- Email: [email protected]
Effects of exhaust temperature on helicopter infrared signature
Applied Thermal Engineering 51 (2013) 529e538
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Effects of exhaust temperature on helicopter infrared signature Pan Cheng-xiong*, Zhang Jing-zhou, Shan Yong College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nan-Jing Yu-Dao Street 29th, Nanjing 210016, China
h i g h l i g h t s < The < The < The < The < The
effect of exhaust temperature on infrared signature for a helicopter is numerically investigated. impact of exhaust temperature on helicopter skin temperature is revealed. impact of exhaust temperature on plume radiation characteristics is revealed. impact of exhaust temperature on helicopter skin radiation is revealed. impact of exhaust temperature on helicopter’s total infrared radiation intensity is revealed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2012 Accepted 16 September 2012 Available online 27 September 2012
The effects of exhaust temperature on infrared signature (in 3e5 mm band) for a helicopter equipped with integrative infrared suppressor were numerically investigated. The internal flow of exhaust gas and the external downwash flow, as well as the mixing between exhaust gas and downwash were simulated by CFD software to determine the temperature distributions on the helicopter skin and in the exhaust plume. Based on the skin and plume temperature distributions, a forwardebackward ray-tracing method was used to calculate the infrared radiation intensity from the helicopter with a narrow-band model. The results show that for a helicopter with its integrative infrared suppressor embedded inside its rear airframe, the exhaust temperature has significant influence on the plume radiation characteristics, while the helicopter skin radiation intensity has little impact. When the exhaust temperature is raised from 900 K to 1200 K, the plume radiation intensity in 3e5 mm band is increased by about 100%, while the skin radiation intensity is increased by only about 5%. In general, the effects of exhaust temperature on helicopter infrared radiation intensity are mainly concentrated on plume, especially obvious for a lower skin emissivity case. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Helicopter CFD Heat transfer Stealth Infrared suppressor
1. Introduction Helicopters are platforms of battlefield force transferring and anti-tank missions. They also play important roles in air to ground fire covering and short distance air to air fights [1]. Due to their high maneuverability, helicopters are of increasing importance in local conflicts and counter terrorism military actions in recent decades. But it is also observed that MAN Portable Air Defense Systems (MANPADS) especially infrared (IR) guided missiles have caused severe casualties to helicopters in recent warfare such as Gulf war and Afghanistan war [2]. Under the threat of MANPADS, to enhance helicopters’ survivability in battlefield, especially their infrared stealth capability has become a major factor in modern helicopter design and manufacturing.
* Corresponding author. E-mail address: [email protected] (P. Cheng-xiong). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.09.016
There have been a great amount of researches and investigations about aero vehicles including helicopters, focusing on their infrared signal characteristics and suppression. As for aero vehicles exhaust system and plume IR signal, Ponton et al. [3] demonstrated a helicopter IR suppressor which balances engine exhaust signal and installation penalty; Rao et al. [4] modeled spectral IR signal from an aircraft exhaust plume and evaluated the effect of engine bypass ratio on it; Bettini et al. [5] presented a fluid dynamic analysis of an infrared suppressor system for a helicopter engine and compared the results to available experimental data; Wang and Li [6] did computational research on an exhaust system with a heat shelter nozzle used on helicopters; Shan and Zhang [7] used numerical calculation to investigate the IR radiation difference between three mixer configurations of a turbofan engine; Shan et al. [8] presented IR signal of an IR suppressor used on helicopters with both numerical calculation and experimental data; Liu et al. [9] reported the exhaust system and plume flow field and IR characteristics of an aircraft; Eriqitai et al. [10] researched the IR
530
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
signal difference between two specific exhaust system for turbofan engine; Shan and Zhang [11] presented the relationship of IR signals between IR suppressors with the same structure but different scale size; As for IR signal characteristics of aero vehicle fuselage skin, Mahulikar et al. [12] investigated the impact of sun, sky and earth radiation on an aircraft IR signal; considering that fuselage skin temperature has a tremendous impact on aircraft IR radiation, Xia et al. [13] analyzed the impact of transient temperature fields of fuselage skin on total IR signal of an aircraft; Luand Wang [14,15] modeled and investigated the effects of temperature and emissivity of an aircraft fuselage skin on its IR radiation characteristics and pointed out the impact of different parts of the fuselage on the whole fuselage IR signal; Chen et al. [16] investigated the impact of helicopter skin temperature on its IR characteristics, the temperature was simulated by virtual heat sources on the fuselage skin. Recently, as the ratio of turbo-shaft engine horse power to its weight increases tremendously, the total temperature at the exit of thermodynamic cycle for aero-engine boosts which makes the IR signal of helicopters augments intensively. In this paper, the effect of exhaust temperature on infrared signature (in 3e5 mm band) for a fictitious helicopter equipped with an integrative infrared suppressor will be numerically investigated. The Fluent-CFD software is used to simulate the internal flow of exhaust gas and external flow of downwash, as well as the mixing between exhaust gas and downwash, to determine the temperature distributions on the helicopter fuselage and in the exhaust plume. When the temperature distributions on the skin and plume are acquired, a forwardebackward ray-tracing method is used to calculate the infrared radiation intensity of the helicopter using a narrow-band model in which the absorption coefficients are determined according to the Handbook of Infrared radiation from combustion gases [17].
2. Physical model The helicopter model is shown in Fig. 1 composed by head, pilot cabin, engine cabin, exhaust system, rotor and the tail. It is 5170 kg in weight with a size of 15.8 m in length, 2.55 m in width and 3.36 m in height. The diameter of the main rotor is 13.8 m, and the main rotor incidence angle is 10 . The angle between rotation axis and y-coordinate is set as 4 . Two turbo-shaft engines are installed in both sides of the helicopter’s engine cabin. The mass flow rate of each engine is 2.5 kg/s and the exhaust temperatures are presumed as 900 K, 1000 K and 1200 K, respectively. Two sets of exhaust tailpipes composed of lobed nozzle and mixing tailpipes are embedded inside the rear airframe of the helicopter as seen in Fig. 2. The exhaust gas firstly flows into the lobed nozzle then flows out to the mixing tailpipe that follows the lobed nozzle and exits from the slot of tailpipe. The mixing tailpipe has an inlet area of 0.0113 m2 and an outlet area of 0.01822 m2 with a slot shape. The outlet slot section length-to-width ratio is 24. A pair of hatches are set on top surface of the rear airframe to let downwash flow into the rear airframe to cool the exhaust tailpipes down and dilute hot exhaust plume. The rear airframe wall is designed as doublelayered. The inner layer is added to suppress the radiation heat transfer between the tailpipes and outer layer of rear airframe skin. A detailed picture of its exhaust system and flow/heat exchange condition is shown in Fig. 3. The whole computational domain can be seen in Fig. 4. 3. Computational method 3.1. Flow field simulation The mass fractions of N2, CO2 and H2O in the exhaust gas are obtained from kerosene (C8H16) reaction equations, their values are
Fig. 1. Fictitious helicopter model.
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
531
Fig. 2. Detail of helicopter exhaust system and grid meshes.
