Indexed in Scopus Compendex and Geobase Elsevier, Chemical Abstract Services-USA, Geo-Ref Information Services-USA www.cafetinnova.org ISSN 0974-5904, Volume 10, No. 01 SPL March 2017, P.P. 18-23 Development and Performance Investigation of Supersonic Blow-down Wind Tunnel KALAKANDA ALFRED SUNNY1, NALLAPANENI MANOJ KUMAR2, HARITHRA. M1, AND RAJAKUMAR S. RAI3 2 1 Department of Aerospace Engineering, Karunya University, Coimbatore-641 114, India Department of Electrical & Electronics Engineering, Bharat Institute of Engineering and Technology, Mangalpally, Ibrahimpatnam, Ranga Reddy-501 510, India 3 Department of Mechanical Engineering, Karunya University, Coimbatore-641 114, India Email: alfredsunny@karunya.edu, nallapanenichow@gmail.com, harithram@karunya.edu.in, rajakumars@karunya.edu Abstract: Wind tunnel is an apparatus used to investigate the aerodynamic behavior of flow over the solid bodies. The importance of this facility is its ability to produce an airstream that simulates real-time condition and its accurate means for aerodynamic research. A supersonic wind tunnel is a type of wind tunnel providing high-speed flow regimes up to Mach number 5, which is helpful in analyzing the flight conditions of space vehicles. In this regard, to analyze the flight conditions in a laboratory level, a supersonic blow-down wind tunnel is developed in Department of Aerospace Engineering, Karunya University. But the major concern is its performance. The performance of this supersonic blow-down wind tunnel is analyzed theoretically and experimentally. Standard methodologies were followed while developing the wind tunnel facility and a brief description is given about its configuration in this paper. Runtime based test was adopted for the theoretical and experimental performance investigation of this facility. Results shows that the developed supersonic blow-down wind tunnel performs better and it is observed that there is no much difference between the experimental and theoretical values. Keywords: wind tunnel, aerodynamic research, supersonic blow down a wind tunnel, Mach number, runtime, mass flow rate. 1. Introduction: The wind tunnel is an experimental setup designed for aerodynamic studies of reduced scaled models, based on achieving geometric, kinematic and dynamic similarities, over full scaled objects. Wind tunnel consists of a tubular passage with the object under test, mounted in the middle. Air is made to move past the object by a powerful fan system or other means. The test object is instrumented with suitable sensors to measure aerodynamic forces, pressure distribution, or other aerodynamic-related characteristics. Application of wind tunnel ranges from aeronautical, automotive, aeroacoustic, aquadynamic to wind study. Wind tunnel is indispensable to the development of modern aircraft. The advances in computational fluid dynamics (CFD) modeling on high-speed digital computers has reduced the demand for wind tunnel testing. However, CFD results are still not completely reliable and wind tunnels are used to verify these predictions [1]. There are several different ways to classify wind tunnels, but they are most often use the ratio of the speed of the fluid, or of any other object, and the speed of sound designated by Mach number in order to emphasize relative importance of compressibility. Each type has its advantages and disadvantages, making certain wind tunnels more suitable for some purposes than others. Wind tunnel that operates at the Mach range of 1.5-5 is called as a supersonic wind tunnel. The power to drive a low-speed wind tunnel varies as the cube of the velocity in the wind tunnel. Although this rule does not hold into the high-speed regime, the implication of rapidly increasing power requirements with test speed is correct. Because of the power requirements, the high-speed wind tunnels are often of the intermittent type, in which the energy is stored in the form of pressure or vacuum and is allowed to drive the tunnel only a few seconds out of each pumping hour. There is a choice in the type of intermittent tunnel to be used at the higher Mach numbers. The operation of the wind tunnel with atmospheric inlet pressure is the in draft tunnel or with atmospheric discharge pressure is the blow-down tunnel. Based on the discussion and available resources, the objective of this paper was to develop a supersonic blow-down type wind tunnel to #021001SPL04 Copyright © 2017 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved. KALAKANDA ALFRED SUNNY, NALLAPANENI MANOJ KUMAR, HARITHRA. M, AND RAJAKUMAR S. RAI serve as an educational and research tool [2] to analyze basic flow principles. 