The Wonderwerk TLUD natural draft test stoves use a combustor section above the secondary burn area to provide time and space for the secondary flame to burn more completely.
The project started as a way to shorten the overall height of a TLUD stove by adding an improved combustor. In the process of Kirk Harris improved the efficiency of the stove.
His write-up: http://stoves.bioenergylists.org/files/wws_tlud_combustor_paper102714.pdf
Kirk modified the original TLUD design ( http://www.drtlud.com ) to include a concentratior and used Venturi principles to blend secondary air to improved combustion.