Heat transfer from gas to liquid during compression

[kbka]

Hi,

Im working on a model where I need to look into the rate of heat transfer from a gas that's being compressed by a "liquid piston" of water.

So basically a closed container containing a fixed amount of air n of volume V, is filled with water from the bottom until the volume of the air is 1/2*V. Still same amount n.
How do I determine the rate of heat transfer from the gas to the water? eg. the overall heat transfer coefficient h for a gas-liquid interface...

Any answers or litterature suggestions would be deeply appreciated.

regards,
kbka

 

[kbka]

Anyone?

 

[Q_Goest]

There shouldn't be any difference between a gas/liquid interface and a gas/solid interface. The convective heat transfer coefficient is the same.

What you'll find is that a simple liquid piston arrangement doesn't have sufficient surface area to reject the heat created during compression unless the compression rate is extremely slow. To get around this issue, heat exchangers are put inside the cylinder. These HX can be passive. The idea is to put a large amount of surface area into the cylinder and use the thermal mass of that material to absorb the heat. As the water or other liquid passes over this material, it cools the material down. That heat can then be rejected to atmosphere by circulating the water through another HX as the cylinder is being emptied.

The only problem with the above arrangement is that the rate of work input exponentially increases as liquid level increases, meaning that the rate of heat input to the gas increases more and more rapidly as the gas is squezed down into a smaller and smaller volume (assuming a constant liquid flow into the cylinder). As this volume decreases, so does the surface area of your internal HX. So just when you need the most surface area to reject the heat, your surface area is being covered up by the liquid. The obvious solution is to try and have as much surface area in the uppermost location of the cylinder as possible, or slow down the rate of compression at the very end of the stroke.

There are other issues with liquid piston compressors. You might want to look at a few patents to go over various issues.
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[kbka]

Thank you very much! Also, I appreciate the insights on the internal heat exchangers.

 

[h2oski1326]

kbka,

Are you assuming dry air in the cylinder (i.e. neglecting evaporation)?

Are you attempting to find a method to get closer to isothermal compression?