3 replies to this topic

[abdullah92]

Hello,

I'm working on a lab experiment where we dry a wet sand at different air flow and temperature settings. we did the experiment at 4 settings, low air flow low temperature, low air flow high temperature, high air flow low temperature, and high air flow high temperature. we calculated the heat transfer coefficient values, and it seems that the air flow has more effect on the value of h than the temperature does. the two values of h at high flow rate are about 30 w/m²k while low air flow rate are about 20 w/m²k. I have 2 questions

 

Q1 why the higher the air flow the higher the heat transfer coefficient?

Q2 why the air flow rate has more effect on h than the temperature does?

thank you

[USR]

Higher the air flow represents higher turbulence in the system and heat transfer coefficient are direct functions of reynold's number. For higher turbulence the reynold's number is high hence the heat transfer coefficients are also high. Simply speaking increasing air flow leads to increased turbulence and one of the important characteristic of turbulence in the system is better mixing and enhanced transport of heat, mass and momentum.

[breizh]

^{Abdullah 92 ,}

^{Take a look at any correlation on convection process it will help you to understand  what USR has explained in his reply  :}

^{Nusselt number = f ( Re,Pr).}

^Breizh

 

Edited by breizh, 22 April 2014 - 07:45 PM.

[abdullah92]

Thank you very much both of you, now it is very clear to me.