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Slot film cooling performance in combustor with flame holders
Energy 37 (2012) 533e539
Contents lists available at SciVerse ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Slot film cooling performance in combustor with flame holders Jiwoon Song, Keon Woo Lee, Kyung Min Kim, Hyung Hee Cho* Department of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 June 2011 Received in revised form 28 October 2011 Accepted 30 October 2011 Available online 7 December 2011
This experimental study was conducted to investigate the effect of a flame holder on multi-slot film cooling in a ramjet combustor. A prediffuser-type ramjet combustor equipped with a flame holder was utilized in this study. The change in adiabatic film cooling effectiveness on the downstream wall was measured using thermochromic liquid crystals. Experiments were conducted using five different blowing ratios, ranging from 0.5 to 1.5. The effect of the flame holder on heat transfer shows different characteristics between the first and second slots. In the first slot, the accelerated flows by blockage effect suppress the ejected secondary flows resulting in better cooling performance in case with the flame holder. However, the second slot represents the low cooling performance over all the regions by the disturbed flows with the flame holder compared to that without the flame holder. And the difference in cooling performance is enlarged with the increase of blowing ratio. The results reveal that it is essential to consider the effect of a wake behind the flame holder. Therefore, it should be considered in the design of film cooling systems for a practical application in ramjet combustors. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Slot film cooling Film cooling effectiveness Ramjet combustor Flame holder
1. Introduction A ramjet combustor is a thermal engine that has efficient flight in high-speed flow. It ejects and burns fuel in a high-speed flow. It makes sufficiently compressed air by its forward high-speed flow of vehicle intake instead of moving parts such as compressor. Eliminating the moving parts makes the ramjet lighter. It makes possible efficient flight and power. To stabilize the flame in a high-speed flow, a flame holder is essentially required. The flame holder is placed at the entrance of a combustion chamber which mixes the burned hot gas with the unburned cool gas for efficient combustion. The flame holder slows down the flame velocity and stabilizes the continuous flame. The flame holder also generates a wake. The wake at the rear side of the flame holder is classified as being in either the recirculation zone or the mixing zone. The mixing zone has strong shear stress, abrupt temperature gradient, and a high chemical reaction rate. The mixing zone generated by the flame holder affects the combustor wall directly and leads to serious thermal problems, such as thermal crack and fatigue [1]. Therefore, cooling techniques for ramjet combustors are a core part of engine in the thermal design. Cooling refers to blocking off the combustor wall from the heat, generated during combustion, and maintaining the combustor structure at a low and safe temperature. In general,
* Corresponding author. Tel.: þ82 2 2123 2828; fax: þ82 2 312 2159. E-mail address: [email protected] (H.H. Cho). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.10.051
there are two types of cooling techniques: ablation and film cooling. Ablation is a coating technique in which the combustor wall is coated. Char ablation is a sacrificial method in which an oxide such as a carbon/carbon or silica/carbon composite is blazed with hot gas, and then a char layer, which has good thermal barrier properties, remains on the body surfaces and protects the target body from high thermal conditions. In contrast, in the film cooling technique, which is the other cooling technology, a coolant is injected through holes or slots on the combustor walls. This creates an isolated film between the hot combustion gases and the material, and this film protects the combustor walls from the hot combustion gas [2]. Film cooling is more complex system than ablation. However, film cooling is preferred in systems that are operational for extended periods because of its ability to deliver high cooling performance and efficiency over extended periods. Over the last few decades, many researchers [3e20] have studied the characteristics of fluid flow, heat transfer, and film cooling effectiveness in order to investigate advanced film cooling systems in company with internal cooling methods [10,11]. For slot geometries, an experimental device with the actual size of a commercial combustion chamber was embodied [12,13]. The effects of the blowing ratio and various slot heights were considered. On the basis of the results, the film cooling correlation of each factor was reported. The slot with discrete holes was used to reduce the thermal load concentrating on the wall by three-dimensional velocity distribution [14]. Furthermore, to enhance the cooling performances, shaped cooling was also studied [15,16]. By using the trace gas method, film
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cooling effectiveness was measured under combined parallel and vertical injection. The effect of slot lip length on film cooling effectiveness in the downstream region was studied [17]. The optimized lip length was found to be t/s ¼ 0.25. In the case of short lip length, coolant easily mixed with the mainstream owing to non-uniformity of the second flow; this caused a decrease in film cooling effectiveness. In the case of long lip length, the pressure drop of the inner slot increased. Recently, an alternative method for injecting the coolant into the slot has been studied [18e20]. Many studies have examined conventional film cooling systems. Moreover, most of them are for gas turbine application. However, a system equipping a flame holder has complex flow characteristics and can thus handle unexpected thermal conditions. Therefore, it is necessary to study the characteristics of the fluid flow and the heat transfer in the system to confirm the thermal safety of ramjet combustors. In addition, it is also necessary to study any film cooling adopted system, such as ramjet combustor, that has encountered a lowering of efficiency due to bypassing the main flow for cooling. Therefore, the efficiency of ramjet vehicle is highly related to the film cooling efficiency. The main objective of this study is to determine the effect of a flame holder in a prediffuser-type ramjet combustor by injecting coolant through the film-cooling slot. In order to determine heat transfer characteristics on the combustion wall, we performed numerical computations using a commercial package (FLUENT 6.2), to analyze the flow characteristics. To obtain detailed information regarding the film cooling effectiveness, heat transfer experiments using a thermochromic liquid crystal (TLC) method were conducted. From the resultant values, we determined the effect of flame holders, based on film cooling effectiveness. 