0.706, 0.209 and 0.085, respectively. At boundaries surrounding the helicopter, the atmosphere is composed of N2 and O2, and the mass fractions are 0.756 and 0.244, respectively. For the boundaries surrounding the helicopter, as seen in Fig. 4, they are more than 10 times of the main rotor diameter away and conditions at those boundaries are considered to be approximately undisturbed atmosphere [18]. The flow condition (axis and tangent components) of rotor downwash is obtained from a rotor downwash model [19]. Unstructured grids generated by ICEM-CFD software are adopted to discretize the computational domain. Yþ on lobed nozzle, mixing tailpipe and exhaust system airframe walls are between 60 and 150. Grids of exhaust system are displayed in Fig. 2. The computation is performed in Fluent software. In the computation, the governing equations in CFD method to acquire helicopter fuselage and plume temperatures and species distributions include conservation of mass, momentum and energy, as well as species transport and radiative heat transfer equations. Those equations are listed as follows:
V$ðr! vÞ ¼ 0
(1)
rð! v $VÞ! v ¼ Vp þ V$s
(2)
2 V$½! v ðrE þ pÞ ¼ V$4leff VT
X
(3)
j
! V$ r! v Yj ¼ V$ J j
(4)
h . i sT 4 ! ! ! ! V$ L r ; s s þ ka Lð r ; s Þ ¼ ka
p
3.2. Infrared signature modeling With the temperature and gas species fields predicted above, a forwardebackward ray-tracing method [7,8,20,21] has been adopted for infrared radiation characteristic analysis in the present paper. Ray-tracing is a statistical method which traces rays of light from a source or detector to a target. In the forward ray-tracing process, rays originating from a detector are targeted toward the helicopter at a certain solid angle shown in Fig. 5. Specifically the solid angle covering the target is discretized into several hundred azimuthal angles and latitudinal angles with D4 and Dq respectively. There are three kinds of routes of the forward rays. Some rays have intersection with neither the fuselage, nor the exhaust plume, thus, the corresponding radiation energy is ignored. While some rays may pass through exhaust plume but have no intersections with fuselage so that their radiations are controlled by radiation transfer equation in absorbinge emitting medium as below:
dLl ¼ kal ðLl Lbl Þ dl
3
! hj J j 5
In the above equations, all convection terms are discretized by second-order upwind scheme while diffusion terms are second order central differenced. Pressureevelocity coupling algorithm SIMPLEC is adopted.
where kal is spectrum absorption coefficient determined by pressure, mole fraction of radiative gas species and temperature according to the CFD results deduced by narrow-band model, Ll is spectrum radiance in ray direction. Some rays arrive at fuselage wall surfaces. Thus radiance of wall surface i is,
(5)
where r is density of gas, ! v is velocity vector, p is static pressure, s is stress tensor, E is total energy, leff is effective conductivity (leff ¼ l þ lt , where lt is turbulent thermal conductivity), T is ! temperature, hj and J j represent enthalpy and diffusion flux for . species j respectively, Yj is local mass fraction of species, r is ! position vector, s is direction vector, ka is absorption coefficient, s is StefaneBoltzmann constant, L is radiance. The emissivity for tailpipes is set as 0.8 and absorption coefficient of plume as 0.1 m1.
(6)
Ll;i ¼
3 i Lb;l ðTi Þ
þ
1 3i
p
Hl;i
(7)
where the first term on the right side is the radiance of wall i itself, the second term is reflection of radiance from other walls to wall i, the walls are all assumed as diffusive and gray, 3 i is the emissivity of wall i. The rays which interact with plume or fuselage wall surfaces are recorded separately. In the backward process, every beam of energy emitted from target toward the receiver (detector) is summed along the optical
532
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
Fig. 3. Internal flow and heat transfer process inside helicopter exhaust system.
Fig. 4. Whole computational domain.
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
533
Table 2 Trail testing of Nq and N4. Nq ¼ 120, Nq ¼ 120, Nq ¼ 120, Nq ¼ 60, Nq ¼ 30, N4 ¼ 360 N4 ¼ 180 N4 ¼ 90 N4 ¼ 360 N4 ¼ 360 Plume infrared 6.61372 radiation intensity (W/Sr) Relative error e Time consumption (h) 120
6.58727
6.81213
6.7460
6.21689
0.4% 60
þ3% 30
þ2% 60
6% 30
The above infrared radiation computational method was validated by previous works by Shan et al. [7,8,11]. An intensive description of computational procedure and validation with experiments can be found in study by Shan et al. [7,8]. 4. Results and discussion 4.1. Flow field simulation validation
Fig. 5. Sketch of solid angle and optical path.
path of the recorded rays. The receiving route is divided into many segments of Ds also shown in Fig. 5 to consider the absorption and emission effects in the radiation transfer route as follows:
Ll ¼ L0l s1l s2l /snl þ L1bl ð1 s1l Þs2l s3l /snl þ / þ Ln1 bl 1 sðn1Þl snl þ Lnbl ð1 snl Þ
(8)
where sil ¼ expðDs kal;i Þ. After backward process, the detector receives all recorded rays, the irradiance of detector can be determined according to,
H ¼
NB X N X j¼1 i¼1
Lnl;ði;jÞ coswi DUi Ddj
(9)
where NB is the total band number, N is the number of recorded rays, wi is the angle between ray and the normal of wall i, DUi is the solid angle of i th ray, Ddj is the spectrum interval. Furthermore, assuming that the target is regarded as a point source, the radiant intensity of the target can be obtained as,
I ¼ H$R2
The predicted results by CFD method are compared with experiment [19] to test the mesh quality, turbulence model and rotor downwash model used in this paper. Fig. 6 displays temperature distribution at the mixing nozzle outlet. In the figure, ske represents standard ke3 turbulence model, rng represents renormalization group ke3 turbulence model, rke represents realizable ke3 turbulence model and sst represents shear stress transport keu turbulence model. Black spots represent values from the experimental results. X represents the distance along the mixing tailpipe outlet length direction, D represents the mixing tailpipe inlet diameter. From Fig. 6 it is quite clear that shear stress transport keu turbulence model fits very well with experimental results. And the predicted results from both 5.5 million and 10 million grids agree with each other, which means the mesh structure with 5.5 million grids is already mesh independent. From this figure it can also be observed that the temperature is relatively lower at the leading edge of the mixing tailpipe outlet while higher at the trailing edge which means the turbulent mixing phenomena or turbulent viscosity predicted by different turbulence models are
(10)
where R is the distance between detector and helicopter. Apparently the discretization number of azimuthal angle (N4), latitudinal angle (Nq) and optical path segments Ds are of great importance on predicted results of helicopter infrared signature. More discretization numbers lead to precise predicted results but end up with unacceptable time consumption. A trial test is carried out on 900 K exhaust temperature case. The detector is deployed at 90 azimuth angle (as seen in Fig. 1). Firstly, keeping Nq ¼ 120 and N4 ¼ 360 while changing Ds. Secondly, keeping Ds ¼ 0.06 m while changing Nq and N4. Results are displayed in Tables 1 and 2. Through the trial test, a favorable choice of Ds ¼ 0.06 m, N4 ¼ 180 and Nq ¼ 120 is adopted in the present paper.
