Bulk mean temperature of the fluid is generally calculated by taking the average of inlet and exit temperatures of the fluid for constant wall temperature boundary condition.but in case of constant heat flux boundary condition; the temperature of the fluid is rapidly increasing even in the thermally fully developed region.Therefore, there is a big difference between inlet and outlet temperatures for constant heat flux boundary condition. So, how to account for the property variations with this large temperature difference??
I do not know how you do your computation; the evolution of the properties may be incorporated directly along the tube.
However you can also use a stepwise approach: Just divide your tube into several sections along the streamwise direction, and apply your mean-value philosophy for the successive sections.
Thermophysical property variation in a brief sense you are talking about thermal conductivity(k) and absolute viscosity(mu) variation mainly. As Markus suggested, you can either use stepwise approach or I assume you can study k variation using some linear profile (as a beginner) variation by incoporating it's effect during your simulation. Obviously u can also make you use of nondimensional parameters too.
Best Regards
Akr
Thank you Prof. Markus
So,if i want to determine the average heat transfer coefficient then i need to take the average of all the "sectional" heat transfer coefficients. ..right??..Thank you once again.
Thank you Arun and Mojtaba for your reply.
I believe, the best way is to use the energy equation which takes in to account the property variation. Eg: del/delX of (k delT/delX) instead of k( del 2T/Delx2). Use auxiliary equations to describe the variation of viscosity and thermal conductivity with temperature. It's almost like compressible flow, except, you do not bother about density variation with pressure.
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