Multiple stage drag turbine downhole motor

A multistage drag turbine assembly is provided for use in a downhole motor, the drag turbine assembly comprising an outer sleeve and a central shaft positioned within the outer sleeve, the central shaft having a hollow center and a divider means extending longitudinally in the hollow center for forming first and second longitudinal channels therein. A stator is mounted on the shaft. The stator has a hub surrounding the shaft and a seal member fixed to the hub, wherein the hub and the shaft each have first and second slot openings therein. A rotor comprising a rotor rim and a plurality of turbine blades mounted on the rotor rim is positioned within the outer sleeve for rotation therewith with respect to the stator such that a flow channel is formed in the outer sleeve between the turbine blades and the stator. A flow path is formed in the turbine assembly such that fluid flows through the turbine assembly flows through the first longitudinal channel in the central shaft, through the first slot openings in the shaft and the stator hub, through the flow channel wherein the fluid contacts the edges of the turbine blades for causing a drag force thereon, and then through the second slot openings in the stator hub and the shaft into the second channel.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a multiple stage turbine for use as a 
downhole motor on a drilling string, and more particularly, to a multiple 
stage turbine downhole motor which is driven by the drag or shear stress 
force of the fluid flowing through the turbine acting on the edges of the 
turbine blades. 
2. Description of the Prior Art 
Prior art downhole motors for use on drilling strings convert the kinetic 
energy of a mass of a fluid against the face surface of turbine blades 
into power for turning a drill string and thereby a drill bit attached to 
the bottom of the drill string. The turbines rely solely on the dynamic or 
impulse force of the fluid against the face surface of the turbine blade. 
Prior art downhole motors of this type are generally required to be 
relatively long in order to have sufficient turbine blade surface area for 
generating enough power to turn the bit at the proper speed with 
sufficient torque. However, because the downhole motor itself is quite 
long, it is difficult for the drill string to move through curves and thus 
it is much more difficult to control the direction of drilling. 
Another disadvantage of the dynamic force type downhole motors, is that 
maximum power and efficiency occur at rather high rotational speeds; 
higher than the range of operational speed for most mechanical drill bits, 
like tricone bits. The reason for this characteristic is that the 
functions of power and efficiency, in terms of the velocity of the flow is 
proportional to the square of the velocity. The function is a parabola in 
which the apex is approximately midway between zero and runaway or no load 
speed. 
Still another disadvantage of prior art downhole turbine motors is that the 
turbine blades are internal with respect to the drilling shaft. In order 
to drive the turbine, fluid must flow through the internal structure of 
the drill string and can cause damage to the bearings, seals and other 
internal parts of the downhole motor. 
SUMMARY OF THE INVENTION 
It is the primary object of the present invention to provide a multiple 
stage turbine which operates by using the shear force of the fluid on the 
edges of the blades of the turbine. 
It is another object of the present invention to provide a downhole motor 
for use in turning a drill string, and thereby a drill bit on the end of 
the drill string, which operates at a relatively slow speed of 300-500 rpm 
and produces high torque, with no torque on the pipe of the drill string 
itself. 
It is another object of the present invention to provide a multiple stage 
turbine in which the rotor having the turbine blades, is external to the 
central shaft of the drill string and thus the moving parts are external 
to the central shaft. Further, because the blades are attached to an 
external movable part, the generated forces are farther away from the axis 
of the turbine, giving more leverage and hence more torque. 
The present invention is directed to a multistage drag turbine assembly for 
use in a downhole motor, the drag turbine assembly comprising an outer 
sleeve and a central shaft positioned within the outer sleeve, the central 
shaft having a hollow center and a divider means extending longitudinally 
in the hollow center for forming first and second longitudinal channels 
therein. A stator is mounted on the shaft. The stator has a hub 
surrounding the shaft and a seal member fixed to the hub, wherein the hub 
and the shaft each have first and second slot openings therein. A rotor 
comprising a rotor rim and a plurality of turbine blades mounted on the 
rotor rim is positioned within the outer sleeve for rotation therewith 
with respect to the stator such that a flow channel is formed in the outer 
sleeve between the turbine blades and the stator. A flow path is formed in 
the turbine assembly such that fluid flowing through the turbine assembly 
flows through the first longitudinal channel in the central shaft, through 
the first slot openings in the shaft and the stator hub, through the flow 
channel wherein the fluid contacts the edges of the turbine blades for 
causing a drag force thereon, and then through the second slot openings in 
the stator hub and the shaft into the second channel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is directed to a multiple stage drag turbine which 
comprises a plurality of single stages, which may be grouped for parallel 
and series flow, each of which operates on the principle of the shear 
stress of fluid flowing in passages or channels in each turbine stage 
against the edges of the turbine blades. The shear stress produces drag 
forces on the blades. The volume of flow is not a direct factor, rather 
only the shear forces on the edges of the turbine blades. The power 
produced by the shear force is a function of the relative velocity of the 
fluid and the blade and drag surface, the drag surface being the edge of 
the turbine blades, and not the face surface of the blade itself. The use 
of the shear force results in a higher torque than a conventional turbine 
rotor of the same dimensions. This enables the motor of the present 
invention to generate sufficient torque using less stages which in turn 
enables it to be shorter in length than conventional turbine motors. 
