Design of a High-Effective Wavy Channel Heat Exchanger for Cryogenic Applications
Authors: Shelby Jason L. , Kota Krishna 
Published: March 2016

Long duration thermal control of cryogenic propellants is a high priority mission activity for NASA. A 20 K cryocooler capable of 20 W of refrigeration would offer a significant mass savings in cryogen storage by facilitating propellant zero boil-off and hence long term cryogen storage, which would provide scope for long duration missions beyond low Earth orbit (LEO). For direct current (DC) type cryocoolers, as well as open cycle liquefaction systems, effective recuperative heat exchange is a highly critical function. Coefficient of Performance (COP) of cryocoolers is greatly sensitive to heat recuperator effectiveness, more so than to the efficiencies of other key components such as compressor, turbine and cold head. A heat recuperator with high effectiveness is extremely important for achieving a high COP for cryocoolers.

Current recuperators tend to be the heaviest and largest components in space cooling systems. Primary issues in miniaturization of a heat recuperator that adversely affect the performance are the longitudinal/axial heat conduction in the wall and insufficient flow length (size) to achieve complete heat transfer between the hot and cold fluids. Further, frictional pressure drops are usually large as a result of decreased size of the flow channels. In addition, it is worth noting that, in general, attempts to improve heat transfer would almost always result in increasing the pressure drop (or otherwise, attempts to decrease pressure drop usually result in lowering the heat transfer performance).

This paper discusses a split wavy channel heat recuperator design to overcome the above mentioned technical show-stoppers. A split wavy channel design has the advantage of increased flow length and improved heat transfer in the same form factor as a heat exchanger with straight channels without causing a large increase in the pressure drop. So far, this concept has never been studied and exploited for a cryogenic heat recuperator.

High fidelity numerical simulations were used to study the three-dimensional flow of working fluid through hot and cold wavy channels of the heat recuperator separated by a wall. A two channel scheme was modeled for the analysis. Wavy geometry consisting of two sinusoidal parallel channels was built in a pre-processor for the solver. Governing equations in mass, momentum and energy at steady-state were implicitly solved on a computational grid using a commercial finite element solver to obtain the pressure, velocity and temperature fields.

The developed numerical model was employed to perform a systematic parametric study. Two working fluids (helium and nitrogen) were considered addressing both liquid oxygen and liquid hydrogen propellant boil-off temperatures. Wavy wall amplitude was varied between 125 μm, 250 μm, and 500 μm with a goal of identifying optimal configuration that will have maximum effectiveness for the size. Entrance Reynolds numbers of 300 and 600 were considered. The performance of various configurations were compared using a performance factor, and traditional recuperator effectiveness.

The results showed that wavy channels, due to a unique fundamental physical phenomenon of geometry governed boundary layer thinning, help in augmenting heat transfer and in realizing a compact cryogenic heat exchanger. An interesting observation was that wavy channels which were previously found by researchers to enhance heat transfer substantially (up to 55% or more) at room temperature for liquids were found to improve heat transfer performance only by up to 10% for gases at cryogenic temperatures, which could be attributed to the drastic variation in thermo-physical properties of gases at these temperatures suggesting the consideration of a different design methodology compared to design for room temperature applications.