This experimental study investigated the change in cross-sectional area (csa) of thermosyphon's condenser to it's a constant perimeter (RAP) and effect of that at the thermal performance. Geometries shapes deform from circular to elliptical and flat to make a change in csa at a same perimeter of all geometries shapes where a different value of RAP are got. At each RAP a different rate of heat input in evaporator and
different filling ratio of working fluid (distill water) are applied. Threshold angle (working angle) where the condensate of distilled water vapor begin to return to evaporator, this angle are examined with the horizontal level for three geometries shapes (three RAP). The results indicated that the flat two phase closed thermosyphon( FTPCT)(low RAP value) and elliptical two phase closed thermosyphon (ETPCT) had a higher and more stable heat transfer coefficient h (condenser air side) with the high and mid filling ratio from than of the circular two
phase closed thermosyphon( CTPCT). That refer to more useful length of condenser to absorb heat from evaporator and prevent it from causing a rise in temperature. The high filling ratio with low rate of heat level in evaporator lead to unuseful area in thermosyphon's condenser and lowering its pressure, while low filling ratio with high
rate of heat caused a chaotic flow inside TPCT and rising the evaporator and condenser temperatures.