluid hav.ing the same temperature
and pressure characteristic~ of the fluid flow in conduit 24.
Chamber 48 ommunicates with mass fluid flow in conduit 24 via
a port 64 and a passageway 66 i.n the housing.
In operation, mass fluid flow passes through conduit
24 and the section 26 in a left to right direction as viewed
in Fig. 2. Within the desired opexating range of the mass flow
sensor, section 26 is sized such that the ~eynold'~ number re-
mains above 600. In flowing past bluff body 28, Karmann vortex
sheets shed off of the bluff body, and the pressure fluctuations
created by the periodically shedding vortices are sensed through
openings 36 in sensor tubes 34. These pressure Eluctuations are
then amplified by fluidic amplifier 40, and transmitted to drive
the pieZG ceramic transducer 44. The pressure fluctuations
acting upon the piezo ceramic transducer generate a voltage
a~ss the piezo ceramic transducer so that an electric~ output
signal rom connection 46 is cxeated who~e frequency is indica-
tîve of the frequency of vortices shed by bluff body 28~
Bellows 50 compensates for changes in density of the
mass ~low by correspondingly contracting and expanding to
respectively drive the sidewalls 30, 32 toward and away from
one another to ther~by ch~nge the characteristic dimension d
of the blu.EE body 28. In this manner the frequency of the
periodic vortices is varied ln response to changes in d~nsity
such that the sensed frequency of the vortices is indicative
of tha mass flow itsel~ through section 26. For instance, as-
suming the mass- fluid flow through conduit 2~ to be a substan-
tially incompressible liquid mass ~low, th~ density of such
liquid flow is responsive substan~ially only to changes in tem-
pe.rature of thi.s liquid. Upon an increase of temperature of a
mass flow, which accordingly red~uces density of the liquid,
in~reased temperature in chamber 48 causes expansion of liquid
trapped within interior 58 of the bellows causing the bellows
to expand. This causes the bowed sections o:f the s-truts 68t
70 to expand and move the sidewalls 30~ 32 farther away from
one another toward a configuration as illustrated in Fig. 5,
In this manner a decrease in density of the liquid flow increases
the characteristic dimension d such that the Erequency of the
shed periodic vortices i.s indicative of the mass fluid flow.
If the conduit 24 is carrying a relatively compres-
sible gaseous fluid flow, the density of this gas flow is a
function of both changes in pressure and temperature. A trapped
volume of gas having temperature and pressure characteri~tics
like that of the gas flow in conduit 24, and preferably tha
same yas as in conduit 24, is then contained in the interior
of sealed bellows 50~ The trapped volum0 of gas in int~rior
58 of the bellows causes expansion and contraction of the bellows
both in response to changes in pressure of the gas flow as well
as change~ in temperature of the gas delivered through passage-
way 66 to chamber 48~ Temperature increase in chamber 48 causes
~$~
expansion oE the bellows to increase d in respo~se to the
reduction in density, and pressure incxease in chamber 4~
(indicative of increased density) causes contraction of the
bellows to reduce dimension d. Thus, similarly to the discus-
sion above with respect to a liquid Elow, the change in density
of the gas flow causes a corresponding expansion or contraction
of bellows 50 and resulting change in the characteristic dimen-
sion d of ~he bluff body such that the frequency of the periodic
vortices is indicative of the mass flow itself.
More specifically, the frequency of the peri~dic vor-
tices being shed by the blu:EE body 28 is determined by the
following equation:
t 1 )
where:
f is the vortex shedding frequency;
V is the stream flow velocity past the bluff body;
d is the characteristic dimension of the bluf
body;
K is a constant related to the Strouhal number.
The stream ~elocity and the mass flow rate are defined by -~he
following well known equations (constants be.ing delet d):
~2) V ~ ~ ,
(31 m = pQ , where-
Q is th~ volumetric flow rate of the fluid;
A is the duct cross-sectional area;
m is the mass flow rate;
p is the fluid density.
Straight~orward substitution oE the second equation into the
first equation provides the following relat.ionship:
(4) f = KQ
g ~
Accordingly it is seen that the frequency of the periodic vor-
tices is an inverse function of the product quanti ty A x d .
The pxesent invention includes th~ compensator in the
form of bellows 50 in order to vary the quantity product A x d
in inverse proportional proport.ion to the fluid density p:
(5~ Ad
By then substituting equation 5 into equation 4, the following
res~lts
(6) f ~ XpQ .
By comparing equations 3 and 6 it is seen that:
(7) f = Km .
Thus, the frequency of the periodic vortices devel-
oped in the present invention is indicative of the mass flow
rateO It will be noted by reference to Figs. 3 and 4 that upon
change o~ the characteristic dimension d by expansion and con-
traction of the walls 30 and 32, a slight change in area A also
results. Accordingly, the bellows 50 and associated actuating
structure is arranged such that the product quantity A x d
changes in inverse proportion to the density p. By arranging
the bluEf body 28 relative to section 26 in an appropriate
manner, such as by assuxing that the width of the section ~6 in
the direction of dimension d is approximately three times the
length of dimension d, the percentage change of area A as a
result of change in characteristic dimension d, is relatively
small in comparison to the percentage change of dimension d it-
self. In this manner, for instance, the density compensator
is arranged such that the characteristic dimension d is changed
at a rate slightly greater than being simply inver~ely propor-
tional to the change in density, in order to compensate for the
~mall decrease in area A, all such that the resulting relation-
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ship is that the produc~ quantity A x d is inversely propor-
tional to the change in density.
