Closing the browser will not end/expire the session. That is done after user is idle for 30 mins time by default.
But you can adjust that from adjusting "timeout" parameter of the following tag in the web.config file
<authentication …> <forms name=".ASPXAUTH" cookieless="UseCookies" timeout="2" /> </authentication>
I have blog about the WFFM data flow process for sc 8 versions in the following links in case you need to get full cycle of the data flow.
Need to clear session while user is closing browser, so that interaction will push to Mongo DB.