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Optimization of the tube in tube counter-flow heat exchanger in a 4.5 K hybrid J-T cryocooler to be used in space

Hybrid J-T cryocooler is commonly employed in space detective missions
requiring liquid helium temperature. In fact, nearly all the space
applications of mechanical cryocoolers working at 4.5 K, having been
launched or under development are hybrid J-T cryocoolers. For instance,
mechanical cryocoolers used in Planck, JWST, SMILES, SPICA and so
on, are all J-T cryocoolers precooled by adsorption cryocooler, Stirling
cooler or pulse tube cooler. But there is hard any research on space
hybrid 4.5 K J-T cryocooler in China. Key Laboratory of Space Energy
Conversion Technologies in Technical Institute of Physics and Chemistry
has developed a 4.5 K J-T cooler precooled by three stages of pulse
tube cryocooler. Base on this cryocooler, design optimization is carried
out on the tube-in-tube counter flow heat exchanger to improve its
performance. Counter flow heat exchanger is one of the key components
of the 4.5 K hybrid J-T cryocooler. Spiral tube-in-tube heat exchanger
is widely used in 4 K class hybrid J-T cryocooler because of its advantages
of light weight and small size. The impact of tube-in-tube heat exchanger
on J-T cycle is analyzed. And J-T cycle is able to provide more cooling
capacity and needs less precooling power as the efficiency of the
counter flow heat exchanger increases. Then, for the purpose of improving
heat transfer efficiency of the counter flow heat exchanger, design
optimization is carried out on the basis of theoretical analysis.
The main heat transfer resistance of the counter flow heat exchanger
is the convection heat transfer between return gas and the outer wall
of inner tube. Thus, smaller tube is used to decrease the main heat
transfer resistance. Besides, the centrifugal force of the moving
helium flow in the spiral pipe brings about secondary flow. The development
of secondary flow results in enhancement of heat transfer of the counter-flow
heat exchanger. So heat transfer coefficient of the heat exchanger
increases with decreasing diameters of the tubes and the spiral diameter.
On the other hand, when the efficiency of the counter flow heat exchanger
rises, the high pressure flow will be precooled much more fully, which
means less precooling power from the precooling cooler will be consumed.
As a result, the power consumption of the whole hybrid J-T cooler
will decrease and its efficiency will be improved. Then, experimental
research is conducted and the experimental results agree well with
theoretical analysis. The temperature before throttling decreases
as the efficiency of the counter flow heat exchanger increases. At
the same time, the cooling capacity increases while the precooling
power decreases. However, the cooling capacity and precooling power
are smaller than theoretical analysis which attributes to the mass
flow rate of calculation is much larger than measured one. Consequently,
the benefit of improving heat transfer efficiency is verified. As
the efficiency of the counter-flow heat exchanger increases, the cooling
capacity of the J-T cooler is improved while the precooling power
is cut down. Eventually, the cooling capacity of the hybrid J-T refrigerator
at 4.7 K is improved from 15 to 32 mW.