Hello,
I have done a simulation using CFX in which there are several spheres as fuel and a fluid moves through them and collects heat from them . In order to get some contours of Nu, I wrote some expressions for Nu and htc and when I drew contours I got negative values for them. Here are my expressions
Myhtc=Wall Heat Flux/(T-780[K])
Nu=Myhtc*0.06[m]/0.31[W m^-1 K^-1] (d=0.06 and k=0.31)
I was wondering if there is any problem related to my expressions? How can I fix it?
Thanks in advance
In Fluent, the wall heat flux is defined to be negative if heat is transferred from the fluid to the wall, and positive if heat is added to the fluid from the wall. I suppose that CFX uses the same definition. You can take the absolute value of your Nusselt number equation if you want it to be always positive.
Thanks Dear Saad , Niko and Ibrahim,
I know that Nu must be positive and also I defined
T_wall - T_fluid properly . But I still get negative values for Nu. Maybe I have to follow Niko's comment to get positive values for Nu.
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