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I'm working on an undergraduate project involving the investigation of the flow of heat on very short timescales (~10-20 ms). This involves aluminum shapes with very fine thermocouples embedded as near the surface as possible (around .25mm deep at most). The thermocouples are in distinct areas of the shapes. For example, we have a cube with one in the center of a face, one in the center of an edge, and one directly on a vertex. We then pour ice water over each thermocouple individually and record the voltage change with an oscilloscope. We did have a plunging system going on previously, but couldn't eliminate the splash effects (within a reasonable budget, anyway), which is why we pour water over each sensor.

Now, we are assuming that with such a short time scale, the heat flow is nearly entirely local, within a couple mm of the sensor. This seems to be confirmed by the data, but we want a way to more rigorously test it.

My idea to test the locality of heat flow is to take a large aluminum bar (because we're already working with aluminum), and embed 3 thermocouples in it, evenly spaced. I'd like to use more, but the equipment we are using now can only work with three channels at one time. Like this:Proposed apparatus

Then to use our pouring procedure to pour water over one of the end sensors (flowing away from the others). We could then compare the rate of temperature change in that sensor to the temperature change of the others. I'm fairly certain this would give us a reasonably good idea of how local the short term changes are, but I wanted to make sure I'm not missing some stupidly obvious mistake.

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I am assuming you are talking about heat transfer from cold water to aluminum block. If this is the case then conduction is the mechanism of heat transfer within the block. If $d$ is the bead size of thermocouple, and $\alpha$ is thermal diffusivity of aluminum, then heat will flow a distance (in order-of-magnitude sense) $d$ in time $t\sim \frac{d^2}{\alpha}$. When I say "heat will flow a distance..." I mean that temperature will change significantly only within that distance due to heat transfer. So if your measurement time is much less than $\frac{d^2}{\alpha}$, you may conclude with reasonable confidence that your measurement is local.

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