Processdefinition ID != name afaik.
The same you supply in your process definition and the version number form the processDefinitionId. So the processDefinitionId in this query is always made of the supplied name and a number.
Maybe the TaskInstanceQuery should be extented to make it possibile to search for all instances with a specific processDefinitionKey.
Be careful not to mix up the processDefinitionKey and the processInstanceKey. The processDefinitionKey is a key supplied for the process definition which stays the same for all versions of the process definition.
To me it seems a good idea to also offer searching for the criteria processDefinitionKey.