2. Experimental setup and numerical 2.1. Numerical calculation To determine the flow in the ramjet combustor, numerical simulations were conducted by the commercial code FLUENT 6.2. Fig. 1 depicts the simulated domain with flame holder. Domain simulates the axi-symmetric three-stage multi-slot system. Both domains have an inlet radius R0 ¼ 150 mm and a combustion inner radius R ¼ 256 mm. The shapes and location of the flame holders are the same as those in the actual experimental setup. To reduce the recirculation effect generated in the expansion part of the coaxial ramjet, 7 slopes were designed. The numerical domain has 200,000 cells, as confirmed by a mesh independent test. The nominal velocity of the main flow is 15 m/s and the operating pressure is the atmospheric pressure, which are the same as the experimental conditions. The conducted turbulence model is the renormalization group (RNG) ke3 model. 2.2. Experimental apparatus Fig. 2 presents schematic views of the experimental setups for heat transfer measurements. The heat transfer experimental setup
is made up of five parts, including the contraction part, the suction of the secondary inlet flow, the secondary injection flow section, the test section, and the diffuser part. The suction section of the secondary inlet flow consists of a 3.75 kW blower and an orifice flow meter. The mainstream temperature is controlled by the heat exchanger, which is connected to a constant temperature reservoir. The secondary injection system is composed of an orifice flow meter, a 3.75 kW blower, a heat exchanger, and a plenum. The heat exchanger is connected to a constant temperature reservoir to control the secondary flow temperature. Blowers apply to the suction and the secondary injection system is repeatedly controlled by inverters [19]. The flow rates for the suction and the secondary injection system are measured by each orifice flow meter. The air from the heat exchanger flows into the plenum chamber via two manifolds. With this system, the temperature can be controlled within a 0.1 C temperature difference. The air in the plenum chamber is injected through the slots into the mainstream. The insulating material (5-mm-thick Styrofoam) is attached to each side wall of the plenum chamber to reduce heat loss and to keep a constant temperature.
2.3. Slot and flame holder geometry Fig. 3 depicts the slot geometry and schematic view in the ramjet combustor. The flame holder and slot geometry are decided based on an actual ramjet combustor. For satisfying the bypassing mass flow rate, the upper part of the film-cooling slot for bypassing from the main flow to the secondary flows is designed based on the results of the numerical analysis performed using a commercial package, Fluent v6.2 as shown in Fig. 3. Fig. 3 also shows the location and geometry of the flame holder. Two flame holders are designed to have a v-gutter shape. The first flame holder is installed 68.5 mm ahead of the first slot. The second flame holder is installed 50 mm ahead of the first flame holder. Both flame holders have a 60 slope and a ‘V’-shaped structure. Polycarbonate is used as the slot material because it has high hardness and it does not bend easily, which is useful in maintaining the characteristics and the geometry of the slots. The height of each slot is designed on the basis of the numerical results with geometry analogy. The length between the slots is selected to be the radius of combustor (150 mm). The height, thickness, and length of each slot are listed in Table 1. In the heat transfer experiment, the coolant flows must be controlled to measure the adiabatic film cooling effectiveness. For this reason, a separate coolant passage and suction passage are needed which maintain the same flow condition in the combustion chamber as shown in Fig. 3. The temperature difference between the mainstream and the secondary injection flow is 20 C. The temperatures of both the mainstream and the coolant flow are controlled by their respective constant temperature reservoirs. The suction system consists of the suction passage that blocks the precoolant passage at one corner, an orifice flow meter to
Fig. 1. Schematic view of the ramjet combustor: numerical model with flame holders.
J. Song et al. / Energy 37 (2012) 533e539
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Fig. 2. Schematic view of experimental setup [19].
Fig. 3. Schematic view of the suction and injection system and location of flame holders.
ensure accurate flow control, and a suction-type, 3.75-kW blower that is controlled by the inverter. The injected flow from the plenum chamber runs along the straight coolant passage below the supplement guide wall. The plenum chamber and the coolant passage are connected with curved surfaces (polyethylene) of constant curvature to ensure uniform flow. The test plate is divided into two parts made of Bakelite. On the upper Bakelite surface, five J-type thermocouples are mounted to monitor the steady state during the test. Each Bakelite plate has low
thermal conductivity (k ¼ 0.35 W/m K) to minimize heat conduction to the measurement plate. The measurement plate is composed of five layers: a TLC sheet (R20C20W Hallcrest Ltd. 300 mm 300 mm) to measure the wall temperature, two Bakelite plates, and a 50-mm-thick Styrofoam layer for insulation. Between the two Bakelite plates, there is a 5-mm air gap filled with fiberglass. The air layer is the most inductive way to maximize the effect of the insulation during the experiment. A 50-mm-thick piece of Styrofoam is attached below the lower Bakelite plate. 2.4. Data acquisition
Table 1 Geometry of multi-slot system. Slot No.