Fig. 6. Temperature distribution at mixing tailpipe outlet.
Table 1 Trail testing of Ds.
Plume infrared radiation intensity (W/Sr) Relative error Time consumption (h)
Ds ¼ 0.02 m
Ds ¼ 0.04 m
Ds ¼ 0.06 m
Ds ¼ 0.08 m
Ds ¼ 0.1 m
Ds ¼ 0.2 m
6.63997 e 360
6.66632 þ0.4% 180
6.61372 0.4% 120
6.77171 þ2% 90
6.48187 2% 72
6.16568 7% 36
534
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
Fig. 7. Temperature distribution at the ridge of mixing tailpipe.
of crucial importance as to simulate the flow field structure correctly. The shear stress transport keu turbulence model has a potential of predicting the proper turbulent viscosity in such a condition because of its fame for “the low Reynolds number turbulence model” and “more accurate and reliable for adverse pressure gradient flows, vortex and streamline curvature” (FLUENT Users’ Manual) while ke3 turbulence model is a classical high Reynolds number model with great turbulent mixing in the flow field so the results from Fig. 6 agree quite well with past experience. The reason is that the internal flow inside the mixing tailpipe is affected by centrifugal force during flow path curvature (also can be seen in Figs. 10e12). Fig. 7 shows the temperature distribution on the ridge of the mixing tailpipe. It should be mentioned that the downwash velocity was evenly distributed downwards by a low pressure blower in the experiment while in the downwash model it has a unique axis and tangent components resembling a real hovering helicopter. So there is a minor difference between them. According to the rotor geometry and weight of the helicopter, the downwash averaged velocity is about 12 m/s in the downwash model. But still they match well as displayed in Fig. 7.
4.2. Flow field and temperature distribution Fig. 8 shows temperature contours in the middle sectional plane on the rear airframe of helicopter with three different exhaust temperatures. It can be seen that exhaust plume distributions for
Fig. 9. Flow field inside mixing tailpipe under exhaust temperature of 900 K.
both sides are not symmetrical due to tangent component of downwash velocity. The effect becomes weaker for the higher exhaust temperature case. As the exhaust temperature rises, the exhaust velocity will be increased under the same exhaust mass flow rate, thus the impact of downwash on plume flow is weakened. With the mixing of rotor downwash, the exhaust plume is diluted soon after it flows out of the exhaust outlet. Figs. 9e11 show the flow fields inside mixing tailpipe under three different exhaust temperatures. Since the flow section of the mixing tailpipe is transited from round to slot with a 90 bend, the exhaust plume with high inlet momentum is difficult to deflect at the leading edge of the bend-corner, resulting in a higher exhaust velocity and temperature at the trailing edge than at the leading edge. As exhaust temperature rises, adverse pressure gradient from the trailing edge to the leading edge is more obvious, and the velocity and temperature inside the mixing tailpipe are increased.
Fig. 8. Plume flow field.
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
535
Fig. 12. Temperature distribution on mixing tailpipe wall. Fig. 10. Flow field inside mixing tailpipe under exhaust temperature of 1000 K.
Fig. 12 shows temperature distribution on the mixing tailpipe wall under three different exhaust temperatures. As exhaust temperature is raised from 900 K to 1200 K, the maximum temperature on mixing tailpipe wall is increased by about 100 K. Although the mixing tailpipe is not directly detectable by the infrared detector since it is embedded inside the rear airframe of helicopter, the radiative heat transfer between mixing tailpipe and helicopter skin will lead to temperature increase on the helicopter skin.
Fig. 11. Flow field inside mixing tailpipe under exhaust temperature of 1200 K.
Figs. 13e15 show temperature distributions on outer surface of helicopter rare airframe under three different exhaust temperatures. The helicopter skin and inner layer shelter surface emissivity are set to 0.2, 0.5 and 0.8, respectively. Because the gap between
Fig. 13. Temperature on helicopter skin with emissivity of 0.2.
536
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
Fig. 14. Temperature on helicopter skin with emissivity of 0.5.
Fig. 15. Temperature on helicopter skin with emissivity of 0.8.
tailpipe and helicopter skin is very small, the helicopter skin near the tailpipe is subjected to serious radiative heating. As exhaust temperature rises, the temperature on helicopter skin is increased, however quite slightly, owing to the fact that helicopter skin is sheltered by an inner layer. When exhaust temperature is 1200 K, the local skin temperature near the tailpipe outlet is 15e30 K higher than atmospheric temperature when the wall surface emissivity is assumed to be 0.8. Once the lower emissivity surface is used, the local skin temperature near the tailpipe outlet is 7e20 K higher than atmospheric temperature, indicating the radiative heat flux between the tailpipe and helicopter skin is suppressed.
higher than that at 270 azimuth angle due to the action of tangent velocity component of downwash. Fig. 17 shows helicopter skin radiation intensity in band 3e5 mm. It can be seen that helicopter skin radiation intensity is slightly
4.3. Infrared signature In the following part, the infrared radiation intensity characteristics in spectrum 3e5 mm of the helicopter will be discussed in XZ plane (as shown in Fig. 1). In this plane there are 36 infrared detectors deployed around a circle 100 m away in radius from the helicopter rotor center. The effect of atmosphere attenuation is ignored here to preserve the original characteristics of the helicopter infrared radiation signature. Fig.16 shows helicopter plume radiation intensity in band 3e5 mm under three different exhaust temperatures. It is clear that the exhaust temperature has a very dominant impact on plume radiation characteristics. When the exhaust temperature is raised from 900 K to 1200 K, plume radiation intensity is increased by about 100%. Therefore in real practice, it is of vital importance to control helicopter exhaust temperature to its lowest level. Another phenomenon worth noting is that radiation intensity at 90 azimuth angle is about 5%
Fig. 16. Plume infrared radiation intensity in 3e5 mm band.
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
537
Table 3 Helicopter skin infrared radiation intensity at 90 azimuth angle (W/Sr).
3 3 3
¼ 0.2 ¼ 0.5 ¼ 0.8
900 K
1000 K
1200 K
6.63 16.99 28.34
6.68 17.14 28.80
6.74 17.65 29.80
radiation intensity is increased by about 25%. When the skin emissivity is higher, the contribution of skin radiation to total infrared radiation intensity predominates over that of the plume. Once the skin emissivity is lowered to 0.5, the contribution of skin radiation to total infrared radiation intensity is equivalent to that of plume. Furthermore, when the skin emissivity is lowered to 0.2, the contribution of skin radiation to total infrared radiation intensity is obviously weaker than that of the plume (as seen in Fig. 18(b)).
Fig. 17. Skin infrared radiation intensity in 3e5 mm band.
influenced by exhaust temperature for a given skin emissivity. When the exhaust temperature is raised from 900 K to 1200 K, skin radiation intensity is increased by only about 5%. Because the helicopter skin is sheltered by an inner layer, the temperature differences on the skin surface for different exhaust temperatures are not big enough to affect the helicopter skin radiation intensity. It is also notable that the skin emissivity is a key factor affecting the helicopter skin radiation intensity. On one hand, lower skin emissivity is beneficial for suppressing radiative heat flux between the tailpipe and helicopter skin, while on the other hand, the infrared radiation capacity is proportion to skin emissivity. Table 3 shows the helicopter skin infrared radiation intensity at 90 azimuth angle. Fig. 18(a) shows the helicopter total infrared radiation intensity in 3e5 mm band when the skin emissivity is set at 0.8. As the exhaust temperature rises from 900 K to 1200 K, total infrared
Fig. 18. Total infrared radiation intensity in 3e5 mm band.
