Most of scientists are employing thermocouple on the outer surface of the circular test tube. Is there any scientist/researcher who inserted thermocouples inside the wall for the determination of HT Coefficient of Nanofluids?
Actually there is not much difference. You can do the calculation but in the mm size of the tube, the difference is very little.
I agree with Azadeh, but if you want to be more rigorous if you know the tube material you can calculate the heat dissipation/resistance through the thickness of the tube. From my experience, the difference in the heat transfer coefficient in a pipe of stainless steel 2mm thick is approximately 0,004 W/km2. If you are trying to measure the heat transfer coefficient i don't advise you to put thermocouples inside the tube because you will disturb the flow inducing eddies in the flow affecting your readings.
Want i did was inserting thermocouples directly into the flow, not to the inner part of the tube (these results aren't published yet because i'm still testing samples). I want to study if the fluid temperature across the length of the tube and compared with known equations for the energy balance for inner flows subjected to heat flux. If i'm not mistaken i have read about someone (don't remember the name from the 300 articles about nano fluids) who tried this and justify (one possibility) for the higher increased in heat transfer of nanofluids was duo to eddies in the thermocouples. If you want to insert thermocouples use the most fine, but they have to be strong enough in order to hold the position independently of the flow velocity.
The thermocouples are comparatively large and the well usually is thin, how can you insert them in the wall?
The wall thickness is small and material is having higher themalconductivity the difference is much less or you can apply steady state equation to find inside temp.
Inserting thermocouple inside the tube may create disturbance to flow some times.(if the number of sensors are more and also you need face the possible leaking problem.
For calculating the heat transfer coefficient, you do need the wall temperature. All wall temperature measurements have uncertainties. Therefore, if you are looking for a method of high accuracy, do the following:
I presume you are using a hot nanofluid and cooling it through the tube wall. Instead, try to heat the nanofluid by having a saturated vapor (say, pure steam), measure its pressure as accurately as possible and get the temperature from pr-temp relationship. (Pressure measurement is inherently more accurate than wall temperature measurements by thermocouples.)
Because of the very high condensation coefficients (hence low thermal resistance), you can assume the outside wall temperature to be equal to the condensing temperature and you can apply correction for the inner wall temperature (as already said by Bruno). When measuring the outlet temperature of the fluid, use a mixing arrangement (say, a mixing chamber) to get the correct mean fluid temperature.
Dear Asim,
You didn't mention anything about your setup. We use a tube with constant heat flux for our experiments. We put the sensors on the surface and then we insulate the tube (including the sensors and electrical heater).
Are you going to perform your test for the case of wall with constant heat flux or the case of wall with constant temperature?
I think the suggestion proposed by Vijay Raghavan is applicable for the case of constant wall temperature.
Therefore, in agreement with Azadeh, I think if the tube wall is thin and insulation layer is strong, the error due to position of sensors can be neglected.
Asim, Mohammad,
I admit I digressed from the original question posed, viz., whether to place it on the inside or outside. My point was more about the inherent error in measuring wall temperatures. I have seen papers reporting of fantastic increases in HTC with nanofluids which were not substantiated in my experiments. If there are in fact only small increases as I suspect, then accuracy will become very important. This has been stated often enough in the literature. That is why I suggested a more accurate method. Asim, wish you good luck in your experiments.
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