Experimental investigations on impaction pin nozzles for inlet fogging system

Transcription

1 Journal of Mechanical Science and Technology 25 (4) (2011) 839~845 DOI /s Experimental investigations on impaction pin nozzles for inlet fogging system Abhilash Suryan 1, Yong Kwan Yoon 2, Dong Sun Kim 2 and Heuy Dong Kim 1,* 1 School of Mechanical Engineering, Andong National University, Andong , Korea 2 FMTRC, Daejoo Machinery Co., Daegu , Korea (Manuscript Received February 21, 2010; Revised December 14, 2010; Accepted January 27, 2011) Abstract Increasing power demands have necessitated the development of energy efficient systems in the industrial sector. At present, about 10% of the overall electric power used by large industrial plants is consumed by high-capacity compressors supplying compressed air. Likewise, in a gas turbine power plant, nearly half the generated power is used for driving the compressor. The work of compression is proportional to inlet air temperature, and cooling the inlet air can save considerable amount of power in large turbo machines during hot summer months. Inlet fogging is a popular means of inlet air cooling, and fog nozzles are the most critical components in an inlet fogging installation. Majority of these installations employ impaction pin nozzles. In the present work, experiments are conducted over a wide range of operating parameters in variable length wind tunnels of different cross sections in order to investigate the performance of impaction pin nozzle in inlet fogging. Flow visualization and measurements are carried out to analyze the fog behavior and identify suitable nozzle locations in typical air ducts. The results show that impaction pin nozzles are suitable for inlet fogging applications. Keywords: Energy saving; Evaporative cooling; Impaction pin nozzle; Inlet fogging Introduction Energy conservation is one of the most economical and environmentally friendly solutions to meet growing energy demands. It can be achieved through the efficient use of energy, wherein the same output is achieved at reduced energy consumption. At present, high-capacity compressors in industrial plants consume large amounts of power. It is estimated that about 10% of total power consumption in such plants is utilized to generate compressed air. Eq. (1) gives the expression for specific work of compression [1]. γ-1 ct γ p 1 p2 w= c -1 η c p1 Specific work of compression, w c, is directly proportional to the inlet air temperature, T 1. As a result, compressor efficiency is reduced during hot summer months when the ambient air temperatures are high. In gas turbine power plants, almost 70% of the work produced by the turbine is absorbed by the compressor. In addition, high inlet air temperature leads to a This paper was recommended for publication in revised form by Associate Editor Jun Sang Park * Corresponding author. Tel.: , Fax.: address: kimhd@andong.ac.kr KSME & Springer 2011 (1) drop in the density of air, resulting in reduced air mass flow rate through the turbine. Overall, the efficiency and power output of power plants are significantly reduced during the summer seasons. Cooling the inlet air can reduce the compressor power consumption and augment the gas turbine power output [2]. Different methods available for inlet air cooling of turbomachinery include refrigerated cooling with vapor compression or absorption systems, thermal energy storage systems, and different types of evaporative coolers. In addition, the selection of a suitable method for a given plant site has to be made based on economic considerations, environmental impact, and the maximum possible power gains attainable [3]. Inlet fogging is a popular evaporative cooling technique. The method is characterized by low initial costs, simplicity of installation, and the ease of operation and maintenance. The principle of an inlet fogging system of an air compressor is illustrated in Fig. 1. In this method, water is sprayed into an inlet air stream in the form of tiny droplets using atomizing nozzles located downstream of the inlet air filters [4]. Fog droplets quickly take up the velocity of air flow within a few centimeters from the nozzle exit and remain airborne for longer durations due to the Brownian movement and random collisions with the air molecules. These create a large evaporative surface area within the intake duct. As they proceed towards the compressor

