J. H. Baik

An exact solution for shuttle heat transfer

A new analytical solution has been obtained for the shuttle heat transfer rate in displacer/cylinder systems having an axial temperature gradient. The temperature oscillations of both the displacer wall and the cylinder wall were considered in the analysis. After the cyclic steady state solution was obtained for the temperatures of the walls by introducing complex temperatures, a simple mathematical expression was derived to calculate the shuttle heat transfer. The usefulness of the results was justified by approximate solution of previous work.

An Exact Expression for Shuttle Heat Transfer

A new and general analytical expression is obtained for the shuttle heat transfer rate in displacer-cylinder systems. The shuttle heat transfer occurs when the displacer has a reciprocating motion over the cylinder with an axial temperature gradient. The heat transfer might be important in small cryogenic refrigerators because it represents a loss of refrigeration in addition to the wall conduction. The oscillating temperature distributions of both the displacer wall and the cylinder wall are exactly obtained as functions of time and space from the conduction equations by introducing the complex temperatures. From the cyclic steady state solution of the temperatures, a simple mathematical expression is derived to calculate the shuttle heat transfer. The expression includes the axial temperature gradient, the stroke and the angular speed of the reciprocating motion, the heat transfer coefficient between two walls, and the thermal properties (such as thermal conductivity, density, or specific heat) of two walls. The usefulness of the results is justified by the approximate solutions of previous works and the physical interpretations are presented.

Operational Testing of Densified Hydrogen Using G‐M Refrigeration

Propellant densification has many beneficial properties when space launch systems are considered. Among these are reduced tank volumes, decreased vapor pressures, and increased enthalpy gain before boil off. Previous NASA investigations have focused on advanced methods of producing densified propellants, but not much work has been accomplished in the area of storing and handling densified propellants. NASA KSC has 50+ years experience in handling cryogenic propellants, but all that experience is with saturated liquids. This work is focused on using existing cryogenic refrigeration technology to subcool hydrogen, and to develop a testbed where propellant handling techniques are researched. Among these topics include continuous operation, zero boil off storage, densification, pressurization techniques, handling of stratification layers, liquefaction, and recovery of boil off losses from chill down procedures.

Initial Test Results of Laboratory Scale Hydrogen Liquefaction and Densification System

Using densified liquid hydrogen as a cryogenic propellant for launch vehicle applications can reduce fuel tank volumes, decrease vapor pressures, and improve cooling capacity over the normal boiling point propellant. A densified liquid hydrogen test bed has been developed using Gifford‐McMahon cryocooler to refrigerate hydrogen inside the 150L storage tank at the Florida Solar Energy Center (FSEC). This work is a collaborative effort amongst researchers at FSEC and NASA Kennedy Space Center. The test bed has an integrated refrigeration and storage system with multiple capabilities including hydrogen liquefaction, densification, and zero‐boil‐off (ZBO) storage test. This paper focuses on the design considerations, the detailed system descriptions, and the results obtained during initial hydrogen liquefaction and densification tests.

Design and Operation of a Small-Scale Hydrogen Liquefier

>> In order to accelerate hydrogen society in current big renewable energy trend, it is very important that hydrogen can be transported and stored as a fuel in efficient and economical fashion. In this perspective, liquid hydrogen can be considered as one of the most prospective storage methods that can bring early arrival of the hydrogen society by its high gravimetric energy density. In this study, a small-scale hydrogen liquefier has been designed and developed to demonstrate direct hydrogen liquefaction technology. Gifford-McMahon (GM) cryocooler was employed to cool warm hydrogen gas to normal boiling point of hydrogen at 20K. Various cryogenic insulation technologies such as double walled vacuum vessels and multi-layer insulation were used to minimize heat leak from ambient. A liquid nitrogen assisted precooler, two ortho-para hydrogen catalytic converters, and highly efficient heat pipe were adapted to achieve the target liquefaction rate of 1L/hr. The liquefier has successfully demonstrated more than 1L/hr of hydrogen liquefaction. The system also has demonstrated its versatile usage as a very efficient 150L liquid hydrogen storage tank.

