I have a finned heat pipe (copper) through which heat is transfered to the shell material. The shell material is a solid having a thermal conductivity of 1.5 W/(m*K). I am developing a 3D model to study the flow and heat transfer characteristics. For simplifying the numerical model, whether the fins can be replaced/ignored by employing effective thermal conductivity (% copper + % shell material) to the shell material? Are there any previous works reported similar to this?
Dear Hakeem,
If your objective is to transfer heat to the shell material which has a stated thermal conductivity > than the actual heat flux thru the fin shell contact in the real case. Also be sure to model the loss due to convection of the fluid in the inner space as it may well be greater than the loss due to contact with the fins.
Interesting problem
Brian
Can you clarify? Are the fins inside the heat pipe and used for wicking or are the fins outside the shell and used for convection?
@ Ralph - There are 2 tubes. The inner tube is having fins on the outer side to promote heat transfer to the material present in the shell (inside the outer tube). As the fins are thin, I am finding issues in meshing. So whether we can convert this model to just 2 concentric tubes without fins and increasing the thermal conductivity of the shell material (effective thermal conductivity)?
@ Zeeshan - Could you please share the links/data of analytical studies for the simplified cases.
Hi Hakeem,
If I understand correctly, you have an inner tube that has radial fins on the outer surface. You have an outer tube that is coaxial with the inner tube. The radial fins from the inner tube contact the inside surface of the outer tube. If this is correct then the thermal resistance is just based on the difference between having a solid ring that connects the inner and outer tubes, and fins that connect the inner and outer tube. which is just based on the cross-sectional area for heat flow. A more exact formula is the equation for heat transfer in a composite cylinder, if you equate the conductivity of one of the cylinders to your fins. see Remsburg, thermal design of electronic equipment, pg 37.
However, if you have any airflow within the radial fins it turns into a more complex problem, because you have a parallel path of conduction and convection (and possibly radiation) heat transfer.
I think it can be done. Basically, heat transfer between the substances inside the inner tube and outside the inner tube is proportional to conductivity of inner tube material and heat transfer area which is higher in finned tube compared to the plain tube. As both are directly proportional, the degree of accuracy over a range of parameters are only to be seen.
Dear Hakeem,
A schematic view of the problem could help.
I assume the heat transfer inside the tube is of interest. Then, I think you can use the available analytic relations to compute the convective heat transfer (conductivity of the shell and fins) and then use the effective convective coefficient as the boundary condition.
If there is no flow motion between the inner-outer tubes, then it is a pure conduction for the two tubes composite. It can be model using the thermal resistance theory.
Dear Hakeem,
If your objective is to transfer heat to the shell material which has a stated thermal conductivity > than the actual heat flux thru the fin shell contact in the real case. Also be sure to model the loss due to convection of the fluid in the inner space as it may well be greater than the loss due to contact with the fins.
Interesting problem
Brian
Every type of heat sink comes with specified thermal resistance. You can use this value in the equation Q = {Th - Tair)/Rth, (Q = heat flow, Th = sink temperature, Tair = air temp and Rth = thermal resistance). This should simplify your simulation significantly and give you a reasonable answer.
In fact, you can add all the thermal resistances to the source of heat and work with the thermal difference between the heat source and the sink (air in your case, if I understand your question correctly)
I found this to work quite well in thermosyphon designs (which is quite a challenge), that I've built, and got within 1 - 2 degree accuracy.
2 Recommendations
1st Jan, 2015
David Rathnaraj Jebamani
Sri Ramakrishna Engineering College
No Fins are used to enhance heat transfer. Use of effective thermal conductivity will give you huge deviation in simulation. Try to calculate the effectiveness of the fins and include in the heat transfer calculations .
1st Jan, 2015
Pritam LAL Sahu
RINL, Visakhapatnam, India
Still there is an element of doubt in presentation of question. In my earlier reply I presumed that its a kind of double pipe heat exchanger with inner copper tube with integral (built-in) fins. Mr hakeem to clarify / correct.
2nd Feb, 2015
Naouel Daouas
National Engineering School of Monastir
I agree with M. Zeeshan Ahmad.The accuracy of the model should me checked by comparing with exact analytical relations for single fins. Here is an interesting link for your help.
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