If a thermocouple is brazed to a cylinder and a fluid is flowing over the cylinder, dose the thermocouple read the true value of the cylinder surface temperature? or the value is affected by the fluid temperature?
if you use thermocouple wire (embedded on the surface of the cylinder).
if the fluid temperature is more than cylinder temperature then surly there is an effect of fluid temperature (because heat transfer takes place from hot body to cold body).
if you use bead like structure (sheath material, with grounded thermocouple). then it will measure the cylinder temperature at a particular point.
It's not easy task ! Moving fluid flow over the cylinder makes a forced convection heat transfer mode takes place including the thermocouple points. Either to bead and isolated or to change the technique of measurements.
If possible take several measurements of the themocouple readout for various flow velocities. Theoretically the slowest velocity would have more residence time of the fluid on the surface of the cylinder than other higher velocities due to the possibility of obtaining equilibrium. Make a graph of temperature read out vs. flow velocity and see if you observe a saturation point.
Dear Mahmud,
I agree with Vijay Kumar Shankar,
I our Laboratory, we fix the thermonuclear junction on the surface and the cover it by resin. The resin acts as a fixer and insulator.
Dear Mahmud,
In agree with Vijay Kumar Shankar,
I our Laboratory, we fix the thermonuclear junction on the surface and then cover it by resin. The resin acts as a fixer and insulator.
I agree with muneer sir. Same procedure I did in my laboratory. Insulator is one the important factor can be consider for this case. Are els you can go for RTDs.
The result of such a measurements depends on the way the thermocouple is fixed to the cylinder and to the material of the cylinder. Ideally the thermocouple should be very thin compared to the cylinder diameter (thermocouples with wire diameter of 10 micro-meter or 30 micro-meter exist). It should be "embedded in the surface (see Vijay Kumar Shankar) in a narrow groove limiting disturbances of the surface to a minimum. The thermocouple wires should be embedded in the cylinder body. The cylinder should have the same heat conductivity as the thermocouple material (be a metal cylinder).
If the cylinder is made of a poorly conducting material (plastic, wood...) the heat flow along the thermocouple wires can significantly perturbe the temperature distribution on the surface of the cylinder.
When the cylinder is large one should take into account that there is a large time lap needed to reach a "steady" state temperature distribution.
With thermocouples one can directly measure the temperature difference between the cylinder surface and the air flow temperature. To do this: one thermocouple junction is imbedded on the surface of the cylinder (say Copper-Constantan type K thermocouple). The Copper wire i s extended to the air flow (far from the cylinder wall) and there a second junction with a Constantan wire is formed. The voltage measured between the two Constantan wires (the one in the cylinder and the one in the air flow) is a very accurate measurement for the temperature difference between the cylinder surface and the airflow. Sensitivities of 0.01K is easily obtained. Note that no reference in ice water mixture is needed as one measures directly the relevant temperature difference. Absolute temperatures are not very interesting. Of course the thermocouple wires in the air flow should be very thin and long. One can calculate the temperature errors made by heat conductivity along the wires (see references in attached paper). At high temperature, radiation heat transfer is also important. Note that at high flow speeds the thermocouple temperature is higher than the gas flow velocity. See attached paper on recovery temperature of cylinders.
It is necessary to achieve a complete thermal isolation between the thermocople and the external fluid stream. In the past, in a nuclear power plant, we measured the temperaturare of the hot water inside a piping, attaching externally different thermocouples. The measured temperaturare was very similar to the process temperature when the thermocouples were completely isolated of the external air of the cooling units.
This process is basically a convective heat transfer mode, but your question does not state if the fluid flow is naturally flowing or forcefully flowing. Many assumptions has to be made like assuming the cylinder as a lump system and so, in this case of lump system, the thermocouple will measure at one point and based on assumption, other points at that particular time will bear the same value of temperature being measure. For your question on true value, the fluid flow will certainly affect the temperature of the cylinder, except you have made an assumption that the cylinder is thermally insulated, hence no lost or gain of thermal energy.
When considering the SURFACE temperature of a cylinder submerged in a flow, you should place the thin thermocouple wires in a trough/channel/ditch along the cylinder surface. you should embed the wire in a paste which has heat conduction property similar to those of the cylinder. Using a resin, as proposed by Muneer Ismael, is a solution when the cylinder is made out of some plastic. When considering a metal cylinder, I would weld or solder the thermocouple within the trough.
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