Self-cooling/heating

Fig. 9. Stored energy release curves for CSF graphite irradiated at 30°C in the Hanford K reactor cooled test hole [64], Note, the rate (with temperature) of stored energy release (J/Kg-K) exceeds the specific heat and thus under adiabatic conditions self sustained heating will occur.

Figure 246. Self-cooling beer keg as an indirect heat storage...

Auto-oxidation is the process of slow oxidation with accompanying evolution of heat, sometimes leading to autoignition if the energy is not removed from the system. Liquids with relatively low volatility are particularly susceptible to this problem. Liquids with high volatility are less susceptible to autoignition because they self-cool as a result of evaporation. 

Conversion to the para form takes place at a relatively slow rate and is accompanied by the release of heat. For each pound of rapidly cooled hydrogen that changes to the para form, enough heat is liberated to vaporize approximately 1.5 1b of liquid hydrogen. However, if a catalyst is used in the liquefaction cycle, para-hydrogen can be produced directly without loss from self-generated heat. 

Reaction 15 is endothermic and the electrolyzed water will undergo self cooling unless external heat is supplied. The enthalpy balance and its related thermoneutral... 

Conditions of qsoiar > t ph0t can be shown to place specific restrictions on the photoabsorber. When Hi20 < Etneut, heat must flow to compensate for the self cooling which occurs at the electrolysis rate. That is, for an enthalpy balanced system any additional required heat must flow in a flux equivalent to iieai = imo, and at an average power dieat, such that ... 

Several instances of compensation behaviour have been described, typically for endothermic, reversible decompositions. Kinetic characteristics are sensitive to reaction conditions, most notably the effects of the presence of the volatile product and heat flow to and within the reaction zones undergoing self-cooling. The following are examples of reactions studied in detail. 

Kinetic data measured for the decomposition of calcium carbonate under isothermal and under programmed-temperature conditions [11] and varied reaction environments influencing the ease of removal of the CO2 product, show that the apparent values of the kinetic parameters k, A and may be influenced by sample heating rate, reactant self-cooling, sample mass, geometry and particle size, which determine the rate because of the reversible nature of the decomposition [12]. These effects can lead to compensation behaviour [13]. 

Self-Cooling or Self-Heating During Reaction of a Solid.175... 

Reactant temperature within the reaction zone may be appreciably different from that measured for the controlled furnace reaction vessel because of local self-cooling or self-heating as a consequence of the reaction enthalpy. The significance of self-cooling in dehydrations has been discussed by Bertrand et al. (19). L vov et al. (20) have developed a computer model to represent the effect with reference to the endothermic dehydration of Li2S04 H20. Not all research reports discuss the possible consequences of reaction enthalpy in influencing reactant temperature. 

Temperature The sensor is located as near the sample as is practicable, and the reliability of measurements is increased by calibration, such as in the Curie point (magnetic) method. There may be delays in the transfer of heat to the sample due to conductivity and thermal inertia of both reactant and container, a temperature programmed change, the gas atmosphere flow, and reactant self-cooling or self-heating during the reaction. 

SiC/SiC composites were considered several years ago in the ARIES-1 ARIES-IV and PROMETHEUS power plant studies in the US and more recently in other international studies. Examples also include the TAURO a self-cooled Pb-17Li blanket (SCLL), whose latest reference design assumes a surface heat flux of 0.5 MW/m, and shows a Pb-17Li outlet temperature of 950 C with an estimated conversion efficiency of -55% the He cooled pebble bed (HCCB) concept—DREAM Veactor study in Japan, with assumed inlet/outlet He coolant temperatures of 600/900°C and a gross thermal efTiciency of -50% and the latest SCLL... 

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