2. Design Methodology of Supersonic Wind Tunnel The intermittent blow-down tunnel usually consists of a basic circuit of the compressor, air storage tank, valves, settling chamber, test section, and exhaust or diffuser. The sizing of components and the selection and matching of components is a large portion of the science of blown tunnel design. Often the design of blow-down tunnel is greatly influenced by some special condition. For example, some major component is already available must be used for economy. In this case, the gas dynamic facility that consists of a compressor and an air storage tank of volume 14.14 m3 with maximum operating pressure of 20 bar which was already available in the laboratory was used. Under such condition, the size, operating range and versatility of the tunnel can be achieved easily. However, the primary consideration in developing the wind tunnel is the design of test section. Designers should make sure that the test section can enclose a relatively large sub-scale model. Therefore, an enclosed conventional solid wall design was chosen. With respect to the available gas dynamic facility a square test section of cross section area 100 Sq. mm was established. The exit of the test section is typically a free jet to the atmosphere through a diffuser. Variable area nozzles that can produce a Mach number range of 1.5 to 5 was designed based on the test section dimensions using the method of characteristics [3-5]. Next, the design for the settling chamber was done. A settling chamber is usually a cylindrical shell, one meter long which accepts the air from the storage tank through a pressure regulating valve which provides a length for settling to obtain uniform flow and then exhausts into the inlet of the nozzle. Based on this principle a settling chamber of 1 meter long and with 34 cm diameter was designed. After designing the components, pipes and valves are needed to connect them. The pipes and valves have to be selected wisely because of the high-pressure losses. The piping should be selected such that for the maximum mass flow, the Mach number will be below 0.4. Also, the pressure drop in the piping and the valves between the storage tank and the pressure regulator valve should be calculated because if the air storage capacity is marginal, this pressure drop may result in a significant reduction in available run time. Taking these into considerations, a piping of 4 inches and a pressure regulating valve of 6 inch with 300 class steel was selected for installation. Characterized by this design methodology, Karunya supersonic wind tunnel was developed. 3.1 Air Supply System The high-pressure air supply comes from two 20 HP electric powered, high-pressure 2-stage compressor. The air is pumped up to 150 psi, which is the maximum pressure of the storage tanks, see in Fig. 1. Motor has totally enclosed fan cooled, Squirrel cage induction type with a speed of 1440 RPM. The overall dimension of the motor is 1725*810*830 mm. Input voltage is 415V and frequency is 50 Hz. Inter-stage coolers and a twintower dryer are used to remove moisture from the air. After passing through an air-cooler, the air is stored in three storage tanks, with a total volume of 14.14 m3. It takes about 4 hours to charge all the storage tanks at the maximum pressure. Fig. 1. High pressure air supply system 3.2 Heat Flow Air Dryers The dryer consists of 2 desiccants towers, a pre-filter, and oil removing filter and after the filter. The wet compressed air enters the pre-filter where the moisture and dust content are removed by 5 microns. The prefilter is fitted with a double seat automatic drain valve. Then the air passes through the oil removing filter where the oil content in the air is removed. Heat flow air dryer is shown in Fig. 2. Fig. 2. Heat flow air dryers 3.3 Air Coolers The working pressure is 30 Kg/cm2 with an operating capacity of 35 Cubic feet per minute. Electric power required for operation is 220 V AC. The model number of the air coolers is AAC-004-6 and is manufactured by GEM equipment, see in Fig. 3. 