# Computing Nusselt number

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 February 15, 2016, 14:16 Computing Nusselt number #1 Senior Member   Shamoon Jamshed Join Date: Apr 2009 Location: Karachi Posts: 270 Rep Power: 14 http://www.cfd-online.com/Forums/ima.../icon3.gifDear Fellows, I have modeled a quarter pipe with its solid thickness modeled too. I have applied the flux over wall as 3500 W/m2. I have the formula for havg= flux/(Tavg,fluid-Tinside). The T inside is the temperature at the lower end of wall, the region that touches fluid. In experiment Tinside is measured via thermocouples that are installed across the tube length wherever the flux is applied. These TC however measures the inner temperature of the wall. Nusselt number is Nu=havg*Di/k. Di=inned dia, and k=thermal conductivity of fluid. My computed Nu is very low from experiment. I am using Fluent, and applied flux on top wall. and specify coupled BC on interface. This creates wall shadow.I do not use any heat value (flux or temp) except on top wall. I saw as well "via System coupling" written in these BC. do I need to turn it on too? In fluent I take Area weighted average for inlet and outlet (Ttotal) while for Thermocouples I made Isolines at TC positions and took Temp (total) with Area weighted average. The error is more than 10 % from experiment. http://www.cfd-online.com/Forums/images/icons/icon3.gif

 February 18, 2016, 10:52 #2 Senior Member   Shamoon Jamshed Join Date: Apr 2009 Location: Karachi Posts: 270 Rep Power: 14 Please help, anyone here?

 February 22, 2016, 04:11 #3 Senior Member   Shamoon Jamshed Join Date: Apr 2009 Location: Karachi Posts: 270 Rep Power: 14 Please help me guys, no heat transfer expert here?

February 25, 2016, 13:51
#4
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Shamoon Jamshed
Join Date: Apr 2009
Location: Karachi
Posts: 270
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Quote:
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 March 3, 2016, 01:08 #5 Senior Member   Shamoon Jamshed Join Date: Apr 2009 Location: Karachi Posts: 270 Rep Power: 14 Please reply, anyone expert in heat transfer?

 March 3, 2016, 01:13 #6 Senior Member   Shamoon Jamshed Join Date: Apr 2009 Location: Karachi Posts: 270 Rep Power: 14 Ok, I figured out one way, For inner wall temperature I used the relation with some proprer understanding of it (that previously was not ) and managed to get results close to experiment. But here are two comments: 1. If for Tavg, fluid I take only pressure outlet massweighted temperature, I come close to experiment. If I take the mean of inlet and outlet , My error remains as low as 7 % and as high as 18%. 2. Turbulence modelling ================= The results of k-omega SST matches (somewhat cloase max error 11%) at low Re, while k-epsilon Standard matches at high Reynolds number. Whats the point?

 October 28, 2016, 08:44 Nusselt and friction factor with two turbulence models #7 Senior Member   Shamoon Jamshed Join Date: Apr 2009 Location: Karachi Posts: 270 Rep Power: 14 This is of much surprise that no one is replying. Let me qutoe my problem again: I am comparing heat transfer and friction factor results for a pipe with grooves in it. I am comparing Nusselt and friction factor in the two cases. The results are very surprising. The nusselt is comparable to experiment with k epsilon while friction is good in case of k omega. Could you please tell me the discrepancy? Please help me as I am not getting why two models are behaving differently.

 Tags havg, nusselt, thermocouples, tinside