Experimental Performance Evaluation of a Cavitating ...

Experimental Performance Evaluation of a Cavitating Venturi

A. Abedini E., A. Ashrafizade, H. Karimi M. & M. Madandar A.

Arabian Journal for Science and Engineering ISSN 1319-8025 Arab J Sci Eng DOI 10.1007/s13369-013-0662-6

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Author's personal copy Arab J Sci Eng DOI 10.1007/s13369-013-0662-6

RESEARCH ARTICLE - MECHANICAL ENGINEERING

Experimental Performance Evaluation of a Cavitating Venturi A. Abedini E. · A. Ashrafizade · H. Karimi M. · M. Madandar A.

Received: 15 September 2011 / Accepted: 4 July 2013 © King Fahd University of Petroleum and Minerals 2013

Abstract Cavitating venturi is known as a robust mass flow control device in many applications. In liquid propellant engines the venturi provides a constant flow rate of propellant to the thrust chamber in spite of the engine pressure transient and oscillations. An experimental set-up is designed and built to study the performance of a manufactured venturi and to also observe the flow phenomena in the device. The set-up has new features, as compared with the similar test facilities, and hence provides more information regarding the performance of the cavitating venturi. Experimental results agree well with the available data and the venturi maintains the prescribed mass flow rate in a range of working conditions as expected. Keywords

Flow control · Chock · Cavitating venturi

A. Abedini E. (B) Department of Mechanical Engineering, Semnan Branch, Islamic Azad University, Semnan, Iran e-mail: [email protected] A. Ashrafizade Department of Mechanical Engineering, K.N. Toosi University of Technology, Tehran, Iran e-mail: [email protected] H. Karimi M. Department of Aerospace Engineering, K.N. Toosi University of Technology, Tehran, Iran e-mail: [email protected] M. Madandar A. Department of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran e-mail: [email protected]

1 Introduction Cavitating venturi is a simple, reliable flow passage that can be used to control the liquid flow rate in a range of industrial applications. In particular, this devise can be used to meter the flow of propellants in liquid engines [1,2]. In such an application, the cavitating venturi keeps the propellant mass flow rate fixed, even when the downstream pressure changes rapidly due to the start-up transient or combustion oscillations. Figure 1 shows the performance curve of a cavitating venturi. By continuously reducing the downstream pressure, the liquid mass flow rate increases initially. However, for a sufficiently low pressure ratio (point B in Fig. 1), the venturi is choked and downstream pressure can no longer affect the mass flow rate. This means that for a sufficiently high upstream pressure, the mass flow rate is effectively constant across a wide range of downstream pressures. In practice, the venturi is designed to maintain a specified mass flow rate in the operational range of the pressure ratio. There are a number of publications in which the performance characteristics of cavitating venturis are discussed. Xu and Heister [3] used a two-phase flow computational model and found that the venturi’s mass flow rate was a

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Author's personal copy Arab J Sci Eng

age, one-dimensional, context. The experimental set-up and the test procedure are introduced next, followed by a discussion on the test results. The paper is wrapped up with some conclusions.

2 Theory The performance curve of a cavitating venturi was introduced in the previous section. The pressure ratio (PR) is defined as follows: PR = Fig. 1 Performance curve for a cavitating venturi

weak function of back pressure. Sun and Yang [4] studied the flow in a converging–diverging passage, using an experimental set up, and focused on the oscillatory behavior of the flow and erosion of the walls. Ishii and Murkami [5] conducted a series of experiments on the flow of liquid Helium in a cavitating venturi with a rectangular cross section. They used high-speed camera pictures to investigate the onset and collapse of cavitating bubbles. Kumar and Pandit [6] found out that the location of the onset of cavitation depended on the cavitation number. At high cavitation numbers, cavitation started near the throat, but at low cavitation numbers bubbles were generated far downstream of the throat. The intensity of cavitation was found to be related to the pressure level. Jarman and Taylor [7] investigated the light flashes and shocks due to the cavitation in cavitating venturis. Preston et al. [8] developed a numerical model and identified four different flow regimes in a cavitating venturi at different back pressures. An experimental study at a single pressure ratio was also conducted by Ulas [9]. This recent experimental study proved that cavitating venturis could be used, reliably, to control the propellant mass flow rate in a liquid engine. The present paper provides more experimental results regarding the performance of cavitating venturis and may be considered as a companion to the experimental study reported in [9]. While a venturi similar to the one reported in [9] is used in this study for the purpose of comparison, attempts have been made to broaden the range of the variation of parameters and to also provide more detailed information. For example, 600 L of water is stored at a pressure as high as 100 bars and the venturi is transparent such that the flow regimes and the cavitation phenomenon can be easily observed. The duration of the tests, therefore, can be considerably higher than the test durations reported in [9] which can help stabilize flow and measurements are so accurate. The next section briefly reviews a simple theory which predicts the performance of a cavitating venturi in an aver-

