Visualization of the flow of fluid within a centrifugal pump

Item Type text; Thesis-Reproduction (electronic)

Authors Linka, James Edward, 1929-

Publisher The University of Arizona.

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VISUALIZATION OF THE FLOW OF FLUID WITHIN A CENTRIFUGAL PUMP ~

byJames Eo Linka

A Thesis Submitted to the Faculty of theDEPARTMENT OF MECHANICAL ENGINEERING

In Partial Fulfillment of the Requirements For the Degree ofMASTER OF SCIENCE

In the Graduate CollegeTHE UNIVERSITY OF ARIZONA

1 9 6 5

STATEMENT BY AUTHOR

This thesis has been submitted in partial ful­fillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library„

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made„ Requests for permis­sion for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarshipo In all other instances, however, permission must be obtained from the author0

SIGNEDs

APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below:

Mechanical Engineering

J—/ Date

ACKNOWLEDGMENTS

The author wishes to acknowledge the assistance of Associate Professor A. Ralph Yappel who initially suggested this investigation* His guidance and counsel­ling assisted materially throughout the experimental phase of this study*

Acknowledgment is due also to Professor A* G* Foster and Messrs* Otis B* 0*Brien and Gerard B* Fields for their assistance rendered during the fabrication of items used in this investigation*

TABLE OF CONTENTS

LIST OF ILLUSTRATIONS „ o - o o o o o a o o o o o - o o

o o o o o o o o o o o o o o o o o o o o o

1 HISTORY OF CENTRIFUGAL PUMPS = » „ = = = =2 PHILOSOPHY OF CENTRIFUGAL PUMP DESIGN . .3 DESIRABILITY OF FLOW VISUALISATION = „ » =4 PREVIOUS ATTEMPTS TO VISUALIZE FLOW

PATTERNS o o o o o o o o o o o o o o o o o

5 DESCRIPTION OF EQUIPMENT AND PHOTOGRAPHICTECHNIQUE EMPLOYED ......................A» Pump Test Equipment Bo Phet©graph!o Technique Co Injection Technique

o o o o o o o o o

o o o o o o o o

o o o o o o o o o

RESULTS OBTAINED o o o o o o o o o o o o o

A o General o o o o o o - o o o o o o o o o B o Discussion for the Three-Blade Impel-

. ler o o o o o o o o o o o o o o o o o

Go Discussion for the Four-Blade Impeller Do Discussion for the Six-Blade Impeller

V CONCLUSIONS © O O . O O O O O O O O O O O O

8 SUGGESTED AREAS FOR FURTHER INVESTIGATIONLIST OF REFERENCES © o o o o o o o o o o o o o o o

LIST OF ILLUSTRATIONS

O O P o o o

O O O O O O O O O O O O O P O

O O P O O P O P P O O O O P O . O O

O O O P O O O P O O O O O O

Figure lumber1 Pump and Driver » 0 = 0 » » »2 Impellers Used3 Tachometer4 Injection Scheme5 Three-Blade Impeller; Original Configuration6 Three-Blade Impeller, Six Holes Through

F an e S - o o o o o o o o o o o o o o b o o o o

7 Three-Blade Impeller, Ho Holes, Excess Air Admitted to Pump o o o o - o » o o < > o o o o o

8 Three-Blade Impeller, 12 Holes, Excess AirAdmitted to Pump o o o o o <> o o o o o » o o

9 Three-Blade Impeller, Tapered Tip, Ho Holes10 Three-Blade Impeller, Tapered, Holes in Tip11 Three-Blade Impeller,"Tapered, Holes in

Tip, Excess Air Admitted < > < , « , . o o e o o o e

12 Four-Blade Impeller, Original Configuration13 Four-Blade Impeller, Trailing Edge Modified14 Four-Blade Impeller, Original Configura­

tion, Excess Air Admitted * o * * * * * . *15 Four-Blade Impeller, Trailing Edge Modified,

Excess Air Admitted o o o o o o o o o o o o

16 Six-Blade Impeller o o o o o o o o o o o o o

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ABSTS&GT

This paper presents a method of photographic analysis of the flow occurring within a centrifugal pump while being operated at normal speeds and under normal conditions of head and discharge^ The technique employed uses a very short pulse of light from a stroboscopic light triggered by the rotation of the pump itselfo Flow is visualised by admitting a fine stream of air bubbles= These bubbles were photographed against a black background using a standard Polaroid camera with close-up lenses»

4 partial analysis of the flow shown in repre­sentative photographs is given<> Three impellers of vary­ing configurations are showno Changes in flow patterns as a result of impeller modifications are described*

The paper concludes with a list of suggested modifications which could lead to more detailed research effort as a logical extension of the work presented*

CHAFTER 1

HISTORY OF CENTRIFUGAL PUMPS

The actual origin of the first centrifugal pump is unknowno Khat records as do exist cite one Johann Jordan as being the first to have successfully applied the cen­trifugal principle for the purpose of pumping water in 1680o In the United States, the first known practical centrifugal pump was a four bladed, double suction casing device. The blades, or vanes, were straight. This pump had an efficiency of from fifteen to twenty percent.Built in Boston in 1818, it is known as the Massachusetts Pump,

Some twenty years later, in 1839» the introduction of curved impeller vanes gave a substantial boost to the centrifugal pump by raising its efficiency by a factor of three ever that of the straight vanes, The turn of the century in 1900 saw the beginning of large production of centrifugal pumps. These pumps were limited to relatively low speeds, being directly coupled to the steam power

* Numbers in parentheses refer to LIST OF REFER- ■ ENCES, ..........

