Why can't you measure the temperature of multiphase flow's?

[Topher925]

I've read from many sources that thermocouples or any other type of thermal transducer can't accurately measure the temperature of multiphase flows but none of them explain why. For instance, a mixture of air and ethanol going through a fuel line at 80 C. Why is it that if you stick a thermocouple in the fuel line to measure temperature, it won't give you an accurate result? At 80C there shouldn't be much of an effect of radiative heat transfer and given a steady flow and temperature there should be no issues with changes in convective heat transfer. So what gives, why can't you do this?

 

[Navin A S]

A:The problem is that the thermocouple measures the temperature of the material it is in contact with. In a two-phase system, this could be either liquid or vapor, or a combination of the two. It's impossible to determine which phase the thermocouple is in contact with without additional instrumentation, so it's not possible to know what temperature is being measured. In addition, since the thermocouple is measuring the temperature of just one portion of the flow, any changes in the composition of the flow upstream or downstream can affect the temperature reading. For example, if the flow is a mixture of air and ethanol, the temperature reading could be affected by changes in the relative proportions of the two components. Finally, thermocouples are not designed to measure the temperature of two-phase systems; they are designed to measure the temperature of single-phase systems. If a two-phase system is too turbulent or has too much velocity, the thermocouple may not be able to accurately measure the temperature.