Gas laser having improved crossflow blower arrangement

A gas laser of the type in which gas is impelled transversely through a region where lasing of the flowing gas occurs, employs a crossfield blower to propel the gas around a closed loop. The crossfield blower has an elongate impeller that is substantially the same length as the lasing region. The impeller generates at least one vortex in the gas flow. A part of that vortex is situated outside the path of the main gas flow to the lasing region. To remove heat from the gas without appreciably interfering with the main gas flow to the lasing region, cooling means are situated in that part of the vortex that is outside the path of the main gas flow to the lasing region.

FIELD OF INVENTION 
This invention relates in general to lasers of the type using a flowing gas 
as the lasing medium. More particularly, the invention pertains to gas 
lasers of the kind employing a crossflow blower to propel the flowing gas. 
BACKGROUND OF THE INVENTION 
In a laser of the kind employing flowing gas as the lasing medium, the gas 
flows through a discharge cavity where an electric field causes an 
electric discharge in the gas that produces an emission of light. The 
amount of light produced is related to the volume of gas that can be made 
to lase. In moderate and high power gas lasers, the volume of gas that can 
be made to lase is increased by propelling the gas through the discharge 
region at high velocity. In high power gas lasers, the gas flow may exceed 
sonic velocity. To conserve gas, it is conventional to cause the gas to be 
recirculated in a closed system and to add gas to replace lost gas. 
Lasing of the gas causes the gas to become very hot and the gas flowing out 
of the discharge cavity, therefore, is at a much high temperature than the 
gas entering the discharge cavity. In a closed system where the gas is 
recirculated, the hot gas from the discharge cavity flows to a heat 
exchanger where the gas is cooled to restore the population of gas 
molecules to levels that are appropriate for again permitting the 
stimulation of lasing. The cooled gas is then accelerated to increase its 
velocity and the gas is again directed into the discharge region for the 
stimulation of lasing. 
In gas lasers of the kind in which the gas flows transversely through the 
discharge region, it is usual to employ a crossflow blower as the device 
for propelling the gas around its recirculatory path. The crossflow blower 
is especially suited for use in the transverse flow gas laser because the 
length of the blower's impeller can be matched to the length of the 
discharge region. 
The crossflow fan appears to have been invented by Mortier in the 1890's 
and is described in his U.S. Pat. No. 507,445 which was granted in 1893. 
An improvement on the crossflow fan is disclosed in Datwyler's U.K. Pat. 
No. 988,712. In that improved fan, the vortex is free to move 
circumferentially around the fan and the vortex thus is able to adjust its 
position to prevailing flow conditions. For a discussion on the crossflow 
fan, see the monograph titled "A Study Of The Cross Flow Fan" by A. M. 
Porter and E. Markland in the Journal of Mechanical Engineering Science, 
Vol. 12, No. 6, 1970. That monograph is here incorporated by reference. 
U.S Pat. No. 4,099,143 describes a flowing gas laser of the transverse gas 
flow type having a gas tight cylindrical housing enclosing a crossflow 
blower, a heat exchanger and means forming a discharge region, together 
with baffles and vanes for causing the gas to flow in a closed loop. In 
that arrangement, the crosssflow blower extends longitudinally within the 
housing along substantially the same length as the discharge region. 
Consequently, the blower propels the gas transversely through the 
discharge region along the entire longitudinal extent of that region. In 
the arrangement disclosed in the patent, the crossflow blower and its 
baffles are disposed in accordance with conventional practice. 
It has been found that the throughput (i.e. the volume of flow) of the 
crossflow blower is adversely affected where the inlet flow conditions or 
the outlet flow conditions or both are such as to cause throttling of the 
flow to occur inside the blower's impeller. Throttling occurs where the 
center of the vortex generated by the blower is inside the impeller's 
cage, as described in the Porter and Markland monograph. To assure proper 
inlet flow conditions, the velocity of the inlet flow must be high enough 
to prevent the inlet flow from hugging the interior circumference of the 
impeller's cage. That is, where the velocity of the inlet flow, after once 
passing through the blades of the impeller, is too low, the flow tends to 
hug the inside circumference of the cage formed by the blades of the 
impeller. 
It is known from the technical literature that a well behaved crossflow 
blower generates two vortices and that the main throughput flow of the 
blower passes between those vortices through a channel bounded by the 
separation streamlines. The exact location and shape of those vortices are 
subject to external factors such as ducting and back pressure and in 
general are not known. 
