| dc.description.abstract | This thesis deals with the development of a laboratory model of an activated?charcoal杗itrogen cryocooler to cater to heat loads of the order of 1 W in the temperature range 80�0 K. Among other aspects, adsorption and thermal characterization of compressors are discussed.
Isotherm parameters of two granular specimens (Chemviron and Alliston) of charcoal are generated using desorption studies. In this method, which can be a viable alternative to other conventional methods, the adsorption parameters are estimated by measuring the concentration difference between two thermodynamic state points, namely pressure and temperature, of the adsorbate. The efficacy of this method is established through comparison of results with volumetric?method experimental data on a Fluka specimen.
Extensive thermodynamic analysis has been carried out on single?stage and two?stage systems. Calculations are done with six specimens of charcoal-namely Fluka, Sarabhai, Saran, BCGI, Alliston, and Chemviron. Optimal operating conditions for the cryocooler cycle are established in terms of packing density and cycle operating conditions. A two?stage system is necessary if refrigeration is required at 80 K or lower for all specimens of charcoal. No tangible benefit is observed in using a two?stage system if the cooling temperature is 117.5 K. The effect of void volume is less evident in the case of a two?stage system.
A thermal?conductivity model of the charcoal杗itrogen bed is proposed, assuming that the adsorbed gas in the micropores and the solid matrix form parallel paths for heat conduction. The model is verified through measurements at 300 K and 0.1 MPa, at which the thermal conductivity is found to be 0.12 W/m稫. It is shown that granular specimens of charcoal have higher thermal conductivity than powdered specimens. The thermal conductivity can be increased by 25% by adding 10% (by volume) copper powder. Once again, it emerges that a high packing density augments the thermal conductivity of the bed.
A vacuum?brazed perforated?plate heat exchanger has been designed and fabricated. The design calculations show that a spacer?thickness?to?hole?diameter ratio of 1 is optimal from the point of view of heat transfer and pressure drop.
Thermal characterization of the compressor shows that a cycle time of 8 minutes, equally partitioned between heating and cooling, can be achieved. With adsorption at 150 K and 1 MPa and desorption at 373 K and 5.2 MPa, a gas outflow of 1.75 slpm was achieved for 2 minutes. When four compressors are operated 90� out of phase, a continuous flow of 1.75 slpm gas at 5.2 MPa is obtained. It is shown that there is no advantage in increasing the cycle time. The laboratory model is operated continuously with a cycle time of 8 minutes, and the cooling temperature obtained is 124 K at 0 W of cooling load but with 0.36 W of parasitic load.
The specific power requirements of the laboratory model were found to be higher than those predicted by analysis. Nonetheless, this cooling technology opens up new possibilities for space and terrestrial CFC?free refrigeration applications. | |