s (mm)
t (mm)
ls (mm)
1st 2nd 3rd
2.5
1
78.2 150 150
TLC (R20C20W, ranging from 20 to 40 C, Hallcrest) was used to measure the local temperature on the film-cooled wall. The calibrations were conducted under the same condition of the main experiments and were repeated with various temperature ranges. Some parts of the temperature ranges are superposed. TLC images were captured using a charge-coupled device (CCD) camera when
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the temperature was in a steady state; the temperature fields from each thermocouple were transferred simultaneously and saved to a PC using a data logger (Model No. 34970A, Agilent Technologies, USA) with a 22 channel-multiplexer (34901A). Hydrargyrum quartz iodide (HQI) lamps were switched on only when recording images to minimize the effect of radiation heat. The capturing of images and the determination of temperature from the thermocouples and controlling lamps are managed by software. A steady state is defined as when temperature variation is less than 0.1 C. The huetemperature calibration data are fitted to a sixth-order polynomial curve, and the results from calibration show good agreement with data from the correlation. 2.5. Operating conditions The velocity of the mainstream is fixed to 15 m/s during the experiment. This study is focused on the parallel slot film cooling in the prediffuser-type ramjet combustor. Experiments were conducted using five different blowing ratios ðM ¼ rj Uj =rN UN Þ ranging from 0.5 to 1.5. The angle of injection into the combustion chamber is 0 in all cases. 2.6. Film cooling effectiveness To estimate the performance of film cooling, film cooling effectiveness (h) is presented. It is calculated from the measured adiabatic wall temperature (Taw) by TLC methods. The film cooling effectiveness is defined as equation (1) for low speed and constant property flows:
Fig. 4. Flow distributions and velocity profiles of mainstream of the first slot without flame holder.
h¼
Taw TN T2 TN
(1)
The range of film cooling effectiveness is from 0 to 1. The value of 0 means no effect on cooling when adiabatic wall temperature is the same as the mainstream temperature ðTN Þ. The value of 1 means a perfect cooling on a wall when wall temperature is the same as secondary flow temperature (T2). 3. Results and discussion To determine the flow characteristics near the slot and the effect of the flame holder, numerical simulations were carried out using Fluent v6.2. Figs. 4 and 5 show the results of velocity contour and profile at the first, second, and third slots. The effective length of each slot is 150 mm. The x-directional velocity at three points every 50 mm is plotted. Near the flame holder at the first slot region, there is a flow acceleration point due to blockage effect by the flame holder, as illustrated in the velocity distributions of the first slot in Fig. 5; it is not shown in any flame holder case. The wakes depicted in Fig. 5 are obviously generated when the flame holders are installed. The acceleration and wakes affect the main flow near the first slot and cause a change in film cooling effectiveness. In the second slot, there is a transition region. The main flow velocity is altered at the upstream of the second slot with the flame holder. However, after the middle of the second slot, the main flow velocity regresses to the velocity profile at no flame holder case. The flame holder affects a little on the velocity distribution at the third slot. To confirm the effect of the horizontal flame holder at the first slot, experiments for film cooling effectiveness are conducted for
Fig. 5. Flow distributions and velocity profiles of mainstream of the first slot with flame holder.
J. Song et al. / Energy 37 (2012) 533e539
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cases with and without flame holder. The results of film cooling effectiveness, as presented in Figs. 6e9, were acquired by translating the temperature data obtained by TLC. The first slot is located on the right after the flame holder, and accelerated flows and turbulence mixing flows caused by the flame holders significantly affect the cooling performance of the first slot. High turbulence flows caused by the flame holder mix the main and the secondary flow, and decrease the film cooling effectiveness. In the first slot cases presented in Fig. 6, film cooling effectiveness is abruptly decreased in both cases. It is because the recirculation flows generated at the diffusing inlet part of the ramjet combustor interrupt the main flow and the secondary injection flow, breaking the injected cooling film. But it is a commonly estimated result. There are two distinctive results compared to other slots by flame holders. First of all, the film cooling performance is temporarily better with the flame holder cases than that without the flame holder cases among the x/s1 ¼ 25e45 regions for various blowing ratios in the first slot cases. The better film cooling performance at the first slot is obtained with the flame holder, because the main flows accelerated by the flame holder suppress the secondary flow. Hence, the secondary flow stays at the cooling surface closer and remains there longer, resulting in temporarily improved cooling performance with the flame holder. However, film cooling effectiveness is lower with the flame holder cases than without the flame
Fig. 7. Comparison of film cooling effectiveness in the second slot with flame holders and without flame holders (blowing ratio: M ¼ rj Uj =rN UN ).