2 840 A. Suryan et al. / Journal of Mechanical Science and Technology 25 (4) (2011) 839~845 INLET AIR AIR FILTER FOG LINE WEATHER STATION FOG NOZZLE ARRAY FOG EVAPORATES; COOLING INLET AIR COMPRESSOR Fig. 1. Principle of the inlet fogging of an air compressor. Saturation curve 2 1 Evaporative Cooling T Dry bulb temperature w Fig. 2. Psychrometrics of evaporative cooling techniques. inlet, the tiny water droplets absorb heat from the surrounding air and reduce in size due to evaporation. Fog droplets not completely evaporated in the inlet duct are carried into the compressor where the higher temperatures allow greater moisture holding capability for air. These droplets become evaporated in the compressor, which cools the air and makes it denser, thereby reducing power requirement further. Thus, inlet fogging offers greater savings compared with other evaporative cooling methods [5]. The theory behind evaporative cooling techniques is illustrated with a psychrometric chart in Fig. 2. As water evaporates, it absorbs the latent heat of vaporization from surrounding air, thereby cooling it. In the inlet fogging method, the fog provides the cooling function as it evaporates in the inlet air duct. However, the limiting temperature attainable by evaporative cooling methods is the wet bulb temperature () corresponding to given ambient air conditions. Air which is initially at a higher temperature (state 1) is cooled ( T) to the saturation state (state 2) by the evaporation of water spray (Fig. 2). The amount of water required is determined from the corresponding change in humidity ratio ( w). Typically, evaporative cooling systems have an effectiveness of However, inlet fogging systems are known to provide an effectiveness of nearly 1.0, ensuring a relative humidity value of 100% at the compressor inlet [3]. Detailed climatic data analysis is carried out to determine the evaporative cooling potential of any given site to check the feasibility and economic viability of installing inlet fogging system. Climatic analyses of several locations across the world using the concept of equivalent cooling degree hours Humidity ratio (g/kg),, :59 0:59 5:59 5:59 10:59 10:59 TIME TIME (ECDH) have revealed that there is a sizeable evaporative cooling potential all over the world, even in very humid climates [6]. ECDH is obtained as the evaporative cooling degree multiplied by the number of hours that they occur over a given period of time for a particular site. In this work, large amounts of quantum of weather data from different locations in Korea were analyzed on an hourly basis to compute the evaporative cooling potentials and site power gains [7]. Erroneous data were filtered out by ensemble mean averaging. A computer program was written for the estimation of wet bulb depression ( T) and the humidity ratio ( w) based on psychrometric equations [8, 9]. Fig. 3 illustrates the climatic data for two typical summer days in Korea. Evaporative cooling potential is highest when the difference between and is the maximum. Fig. 3 also shows that there is a sizeable evaporative cooling potential at the site, and that the evaporative cooling potential is higher when ambient air humidity is low. The highest evaporative cooling potential during the course of a single day exists around the mid day hours. While and values show fluctuations throughout the course of day, the remains relatively constant. The choice of suitable fog nozzle is critical in converting the entire evaporative cooling potential at the site to power gains. The nozzle should generate droplets that are small enough to fully evaporate before reaching the compressor inlet. Large droplets diminish evaporation efficiency and can cause compressor blade erosion. However, using nozzles with very small orifice diameters can lead to frequent plugging and relatively higher pressure drops on account of greater number of nozzles required. An experimental study on two fluid nozzles has been conducted earlier to verify the evaporation characteristics [10]. Numerous inlet fogging installations employ impaction pin nozzles. The objective of the present work is to experimentally verify the operating characteristics of impaction pin nozzles and determine the cooling achieved by fogging in a typical air 15:59 15:59 2 (a) low humidity 20:59 20: (b) high humidity 2 Fig. 3. Climatic data for two typical summer days in Korea. (%) (%)