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Po − Ps Pi − Ps

(1)

where Po is the venturi’s outlet (downstream) pressure, Pi the venturi’s inlet (upstream) pressure, and Ps the saturation pressure corresponding to the working fluid temperature. The pressure ratio corresponding to the point B in Fig. 1 is called the critical pressure ratio. Assuming one-dimensional flow in thermodynamic equilibrium state, the static pressure is related to the velocity, and, therefore, mass flow rate, through the Bernoulli’s equation. Hence, the choked mass flow rate at the throat can be easily calculated using an equation based on the Bernoulli’s equation corrected by an experimental factor K: (2) m˙ choked = K A 2ρf (Pi − Ps ), in which A is the throat area, K the inlet loss coefficient, ρf the fluid density.

3 Experimental Set Up and the Test Procedure The venturi is designed to pass 14.4 kg/s of water when the upstream pressure is 100 bar. The throat diameter is 11.3 mm [9]. The half angles of the converging and diverging parts of the venturi are the nominal values of 15 and 7, respectively [9–12]. To be able to observe the flow details in the venturi, it is made up of Plexiglas. Care is needed to construct a sufficiently strong, accurate, smooth, and transparent venturi as shown in Fig. 2. The test set-up is shown in Fig. 3 and consists of the following main parts: 1. 2. 3. 4. 5. 6. 7. 8.

Pressurized air tank Regulator Water tank Solenoid valve Flow meter Pressure gauge Cavitating venturi Hand valve


Fig. 2 The transparent (Plexiglas) venture Fig. 4 Vapor formation at fully open valve

the pressure ratio and, consequently, the flow pattern inside the venturi. Each test starts by setting up the tank pressure first and then the solenoid valve is activated. Data collection starts afterwards using a flow meter and pressure gauges. Tests are conducted at various tank pressures and different downstream pressures controlled by the exit hand valve. The features of the experimental setup used in this study as compared with the one used in [9] can be summarized as follows:

Fig. 3 Schematic of the test set-up

High-pressure air is stored in a large tank. Regulator is located after pressurized air tank and before water tank to adjust the pressure inside the water tank. Water tank is made of thick steel and its volume is about 600 L. A pressure gauge is used to measure the pressure inside the water tank. A solenoid valve, which only has two on and off working conditions, is followed by a flow meter and a pressure gauge and connects the water tank to the cavitating venturi. Flow meters and gauges are located far from the valves such that flow disturbance does not deteriorate the accuracy of measurements. The operating frequency of measuring devices is 150 Hz. The venturi is made up of Plexiglas so that the cavitation phenomenon can be observed as shown in Fig. 4. A hand valve at the end of the experimental setup allows changing

1. The pressure inside the water tank in [9] dropped as much as 5 bars during the test period (about 0.3 s). In our test rig, the regulator installed in the water tank keeps the water pressure constant during the test period. 2. The allowable test duration, which was less than a second in [9], can be increased up to 100 s in our test facility. 3. The downstream pressure was fixed in [9]. Here the downstream pressure can be changed continuously during the test for extracting venturi performance curve. 4. The instantaneous mass flow rate can be measured in the test facility introduced in this section as compared with the average mass flow rate reported in [9]. 5. A Plexiglas venturi is used in this study. 6. Provisions are made in the storage tanks and also in the test procedure such that the dissolved air in the water is minimized in our experimental study. 4 Results The data corresponding to the first test are as follows: Water tank pressure = 30 bars Venturi inlet pressure = 17.25 bars Venturi outlet pressure = 1 bar.

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Figure 5 shows the pressure and mass flow rate for the above operational conditions. Note that there are short transient parts in both curves, shown as part I in Fig. 5. The sudden discharge of the water from the tank and the regulator time delay are the main reasons for this transient part which is also associated with a considerable loss of head. The second part of the diagram, part II, corresponds to the steady-state operating condition. It is seen that the mass flow rate is basically constant due to the occurrence of the cavitation, which is observable as shown in Fig. 4. High-frequency oscillations,

Fig. 7 Pressure and mass flow diagrams for variable downstream pressures

Fig. 5 Pressure and mass flow diagrams at fully open valve

Fig. 6 Mass flow diagram Table 1 Comparison of the experimental and theoretical mass flow rates

