This the command that i use to import data :
Lambda= Import["C:\\Users\\Documents\\Modélisation\\1_\\mathematica\\condu_therm_rer.xlsx"];
and this is how the data are displayed:
{{{707.637, 0.0556025}, {1280.52, 0.092702}, {1415.35, 0.0927435}, {1651.15, 0.129739}, {1887.03, 0.148273}...