Overall heat transfer coefficient in function of flow rate?
Overall heat transfer coefficient in function of flow rate?
(OP)
I did a study on how good the HE could cool by changing both the volume rate.
What I experimentally found out is that
Q = U A ΔTLM
the U (overall heat transfer coefficient) can be written in a function of one of the fluidums mass rate when I keep the other fluidum's mass rate constant:
so U(mass rate fluidum 1)=(a*M+b)/(M+c). So its a rational function with both first order polynoms.
And when I curve fitted it from my experimental data, it had an R square of at least 0,999.
So I know its the correct function, but I have no idea why. I did some research but i couldn't find it anywhere what the correlation is of the OHTC and the mass rate of the fluidums.
Thanks!!
Alain Vanhille
What I experimentally found out is that
Q = U A ΔTLM
the U (overall heat transfer coefficient) can be written in a function of one of the fluidums mass rate when I keep the other fluidum's mass rate constant:
so U(mass rate fluidum 1)=(a*M+b)/(M+c). So its a rational function with both first order polynoms.
And when I curve fitted it from my experimental data, it had an R square of at least 0,999.
So I know its the correct function, but I have no idea why. I did some research but i couldn't find it anywhere what the correlation is of the OHTC and the mass rate of the fluidums.
Thanks!!
Alain Vanhille





RE: Overall heat transfer coefficient in function of flow rate?
RE: Overall heat transfer coefficient in function of flow rate?
The basic physical process is dependent on mass flow, so there should be no surprise that there's a strong dependence on the momentum flow of the fluid. However, the fact that you COULD fit to a polynomial means little if you have no predictable way of determining your fitting coefficients a, b, and c, or any rationale for why they exist. I suggest that you consult your HT texts and read up on Nusselt and Reynolds numbers in particular. If you cannot afford to buy one, there is a rather verbose and obtuse one that can be downloaded for free: http://web.mit.edu/lienhard/www/ahtt.html.
Now, admittedly, the Nusselt, Reynolds, and Prandtdl number approach are similarly heuristic, in that they are not solutions to obvious equations derived from physics, but, they are mostly based on the apriori, known physical parameters of the scenario.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Overall heat transfer coefficient in function of flow rate?
RE: Overall heat transfer coefficient in function of flow rate?
Nu=a*Pr^b*Re^c
and Nu=h*D/k and solve
With regards to fitting: Its very fine, but given enough data point and a sufficient polynomial you can always get Rsq very low.
Best regards
Morten
RE: Overall heat transfer coefficient in function of flow rate?
If you are maintining one mass flowrate and varying the other, the ratio of (W*Cp1)/(W*Cp2) will vary, so one would expect a change in heat exchanger effectiveness "e", as per the "compact heat exchanger theory", sometimes called the e-NTU method. I would compare the results of your experiment with that predicted by the e-NTU method.
RE: Overall heat transfer coefficient in function of flow rate?
i know the part that the heat resistance film is getting thinner as the fluid is more turbelent, which is more flow rate.
I retried some fittings, and thought about a very logic function and found a new, actually more believable function of
U=a*M/(M+b). So if the flow rate is o, you don't transfer heat so U=o kW/m²K
If the flow rate is infinite the heat transfer is U=a kW/m²K. with in my back mind that you can cool the hot fluid to the temperature of the cold fluid' entrance. This is the maximum heat transfer possible in the cooler.
Does my logic have any flaws?
RE: Overall heat transfer coefficient in function of flow rate?
this is what the standard model says:
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Overall heat transfer coefficient in function of flow rate?
I can't find it anywhere..
RE: Overall heat transfer coefficient in function of flow rate?
http://en.wikipedia.org/wiki/Dittus-Boelter_equation#Dittus-Boelter_equation
Best regards Morten
RE: Overall heat transfer coefficient in function of flow rate?
I have another question though.
The correction factors, whichs exists for all heat exchangers.How are they determined? Is it experimentally or proven science?
Thanks!
RE: Overall heat transfer coefficient in function of flow rate?
I don't believe it unless you had maybe two or three data points.
RE: Overall heat transfer coefficient in function of flow rate?
And, please, don't forget that he value of U can never be greater than the lowest of both heat transfer coefficients in the given heat exchanger.