1

plants of the times0 Heads developed were from five to fifteen feet* All pumps at this time were single stage*

The advent of World War 1 and the attendant rapid expansion of United States commercial industry in general, and the United States Navy in particular, generated a tremendous demand for centrifugal pumps* At the same time, the widespread availability of large electric motors running at 36OO revolutions per minute gave centrifugal pumps the capability of producing the high pressure heads required for boiler feed water application*(3)

Since World War I, centrifugal pumps have increased in capacity, complexity, and application* Multistage pumps employing six to eight stages are not uncommon, nor are pumps developing several thousand feet of head* Centrifu­gal pumps today are used to pump liquids and suspended solids mixtures, hot and cold substances, food products and highly corrosive mixtures, and are even used to lift fully grown fish over dams placed in streams and rivers*

CHAPTER 2

PHILOSOPHY OF CENTRIFUGAL PUMP DESIGN

R0 Lo Daugherty stated in 1915sThe design of a centrifugal pump impeller is

ultimately based upon the performances of other impellers„ The theory indicates what would be the general effect of altering certain dimen­sions » Hence successful design consists of modifying or changing the design of impellers which have been tested out rather than the creation of entirely new patterns0(5)

This approach has not changed greatly in the past fifty yearso Some sixty percent of all pumps in use today are centrifugal pumps» More than three-quarters of these are in a head and capacity range that can be met by currently standardized end suction pumps0 From a practi­cal engineering standpoint, most pumping requirements may be satisfactorily met by a judicious grouping of standard­ized componentso As an example, given one hundred sizes of centrifugal pumps, using interchangeable parts, more than 60,000 different standard combinations can be devel­oped to fit specific needso

As a result of this tremendous diversification of available pumps, current design practice is to:

3

(1) Establish the required head and capacity con­ditions = Once established, these are plotted, head versus capacityo

(2) Determine a suitable specific speed* Nor­mally, the specific speed will be as large as possible, since a larger specific speed permits smaller pumps and cheaper drivers, (or motors), to be used* Considerations involved in selecting the specific speed include the type of driver available, the efficiencies to be realized at different specific speeds, and the number of.pumping stages to be used*

(3) The next step is usually a search through pump catalogues to select an impeller whose performance characteristics match those desired at the same specific speeds*

' (4) If the desired performance characteristics cannot be met with an existing impeller, one can be found normally whose head versus capacity curves at the desired specific speeds parallel those developed from requirements* By the use of affinity laws, a reduction or multiplication of the characteristics of the existing impeller is then made to develop the desired item*

When no satisfactory model impeller is available, a snews impeller must be designed* Even then, the design is based almost solely upon experimentally established

factors= These have been developed from successful impel­lers and will provide direct relationships of expected performance to the size and geometry of the desired impel­ler.

*»New® impeller design begins with estimating a head versus capacity curve for the desired specific speed, lextp the impeller vane discharge angle is selected. This is normally about 22| % plus or minus 5*, determined from previous impeller designs, i. speed constant or head coefficient, based on empirical data, is used to define the relationship between the pump total head and the impel­ler peripheral velocity. By use of the speed constant, the impeller diameter, number of vanes, relative diameters of the inlet to the periphery, angle of vanes at the inlet, and the profile of the impeller may be determined. All of these determinations normally are based upon empirically developed constants and graphs.

The effects ©f leakage past the impeller due to the required clearances between the impeller and the eas­ing are neglected. These effects are presumed to be negligible, or are accounted for in the empirical con­stants used. Losses which might be attributed to the fric­tion between fluid and impeller during fluid passage, and losses due to Reynold8 s lumber effects are likewise neglected.

This brief summary ©f the current philosophy of centrifugal pump design has been drawn largely from Stepanoffy who remarks:

In connection with the design procedure out­lined in this chapter (Specific Speed and Design Constants) it should be noted that if all new — impellers.were built by multiplication of exis­ting types9 there would be no progress in per­formance of pumpso Designing new impellers from basic design always involves some degree of experimentation,, It depends upon the skill of - the designer to sort out values leading to opti­mum efficiency0

CHAPTER 3

DESIRABILITY OF FLOW VISUALIZATION

The previous chapter dwelt heavily upon the empir­ical nature of all centrifugal pump designo The point that losses occurring within the pump were grouped together, rather than identified and analysed as being specifically the result of leakage, friction, or Reynold?s Number effects, was also stressed*

As with most mechanical developments, centrifugal pump evolution has preceded the development of its mathe­matical analysis* The *®eut and try* approach has suc­ceeded in gradually raising pump performance to the point that pump efficiencies of 90% have been attained* This success of empirical design may be attributed to the fact that the centrifugal principle is as simple as swing­ing a bucket of water in an overhead arc* The absence of mathematical analysis is simply because the actual fluid flow through the pump virtually defies description* Fluid enters the pump in the turbulent regime* It undergoes a 90d change of direction* Patterns of circulation are set up as it becomes even more turbulent by the acceleration as a result of the centrifugal action* It then changes

direction by another 90® and is decelerated while the pres­sure increaseso 411 this time, it is being rotated at high speedso

To date, no mathematical analogy of this somewhat violent flow has been developed for the three dimensional case* Where attempts have been made, in a two dimensional effort, the final product has included empirical assump­tions as a means of relaxing conditions and of effecting

(7)simplification to permit analysis„A thorough theoretical solution to the flow path

of the fluid during its passage from inlet, through the impeller, and past the pump outlet does not appear to be imminently forthcoming« Hence, any fresh approach to pump design or even to further improvements in efficiency will probably be based largely upon experimental analysis*

