I’m currently doing undergrad research on using a gravel (basalt 2-5mm grain size) bed as an heat storage. In order to heat up the gravel inside, hot (~100°C) and dry (RF<4%) air with a pressure of 10bar is inserted from above (->see T0). I implemented thermometers, measuring the temperature at different positions. The thermometer T1 is slightly above the gravel, while T2 and T3 are inside the gravel. Obviously, thermometers close to the top (close to the incoming air) heat up first. The temperature increase looks like a logistic function for T0 and T1. T2 and T3 behave a little different, though. At a certain temperature (~50°C) the logistic increase saturates and a linear increase follows. This linear increase takes a lot of time and finally saturates close to the equilibrium temperature. My first guess was, that the saturation of the first curve is due to water condensating inside the gravel bed. Now, this seems rather unlikely, since the incoming air is very dry, and no moisture was found when looking into the gravel bed. Since my advisors are not very experienced with gravel beds, they could not come up with a solution as well. Still, I do believe, I’m not the first one with such an observation, and therefor wanted to ask, whether anyone of you knows what could be happening here. I appreciate every theory, you can come up with (and can be tested).
The other temperature curves are from different thermometers, which are not of any importance in this setup.