538
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
Based on the above analysis, it is deduced that the effect of exhaust temperature on total infrared radiation intensity is mainly concentrated to the plume. And this effect is more obvious for the lower skin emissivity case.
l r 3
s w
5. Conclusions The effects of exhaust temperature on infrared signature of a helicopter equipped with integrative infrared suppressor are investigated. The results are summarized as follows: 1) As the exhaust temperature is raised from 900 K to 1200 K, the maximum temperature on mixing tailpipe wall is increased by about 100 K. The helicopter skin temperature is slightly impacted by exhaust temperature owing to that the helicopter skin is sheltered by an inner layer. 2) The exhaust temperature has a very dominant influence on plume radiation characteristics. When the exhaust temperature is raised from 900 K to 1200 K, plume radiation intensity in 3e5 mm band is increased by about 100%. 3) The helicopter skin radiation intensity is slightly impacted by exhaust temperature. The key factor affecting the helicopter skin radiation intensity is skin emissivity. Lower skin emissivity is beneficial for suppressing radiative heat flux between the tailpipe and helicopter skin, and weakening the skin infrared radiation capacity at the same time. 4) The effects of exhaust temperature on helicopter’s total infrared radiation intensity are mainly concentrated to the plume. And this effect is more obvious for the lower skin emissivity case. Acknowledgements This work was supported by funding of Jiangsu Innovation Program for Graduate Education (Fund No. CXZZ11_0222). Also we would like to express our gratefulness to Benjamin Ames for his kind offering of time in editing this manuscript to make it fluent and native in expressions. Nomenclature E h H I ! J ka L N NB p . r R ! s Ds T ! v Y
specific total energy (J/kg) specific enthalpy (J/kg) irradiance (W/m2) radiation intensity (W/Sr) diffusion flux (kg s/m2) absorption coefficient (1/m) radiance (W/(Sr m2)) number (e) band number (e) pressure (Pa) position vector (m) distance between detector and helicopter (m) direction vector (m) optical path segments (m) temperature (K) velocity vector (m/s) mass fraction (e)
Greek symbols s stress tensor (Pa)
Dd DU
effective conductivity (W/(m K)) density (kg/m3) emissivity (e) StefaneBoltzmann constant (5.67 108W/(m2 K4)) angle between ray and the normal of wall (degree) spectrum interval (mm) solid angle (Sr)
Subscripts l spectrum 4 azimuthal direction q latitudinal direction Abbreviations HITRAN high resolution atmospheric transmission CFD computational fluid dynamics References [1] Zhenbo Wu, Zhe Wu, Research on infrared stealth system for armed helicopter, Journal of Beijing University of Aeronautics and Astronautics 29 (7) (2003) 588e592 (in Chinese). [2] S.P. Mahulikar, H.R. Sonawane, G.A. Rao, Infrared signature studies of aerospace vehicles, Progress in Aerospace Sciences 43 (2007) 218e245. [3] T. Ponton, G. Warnes, Helicopter IRS engine integration for the “first” technology demonstrator programme, ASME GT2007-27408. [4] G.A. Rao, J.P. Buijtenen, S.P. Mahulikar, The effect bypass ratio on aircraft plume infrared signatures, AIAA ISABE-2009-1194. [5] C. Bettini, C. Cravero, S. Cogliandro, Multidisciplinary analysis of a complete infrared suppression system, ASME GT2007-27721. [6] S.F. Wang, L.G. Li, Investigations of flows in a new infrared suppressor, Applied Thermal Engineering 26 (2006) 36e45. [7] Y. Shan, J.Z. Zhang, Numerical investigation of flow mixture enhancement and infrared radiation shield by lobed forced mixer, Applied Thermal Engineering 29 (2009) 3687e3695. [8] Yong Shan, Jingzhou Zhang, Liguo Li, Numerical calculation and experimental verification for the infrared radiation characteristics of helicopter infrared radiation suppressor, Journal of Infrared and Millimeter Waves 25 (2) (2006) 95e100 (in Chinese). [9] Youhong Liu, Wanren Shao, Jinxiu Zhang, Numerical simulation of flowfield and infrared characteristics of an aeroengine exhaust system and its plume, Journal of Aerospace Power 23 (4) (2008) 591e597 (in Chinese). [10] Eriqitai, Qiang Wang, Weipeng Chen, Comparative investigation of the infrared characteristics for two exhaust systems of a turbofan engine, Journal of Propulsion Technology 24 (4) (2003) 334e337 (in Chinese). [11] Yong Shan, Jingzhou Zhang, Effect of scale factor on infrared radiation characteristics of helicopter infrared radiation suppressor, Journal of Aerospace Power 23 (2) (2008) 221e226 (in Chinese). [12] S.P. Mahulikar, S.K. Potnuru, G.A. Rao, Study of sunshine, skyshine, and earthshine for aircraft infrared detection, Journal of Optics (2009) 045703. [13] Xinlin Xia, Qing Ai, Depeng Ren, Analysis on the transient temperature fields for infrared radiation of aircraft skin, Journal of Infrared and Millimeter Waves 26 (3) (2007) 174e177 (in Chinese). [14] Jianwei Lu, Qiang Wang, Aircraft skin infrared radiation characteristics modeling and analysis, Chinese Journal of Aeronautics 22 (2009) 493e497. [15] Jianwei Lu, Qiang Wang, Effect of temperature and emissivity of aircraft skin on infrared radiation characteristics, Opto-electronic Engineering 36 (2) (2009) 50e54 (in Chinese). [16] Haitao Chen, Enrong Bian, Wenhe Liao, Study on numerical simulation method of infrared radiation calculating form military helicopter body, Journal of Basic Science and Engineering 17 (4) (2009) 614e621 (in Chinese). [17] C.B. Ludwig, Handbook of Infrared Radiation from Combustion Gases, NASA SP 1973-3080. [18] Zi-li Tong, Mao Sun, NaviereStokes analysis of the aerodynamic properties of coaxial rotors, Acta Aeronautica et Astronautica Sinica 20 (4) (1999) 348e350 (in Chinese). [19] Chengxiong Pan, Jingzhou Zhang, Yong Shan, Modeling and analysis of helicopter skin temperature distribution, Acta Aeronautica et Astronautica 32 (2) (2011) 249e256 (in Chinese). [20] M. Johansson, M. Dalenbring, SIGGE, a prediction tool for aeronautical IR signatures, and its applications, AIAA Paper 2006-3276. [21] A.G. Voloboi, V.A. Galaktionov, K.A. Dmitriev, et al., Bidirectional ray tracing for the integration of illumination by the quasi-Monte Carlo method, Programming and Computer Software 30 (5) (2004) 258e265.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Effects of exhaust temperature on helicopter infrared signature Pan Cheng-xiong*, Zhang Jing-zhou, Shan Yong College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nan-Jing Yu-Dao Street 29th, Nanjing 210016, China
h i g h l i g h t s < The < The < The < The < The
effect of exhaust temperature on infrared signature for a helicopter is numerically investigated. impact of exhaust temperature on helicopter skin temperature is revealed. impact of exhaust temperature on plume radiation characteristics is revealed. impact of exhaust temperature on helicopter skin radiation is revealed. impact of exhaust temperature on helicopter’s total infrared radiation intensity is revealed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2012 Accepted 16 September 2012 Available online 27 September 2012