Efforts to increase the already relatively high efficiency of all centrifugal pumps, and particularly to develop small light-weight pumps of increased capacity and head for space age application, would be aided by a better understanding of the internal flow* With such an increased understanding, efforts could be concentrated upon isolating and systematically decreasing the losses known to exist, but which have not been evaluated* It would appear that as detailed knowledge develops, small but consistent improvements in overall pump performance

may be realized* In addition, this better understanding should lead to more accurate prediction of the capabili­ties of a new impeller design* The attendant advantages of more precise and accurate design procedures are obvi­ous o

To this end, considerable time and effort have been expended to visualize the actual fluid flow through a centrifugal pump* In the remainder of this paper, pre­vious attempts to visualize this flow will be discussed*. 4 simple technique, believed to be unique, will then be presented* This technique permits visual investigation and recording of the fluid flow under normal pump opera­ting conditions*

CHAPTER 4

PREVIOUS ATTEMPTS TO VISUALIZE FLOW PATTERNS

Any search into efforts to visualize fluid flow will uncover a great variety of techniques which have been usedo Probably one of the most popular techniques today is the use of hot wire anemometers. This technique^ how­ever, is particularly adapted for use with slow moving fluids. It was believed that this method would not be practical inside a pump.

In 1931$ photographs of the flow within a centri­fugal pump were made by injecting a dark dye. The inlet to the pump was sealed to the glass front shroud of the impeller. The camera was rotated with the impeller. A stroboscopic flash effect was obtained by rotating a reflecting prism synchronously with the impeller. These early photographs showed clouds of dye following both the leading and trailing edges of the v a n e s . T h e lack of turbulence in these pictures suggests that pump speeds were probably quite slow.

The use of pitot tubes and pressure taps placed at various locations in a pump have aided in the determin­ation of flow patterns. This approach has indicated the

10

11pressure difference existing between the leading and trail­ing edges of the impeller* As would be expected, higher pressures are on the leading edge rather than on the trail­ing edge* In addition, the pressure at each edge increases radially outward from the eye of the pump toward the impel­ler t i p s 51

While the use of pressure taps has permitted con­siderable theory to be developed regarding the flow path taken by fluids as they pass through a centrifugal pump, the flow patterns which have been developed in this manner tend to be idealized*

An interesting approach was tried in 1955 with the use of an oil soaked cigar as a smoke generator* The smoke patterns produced within an ®empty® but rotating centrifugal pump were photographed*^"^ While turbulence was shown, it was felt that direct application of these results to water flow through a pump required a great number of assumptions due to the differences in viscosity and density*

Osborne and Morelli obtained photographs of the flow within a transparent impeller using an immiscible solution of dibutyl phthalate dissolved in kerosene* The impeller was submerged in a large tank of water and rotated at 150 revolutions per m i n u t e * ^ At these low speeds, little turbulence was developed, and this method did not

12realistically reflect any effects ©f introducing the fluid into the impeller, as in a normally operating pump. Fur­ther, no effects of the pump easing were reflected in the flow observed.

In addition to visual studies performed on cen­trifugal pumps, some of the techniques used to visualize flow patterns in basins, tubes, and channels were consid­ered for application to pump studies. These included the use of paper confetti, starch and iodine solutions, ̂ 5) aluminum c h i p s , a n d fluorescein dyes.(^7)

The use of doubly refracting substances such as benz©purpurin, ̂ Bentonite Clay, ̂ and Eosine^O)were considered. Their use would have required the capa­bility of directing a polarized light source completely through the front side of the pump easing, the fluid mix­ture, and the back side of the easing. This technique would provide a visual indication of the stress patterns present in the fluid at the instant being photographed. Although the use of doubly refracting substances has been limited to flow within transparent pipe and in jet streams, it is believed that this application to centrif­ugal pump investigation is feasible.

CHAPTER 5

DESCRIPTION OF EQUIPMENT AND PHOTOGRAPHIC TECHNIQUE EMPLOYED

A. Pump Test EquipmentThe fabrication of the centrifugal pump test facil­

ity was done in conjunction with another Master of Science thesis investigating the effects of varying impeller design upon the flow within a centrifugal pump.

The pump selected was a Worthington model ijCGlA radial flow centrifugal pump driven by a Westinghouse ’’Life Line” constant speed electric motor, design B. The motor was rated at one horsepower, using three phase 220 volt power.

*Figure 1. Pump and Driver

13

14This particular pump and driver combination was

selected for the following reasons:(1) Both were readily available locally and

could be used at no cost.(2) The impeller had an eight inch diameter.

This permitted photographs to be taken through the inletside of the pump casing without obstruction from the inlet pipe.

(3) The impeller thickness, seven-eighths inch, was adequate to permit modification of test impellers.

(4) A variable diameter drive pulley was mountedon the motor. This provided an infinite pump speed selec­tion through the belt drive. Pump speeds could be varied through a range of from 430 to 1210 revolutions per minute.

(5) The pump was originally manufactured for use with dairy products, and was constructed, therefore, entirely of stainless steel. This allowed a wide latitude in considering substances to be injected to visualize the flow, and precluded any rust contamination.

A 175-gallon tank served as a reservoir. An aluminum plate baffle was placed in the tank to minimize turbulence and air entrainment in the water furnished the pump. Connections between the reservoir and the pump were two 6-foot lengths of 2-inch conduits, forming a complete circuit. The discharge conduit contained a flat plate

15orifice which was calibrated to provide a means of deter­mining flow rate using differential manometers, A gate valve was installed at the reservoir end of the discharge conduit to allow the pump head to be varied.

The inlet side of the pump casing was replaced by a solid plexiglass block machined to the internal contour of the original casing. A Ig-inch inside diameter plexi­glass pipe two feet long was bonded to this new casing.An aluminum bracket was added for rigidity. Connection between the plexiglass inlet pipe and the conduit was an automobile radiator hose. This isolated the vibrations of the pump-driver combination, preventing undue stressing of the inlet pipe bond, and assisted in easy disassembly of the pump.