3. Configuration of Supersonic Wind Tunnel The aerodynamic configuration that is finally selected and developed for the wind tunnel is described below: Fig. 3. AAC-004-6 Air coolers International Journal of Earth Sciences and Engineering ISSN 0974-5904, Vol. 10, No. 01 SPL, March 2017, pp. 18-23 Development and Performance Investigation of Supersonic Blow-down Wind Tunnel 3.4 Storage Tank The size and maximum pressure of the tank, limits the run time of the tunnel. A storage tank was available for the supersonic wind tunnel. This tank is 1.5 m in diameter, 8 m in length, has a volume of 14 m3 and is rated for a pressure of 150 psi. By charging the main storage tanks, it is estimated that the supersonic wind tunnel is capable of achieving the required Mach number. The storage tank shown in Fig. 4. is filled from the high-pressure compressor by two solenoid valves at an air distribution panel. The high-pressure air passes through a pipe from back of the panel to air storage tank. The pressure in the tube is closely monitored during the air filling process. 10 bar. The settling chamber is designed to be 34 cm in diameter and 1 m in length. The maximum flow speed is less than the 80-100 ft/sec as recommended by Pope and Goin [1]. The lower limit is less than the recommended value of 10 ft/sec, which causes some heat convection from the wall to the adjacent air. 3.7 Nozzle The nozzle is hand actuated and has a Mach number range of 1.5 to 4.0. The maximum pressure rating of the nozzle is 200 psi. It is symmetric in the vertical and horizontal direction. The area converges horizontally at the entrance and vertically at the throat. The nozzle consists of a rigid contoured throat section and downstream flexible plates. The throat area can be preset to a fixed position prior to a test for fixed Mach number operation. This provides a uniform exit flow over the entire Mach number range of the nozzle. The nozzle exit has a 100 mm square cross section. Fig. 7. illustrates the nozzle components upper plank and lower plank. Fig. 4. Storage tank 3.5 Gate Valve The storage tank that is located upstream of the supersonic wind tunnel is isolated from the rest of the tunnel by 6 inch diameter gate valve which can regulate pressure up to 300 psi, the gate valve is shown in Fig. 5. Fig. 7. Nozzle upper plank and lower plank 3.8 Test Section Fig. 5. Gate valve 3.6 Settling Chamber A side view of the test section is shown in Fig. 8. The test section is enclosed by a 2 cm casing, with glass windows on the two sides of the test section to provide substantial optical access. A model can be attached to a strut and installed from the top opening of the test section. It is also possible to mount models through attachments in the diffuser. Fig. 8. Test section Fig. 6. Settling chamber The configuration of the settling chamber is shown in the Fig. 6. The settling chamber has a pressure rating of 3.9 Diffuser The diffuser captures the flow from the test section. One of the factors that contribute to an increase in the power International Journal of Earth Sciences and Engineering ISSN 0974-5904, Vol. 10, No. 01 SPL, March 2017, pp. 18-23 KALAKANDA ALFRED SUNNY, NALLAPANENI MANOJ KUMAR, HARITHRA. M, AND RAJAKUMAR S. RAI requirement of a supersonic wind tunnel is the aerodynamic irreversibility in the diffuser. A typical constant area diffuser consists of a conventional subsonic geometry diffuser preceded by a long constant area duct. This type of diffuser shown in Fig. 9. gives nearly the same recompression ratio as a normal shock even though the geometry is quite simple compared with a variable area diffuser. The compression occurs through a system of oblique shocks interacting with the boundary layers. This is not the most efficient way to recover the pressure but it is often more practical and very stable under different Mach numbers. due to choked flow in the throat. Equation. 2 gives the mass flow rate for ideal conditions which is a function of stagnation conditions and gas constant R, which is 287 KJ/kg-K for air.  1 P A *   2   1 m 0   T0 R    1   (2) The throat area was calculated using the Mach-Area relations to determine the throat area as a function of the test section area At = 0.01 Sq. m, using Equation. 