Possibly the anomalies in the thermocouple are due to electrical interference? Did you post details about the electrics of the experiment setup?
In this setup the TC is in contact with the Nickle container. No contact with Alumina. (As far as I can see from the drawing).
Also see the calibration curve.
From past experience with AC heater coils, it is possible to generate a large common mode AC signal on the thermocouple by coupling through the capacitance between the coil and the thermocouple; and, in this case, to the nickel container. This can be mitigated to some extent by using 100 ohm resistors from each of the thermocouple connections to ground and use of differential measurement of the thermocouple.
However, in this case, I don't think that is the source of the rapid fluctuations. Below 200C and above about 900C, the measurements smooth out even though above 900C the voltage driving the heater coil is highest and would produce the highest coupled common mode signal. The fluctuations are most noticeable around 280C and between 600C and 800C. Around 280C the first breakdown of the LAH occurs and between 600C and 800C another melting phase occurs. When these phase changes occur, H2 is released and the LAH foams and sputters. It is likely that as the LAH foams and sputters, hot spots come and go on the tube. Since only one spot is being measured, I think you are seeing the result of the LAH foaming/sputtering inside the tube.
To eliminate the rapid variation, the best solution I can think of is to use a more distributed temperature sensing or multiple thermocouples along the length of the fuel tube averaged together. One way to do the distributed sensing would be to wrap the length of the tube with an insulated wire (a little problematic at these temperatures) and measure the resistance as a function of temperature (a large coiled RTD).
In the Glowstick experiments, I found several additional sources of thermocouple error. The electrical conductivity of ceramics increasing with temperature also applies to typical refractory cements. I found that silica+alumina types were a particular problem when approaching 1000°C, when the silica component liquifies and migrates into any available void or crack. This loosens the bond, and forms a potential conduction path through the body of the cement.
Another problem I noted was the sensitivity of the thermocouple to minute changes in the thermal conduction path at its attachment point. Most cements will shrink as they cure and this can break all or part of the bond at the substrate. The thermal path to the thermocouple is thus changed from conduction to a mix of conduction and radiation across the gap, however small it might be. There may also be a subsequent variation in surface contact due to differential thermal expansion of the cement, substrate and thermocouple metals. It may be even more pronounced in this experiment, where the thermocouple is attached directly to the metal fuel tube.
This has proved to be a difficult engineering problem, resulting in rather large error bars for the temperature measurement, as much as ±50°C at 1000°C operating temperature. As a consequence, measurement of thermal gain less than 1.1 is not possible. I would want to see greater than 1.2 in my experiments before claiming anything. I'm working on better thermal coupling technique and welcome any suggestions.
AlanG
I wonder if a post test rerun would show the same behaviour in the same temperature range? If it is due to chemical or other fuel changes in the initial run then maybe the behaviour would different in the rerun. If it is the same then maybe it would indicate a problem with the thermocouple? Is this kind of test or a post test calibration planned or worth performing?
such as zirconia or boron carbide that dont become conductive at high temperatures.
such as zirconia or boron carbide that dont become conductive at high temperatures.
Zirconia has very interesting behavior at high temperature, it conducts oxygen ions. The oxygen sensor in your car relies on this phenomenon.
I'm working on better thermal coupling technique and welcome any suggestions.
The only solution I can think of is to use many TCs and average the readings.
Even better way to avoid the contact issues/H2 poisoning/EMF noises/TC meltdowns/out of range clipping and many more issues, is to give up the direct temperature measurement method and switch to proper calorimetry.
From past experience with AC heater coils, it is possible to generate a large common mode AC signal on the thermocouple by coupling through the capacitance between the coil and the thermocouple; and, in this case, to the nickel container. This can be mitigated to some extent by using 100 ohm resistors from each of the thermocouple connections to ground and use of differential measurement of the thermocouple.
However, in this case, I don't think that is the source of the rapid fluctuations. Below 200C and above about 900C, the measurements smooth out even though above 900C the voltage driving the heater coil is highest and would produce the highest coupled common mode signal. The fluctuations are most noticeable around 280C and between 600C and 800C. Around 280C the first breakdown of the LAH occurs and between 600C and 800C another melting phase occurs. When these phase changes occur, H2 is released and the LAH foams and sputters. It is likely that as the LAH foams and sputters, hot spots come and go on the tube. Since only one spot is being measured, I think you are seeing the result of the LAH foaming/sputtering inside the tube.
To eliminate the rapid variation, the best solution I can think of is to use a more distributed temperature sensing or multiple thermocouples along the length of the fuel tube averaged together. One way to do the distributed sensing would be to wrap the length of the tube with an insulated wire (a little problematic at these temperatures) and measure the resistance as a function of temperature (a large coiled RTD).
Or a few maybe 3 different Tc in different locations along the fuel tube. This may help to characteris the transient and localised effects if present and may be show flow effects if present, and could also be used to find an average and help identify failed TC if one fails.
Ahh I just saw Taruns post he just beet me to it I guess 
I am confident that during the cycle of the wave that apply the supply to the heater, during the off time, the ends of the load should be shorted, to allow the joule effect from the persistent voltage that has been observed also by Celani.
Let us know...
Alessandro Coppi
Launched spent reactor test thermocouple, this time everything was clean.
A few months later, I finally decided to break up the handset from the May experience Firax Tech replic series and see the contents. Here are a few photos.