Performance test of a 6 L liquid hydrogen fuel tank for unmanned aerial vehicles

A 6 L liquid hydrogen fuel tank has been designed, fabricated and tested to optimize boil-off rate and minimize weight for a 200 W light weight fuel cell in an unmanned aerial vehicle (UAV). The 200 W fuel cell required a maximum flow rate of 2.3 SLPM or less liquid hydrogen boil-off from the fuel tank. After looking at several different insulation schemes, the system was optimized as two concentric lightweight aluminum cylinders with high vacuum and multi-layer insulation in between. MLI thickness and support structures were designed to minimize the tank weight. For support, filling and feed gas to a fuel-cell, the system was designed with two G-10 CR tubes which connected the inner vessel to the outer shell. A secondary G10-CR support structure was also added to ensure stability and durability during a flight. After fabrication the fuel tank was filled with liquid hydrogen. A series of boil-off tests were performed in various operating conditions to confirm thermal performance of the fuel tank for a 200 W fuel cell.

Development of 1 L hr−1 scale hydrogen liquefier using Gifford-McMahon (GM) cryocooler

Korea Institute of Science and Technology (KIST) and Florida Solar Energy Center (FSEC) have collaborated to develop a demonstration-scale hydrogen liquefier for future liquid hydrogen research in Korea. A 1 L hr−1 liquefaction rate, direct-cooling type hydrogen liquefier using a commercially available GM cryocooler has been designed, fabricated, and tested at KIST. The liquefier consists of a GM cryocooler, finned heat pipe, liquid nitrogen precooler, ortho-para hydrogen converter, and vacuum jacketed internal storage tank. The system successfully demonstrated more than 1 L hr-1 of hydrogen liquefaction rate from ambient temperature gas. A detailed design method, loss analysis, overview of component fabrication, and experimental results are discussed in this paper.

A First Order Model of a Hybrid Pulse Tube/Reverse-Brayton Cryocooler

This paper describes a cryogenic refrigeration cycle that combines a recuperative lower stage with a regenerative upper stage. The resulting cryocooler avoids the inherent thermal saturation and void volume losses that are associated with a regenerator and therefore has the potential for high performance at low temperature. The hybrid concept also has advantages relative to thermal, mechanical, and electrical integration as well as reliability.

Performance of a 5 L Liquid Hydrogen Storage Vessel

Abstract >> In the face of the world’s growing energy storage needs, liquid hydrogen offers a high energy densitysolution for the storage and transport of energy throughout society. A 5 L liquid hydrogen storage tank has beendesigned, fabricated and tested to investigate boil-off rate of liquid hydrogen. As the insulation plays a key roleon the cryogenic vessels, various insulation methods have been employed. To reduce heat conduction loss, theepoxy resin-based insulation supports G-10 were used. To minimize radiation heat loss, vapor cooled radiationshield, multi-layer insulation, and high vacuum were adopted. Mass flow meter was used to measure boil-off rate of the 5 L cryogenic vessel. A series of performance tests were done for liquid nitrogen and liquid hydrogento compare with design parameters, resulting in the boil-off rate of 1.7%/day for liquid nitrogen and 16.8%/dayfor liquid hydrogen at maximum.Key words : Liquid Hydrogen(액체수소), Cryogenic Storage Vessel(극저온 저장용기), Insulation(단열), Boil-offRate(증발율)

Thermo-physical performance prediction of the KSC Ground Operation Demonstration Unit for liquid hydrogen

NASA Kennedy Space Center (KSC) researchers have been working on enhanced and modernized cryogenic liquid propellant handling techniques to reduce life cycle costs of propellant management system for the unique KSC application. The KSC Ground Operation Demonstration Unit (GODU) for liquid hydrogen (LH2) plans to demonstrate integrated refrigeration, zero-loss flexible term storage of LH2, and densified hydrogen handling techniques. The Florida Solar Energy Center (FSEC) has partnered with the KSC researchers to develop thermal performance prediction model of the GODU for LH2. The model includes integrated refrigeration cooling performance, thermal losses in the tank and distribution lines, transient system characteristics during chilling and loading, and long term steady-state propellant storage. This paper will discuss recent experimental data of the GODU for LH2 system and modeling results.