Transcript Slide 1

Department of Fire Protection Engineering
Thermocouples: Capabilities & Challenges
Ajay V. Singh
Graduate Student
Department of Fire Protection Engineering
University of Maryland College Park, MD
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
 The basis of thermocouples was established by
Thomas Johann Seebeck in 1821 when he
discovered that a conductor generates a voltage
when subjected to a temperature gradient.
 To measure this voltage, one must use a second
conductor material which generates a different
voltage under the same temperature gradient. The
voltage difference generated by the two materials
can then be measured and related to the
corresponding temperature gradient.
 It is thus clear that, based on Seebeck's principle,
thermocouples can only measure temperature
differences and need a known reference
temperature to yield the absolute readings
 Thermocouple is a relative not an absolute
temperature sensor. In other words, a
thermocouple requires a reference of known
temperature
The temperature at the probe tip can then be related to
the voltage output as,
The above formula is effective only if the reference
temperature TRef in the experiment is kept the same as
the reference temperature specified on the data sheet.
A.
JAMES
CLARK SCHOOL
of ENGINEERING
UNIVERSITY of MARYLAND
July
20, 2015
Dynamics
of Boundary Layer●Flames
2
Typical
range (deg
C)
Type
Materials
Suitable environment
Comments
T
Copper (Cu) vs
Constantan
Vacuum, oxidizing,
reducing, and inert
-270 to 400
High stability at sub zero and cryogenic temperatures
J
Iron (Fe) vs
Constantan
-210 to 1200
Vacuum, oxidizing,
reducing, and inert
Heavier gauge wire is recommended for long term life
above 540 degC since Iron oxidizes at high temperatures
K
Chromel vs Alumel
-270 to 1370
Oxidizing or inert
Should not be used in alternating reducing or oxidizing
atmospheres
E
Chromel vs
Constantan
-270 to 1000
Oxidizing or inert
Not recommended for alternating oxidizing or inert
atmospheres
S
(Pt-10% Rh) vs Pt
-50 to 1768
B
(Pt-13% Rh) vs (Pt6% Rh)
0 to 1820
R
(Pt-13% Rh) vs Pt
-50 to 1768
N
(Ni-Cr-Si) vs (NiSi-Mg)
-270 to 1300
Oxidizing or reducing
Relatively strong. Stable calibration. Very accurate at
high temperatures
Oxidizing or reducing
Relatively strong. Stable calibration. Very accurate at
high temperatures
Oxidizing or reducing
Relatively strong. Stable calibration. Very accurate at
high temperatures
Oxidizing, dry reducing or
inert
Very reliable and accurate at high temperatures. Can
replace type K thermocouples in many applications.
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
How to make a thermocouple: Best Practice
Very common design. However, errors become
substantial due to wire extension in flames and
other combusting environments [13]
Probably, the best way to go
Thermocouple wire extension in a flame [13]
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
Thermocouples: Data Acquisition
2D traverse
mechanism
NI cDAQ 9171
Mettler Toledo
load cell
Stepper Motor
Controller
NI 9214 has in-built signal conditioner and
Cold-junction compensation (CJC) module
NI-9214, 16 channel, 24 bit ADC
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
 Support wire diameters should be at least 4 times the wire diameter, so that the
resistance per unit length of the fine wires is significantly higher than that of support
wires
 Bead diameters and shape vary widely, depending on the technique and care used to
join the wires
 In practice, most thermocouples have bead diameters in the range 1.5dw  db  2.5dw
 Formed junction is not a “sphere” and resembles a “truncated sphere”
(left) Schematic of a typical thermocouple arrangement for combustion-system measurements. Relatively large lead wires are placed in a
protective, ceramic rod and then bent outward in order to provide tension for the thermocouple wires themselves and to allow for
sufficiently long thermocouple wire segments to reduce conductive heat transfer back to the leads [1]. (right) Electron micrograph (180 X)
of the Pt/Pt-13%Rh thermocouple bead. Dark patches are the remnants of soot. [2]
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
Thermocouples: Conduction loss
1) Conduction loss
2) Radiation loss
3) Uncertainties due to catalytic effects
 Heat conduction to support wires is assumed to have negligible effect on the
temperature of the junction if L  200d w , where L is the length of the fine wire [3].
Can be quantified and is negligible for L  200d w
 It is reasonable to assume that there are no radial temperature gradients in the wire,
2
since it can be shown that radial thermal diffusion rw kw is always at least one order of
magnitude less than the time constant  w for the wire diameters of 50  m [3].
 Conduction losses can be reduced by placing the thermocouple along an isotherm.
However, in flames it’s a difficult task to achieve (especially in turbulent flames)
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
 Errors in thermocouple measurements due to catalysis can be reduced by coating the
wires with a thin layer of non-catalytic coating
 Often, silicon dioxide (SiO2) or yttrium oxide (Y2O3, toxic in nature) coatings are used
to prevent exothermic reactions on the possibly catalytic platinum surface [2]
 Although coating reduces the catalytic effect, it increases the thermocouple diameter,
the response time of the thermocouple, and the radiation losses. It also changes the
convective and conductive heat transfer of the thermocouple. Since the coating changes
the heat transfer properties of the thermocouple, it is difficult to quantify the effect of
catalysis.
 SiO2 coatings in a reducing atmosphere can lead to the formation of silicon solid
solution in the legs of a thermocouple. This would shift the Fermi-energy levels of the
two components of the junction and can de-calibrate the potential difference output [2].
Thermocouple coating microscopic images []
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
Can be quantified precisely and accurately.
An energy balance on the thermocouple takes
the following form [1]:
0
0 dT
dT
(1)
Q  Q  Q  Q
m c
Vc
b
cat
conv
rad
cond
b p
dt
b
b b p
dt
For an unsteady system, it becomes
hAb (Tg  Tb )  mb c p
Tg  Tb   (Tg , U )

