Boelter, Llewellyn Michael Kraus

Elswick, W R

Rubesin, M W

Sanders, V D

English

NASA Technical Reports Server

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NATIONALADVISORYCOMMITTEE
FOR AERONAUTICS
TECHNICAL NOTE 2428
AN INVESTIGATION OF AIRCRAFT HEATERS
XxxvI - PRELIMINA RY INVESTIGATION OF A
COMBUSTION-TYPE AIRCRAFT HEATER
By L. M. K. Boelter, W. R. Elswick, V. D. Sanders
and M. W. RJIbeSi12
University of C alifomia
Washington
I
August 1951
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NATIONAL ADVBORY COMMITTQ3 FOR AERONAUTICS
TECHNICAL NOTE 2428
AN INW3STIOATION OF AIRCRAFT HEATERS
XXXVI - PRELIMINARY INVESTIGATION OF A
COMBUSTION-TYPE AIRCRAJ?THEATER
By L. M. K. Boelter, W. R. Elswick, V. D. Sanders
and M. W. Rubesin
SUMMARY “
CIUL5L7J
Expertintal results on the thermal performance of an aircraft
combustion-typeheater are presented. The heater is rated at SO,000 Btu
per hour and is a type used for cabin heating and anti-icing systems.
The performance of this combustion-typeheater with a counterflow and
a parallel-flow return passage on the gas side does not appear predict-
able by use of efiting equations for the following reasons: (1) The
mechanism of heat transfer for swirling gases inside the combustion
chaniberis not accurately known; and (2) accurate mixed-mean tempera-
ture measuments of the burning fuel inside the combustion chamber
were difficult to obtain.
The thermal conductance for the gases within the combustion chain
ber is predicted to be many times lower, by use of existing equations,
than the experimmtal data indicate.
To compare the relative effects of ~aseous radiation and turbulence
due to the process of combustion with that of forced convection alone
in the absence of combustion, preheated-gas runs were made as wdl as
‘hco~wtion;r rllns. Evaluation of the thermal effectiveness of the
heater for these two types of tests reveals that the effects of mseous
radiation and turbulenc~
negligible.
The combustion-type
Engineering Laboratories
due to the process of combustion are -
INTRODIJCTION
aircraft heater was tested in the Mechanical
of the University of CalMornia. This heater
-. —— . ...— —
—
.
.2 NACATN 21128
is designed to be used for cabin heating and wing- and tail-surface
anti-icing systems.
At the present time, methods of thermal analysis for this type of
combustion heater have failed to yield results which compare with meas-
ured values. Two reasons for this failure are
1. Inadequate lmowledge of the mechanism of heat transfer from the
burning, stiling gases inside the combustion chamber to the chamber
walls; that is: (a) The unit thermal conductance for a jet of inert
fluid passing along the inside of a cylinder in a spiral path is not
known; (b) eddies and turbulence caused by combustion of fuel probab3y
increase the effective thermal conductance over that for unburned gases;
and (c) radiation from solid or gaseous constituents of the gases to
the combustion chamber walls may yield added heat-transfer rates
2. Dtificulty in spec= the mixed-mean temperature of the cow
bustion gases in the combustion chamber to be usedin the prediction
equations
The total effect of items l(b), l(c), and 2 was determined by the
following procedure: After the tests utilizing a gasoline and air mix-
ture (referred to as llcomb~tionlltests) were completed, further tests
were made with hot (but not burning) gases passtig through the combustion
chamber (referred to as “preheated-gas testslf). The latter tests were
thus made in the absence of turbulence due to combustion (item l(b)),