Fig. 6. Comparison of film cooling effectiveness in the first slot with flame holders and without flame holders (blowing ratio: M ¼ rj Uj =rN UN ).
holder cases in the downstream region. Flow runs in after downstream, and accelerated flows lose their momentums. And substituting the accelerated flows, high turbulence flows are dominant. Then, film cooling effectiveness decreases rapidly with the flame holder. Another interesting point is that the intersecting points move downstream as the blowing ratios increase as shown in Fig. 6. The reason is that the flows are accelerated and so the velocity of the main flows and coolant flows becomes similar locally. The flow velocity around the first slot increases as a result of the blockage effect of the flame holder shown in velocity distribution at the first slot. Subsequently, with increasing blowing ratio, the local velocity difference between the main flow and the coolant flow becomes smaller. In film cooling method, the large velocity difference between the main flows and the coolant flows has a negative effect because of flow disturbance. Hence, augmentation of film cooling performance is expected at the high blowing rate due to a parallel-going flow characteristic. At the second slot, the effectiveness decreases rapidly as shown in Fig. 7. Unlike the first slot case, there is no intersecting point between the cases with and without the flame holder as shown in Fig. 7. Instead of the intersecting point, the decrease in cooling effectiveness is much larger than that with the flame holders as the blowing ratio increases. At a high blowing ratio, the difference in the velocity increase between the main flow and the secondary flow leads to an increase in the flow disturbance. The high flow
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disturbance due to the flame holder interferes the coolant stability. Therefore, with the flame holder cases, the film cooling performance decreases because the deterioration of the coolant flow caused by the high turbulence affects the improvement in cooling performance more seriously than the increase in the coolant flow at the high blowing ratio. In the third slot, the flame holder decreases a little film cooling effectiveness, as shown in Fig. 8. High disturbed and accelerated flow caused by the flame holder does not reach to the third slot, so there are parallel flows such as film cooling appearance. Therefore the higher blowing ratio results in the better cooling performance. However, the improvement rate is not linear. Although the coolant flow rate increases twofold, the cooling effectiveness does not increase twofold. From the results shown in Fig. 6, the first slot has better cooling performances region in the flame holder installed case than that without the flame holder. However, in the second slot, the flame holder has a negative effect on cooling effectiveness continuously with the increase of blowing ratios as shown in Fig. 9. Therefore, thermal design in the second slot should consider seriously this degradation with the flame holder. In case of high blowing ratio for thermal robust design, there is an advantage that the improvement of film cooling effectiveness region is extended by the flame holder in the first slot. However, there is also a disadvantage in the high blowing ratio case, which is the sudden decrease of film cooling effectiveness in the second slot by the flame holder. Therefore, moderate blowing ratios should be selected in each slot by thermal design for effective operation. 4. Conclusions In the present study, the effect of a flame holder on film cooling effectiveness was investigated for a multi-slot film cooling system in a ramjet combustor. Film cooling effectiveness was measured using TLC experiments. The conclusions are summarized as follows: Fig. 8. Comparison of film cooling effectiveness in third slot with flame holders and without flame holders (blowing ratio: M ¼ rj Uj =rN UN ).
1. Main flows are accelerated around the first slot due to the blockage effect of the inserted flame holder. 2. In the first slot, film cooling effectiveness has an intersecting point and it recedes from the slot starting point due to the suppressed coolant as a result of the accelerated flow and the high disturbance of the wake. 3. In the second slot, the difference in film cooling effectiveness between with and without the flame holder cases becomes higher with the increase in the blowing ratio because the high disturbed flow has a significant effect. 4. Unlike the first slot, film cooling effectiveness decreases significantly with the flame holder in the second slot. It should be considered in thermal design for multi-stage film cooling, especially at the high blowing ratio. 5. In the third slot, the flame holder affects film cooling effectiveness a little. High blowing rate makes better cooling performance in both cases such as ordinary film cooling systems. Hence, in thermal design of film cooling in the third slot, the result from the flame holder does not need to be considered seriously. 6. For effective operation in ramjet combustor and prevention of the thermal defect, proper blowing ratio in each slot should be selected and designed. References
Fig. 9. Film cooling effectiveness at x/s2 ¼ 50 with and without flame holders in the second slot with various blowing ratios.
[1] Lee CW, Moon SY, Sohn CH. Spray and combustion characteristics of a dumptype ramjet combustor. KSME Int J 2003;17:2019e26. [2] Goldstein RJ. Film cooling. Adv Heat Transf 1971;7:321e79.
J. Song et al. / Energy 37 (2012) 533e539 [3] Goldstein RJ, Eckert ERG, Burggraf F. Effects of hole geometry and density on three-dimensional film cooling. Int J Heat Mass Transf 1974;17: 595e607. [4] Leylek JH, Zerkle RD. Discrete-jet film cooling: a comparison of computational results with experiments. ASME J Turbomach 1994;116:321e79. [5] Schmidt DL, Sen B, Bogard DG. Film cooling with compound angle holes: adiabatic effectiveness. ASME J Turbomach 1994;118:807e13. [6] Cho HH, Goldstein RJ. Heat (mass) transfer and film cooling effectiveness with injection through discrete holes: part I e within holes and on the back surface. ASME J Turbomach 1995;117:440e50. [7] Cho HH, Goldstein RJ. Heat (mass) transfer and film cooling effectiveness with injection through discrete holes: part II e on the exposed surface. ASME J Turbomach 1995;117:451e60. [8] Goldstein RJ, Cho HH, Jabbari MY. Effect of plenum crossflow on heat (mass) transfer near and within the entrance of film cooling holes. ASME J Turbomach 1997;119:761e9. [9] Cho HH, Kang SG, Rhee D-H. Heat/mass transfer measurement within a film cooling hole of square and rectangular cross section. ASME J Turbomach 2001; 123:806e14. [10] Kim KM, Kim BS, Lee DH, Moon H, Cho HH. Optimal design of transverse ribs in tubes for thermal performance enhancement. Energy 2010;35: 2400e6.