OBJECTS OF THE INVENTION 
The primary object of the invention is to obtain proper performance of the 
crossflow blower employed in a flowing gas laser by insuring that the 
inlet and outlet conditions do not cause throttling of the gas flow 
through the impeller of the blower. 
Another object of the invention is to eliminate or reduce the adverse 
effects of an upstream heat exchanger on the gas flow produced by a 
crossflow blower employed in a flowing gas laser. 
A further object of the invention is to eliminate the need for an upstream 
heat exchanger in a flowing gas laser while enabling the heat added to the 
gas by the work of a crossflow blower to be removed by cooling means 
situated outside the main gas flow on the outlet side of the blower.

DETAILED DESCRIPTION OF INVENTION EMBODIMENTS 
Referring now to FIG. 1 of the drawings, there is shown a laser whose 
internal structures are situated in a gas tight enclosure formed by a 
hollow cylindrical housing 1 that is closed at one end by a cap 2 and is 
closed at its other end by a cap 3. Extending through end cap 3 is a 
flexible conduit 4 through which electrical power lines, water supply 
hoses, and other lines are brought into the housing without impairing the 
gas tight integrity of the enclosure. Protruding from end cap 2 is a 
cylindrical tube 5 in which is mounted the optically transparent window 
element through which the laser beam passes out of the gas tight 
enclosure. 
FIG. 2 of the drawing schematically shows a conventional arrangement of the 
structures inside the housing 1 of the laser. In the illustrated 
arrangement, the discharge region 10, in which lasing of the gas occurs is 
situated between two dielectric plates 11 and 12 made of a ceramic 
material such as cordierite which has a low coefficient of expansion and 
which can withstand high temperatures. Rows of electrodes 13 and 14 extend 
through the dielectric plates and establish an electric field that causes 
the gas in the discharge region to lase. Housed within ballast modules 15A 
and 15B are electrical components that are associated with the electrodes 
13 and 14 that establish the electric field in the discharge region. 
Cooling elements may also be situated in the ballast modules. The hot gas 
leaving the discharge region flows through a downstream heat exchanger 16 
which cools the gas to restore the population of gas molecules to the 
appropriate levels that permit the gas, when recirculated, to be again 
stimulated into lasing activity. The cooled gas then flows to the intake 
side of a crossflow blower 17. That device is sometimes referred to as a 
"squirrel cage" blower because its impeller has a single circular row of 
blades forming a cage that rotates about its central axis. The gas 
pressure at the outlet side of the crossflow blower is higher than the gas 
pressure at the inlet side. The crossflow blower compresses the gas and 
accelerates its velocity and in so doing heats the gas. To remove the heat 
added to the gas by the work of the crossflow blower, the gas from the 
outlet side of the blower is cooled by causing it to pass through an 
upstream heat exchanger 18 before the gas enters the discharge region 10. 
The heat exchangers 16 and 18 may be of any convenient type, such as the 
conventional kind that employs water cooled tubes with multitudes of fins 
that increase the effective heat exchange surface area. The path of the 
gas flow in the FIG. 2 arrangement is indicated by arrows and it is 
evident that the gas circulates in a closed loop. 
Referring now to FIG. 3, the optical elements 19 and 20 of the laser's 
resonant optical cavity are shown disposed at the ends of the discharge 
region 10. For clarity, optical element 19 of the resonator has been 
omitted in FIG. 2. The impeller of the crossflow blower 17 is 
substantially coextensive with the length of the discharge region 10 of 
the laser. The crossflow blower has a substantially uniform gas flow 
pattern over the entire longitudinal extent of the impeller and thus is 
able to propel the gas with uniform velocity transversely across the 
discharge region throughout that region's entire longitudinal extent. The 
impeller of the crossflow blower is driven by an electric motor 21 mounted 
on a support 22 attached to the housing 1. In a similar manner, the other 
end of the impeller of the crossflow blower is supported in a bearing 23 
mounted on a support 24 secured to the housing. 
Referring again to FIG. 2, it can be seen that the outlet flow of blower 17 
is confined between a vortex wall 26 and a rear wall 27 which direct the 
gas flow toward the upstream heat exchanger 18. The primary function of 
that heat exchanger is to remove the heat added to the gas by the work of 
blower 17. Each heat exchanger is shown in FIG. 2 as employing two rows of 
water cooled pipes having fins to increase the effective heat exchange 
area. The heat exchanger 18 impedes the flow of gas by its "drag" that 
reduces the gas velocity. Obviously, the removal of the upstream heat 
exchanger from the flow path of the gas is a highly desirable objective if 
its removal can be achieved while enabling the heat added by the work of 
the blower to be removed from the gas. 
