Centrifugal fan with accumulating volute

Centrifugal blowers which maintain a substantially constant (usually .+-.5%) static pressure field around the circumference of the blower's impeller, notwithstanding at least one abrupt radial or axial discontinuity in the volute of the blower, e.g., due to one or more external axial and/or radial constraints in an irregularly shaped package. The blower accommodates such constraints by including discontinuities in the volute; therefore the blower takes advantage of relatively unconstrained segments of the package to have an overall large size. Notwithstanding the volute discontinuities, a substantially constant pressure field around the impeller is achieved by maintaining a specific relationship between G(.THETA.) and H(.THETA.), G(.THETA.) being radial extent of the volute as a function of the angular displacement .THETA. around the impeller's circumference and H(.THETA.) being the axial extent of the volute as a function of .THETA., angular displacement around the volute.

BACKGROUND OF THE INVENTION 
This invention relates to the housing (volute) surrounding a centrifugal 
blower or fan. 
Centrifugal blowers and fans generally include an impeller that rotates in 
a predetermined direction in a housing and is driven by a motor. Such 
blowers are used in a variety of applications where energy consumption, 
efficiency, noise, and space constraints are important. Various prior 
housing designs have attempted to meet predetermined space constraints 
while maintaining the desired performance. 
Generally, a volute may be included around the circumference of a 
centrifugal fan to accumulate the flow generated by the impeller, 
particularly for fans with backward curved impeller blades. Volutes add 
substantially to the overall blower package size, forcing a tradeoff of 
increased efficiency from the volute aerodynamics, on the one hand, versus 
reduced motor and impeller size, resulting in increased energy consumption 
and noise on the other. 
Japanese patent (#52-86554) describes a housing or volute which expands 
with angle in the axial direction. 
U.S. Pat. No. 3,246,834 describes a housing which expands significantly in 
the axial direction. 
In many instances, the blower must be accommodated in a space that includes 
significant discontinuities, e.g. due to packaging constraints from other 
equipment. Specifically, for automobile blowers positioned in tightly 
configured spaces, such discontinuities are common. 
SUMMARY OF THE INVENTION 
The invention features centrifugal blowers which a substantially constant 
(usually .+-.5%) static pressure field around the circumference of the 
blower's impeller, notwithstanding at least one abrupt radial or axial 
discontinuity in the volute of the blower. An abrupt discontinuity is 
generally characterized by at least a 5% change in the first derivative of 
the function in question (G(.THETA.) or H(.THETA.) as defined below) over 
an angular change of 30.degree. or less. The design according to the 
invention is particularly useful for blowers installed in irregularly 
shaped packages, where a regularly shaped blower would be considerably 
smaller due to one or more external axial and/or radial constraints. 
According to the invention, the blower accommodates such constraints by 
including discontinuities in the volute; therefore the blower takes 
advantage of relatively unconstrained segments of the package to have an 
overall large size. Notwithstanding the volute discontinuities, a 
substantially constant pressure field around the impeller is achieved by 
maintaining a specific relationship described below between G(.THETA.) and 
H(.THETA.), G(.THETA.) being radial extent of the volute as a function of 
the angular displacement .THETA. around the impeller's circumference as 
shown in FIG. 2, and H(.THETA.) being the axial extent of the volute as a 
function of .THETA.. By maintaining the relationships G(.THETA.) and 
H(.THETA.) described below, the invention avoids the undesirable 
alternatives in which: a) the volute is smooth, but must be relatively 
small as dictated by the most restrictive point in the flow path; or b) 
flow separation (with resulting inefficiency) occurs due to an extreme 
discontinuity. 
The goal of a uniform pressure field around the impeller circumference can 
be analyzed in terms of conservation of angular momentum around the 
impeller. As long as the viscous forces are small, they cannot have a 
significant impact on the angular momentum of the fluid in the short time 
it is contained within the volute. If the impeller sees a uniform pressure 
field around its circumference, there is no pressure gradient in the 
tangential direction which would cause a change in the fluid's angular 
momentum. 