solid and gaseous radiation (item l(c)), and difficulty in measuring
hot-air temperature h the combustion chamber (item 2). A comparison
of the preheated-air tests with combustion tests therefore reyeals the
total effect of these three items.
The results reveal (fig. 1) that the heat-transfer rates for the
combustion tests were greater than those for the preheated-gas tests
only by the ratio of the respctive inlet-temperaturedifferences. Thus,
the effects of items l(b), l(c), and 2 can be said to be negligible.
The following measurements were made:
1. Weight rates of combustion gas, preheated air, and ventilating
air ,
2. Weight rate of fuel (gasoline) to heater (for combustion tests)
3. Inlet and outlet
and ventilating air
This investi~tion,
craft heat exchangers at
temperatures of combustion gas, preheated gas,
part of a research program investigating air-
the University of California, was conducted
.
—.—
— —— –—
..!
NACATN 2428 3
under the sponsorship and with the financial assistance of the National
Adviso~ Committee for Aeronautics.
sYmoIs
cPa
cPg
qa
Wa
WC
Wf
‘g
‘sin
Taout
‘qn
heat capacity, at constant.pressure,of air, Btu/(lb)(OF)
heat capacity, at constant pressure, of gas, Btu/(lb)(°F)
heat transferred to ventilating air, Btu/(hr)
weight rate of
weight rate of
weight rate of
weight rate of
temperature of
temperate of
temperature of
ventilating air, (lb)/(hr)
combustionair, (~)/(hr)
fuel, (lb)/(hr)
combustion gases,
inlet ventilating
(M)/(hr), @c+~p
air, ‘F
outlet ventilating air, ‘F
enter”figpreheated air, ‘F
DESCRIPTION OF BEATER &D TESTING PRCHXDURES
The SO,OO~Btu-per-hour combustion-typeheater with para12el and
counterf16w passages is shown in figure 2. The heater is cylindrical,
in form, 18 inches long and 7 inches in diambter.
Combustion takes place inpide a cylindrical combustion chamber
which lies within two concentric cylinders. The conibustionchaniberand
passages are constructed of heat-resisting alloy steel and are fully
welded. The fuel and air inlets
the upstream end of the heater.
combustion chamber to the return
. end of the heater.
.
and exhaust-gas outlet are placed near
Four cross-over passages connecting the
passage are placed at the downstream
—.. — ... ..—. —-———
_——. —.
4A removable head,
igniter mounted at its
which holds a vaporizing wick with
center, covers the upstream end of
chamber. Fuel is fed to the wick and’is vaporized inside
A stainless-steel jacket surrounds the combustion chamber
NACA TN 2428
.
a glow-coil
the combustion ,,
the chamber.
and return
passage. A diagr& of the heater test stand is given in figure 3. Air
enters tangentially into the combustion chamber just downstream from the
wick, thereby inducing added turbulence to allow vaporization of the
fuel.
All regulating devices were removed from the heater for.these tests.
A 1~- inch bellmouth o’fiice was used to meter the ventilating air
2
and a 5/8-inch bellmouth ortiice was used to meter the combustion air.
Measurement of fuel rates was made with a calibrated rotamter.
The inlet temperatures of the combustion air and ventilating air
were measured by mans of thermocouples. outlet temperatures were also
measured by means of traversing thermocouples (see fig. 3).
Preheated gas was introduced into the heater as shown in figure 3.
Temperature measurements of the entering preheated gas were made by
means of a shielded thermocouple.
HEAT-TRANSFER RESIIL1’S
The results of the combustion tests are given in figures 4 and ~.
These results are for runs of three air-fuel ratios of 26, 20, and I-6,
..
with three fuel rates 2.5, 3.0, and 3.5 pounds per hour. The ventilating ,
