design porosity. Under nucleate boiling conditions, the heat transfer coefficient increases as the wick porosity tends towards 100%, and increased wick fiber diameter increases heat transfer even more rapidly. Taking account of the results derived above, we are able to understand the influence of various heat pipe design parameters on the heat transfer coefficient well enough to use them as a guide in designing heat pipes for use in heat rejection radiators for spacecraft thermal control. Power Density and Dynamic Heat Pipe Characteristics It is interesting to determine the heat pipe radiator power dissipation density as well as the characteristic time of response to changes in heat input to the heat pipe. Considering the radiator mass to be the sum of heat pipe mass and fin masses, and using literature relations for the dependence of radiator heat rejection on geometrical and thermophysical factors [8] the power density can be written as: Computational studies of this relation show that the value of £e increases monotonically towards its asymptote as the distance between the heat pipes (Z/dH) increases. Depending on the fin thickness 3, Z/dH must be selected to minimize construction mass. Z/dH falls between 2 and 6. Heat pipe dynamic behavior must be examined as a function of Tk, the rate of temperature change in the condenser due to a heat flow perturbation caused by the control system. The heat pipe is treated as three elements in series: the convective heat exchanger and working fluid evaporator, the isothermal vapor transport line, and the finned radiator and condenser. Equations describing the thermal transfer dynamics of each element of this system can be combined as described in the literature [9], and manipulated (via linearization, recording in relative deviations and laplace transform) to obtain the following expression for the system transfer function: which determines the relationship between the thermal flux transferred by the heat pipe and its temperature. The dynamics of this system are determined to a considerable extent by the heat exchange process in the condenser. Our analysis shows that the heat pipe temperature will either fluctuate or immediately be damped, depending on the working fluid used. The condition necessary to ensure damping is: If the damping time is determined by the radiator design specification, the external heat pipe diameter can be determined from the condition
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