What are the design considerations for a heat exchanger for an MX5 sports car?

[R.C]

Hello,

I have to design a heat exchanger for my uni course and am a little stuck with where to start. I know a little about LMTD and OHTC etc etc. my main starting problem is finding material properties for ethylene glycol, I have tried Engineering Toolbox but for the maximum temperatures I am a little stuck for a 50/50 mixture (the reason it is 50/50 is because of a previous assignment using this ratio). I could do with some pointers on this if any of you could help.

Here is the proposed problem:
"

A heat exchanger design is required to remove heat from the engine coolant of an MX5 sports car. The constraints for the problem are as follows:

• The space envelope available is 850 mm by 400 mm by 45 mm
  
• The coolant enters at 110oC, leaves at 70oC and the exchanger is required to remove 99 kW of heat.
  
• The heat is dissipated into atmospheric air traveling at 100 km.h-1 through the heat exchanger.
  

Aspects of the design that need to be determined are:

• The material properties and flow rate of the coolant.
  
• The size, shape and material for the heat exchanger design.
  
• The thermal properties of the coolant, air and solid material.
  
• The overall thermal performance of the system."

Also any units as a UK standard please, if you don't mind.

• 
  

 

[Chestermiller]

Are you saying that you can't find thermal properties on a 50/50 mixture of ethylene glycol in the temperature range 70C to 110 C?

Chet

 

[SteamKing]

Hello,

I have to design a heat exchanger for my uni course and am a little stuck with where to start. I know a little about LMTD and OHTC etc etc. my main starting problem is finding material properties for ethylene glycol, I have tried Engineering Toolbox but for the maximum temperatures I am a little stuck for a 50/50 mixture (the reason it is 50/50 is because of a previous assignment using this ratio). I could do with some pointers on this if any of you could help.

Here is the proposed problem:
"

A heat exchanger design is required to remove heat from the engine coolant of an MX5 sports car. The constraints for the problem are as follows:

• The space envelope available is 850 mm by 400 mm by 45 mm
• The coolant enters at 110oC, leaves at 70oC and the exchanger is required to remove 99 kW of heat.
• The heat is dissipated into atmospheric air traveling at 100 km.h-1 through the heat exchanger.

Aspects of the design that need to be determined are:

• The material properties and flow rate of the coolant.
• The size, shape and material for the heat exchanger design.
• The thermal properties of the coolant, air and solid material.
• The overall thermal performance of the system."

Also any units as a UK standard please, if you don't mind.

Various physical and thermal properties for different ethylene glycol mixtures are contained in this brochure:

http://www.meglobal.biz/media/product_guides/MEGlobal_MEG.pdf

If you seek other references, Google "thermal properties ethylene glycol mix" or similar to find what you want. ;)

 

[R.C]

Ok this is a great start thanks guys. Any suggestions for how I can best approach this problem?

 

[Chestermiller]

Ok this is a great start thanks guys. Any suggestions for how I can best approach this problem?

I have no suggestions. Let's see what you can do first.

Chet

 

[CFDFEAGURU]

First, do a simple heat balance. I am assuming that you have 3 known temperatures... If not then you are going to have to use NTU method to obtain the other two needed temperature values.

 

[R.C]

Ok so my solution came out as 220 pipes in 2 separate "cores". pipes were of dimensions 20mmx2mm (rectangular) and 350mm long with 0.5mm wall thickness. Each pipe had 400 uniform flat fins, 200 per side that extended out 2mm with thickness 0.5mm. Turns out without the fins at an inlet temperature of 30 degrees C my pipes could only remove about 120W of heat, with fins its around 470W. Pretty sure I have gone wrong somewhere...

 

[R.C]

Inlet temperature of the air (cold fluid) that is.

 

[CFDFEAGURU]

Why would you use a rectangular "pipe" ?? Pipes are round. What is the pressure drop thru the unit? Is this a gas to gas exchanger?

 

[R.C]

Square pipe allows for larger surface area to volume ratio. This was a liquid to gas exchanger.

 

[Highspeed]

No disrepect intended at all...but have you looked at a modern vehicle radiator? I would suggest that you ask yourself what does the most heat transfer - the tubes, the fins, or the tanks? If you haven't already, take a good look at the fins from a radiator. Hint: How many sets of fins are there? How thick are those fins? Good luck with your project!

 

[R.C]

Yeah I added fins and it made it work nicely. I'll post the whole solution that I got up soon. Unfortunately I only had a week to do this project alongside other uni committments. Not my finest hour of thermodynamics I'll admit.