Thermocouples: Capabilities & Challenges Ajay V.

How to make a thermocouple: Best Practice Thermocouple wire extension in a flame [13] Probably, the best way to go Very common design. However, errors become substantial due to wire extension in flames and other combusting environments [13]<br>

Thermocouples: Data Acquisition NI-9214, 16 channel, 24 bit ADC Mettler Toledo load cell 2D traverse mechanism Stepper Motor Controller NI cDAQ 9171 NI 9214 has in-built signal conditioner and Cold-junction compensation (CJC) module<br>

Thermocouple: Concepts 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
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]<br>

Thermocouples: Conduction loss Conduction loss
Radiation loss
Uncertainties due to catalytic effects<br>

Thermocouples: Catalytic effects 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 []<br>

Thermocouples: Radiation Loss 0 0 An energy balance on the thermocouple takes the following form [1]: For an unsteady system, it becomes where (1) (2) (3) For a steady system, it becomes Emissivity of Pt can be represented as a function of absolute temperature [2] (4) Appropriate Nusselt number correlation should be used to model the convective heat transfer about the thermocouple. Here ‘d’ represents the diameter of fine wire or thermocouple bead depending on whether Cylindrical or Spherical Nu assumption is used Can be quantified precisely and accurately.<br>

Spherical Nusselt number Correlations<br>

Cylindrical Nusselt number Correlations<br>

Conclusions 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 ).

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.<br>

References [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.<br>

Questions? 14 Dynamics of Boundary Layer Flames June 25, 2014<br>