1、- NASA TECHNICAL NOTE N o* m P n z NASA TN D-4592 L. / -I“ LOAN COPY: RETU!?N IO AFWL (WLIL-2) KIRTLANO AFB, N MEX CAVITATING PERFORMANCE OF TWO LOW=AREA=RATIOWATER JET PUMPS HAVING THROAT LENGTHS OF 7.25 DIAMETERS NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. MAY 1968 c Provided b
2、y IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-TECH LIBRARY KAFB. NM 0333049 NASA TN D-43YZ CAVITATING PERFORMANCE OF TWO LOW-AREA-RATIO WATER JET PUMPS HAVING THROAT LENGTHS OF 7.25 DIAMETERS By Nelson L. Sanger Lewis Research Center Cleveland, Ohio NATIONAL
3、 AERONAUT ICs AND SPACE ADMlN ISTRATION For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 - CFSTI price $3.00 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-11111111111111 1111 Provided by IHS
4、Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-. CONTENTS SUMMARY . INTRODUCTION MECHANISM AND ANALYSIS OF CAVITATION . Mechanism of Cavitation in Jet Pump Flow Analysis . Previously reported analyses Present analysis . APPARATUS AND PROCEDURE . Apparatus Experime
5、ntal Procedure Cavitation Criteria . Air content Incipience . Time delay effect . RESULTS AND DISCUSSION Noncavitating Performance . Overall Cavitating Performance Effect of Flow ratio Effect of nozzle spacing . Photographs of cavitation . Prediction Parameters . Cavitation prediction parameter . Ef
6、fect of nozzle spacing Comparison with previously reported results . Alternate cavitation prediction parameter SUMMARY OF RESULTS . APPE NDME S A-SYMBOLS B .DEVELOPMENT OF JET PUMP CAVITATION ANALYSES I. Gosline and OBrien Analysis 11. Rouse Parameter . III. Bonnington Modified Rouse Parameter . IV
7、. Cavitation Prediction Parameter V . Alternate Cavitation Prediction Parameter . REFERENCES Page 1 2 7 7 9 9 9 10 10 10 10 12 15 16 18 21 21 22 22 24 26 28 30 30 32 32 33 35 36 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-CAVITATING PERFORMANCE O
8、F TWO LOW-AREA-RATIO WATER JET PUMPS HAVING THROAT LENGTHS OF 7.25 DIAMETERS by Nelson L. Sanger Lewis Research Center SUMMARY Cavitation performance (total headrise as a function of pumped fluid inlet pressure) of two jet pumps was evaluated in a closed-loop facility using room-temperature, deaer a
9、ted water. Objectives of the investigation were to study the cavitation performance of jet pumps having low ratios of nozzle to throat area and to examine methods of cavitation prediction in jet pumps. Experimental performance was obtained with two nozzles operated separately in one test section. Th
10、e test section had a throat diameter of 1. 35 inches (3.43 cm), a throat length of 7.25 diameters, and a diffuser included angle of 86 (0.141 rad). The nozzles had exit diameters corresponding to nozzle- to throat-area ratios of 0.066 and 0.197. Each nozzle was operated at three spacings of the nozz
11、le exit from the throat entrance. At each nozzle spacing, tests were conducted at four values of seconary- to primary-flow ratio, while secondary (pumped fluid) inlet pressure was varied. Extensive amounts of cavitation were observed before performance was affected. However, when the head ratio dete
12、riorated, it did so quite sharply. At a fixed nozzle position, an increase in secondary- to primary-flow ratio resulted in a greater required secondary fluid inlet pressure in order to suppress cavitation. At any fixed flow ratio, less secondary fluid inlet pressure was required to suppress cavitati
13、on as the nozzle was retracted from the throat entrance. For the test section considered in this investigation, a nozzle spacing of approxi mately 1throat diameter best satisfied the two major performance requirements of high efficiency and cavitation resistance. The design of the secondary inlet re
14、gion was im-Smooth hydrodynamic streamlining of thisportant to jet pump cavitation performance. region and a thin nozzle wall at the nozzle exit would reduce cavitation susceptibility. Two related parameters are proposed which are useful in predicting the conditions at which total headrise deteriora
15、tes because of cavitation. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-I NTRODUCTlON Future space vehicles will require large quantities of electric power. One means of meeting these requirements is through the use of a Rankine cycle system havin
16、g a liquid metal as the working fluid. Jet pumps have several possible applications in such systems (refs. 1 to 3). In order to achieve high system efficiencies, high boiler temperatures and pressures and low radiator temperatures and pressures are necessary. This combination, in addition to a requi
17、rement for low power absorption by the jet pump, results in jet pumps having low ratios of nozzle exit area to throat area (jet pump area ratio R). Jet pumps having low area ratios require a relatively low quantity of flow to be recirculated to the nozzle by the main-stage pump (primary or high pres
