The Effect of Fluid Property Variations on the Performance of Cryogenic Helium Heat Exchangers

Balanced flow coaxial tube counterflow heat exchangers have application to a number of helium refrigeration systems. Variations in the helium fluid properties in heat exchangers of this type are large enough to cause concern over the accuracy of simplified constant property analyses. The effect of fluid property variations on the performance of such heat exchangers are investigated with a finite difference thermal model. The model uses temperature dependent values of specific heat, thermal conductivity, and viscosity to evaluate heat exchanger performance, but neglects the effects of pressure losses, internal wall thermal resistance and axial conduction. Ratios of analytically predicted to numerically determined heat exchanger effectiveness versus the design N_tu are presented. For values of N_tu of interest in most cryogenic applications (i.e., greater than 3), predicted performance using variable fluid properties is two percent greater than that predicted using mean properties.