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[11] Lee DH, Rhee D-H, Kim KM, Cho HH, Moon HK. Detailed measurement of heat/ mass transfer with continuous and multiple V-shaped ribs in rectangular channel. Energy 2009;34:1770e8. [12] Papell SS, Trout AM. Experimental investigation of air film cooling applied to an adiabatic wall by means of an axially discharging slot. National Aeronautics and Space Administration Technical Note D-9; 1959. [13] Papell SS. Effect on gaseous film cooling of coolant injection through angled slots and normal holes. NASA Technical Note D-299; September 1960. [14] Folayan CO, Whitelaw JH. The effectiveness of combined tangential and normal film-cooling slots with finite lip. ASME Paper No. 76-HT-30; 1976. [15] Baheri S, Alavi Tabrizi SP, Jubran BA. Film cooling effectiveness from trenched shaped and compound holes. Heat Mass Transf 2008;44:989e98. [16] Bunker RS. A review of shaped hole turbine film cooling technology. J Heat Trans-T ASME 2005;127:441e53. [17] Seban RA. Heat transfer and effectiveness for a turbulent boundary layer with tangential fluid injection. ASME J Heat Transf 1960 Nov;303e12. [18] Sivasegaram S. Film cooling slots: the importance of lip thickness and injection angle. J Mech Eng Sci 1969;11:22e7. [19] Park KH, Yang KM, Lee KW, Cho HH, Ham HC, Hwang KY. Effects of injection type on slot film cooling for a ramjet combustor. J Mech Sci Technol 2009;23:1852e7. [20] Cho HH, Ham JK. Influence of injection type and feed arrangement on flow and heat transfer in an injection slot. ASME J Turbomach 2002;124:132e41.
Contents lists available at SciVerse ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Slot film cooling performance in combustor with flame holders Jiwoon Song, Keon Woo Lee, Kyung Min Kim, Hyung Hee Cho* Department of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 June 2011 Received in revised form 28 October 2011 Accepted 30 October 2011 Available online 7 December 2011
This experimental study was conducted to investigate the effect of a flame holder on multi-slot film cooling in a ramjet combustor. A prediffuser-type ramjet combustor equipped with a flame holder was utilized in this study. The change in adiabatic film cooling effectiveness on the downstream wall was measured using thermochromic liquid crystals. Experiments were conducted using five different blowing ratios, ranging from 0.5 to 1.5. The effect of the flame holder on heat transfer shows different characteristics between the first and second slots. In the first slot, the accelerated flows by blockage effect suppress the ejected secondary flows resulting in better cooling performance in case with the flame holder. However, the second slot represents the low cooling performance over all the regions by the disturbed flows with the flame holder compared to that without the flame holder. And the difference in cooling performance is enlarged with the increase of blowing ratio. The results reveal that it is essential to consider the effect of a wake behind the flame holder. Therefore, it should be considered in the design of film cooling systems for a practical application in ramjet combustors. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Slot film cooling Film cooling effectiveness Ramjet combustor Flame holder
1. Introduction A ramjet combustor is a thermal engine that has efficient flight in high-speed flow. It ejects and burns fuel in a high-speed flow. It makes sufficiently compressed air by its forward high-speed flow of vehicle intake instead of moving parts such as compressor. Eliminating the moving parts makes the ramjet lighter. It makes possible efficient flight and power. To stabilize the flame in a high-speed flow, a flame holder is essentially required. The flame holder is placed at the entrance of a combustion chamber which mixes the burned hot gas with the unburned cool gas for efficient combustion. The flame holder slows down the flame velocity and stabilizes the continuous flame. The flame holder also generates a wake. The wake at the rear side of the flame holder is classified as being in either the recirculation zone or the mixing zone. The mixing zone has strong shear stress, abrupt temperature gradient, and a high chemical reaction rate. The mixing zone generated by the flame holder affects the combustor wall directly and leads to serious thermal problems, such as thermal crack and fatigue [1]. Therefore, cooling techniques for ramjet combustors are a core part of engine in the thermal design. Cooling refers to blocking off the combustor wall from the heat, generated during combustion, and maintaining the combustor structure at a low and safe temperature. In general,
* Corresponding author. Tel.: þ82 2 2123 2828; fax: þ82 2 312 2159. E-mail address: [email protected] (H.H. Cho). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.10.051
there are two types of cooling techniques: ablation and film cooling. Ablation is a coating technique in which the combustor wall is coated. Char ablation is a sacrificial method in which an oxide such as a carbon/carbon or silica/carbon composite is blazed with hot gas, and then a char layer, which has good thermal barrier properties, remains on the body surfaces and protects the target body from high thermal conditions. In contrast, in the film cooling technique, which is the other cooling technology, a coolant is injected through holes or slots on the combustor walls. This creates an isolated film between the hot combustion gases and the material, and this film protects the combustor walls from the hot combustion gas [2]. Film cooling is more complex system than ablation. However, film cooling is preferred in systems that are operational for extended periods because of its ability to deliver high cooling performance and efficiency over extended periods. Over the last few decades, many researchers [3e20] have studied the characteristics of fluid flow, heat transfer, and film cooling effectiveness in order to investigate advanced film cooling systems in company with internal cooling methods [10,11]. For slot geometries, an experimental device with the actual size of a commercial combustion chamber was embodied [12,13]. The effects of the blowing ratio and various slot heights were considered. On the basis of the results, the film cooling correlation of each factor was reported. The slot with discrete holes was used to reduce the thermal load concentrating on the wall by three-dimensional velocity distribution [14]. Furthermore, to enhance the cooling performances, shaped cooling was also studied [15,16]. By using the trace gas method, film