3 A. Suryan et al. / Journal of Mechanical Science and Technology 25 (4) (2011) 839~ Inverter Water filter Pump Header Inverter 11 HP 1750 RPM 15 lpm 275 bar Control valve Water Tank Data PC Acquisition System PLC Hygrometer Fig. 4. Typical impaction pin nozzle. duct. Thus, tests were conducted to determine the influence of important parameters such as water supply pressure and flow rate, air flow velocity, and the ambient temperature and humidity on the evaporation of fog droplets and the cooling attained within the duct. A typical impaction pin nozzle, which is made of high grade stainless steel, is illustrated in Fig. 4. Water is forced through a smooth orifice with a diameter of about mm. High velocity jet from the orifice is directed at the impaction pin located downstream and fixed above the orifice on the nozzle body. Impaction of water jet on the pin generates a thin conical sheet of water. The thickness of the water sheet depends on orifice diameter, impaction pin geometry, and supply pressure. As the conical sheet extends away from the orifice, its surface area is expanded and the sheet becomes thinner. Eventually, surface tension forces cause the sheet to break down into fingers of water and aerodynamic instabilities cause theses fingers to break into ligaments and then finally into droplets. Increasing the supply pressure can cause a velocity increase, leading to sheet thinning and, subsequently, decrease in droplet size. Typically, impaction pin nozzles generate fog droplets within the range of 3-30 µm [11]. 2. Experimental setup The experimental set up is described with a schematic diagram in Fig. 5. The set up was installed in an air conditioned space and consisted of a centrifugal blower for supplying air at required flow rate, two separate air ducts of variable length, temperature and humidity sensors, an air system analyzer, water pump, water filter, and water tank. The air ducts had a cross section area measuring 40 x 40 cm and 1 x 1 m. Test nozzles were mounted in co-flow position inside the duct close to centrifugal blower. The nozzles were mounted sufficiently away from each other and also from walls when multiple nozzles were tested. This was done so as to reduce the possibility of droplet collision and coalescence which might create larger droplets [12]. In the system, air blower supplied air in the required conditions at velocities ranging from 0-10 m/s. Air velocities (u a ) were calibrated and measured with an anemometer as well as a pitot tube traverse system. An 11 HP, 15 lpm pump supplied water to fog nozzles. Flow rate was monitored continuously during tests. Water supply pressure (p s ) was varied from 7 Blower Fogging Nozzle Fig. 5. Schematic diagram of the experimental set up. Q w x 10 6 (m 3 /s) Temperature Sensors 0 Fig. 6. Water flow rate for an impaction pin nozzle. MPa to 21 MPa, representing the typical operating range for impaction pin nozzles. An electronic control system maintained steady supply pressure without pressure fluctuation. Temperature and humidity measurements were obtained at different sections downstream of the nozzles. The measurement probe used in the experiments had an accuracy of ±2% from 2% to 98% and ± 0.4 o C between -10 and 50 o C. Temperature sensors might indicate the wet bulb temperatures long before the air is actually cooled to the because of water wetting [13]. Hence, special care was taken to shield temperature sensors from the free moisture. Sintered PTFE filter was used for the purpose. Data acquired were analyzed using an air system analyzer. 3. Results and discussion Experiments were conducted to verify the suitability of impaction pin nozzles for the inlet fogging of an air compressor. Nine different nozzles were tested under same ambient conditions to ensure measurement repeatability. Cooling trends and humidity patterns were obtained. Measurement section was moved by increasing the duct length to ascertain evaporation distance under each test condition based on supply pressure and air flow velocity. Psychrometric computations were also carried out for actual ambient conditions simulated for the experiment. Cooling water requirement for each test condition was estimated from the following equation:

4 842 A. Suryan et al. / Journal of Mechanical Science and Technology 25 (4) (2011) 839~ m/s 3 m/s 5 m/s 7 m/s 9 m/s 10 m/s 1 m w_act / m a Fig. 7. Change in flow rate with water supply pressure. T TEXIT deltat Effectiveness TIME Fig. 8. Temperature and relative humidity variation for different test conditions (p s =14MPa, u a =5m/s) , (%) 1 m/s 3 m/s 5 m/s 7 m/s 9 m/s 10 m/s Fig. 9. Cooling trend with supply pressure: smaller duct (single nozzle). 1 7 MPa 10 MPa 14 MPa 17 MPa 20 MPa 21 MPa u a (m/s) Fig. 10. Cooling trend with air velocity: smaller duct (single nozzle). w, (2) m w= ma 1 +w where m is water mass flow rate, m is the air mass flow rate, w a and w is the change in humidity ratio. Initially, tests were conducted in smaller air ducts (40 x 40 cm) to determine water flow rate through a single nozzle at different operating pressures. Results are shown in Fig. 6. The different water consumption rates for the different test conditions are plotted in Fig. 7. The actual water flow rate m w-act was non-dimensionalized with the air mass flow rate m a. Tests were conducted under different ambient conditions. Fig. 8 gives the variations in temperature and relative humidity for different ambient conditions during the tests. The and the exit temperature of the duct remained nearly constant even when the and values changed. Ambient conditions were maintained nearly constant to compare the performance under different air flow velocities and supply pressures. The results presented herein correspond to ambient conditions of =34.9 o C, =44%. For this condition, was evaluated as o C. Once the air was saturated, no further cooling of air was possible and excess water spray escaped as fog droplets without getting evaporated. Temperature and relative humidity measurements were obtained at different sections along the duct length and along the cross section. Each test was followed by a free blowing of air without fogging to restore the initial ambient conditions. Figs. 9 and 10 give the cooling trends obtained by fogging with a single nozzle in the 40 x 40 cm duct. When lowest possible temperature was attained, fogging was regulated by controlling the supply pressure of water pump. When flow velocity was in the range of 1-5 m/s, minimum possible temperature of 24.7 o C was reached by fogging in the pressure range of MPa. Fig. 10 shows cooling trend with changes in air velocity. When velocity was 1 m/s, air mass flow rate was very low. Hence the air was saturated at a low rate of fogging and air temperature reached the. No further change in temperature was recorded as the temperature sensors are shielded from free moisture in flow. At higher air velocities, the amount of water supplied was not sufficient to saturate the air. Hence, air was not cooled to the at low supply pressure levels. Figs. 11 and 12 give the cooling trends in a larger air duct with a cross section area measuring 1 x 1 m equipped with 9 fog nozzles. The supply pressure was increased step by step to meet the water flow requirements at increased air velocities and flow rates. The minimum temperature very nearly approached the theoretical wet bulb temperature under air velocities up to 9 m/s within the normal operating pressure range