Figure 2. Impellers Used

16Figure 2 shows the final configurations of the four

test impellers fabricated. Impellers a, b, and c were made from aluminum plate. Initially, impellers a and b had simple radial vanes similar to those of c. Impellers b and c incorporate a back shroud. Impeller d is a 65-inch diameter fully shrouded bronze impeller. The front shroud was removed and replaced by one of plexiglass, machined to preserve the original internal contour.

Instrumentation in addition to the previously described orifice included manometers to measure discharge pump head and suction head pressure.

B. Photographic TechniquePhotographs were taken with a Polaroid model 95A

"Highlander" camera. Polaroid closeup lenses three plus two were used. The area photographed was three and one- half by four and one-half inches. Distance from camera to subject (measured to the center of impeller width) was seven inches.

Illumination was obtained from a Tachlite tach­ometer, model 832.

This tachometer produced direct readout of impel­ler speed (rpm) and "froze" the impeller with a brilliant white light from a xenon bulb. The light pulse duration was eight microseconds. The tachometer lamp was triggered by the pump drive wheel by using the external input mode

17

Tachometer

of operation. In this mode, a small light source and a photoelectric cell incorporated into the pickup, shown on the right in Figure 3> were aimed at the pump drive wheel. A l/8-inch wide strip of aluminum foil was cemented to one of the spokes of the pump drive wheel. This actuated the tachometer lamp by reflecting from the light source to the photoelectric cell. Use of this mode provided a more stable apparent "freezing" of the impeller than normally obtained from stroboscopic lights. Any perturbations in pump speed did not change the apparent position of the impeller. By adjusting the position of the pickup head around the drive wheel, any given vane of the impeller could be located precisely.

Type 47 Polaroid film, ASA 3000, was used. This film permitted pictures to be taken with a single pulse of light and had the convenience of ten second development

time. Tests were being conducted at a pump speed of 1000 revolutions per minute9 and all photographs were taken at that speed* This provided one pulse of light approxi­mately each one-sixteenth of a second* Since the inten­sity of illumination could be varied by moving the tach­ometer lamp in relation to the pump, a camera setting of »1** was used* This setting corresponds to f = 8*8 at one- twelfth of a second* The relative times between open shutter and pulses of light assured that at least one pulse would occur while the shutter was open, but never more than two* With the shutter of the camera fully opened, depth of field was very limited* Focusing was correspondingly critical* A cone was fabricated which slipped over the camera lens and supported a threaded rod perpendicular to the lens* This not only helped to main­tain camera distance accurately, but also served as an aiming device*

Co Injection TechniqueThe method of injecting a substance to be photo­

graphed took advantage of the sub-atmospheric pressure condition existing at the center or eye of the impeller* The inlet pipe was modified to accept a piece of brass pipe extending for twelve inches from the inner face of the plexiglass housing toward the inlet conduit« The

19pump end of this brass pipe was fitted with a nylon bush­ing which fitted over a short brass tube screwed into the pump shaft.

Figure 4* Injection Scheme

The substance to be injected followed the path indicated in Figure 4 by the dashed lines from A to B.The brass tube shown protruding from the cone in the fore­ground directed the injected substance into the center drilled pump shaft, thence outward radially into a cavity in the brass cone. This cavity matched up with holes in the impeller which led to outlet holes at the root of the impeller blades. Injection was accomplished by simply releasing the screw hose clamp and inserting the free end of the tubing into the dye. Atmospheric pressure forced the substance into the pump.

20Initial attempts to obtain photographs were frus­

trated by the glare from the highly reflective surface of the back wall of the pump, 4 yellow-green primer was applied which successfully eliminated most glareo The yellow-green was chosen since a supersaturated solution of potassium permanganate was to be used* The turbulence within the pump caused this solution to so completely dif­fuse throughout the pump cavity that no indication of flow was possible0 The water in the pump cavity changed vir­tually instantaneously from a clear to a uniformly light purple solution,

in immiscible solution of Dupont Oil Blue A dis­solved in mineral oil was next tried. It was hoped that this type solution would not diffuse, but would break up into small dark beads moving with the water in the pump.The extreme turbulence, however, resulted in dye par­ticles too small to show up in photographs,

It was noticed that the air bubbles inadvertently admitted prior to dye injection seemed to stand out fairly well, A smoke generator was constructed from an oil soaked, inexpensive cigar wedged into a glass tube and connected to the injection tubing, T/fhen ignited, the cigar burned furiously, but failed to cause the bubbles to stand out more clearly.

21Finallys the back wall of the pump casing was

painted a dull black= The intense white illumination from the tachometer lamp against the black background permitted easy photography of the individual bubbles as they passed through the pump0 The use of air bubbles had the advan­tages of unlimited availability and no system fluid con­tamination 0 The size of the individual bubbles arid the total amount of air admitted could be controlled to some extent by adjusting the hose clamp on the injection tubing0

It is believed that use of a camera having adjus­table time and lens opening settings would give greater flexibility in picture taking. Using such photographic equipment, the depth of field problem would not be as critical as with the Polaroid camera used in this inves­tigation.

CHAPTER 6

RESULTS OBTAINED

Ao GeneralThis chapter will present seme of the phetegraphs

which were obtained using the technique described in the previous chapter. The description of these photographs will emphasize the ability to discern changes in the flow pattern as a result of geometric modifications to the impellers used, While a complete analysis of the entire flow pattern will not be presented, wherever confirmations of particular aspects of centrifugal pump theory are illustrated, they will be pointed out.

