Stagnation Properties

[TangoCleveland]

Question about measuring stagnation temperatures: I can use total pressure probes and static pressure probes to measure total and static pressures, and infer the fluid velocity in a pitot-static tube. Is there an analogous measurement for temperatures? Does a thermocouple (or RTD) measure static, or stagnation temperature?

Larry

 

[hacksaw]

Yes, but they are far more difficult to get right because there are no simple corrections that can be made without rather comprehensive bench marking on a case by case basis.

Good query

 

[bmsg]

What do you mean by 'static temperature'?
By using temperature sensors like thermocouples or resistance temperature detectors (RTDs) you can measure the temperature of the medium.
Typically, temperature measurement has dead-time (depending on your measurement configuration.

 

[hacksaw]

to bmsg,

velocity dependent temperatures are involved; the time response is an entirely different matter.

 

[aviat]

Your thermocouple would read total air temp TAT or stagnation temp which is static air temp plus ram rise. Ram temp occurs mostly at higher speeds due to compressibility of air.
Depending on what you want to measure and the accuracy desired you may need a special probe designed for measuring TAT.
See: thread1-102277

 

[TangoCleveland]

Here's my situation. We have air at 1600F and 250 psig (stagnation) flowing in a 2" pipe with velocity about 0.8 Mach. I am doing a flow analysis that gives static and stagnation temperatures. If the user has a standard thermowell in the pipe, will their thermocouple read closer to the static or stagnation temperature? One reference I have says it will read somewhere in between, unless the probe is specificatlly designed to read stagnation temperature (or Total Air Temperature, TAT).

Thanks for responding. One neat thing about Eng-Tips is that it helps you forumulate the correct question. It's usually easier to get the correct answer that way.....

Larry

 

[hacksaw]

all depends on the recovery factor for the well. the indicated temperature indeed is somewhere in between the two limts, but with the process arrangement you've described and the flow velocities you have a more serious problem.

good luck