3.  1 At 1  2   1 2  2( 1) M )  ( 1    A* M    1 2  (3) Throat area calculations are carried out for Mach numbers ranging from 1.5-3.5 with different stagnation pressures and are tabulated below in Table 1. Table 1: Throat area for different Mach number M 1.5 2.0 2.5 3.0 3.5 Fig. 9. Diffuser 4. Performance Test Wind tunnel runtime is normally the amount of time the wind tunnel operates for a specific Mach number. For any tunnel with limited time operation, the duration of a run is of vital importance, as it imposes limitations on the type of tests that can be performed. A run was considered to start when the valve opened, allowing air to flow through the tunnel. A preliminary runtime calculation was carried out before the design was implemented. After developing, wind tunnel was made to run for different Mach number and stagnation pressure combinations and the time was recorded and the performance is compared. 4.1 Theoretical Runtime The runtime of the blow down supersonic wind tunnel is represented by Equation. 1 and is the function of mass flow rate ṁ, the initial pressure Pi, the final pressure Pf, the initial temperature Ti, the tank volume Vt, polytrophic coefficient n and the gas constant R. Assuming a reasonable 50% pressure loss through the pressure regulating valve the final pressure is obtained. The polytrophic coefficient was chosen to be 1.  P Vt Pi  t  1   f   Pi m RTi  1  n      (1) The first parameter required before for the runtime calculation was the mass flow rate, which is a constant A/A* 1.002 1.687 2.637 4.235 6.789 A* (Sq. m) 9.98*10e-3 5.93*10e-3 3.80*10e-3 2.36*10e-3 1.47*10e-3 Table 2. Represents the mass flow rate values and they are estimated by carrying out calculations for Mach numbers ranging from 1.5-3.5 with different stagnation pressures Po (Po = 4, 6, 8) at stagnation temperature T o (To=300 K). Table 2: Mass flow rate calculation M 1.5 1.5 1.5 2 2 2 2.5 2.5 2.5 3 3 3 3.5 3.5 3.5 Po (bar) 4 6 8 4 6 8 4 6 8 4 6 8 4 6 8 To (K) 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 A* (Sq. m) 9.98*10e-3 9.98*10e-3 9.98*10e-3 5.93*10e-3 5.93*10e-3 5.93*10e-3 3.80*10e-3 3.80*10e-3 3.80*10e-3 2.36*10e-3 2.37*10e-3 2.36*10e-3 1.47*10e-3 1.47*10e-3 1.47*10e-3 ṁ (kg/sec) 9.32 13.98 18.64 5.54 8.31 11.08 3.55 5.32 7.09 2.20 3.31 4.41 1.37 2.06 2.74 Table 3. Represents the theoretical runtime, which was calculated as per the Equation. 1. While calculating this mass flow rate ṁ, the initial pressure Pi, the final International Journal of Earth Sciences and Engineering ISSN 0974-5904, Vol. 10, No. 01 SPL, March 2017, pp. 18-23 Development and Performance Investigation of Supersonic Blow-down Wind Tunnel Table 3: Theoretical Runtime calculation M 1.5 1.5 1.5 2 2 2 2.5 2.5 2.5 3 3 3 3.5 3.5 3.5 V (m3) 14.14 14.14 14.14 14.14 14.14 14.14 14.14 14.14 14.14 14.14 14.14 14.14 14.14 14.14 14.14 ṁ (kg/sec) 9.32 13.98 18.64 5.54 8.31 11.08 3.55 5.32 7.09 2.20 3.31 4.41 1.37 2.06 2.74 Ti (K) 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 Pi (bar) 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 Pf (bar) 6 9 12 6 9 12 6 9 12 6 9 12 6 9 12 t (sec) 25 13 7 42 22 12 65 34 18 104 55 30 168 88 48 4.2 Experimental Runtime The wind tunnel is made to be operated at Mach numbers ranging from 1.5-3.5 with different stagnation pressures Po (Po = 4, 6, 8). At this time the run time of the tunnel was obtained by conducting the runtime test and the experimental runtime values are tabulated below in Table 4. Table 4: Experimental runtime at different mach numbers and stagnation pressures M 1.5 1.5 1.5 2 2 2 2.5 2.5 2.5 3 3 3 3.5 3.5 3.5 Po (bar) 4 6 8 4 6 8 4 6 8 4 6 8 4 6 8 t (sec) 17 10 6 36 14 9 43 23 14 84 34 23 112 65 35 4.3 Comparison of Theoretical and Experimental Runtime It is observed that the tunnel operates at the range as predicted during the preliminary calculations. In addition to this, an account of heat, pressure and friction losses, shockwave formations the wind tunnel show a steady operation at the Mach range of 1.5-3.5. Table 5. Shows the comparison of theoretical and experimental runtime for the designed supersonic blow down wind tunnel. It has been observed that a slight variations and at certain stagnation pressures there is deviation. Table 5: Comparison