May be check below areas:
Qlik Sense reserves RAM for itself and it doesn't have to be a problem unless you need to run another application in the same server, which needs lots of RAM (which I think is not recommended).
Memory will be consumed by:
1. The data model itself,
2. Session (selection state) storage for each user.
3. Cached results as charts are recalculated when a user make selections. Each new filter will generate a new cache entry. This is the primary cause of memory growth as the day goes on. QS should manage the cache to not cause a fatal problem in the server.
4. Temporary (transient) memory required to calculate a given chart (see below).
Users developing visualizations can consume a lot of memory by creating wacky (that's a tech term) charts. Again, QS should manage this so it does not cause a failure in the server.
You looking at memory after closing your apps? if yes then no worries .The Qlik sense engine will not release the memory after closing of your app..If you try to open another app that memory automatically allocates to that app..Some time the Sync b/w Central and nodes will happen that time also memory is to high ...