A valve for automatically modulating fluid flow includes a flexible flow adjustment member having a variable diameter passage therethrough and an outer surface and a shell surrounding at least a portion of the outer surface and creating a void between the shell and the outer surface. The shell is in contact with the flow of fluid. A thermally reactive material that expands when heated and contracts when cooled is a substantially non-volatile heat transfer liquid that does not undergo a phase change in use and contains inert particulate filler within the void and is in contact with the outer surface of the flexible flow adjustment member. The thermally reactive material exerts pressure on the adjustment member in response to heating the thermally reactive material to cause the passage to narrow and relieves pressure thereon in response to cooling the thermally reactive material to permit the passage to widen.