The effects of exhaust temperature on infrared signature (in 3e5 mm band) for a helicopter equipped with integrative infrared suppressor were numerically investigated. The internal flow of exhaust gas and the external downwash flow, as well as the mixing between exhaust gas and downwash were simulated by CFD software to determine the temperature distributions on the helicopter skin and in the exhaust plume. Based on the skin and plume temperature distributions, a forwardebackward ray-tracing method was used to calculate the infrared radiation intensity from the helicopter with a narrow-band model. The results show that for a helicopter with its integrative infrared suppressor embedded inside its rear airframe, the exhaust temperature has significant influence on the plume radiation characteristics, while the helicopter skin radiation intensity has little impact. When the exhaust temperature is raised from 900 K to 1200 K, the plume radiation intensity in 3e5 mm band is increased by about 100%, while the skin radiation intensity is increased by only about 5%. In general, the effects of exhaust temperature on helicopter infrared radiation intensity are mainly concentrated on plume, especially obvious for a lower skin emissivity case. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Helicopter CFD Heat transfer Stealth Infrared suppressor
1. Introduction Helicopters are platforms of battlefield force transferring and anti-tank missions. They also play important roles in air to ground fire covering and short distance air to air fights [1]. Due to their high maneuverability, helicopters are of increasing importance in local conflicts and counter terrorism military actions in recent decades. But it is also observed that MAN Portable Air Defense Systems (MANPADS) especially infrared (IR) guided missiles have caused severe casualties to helicopters in recent warfare such as Gulf war and Afghanistan war [2]. Under the threat of MANPADS, to enhance helicopters’ survivability in battlefield, especially their infrared stealth capability has become a major factor in modern helicopter design and manufacturing.
* Corresponding author. E-mail address: [email protected] (P. Cheng-xiong). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.09.016
There have been a great amount of researches and investigations about aero vehicles including helicopters, focusing on their infrared signal characteristics and suppression. As for aero vehicles exhaust system and plume IR signal, Ponton et al. [3] demonstrated a helicopter IR suppressor which balances engine exhaust signal and installation penalty; Rao et al. [4] modeled spectral IR signal from an aircraft exhaust plume and evaluated the effect of engine bypass ratio on it; Bettini et al. [5] presented a fluid dynamic analysis of an infrared suppressor system for a helicopter engine and compared the results to available experimental data; Wang and Li [6] did computational research on an exhaust system with a heat shelter nozzle used on helicopters; Shan and Zhang [7] used numerical calculation to investigate the IR radiation difference between three mixer configurations of a turbofan engine; Shan et al. [8] presented IR signal of an IR suppressor used on helicopters with both numerical calculation and experimental data; Liu et al. [9] reported the exhaust system and plume flow field and IR characteristics of an aircraft; Eriqitai et al. [10] researched the IR
530
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
signal difference between two specific exhaust system for turbofan engine; Shan and Zhang [11] presented the relationship of IR signals between IR suppressors with the same structure but different scale size; As for IR signal characteristics of aero vehicle fuselage skin, Mahulikar et al. [12] investigated the impact of sun, sky and earth radiation on an aircraft IR signal; considering that fuselage skin temperature has a tremendous impact on aircraft IR radiation, Xia et al. [13] analyzed the impact of transient temperature fields of fuselage skin on total IR signal of an aircraft; Luand Wang [14,15] modeled and investigated the effects of temperature and emissivity of an aircraft fuselage skin on its IR radiation characteristics and pointed out the impact of different parts of the fuselage on the whole fuselage IR signal; Chen et al. [16] investigated the impact of helicopter skin temperature on its IR characteristics, the temperature was simulated by virtual heat sources on the fuselage skin. Recently, as the ratio of turbo-shaft engine horse power to its weight increases tremendously, the total temperature at the exit of thermodynamic cycle for aero-engine boosts which makes the IR signal of helicopters augments intensively. In this paper, the effect of exhaust temperature on infrared signature (in 3e5 mm band) for a fictitious helicopter equipped with an integrative infrared suppressor will be numerically investigated. The Fluent-CFD software is used to simulate the internal flow of exhaust gas and external flow of downwash, as well as the mixing between exhaust gas and downwash, to determine the temperature distributions on the helicopter fuselage and in the exhaust plume. When the temperature distributions on the skin and plume are acquired, a forwardebackward ray-tracing method is used to calculate the infrared radiation intensity of the helicopter using a narrow-band model in which the absorption coefficients are determined according to the Handbook of Infrared radiation from combustion gases [17].
2. Physical model The helicopter model is shown in Fig. 1 composed by head, pilot cabin, engine cabin, exhaust system, rotor and the tail. It is 5170 kg in weight with a size of 15.8 m in length, 2.55 m in width and 3.36 m in height. The diameter of the main rotor is 13.8 m, and the main rotor incidence angle is 10 . The angle between rotation axis and y-coordinate is set as 4 . Two turbo-shaft engines are installed in both sides of the helicopter’s engine cabin. The mass flow rate of each engine is 2.5 kg/s and the exhaust temperatures are presumed as 900 K, 1000 K and 1200 K, respectively. Two sets of exhaust tailpipes composed of lobed nozzle and mixing tailpipes are embedded inside the rear airframe of the helicopter as seen in Fig. 2. The exhaust gas firstly flows into the lobed nozzle then flows out to the mixing tailpipe that follows the lobed nozzle and exits from the slot of tailpipe. The mixing tailpipe has an inlet area of 0.0113 m2 and an outlet area of 0.01822 m2 with a slot shape. The outlet slot section length-to-width ratio is 24. A pair of hatches are set on top surface of the rear airframe to let downwash flow into the rear airframe to cool the exhaust tailpipes down and dilute hot exhaust plume. The rear airframe wall is designed as doublelayered. The inner layer is added to suppress the radiation heat transfer between the tailpipes and outer layer of rear airframe skin. A detailed picture of its exhaust system and flow/heat exchange condition is shown in Fig. 3. The whole computational domain can be seen in Fig. 4. 3. Computational method 3.1. Flow field simulation The mass fractions of N2, CO2 and H2O in the exhaust gas are obtained from kerosene (C8H16) reaction equations, their values are
Fig. 1. Fictitious helicopter model.
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
531
Fig. 2. Detail of helicopter exhaust system and grid meshes.