On the above assumption--i.e., that the fluid's angular momentum is 
conserved about the axis of rotation--the cross-sectional shape of the 
volute at a given angle is designed as follows. First, the assumption that 
angular momentum is conserved about the axis leads to the conclusion that 
the tangential velocity of the fluid is proportional to 1/radius. The 
volute is designed to accumulate the tangential velocity, placing a 
constraint on the two functions G(.THETA.) and H(.THETA.)--i.e., the 
functions are not independent, and they are related as follows: 
EQU G(.THETA.)=g.sub.o (e.sup.h.tan.alpha./H(.THETA.) -1), 
where 
g.sub.o is a constant, 
h is the axial dimension of the volute at the volute origin; and 
.alpha. is the average angle of airflow exiting the impeller. 
Thus, in one aspect, the invention generally features a centrifugal blower 
in which G(.THETA.), H(.THETA.), or both, is characterized by an abrupt 
discontinuity, and the volute has a cross-sectional area which maintains a 
substantially constant pressure field around the impeller at the design 
point for the blower, e.g. when the blower is producing an airflow at the 
volute exit which is within a pre-designed range. 
Another aspect of the invention generally features a centrifugal blower in 
which G(.THETA.), H(.THETA.), or both, is characterized by an abrupt 
discontinuity, and the functions G(.THETA.) and H(.THETA.) are related as 
specified above. 
We have also discovered that such volutes can be designed to accumulate a 
significant portion of the flow rate in a space (a subvolute) axially 
offset from the impeller and characterized by an inner radius which is 
less the outer radius of the impeller. This subvolute region preferably 
extends over most (preferably at least 90.degree.) of the blower's 
circumference and accommodates a significant portion (at least 20%) of the 
volumetric flow in the volute. For example, the subvolute region extends 
from .THETA..ltoreq.30.degree. to the volute exit. The inner radius of the 
subvolute is less than 90% of the impeller radius over at least 45.degree. 
of the blower circumference. Such designs are particularly appropriate for 
axially extended volutes (e.g. the axial extent of the volute is at least 
twice the axial extent of the impeller over at least 15.degree. of the 
blower's circumference). Also preferably, the discontinuous function is a 
Fermi function, or a superposition of multiple Fermi functions. 
The invention thus provides improved performance by purposely introducing a 
discontinuity to accommodate an axial or radial restriction that would so 
substantially limit the cross-sectional area of a "smooth" volute--e.g. a 
volute whose cross-sectional area expands linearly with increasing angle. 
Such a "smooth" volute would exhibit substantially poorer performance, 
e.g., in terms of power consumption for a given impeller and flow rate or 
in terms of noise for a given flow rate and a smaller impeller. 
Thus, the invention recognizes that "smoothness" in the volute may be 
sacrificed to accommodate a tortuous package constraint, to yield a larger 
volute and, overall, a more efficient volute design. 
Other features and advantages of the invention will be apparent from the 
following description of the preferred embodiment.

STRUCTURE 
In FIGS. 1A and 1B, blower 10 includes an impeller impeller having 
conventional blades (not shown) driven by a motor (not shown) to draw air 
axially into the impeller inlet 24. The blades expel air radially into 
volute 30 which surrounds the impeller. Volute 30 encounters certain axial 
and/or radial constraints, illustrated in other figures. FIG. 1B is a 
sectional view, partly broken away, along 1B--1B of FIG. 2. The 
circumference of the impeller is indicated by two lines, 20 and 21, 
representing the inlet side and the motor side of the impeller 
respectively. 
FIG. 2 is a graph based on a section perpendicular to the axis of a 
generalized blower. FIG. 2 has been generalized to show variables 
discussed below, and FIG. 2 is not necessary drawn to scale. In FIG. 2, 
the outer wall of volute 30 is labeled OW and the inner wall of volute 30 
is labeled IW. Specifically, FIG. 2 shows G, the volute's radial 
dimension, as a function of .THETA., the angular displacement from 
.THETA..sub.o, the volute exit plane. In the equation given above relating 
G(.THETA.) and H(.THETA.), "h" is the axial dimension of the volute at 
R.sub.o. H and h are shown in FIG. 1. .alpha. is an angle formed between a 
tangent T to the airflow streamline SL and a line L perpendicular to the 
radius at that tangent. .alpha. will be characteristic of a given 
impeller, primarily as a function of the blade angle (forward versus 
rearward sweep). Circles 20 representing the circumference of the 
impeller, is shown by a broken line in the region over which the inner 
radius of volute 30 is less than the outer radius of the impeller. 