Referring now to FIG. 4, the impeller of the crossflow fan is shown 
together with vortex wall 26 and rear wall 27. It can be seen that the 
inflow can occur over a wide arc whereas the outflow is restricted to a 
much smaller arc. That is the conventional manner of employing a crossflow 
fan and is the arrangement depicted in FIG. 2. 
Referring now to FIG. 5, there is shown a modification of the FIG. 2 
arrangement that results in improved performance. In the FIG. 5 
arrangement, the inlet flow to the crossflow blower 17 is confined by a 
tapered nozzle 28 to a smaller sector of the periphery of the impeller. In 
addition, the vortex wall 26, shown in FIG. 2, has been removed. The 
tapered nozzle 28 increases the velocity of the gas flowing into crossflow 
blower. 
Referring now to FIG. 6, there is shown a portion of the impeller of a 
crossflow blower having blades 17A and 17B which are circular arcs. The 
ideal inlet gas flow is tangent to the top of the blades, as indicated in 
that figure by the arrow X, so that the ideal angle of attack is zero. 
Relative to the tangent to the outside periphery of the impeller, the 
angle .alpha. of 30.degree. gives a zero angle of attack. 
To prevent the inlet flow from hugging the internal circumference of the 
impeller, the exit velocity V.sub.1 of the gas must have sufficient radial 
velocity to cause the gas to flow substantially in the direction indicated 
by the arrow Y. 
The tapered nozzle 28, in addition to confining the inlet flow to the 
crossflow blower 17 causes the inlet flow angle of attack to approach the 
ideal and also, because of the taper, increases the inlet velocity. 
Referring now to FIG. 7, there is schematically shown the result of 
increasing the velocity of the inlet flow and confining the inlet flow to 
a smaller sector of the impeller's circumference. The main flow through 
the impeller is in a channel between the separation streamlines indicated 
in FIG. 7. That main flow channel in the impeller is confined by two 
vortices, one of which has its center on the impeller blades. The nozzle 
28 tapers to a width that is approximately the same as the height of the 
channel through the electric discharge region. 
The major portion of the heat generated in the fan's compression of the 
gas, is generated in the two vortices and not in the throughput flow of 
the main channel. In the FIG. 2 arrangement, all the heat generated in 
vortices must diffuse into the main flow before being removed by the 
upstream heat exchanger. 
Referring now to FIG. 8, there is schematically shown the preferred 
arrangement of the internal structures of the laser. In that preferred 
embodiment of the invention, the upstream heat exchanger has been entirely 
eliminated. Cooling to remove the heat generated by the crossflow blower 
is provided by finned cooling tubes 30 and 31 disposed in the path of the 
gas circulating in the two vortices. The tubes 30 are positioned in the 
path of one vortex and the tubes 31 are positioned in a the path of the 
other vortex. The placement of the cooling tubes and the number of those 
tubes can, of course, be varied to provide the requisite cooling. Further, 
the placement, size, and number of those cooling tubes can be adjusted to 
minimize or eliminate any undesired effects on the throughput of the main 
flow channel. 
The tapered nozzle 28 shown in FIG. 8 can be made of sheet metal or of any 
other suitable sheet material. If desired, curved vanes 32 may be employed 
to smoothly turn the flow of gas into the discharge region. 
FIG. 9 schematically shows a hybrid embodiment of the invention that 
retains the upstream heat exchanger while utilizing the improved crossflow 
blower arrangement of FIG. 8. In the FIG. 9 embodiment, some of the heat 
added by blower 17 is removed by upstream heat exchanger 32. However to 
lessen the back pressure of heat exchanger 32, the impedance to gas flow 
through that exchanger is greatly reduced by using water cooled tubes that 
offer little resistance to the gas flow. The major part of the heat added 
by blower 17 is, in the FIG. 9 arrangement, removed by the finned cooling 
pipes 33 and 34 which are disposed in the path of the vortices. Because 
some of the heat load is carried by the upstream heat exchanger, the heat 
load on cooling pipes 33 and 34 is reduced and consequently fewer of those 
cooling pipes need be employed. 
Inasmuch as the invention can be embodied in various forms, it is not 
intended that the scope of the invention be limited only to the 
embodiments herein described. Rather, it is intended that the scope of the 
invention be construed in accordance with the appended claims, having due 
regard for obvious changes that do not alter the essential features of the 
invention.