All photographs were taken with a pump speed of 1000 revolutions per minute, and the discharge valve was fully opened. As viewed in the photographs, impeller rotation is counterclockwise. Although test runs and photographs were made with each of the four impellers shown in Figure 2 of Chapter 5, no modifications were per­formed on the five-blade impeller. Therefore, no photo­graphs of this impeller will be presented. It is to be remembered that the three-blade impeller was completely

23open, the four-blade impeller incorporated a flat plate back shroud, and the six-blade impeller had both front and rear shrouds»

Figures 5 through 11 are photographs of the three- blade impeller. Figures 12 through 15 are of the four- blade impeller, and Figure 16 is of the six-blade shrouded impeller»

Two general types of photographs will be presented,, Those showing separate bubbles and bubble formations against a black or gray background were taken with a small amount of air being admitted. The pictures which show an abrupt change in appearance from some well-defined bubble forma­tion to a frothy looking mixture were made while the pump was being ^flooded** with air. Those showing the froth were made with the clamp completely removed from the injec­tion hose,

B» Discussion for the Three-Blade ImpellerIn Chapter 4? a brief discussion was given indi­

cating the presence of a region of low pressure along the surface of the trailing edge of the impeller vane. In addition, a boundary layer of fluid would be expected due to the movement of fluid along this surface. Within this boundary layer, the velocity of the fluid toward the impel­ler tip would be relatively slow. Since energy is con­stantly being dissipated within this layer, the flow tends

to become progressively more sluggish as it proceeds out­ward. The definite trace of bubbles observed in Figure 5 just above the impeller vane seems to confirm the presence of this boundary layer.

Figure 5• Three-Blade Impeller,Original Configuration

It was noticed while photographs were being taken that this trail of bubbles, apparently staying in about the position shown in the photograph, was not stationary, but undulated back and forth with respect to the vane.The trail would separate a small distance from the vane, and then move back toward it again. Small perturbations

25in the flow would seem to cause the boundary layer to vary its thickness. This probably caused the undulations observed.

This trail of bubbles, while appearing to remain quite constant, seemed to emanate from the base of the impeller vane, and due to the perturbations, "dance" out­ward along the vane. This continual feeding of the trail from the base of the impeller vane and hence outward appears to be confirmed in Figure 5.

An indication of the generally turbulent nature of the flow through the pump is given by the cloud of bub­bles shown in the upper and upper right of the picture.A pressure differential exists between the leading and trailing edges of the vane, as brought out earlier. A similar pressure difference exists between the trailing edge of one vane, and the leading edge of the adjacent vane. This pressure difference would suggest the possi­bility of a pattern of circulation between the vanes.This, in fact, is the case.^^

Since it was impossible to see the bubbles except under the very brief pulse of light from the Strobotac, it is not known that the cloud in the upper portion of the picture rotated. However, the known presence of a pattern of circulation and the shape of this cloud suggest that the bubbles are in the act of rotating in a clockwise direction.

26

Figure 6. Three-Blade Impeller, Six Holes Through Vanes

Figure 6 shows virtually the same operating con­ditions as in Figure 5- The three-blade impeller was modi­fied by drilling six holes, one-sixteenth inch in diameter, through the vanes of the impeller. Holes were drilled from the leading edge, outward, through the vane, to the trailing edge. These holes were drilled at an angle of approximately 45° with respect to the radial center line of the vane. As seen in the figure, the arrangement of holes was two rows of 3 holes each.

The bubble trace appears to be substantially as shown in Figure 5 in the vicinity of the eye of the pump.

27In the region of the two rows of holes and beyond, however, it is significantly diminished. It is suggested that fluid streamed through the holes, flowing from the higher pres­sure side (leading edge) of the vane to the lower pressure side. Fluid flow through the holes also was aided by centrifugal effects.

This stream of fluid, having disrupted the boun­dary layer, probably increased the rate of flow of fluid adjacent to the vane. The result could very well be the breakup of the boundary layer effects by imparting addi­tional energy to this sluggish part of the flow, and causing its velocity to increase. This would account for the thinning out of the bubble trace from the point of fluid injection into the boundary layer, parallel to the vane and extending outward to the tip.

The region of turbulence between the vanes is quite similar in appearance to that of Figure 5• The swirling appearance seems to have been retained. How­ever, although the general appearance has not changed greatly, the turbulence appears somewhat more orderly as a result of energizing the boundary layer.

28

Figure 7. Three-Blade Impeller, NoHoles, Excess Air Admitted to Pump

The impeller shown in Figure 7 is the same as that shown in Figure 5, that is, the three-blade impeller with no fluid injection holes. This photograph was taken when the air bubble supply was increased to a maximum. A com­parison of this figure with Figure 5 suggests a greater flow of fluid toward the leading edge of the vane due to the presence of a greater concentration of bubbles in this area.

An eddy motion appears in the region of the vane tip on the trailing edge. Since this is a region of very

29low energy, resulting in fluid separation, this eddy action seems logical.

The gray area in the region of the trailing edge of the impeller vane implies that a wake is being formed behind the vane. This wake had a definite effect upon the boundary layer believed to exist there in normal operation. The frothy mixture appears to occur first at the tip of the impeller, and then fold itself around a more dense solu­tion of water. The quantity of air entering this region appears to be small when compared to the region of the lead­ing edge of the vane.

In the upper right corner of the picture, it appears that the mixture outside of the froth and extending into the volute contains considerably more water than does the froth. In this region, the fluid is probably in a rela­tively steady motion as it travels toward the pump dis­charge. This steady motion would tend to thin out the accumulation of bubbles.

