Hello About problem 13, chapter 1 in Fundamentals of Heat and MassTransfer by In

Question

Hello

About problem 13, chapter 1 in Fundamentals of Heat and MassTransfer by Incropera and Dewitt:
Given are velocity in air and water, the temperature of the handand the medium (air and water) and the respective convectioncoefficients. The heat flux in both cases is to be found.
Why do they give velocity at all when it is obviously not used? I(completely sure that convection depends on velocity), spend a lotof time calculating Nusselt numbers etc. being sure that this willhelp me find the right convection coefficient only to find out (when comparing my results to the solution in Cramster) thatvelocity is not used at all. What is the idea of giving thevelocity then? Does it influence the final solution at all? If yesin what way?

Thank you.

Explanation / Answer

I don't know what solution you are referring but I am telling youthe answer Q = hAT or, q = Q/A = hT qwater = hwater*(Thand - Twater)            =(900 W/m2-K )*(30-10)K = 18000W/m2 qair = hair*(Thand - Tair)    = (40 W/m2-k)*(30-(-5))K    = 1400 W/m2 as qwater > qair, Hand will feal coller inwater Now, to things to visualize: 1. air is at lower temperature (Tair = -50C)comapred to water at 100C, still qwater ismore. 2. vair >vwater still qwater ismore. Now, your confusion why at all the speed is given and convectiveheat transfer depends on speed of the medium? Answer is Yes, convective heat transfer depends on the flow of themedium. but convection heat transfer coefficient (h) takescare of this.Now, h is dependent on the type of media, gas orliquid, the flow properties such as velocity, viscosity and otherflow and temperature dependent properties. and h is given so speedplays no role in calculation. then why speed is given.. this is for visualization purpose.

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