You could use thermocouples with a rotary coupling (slip ring).

The main idea is to distribute the liquid flowing inside the pipe on the inner wall as a thin film. (the pipe is only partially filled with water). The final goal is evaporation of the water. So, constant heat is applied by passing a high current through the pipe (I^2R). I am expecting parts of the inner wall will dry up and sections of it will remain wet. This will lead to temperature variations or hot spots and cold spots all along the length of the inner wall. If I choose to go with thermocouple I will need a lot of them to make an accurate profile.

Other than that, If I use the thermocouples, how do I place them inside the pipe, so that I can accurately measure the inner wall temperature only and not the water film temperature?

Is there a possibility of embedding thermocouples in such a way, that the flow hydrodynamics is not disturbed?

It would be helpful to give major constraints like this in your initial question. What temperatures do you expect to reach in the pipe? to what degree of granularity do you need to know the temperature? Have you done any modeling of the system?

You probably will see an effect on the hydrodynamics of the system with internal thermocouples. You could mount them on the outside of the pipe, embedding the thermocouple beads in the pipe.

I am looking at boiling regime for water. The outside temperature will be varied in the range of 250-600 deg C. And based on the evaporation rates, the inner temp may vary to say 300 deg C, which will depend on dry out region, rotation rate, liquid pool volume in the pipe etc. +/- 5 deg C Accuracy may be acceptable (would you suggest some value may be based on experimental experience).

Initial modelling based on available literature data has been done but the results are at best not satisfactory.

I like the idea of mounting them outside and embedding just the tips inside, just enough that it wont effect the hydrodynamics. I will look into it.

Somenath Chatterjee,

I am Dr.S.Ravindran. I finished Ph.D. at IIT Bombay in the year 1988, Flow boiling was my topic. Prof.A.Jaganmohan and Prof.BS Jagadisg were my guides. I too used ohmic heating of SS tube for heating. I wanted to share my ideas with you.

As I have understood, following is the arrangement of your setup:

1) Horizontal tube, rotated at 1 to 500 rpm

2) ohmic heating may need over 1000 Amp

3) water will be allowed in a predetermined quantity and temperature. As the tube rotates, it shall adhere to the inner surface and form a thin annular film of water, which will evaporate.

4) The outer surface temperature is expected to fall in the range of 250- 600 deg C and inner temperature is expected to be in the order of 300 deg C.

My suggestions are listed below:

1) Wires (without varnish) covered with fiber dust and double braided thermo-couples may be suitable for your application.

2) Thermo-couple beads be prepared and fix them to have direct contact with water and hot surface. for better thermal conductivity you may use Silver solder to fix the thermo-couples.

3) 1 meter long SS tubes of nearly 45 to 25 mm dia and 1.5 to 2 mm thick wall have electric resistance in the order of 6 to 12 micro ohms only. If there is no technical problems, I suggest you to use Kenthal or Microthal tubes as the test section.

4) As inner temperature is expected to be in the order of 300 deg C, at low speeds of rotation, there is a possibility of dry-outs to occur and Leidenfrost phenomenon of boiling to take place at low flows. If it occurs, the thermo-couple out put may vary at random. Have a good data logger for the temperature measurements. It should not be a problem for BARC to help you with instrumentation.

5) Plan for a short test section to help you in fixing the thermo-couples. If you are planning for a long test section, then cut the tube in to small bits of about 30 cm; after fixing thermo-couples, you may join them together with silver soldering.

pl feel free to reach me over my e-mail ([email protected]) or mobile: (9840864619).

Regards,

Dr.S.Ravindran

Thank you Prof.Ravindran. The problem desciption is spot on. I will discuss the details with you as soon as the preliminary design is ready for experiments.