5 A. Suryan et al. / Journal of Mechanical Science and Technology 25 (4) (2011) 839~ m/s 3 m/s 5 m/s 7 m/s 9 m/s 10 m/s Table 1. Cooling effectiveness. (%) T EXIT Degree of cooling Cooling effectiveness η Fig. 11. Cooling trend with supply pressure: larger duct (9 nozzles). Τ 1 7 MPa 10 MPa 14 MPa 17 MPa 20 MPa 21 MPa 36.8 C, 44.9% 1 nozzle 34.1 C, 52.5% 1 nozzle 32.1 C, 61.4% 1 nozzle 36.8 C, 44.9% 2 nozzles 34.1 C, 52.5% 2 nozzles 32.1 C, 61.4% 2 nozzles Fig. 13. Cooling trends obtained from repeated tests. of impaction nozzles. When air velocity was increased to 10 m/s, water supply was insufficient to cool the air to the. Fig. 12 shows the cooling trend with respect to the changes in air velocities. In addition to the low rate of spraying, the poor evaporation of large droplets also contributed to the low performance at supply pressures below 14 MPa. The experimental result obtained was then compared with the theoretical psychrometric computation in Table 1. The air flow velocity considered was 5 m/s, and the supply pressure was set to 14 MPa. Exit temperature was measured after 60 seconds after the commencement of fogging with a relative humidity of 100%. Fogging was continued for 5 minutes. To ensure measurement repeatability, the temperature and relative humidity measurements were noted at 30-second intervals. The temperature at the exit of the duct was very nearly equal to that of the theoretical wet bulb temperature. Cooling effectiveness was estimated as: -T η = EXIT u a (m/s) Fig. 12. Cooling trend with air velocity: larger duct (9 nozzles)., (3) where is the dry bulb temperature, is the wet bulb temperature, and T EXIT is the temperature of the duct exit. Table 1 also presents the results obtained from tests at a few other ambient conditions. Fig. 13 compares the cooling trends obtained under different ambient conditions at an air velocity of 7 m/s. The figure gives the idea on the number of nozzles engaged under different test conditions. A second nozzle was engaged when the water supply was insufficient to saturate the air. Subsequently, the supply pressure was also reduced to prevent excess spray. Thus, the minimum possible temperature was reached under all test conditions by controlling the number of fog nozzles engaged and the supply pressure. 4. Conclusions Experiments were conducted to verify the performance characteristics of impaction pin nozzles for inlet fogging applications. Different operating conditions were simulated in experimental air ducts in the laboratory. A single nozzle was initially tested in a duct with a smaller cross-sectional area to verify the water flow rate at different supply pressure levels. The respective numbers of nozzles required for fogging under different ambient conditions were determined from theoretical psychrometric computations. Fogging was carried out under different supply pressures, water flow rates, and air velocities. Temperature and humidity measurements were obtained at different sections in the duct, and the drop in temperature was noted. Actual water flow rate was continuously monitored and compared with the theoretical estimation. The suitable location for spray nozzles within actual compressor air intake duct was identified by observing the fog patterns and the evaporation dynamics. Most suitable range of operating pressure for the nozzle was found to be between MPa. The effect of

6 844 A. Suryan et al. / Journal of Mechanical Science and Technology 25 (4) (2011) 839~845 the nozzle locations on the evaporation efficiency was verified. The nozzles were placed at distances of 10 cm from each other as well as 10 cm from the walls. The experimental results were compared with the theoretical psychrometric computations. Cooling effectiveness obtained was greater than 0.96 in all tests. The trends obtained showed that it was possible to cool the air to the wet bulb temperature values in most of the test conditions attempted. Experimental results, along with the observation of the evaporation dynamics through flow visualization, indicate that impaction pin nozzles are suitable for the inlet fogging applications for air compressors. Acknowledgment This research was financially supported by the Ministry of Education, Science, Technology (MEST) and by the Korea Institute for Advancement of Technology (KIAT) through the Human Resource Training Project for Regional Innovation, Korea. Nomenclature c p : Specific heat of air at constant pressure (J/kg/K) : Dry bulb temperature m w : Required