Setting up a simple issue search for open cases sorted by ranking produces the following query

project = TIMEREG AND status in (Open, "In Progress", Reopened) ORDER BY cf[10000] ASC, key DESC

trying to list only cases above a certain ranking doesn't work, auto complete inserts the fieldname Ranking

the query

project = TIMEREG AND status in (Open, "In Progress", Reopened) AND Ranking <= 15 ORDER BY cf[10000] ASC, key DESC

returns nothing

changing fieldname Ranking to cf[10000] still returns nothing

Workaround

Use a plugin that enables you to sort by rank in Advanced Search.
Please check here:
https://github.com/matthewlmcclure/jira-rankof-jql-function