analys a dimpled microchannel
 

Hi every one
i'm analysing a article that's about heat transfer in dimpled microchannel .
but i reach Nusselt number 10 % lower than based article and i couldn't get true Nu Number . i've tried different mesh but also no change . i should point that in article the Nu number is based on hydrulic diameter and i set hydrulic diameter in refrence value (length). article nusselt number is 8.21 and my Nu number is 7.2 .
i even have this promblem in flat microchannel withot any dimple .
i will attache some picture about this .
Thanks if anyone can help me.