Heat exchanger question

rideway

Hello everybody

The problem is about a very simple heat exchanger.

One pipe with cold water at the inlet and at some points heat is applied uniformly. Assumed no buoyancy. Wall is smooth and with no slip.

I simulate different a matrix of volume flow rates and inlet water temps. For 5 l/min, water inlet temp varies from 50°C to 150°C; same for 10 l/min and 20 l/min.

My questions are:

1.- If the water properties (density, viscosity, heat capacity, thermal conductivity) are constant, should the pressure drop be constant for a given volume flow rate?
2.- If I simulate water accordingly with variable density and viscosity, should the pressure drop increase with the increase of density or decrease?

I dont know if I explained correctly. Dont hesitate to ask if needed.

Thank you very much in advance!
Regards

rideway

Anyone can help me? Im still stucked with it.
Thanks!!

Q_Goest

Hi rideway. Fluid flow and pressure drop through pipes and networks of pipes including through pipes making up heat exchangers can be done using the Darcy Weisbach equation. If you're not familiar with that you can look through the Pipe-Flo document I posted here:
http://www.physicsforums.com/showthread.php?t=179830

I did a quick analysis using a program I have to see how pressure drop changes for water over a large range of temperature as well as looking at properties. First, your temperature of 150 C means the water is above atmospheric pressure, so I arbitrarily selected a pressure above the saturation point so it would remain liquid for the analysis. I found the viscosity changed considerably over a large change in temperature but density not so much. The resulting pressure drop difference was relatively small so my conclusion was that viscosity doesn't have a large impact on pressure drop, but that was a relatively superficial analysis assuming subcooled water.

Note that the heat capacity and thermal conductivity of water doesn't enter into the Darcy Weisbach equation so it can't affect pressure drop with the exception that those factors influence the water's change in temperature as it flows through a pipe with heat transfer. Hope that helps.

rideway

Thank you very much! It really helped a lot

That pdf document is really good, it confirmed what i was thinking.

I realized with my calculations that if I define all properties constant, the pressure drop is not changing. Then I set the water properties as IAPWS EOS (International Association for the Properties of Water and Steam Equations Of State) and as I thought, pressure drop changed for every case:

I post some conclusions that might help.
- The bigger the inlet temperature, smaller the density and smaller the pressure drop. By high volume flow rates up to 500Pa for a 80°C delta in a 2 meter long pipe. The density change was aprox 1%.
- The bigger the volume flow rate, the bigger the pressure drop. This one was quite obvious. It was an almost a quadratic result.

Thank you very much for your help!! :)