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Pool boiling experiment

brunopserrao

Dear community,

My research group is developing a pool boiling experiment. It consists basically by a heater (a Stainless Steel wire in our case) placed in a working fluid and a power supply connected to it. In a pool boiling experiment, different bubble regimes are observed depending on the temperature difference between the heater and the working fluid. Our goal is to measure the voltage across the heater in order to calculate the heater resistance and use the temperature x resistance relation to get its temperature.

Our experimental setup consists of a 5000 Watts DC power supply (0-20V and 0-250A) connected to copper wires and then connected to a 316L Stainless Steel wire (this one stays in a water tank).

We have already run this experiment a few times, but we never got a reliable voltage measurement. Our calculated resistance for the heater in general decreases with increasing the power in the power supply, what is not true for two reasons: (1) we can observe the different regimes using a high speed-camera, and (2) increasing power means increasing the heater temperature that means increasing resistance according to the temperature x resistance relation.

In addition, our power supply display is not reliable, so we measure our system current using a clamp digital ammeter. Our voltage across the heater is measured by a 6 1/2 DMM. Can someone help me to understand what we are doing wrong?

Regards,

Bruno

Parents

  • I tried the last suggestion to measure the voltage across the heating element (30 AWG 316L SS wire) only. At the same time, we took voltage measurements on three other points in the system: (1) between the two terminals of the power supply (positive and negative), (2) in the positive side only (between the positive terminal and the positive side of the heating element), and (3) in the negative side only (between the negative terminal and the negative side of the heating element.

    Then, the total voltage across the power supply (measurement 1) should be equal to the sum of the other three measurements. Based on that we observed the following:

    (1) The voltage measurements across the terminals (total) and across the positive side showed a stable behavior (measurements do not vary much).

    (2) The voltage measurements across the negative side and across the heating element showed an unstable behavior (measurements vary considerably).

    (3) The negative side copper block in the test section was generating a big amount of bubbles compared with the positive side copper block. That means the negative side of our system is getting hotter than our positive side.

    Note 1: Based on (2) and (3), we believe the unstable behavior happens because both, the negative copper block (part of the negative side section) and the 316L SS wire, are transferring more heat to the water than the positive side. That make sense for me if we think the electrons move from the negative to the positive terminals (electrons move in the opposite direction of the current flow). That means the power supply is providing power to the system from the negative to the positive terminals, across our system. Hence, the negative side gets hotter and most of the heat is transferred to the tank  through the copper block (due to the temperature difference between this copper block and the water) and the SS wire (since its temperature increases much faster than the copper blocks). Then, just a small portion of the power actually reaches and is released in the positive side.

    Note 2: Based on (2), we believe the instability observed in the measurements means those two section's voltages are balancing each other. That happens because the bubbles coming out of the copper block and the heating element help to cool down those parts, according to the pool boiling curve and bubble dynamics. According to bubble dynamics, when a single bubble (hot bubble) departs from a surface, it takes that heat out of the surface and the water comes in contact with the surface at this spot, increasing the heat transfer mechanisms there.

    Please give me some comments/feedbacks on the above discussion.

  • in reply to brunopserrao

    If one end of the wire is getting hotter, it may be due to a poor connection. With this much current, a little extra impedance at the connection will cause significant heat. Alligator clips don't provide much contact area - for a good connection, you need more total contact area than the cross-sectional area of the wire. Contact means direct metal-to-metal contact - no air or water gaps. This can be quite a problem since many connectors do not meet this criterion.

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  • in reply to brunopserrao

    If one end of the wire is getting hotter, it may be due to a poor connection. With this much current, a little extra impedance at the connection will cause significant heat. Alligator clips don't provide much contact area - for a good connection, you need more total contact area than the cross-sectional area of the wire. Contact means direct metal-to-metal contact - no air or water gaps. This can be quite a problem since many connectors do not meet this criterion.

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