18、sure “booster“ flow Q1), thus keeping the main-stage-pump size, weight, and power requirements low. In a previous report, jet pump design considerations were explored, both analytically and experimentally, for the case of noncavitating operation (ref. 3). However, in Rankine cycle space systems, cav
19、itation in the pumps represents a serious problem. Radiator condensate pumps and boiler recirculation pumps must handle fluid quite near saturation temperature. Cavitation can be suppressed by subcooling the fluid. But utilizing subcool ing as the only method of cavitation supression results in an u
20、nacceptable system weight penalty due to the need for additional radiator-condenser sections. One solution to this problem is the use of a limited amount of subcooling and a cavitation-resistant auxiliary pump to boost inlet pressure to the main-stage pump. If a jet pump is used as an auxiliary unit
21、, or in certain applications as a main-stage unit, a knowledge of jet pump cavitation performance will be necessary to optimize system weight and performance. No single method of predicting the cavitation-imposed operating limits of jet pumps has yet been agreed on. The mechanism of cavitation in a
22、jet pump is closely related to the turbulent mixing process. This process is not yet fully understood, particularly for the case of a ducted jet. Jet pump cavitation was first discussed in reference 4 for the condition at which cavi tation caused total headrise to drop off. Limiting secondary (pumpe
23、d) flow Q2 was pre dicted by application of the one -dimensional energy and continuity relations. With room-temperature water as the test fluid, a general but uneven correlation between theory and experiment was achieved. Rouse (ref. 5), working also with room-temperature water, investigated cavitat
24、ion produced by a submerged jet ejecting into a large tank of quiescent water. He was able to correlate audible incipient cavitation at different flow rates by using a conventional cavitation number. In reference 6, Bonnington attempted to modify the Rouse parameter to apply to the ducted flow of a
25、jet pump. His experimental data, which corresponded to the condition of total headrise dropoff and not incipience, did not correlate with the modified Rouse parameter. Contrary to these results, experimental data published by Mueller (ref. 7), also for the condition of total headrise dropoff, agreed
26、 2 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-with the modified Rouse parameter. Other cavitation prediction parameters have been suggested (refs. 8 and 9) but have not been used widely. A summary presentation of these parameters is given in ref
27、erence 7. The present investigation was conducted to study the cavitation performance of jet pumps having low area ratios and to examine methods of cavitation prediction in jet pumps further. Specifically, cavitation performance was investigated in terms of the charac teristics of the jet pump total
28、 headrise at constant values of secondary- to primary-flow ratio M as the inlet pressure of the secondary fluid Q2 was reduced. Although the conditions at which total headrise deteriorated were of principal interest, performance conditions were obtained for all stages of cavitation. Experimental per
29、formance at two area ratios, R = 0.197 and 0.066, was recorded by operating two nozzles separately in one test section. Three different spacings of the noz zle exit upstream from the throat entrance were investigated for each area ratio. De aerated, room-temperature tap water was used as the test fl
30、uid. The acrylic plastic test section was constructed with a circular bell-mouth entry, a constant diameter throat having a length of 7.25 diameters, and a diffuser of 86 (0.141 rad) included angle. Operating conditions included primary flow rates of 33 and 75 gallons per minute (2.08 and 4.7410-m3/
31、sec), secondary flow rates of 85 to 150 gallons per minute (5.36 to 9.4710m3/sec), and secondary inlet pressures of 4 to 25 pounds per square inch absolute (2. 76 to 17. 2X104 N/m2 abs). MECHANISM AND ANALYSIS OF CAVITATION Mechanism of Cavitation in Jet Pump Flow To interpret experimental results a
32、ccurately requires some knowledge of the mecha nism of cavitation inception and development in a jet pump. As defined by Holl and Wislicenus (ref. lo), “The term cavitation shall denote the formation of vapor or gas filled voids within a liquid under the influence of local pressure reductions produc
33、ed by dynamic action. The model of cavitation inception that has gained the widest acceptance is the nuclei theory (ref. 11). Theoretical analyses (ref. 12) predict that a pure liquid can sustain considerable tensile stress before fracturing. Experimental investigations of highly purified and deaera
34、ted water (refs. 13 and 14) have confirmed the existence of liquid tension, but of a magnitude less than theoretically predicted. Other investigations (refs. 15 and 16), which used unmodified water, have reported even smaller tensions,2but still of the order of several pounds per square inch (N/m ).