The presence of the very frothy region appears to indicate a tendency for fluid recirculation. The presence of the wake on the trailing edge of the vane seems to reinforce the argument in favor of recirculation taking place.

Earlier it was stated that fluid streamed through the holes which had been drilled in the impeller vanes.

30By admitting the maximum amount of air, a clear indication was given that this, in fact, did occur.

Figure 8. Three-Blade Impeller,12 Holes, Excess Air Admitted to Pump

Figure 8 shows the three-blade impeller with four rows of 3 holes each drilled through the vanes as described above. A comparison with Figure 7 indicates no sign of the eddy motion seen previously. It appears that one of the effects of fluid injection into the boundary layer region is to improve flow in the region of the impeller tip.

Again comparing Figure 8 with Figure 7, it appears that the generation of froth was caused by the greater amount of flow occurring along the leading edge of the

31vane compared to the much smaller flow along the trailing edge. This observation tends to reinforce the idea of a relatively "dead** area of sluggish fluid behind the vane.A further indication of this is seen from the fact that the froth line is clearly moved away from the impeller vane by the fluid streams from the injection holes.

A later modification of the three-blade impeller consisted of tapering the trailing edge of each vane toward the leading edge. This configuration is shown in Figure 9*

Figure 9* Three-Blade Impeller,Tapered Tip, No Holes

No modification was made to the leading edge. All holes previously drilled were filled with "liquid" aluminum. Comparing this photo with Figure 5> the bubble trace still shows behind the trailing edge of the vane, but is less

32defined. The previously seen area of turbulence (Figures 5 and 6 ) still exists between the vanes. While still in about the same position, there seems to be a lesser degree of disorder in the pattern, and the concentration appears to be smaller.

The tapering of the impeller vane to bring the trailing edge forward to the leading edge at the tip has long been known to be a method to raise both capacity and h e a d . (21) jn this case, both were raised substantially, and the efficiency increased correspondingly, whereas, in some earlier studies the efficiency dropped considerably. The undulating effect observed earlier, before the trailing edge was shaved off, was still present.

Perhaps the best explanation of the increased per­formance is that the tapering eliminated an area which contributed nothing to performance. That is, the blunt extremity of the impeller before modification could impart no energy to the fluid. This blunt end probably accounted for some friction loss by having a fairly large area in contact with the fluid in the volute and also provided a relatively large "dead1* space. Tapering the trailing edge forward may have caused the virtual elimination of the pressure differential recorded between the leading and trailing edges of the impeller in the region of the vane

33tip. Although this reduction of pressure gradient occurred only at the tip, it must have had sufficient effect to improve pump performance.

In Figure 10, the tapered three-blade impeller had two rows of 3 holes drilled through the vane close to the tip. These holes can be seen, with careful scrutiny, in the figure. As shown in Figure 11, the fluid streaming through the vane occurs approximately normal to the trail­ing edge at the point of emergence. A careful inspection of Figures 9 and 10 would seem to indicate that the angle

Figure 10. Three-Blade Impeller,Tapered, Holes in Tip

of slip, or flow direction with respect to the vane is angled somewhat in the counter-rotation direction in

34

Figure 11, Three-Blade Impeller,Tapered, Holes in Tip,Excess Air Admitted

Figure 10, It is believed that the presence of the holes and the path of the fluid passing through the holes might have caused this. In Figure 11, as in Figure 8, the froth formation boundary is moved well rearward of the tip of the impeller vane.

As inferred earlier in this chapter, the purpose of drilling the holes through the impeller was to add energy to the boundary layer along the trailing edge of the impeller vane. The pattern of fluid streaming through the holes seems to indicate clearly that this boundary was broken up. However, to add more energy to the boundary layer, it appears desirable to have the holes exit more

35nearly parallel to the trailing edge of the vane* In so doing, the fluid coming through the holes should replace the slower moving boundary layer with a stream of rapidly moving fluid„

Two distinct results were noted with the drilling of the holeso When the holes were placed fairly close to the eye of the pump, in the manner of Figures 6 and 8, pump performance was degradedo Lower capacity, heads, and efficiencies were obtained* This degradation was consis­tent in that, as the number of holes was increased, all three performance parameters decreased* However, with the holes drilled near the tip, as shown in Figures 10 and 11, an increase in performance was obtained* 411 measure­ments of head, capacity, and the resultant efficiency determinations were based on operations with no air being deliberately admitted*

C* Discussion for the Four-Blade ImpellerFigure 12 shows the four-blade impeller in its orig­

inal configurat ion * No holes had been drilled, and the trailing edge was straight * The bubble trace expected along the trailing edge of the vane does not show as dis­tinctly as with the open three-blade impeller of Figures 5 and 6* In addition, the area of general turbulence appears to be concentrated closer to the eye of the pump, and does not have the ragged appearance of that shown in Figure 5°

36

J

Figure 12. Four-Blade Impeller,Original Configuration

The presence of the back shroud, rotating with the impel­ler, is believed to be instrumental in tending to confine this turbulence. In addition, the addition of another impeller vane to more closely direct the fluid radially should result in a less turbulent flow pattern than observed with the three-blade impeller.

It was noted that head, capacity, and efficiency were all less with this impeller than with the three-blade impeller under the "no modifications" condition. This was anticipated to some extent, since the vanes of the four- blade impeller were wider than those of the three-blade design. This larger amount of metal in the impeller

37displaced an equivalent amount of fluid, decreasing the capabilities of the pump, while also providing more "dead" space effects.

