Flow Rate impact due to thermocouple obstruction

[jondesousa]

TL;DR

Looking to see if there are rules of thumb for acceptable thermocouple obstruction in tubing for liquid flow applications

I'm working on an ASME committee where we are trying to establish guidelines for the level of obstruction that would be acceptable for a thermocouple to protrude into a flow path and still be able to take accurate measurements without overly impacting the flow rate or pressure drop.

I've seen a rule of thumb saying thermocouples shouldn't take more than 2-5% of the flow path for gas applications but am not sure about liquids given the variations in viscosity and density of different liquids.

First thoughts I tried evaluating via Reynold's number but the application space has measurement in both laminar and turbulent flow conditions.

If anyone has thoughts, it would be greatly appreciated.

 

[.Scott]

You could consider the maximum force placed on the thermocouple by the flow of the fluid under worse-case flow, temperature, and viscosity conditions. That way, you wouldn't have to factor those conditions into the standard. And you wouldn't have to consider shape and other geometry.

 

[Baluncore]

I think we are assuming that temperature is being measured, yet it could be mass flow rate.

Why does the thermocouple need to obstruct the flow? The wall of the tube will give a good indication of fluid temperature. It is the response time that will depend on the penetration into the flow. The faster flow in the middle of the channel will give the fastest response, with the least influence from the wall, and the external temperature.

In an electrical distribution system, the limit is on loss of voltage, (potential or pressure), and is set at about 5% of the input. If the loss is greater than that at maximum flow, then buy thicker wires. Electricity is not a viscous flow, so maybe a 5% pressure drop does not hold in fluid filled tubes.

In plumbing, at maximum specified flow, there is a head loss specified for each unit length of tube, and for each type of manufactured junction.

Maybe we need to know the application.

 

[jrmichler]

TL;DR: Looking to see if there are rules of thumb for acceptable thermocouple obstruction in tubing for liquid flow applications

If anyone has thoughts, it would be greatly appreciated.

No rule of thumb, but here is how I would approach the problem of calculating pressure loss.

1) Thermocouples are normally in a thermowell. That well has a diameter and length. Calculate the Reynolds number from the thermowell diameter and fluid velocity.

2) Find the drag coefficient, $C_D$, from a chart such as this one in Fluid Dynamic Drag, by Hoerner.

3) Calculate the drag force from $C_D$ and fluid velocity.

4) Pressure loss is the drag force divided by the flow area.

Use engineering judgement to determine if the pressure loss is acceptable.