Those skilled in the art will recognize that blowers according to the 
invention can be produced using computer assisted design and machinery, so 
that the requisite relationships have been satisfied. Angle .alpha. can be 
measured, e.g. with Pitot tubes. One useful approach for such a design is 
the structuring of H(.THETA.) in terms of a Fermi function illustrated in 
the following example. 
The constant, g.sub.o, described above is determined by boundary 
conditions. Specifically, the flux leaving the volute must equal the flux 
leaving the blower at the design conditions (e.g. the design point for 
airflow). 
FIG. 3 shows the axial dimension of a blower designed in accordance with 
the invention to meet certain axial packaging constraints. The ordinate in 
FIG. 1 is the angular position around the blower's circumference, where 
0.degree. is the theoretical starting angle of the volute. The axial 
constraints are shown at 0.degree.-90.degree., 90.degree.-180.degree., 
180.degree.-270.degree. and 270.degree.-360.degree.. The axial dimension 
of the impeller is constant. The line labeled "Prior Art" in FIG. 3 shows 
the largest possible volute having an axial dimension that increases 
linearly with increasing angle. As demonstrated in FIG. 3, in certain 
packages, the linearly increasing axial dimension produces an 
unnecessarily small, and therefore inefficient, cross-sectional area. 
The invention provides considerable flexibility in satisfying the 
requirement that the volute accumulate (accommodate) the tangential 
velocity, and that the tangential velocity be proportional (to a first 
approximation) to 1/radius. These requirements are achieved without 
adhering to the constraint of a linearly increasing axial dimension. The 
invention achieves cross-sectional area that is relatively larger for any 
given package constraint, by satisfying the relationships G(.THETA.) and 
H(.THETA.) described above. 
In order to use all the space available, a radially constrained volute 
which directs a fraction of the airflow into a radius smaller than the 
impeller results in a more efficient housing at high flow rates. The space 
axially below the impeller at a radius smaller than the impeller can be 
used to accumulate a significant fraction of the flow rate. 
A preferred feature of the invention is the use of a blower characterized 
in that: a) the maximum radial extent of the inside surface of the volute 
is significantly smaller than (less than about 90% of) the maximum 
impeller radial dimension; and b) the axial extent of the housing is 
significantly greater than (at least twice) the impeller's axial dimension 
over some position of the blower's circumference. 
The above described relationships G(.THETA.) and H(.THETA.) can be 
satisfied even where there are abrupt variations in the radial dimension 
of the volute, by designing corresponding opposite variations in the axial 
dimension, thereby limiting the rate of change in the cross-sectional area 
of the volute. The only limit on the design is the abruptness of the 
discontinuity that can be tolerated without suffering flow separation 
The above features are illustrated in FIGS. 4A, 4B and 5, which generally 
correspond to the blower and volute illustrated in FIG. 1. FIG. 4A shows 
G(.THETA.). FIG. 4B shows H(.THETA.). Approaching the terminus of each 
constraint, H increases abruptly, and G has a corresponding (slight) 
decrease. 
These features are particularly useful in a volute which radially is 
constrained and has a radial dimension of the inside surface substantially 
smaller (less than 90%) of the impeller radius, for a substantial (greater 
than 45.degree.) of the volute's circumference. In such a volute, a 
significant fraction of the flow rate can be accumulated in a space 
axially above or below the impeller at a radius smaller than the impeller. 
FIG. 5 illustrates the various coefficients used to develop two overlapping 
fermi functions to describe blower dimensions (e.g. the axial dimension of 
the blower) to accommodate three axial constraints C.sub.1, C.sub.2 and 
C.sub.3, corresponding to coefficients C.sub.1, C.sub.2 and C.sub.3, 
respectively. In FIG. 5, coefficient C.sub.4 defines the rate of 
transition from C.sub.1 to C.sub.2, and coefficient C.sub.5 defines the 
rate of transition from C.sub.2 to C.sub.3. C.sub.6 and C.sub.7 are the 
respective transition midpoints. The function is as follows: 
EQU F(X)=C1+(C2-C1)/[1+e.sup.-C4.multidot.(X-C6 ]+(C3-C2)/[1+e.sup.-C5.(X-C7)] 
Other embodiments are within the following claims.