0.706, 0.209 and 0.085, respectively. At boundaries surrounding the helicopter, the atmosphere is composed of N2 and O2, and the mass fractions are 0.756 and 0.244, respectively. For the boundaries surrounding the helicopter, as seen in Fig. 4, they are more than 10 times of the main rotor diameter away and conditions at those boundaries are considered to be approximately undisturbed atmosphere [18]. The flow condition (axis and tangent components) of rotor downwash is obtained from a rotor downwash model [19]. Unstructured grids generated by ICEM-CFD software are adopted to discretize the computational domain. Yþ on lobed nozzle, mixing tailpipe and exhaust system airframe walls are between 60 and 150. Grids of exhaust system are displayed in Fig. 2. The computation is performed in Fluent software. In the computation, the governing equations in CFD method to acquire helicopter fuselage and plume temperatures and species distributions include conservation of mass, momentum and energy, as well as species transport and radiative heat transfer equations. Those equations are listed as follows:
V$ðr! vÞ ¼ 0
(1)
rð! v $VÞ! v ¼ Vp þ V$s
(2)
2 V$½! v ðrE þ pÞ ¼ V$4leff VT
X
(3)
j
! V$ r! v Yj ¼ V$ J j
(4)
h . i sT 4 ! ! ! ! V$ L r ; s s þ ka Lð r ; s Þ ¼ ka
p
3.2. Infrared signature modeling With the temperature and gas species fields predicted above, a forwardebackward ray-tracing method [7,8,20,21] has been adopted for infrared radiation characteristic analysis in the present paper. Ray-tracing is a statistical method which traces rays of light from a source or detector to a target. In the forward ray-tracing process, rays originating from a detector are targeted toward the helicopter at a certain solid angle shown in Fig. 5. Specifically the solid angle covering the target is discretized into several hundred azimuthal angles and latitudinal angles with D4 and Dq respectively. There are three kinds of routes of the forward rays. Some rays have intersection with neither the fuselage, nor the exhaust plume, thus, the corresponding radiation energy is ignored. While some rays may pass through exhaust plume but have no intersections with fuselage so that their radiations are controlled by radiation transfer equation in absorbinge emitting medium as below:
dLl ¼ kal ðLl Lbl Þ dl
3
! hj J j 5
In the above equations, all convection terms are discretized by second-order upwind scheme while diffusion terms are second order central differenced. Pressureevelocity coupling algorithm SIMPLEC is adopted.
where kal is spectrum absorption coefficient determined by pressure, mole fraction of radiative gas species and temperature according to the CFD results deduced by narrow-band model, Ll is spectrum radiance in ray direction. Some rays arrive at fuselage wall surfaces. Thus radiance of wall surface i is,
(5)
where r is density of gas, ! v is velocity vector, p is static pressure, s is stress tensor, E is total energy, leff is effective conductivity (leff ¼ l þ lt , where lt is turbulent thermal conductivity), T is ! temperature, hj and J j represent enthalpy and diffusion flux for . species j respectively, Yj is local mass fraction of species, r is ! position vector, s is direction vector, ka is absorption coefficient, s is StefaneBoltzmann constant, L is radiance. The emissivity for tailpipes is set as 0.8 and absorption coefficient of plume as 0.1 m1.
(6)
Ll;i ¼
3 i Lb;l ðTi Þ
þ
1 3i
p
Hl;i
(7)
where the first term on the right side is the radiance of wall i itself, the second term is reflection of radiance from other walls to wall i, the walls are all assumed as diffusive and gray, 3 i is the emissivity of wall i. The rays which interact with plume or fuselage wall surfaces are recorded separately. In the backward process, every beam of energy emitted from target toward the receiver (detector) is summed along the optical
532
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
Fig. 3. Internal flow and heat transfer process inside helicopter exhaust system.
Fig. 4. Whole computational domain.
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
533
Table 2 Trail testing of Nq and N4. Nq ¼ 120, Nq ¼ 120, Nq ¼ 120, Nq ¼ 60, Nq ¼ 30, N4 ¼ 360 N4 ¼ 180 N4 ¼ 90 N4 ¼ 360 N4 ¼ 360 Plume infrared 6.61372 radiation intensity (W/Sr) Relative error e Time consumption (h) 120
6.58727
6.81213
6.7460
6.21689
0.4% 60
þ3% 30
þ2% 60
6% 30
The above infrared radiation computational method was validated by previous works by Shan et al. [7,8,11]. An intensive description of computational procedure and validation with experiments can be found in study by Shan et al. [7,8]. 4. Results and discussion 4.1. Flow field simulation validation
Fig. 5. Sketch of solid angle and optical path.
path of the recorded rays. The receiving route is divided into many segments of Ds also shown in Fig. 5 to consider the absorption and emission effects in the radiation transfer route as follows:
Ll ¼ L0l s1l s2l /snl þ L1bl ð1 s1l Þs2l s3l /snl þ / þ Ln1 bl 1 sðn1Þl snl þ Lnbl ð1 snl Þ
(8)
where sil ¼ expðDs kal;i Þ. After backward process, the detector receives all recorded rays, the irradiance of detector can be determined according to,
H ¼
NB X N X j¼1 i¼1
Lnl;ði;jÞ coswi DUi Ddj
(9)
where NB is the total band number, N is the number of recorded rays, wi is the angle between ray and the normal of wall i, DUi is the solid angle of i th ray, Ddj is the spectrum interval. Furthermore, assuming that the target is regarded as a point source, the radiant intensity of the target can be obtained as,
I ¼ H$R2
The predicted results by CFD method are compared with experiment [19] to test the mesh quality, turbulence model and rotor downwash model used in this paper. Fig. 6 displays temperature distribution at the mixing nozzle outlet. In the figure, ske represents standard ke3 turbulence model, rng represents renormalization group ke3 turbulence model, rke represents realizable ke3 turbulence model and sst represents shear stress transport keu turbulence model. Black spots represent values from the experimental results. X represents the distance along the mixing tailpipe outlet length direction, D represents the mixing tailpipe inlet diameter. From Fig. 6 it is quite clear that shear stress transport keu turbulence model fits very well with experimental results. And the predicted results from both 5.5 million and 10 million grids agree with each other, which means the mesh structure with 5.5 million grids is already mesh independent. From this figure it can also be observed that the temperature is relatively lower at the leading edge of the mixing tailpipe outlet while higher at the trailing edge which means the turbulent mixing phenomena or turbulent viscosity predicted by different turbulence models are
(10)
where R is the distance between detector and helicopter. Apparently the discretization number of azimuthal angle (N4), latitudinal angle (Nq) and optical path segments Ds are of great importance on predicted results of helicopter infrared signature. More discretization numbers lead to precise predicted results but end up with unacceptable time consumption. A trial test is carried out on 900 K exhaust temperature case. The detector is deployed at 90 azimuth angle (as seen in Fig. 1). Firstly, keeping Nq ¼ 120 and N4 ¼ 360 while changing Ds. Secondly, keeping Ds ¼ 0.06 m while changing Nq and N4. Results are displayed in Tables 1 and 2. Through the trial test, a favorable choice of Ds ¼ 0.06 m, N4 ¼ 180 and Nq ¼ 120 is adopted in the present paper.
Fig. 6. Temperature distribution at mixing tailpipe outlet.