35、 This inability of a liquid to sustain theoretically predicted tensions has been attri buted to the presence of “weak spots“ or nuclei. A liquid under tension is metastable, 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-and nuclei of sufficient s
36、ize provide the required disturbance to produce instability. The nuclei have been hypothesized as consisting of small volumes of undissolved gas present in crevices of the boundary material and in crevices of microscopic dust particles present in the free stream. A nucleus of sufficient size exposed
37、 to a pressure lower than a critical value grows rapidly. Exposure need only be for short time intervals (e. g. , 10 psec for a spherical bubble 0.001 in. (0.0025 cm) in diameter, ref. 17) to initiate the process. For the case of unseparated flow, in which the minimum pressure occurs at the bound ar
38、y, it has generally been possible, with the exception of certain scale effects (ref. lo), to predict cavitation inception with the aid of the conventional cavitation number. The use of fluid vapor pressure as critical pressure has proved successful for most engineering applications (ref. 11). In sep
39、arated flow and shear flow, however, the minimum pressure does not occur at the boundary but in the shear layer, and experimental results have had to be relied on as the chief source of information. In reference 18, the turbulence level in boundary-layer flow was related to cavitation inception. Inc
40、ipient cavitation was ob served to occur in the center of the boundary layer. This suggests that nuclei were being transported from the wall to the center of the vortical eddies in the turbulent boundary layer. Flow in a jet pump is of the shear type. The primary and secondary fluids are separated b
41、y a shear or mixing layer composed of many small turbulent eddies. The experiments of reference 4 confirmed the existence of low local pressures related to turbulence in jet mixing layers. Cavitation occurs in the mixing layer, and it is likely that the mechanism of occurrence is identical to that o
42、bserved in turbulent boundary layers, with the exception of the source of the nuclei. In the experiments of reference 18, the source of nuclei was the wall next to the boundary layer. In jet pumps, there are two likely sources: free-stream nuclei, and nuclei transported by the boundary layer flowing
43、 over the primary nozzle surfaces. Analysis Because the flow in a jet pump is a shear flow, cavitation inception conditions are not readily predicted. Not enough is presently known of the relation between the minimum local pressures in the mixing layer and important jet pump flow parameters. In jet
44、pump flow, however, cavitation inception is not of primary interest. The conditions at which jet pump total headrise deteriorates as a result of cavitation are the more critical condi tions from the standpoint of design and application. The nomenclature used in the following discussion of analyses i
45、s established in figure 1 and appendix A. The primary fluid Q1 is pressurized by an independent source 4 I Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-t- -Throat -14 -Diffuser + I Secondaryfluid CD-9399 Figure 1. - Schematic representation of jet
46、 pump. and is accelerated to high velocity in the primary nozzle. In a Rankine cycle system, in which the jet pump is used as an auxiliary pump to the condensate pump, the primary fluid is the recirculated fluid, sometimes referred to as the “booster“ flow. The secondary fluid is the pumped fluid an
47、d is entrained by and mixed with the high-velocity primary fluid in the constant-diameter throat section. The mixed fluids pass through a diffuser which converts a part of the velocity head to static pressure. In a Rankine cycle system application, the secondary flow rate is equivalent to the flow r
48、ate through the main cycle. Jet pump performance is commonly expressed by the following parameters: the secondary- to primary-flow ratio, M = Q21; the head ratio, N = (H5 - H2)/(H1 - H5);/Q and the nozzle- to throat-area ratio, R = An/At. Previously reported analyses. - The analysis of reference 4 d
49、id not attempt to account for the character of the mixing process nor conditions at cavitation inception. The analysis is presented in appendix B section I by using nomenclature convenient to this report. Application of the energy and continuity relations to the secondary fluid results in an expression for secondary flow rate. The a
