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Hello Somen,

As I understand it, you are using a RTD (Resistance Temperature Detector) [and not a Resonant Tunneling Diode as appears in the keyword list under your question] to measure the heat transfer taking place in a packed bed with water flowing through it. You are operating the RTD in a pulsed mode, I assume to measure the heat transfer to the flowing water from the self-heating of the RTD. Is that correct? And you are experiencing irregular changes in the value of the Peclet (dimensionless heat transfer parameter) number as derived from the measurements of the RTD as the velocity of the water is increased.

Are you sure that the environment around the RTD is not changing in some irregular way as the water velocity increases? For example, I am not sure how you are mounting the RTD in the bed, but suppose that at low water velocities the RTD is completely surrounded by water, but that at higher water velocities the solid phase of the bed is shifted in such a way that some of the solid phase is touching the RTD. It might be that sometimes the solid phase of the bed shifts with higher water velocity and sometime not. There is also the possibility of a transition from laminar to turbulent flow around the RTD, and that this transition is affected, in an irregular way, by the shifting of the solid phase of the bed as the water velocity increases.

And perhaps you have already thought of all of these possibilities and have taken precautions to mitigate them. It is hard to tell from your description.

It may be that you need to perform a positive control experiment that does not have the complexity of water flowing through a solid phase bed. Suppose you rigidly mount the RTD in a water filled tube with clear walls so that you can view the RTD as the water flow through the tube is increased. In this case, one would expect that the heat transfer would increase with increasing water velocity as the laminar flow increased. At a certain point turbulence will appear, and depending on exactly when you pulse the RTD and its thermal response time you might observe irregular changes in the heat transfer. Another control experiment you might want to try is a negative control experiment, such as mounting the RTD at a stagnation point where the local water velocity does not change even if the overall water velocity does. Does the heat transfer from the RTD, in this case, remain constant?

Regards,

Tom Cuff

Hello, again, Somen,

I just had another thought. The reason most people operate a RTD in the pulse mode is that they are trying to measure the ambient temperature of the substance in which the RTD is immersed, and if they operate the RTD continuously the self-heating of the RTD due to Joule heating will render it very sensitive to any wind chill effect caused by the motion of the fluid in which it is immersed. In other words, if the immersion fluid is moving, the self-heating RTD will cause an under-reporting of the ambient temperature due to transferring of its heat to the fluid. But you, on the other hand, want to measure the wind chill, as it were, due to the motion of the fluid.

It is possible that by utilizing the RTD in the pulse mode, you are making it less sensitive to the wind chill, i.e., heat transfer, and more sensitive to the ambient temperature, which might vary from experiment to experiment.

Anyway, it is just a thought. Why do you use the RTD in the pulsed mode? And what type of RTD are you using: thermistor, platinum resistance thermometer, etc.?

Regards,

Tom Cuff

Please mention the direction of motion of both phases i.e., water and air and tracer inlet position and condition.