534
J. Song et al. / Energy 37 (2012) 533e539
cooling effectiveness was measured under combined parallel and vertical injection. The effect of slot lip length on film cooling effectiveness in the downstream region was studied [17]. The optimized lip length was found to be t/s ¼ 0.25. In the case of short lip length, coolant easily mixed with the mainstream owing to non-uniformity of the second flow; this caused a decrease in film cooling effectiveness. In the case of long lip length, the pressure drop of the inner slot increased. Recently, an alternative method for injecting the coolant into the slot has been studied [18e20]. Many studies have examined conventional film cooling systems. Moreover, most of them are for gas turbine application. However, a system equipping a flame holder has complex flow characteristics and can thus handle unexpected thermal conditions. Therefore, it is necessary to study the characteristics of the fluid flow and the heat transfer in the system to confirm the thermal safety of ramjet combustors. In addition, it is also necessary to study any film cooling adopted system, such as ramjet combustor, that has encountered a lowering of efficiency due to bypassing the main flow for cooling. Therefore, the efficiency of ramjet vehicle is highly related to the film cooling efficiency. The main objective of this study is to determine the effect of a flame holder in a prediffuser-type ramjet combustor by injecting coolant through the film-cooling slot. In order to determine heat transfer characteristics on the combustion wall, we performed numerical computations using a commercial package (FLUENT 6.2), to analyze the flow characteristics. To obtain detailed information regarding the film cooling effectiveness, heat transfer experiments using a thermochromic liquid crystal (TLC) method were conducted. From the resultant values, we determined the effect of flame holders, based on film cooling effectiveness. 2. Experimental setup and numerical 2.1. Numerical calculation To determine the flow in the ramjet combustor, numerical simulations were conducted by the commercial code FLUENT 6.2. Fig. 1 depicts the simulated domain with flame holder. Domain simulates the axi-symmetric three-stage multi-slot system. Both domains have an inlet radius R0 ¼ 150 mm and a combustion inner radius R ¼ 256 mm. The shapes and location of the flame holders are the same as those in the actual experimental setup. To reduce the recirculation effect generated in the expansion part of the coaxial ramjet, 7 slopes were designed. The numerical domain has 200,000 cells, as confirmed by a mesh independent test. The nominal velocity of the main flow is 15 m/s and the operating pressure is the atmospheric pressure, which are the same as the experimental conditions. The conducted turbulence model is the renormalization group (RNG) ke3 model. 2.2. Experimental apparatus Fig. 2 presents schematic views of the experimental setups for heat transfer measurements. The heat transfer experimental setup
is made up of five parts, including the contraction part, the suction of the secondary inlet flow, the secondary injection flow section, the test section, and the diffuser part. The suction section of the secondary inlet flow consists of a 3.75 kW blower and an orifice flow meter. The mainstream temperature is controlled by the heat exchanger, which is connected to a constant temperature reservoir. The secondary injection system is composed of an orifice flow meter, a 3.75 kW blower, a heat exchanger, and a plenum. The heat exchanger is connected to a constant temperature reservoir to control the secondary flow temperature. Blowers apply to the suction and the secondary injection system is repeatedly controlled by inverters [19]. The flow rates for the suction and the secondary injection system are measured by each orifice flow meter. The air from the heat exchanger flows into the plenum chamber via two manifolds. With this system, the temperature can be controlled within a 0.1 C temperature difference. The air in the plenum chamber is injected through the slots into the mainstream. The insulating material (5-mm-thick Styrofoam) is attached to each side wall of the plenum chamber to reduce heat loss and to keep a constant temperature.
2.3. Slot and flame holder geometry Fig. 3 depicts the slot geometry and schematic view in the ramjet combustor. The flame holder and slot geometry are decided based on an actual ramjet combustor. For satisfying the bypassing mass flow rate, the upper part of the film-cooling slot for bypassing from the main flow to the secondary flows is designed based on the results of the numerical analysis performed using a commercial package, Fluent v6.2 as shown in Fig. 3. Fig. 3 also shows the location and geometry of the flame holder. Two flame holders are designed to have a v-gutter shape. The first flame holder is installed 68.5 mm ahead of the first slot. The second flame holder is installed 50 mm ahead of the first flame holder. Both flame holders have a 60 slope and a ‘V’-shaped structure. Polycarbonate is used as the slot material because it has high hardness and it does not bend easily, which is useful in maintaining the characteristics and the geometry of the slots. The height of each slot is designed on the basis of the numerical results with geometry analogy. The length between the slots is selected to be the radius of combustor (150 mm). The height, thickness, and length of each slot are listed in Table 1. In the heat transfer experiment, the coolant flows must be controlled to measure the adiabatic film cooling effectiveness. For this reason, a separate coolant passage and suction passage are needed which maintain the same flow condition in the combustion chamber as shown in Fig. 3. The temperature difference between the mainstream and the secondary injection flow is 20 C. The temperatures of both the mainstream and the coolant flow are controlled by their respective constant temperature reservoirs. The suction system consists of the suction passage that blocks the precoolant passage at one corner, an orifice flow meter to
Fig. 1. Schematic view of the ramjet combustor: numerical model with flame holders.
