This paper aims at improving the understanding of the dynamics of the interactions during water mist fire suppression in tunnels. The work described here makes extensive use of numerical simulations using the Fire Dynamics Simulator (FDS, NIST), on the basis of the numerical reconstruction of an experimental test performed at one third scale. In a first step, the method consists of determining the relevant numerical parameters to accurately model the water mist using current FDS sub-models. The second step consists of calibrating the code, in order to get the best achievable numerical processing of the test. Finally, the third part of the work is devoted to the extraction and the post processing of information from the FDS calculation, in order to quantify the global energy balance within the test tunnel. This makes it possible to evaluate the distribution of energy and notably the heat absorbed by droplets during water mist application. An estimation of the efficiency of the water application is then proposed. On the whole, this paper gives an example of the use of a numerical model to help interpreting experimental results by getting information whose measurement is very difficult to reach.