The shear stress utilized in the present invention is produced by the 
friction or scraping action on the edges of the blades. The shear stress 
or "drag force" is expressed as: 
EQU F.sub.dr =.tau..sub.dr a.sub.dr . . . (1) 
where: 
F.sub.dr =Drag (drive) force (N). 
.tau..sub.dr =Shear stress on the rotor blades (N/cm.sup.2). 
a.sub.dr =Drag where the shear stresses acts (m.sup.2). 
The mechanical power produced by this drag force is expressed as: 
EQU HP=F.sub.dr u=.tau..sub.dr a.sub.dr u/k . . . (2) 
where: 
u=Tangential velocity of the blades (m/s). 
k=10.sup.4 m.sup.2 /cm.sup.2 
Knowing that the hydraulic head in meters due to shear stresses is 
proportional to the square of the relative velocity between the fluid and 
the blades of the rotor, we have: 
EQU .tau..sub.dr =.gamma..lambda..sub.dr v.sup.2 /2 gk . . . (3) 
.gamma.=Specific weight of the fluid (N/m.sup.3). 
.lambda..sub.dr =Drag coefficient (dimensionless) of the rotor blades 
geometrical configuration. 
v=Relative velocity of the fluid with respect to the edge of the blades of 
the turbine (m/s). 
g=Gravity acceleration (m/s.sup.2). 
Simultaneously a hydraulic head is produced due to friction against the 
wall or walls of the passage not covered by blades. 
EQU .tau..sub.fr =.gamma..lambda..sub.fr v.sup.2 /2 gk . . . (4) 
.tau..sub.fr =Shear stress in the stator (N/cm.sup.2). 
.lambda..sub.fr =Friction coefficient (dimensionless) of the stator walls 
(without blades). 
Substituting equation (3) into equation (2) and also substituting the value 
of the relative velocity v=(c-u), the mechanical power will be: 
EQU HP=.gamma..lambda..sub.dr .sub.dr u(c-u).sup.2 /2 g . . . (5) 
c=Average velocity of the fluid through the channels of the turbine (m/s). 
This is the fundamental equation of the turbine of the present invention. 
It can be seen that the output power does not depend on the angle of 
incidence of a mass or volume of a fluid, but rather, depends on other 
parameters, the specific weight of the fluid (.gamma.), the dimensionless 
drag coefficient (drive coefficient) (.lambda..sub.dr), the drag surface 
(a.sub.dr), the velocity of the fluid (v) through the drag passage or 
channel of the turbine and the velocity of the rotor itself (u). 
The input pressure and hydraulic power are calculated as follows: 
The hydraulic head H is the specific energy which is used to circulate the 
fluid through the turbine and is calculated as follows: 
EQU H.sub.in =[.tau..sub.dr a.sub.dr /.lambda.A.sub.m +.tau..sub.fr a.sub.fr 
/.lambda.A.sub.m ].kappa. . . . (6) 
H.sub.in =Input pressure, input head or specific input energy (m). 
A.sub.m =Area of the section of the channels through which the fluid 
circulates with velocity c (m.sup.2). 
a.sub.fr =Friction area where the shear stresses friction acts (m.sup.2). 
The first term of the right hand side of this equation is the head used by 
the rotor and the second term is the friction head lost in the stator 
without producing any power. Using the previous values in equations (3) 
and (4), the input head will be: 
EQU H.sub.in =.lambda..sub.dr a.sub.dr (c-u).sup.2 /2 g A.sub.m 
+.lambda..sub.fr C.sup.2 /2 g A.sub.m . . . (7) 
The input power in hydraulic terms is: 
EQU HP.sub.in =.gamma.H.sub.in Q/K . . . (w) (8) 
or 
##EQU1## 
where: HP.sub.in =Input hydraulic power (w). 
Q=Total volume of the fluid incoming into the turbine (m.sup.3 /s). 
The efficiency is then: 
EQU .eta.=HP/HP.sub.in . . . (10) 
substituting equations (5) and (9): 
##EQU2## 
It can thus be seen that the efficiency depends only on the through flow 
velocity, the rotor velocity and the physical and geometrical 
characteristics of the turbine, i.e., the drag surface, the friction 
surface and their corresponding dimensionless coefficients .lambda.. 
An example of the performance of the turbine, can be seen the graphs shown 
in FIG. 1. 