air weight rates ranged from 600 to MOO pounds per hour.
The curve representing the highest fuel weight Wf shows the
greatest heat-transfer rate (fig. 4). This fact can be explained as
follows: Theoretically, the temperature of the combustion mixbure-is-
only a function of the air-fuel ratio. Then, at a constant ventilating-
air weight rate and constant air-fuel ratio, the heat transferred
will ficre=e with fuel weight rate because of the increased unit thermal
conductance along the combustio~chamher w~XLs (due to ~eater weight
rate of gases). Also a greater over-all temperature difference will be
obtatied between the combustion gas and the ventilating air because of
the smalJ temperature decrease of the combustion gases.
The theoretical air-fuel ratio for complete combustion yielding 7
approximately the greatest adiabatic temperature is 15.3, by weightj
. ..— ——
—
—
NACATN 2&28
based on the following equation (references1 and 2):
\
AU runs were made
because the heater
ratios.
2c#~8 ‘+”2502 + 3-8H20+ 1&02
at air-fuel ratios greater than the
would not operate satisfactorily at
theoretical
lower air-fuel
The lowest air-fuel ratio fields the greatest heat-transfer rate
at a constant fuel rate and ventilating-a~ rate (fig. ~). This could
be the result of at least two phenomena:
1. The lowering of the adiabatic temperature due to dilutionby
the excess combustion air overcomes the increase in unit thermal conduc-
tance and the small temperat- decrease in the combustion gases mm
tioned above.
2. Should there be poor tig of air and fuel within the combus-
tion chamber, even at ratios greater than the theoretically correct
air-fuel ratio, there might be regions of rich mixt~es allowing local
incomplete combustion and radiation from the formation of luminous par-
ticles of carbon. The existence of these regions would tend to increase
the effective thermal conductance along the inside of the combustion
chamber and allow greater heat transfer.
In order to determine the presence of heat transfer due to radi-
ation from luminous carbon particles and also the fraction due to
induced turbulence caused by the combustion mechhismj hot, nonluminous,
~ preburned gases at about 16000 F were drawn through the heater in Place
of the combwtion process wittin the combustion chamber. The results of
this test were plotted in figure 1 together with the results of the
combustion runs. It should be noted that the ordinate of the
T -’-c
aout ati
curve b the heater effectiveness,which is the frac-
‘gti - Tati
tion of the msximmpossible amount of heat that could be transferred
to the air. l%cause the points of both sets of runs tend to fall on the
same curve, the effectiveness of the heater is then the same for the
combustion and preheated-gas runs. This result would tend to indicate
that the luminous radiation of hot carbon particles and induced combus-
tion turbulence (both factors being such as to increase the heater effec-
tiveness) play a negligible role in accomplishing heat transfer in this
heater.
Heat-balance agreements are, on the average, within 15 percent for
both the combustion and preheated-gas tests.
_ .—... . . . —___ . ..———- ——
—— .—- .-
6CONCLUSIONS
NACA TN 2428
From an experimental investigation of an
heater, the following conclusions were drawn:
aircraft combustion-type
1. The effect of added heat transfer from the combustion gases due
to radiation from the solid or gaseous constituents of the gases to the
combustion chamber walls is negligible.
2. The effect of added heat transfer from the combustion gases dti