of Theoretical and Experimental Runtime Runtime in t (sec) P0 (bar) Theoretical Experimental Runtime Runtime 4 25 17 13 6 10 8 7 6 4 42 36 6 22 14 8 12 9 4 65 43 6 34 23 8 18 14 4 104 84 6 55 34 8 30 23 4 168 112 6 88 65 8 48 35 M 1.5 1.5 1.5 2 2 2 2.5 2.5 2.5 3 3 3 3.5 3.5 3.5 5. Investigation and Discussion The variation of mass flow rates and runtime with stagnation pressure was investigated. Fig. 10. shows that the mass flow rate increases with the increase in stagnation pressure. This infers that for a fixed Mach number the mass flow rate can be increased or decreased by varying the stagnation pressure and temperature. Thereby the required velocity can be obtained for a particular test. Mass flow rate (kg/sec) pressure Pf, the initial temperature T i, the tank volume Vt, polytrophic coefficient n and the gas constant R were considered. 20 Mach 1.5 15 Mach 2 10 Mach 2.5 5 Mach 3 0 0 5 Stagnation pressure (bar) 10 Mach 3.5 Fig. 10. Variation of mass flow rate with stagnation pressure Fig. 11 and Fig. 12 shows the variation of run time with respect to stagnation pressure for different Mach International Journal of Earth Sciences and Engineering ISSN 0974-5904, Vol. 10, No. 01 SPL, March 2017, pp. 18-23 KALAKANDA ALFRED SUNNY, NALLAPANENI MANOJ KUMAR, HARITHRA. M, AND RAJAKUMAR S. RAI Runtime (sec) numbers. Reduction in the run time with the increase in the stagnation pressure is seen. This shows that there is always time limitation for higher Mach number tests. In order to obtain high speed for a long time, the tunnel is to operate at the lowest possible stagnation pressure possible. 450 400 350 300 250 200 150 100 50 0 Mach 3.5 Mach 3 Mach 2.5 Mach 2 Mach 1.5 List of Symbols t Vt At A* M ti Pi Pf To Po n γ ṁ R tunnel runtime, sec storage tank volume, m3 test section area, m2 nozzle throat area, m2 Mach number initial temperature in the tank, K initial pressure in the tank, bar final pressure in the tank, bar stagnation temperature, K stagnation pressure, bar polytrophic index(n=1) ratio of specific heats (γ=1.4) mass flow rate, kg/sec gas constant, (R=287 J/kg-K) Acknowledgment 4 6 8 Stagnation Pressure (bar) Fig. 11. Variation of theoretical runtime with stagnation pressure We would like to thank Karunya University for support and fund offered during the establishment of this facility. We also thank V. Kumar, Lab Technician (GRII) and Venkatesh for helping us in setting up the facility. Runtime (sec) References: 120 100 80 60 40 20 0 Mach 1.5 Mach 2 Mach 2.5 Mach 3 Mach 3.5 4 6 8 Stagnation Pressure (bar) Fig. 12. Variation of experimental runtime with stagnation pressure 6. Conclusion A supersonic blow-down wind tunnel, using air from the compressed air plant and exhausting to atmosphere, has been engineered and built based on conventional design principles. Nozzle blocks from Mach 3.5 and parallel duct supersonic diffuser has been installed. Run time varies in seconds for various mach numbers and at stagnation pressures and the Reynolds number can be varied by changing the density level (by changing pressure) in the settling chamber. It has been observed that there is a slight variations in experimental runtimes compared to theoretical runtimes and at certain stagnation pressures there is slight deviation also. However, the developed supersonic blow-down wind tunnel has been operating satisfactorily. [1] Pope A., and Goin K. “High-Speed Wind Tunnel Testing”, John Wiley & Sons; New York, 1965. [2] J. Njock Libii, “Wind tunnels in Engineering Education”, Chapter in Wind Tunnels and Experimental Fluid Dynamics Research, In Tech, 2011. [3] Kelly Butler, David Cancel, Brian Earley, Stacey Morin, Evan Morrison, and Michael Sangenario, “Design and Construction of a Supersonic Wind tunnel”, Major Project Thesis from Bachelor of Science, Department of Aerospace and Mechanical Engineering, Worcester Polytechnic Institute, March 2010. [4] Bhavin K Bharath, “Design and Fabrication of a Supersonic Wind tunnel”, International Journal of Engineering and Applied Sciences, 2 (5), PP. 103107, May 2015. [5] Kenney, J.T., Webb, L.M.: A Summary of the Techniques of Variable Mach Number Supersonic Wind Tunnel Nozzle Design, AGAR Dograph 3,1954. International Journal of Earth Sciences and Engineering ISSN 0974-5904, Vol. 10, No. 01 SPL, March 2017, pp. 18-23