A data center thermosiphon cooling system (100; 200; 300), comprising: a plurality of evaporator modules (123; 227; 327), each evaporator module (123; 227; 327) configured to thermally couple to one or more heat-generating devices (124; 221; 321) mountable in a rack (105; 205; 305) of a data center; a condenser module (115; 215; 315) that comprises a heat transfer surface and is mounted external to the rack (105; 205; 305); and a plurality of transport members, each transport member comprising: a liquid conduit (119; 219; 319) that fluidly couples an inlet of a respective evaporator module (123; 227; 327) to the condenser module to deliver a liquid phase (107; 207; 307) of a working fluid from the condenser module (115; 215; 315) to the respective evaporator module (123; 227; 327); a vapor conduit (117; 217; 317) that fluidly couples an outlet of the respective evaporator module (123; 227; 327) to the condenser module (115; 215; 315) to deliver a mixed-phase (103, 303) of the working fluid from the respective evaporator module (123; 227; 327) to the condenser module (115; 215; 315), the mixed-phase 103, 303) of the working fluid comprising heat transferred from the one or more heat-generating devices (124; 221; 321) to the working fluid through the respective evaporator module (123; 227; 327); a liquid header fluidly coupled to each of the liquid conduits (119; 219; 319) and to an outlet of the condenser module (115; 215; 315); a vapor header fluidly coupled to each of the vapor conduits (117; 217; 317) and an inlet of the condenser module (115; 215; 315); and a respective fluid disconnect that couples each liquid conduit to the liquid header, wherein the fluid disconnect comprises a variable orifice configured to meter an amount of the liquid phase (107; 207; 307) of the working fluid to the respective evaporator module (123; 227; 327) based on an amount of the heat transferred from the one or more heat-generating devices (124; 221; 321) to the working fluid through the respective evaporator module (123; 227; 327). a plurality of evaporator modules (123; 227; 327), each evaporator module (123; 227; 327) configured to thermally couple to one or more heat-generating devices (124; 221; 321) mountable in a rack (105; 205; 305) of a data center; a condenser module (115; 215; 315) that comprises a heat transfer surface and is mounted external to the rack (105; 205; 305); and a plurality of transport members, each transport member comprising: a liquid conduit (119; 219; 319) that fluidly couples an inlet of a respective evaporator module (123; 227; 327) to the condenser module to deliver a liquid phase (107; 207; 307) of a working fluid from the condenser module (115; 215; 315) to the respective evaporator module (123; 227; 327); a vapor conduit (117; 217; 317) that fluidly couples an outlet of the respective evaporator module (123; 227; 327) to the condenser module (115; 215; 315) to deliver a mixed-phase (103, 303) of the working fluid from the respective evaporator module (123; 227; 327) to the condenser module (115; 215; 315), the mixed-phase 103, 303) of the working fluid comprising heat transferred from the one or more heat-generating devices (124; 221; 321) to the working fluid through the respective evaporator module (123; 227; 327); a liquid header fluidly coupled to each of the liquid conduits (119; 219; 319) and to an outlet of the condenser module (115; 215; 315); a vapor header fluidly coupled to each of the vapor conduits (117; 217; 317) and an inlet of the condenser module (115; 215; 315); and a respective fluid disconnect that couples each liquid conduit to the liquid header, wherein the fluid disconnect comprises a variable orifice configured to meter an amount of the liquid phase (107; 207; 307) of the working fluid to the respective evaporator module (123; 227; 327) based on an amount of the heat transferred from the one or more heat-generating devices (124; 221; 321) to the working fluid through the respective evaporator module (123; 227; 327). a liquid conduit (119; 219; 319) that fluidly couples an inlet of a respective evaporator module (123; 227; 327) to the condenser module to deliver a liquid phase (107; 207; 307) of a working fluid from the condenser module (115; 215; 315) to the respective evaporator module (123; 227; 327); a vapor conduit (117; 217; 317) that fluidly couples an outlet of the respective evaporator module (123; 227; 327) to the condenser module (115; 215; 315) to deliver a mixed-phase (103, 303) of the working fluid from the respective evaporator module (123; 227; 327) to the condenser module (115; 215; 315), the mixed-phase 103, 303) of the working fluid comprising heat transferred from the one or more heat-generating devices (124; 221; 321) to the working fluid through the respective evaporator module (123; 227; 327); a liquid header fluidly coupled to each of the liquid conduits (119; 219; 319) and to an outlet of the condenser module (115; 215; 315); a vapor header fluidly coupled to each of the vapor conduits (117; 217; 317) and an inlet of the condenser module (115; 215; 315); and a respective fluid disconnect that couples each liquid conduit to the liquid header, wherein the fluid disconnect comprises a variable orifice configured to meter an amount of the liquid phase (107; 207; 307) of the working fluid to the respective evaporator module (123; 227; 327) based on an amount of the heat transferred from the one or more heat-generating devices (124; 221; 321) to the working fluid through the respective evaporator module (123; 227; 327).