dTb
4
 Ab (Tb4  Tsurr
)
dt
dTb
4
  (Tb4  Tsurr
)
dt
mb c p

hAb
where (3)
Emissivity of Pt can be represented as a
function of absolute temperature [2]

h
For a steady system, it becomes
Tg  Tb 
 d
kNu
Nu 
hd
k
4
(Tb4  Tsurr
)
(2)
  1.507  104 T  1.596 108 T 2
(4)
Here ‘d’ represents the diameter of fine wire or thermocouple bead
depending on whether Cylindrical or Spherical Nu assumption is used
Appropriate Nusselt number correlation should be used to model the convective heat
transfer about the thermocouple.
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
For spherical bead approximation,
1
(1)
Nud ,sph  2.0  0.60 Re d Pr
2
1
3
Ranz and Marshall (1952) [4]
For Reynolds number between 0 and 200. Properties evaluated at T
0.4    
3
2

Nud ,sph  2.0  (0.4 Re d  0.06 Re d ) Pr 
 s 
0.71  Pr  380
3.5  Re  76000
2
1
(2)
Properties evaluated at
temperature
(3)
T
1
.
s
4
Whitaker (1972) [5]
is the gas viscosity evaluated at the surface
Nud ,sph  2  0.37 Re0.6 Pr0.33
[2]
For all Re and Pr numbers of interest in low flow velocities
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
For cylinders, many Nusselt number correlations have been introduced,
(1) For the low Reynolds numbers applicable for fine-wire thermocouple measurements in
combustion systems, the Collis and Williams (1959) correlation is most commonly used
(Cylindrical Nu number correlation preferably that of Collis and Williams should be
used) [6],
Nu d ,cyl
0.45  Tm 
 (0.24  0.56 Re d ) 
 T 
0.17
0.02  Re  44
With the Reynolds number evaluated at the so-called “film temperature”, Tm 
Tb  T
2
(2) Another widely quoted correlation is that due to Kramers (1946) [7]
1
1
0.01  Re  10000
Nud ,cyl  0.42 Pr0.2  0.57 Pr 3 Re d 2
With the gas properties evaluated at Tm 
Tb  T
2
(3) Andrews et. al. (1972) [8] evaluated the following expression for
Nud ,cyl  0.34  0.65Re0d.45
0.02  Re  20 ,
Gas properties evaluated at
Tm 
Tb  T
2
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND

A cylindrical Nu correlation (preferably that of Collis and Williams) should be used,
with estimation of the local flow velocity when it is known.
 It is better not to coat the thermocouple wires with a non-catalytic coating as it increases
uncertainties and errors that cannot be quantified.
 Conduction losses can be avoided if fine wire thermocouples are used (50-75 m ).
 Radiation loss from thermocouples at high temperatures is a major source of error in
thermocouple measurements.
 In turbulent flames, unsteady term comprising the time constant cannot be neglected if
we wish to capture temperature fluctuations at high frequency.
 Emissivity of thermocouples varies with temperature and surface characteristics
including roughness, coating of the metal. Variation of emissivity with absolute
temperature can be estimated for certain thermocouples.
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
[1] C.R. Shaddix, Proceedings of the 33rd National Heat Transfer Conference (1999) Aug 15-17, pp. 1-9
[2] J.A. Ang, P. J. Pagni, T.G. Mataga, J.M. Margle and V.L. Lyons, AIAA Journal 26 (1988), No. 3, pp. 323-329
[3] Yule, A. J., Taylor, D. S., and Chigier, N. A. (1978), J.Energy 2:223.
[4] Ranz, W. E., and Marshall, W. R., Jr. (1952), Chem.
Engr. Progress 48:141-146 and 173-180.
[5] Whitaker, S., AlChE Journal, Vol. 18, No. 2, pp. 361-371, 1972
[6] D.C. Collis, M.J. Williams, J. Fluid Mech. 6 (1959), pp. 357-384
[7] Kramers, H. (1946), Physica 12:61.
[8] Andrews, G. E., Bradley, D., and Hundy, G. F. (1972), Int. J. Heat Mass Transfer 15:1765-1786.
[6] Ballantyne, A., and Moss, J. B. (1977), Combust. Sci.and Tech. 17:63-72.
[7] Bradley, D., and Mathews, K. J. (1968), J. Mech. Engr.Science 10:299-305.
[8] Bradley, D., Lau, A. K. C., and Missaghi, M., Combustion Science and Technology, Vol. 64, pp. 119-134, 1989
[9] Heitor, M. V.; Taylor, A. M. K. P.; Whitelaw, J. H. (1985), Experiments in Fluids 3, 109-121
[10] Lockwood, F. C.; Moneib, H. (1980), Comb. Sci. Tech. 22, 63-81
[11] Lockwood, F. C.; Moneib, H. (1981), Comb. Sci. Tech. 26,177-181
Miles, P. C., and Gouldin, F. C. (1993), Combust. Sci.and Tech. 89:181-199.
[12] Yule, A. J.: Taylor, D. S.; Chigier, N. A. (1978), AlAA paper 78-30
[13] R. Ghoddoussi, An Investigation of Thermal Characteristics of Premixed Counter flow Flames using Micro
thermocouples, MS thesis, University of Maryland College Park, 2005.
[14] M. Jakob, Heat Transfer, Vol. 1, Wiley, New York, 1949.
[15] C. D. Hodgeman (ed.), handbook of Chemistry and Physics, 42nd ed., CRC Pess, Cleveland, 1960
[16] G.G. Gubareff, J.E. Janssen, R.H. Torborg, Thermal Radiation Properties Survey, 2nd ed., Honeywell Research Center,
Minneapolis-Honeywell Regulator Co., Minneapolis, 1960.
A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND
Questions?
July 20, 2015
Dynamics of Boundary Layer Flames
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