M.Tech Seminar Topic EFFECT OF EXHAUST TEMPERATURE ON INFRARED
SIGNATURES OF A HELICOPTER A COMPONENT OF STEALTH TECHNOLOGY
Submitted by:Kamal Raj SharmaRoll No. 12M336Thermal Engineering,
Mechanical Engineering DepartmentNIT, Hamirpur 177005 (H.P.)
DEPARTMENT OF MECHANICAL ENGINEERING, NATIONAL INSTITUTE OF
TECHNOLOGY, HAMIRPUR, HAMIRPUR 177005 (HP)
TABLE OF CONTENTS
1.
INTRODUCTION--------------------------------------------------
3
2. SOURCES OF IR SIGNATURES IN HELICOPTER-------- 4
3. SALIENT FEATURES OF THE IR SIGNATURES---------- 5 4.
LITERATURE REVIEW------------------------------------------ 7
5. EFFECTS OF EXHAUST TEMPERATURES INSIDE
TAILPIPE------------------------------------------------------------
9
6. EFFECT OF TEMPERATURE ON EXHAUST PLUME--- 10
7. EFFECT OF TEMPERATURE ON MIXING TAILPIPE
WALL---------------------------------------------------------------
11
8.
REFERENCES----------------------------------------------------
13
1. INTRODUCTION
STEALTH TECHNOLOGY- INFRARED SIGNATURES OF HELICOPTER
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. Due to their
high manoeuvrability, 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 Defence
Systems (MANPADS) especially infrared (IR) guided missiles have
caused severe casualties to helicopters in recent warfare such as
Gulf war and Afghanistan war.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. 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. demonstrated a helicopter IR suppressor which
balances engine exhaust signal and installation penalty. Rao et al.
modelled spectral IR signal from an aircraft exhaust plume and
evaluated the effect of engine bypass ratio on it; Bettini et al.
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 did computational research on an
exhaust system with a heat shelter nozzle used on helicopters; Shan
and Zhang used numerical calculation to investigate the IR
radiation difference between three mixer configurations of a
turbofan engine; Shan et al. presented IR signal of an IR
suppressor used on helicopters with both numerical calculation and
experimental data; Liu et al. reported the exhaust system and plume
flow field and IR characteristics of an aircraft; Eriqitai et al.
Researched the IR signal difference between two specific exhaust
system for turbofan engine; Shan and Zhang 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.
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. analyzed
the impact of transient temperature fields of fuselage skin on
total IR signal of an aircraft; Luand Wang modelled 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. 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 article, the
effect of exhaust temperature on infrared signature (in 3-5 micron
band) for a fictitious helicopter equipped with an integrative
infrared suppressor is studied 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 forward backward 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.
2. SOURCES OF IR SIGNATURES IN HELICOPTER
The main sources of IR signatures in the helicopter are:-
Engine Exhaust Duct (Tail pipe) & Hot Parts (e.g. Turbine
blades).
Tail-Boom Heated by Plume.
Plumes.
BREAKDOWN OF IR SIGNATURE LEVEL OF A UH-1H HELICOPTER FROM
DIFFERENT SOURCES IN 3-5 M BAND
3. SALIENT FEATURES OF THE IR SIGNATURES
The IR signature level (IRSL) of helicopter is not visible from
the front up to 90 degrees. The IRSL of helicopter is minimum at
the axis because the exhaust from the helicopter is not on axis
also the signatures are symmetric as there are two exhaust pipes
out of the helicopter. The area of a particular IR signature shows
its specific importance. Thus more the area more is the need of
suppression to avoid detection by IR detectors in 3-5 m band. The
detection of helicopters is done more often in 3-5 m band as there
is no aerodynamic heating of the airframe and structure since the
speed of helicopters is not more. The IR signature level (IRSL) of
plume heated tail boom is more than that of plumes itself because
plumes do not emit radiations continuously in all bands where as
plume heated tail boom emits radiations at almost all
wavelengths.