water mass flow rate (kg/s) m w act m a : Air mass flow rate (kg/s) p 2 / p 1 : Compressor pressure ratio p s : Water supply pressure (MPa) Q w : Measure water volume flow rate (m 3 /s) : Relative humidity T 1 : Air temperature at compressor inlet (K) T EXIT : Air temperature at duct exit u a : Air flow velocity (m/s) : Wet bulb temperature w c : Specific work of compression (J/kg) γ : Ratio of specific heats for air T : Wet bulb depression w : Change in humidity ratio (kg/kg of dry air) η : Cooling effectiveness η c : Isentropic efficiency of compressor φ : Diameter Generation, Proc. of ASME Turbo Expo 2000, ASME Paper No GT-307, Munich, Germany (2000). [4] C. B. Meher-Homji and T. R. Mee III, Inlet Fogging of Gas Turbine Engines-Part B: Practical Considerations, Control and O&M Aspects, Proc. of ASME Turbo Expo 2000, ASME Paper No GT-308, Munich, Germany (2000). [5] C. B. Meher-Homji and T. R. Mee III, Gas Turbine Power Augmentation by Fogging of Inlet Air, Proc. of 28 th Turbomachinery Symposium, Houston, Texas, USA (1999). [6] M. Chaker and C. B. Meher-Homji, Inlet Fogging of Gas Turbine Engines: Climatic Analysis of gas Turbine Evaporative Cooling Potential of International Locations, Proc. of ASME Turbo Expo 2002, ASME Paper: 2002-GT-30559, Amsterdam, Netherlands (2002). [7] A. Suryan, D. S. Kim, H. D. Lee, J. K. Kwon and H. D. Kim, Analytical Study on Evaporative Cooling Potential and Power Gains of Air Compressors by Inlet Fogging, Proc. of KSME Autumn Conference, Pyeongchang, Korea (2008) [8] R. Parsons, ASHRAE Handbook- Fundamentals, ASHRAE Ed. Atlanta, Georgia, USA (2001). [9] H. S. Ren, Construction of a Generalized Psychrometric Chart for Different Pressures, Int. Journal of Mech. Engg. Education, 32/2, Manchester University Press, Manchester, U. K. (2005) [10] A. Suryan, D. S. Kim and H. D. Kim, Experimental Study on Inlet Fogging System using Two-fluid Nozzles, Journal of Thermal Science, Vol. 19, No. 2, Springer (2010) [11] M. Chaker, C. B. Meher-Homji and T. R. Mee III, Inlet Fogging of Gas Turbine Engines-Part B: Fog Droplet Sizing Analysis, Nozzle Types, Measurement and Testing, Proc. of ASME Turbo Expo 2002, ASME Paper: 2002-GT-30563, Amsterdam, Netherlands (2002). [12] M. Chaker, C. B. Meher-Homji and T. R. Mee III, Inlet Fogging of Gas Turbine Engines: Experimental and Analytical Investigations on Impaction Pin Nozzle Behavior, Proc. of ASME Turbo Expo 2003, ASME Paper: 2003-GT-38801, Atlanta, Georgia, USA (2003). [13] D. E. Willems and P. D. Ritland, A Pragmatic Approach to Evaluation of Inlet Fogging System Effectiveness, Proc. of International Joint Power Generation Conference, 2003, IJPGC , Atlanta, Georgia, USA (2003). References [1] S. L. Dixon, Fluid Mechanics and Thermodynamics of Turbomachinery, Fourth Ed. Elsevier Butterworth-Heinemann, Burlington, MA, USA (1998). [2] M. Chaker, C. B. Meher-Homji and T. R. Mee III, Inlet Fogging of Gas Turbine Engines-Part A: Fog Droplet Thermodynamics, Heat Transfer and Practical Considerations, Proc. of ASME Turbo Expo 2002, ASME Paper: 2002-GT , Amsterdam, Netherlands (2002). [3] C. B. Meher-Homji and T. R. Mee III, Inlet Fogging of Gas Turbine Engines-Part A: Theory, Psychrometrics and Fog Heuy-Dong Kim received his B.S. and M.S. degrees in Mechanical Engineering from Kyungpook National University, Korea, in 1986 and 1988, respectively. He then received his Ph.D. from Kyushu University, Japan, in Dr. Kim is currently a Professor at the School of Mechanical Engineering, Andong National University, Korea. His research interests include High-Speed Trains, Ramjet and Scramjet, Shock Tube and Technology, Shock Wave Dynamics, Explosions and

7 A. Suryan et al. / Journal of Mechanical Science and Technology 25 (4) (2011) 839~ Blast Waves, Flow Measurement, Aerodynamic Noises, and Supersonic Wind Tunnels. Abhilash Suryan received his B. Tech. and M. Tech. degrees in Mechanical Engineering from the College of Engineering, Trivandrum, University of Kerala, India, in 1995 and 1997, respectively. He is currently pursuing his Ph.D. Degree at the Gas Dynamics Laboratory of the School of Mechanical Engineering, Andong National University, Korea.