to the increase in the effective thermal conductance within the combus-
tion chamber caused by eddies and turbulence induced by the combustion
of fuel is negligible.
3. Further analysis and experimentation are required for the study
of the mechanism of heat transfer and combustion inside the combustion
chamber in order to predict mop accurately the thermal behavior of
this heater.
Department of Engineering,
University of California,
.
Berkeley, Calif., February 26, 1946
REFERENCES
1. Lewis, Bernard, and Von Elbe, Guenther: Combustion, Flames and
Explosions of Gases. Cambridge Univ. Press, 1938.
2. Boelter, L. M. K., Morrin, E. H., Martinelli, R. C., and poppendiek,
H. F.: An Investigation of Aircraft Heaters. XIV - An Air and
Heat Flow Analysis of a Ram-Op-eratedHeater and Duct System. N4CA
ARR4C01,1944.
..— —
—
,I
1
.00
q
a
0
.07
t “
o n
.06 n
t
km
m
.05
0 %dlm
*O = oQBltusLlOn
@
u
t
.Q!l a
WC
w
q
.03 n
c m
n
q o
.02
.01 T- —
0
0 5 10 15 20 2.5 30 35 I@ 45
~a @#g cPg
Figure 1.- Comparison of combustion and preheated-gas tests.
-J
co
claeolhw blat
Vmtil.atti air 7 .
I
vT
1 I
—
I
I I /1
I I /1 Vaporlzhg wlok -~
/ + q ‘
Ventilating d-r
J
‘l—
Figure 2.- Gasoltie-fired aircraft combustion heater. Air-side croea -
sectional area, 0.087 square foot; gae-side combustion-chaniber croBa-
aectlonal area, 0.123 foot; gas-side return-~ssage cross-sectional
area, 0.0~7 square foot. Scale, 1/2 size.
n
3-—-
. ,
P
,N
,Cn
.N
.
From blower
Preagum
taps
g“
-- Air valve T-1
Air in ~ t
lo
T-1 Inlet-air thermocouple
T -2 Hot-inlet-gas shielded
orifice.
1-1/2” bellmouth
thermocouple r
T -3 Oaa (leaving heater) thermocouple
T4 Gas (leaving venturi) thermocouple
T-5 Heated+utlet-airtr averse themmcouple
4
T-5 )&clng orifice T-2 -=@=
‘P
~Air out Heater
-3 -
IQ
Hot gas
Blouer d entering
Ve~uri meter
T-1+
Figure 3.- Schemtic diagram of heater test stand for preheated gases. ‘.o
10
,
56,cw0
52,m
48,0cm
I.lJ.l,ooo
40,0CH)
36,000
32,000
28,mo
a,ooo
20,000
Fnelweigkrtrate,
I Wf(lb/hr) 3s
3.0
2.5
A
o 0
I
Ventilating-airweight rate,W’a,lb/hr
Figure 4.- Variation of
(a) Air-fuel ratio, 26.
thermal output with ventilating-air weight rate.
.
—.— —-—-—.—
11
*
56,c00
52,CCI0
48,000
40 ,mo
36,000
32,m
28,000
21@oo
20,0fxJ
Fuel meight rate,
I
2.5 / 1
o 200 400 @0800, 100012001M0 1600
Ventflating-airweight rate, was lb~
(b) Air-fuel ratio, 20.
Figure 4.- Continued.
7
_—. _.—. .——c — _ _ .——
12 NACA TN 2428
.
. .
56,CU)0
52,0CQ
~8,000
Lll!,,mo
40,000
36,0cQ
32,0m
28,OOO
24,m
20,000
Fuel-weightrate,
Wf
(lb&) —
3.5 ~
/3.0 _
2.5 1
o 200 400 m 800 1000 1.2oo lmo 1(
Ventilating-airweight rate,Wa, lb/hr
(c) Air-fuel ratio, 16.
Figure 4.- Concluded.
xl
.—. .—
—. —-—
13
4
-.
50,000
45,000
40,000
35,000
30jmo
25,000
20,000
2.5 3.0 3s
Fuel r+e, Wf, lb/hr
(a) Ventilating-air weight rate, WO pounds per hour.
Figure ~.- Variation of thermal output with fuel weight rate.
..--. — .— ..+ —. ..
55,000
50,000
45,000
40,0ao
35,000
30,000
25,000
2.5 3.0
Air-fuel
ratio,
Wfif
/
/
/
,/
/“
/ “
0/
16
20
26
3*5
Fuel rate, Wf, lb/hr
(b) Ventilating-air weight rate, 1O(X)pounds per hour.
Figure 5.- Continued.
..— —
..
,
,
.
60,000
55,000
50,000
45,000
40,000
35,000
30,000
2.5
/
/<
/
“/”
L;’,/,//
>’
ratio,
Wflf
/=
Fuel rate, Wf, lb/hr
16
20
26
3.5
(c) Ventilating-air weight rate, 1300 pounds per hour.
Figure ~.- Concluded.
NAcA-LangIeY-B-16-51- l@Jo
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.._ _ —. .—— —.——
—


An investigation of aircraft heaters XXXVI : preliminary investigation of a combustion-type aircraft heater

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