To induce the fluid flow between the impeller vanes to more closely follow the contour of the vanes, the trail­ing edge of the vanes of the four-blade impeller were under cut, or hollowed out as shown in Figure 13. The bubble

Figure 13. Four-Blade Impeller,Trailing Edge Modified

trace tends confirm that this did occur. As a result of this modification, pump performance increased. This increased performance is believed to have been caused by the resulting more uniform velocity distribution and a decrease in fluid recirculation.

Figures 14 and 15 compare the four-blade impeller both before and after hollowing out the trailing edge of the vane with the pump "flooded" with air. As in similar

Figure 14. Four-Blade Impeller,Original Configuration,Excess Air Admitted

pictures of the three-blade impeller, (Figures 7# 8, and 11), the leakage between the impeller vanes and the casing face is demonstrated. The most apparent difference between Figures 14 and 15 is the clear shift of the boundary of the frothy region away from the vane with the scooped out con­figuration. It appears that the predominantly water por­tion of the flow, (as compared to the frothy portion), in Figure 15 is directed more nearly radially outward in the region between the vanes.

39

Figure 15• Four-Blade Impeller,Trailing Edge Modified,Excess Air Admitted

The right center of Figure 14 and the lower right portion of Figure 15 show an area of extreme turbulence.To the naked eye this appeared to be almost a pocket con­taining air only. In Figure 15, this pocket seems to be a bit closer to the eye of the pump, probably because of better flow conditions with this impeller configuration.No explanation is offered for this rather interesting area of the flow pattern, but it should be noted that it occurred only with the impellers having a back shroud.

40D. Discussion for the Six-Blade Impeller

Figure 16 shows the six-blade, fully shrouded impeller.

Figure 16. Six-Blade Impeller

As anticipated from current centrifugal pump theory, and also from the results with the four-blade impeller, the fluid flow tended to follow quite closely the contours of the vanes, at least to a greater degree than observed pre­viously. The presence of a clockwise circulatory pattern appears between the leftmost pair of vanes. The different quantities of air appearing between the vane pairs is caused by variations in the size and relative positions of the air admitting channels used with this impeller.

GHIPTSR 7

CONCLUSIONS

With this discussion of some of the photographs taken during this investigations, it is not presumed that a complete analysis of the flow pattern inside the centrif­ugal pump has been presented» This photographic technique is not an end in itself, but must be coupled with the actual measurements necessary to determine pump perfor­mance » It is felt, however, that this technique, and the photographs presented in the preceding chapter can be a valuable tool to future investigators„

This thesis project has enabled visual verifica­tion of some aspects of current pump theory0 These accom­plishments have been made while operating a centrifugal pump under conditions which are believed to have realis­tically approximated normal pump operation. In this respect, the technique is believed unique, 4s cited in Chapter 4» previous attempts to photograph the flow within a centrifugal pump have been at abnormally low speeds.In some cases, the impeller was not realistically housed in a pump easing.

41

42From a practical poimfc of view, the tapering of the

impeller vanes appears to offer the greatest single gain in pump performanceo The six-blade impeller introduced arti­ficialities in that it was a 6-l/2-inch impeller placed into a housing meant to accommodate an 8-inch impeller0 In addition, the relatively thick plexiglass front shroud eliminated a considerable amount of space normally occu­pied by the fluid being pumped,

Ihile the photographs do not at this point allow any sweeping statements to be made regarding the optimum configuration of an impeller, it is believed that by following the technique outlined herein, specific areas of research may be pinpointed. Further investigations may be made with a greater facility than possible without the use-of photographs. The technique is believed to be inherently adaptable to a wide range of investigations with centrifugal pumps.

CHSPTER 8

SUGGESTED AREAS FOR FURTHER INVESTIGATION

This thesis has been limited to developing a tech­nique which would permit the visual recording of fluid flow within a centrifugal pump. Included in this scope was the collection, fabrication, and assembling of the necessary pump and ancillary components,. During this effort, and that of a concurrent thesis project, it became apparent that the test facility possessed a potential which would not be fully developed®

While it is difficult to determine the ultimate worth of the current test facility for experimental work, it is believed that pursuit of the following suggestions will prove to be of interest and value® All suggestions take into account such mundane, but necessary, considera­tions as cost and the availability of shop facilities®In each case, the modification suggested could be incor­porated into the present assembly at a cost which probably would not exceed twenty dollars for new materials® All work required could be done using the currently available facilities of the Aerospace and Mechanical Engineering

43

44Department» Fabrication times in the order ef ten to twenty hours per modification are believed, realistic«

4 section of the stainless steel casing and the pump mounting bracket could be removed and that portion of the casing replaced with a clear substance, say plexiglass» A segment encompassing about 120s could be so modified without weakening the pump and support significantly o It would then be possible to view through a major section of

' the pump during operations» The use of Eosine or Benton­ite Clays in conjunction with a polarized stroboscopic light should result in visible stress patterns being devel­oped »

Investigations of the effects of varying the num­ber, the configuration, and the surface roughness of impeller vanes could be conducted with relative ease* It is suggested that a basic vane configuration be determined, and a small mold be made in which to cast individual vanes„ These vanes could then be mounted on a thin circular plate to form a semi-shrouded impeller0 This approach could lead to a determination of the optimum number-of vanes to be used for greatest pump capacities, efficiencies, or headso Studies also could be conducted on the effects of varying the entrance and vane tip angles while maintaining a given vane design* The casting mold could next be modi­fied to see if an optimum vane configuration can be

45establishedo In addition* the surface finish of the lead­ing and trailing edges of the vanes could be variedo kn investigation following these lines of research might help to evaluate the effects, of friction between the fluid and the impeller vanes* and could lead to a determination of the magnitude of friction losses=

Another relatively simple modification could be per­formed in conjunction with the vane investigation suggested above. The effects of contouring the back shroud to direct the fluid more smoothly into its first 90° change of direc­tion* as opposed to its impacting normal to the shroud* could be determined. This could lead to establishing an optimum shroud contour for particular pump parameters*(head* capacity* or efficiency). The use of the locally available Cincinnati lathe with the Hydroguide attachment is recommended.