Table 1 Trail testing of Ds.
Plume infrared radiation intensity (W/Sr) Relative error Time consumption (h)
Ds ¼ 0.02 m
Ds ¼ 0.04 m
Ds ¼ 0.06 m
Ds ¼ 0.08 m
Ds ¼ 0.1 m
Ds ¼ 0.2 m
6.63997 e 360
6.66632 þ0.4% 180
6.61372 0.4% 120
6.77171 þ2% 90
6.48187 2% 72
6.16568 7% 36
534
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
Fig. 7. Temperature distribution at the ridge of mixing tailpipe.
of crucial importance as to simulate the flow field structure correctly. The shear stress transport keu turbulence model has a potential of predicting the proper turbulent viscosity in such a condition because of its fame for “the low Reynolds number turbulence model” and “more accurate and reliable for adverse pressure gradient flows, vortex and streamline curvature” (FLUENT Users’ Manual) while ke3 turbulence model is a classical high Reynolds number model with great turbulent mixing in the flow field so the results from Fig. 6 agree quite well with past experience. The reason is that the internal flow inside the mixing tailpipe is affected by centrifugal force during flow path curvature (also can be seen in Figs. 10e12). Fig. 7 shows the temperature distribution on the ridge of the mixing tailpipe. It should be mentioned that the downwash velocity was evenly distributed downwards by a low pressure blower in the experiment while in the downwash model it has a unique axis and tangent components resembling a real hovering helicopter. So there is a minor difference between them. According to the rotor geometry and weight of the helicopter, the downwash averaged velocity is about 12 m/s in the downwash model. But still they match well as displayed in Fig. 7.
4.2. Flow field and temperature distribution Fig. 8 shows temperature contours in the middle sectional plane on the rear airframe of helicopter with three different exhaust temperatures. It can be seen that exhaust plume distributions for
Fig. 9. Flow field inside mixing tailpipe under exhaust temperature of 900 K.
both sides are not symmetrical due to tangent component of downwash velocity. The effect becomes weaker for the higher exhaust temperature case. As the exhaust temperature rises, the exhaust velocity will be increased under the same exhaust mass flow rate, thus the impact of downwash on plume flow is weakened. With the mixing of rotor downwash, the exhaust plume is diluted soon after it flows out of the exhaust outlet. Figs. 9e11 show the flow fields inside mixing tailpipe under three different exhaust temperatures. Since the flow section of the mixing tailpipe is transited from round to slot with a 90 bend, the exhaust plume with high inlet momentum is difficult to deflect at the leading edge of the bend-corner, resulting in a higher exhaust velocity and temperature at the trailing edge than at the leading edge. As exhaust temperature rises, adverse pressure gradient from the trailing edge to the leading edge is more obvious, and the velocity and temperature inside the mixing tailpipe are increased.
Fig. 8. Plume flow field.
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
535
Fig. 12. Temperature distribution on mixing tailpipe wall. Fig. 10. Flow field inside mixing tailpipe under exhaust temperature of 1000 K.
Fig. 12 shows temperature distribution on the mixing tailpipe wall under three different exhaust temperatures. As exhaust temperature is raised from 900 K to 1200 K, the maximum temperature on mixing tailpipe wall is increased by about 100 K. Although the mixing tailpipe is not directly detectable by the infrared detector since it is embedded inside the rear airframe of helicopter, the radiative heat transfer between mixing tailpipe and helicopter skin will lead to temperature increase on the helicopter skin.
Fig. 11. Flow field inside mixing tailpipe under exhaust temperature of 1200 K.
Figs. 13e15 show temperature distributions on outer surface of helicopter rare airframe under three different exhaust temperatures. The helicopter skin and inner layer shelter surface emissivity are set to 0.2, 0.5 and 0.8, respectively. Because the gap between
Fig. 13. Temperature on helicopter skin with emissivity of 0.2.
536
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
Fig. 14. Temperature on helicopter skin with emissivity of 0.5.
Fig. 15. Temperature on helicopter skin with emissivity of 0.8.
tailpipe and helicopter skin is very small, the helicopter skin near the tailpipe is subjected to serious radiative heating. As exhaust temperature rises, the temperature on helicopter skin is increased, however quite slightly, owing to the fact that helicopter skin is sheltered by an inner layer. When exhaust temperature is 1200 K, the local skin temperature near the tailpipe outlet is 15e30 K higher than atmospheric temperature when the wall surface emissivity is assumed to be 0.8. Once the lower emissivity surface is used, the local skin temperature near the tailpipe outlet is 7e20 K higher than atmospheric temperature, indicating the radiative heat flux between the tailpipe and helicopter skin is suppressed.
higher than that at 270 azimuth angle due to the action of tangent velocity component of downwash. Fig. 17 shows helicopter skin radiation intensity in band 3e5 mm. It can be seen that helicopter skin radiation intensity is slightly
4.3. Infrared signature In the following part, the infrared radiation intensity characteristics in spectrum 3e5 mm of the helicopter will be discussed in XZ plane (as shown in Fig. 1). In this plane there are 36 infrared detectors deployed around a circle 100 m away in radius from the helicopter rotor center. The effect of atmosphere attenuation is ignored here to preserve the original characteristics of the helicopter infrared radiation signature. Fig.16 shows helicopter plume radiation intensity in band 3e5 mm under three different exhaust temperatures. It is clear that the exhaust temperature has a very dominant impact on plume radiation characteristics. When the exhaust temperature is raised from 900 K to 1200 K, plume radiation intensity is increased by about 100%. Therefore in real practice, it is of vital importance to control helicopter exhaust temperature to its lowest level. Another phenomenon worth noting is that radiation intensity at 90 azimuth angle is about 5%
Fig. 16. Plume infrared radiation intensity in 3e5 mm band.
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
537
Table 3 Helicopter skin infrared radiation intensity at 90 azimuth angle (W/Sr).
3 3 3
¼ 0.2 ¼ 0.5 ¼ 0.8
900 K
1000 K
1200 K
6.63 16.99 28.34
6.68 17.14 28.80
6.74 17.65 29.80
radiation intensity is increased by about 25%. When the skin emissivity is higher, the contribution of skin radiation to total infrared radiation intensity predominates over that of the plume. Once the skin emissivity is lowered to 0.5, the contribution of skin radiation to total infrared radiation intensity is equivalent to that of plume. Furthermore, when the skin emissivity is lowered to 0.2, the contribution of skin radiation to total infrared radiation intensity is obviously weaker than that of the plume (as seen in Fig. 18(b)).
Fig. 17. Skin infrared radiation intensity in 3e5 mm band.
influenced by exhaust temperature for a given skin emissivity. When the exhaust temperature is raised from 900 K to 1200 K, skin radiation intensity is increased by only about 5%. Because the helicopter skin is sheltered by an inner layer, the temperature differences on the skin surface for different exhaust temperatures are not big enough to affect the helicopter skin radiation intensity. It is also notable that the skin emissivity is a key factor affecting the helicopter skin radiation intensity. On one hand, lower skin emissivity is beneficial for suppressing radiative heat flux between the tailpipe and helicopter skin, while on the other hand, the infrared radiation capacity is proportion to skin emissivity. Table 3 shows the helicopter skin infrared radiation intensity at 90 azimuth angle. Fig. 18(a) shows the helicopter total infrared radiation intensity in 3e5 mm band when the skin emissivity is set at 0.8. As the exhaust temperature rises from 900 K to 1200 K, total infrared
Fig. 18. Total infrared radiation intensity in 3e5 mm band.
