Heat conductivity depending on temperature

[Oli Hall]

Hi, I'm working on my final year project for a part time mechanical engineering design degree course. I've built and tested a heat exchanger which is used for controlling the temperature of N2 gas.

I'm currently working through the calculations so I can compare real against calculated values.

I'm struggling to calculate the thermal conductivity of N2 depending on temperature. An Engineer from work gave me a Visual basic file which had the following expression which works really well. The problem is I can't work out what the components are - and I'm not happy using it if I don't understand what it's doing


Visual basic expression.

Function lambda_N2 (Temp As Double)

' Calculation of heat conductivity (W/(m*K)) depending on temperature
' for gaseous N2 - source Malkov

lambda_N2 = 0
lambda_N2 = -0.000000054111 * Temp * Temp + 0.000102445 * Temp - 0.00016339

End Function

I assume that the values he's used are constants - but as yet I've been unable to trace them. Could someone explain what some or all of it is doing?

Or is there another way of calculating thermal conductivity?
(Note. I want it for an individual temperature not for ∆T)

Many thanks in advance
Oli

[Mapes]

Hi Oli, welcome to PF.

The equation looks like an empirical fit to experimentally measured results (and as such, has little meaning without a temperature and pressure range). The constants are likely selected just to make the resulting curve fit well to measured values.

A much more involved equation with physical justification is given here (http://www.boulder.nist.gov/div838/theory/refprop/NAO.PDF), which may be what you're looking for. The document also includes many references to experimental work, which may be useful.