The present plexiglass inlet side of the casing is believed to be substantially thicker than required to resist the pressures developed in the pump. By means of the lathe with the Hydroguide attachment* the internal con­tour of this part of the casing could be varied as desired.

The viscosity of water being pumped could be varied by a factor of at least two by varying its temperature within easily attained limits. An immersion heater would be required in the reservoir to elevate water temperature.

46Blocks of ice added to the reservoir could adequately depress the temperature» It is recommended that a screen be placed over the reservoir outlet to prevent ice from marring the plexiglass portion of the pump casing»

Prerotation might be observed simply by partially closing the gate valve at the reservoir outlet» Tufts of yarn or thread cemented to the inside of the pump inlet pipe -would probably give the best visual indication.

More sophisticated studies regarding cavitation could be undertaken by sealing the system and evacuating a portion of the air above the water level of the reservoir. The reservoir could be covered by a flat plate. Gaskets would be required around the rim of the reservoir and around the pump discharge pipe. mGm clamps could secure the plate and adequately compress the gasket around the rim to preclude the admission of air. All pipe joints should be checked for leaks.

All research done during this project used.a pump speed of 1006 revolutions per minute. The variable speed capability of the pump and driver combination could be incorporated into each of the above suggestions. In addi­tion, the speed range could be greatly extended by obtain­ing pump drive wheels both smaller and larger than that now being used. Ho other modifications would be necessary to gain this extended speed range.

47The present configuration of the pump test facility

is conducive to further investigations, with only the most basic and minor modifications« It is believed, however, that a wide range of extensive and more sophisticated investigations may be opened up by incorporating the sug­gestions listed® Undoubtedly, further investigations will suggest further modifications and areas of interest to be investigated® The limits to which the existing facility may be modified cannot be predicted® Likewise, the degree to which the photographic technique may be exploited is unknown® Such limits and degrees will be governed only by the desires and imaginations of future investigators ®

LIST OF REFERENCES

lo Centrifugal Pump ProblemsGardner-Denver Company, October 1936.

2<> Finch, Volney C o , Pump Handbook0 Millbraes National Press, 1948o

3o 100 Y e a r s 1840-1940, Worthington0 Worthington Pump and Machinery Company, 1940c

4= Karassik, lo Jo, and Carter, R0, Centrifugal Pumps0 New York: F» W® Dodge Corp., 19o0o

5o Daugherty, R® L®, Centrifugal Pumps® New York: McGraw-Hill Book Go®, 1915®

60 Stepanoff, 4® J®, Centrifugal and Axial Flow Pumps®New York: John Wiley and Sons, 1957®

7® Streeter, V® L®, Fluid Mechanics® New York: McGraw- Hill Book Co®, 1952®

8® Fischer, K®, Untersuchung der Stromung in einer Zentrifugal Pumpe9 Mitteilungen des Hyd® Inst® der Technishe Hochsehule, Munich, 1931, as referenced by Macmeeken, J® W®, Turbulence in Centrifugal Pumps® Trans® of the ASME, 1932, Hyd® 54-4, PP® 47-64®

9® Binder, R® G®, and Knapp, R® T®, Experimental Deter­mination of the Flow Characteristics in the Volute of Centrifugal Pumps, Trans® of the ASME, 1930, P° 649®

10® Deck, J® F®, Investigations Concerning Flow Conditions in a Centrifugal'Pump and the Effect of Blade Loading • on Head and Slip® Inst® of Mech® Eng® Proc®, Jan® 1951, pp® 1-15°

11® Ellis, G® 0®, A Study of Induced Vorticity in Cen­trifugal Compressors® Journal of Eng® for Power, Jan® 1964, pp® 63-760

48

12.

13 <»

15 -

16.17 o

18.

19 o

20 c

2 1 c

49Herzigj, H= Z.j, and Hansen, Ju Go 9 Visualization Studies of Secondary Flows with Application-t© Turbo- machinesc Trans» of the ASME9 1955 a PP ° 249-266»Osborne, "W= G=, and Morelli, Do A., Head and Flow Observations on a High-Efficiency Free Centrifugal- Pump Impellerc Trans, of the ASME, 1950, pp. 999-1007 oDornaus, Wo L., Flow Characteristics of a Multiple- Cell Pump Basin9 .TranS. of the ASME, 1958, pp. 1129- 113 7 oKolin, A., Demonstration of Parabolic Velocity Dis­tribution in Laminar Flow. Am. Journal of Physics7 1953, pp."''6i9V62̂ :General Discussion of Heat Transfer. ASME, 1951, pp. 218—219oArons, A. Bo, Ingersbll, A. P., and Green, T. III, Experimentally Observed Instability of a Laminar Ekman Flow in a Rotating-Basing Tellus, Feb. 1961, pp. 31-39°Binnie, A» M o , and-Fowler, J. S o , Turbulence in a Long Circular Tube, Proco of the loyal S©e» of London, 23 Dee. 1947, pp. 32-44°Ullyot, P., Investigation of Flow in Liquids by Use of Birefringent Colloidal Solutions of Vanadium Pentoxide9 Transo of the ASME, 1947, pp° 245-252.Reynolds, A. J., Observations of a Liquid into a ■ Liquid Jet. Journal"of Fluid Mech., 1962, pp. 552- 557°Kristal, F. A=, and Annett, F. A., Pumps. New York: McGraw-Hill Book Co., 1953°