The rise time (0 is defined as the time required to raise the temperature of the evaporator to THi from room temperature after the input power Q, is applied. This is obtained from this plot (Fig. 12) by finding the time (after power ‘on') at which TC #4 attained a temperature within 2°C of the rise temperature (TH1). The startup time (ts) is defined as the time taken after t; for the hot front to move to the beginning of the condenser, i.e. to move through the adiabatic length. ts is obtained by a similar procedure applied to TC #11 counting the time after the rise time. The initial hot zone temperature or rise temperature (T^) is the temperature at which the sodium vapor pressure (Pv) is equal to the adjusted initial gas charge pressure P,( Tc) such that where Tc = room temperature LP = heat pipe length, and LE = evaporator length. The startup temperature (THs) is the temperature of the hot zone when the hot front has reached the beginning of the condenser. THi and THs are obtained from the graphs as marked (Figs 12 and 13). The pipe started up from the frozen state for all power inputs up to Q = 564 W (Qo = 306 W) when applied as a sudden load. Beyond this power level, only gradual increments of 25 W were possible. In addition, the nichrome heater operating temperature limitation (1000°C) restricted the heat input level. Hot Zone Temperature and Length Variation The hot zone temperature and length variations as functions of time are plotted for Qo = 306 W in Figs 14 and 15, respectively. In the time axis, r=0 corresponds to the rise time (tj). The 3000 s of the transient test are counted after the lapse of t^ The agreement between the experimental and analytical results is good. The small deviation in each graph is attributed to the limitations of length determination (± 0.05 m uncertainty) and temperature measurement (±4.5°C uncertainty).
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