FIGS. 2A-2F are a sectional view of a downhole motor of the present 
invention. Downhole motor 1 includes a hollow inner shaft 3 and an outer 
sleeve or housing 5, and has a seal structure 7 and bushing 9 at the input 
end. A turbine assembly 11 comprises a plurality of turbine stages which 
may be divided into a plurality of groups. Each stage comprises a stator 
assembly 13 and a rotor assembly 15. The stator assembly 13 includes a 
seal member 13a and a hub 13b, and the rotor assembly 15 includes a 
plurality of blades 15a and a rotor rim 15b. The bottom end of the turbine 
assembly is sealed by a second seal assembly 17 which includes a bushing 
19. The top seal assembly 7 is a much heavier seal then the bottom seal 
assembly 17. The bushings 9 and 19 provide support and maintain alignment 
of the inner shaft 3 and outer sleeve 5. A roller bearing assembly 21 
carries the thrust loads and radial loads and assists in maintaining the 
alignment between the inner shaft and outer sleeve. Although a roller 
bearing assembly is shown, other bearing assemblies such as ball bearings 
can also be used. The bearing structure also includes a self-contained 
lubricating system which may include a pressure compensator 23, if 
required. The turbine assembly and seals are loaded and held together by 
means of nuts 25 and 27, and the bearing assembly is held in place by nuts 
29a and 29b. 
Referring to FIGS. 3 and 4A-4B, shaft 3 has an interior divider 31 which 
extends axially along the length of the shaft in the area surrounded by 
the turbine assembly. The purpose of the divider 31 is to divide the space 
in the inner shaft into two channels 33 and 35 for carrying fluid into the 
turbine assembly. Fluid F is pumped into the inner shaft from the top of 
the turbine motor assembly so that it flows down in channel 33. The fluid 
then goes through slotted opening 37 where it is then diverted into 
channel or passage 39 by seal member 13a which is fixed onto hub member 
13b. Hub member 13b is keyed onto inner shaft 3 by means of rod 41 so that 
stator member 13 does not rotate. 
After channel 39 is filled and fluid flows around channel 39 and contacts 
the edges 15c of the blades 15a creating a shear, drag or edge force on 
the blade edges 15c. This drag force rotates the blade assembly or rotor 
15. When the flow in channel 39 reaches seal member 13a, it is diverted 
through slotted opening 43 into channel 35 where it flows downward to the 
next group of stages. Rotation of the rotor 15 rotates the outer sleeve 5 
which is fixed thereto by means of the loading of nuts 25 and 27. A drill 
bit (not shown) is coupled to the lower end of the downhole motor for 
rotation therewith. 
FIG. 5 illustrates the manner in which a plurality of turbine rotors or 
stages 15 are assembled in groups for parallel and serial operations. 
Fluid flows into one channel 33 in inner shaft 3, which in FIG. 5 is the 
upper half. Fluid comes out of the slot openings 37, flows around channels 
39 and then re-enters the inner shaft through exit slot openings 37. 
In the embodiment shown in FIG. 5, ten turbine stages form the first group. 
The flow through all ten stages is in parallel. Interior walls 45 are 
placed in channels 33 and 35 in the interior of shaft 3 to block flow 
through the channel and to cause the flow to go in parallel from the 
channel through the corresponding slot opening into the corresponding 
channel 39. The walls 45 are positioned to divide the turbine stages into 
groups. When the fluid flowing in channel 33 reaches a wall 45, the flow 
in channel 33 is blocked so that fluid flows into the group of ten turbine 
stages. After flowing through the turbine stages, the fluid flows into 
channel 35. Upon reaching an interior wall 45, the fluid is again blocked 
so it flows into the next group of turbine stages. In this next group, the 
slot openings 43 become the input slots. The input slot openings 43 in the 
second group of ten turbine stages are located in the bottom of the seal 
hub 13b, as shown in FIG. 3b. Thus even though the fluid is entering the 
turbine assembly from channel 35, it flows in the same direction as in the 
previous group of ten turbine stages. This alternating series and parallel 
flow continues through the entire turbine assembly. 
The number of turbine stages included in each group and the number of 
groups will depend upon the particular conditions under which the downhole 
motor is used, primarily the required volume and pressure conditions 
necessary for drilling. 
FIG. 6 shows an alternative embodiment of a stage of the turbine assembly. 
The difference between the embodiment of FIG. 2 and the embodiment of FIG. 
6 is in the structure of the blades 115a and 115'a. Corresponding changes 
have also been made to the seal member 113a. In particular, in the 
embodiment of FIG. 3, the blades 15a are in the axial direction, whereas 
in the embodiment of FIG. 6, the blades 115a and 115'a are in the radial 
and axial direction. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential characteristics thereof. The 
presently disclosed embodiments are therefore to be considered in all 
respects as illustrative and not restrictive, the scope of the invention 
being indicated by the appended claims, rather than the foregoing 
description, and all changes which come within the meaning and range of 
equivalency of the claims are, therefore, to be embraced therein.