A mechanical-capillary-driven (Hybrid) Two-Phase Loop (HTPL) is a phase change cooling device that utilizes both mechanical (active) and capillary (passive) pumping. The HTPL consists of a mechanical pump, evaporator, condenser, and liquid reservoir which are connected through two separate loops for liquid and vapor flows. The hybrid (active/passive) pumping and scalable evaporator design of the HTPL make it possible to handle challenging thermal requirements such as high heat flux heat acquisition, large heat transfer area, reliable operation, long distance thermal transport with a small temperature difference. In this paper, the operational characteristics of the HTPL were numerically and experimentally investigated by varying heat inputs, flow rates of mechanical pump, and heat sink temperatures. A pressure relation between liquid and vapor phases in the evaporator was proposed to determine three distinctive boiling modes (flooded, partially flooded, and capillary) and heat transfer limits were discussed. The operating range of the capillary mode, which is a desirable boiling condition with low thermal resistances, is extended by reducing the pressure drop of the liquid supply in the evaporator and increasing a capillary pressure head in the evaporator wick. A reduction of heat sink temperature increases the system thermal resistance of the HTPL, especially in the vapor line which exists from the evaporator to the condenser. A capillary limit of the HTPL, by increasing pump flow rate, approaches a boiling limit which is estimated to be 259.8 W/cm(2). (C) 2018 Elsevier Ltd. All rights reserved.