flow rate and heat analysis

[origen87]

hello all,
i am from electrical background and for a project i need to analyse the flow of oil through a hollow copper tube when the tube is subjected to some amount of heat. the outcome should tell me if the heat will be absorbed by the oil and what would be the stress points when i apply different amount of fluid flow rate. can fluent 6.3.26 be used for this?.if so please let me know how to start, because its totally new to me..
thanks a tonne:)

[CFDFEAGURU]

I am sure Fluent can analyze this problem for you but you don't need to go to that level of detail to determine the stress and outlet temperature of the oil.


... if the heat will be absorbed by the oil

The heat has to go somewhere, so yes the amount of heat not absorbed by the copper tubing will be absorbed by the oil and this will increase the temperature of the oil when it exits the copper tubing.

To calculate the exit temperature simple use the following formula

q = mCpdt

where

m = mass flow rate of the oil (lbs/hr)
Cp = specific heat of the oil at constant pressure (Btu/lb-F)
dt = temperature difference between the inlet and outlet of the oil
q = heat absorbed by the oil (Btu/hr)

Use this equation to perform a heat balance between the oil and the fluid that is doing the heating.

... stress points ...

The stress in the tubing is dependent upon the pressure. So for a constant diameter tubing increasing the pressure will increase the stress in the tube wall. See the ASME (American Society of Mechanical Engineers) Section VIII Div.1 for a standard wall thickness calculation for the tubing. If you don't have access to it, let me know and I can help you out.

If you have turns in the tubing the stress at the elbows is not a straight forward calculation.

I am assuming that you are trying to design a hot oil heater with multiple passes using 180 degree return bends. Am I correct?

Trust me, if you haven't used Fluent before, you will spend more time trying to figure out how to use Fluent then you will doing these simple hand calcs and once you are done you will see the Fluent results are probably within .1% of your simple hand calcs.

Thanks
Matt

[origen87]

Thanks Matt!!
Well these are coils(round, similar to a suspension used in automobiles) one overlapping the other, I am through with the temp calculations but for presenting it to my boss I need to have a model which is simulated and shows the heat at tube at different points and corresponding stress. When such coils are overlapped the heat of one tube will have effect on others too.

[CFDFEAGURU]

So you have a double pipe serpentine coil?

Are you able to show a picture or print of it?

Thanks
Matt

[origen87]

k..i wil sketch a rough figure.

[origen87]

Hey Matt,
very rough sketch though,but will give you an idea of the system, the two coils are separated by some kind of insulation.

[CFDFEAGURU]

Thanks for the print.

Well, the equation in my first reply is still valid for this system.

If you want to calculate a heat flux, you can "unwrap" the coil and calculate an equivalent straight length of tubing and use that in the heat flux calculation. The only problem there would be with calculating an accurate inside and outside convection coefficient. What is the fluid you are using for heating?

What quantities do you want to calculate?

Thanks
Matt

[origen87]

I am using mineral oil..the one used in transformers..i need to find out the stress at every point.

[CFDFEAGURU]

... need to find out the stress at every point.

Is the outer tubing the same diameter from the beginning to the end? If so the calculation for it is simple.

Sh = (p*r)/t

where

Sh = hoop stress (psi)
p = internal pressure (psi)
r = inner radius (inches)
t = tube thickness (inches)

This will be the stress at every location in the outer tube of the serpentine coil

If you specify and allowable stress for the tube material, you can use the above formula to calculate the required minimum tube thickness.

The inner tube is different because it is pressurized externally. See the ASME code Section VIII Div.1 for the procedure for this. If you don't have access to the ASME let me know.

Thanks
Matt

[origen87]

hey thanks matt!!well i dont have access to that, could you please help me with that

[CFDFEAGURU]

What is the inner pressure of the inner tube?
What is the inner pressure of the outer tube?
What material is the tubing? UNS # ? ASTM Spec?
What is the inlet and outlet temperatures of the inner and outer tubing?

Thanks
Matt

[origen87]

I know the needed pressure difference, and that is 3.92 bar,velocity of flow being 0.7m/s.its electrolytic copper(99.9%pure).ASTM spec....inlet temp 80c and outlet 25c

[origen87]

hey sry,,,inlet is 25,,,n outlet is 80c

[CFDFEAGURU]

Oh yeah I forgot to ask you,

What are the inner diameters of both tubes?

Do you have the mean specific heat of the mineral oil?

Thanks
Matt

[origen87]

id=6.5mm,od=8.2mm,,,,,,,,id=4.9mm,od=5.5mm

[origen87]

1.67 kJ/kg.K

[CFDFEAGURU]

origen87, I am out of my office today. I will be able to help you tomorrow.

Thanks
Matt

[CFDFEAGURU]

I know the needed pressure difference,..

Could you get the actual operating pressures of the inner and outer tubes?

Thanks
Matt

[origen87]

i dnt have any clue of d operating pressures. initially i knew only required flow rate and tube dimensions that gave me the pressure difference reqd.