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Fig. 2. Schematic view of experimental setup [19].
Fig. 3. Schematic view of the suction and injection system and location of flame holders.
ensure accurate flow control, and a suction-type, 3.75-kW blower that is controlled by the inverter. The injected flow from the plenum chamber runs along the straight coolant passage below the supplement guide wall. The plenum chamber and the coolant passage are connected with curved surfaces (polyethylene) of constant curvature to ensure uniform flow. The test plate is divided into two parts made of Bakelite. On the upper Bakelite surface, five J-type thermocouples are mounted to monitor the steady state during the test. Each Bakelite plate has low
thermal conductivity (k ¼ 0.35 W/m K) to minimize heat conduction to the measurement plate. The measurement plate is composed of five layers: a TLC sheet (R20C20W Hallcrest Ltd. 300 mm 300 mm) to measure the wall temperature, two Bakelite plates, and a 50-mm-thick Styrofoam layer for insulation. Between the two Bakelite plates, there is a 5-mm air gap filled with fiberglass. The air layer is the most inductive way to maximize the effect of the insulation during the experiment. A 50-mm-thick piece of Styrofoam is attached below the lower Bakelite plate. 2.4. Data acquisition
Table 1 Geometry of multi-slot system. Slot No.
s (mm)
t (mm)
ls (mm)
1st 2nd 3rd
2.5
1
78.2 150 150
TLC (R20C20W, ranging from 20 to 40 C, Hallcrest) was used to measure the local temperature on the film-cooled wall. The calibrations were conducted under the same condition of the main experiments and were repeated with various temperature ranges. Some parts of the temperature ranges are superposed. TLC images were captured using a charge-coupled device (CCD) camera when
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the temperature was in a steady state; the temperature fields from each thermocouple were transferred simultaneously and saved to a PC using a data logger (Model No. 34970A, Agilent Technologies, USA) with a 22 channel-multiplexer (34901A). Hydrargyrum quartz iodide (HQI) lamps were switched on only when recording images to minimize the effect of radiation heat. The capturing of images and the determination of temperature from the thermocouples and controlling lamps are managed by software. A steady state is defined as when temperature variation is less than 0.1 C. The huetemperature calibration data are fitted to a sixth-order polynomial curve, and the results from calibration show good agreement with data from the correlation. 2.5. Operating conditions The velocity of the mainstream is fixed to 15 m/s during the experiment. This study is focused on the parallel slot film cooling in the prediffuser-type ramjet combustor. Experiments were conducted using five different blowing ratios ðM ¼ rj Uj =rN UN Þ ranging from 0.5 to 1.5. The angle of injection into the combustion chamber is 0 in all cases. 2.6. Film cooling effectiveness To estimate the performance of film cooling, film cooling effectiveness (h) is presented. It is calculated from the measured adiabatic wall temperature (Taw) by TLC methods. The film cooling effectiveness is defined as equation (1) for low speed and constant property flows:
Fig. 4. Flow distributions and velocity profiles of mainstream of the first slot without flame holder.
h¼
Taw TN T2 TN
(1)
The range of film cooling effectiveness is from 0 to 1. The value of 0 means no effect on cooling when adiabatic wall temperature is the same as the mainstream temperature ðTN Þ. The value of 1 means a perfect cooling on a wall when wall temperature is the same as secondary flow temperature (T2). 3. Results and discussion To determine the flow characteristics near the slot and the effect of the flame holder, numerical simulations were carried out using Fluent v6.2. Figs. 4 and 5 show the results of velocity contour and profile at the first, second, and third slots. The effective length of each slot is 150 mm. The x-directional velocity at three points every 50 mm is plotted. Near the flame holder at the first slot region, there is a flow acceleration point due to blockage effect by the flame holder, as illustrated in the velocity distributions of the first slot in Fig. 5; it is not shown in any flame holder case. The wakes depicted in Fig. 5 are obviously generated when the flame holders are installed. The acceleration and wakes affect the main flow near the first slot and cause a change in film cooling effectiveness. In the second slot, there is a transition region. The main flow velocity is altered at the upstream of the second slot with the flame holder. However, after the middle of the second slot, the main flow velocity regresses to the velocity profile at no flame holder case. The flame holder affects a little on the velocity distribution at the third slot. To confirm the effect of the horizontal flame holder at the first slot, experiments for film cooling effectiveness are conducted for
Fig. 5. Flow distributions and velocity profiles of mainstream of the first slot with flame holder.