SIDE VIEW OF INTERNAL FLOW INSIDE HELICOPTER EXHAUST SYSTEM
HEAT TRANSFER PROCESS INSIDE HELICOPTER EXHAUST SYSTEM (B-B
SECTIONAL VIEW)
4. LITERATURE REVIEW
1. S.P. Mahulikar, H.R. Sonawane, G.A. Rao, Progress in
Aerospace Sciences 43 (2007) 218-245 The potent threat from
passively guided infrared (IR) homing missiles is articulated, and
the resulting concerns regarding operation in a hostile environment
are elaborated. Though capabilities of IR technology were known
prior to World War I, the initial success of RADAR slowed their
development till 1960. Recent developments in IR sensing technology
have made it virtually impossible to escape IR-detection. Modern IR
imaging systems can differentiate small temperature differences,
and are immune to conventional countermeasures that appear as point
sources of IR-radiation. Antiaircraft missiles with imaging IR
detectors are under development, and are soon likely to find a
place in tactical warfare. Therefore, military forces are demanding
more stringent IR counter-measures (IRCMs) from future
aircraft/helicopters. Survivability against IR-guided threats has
found a place in the design stage itself, leading to an upsurge of
research on several aspects of IR signature prediction and
management. This review summarizes the perspectives that led to
various research, design, and developmental activities in this
field. The most important points are: (i) Conventionally, fuselage
IR signature was neglected; however, it is now realized that the
rear fuselage is the main source of IR signature in 812 mm band.
(ii) Earlier, the aircraft plume was generally considered as the
major source of IR radiation, but research showed that its
significance is restricted to the 4.154.20 mm band. In particular,
the role of atmospheric transmittance of IR in determining this
relatively low importance of plume IR radiation is now known.(iii)
The background IR-radiance plays an important role in determining
IR signature in the 812 mm band. The IR signature due to positive
contrast decreases and due to negative contrast increases, with
increasing background IR-radiance. The atmospheric attenuation
reduces IR signature for both, positive and negative contrast.(iv).
The effect of earthshine on rear fuselage IR emissions in the 812
mm band was identified, and it was shown that earthshine always
makes IR-detection of the rear fuselage possible.2. G.A. Rao, J.P.
Buijtenen, S.P. Mahulikar, AIAA ISABE-2009-1194 Modeled spectral IR
signal from an aircraft exhaust plume and evaluated the effect of
engine bypass ratio on it. 3. Rao & Mahulikar, Aeronaut.
J.(2002) The IR signature level in the 3-5 micron band is a
stronger function of temperature than 8-12 micron band . Hence 8-12
micron band will be more popularly used in the future, as it can
detect aircraft stealthy with respect to IR. 4. Pan Cheng-xiong,
Zhang Jing-zhou, Shan Yong, Applied Thermal Engineering 51 (2013)
529-538 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 3-5 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
helicopters total infrared radiation intensity are mainly
concentrated to the plume. And this effect is more obvious for the
lower skin emissivity case.
5. EFFECTS OF EXHAUST TEMPERATURES INSIDE TAILPIPE The flow
fields inside mixing tailpipe under 900 K exhaust temperature are
shown in figs below. 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.
Temperature contour (K) of mixing tailpipe under the exhaust
temperature of 900 K.
Temperature contour (K) of mixing tailpipe under the exhaust
temperature of 1000 K.
6. EFFECT OF TEMPERATURE ON EXHAUST PLUME
Temperature contours in the middle sectional plane on the rear
airframe of helicopter with three different exhaust temperatures
are shown in figs below. It can be seen that exhaust plume
distributions for 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.
PLUME FLOW FIELD
7. EFFECT OF TEMPERATURE ON MIXING TAILPIPE WALL
Temperature distribution on the mixing tailpipe wall under three
different exhaust temperatures is shown in figs below. 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
radiation heat transfer between mixing tailpipe and helicopter skin
will lead to temperature increase on the helicopter skin.
TEMPERATURE DISTRIBUTION ON MIXING TAILPIPE WALL
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