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This is a simple water/air flow thought experiment I had that I'd like to understand better... The basic idea is that you have a closed loop of water or air standing vertically. One side is heated to make the water/air rise and the other is cooled to make it descend, creating flow (does this work?). If this works which would produce the more efficient work output, air or water? (I'm assuming air because of it's lower heat capacity / speed of temperature change?) And is temperature difference the main factor that would effect the flow rate?

Attached is a very rough sketch to help visualise... grateful for any input that helps me learn!

enter image description here

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  • $\begingroup$ It takes work to produce the heating and cooling. If your objective is to do work with this device, it would be much more efficient to directly use the electricity (assuming all electric devices) to do the work that you want your device to do. $\endgroup$ Jan 2, 2019 at 6:59
  • $\begingroup$ @DavidWhite Yes agreed, but was thinking through more solar heating and large body water cooling kind of device. (Originally was looking at the Stirling engine) $\endgroup$
    – Dan
    Jan 2, 2019 at 7:04
  • $\begingroup$ What is 'g' in your diagram? $\endgroup$
    – Karthik
    Jan 2, 2019 at 7:23
  • $\begingroup$ @KarthikV sorry it’s gravity $\endgroup$
    – Dan
    Jan 2, 2019 at 7:23

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The idea in principle will work. A radiator used for an internal combustion engine is an example. It transfers heat from one location to another. In modern systems a pump is used. But I have seen old cars where radiators did not have water pumps. You are trying to compare liquids and gases during convective heat transfer. But this will depend on the amount of heat energy, not the temperatures. The quantity of heat transported will be lower for gases since the heat capacity is low. But convection is much faster in gases.

So this is an engineering problem. Liquids will not be the superior option under all designs. Neither will gases. The flow rate, the dimensions, the actual temperatures, amount of heat energy, efficiency of heat transfer across walls etc. will all be important.

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