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-   -   Pressure drop in a heat exchanger (http://www.cfd-online.com/Forums/fluent/46610-pressure-drop-heat-exchanger.html)

Diana November 18, 2007 13:31

Pressure drop in a heat exchanger
 
Hi,

I want to find the pressure drop on the shell side of a long heat exchanger with many baffles. I simulated a representative section of the flow domain with few baffles in Fluent. I used mass flow inlet and pressure outlet as BCs. Now,

1. How I can find the pressure drop in this section of the heat exchanger: I believe it is the area-weighted average of the inlet total pressure minus that of the outlet? Or is it the static pressure?confused!

2. How I can find the pressure drop in the entire heat exchanger, can it be simply the pressure drop in the simulated section per unit length multiplied by the whole length of the heat exchanger or something else. Is there a need in this case to use periodic boundary conditions and re-simulation the case?If yes how I can use them!

3. What I can do to improve the solution in order to match the calculated/measured values? Grid adaptation?If yes what kind?

All answeres are welcome, and deep thanks in advance

-Diana


Diana November 26, 2007 04:45

Re: Pressure drop in a heat exchanger--HELP
 
Still...any contrribution is really welcomed :)

-Diana


raj November 27, 2007 17:12

Re: Pressure drop in a heat exchanger
 
I guess ur inlet and outlet BC are O.K...one of my friend who is working on the same prob in Fluent but in Compact HE..he used the same approach but assuemed the fins as rectangular channels in the first iterations...he found out the Pressure drop and Mass flow rate at the end of inlet headers and end of channels....he used incommpressible flow approximation with K-E turbulent model...but u can also use one Equation model(i forgotton the name)...

He used Area weighted avg for preesure drop calculation...once get the total pressure drop and individual pressure drop at the junctions, he replaced the rectangular model with relatively actual fins and corresponding mass flowrated (because the fins locating near the sides recive less mass flow and more flow tends to go to fins located middle).. so he repeated the conidtion for diff fins for diff mass flow rate...

The finaal answer he arrived is 1) from 1st simulation he got inlet header and outlet header pressure drops and mass flow rate vaues at the end of inlet headers and he negleted the pressure drop due to channel assumption...

2) Re analysis of fin portion with reasonably close resemblence of fin models and reasonably close Mass flow rate for each fin and calculated pressure drop across fins...

3) He used Area weighted avg method for pressure drop and also used iso clip isosurface option in first itration to get pressure drop and Mass flow rates..

4) to improve further at the fins , he is now trying to improve his fin model, meshing and exploring the options of porous option analysis to caluclate j and f factors...

5) i hope u have understand something of what i said...i am not involved this so i am sorry if i am awkward in my ans...and also i dont know the config of shell and tube HE...

6) all the best for ur analysis....

Diana December 1, 2007 12:04

Reversed flow vs. convergence
 
Thank you raj for your reply, the heat exchanger has finless tubes. I posted a new message earlier but it didn't appear, may be because of the problem that cfd-online was suffering.

I want to find the pressure drop on the shell side of a heat exchanger. I get reversed flow on pressure outlets (the heat exchanger has more than one pressure outlet). This happens from the start of the simulation, while the residual plots drop in the first few hundred iterations they proceed on becoming straight and don't change even after 4000 iterations. Nevertheless, flux reports show a converged solution (much less than 2% difference in mass flow rate)

1- Why reversed or back flow happens at the outlets? 2- Why residuals don't drop below certain limits? 3- Why reversed flow disappears in non steady simulations? I started with unsteady solver where reversed flow was suppressed, as soon as I switched to steady solver, reversed flow was back 4- Which is more appropriate for use in heat exchangers: static pressure or total pressure, also from point of view of measurements.

-Diana


Diana December 7, 2007 06:29

Re: Reversed Flow Revisited! Suggestions?
 
Hello all,

Are there any suggestions about getting rid of reversed flow in this type of problem? The flow in the heat exchanger is in the vertical direction (from bottom to top) in real situation, so is reversed flow expected in this type of heat exchanger? I read comments about changing the boundary condition from pressure outlet to outflow, extending the calculation domain, and starting with unsteady solver...what would you suggest? Especially, how I can extend the calculation domain in 3D when I have the flow over a tube bank with leaking baffles!

Thanks

-Diana :)


pratish December 13, 2007 10:33

Re: Pressure drop in a heat exchanger
 
i want to know calculation for pressure drop in finned heat xchanger .having 25.4 MM pitch.


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