Thermal Excursions Research Articles

THERMAL FATIGUE: A POSSIBLE SOURCE OF STRUCTURAL MODIFICATIONS IN METEORITES

Periodic thermal excursions, even in a low‐temperature range, may cause internal stresses and give origin to some of the observed plastic and clastic deformations in stony meteorites.An experimental test confirms our hypothesis: lattice distortions, cleavage and cracking were artificially induced in specimens of two H‐chondrites (Mills and Bur‐Gheluai) by low‐temperature “thermal fatigue.” The observed effects appear to be increasing with increasing number of cycles.

An Intercell Planar Heat Pipe for the Removal of Heat During the Cycling of a High Rate Nickel Cadmium Battery

Two 22 A‐h nickel‐cadmium cells were continuously cycled at a 1c charge rate and a 2c discharge rate, with cooling provided by an intercell planar (rectangular cross section) heat pipe. For purposes of comparison, thermocouple measurements were also taken with an aluminum conduction fin substituting for the heat pipe. The aluminum fin and heat pipe were cooled, by room temperature forced air. Thermally insulated cells were also cycled at the same rates. Cell case temperatures were measured during cycling, and a maximum of 29°C with a 5°C thermal excursion was noted with the heat pipe under conditions of thermal equilibrium which were observed after 3 complete cycles. For the aluminum fin configuration a maximum of 42°C with a 7°C thermal excursion was obtained near thermal equilibrium after 5 complete cycles. The insulated configuration yielded a battery case temperature of 83°C after 5 cycles, and thermal equilibrium was never reached. Coulombic efficiency values for the heat pipe cooled battery were found to be several percent greater than ∼95% which was recorded for the aluminum fin configuration. The specific heat of the cells was measured to be 0.27 cal/g°C. From this and the measured values of the total heat generated per cycle, the effectiveness of the heat pipe in removing battery heat was calculated to be approximately 26% greater than the aluminum fin at or near equilibrium. It is surmised that the significantly lower operating temperatures produced by the heat pipe should lead to an important lengthening of battery cycle life and an associated reduction of capacity degradation.

Accuracy of the Quasistatic Treatment of Spatial Reactor Kinetics

The quasistatic approach of treating the spatial dynamics problem is described as one method out of a full sequence of methods that factorize the flux into an amplitude and a shape function. The accuracy of these methods is investigated for a wide range of excursions in fast and thermal reactors by comparison with a full numerical solution. The quasistatic method describes even extreme excursions in fast reactors very accurately. Its application to thermal reactor excursions may, however, lead to appreciable errors. The “improved quasistatic” method reduces the errors for both types of reactors to negligible amounts so that its application to thermal reactors may be also considered.