I am performing a frictional heating test, in which i am rubbing a asbestos material against a cast iron plate. During rubbing action, temperature of plate increases, but after stopping it also the temperature increase for few seconds and then slowly decreases to room temperature. Does anybody know, due to which phenomena or reason, the plate temperature increases even after stopping of rubbing action.?
The reason of this phenomena caused by process of heat transfer and temperature redistribution which is occuring in time. You should take into account the relatively low heat conductivity of asbestos material which provides for good heat insulator of iron plate after stopping of frictional test.
There is no phenomenon of friction, but a simple inertia of the temperature measuring device. The thermometer shows after switching off the friction, the temperature with was measured by a few seconds befor.
Hi,
The temperature rise in your experiment was in its transient stage. In other words, the temperature picked up by your thermometer was not steady state temp. Due to the relatively low thermal conductivity of Asbestos compared to steel, their is this 'thermal gradient effect' (lag) which is shown in the temp rise after stopping robbing action had stopped. It does not contradict the laws of conservation of energy.
Imagine that due to robbing, free electrons have to migrate from region A (Robbing point) to B (Sensing point). If after rubbing stops and a stream of free electrons is about to reach the B, this stream would eventually reach B (due to momentum) and register temperature increase even after robbing has stopped!
Thanks.
It is often a reasonable to consider that the output signal of a temperature sensor ― possibly a thermocouple for the considered application ― is affected by first-order delay, also called 'lag'. The error that hence results from identifying measured temperature (Tmeas) with the actual (real) temperature (Treal) relates to the dynamic response of the temperature sensor, given by the following ordinary differential equation: τ·dTmeas/dt + Tmeas = Treal. Here, τ stands for the 'time constant' of the temperature sensor, which can be often experimentally accessed by imposing a step-change to the actual temperature (*).
(*) Cf. (e.g.): E. B. Nauman, "Introductory Systems Analysis for Process Engineers", Butterworth-Heinemann Series in Chemical Engineering, Reed Publishing Inc., USA, 1990.
Dear Tarup Chordia,
As far as I can see this phenomena takes place because of non-instantaneous heat transfer between the sample and the temperature sensor. Even if the thermocouple or other thermometer has low “lag”, it has no possibility to reach thermodynamic equilibrium with the sample because of border effects – the thermal contact is never ideal. Properties of the material of the sample are also very important if the sensor is far away from the heating point. The time “lag” decreases with an increase of thermal conductivity and thermal capacity and it is very small, for example, for water temperature measuring.
In your case several seconds “lag” is normal for contact sensors.
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