538
P. Cheng-xiong et al. / Applied Thermal Engineering 51 (2013) 529e538
Based on the above analysis, it is deduced that the effect of exhaust temperature on total infrared radiation intensity is mainly concentrated to the plume. And this effect is more obvious for the lower skin emissivity case.
l r 3
s w
5. Conclusions The effects of exhaust temperature on infrared signature of a helicopter equipped with integrative infrared suppressor are investigated. The results are summarized as follows: 1) As the exhaust temperature is raised from 900 K to 1200 K, the maximum temperature on mixing tailpipe wall is increased by about 100 K. The helicopter skin temperature is slightly impacted by exhaust temperature owing to that the helicopter skin is sheltered by an inner layer. 2) The exhaust temperature has a very dominant influence on plume radiation characteristics. When the exhaust temperature is raised from 900 K to 1200 K, plume radiation intensity in 3e5 mm band is increased by about 100%. 3) The helicopter skin radiation intensity is slightly impacted by exhaust temperature. The key factor affecting the helicopter skin radiation intensity is skin emissivity. Lower skin emissivity is beneficial for suppressing radiative heat flux between the tailpipe and helicopter skin, and weakening the skin infrared radiation capacity at the same time. 4) The effects of exhaust temperature on helicopter’s total infrared radiation intensity are mainly concentrated to the plume. And this effect is more obvious for the lower skin emissivity case. Acknowledgements This work was supported by funding of Jiangsu Innovation Program for Graduate Education (Fund No. CXZZ11_0222). Also we would like to express our gratefulness to Benjamin Ames for his kind offering of time in editing this manuscript to make it fluent and native in expressions. Nomenclature E h H I ! J ka L N NB p . r R ! s Ds T ! v Y
specific total energy (J/kg) specific enthalpy (J/kg) irradiance (W/m2) radiation intensity (W/Sr) diffusion flux (kg s/m2) absorption coefficient (1/m) radiance (W/(Sr m2)) number (e) band number (e) pressure (Pa) position vector (m) distance between detector and helicopter (m) direction vector (m) optical path segments (m) temperature (K) velocity vector (m/s) mass fraction (e)
Greek symbols s stress tensor (Pa)
Dd DU
effective conductivity (W/(m K)) density (kg/m3) emissivity (e) StefaneBoltzmann constant (5.67 108W/(m2 K4)) angle between ray and the normal of wall (degree) spectrum interval (mm) solid angle (Sr)
Subscripts l spectrum 4 azimuthal direction q latitudinal direction Abbreviations HITRAN high resolution atmospheric transmission CFD computational fluid dynamics References [1] Zhenbo Wu, Zhe Wu, Research on infrared stealth system for armed helicopter, Journal of Beijing University of Aeronautics and Astronautics 29 (7) (2003) 588e592 (in Chinese). [2] S.P. Mahulikar, H.R. Sonawane, G.A. Rao, Infrared signature studies of aerospace vehicles, Progress in Aerospace Sciences 43 (2007) 218e245. [3] T. Ponton, G. Warnes, Helicopter IRS engine integration for the “first” technology demonstrator programme, ASME GT2007-27408. [4] G.A. Rao, J.P. Buijtenen, S.P. Mahulikar, The effect bypass ratio on aircraft plume infrared signatures, AIAA ISABE-2009-1194. [5] C. Bettini, C. Cravero, S. Cogliandro, Multidisciplinary analysis of a complete infrared suppression system, ASME GT2007-27721. [6] S.F. Wang, L.G. Li, Investigations of flows in a new infrared suppressor, Applied Thermal Engineering 26 (2006) 36e45. [7] Y. Shan, J.Z. Zhang, Numerical investigation of flow mixture enhancement and infrared radiation shield by lobed forced mixer, Applied Thermal Engineering 29 (2009) 3687e3695. [8] Yong Shan, Jingzhou Zhang, Liguo Li, Numerical calculation and experimental verification for the infrared radiation characteristics of helicopter infrared radiation suppressor, Journal of Infrared and Millimeter Waves 25 (2) (2006) 95e100 (in Chinese). [9] Youhong Liu, Wanren Shao, Jinxiu Zhang, Numerical simulation of flowfield and infrared characteristics of an aeroengine exhaust system and its plume, Journal of Aerospace Power 23 (4) (2008) 591e597 (in Chinese). [10] Eriqitai, Qiang Wang, Weipeng Chen, Comparative investigation of the infrared characteristics for two exhaust systems of a turbofan engine, Journal of Propulsion Technology 24 (4) (2003) 334e337 (in Chinese). [11] Yong Shan, Jingzhou Zhang, Effect of scale factor on infrared radiation characteristics of helicopter infrared radiation suppressor, Journal of Aerospace Power 23 (2) (2008) 221e226 (in Chinese). [12] S.P. Mahulikar, S.K. Potnuru, G.A. Rao, Study of sunshine, skyshine, and earthshine for aircraft infrared detection, Journal of Optics (2009) 045703. [13] Xinlin Xia, Qing Ai, Depeng Ren, Analysis on the transient temperature fields for infrared radiation of aircraft skin, Journal of Infrared and Millimeter Waves 26 (3) (2007) 174e177 (in Chinese). [14] Jianwei Lu, Qiang Wang, Aircraft skin infrared radiation characteristics modeling and analysis, Chinese Journal of Aeronautics 22 (2009) 493e497. [15] Jianwei Lu, Qiang Wang, Effect of temperature and emissivity of aircraft skin on infrared radiation characteristics, Opto-electronic Engineering 36 (2) (2009) 50e54 (in Chinese). [16] Haitao Chen, Enrong Bian, Wenhe Liao, Study on numerical simulation method of infrared radiation calculating form military helicopter body, Journal of Basic Science and Engineering 17 (4) (2009) 614e621 (in Chinese). [17] C.B. Ludwig, Handbook of Infrared Radiation from Combustion Gases, NASA SP 1973-3080. [18] Zi-li Tong, Mao Sun, NaviereStokes analysis of the aerodynamic properties of coaxial rotors, Acta Aeronautica et Astronautica Sinica 20 (4) (1999) 348e350 (in Chinese). [19] Chengxiong Pan, Jingzhou Zhang, Yong Shan, Modeling and analysis of helicopter skin temperature distribution, Acta Aeronautica et Astronautica 32 (2) (2011) 249e256 (in Chinese). [20] M. Johansson, M. Dalenbring, SIGGE, a prediction tool for aeronautical IR signatures, and its applications, AIAA Paper 2006-3276. [21] A.G. Voloboi, V.A. Galaktionov, K.A. Dmitriev, et al., Bidirectional ray tracing for the integration of illumination by the quasi-Monte Carlo method, Programming and Computer Software 30 (5) (2004) 258e265.