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cases with and without flame holder. The results of film cooling effectiveness, as presented in Figs. 6e9, were acquired by translating the temperature data obtained by TLC. The first slot is located on the right after the flame holder, and accelerated flows and turbulence mixing flows caused by the flame holders significantly affect the cooling performance of the first slot. High turbulence flows caused by the flame holder mix the main and the secondary flow, and decrease the film cooling effectiveness. In the first slot cases presented in Fig. 6, film cooling effectiveness is abruptly decreased in both cases. It is because the recirculation flows generated at the diffusing inlet part of the ramjet combustor interrupt the main flow and the secondary injection flow, breaking the injected cooling film. But it is a commonly estimated result. There are two distinctive results compared to other slots by flame holders. First of all, the film cooling performance is temporarily better with the flame holder cases than that without the flame holder cases among the x/s1 ¼ 25e45 regions for various blowing ratios in the first slot cases. The better film cooling performance at the first slot is obtained with the flame holder, because the main flows accelerated by the flame holder suppress the secondary flow. Hence, the secondary flow stays at the cooling surface closer and remains there longer, resulting in temporarily improved cooling performance with the flame holder. However, film cooling effectiveness is lower with the flame holder cases than without the flame
Fig. 7. Comparison of film cooling effectiveness in the second slot with flame holders and without flame holders (blowing ratio: M ¼ rj Uj =rN UN ).
Fig. 6. Comparison of film cooling effectiveness in the first slot with flame holders and without flame holders (blowing ratio: M ¼ rj Uj =rN UN ).
holder cases in the downstream region. Flow runs in after downstream, and accelerated flows lose their momentums. And substituting the accelerated flows, high turbulence flows are dominant. Then, film cooling effectiveness decreases rapidly with the flame holder. Another interesting point is that the intersecting points move downstream as the blowing ratios increase as shown in Fig. 6. The reason is that the flows are accelerated and so the velocity of the main flows and coolant flows becomes similar locally. The flow velocity around the first slot increases as a result of the blockage effect of the flame holder shown in velocity distribution at the first slot. Subsequently, with increasing blowing ratio, the local velocity difference between the main flow and the coolant flow becomes smaller. In film cooling method, the large velocity difference between the main flows and the coolant flows has a negative effect because of flow disturbance. Hence, augmentation of film cooling performance is expected at the high blowing rate due to a parallel-going flow characteristic. At the second slot, the effectiveness decreases rapidly as shown in Fig. 7. Unlike the first slot case, there is no intersecting point between the cases with and without the flame holder as shown in Fig. 7. Instead of the intersecting point, the decrease in cooling effectiveness is much larger than that with the flame holders as the blowing ratio increases. At a high blowing ratio, the difference in the velocity increase between the main flow and the secondary flow leads to an increase in the flow disturbance. The high flow
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disturbance due to the flame holder interferes the coolant stability. Therefore, with the flame holder cases, the film cooling performance decreases because the deterioration of the coolant flow caused by the high turbulence affects the improvement in cooling performance more seriously than the increase in the coolant flow at the high blowing ratio. In the third slot, the flame holder decreases a little film cooling effectiveness, as shown in Fig. 8. High disturbed and accelerated flow caused by the flame holder does not reach to the third slot, so there are parallel flows such as film cooling appearance. Therefore the higher blowing ratio results in the better cooling performance. However, the improvement rate is not linear. Although the coolant flow rate increases twofold, the cooling effectiveness does not increase twofold. From the results shown in Fig. 6, the first slot has better cooling performances region in the flame holder installed case than that without the flame holder. However, in the second slot, the flame holder has a negative effect on cooling effectiveness continuously with the increase of blowing ratios as shown in Fig. 9. Therefore, thermal design in the second slot should consider seriously this degradation with the flame holder. In case of high blowing ratio for thermal robust design, there is an advantage that the improvement of film cooling effectiveness region is extended by the flame holder in the first slot. However, there is also a disadvantage in the high blowing ratio case, which is the sudden decrease of film cooling effectiveness in the second slot by the flame holder. Therefore, moderate blowing ratios should be selected in each slot by thermal design for effective operation. 4. Conclusions In the present study, the effect of a flame holder on film cooling effectiveness was investigated for a multi-slot film cooling system in a ramjet combustor. Film cooling effectiveness was measured using TLC experiments. The conclusions are summarized as follows: Fig. 8. Comparison of film cooling effectiveness in third slot with flame holders and without flame holders (blowing ratio: M ¼ rj Uj =rN UN ).
1. Main flows are accelerated around the first slot due to the blockage effect of the inserted flame holder. 2. In the first slot, film cooling effectiveness has an intersecting point and it recedes from the slot starting point due to the suppressed coolant as a result of the accelerated flow and the high disturbance of the wake. 3. In the second slot, the difference in film cooling effectiveness between with and without the flame holder cases becomes higher with the increase in the blowing ratio because the high disturbed flow has a significant effect. 4. Unlike the first slot, film cooling effectiveness decreases significantly with the flame holder in the second slot. It should be considered in thermal design for multi-stage film cooling, especially at the high blowing ratio. 5. In the third slot, the flame holder affects film cooling effectiveness a little. High blowing rate makes better cooling performance in both cases such as ordinary film cooling systems. Hence, in thermal design of film cooling in the third slot, the result from the flame holder does not need to be considered seriously. 6. For effective operation in ramjet combustor and prevention of the thermal defect, proper blowing ratio in each slot should be selected and designed. References
Fig. 9. Film cooling effectiveness at x/s2 ¼ 50 with and without flame holders in the second slot with various blowing ratios.
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