S. Jacob

Investigations into the thermal performance of multilayer insulation (300- 77 K) Part 1: Calorimetric studies

The thermal performance of multilayer insulation (MLI), consisting of double aluminized Mylar radiation shields and nylon net thermal spacers, was evaluated using a double guarded cylindrical calorimeter and a tank calorimeter over the temperature range 300-77 K. The degradation in the effective thermal conductivity of MLI was evaluated to be 1.68 using the calorimeters. The optimum number of layers for the MLI was 40 to 50 at a layer density of 25 layers

Investigations into the thermal performance of multilayer insulation (300-77 K) Part 2: Thermal analysis

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Experimental studies on cool-down and mass flow characteristics of a demountable liquid nitrogen transfer line

. The temperature profile and heat flux through the MLI were obtained as a function of vacuum level for different numbers of insulation layers. The temperature profile of the MLI indicates the relative predominance of the conduction and radiation components of heat transfer through the insulation. It is observed that for a given number of layers, the temperature of a specific layer between the cold and warm boundaries decreases with an increase in chamber pressure and vice versa.

DEVELOPMENT OF CRYOGENIC LOOP HEAT PIPE

Simple analytical methods have been employed for heat transfer analysis of experimental data obtained through calorimetric investigations on multilayer insulation (MLI). Sectional heat transfer analysis has shown that the effective thermal conductivity of the MLI varies from section to section of the insulation structure and it has a peak which lies between the middle and warm boundary regions of the MLI. This could be attributed to a peak in residual gas conduction in this region. The theoretical estimation of heat flux through MLI, using a simple analytical model, is also discussed in this paper. This model takes into consideration the non-linear temperature profile of the insulation. The computed heat flux using this model gives a lower (2 to 4 times) value in comparison with the heat flux estimated from calorimetric measurements. A refined model has been suggested which includes the residual gas conduction also in MLI.

Development of pilot plant for cryogrinding of spices : A method for quality improvement

A demountable transfer line has been designed and fabricated to carry out experimental studies on cool-down time and mass flow characteristics of a liquid nitrogen transfer line held in a horizontal position. The effects of varying degrees of vacuum in the interspace on the cool-down time and mass flow rates have been evaluated. It has been found that during the initial phase of cool-down, lines with coarse vacuum perform better than high-vacuum-insulated lines. The performances of multilayer-insulated lines and lines with interspace medium of carbon dioxide have been evaluated as well as that of bare line exposed to an ambient environment. An experimental correlation has been developed to predict the cool-down time as a function of vacuum level and supply dewar pressure.

Development of moving magnet type linear motor for dual piston compressor for pulse tube cryocooler

The design of a cryogenic loop heat pipe (CLHP) is presented in the paper. As the wick is required only in the evaporator section, very small pore size wicks can be used in applications with high thermal transport requirements and/or where the heat must be transported over a long distance against gravity. A FORTRAN program to solve the mathematical model and to determine the parameters for various boundary conditions has been developed. The CLHP is designed to transfer 5W heat at 70K using nitrogen or oxygen as working fluid. It will be a self priming type device which can operate against gravity with evaporator above the condenser as well as under microgravity condition. A G-M type single stage double inlet pulse tube refrigerator is coupled to the CLHP to test its performance. The mathematical model, design, fabrication integration of the heat pipe with the pulse tube system and testing with stainless steel wick at the evaporator will be described in the paper.

Development of Discrete Array Type Liquid Level Indicator for Cryogenic Fluids

The development of a pilot plant for cryogrinding of spices is discussed in this paper. The unit has an indigenously designed and developed cryogenic screw cooler for optimal utilization of liquid nitrogen along with a digital signal processing system (DSP) for liquid nitrogen spray and automatic temperature control. In the first part, the advantages of cryogrinding over conventional grinding are described. The latter part discusses the details of the pilot plant, along with preliminary experimental studies using pepper as the sample. Experimental results clearly indicate the quality improvement of the cryoground spice in terms of increased volatile oil contents and flavour components.

Evaluation of mechanical losses in a linear motor pressure wave generator

This paper describes the design, fabrication and testing of a moving magnet type linear motor of dual piston configuration for a pulse tube cryocooler for ground applications. Eight radially magnetized segmented magnets were used to form one set of a magnet ring. Four magnet rings of such type were constructed, in which one pair of rings has north-pole on its outer diameter and south-pole on inner diameter, while the other pair is its complementary. The magnets were mounted with opposite poles together on the magnet holder with an axial moving shaft having a piston mounted on both ends of the shaft. The shaft movement was restricted to the axial direction by using C-clamp type flexures, mounted on both sides of the shaft. The force requirement for driving the compressor was calculated based on which the electrical circuit of motor is designed by proper selection of wire gauge and Ampere-turns. The flexure spring force estimation was done through simulation using ANSYS 11.0 and was verified experimentally; while the magnet spring force was determined experimentally. The motor with mounted piston was tested using a variable voltage and variable frequency power supply capable of driving 140 watts of load.

LOX SEPARATION STUDIES USING CRYOGENIC VORTEX TUBE

In large cryogenic storage systems, the level of cryogenic fluids such as liquid nitrogen, liquid oxygen, etc., are usually monitored using capacitance type indicators. They are both heavy and inaccurate. We report here the development of discrete array type liquid level indicators. A special class of analog multiplexer IC, which operates at cryogenic temperatures, is used in the unit. Operating it at liquid helium temperature and switching the channels one million times without any failure tested the reliable operation of the IC at cryogenic temperature. The system is capable of detecting the liquid level of any cryogen. Only a software tuning is required for operation with each fluid. The developed unit has an active length of 400 mm and six regions with sensitivities ranging from ± 5 mm to ± 0.5 mm. Redundancy of level measurement is achieved through software. The interaction of the system that has 138 sensors to ambient electronics is through only 15 wires. The integrated unit has been cycled several times between room temperature and 77 K.

Experimental Studies of Convection in a Single Stage Pulse Tube Refrigerator

A moving magnet linear motor compressor or pressure wave generator (PWG) of 2 cc swept volume with dual opposed piston configuration has been developed to operate miniature pulse tube coolers. Prelimnary experiments yielded only a no-load cold end temperature of 180 K. Auxiliary tests and the interpretation of detailed modeling of a PWG suggest that much of the PV power has been lost in the form of blow-by at piston seals due to large and non-optimum clearance seal gap between piston and cylinder. The results of experimental parameters simulated using Sage provide the optimum seal gap value for maximizing the delivered PV power.