Figs. 6-8 illustrate an alternate embodiment oE the
present inventlon wherein only the cross-~ectional area A is
varied in order to provide density compensation. More parti-
cularly this arrangement includes a housing 80 defininy an in-
terior conduit opening 82 carrying the mass fluid ~low, and a
cylindrical bluff body 84 disposad within conduit 82 such tha t
the characteristic dimension d of bluff body 84 is in a vertical
direction as illustrated in Fig. 6~ A density compensating
bellows 86 is included which along with rigid end walls 88 and
90 define an enclosed, trapped volume which contains fluid
having like pressure and temperature characteristics as the
~luid in conduit 82. Surrounding the bellows is a chamber 93
which communicates with the mass flow in conduit 82 via passage~
way 32. Associated with bluff body 84 is a piston-~ype arrange-
ment presenting a barrier element 94. The integral bluff body
84 and barrier 94 are movably mounted in housing 80 and inter-
connected with end wall 88 to b~ responsive to changes in mass
10w density.
More particularly a change in the mass flow density
causes contraction or expansion of bellows 86 in the same manner
as bellows 50 of the,Fig. 1 arrangement. In response to move-
ment of the bellows, the barrier 94 shift to adju t the cross-
sec~ional area A of the conduit which is carrying the mass flow
past the bluff body 8~. Barrier 94 is appropriately shaped so
that the cross-sectional area A changes in inverse proportion
to chan~es in density of the mass ~low. The characteristic
dimension d of the bluff body remains unchanged, and therefore
the product quantity A x d is varied in inverse proportion to
the density of the mass flow. As a result the frequency of
the shed periodic vortices whose pressure fluctuations are
sensed by openings 98 on the downstream side of the bluff body
and transmitted through sensing tubes 96 to an appropriate trans-
ducer, are propoxtional to mass flow.
Fig. 9 ilLustrates another alternate for~ of density
compensator in combination with a bluEf body substantially simi-
lar to that illustrated in Fig. 1. The Fig. 9 arrangement in-
cludes a housing 100 defining an internal, rectangular, mass
flow carrying conduit 102, and a bluff body generally similar
to configura~ion to that illustrated in Fig. 1 is disposed in
conduit 102. ~ore specifically, the bluff body has spaced
sidewalls 106 defining the characteristic dimension d thexe-
between along with bowing compensator struts 104 therebetween
having one or both opposite ends thereof affixed to the housing
100 .
In contrast to the compensator-type bellows of the
Fig. 1 arrangementt the Fig. 9 structure includes compensator
struts 104 which are composed of a bimetallic, thermally res-
ponsive material. The compensator struts are responsive tochanges in temperature of the fluid in conduit 102, r~spectively
bow.ing .inwardly and outwardly in response to decrease and in-
cxease of temperatu.re of mass flow. Again, the compensator
arrangement is such that the product quan~ity A x d is varied
in inverse proportion to the changes in density such that the
shedding frequency from the bluff body is indicative of the mass
flow rate through the conduit. The Fig. 9 arrangement is parti-
cularly useful in sen~ing the mass flow rate of a substantially
incompressible l.iquid whose density changes substantially only
in response to changes in temperature of the liquid.
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Fig. 10 illustrates a further arrangement contem-
plated by the present invention which includes a housing 110
defining an internal conduit 112 carrying the ma~s fluid flow,
with a bluEf body 114 extending across the conduit 112. Parti-
cularly for applications wherein the bluff body may be substan~
tially smaller than the size of the flow carrying conduit, the
Fig. 10 arrangement is useful in that bluff body 114 has only
a portion 116 thereof which changes in size in ~elation to
changes in density o the mass flow. To prevent interference
by end effects from the bluff body itself upon the frequency
of the vortices, the length of the expandable section 116 in a
horizontal direction as illustrated in ~ig. 10, it is prefer-
ably approximately at least six times the diameter of the sen-
sing opening 118. The Fig. 10 arrangement is constructed in
order to operate along the principles discussed previously.
From the Fig. 10 arrangement it will be apparent therefore that
the entire length of the bluff body need not be expandable, but
rather only a sufficient portion thereof to avoid end effects.
It will be apparent that the present invention pro-
vides an improved method of sensing mass flow which includesstep of producing periodic vortices in the mass flow that are
at a frequency which is indicative of the mass flow, along with
the step of sensing the frequency of the~e periodic vortices.
Den~ity compensation is a part of the step of producing the
desired periodic vortices, and may be accomplished by emplacing
a bluff body in the mass flow and then varying the characteris~
tic dimension d of the bluEf body in relation to the density
of the mass flow. Alternately, density compensation is accom-
lished by varying the cross-sectional flow area A in relation
to the density of the mass flow.
Various alterations and modifications to the fore-
going will be apparent to those skilled in the art. Accordingly,
the foregoing detailed description of preferred arrangements of
the present invention should be considered exemplary in nature
and not as limiting to the scope and spixit of the invention as
set forth in the appended claims.
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