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with very small amplitude, are due to the upstream pressure oscillations. Figure 6 shows the mass flow time trace. Abnormally high mass flow rate at point A, the start up surge, is due to the measurement of the flow before it enters the venturi and is rapidly damped when the flow is established afterwards. The oscillatory behavior between A and B is due to the regulator time response. The venturi maintains a fixed mass flow rate in the time interval BC and the test is finished when the solenoid valve is closed at C. The experimentally measured steady mass flow rate is compared with the calculated mass flow rate using Eq. (2) in Table 1. It is seen that the simple one-dimensional theory predicts the mass flow rate rather in this test case. The second test is conducted with the following upstream pressure values: Water tank pressure = 90 bar Inlet pressure = 52.18 bar. In contrast to the first test, the outlet pressure in this case is not fixed. Experimental set up allows the user to gradually change the downstream pressure and comfortably investigate the variation of mass flow rate and the flow patterns. The storage tank is big enough to provide high-pressure water for a relatively long test. Figure 7 shows the pressure and mass flow time traces while the exit valve is gradually closing. The downstream

Tank pressure (bar)

Inlet pressure (bar)

Outlet pressure (bar)

Experimental mass Flow (kg/s)

Theoretical mass flow (kg/s)

Error %

28.7

17.25

0.25

5.48

5.78

5


Fig. 8 Enlarged mass flow diagram after critical point Fig. 10 Onset of cavitation at the critical pressure ratio 1

Table 2 Comparison of the mass flow rate in the present work with [9]

Mass Flow Ratio

0.8

0.6

Venturi inlet pressure (bar)

Mass flow (present) (kg/s)

Mass flow (previous [9]) (kg/s)

52.18

9.57

9.7

0.4

0.2 Present Work Navickas [10]

0

0

0.2

0.4

0.6

0.8

1

Pressure Ratio

with the measurements reported in [10]. The onset of the cavitation at the throat, corresponding to the critical pressure ratio, is shown in Fig. 10. Table 2 compares the experimental mass flow rate obtained in this study and the mass flow rate obtained in a similar test reported in [9]. Good agreement between the results is observed.

Fig. 9 The performance curve in the second test case

5 Conclusions pressure continuously changes in the time period between t = 2 s to t = 6.7 s. It is seen that both the venturi’s inlet pressure and the mass flow rate do not change while the venturi is choked. Figure 8 shows an enlarged view of the mass flow diagram. Note that after t = 4.95 s, which corresponds to the critical pressure ratio, the venturi is not choked anymore and the mass flow rate decreases by increasing the downstream pressure. As a matter of fact, the mass flow rate is nearly constant up to a very high pressure ratio (PR = 0.9) in this case. This is higher than the usual values reported in the literature (PR = 0.85) and one concludes that the range of the pressure ratios for which the venturi works properly is broader than the reported values. This might be partially due to the smoothness of the venturi used in our tests and accurate design of exit and entrance corners. The performance curve of the venturi in the second test case is shown in Fig. 9 and the test results are also compared

An experimental set-up was designed, built and assembled to study a cavitating venturi used as a liquid flow control device. The test facility has new and improved features as compared with a previous experimental set up [9]. The venturi was made up of Plexiglas so that the phenomenon of cavitation could be observed. Reliable performance of the cavitating venturi in a range of pressure ratios was observed and the results are in excellent agreement with the available experimental data. References 1. Huzel, D.K.; Huang., D.H.: Modern engineering for design of liquid propellant rocket engines. Astronautics and Aeronautics. American Institute of Aeronautics and Astronautics, USA (1992) 2. Harvey, D.W.: Throttling venturi valves for liquid rocket engines. In: AIAA Joint Propulsion Conference and Exhibit, 22 May 1970, Redondo Beach, CA, USA

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Author's personal copy Arab J Sci Eng 3. Xu, C.; Heister, S.D.: Modeling cavitating venturi flows. Propuls. Power 18(6), 1227–1234 (2002) 4. Sun, B.H.; Yang., W.J.: Oscillatory cavitating flow in a convergentdivergent nozzle. Fluid Eng. 177, 95–100 (1993) 5. Ishii, T.; Murakami., M.: Comparison of cavitation flows in He I and He II. Cryogenics 43, 507–514 (2003) 6. Kumar, P.S.; Pandit, A.B.: Modeling hydrodynamic cavitation. Chem. Eng. Technol. 2(12), 1017–1027 (1999) 7. Jarman, P.D.; Taylor, K.J.: Light flashes and shocks from a cavitating flow. Brit. J. Appl. Phys. 16, 675–683 (1965)

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8. Preston, A.; Colonius, T.; Brennen, C.E.: A numerical investigation of unsteady bubbly cavitating nozzle flows. In: Proceedings of ASME FEDSM’00 ASME 2000 Fluids Engineering Division Summer Meeting, June 11–15, 2000, Boston, MA, USA 9. Ulas, A.: Passive flow control in liquid-propellant rocket engines with cavitating venturi. Flow Meas. Instrum. 17, 93–97 (2006) 10. Navickas, J.; Chen, L.L.: Cavitating venturi performance characteristic. Fluid Eng. Div. Publ. FED 177, 153–159 (1993) 11. http://www.flowsystemsinc.com. Accessed 2010 12. http://www.foxvalve.com. Accessed 2010