Thermodynamic properties of liquid ³3He-⁴4He mixtures between 0.15 K and 1.8 K

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 132-135).Thermodynamic property relations for liquid ³3He-⁴4He mixtures between temperatures of 0.15 K and 1.8 K are determined. The relations are valid over the entire concentration range. Thermodynamic properties are first calculated at saturated pressure (which is more or less equal to zero pressure) in the two-phase region, and then extended to the single-phase He-II (up to 1.8 K) and He-I (up to 1.5 K) regions. The calculations at saturated pressure are based on available specific heat data and previously determined sub- 0.15 K properties. The property relations are then extended to higher pressures (up to 10 bar) between 0.15 K and 1.5 K, using available molar volume data. The results are largely in good agreement with some other ³3He-⁴4He mixture property data, though the scarcity of experimental data in large parts of the region of interest precludes a more thorough comparison. Applications of the derived properties to components of sub-Kelvin refrigerators, specifically He-II mixture heat exchangers and He-II mixture throttles, are also discussed.by Gunaranjan Chaudhry.Ph.D

Modeling of a floating piston expander employed in a 10 K cryocooler

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references (p. 81).A single stage of a 3-stage Collins-type cryocooler designed to provide I W of cooling at 10 K was constructed and tested. A single stage of the cryocooler consists of a compressor, a counter-flow heat exchanger, and an expander to expand the working fluid. The work of the expanding cold gas is transmitted up a floating piston and is dissipated by gas flows in and out of a warm volume. Flow through the cold volume is controlled by smart electromagnetic valves. Models were developed to describe the thermodynamic processes that make up the expander cycle. In the first iteration, models were developed to determine the equilibrium states at various points in the cycle by assuming the thermodynamic processes that made up the expander cycle to be quasi-static. These models were used to determine appropriate values of parameters such as the cut-off volume, the recompression volume, and warm end reservoir pressures for expander operation. Experiments were done to determine the efficiency of the floating-piston expander. Tests were also done to determine the characteristics of the heat exchanger and compare them with the design characteristics. Finally, the stage was run as a refrigerator with zero heat-load. It was observed that the quasi-static models did not adequately describe the performance of the expander as most of the processes did not go to equilibrium.(cont.) Therefore, these models were improved by incorporating the dynamics of the piston motion, the fluid flow through the warm and cold volumes, and the fluid flow through the high-pressure passages of the heat exchanger.by Gunaranjan Chaudhry.S.M

Analysis of oxy-fuel combustion power cycle utilizing a pressurized coal combustor

Growing concerns over greenhouse gas emissions have driven extensive research into new power generation cycles that enable carbon dioxide capture and sequestration. In this regard, oxy-fuel combustion is a promising new technology in which fuels are burned in an environment of oxygen and recycled combustion gases. In this paper, an oxy-fuel combustion power cycle that utilizes a pressurized coal combustor is analyzed. We show that this approach recovers more thermal energy from the flue gases because the elevated flue gas pressure raises the dew point and the available latent enthalpy in the flue gases. The high-pressure water-condensing flue gas thermal energy recovery system reduces steam bleeding which is typically used in conventional steam cycles and enables the cycle to achieve higher efficiency. The pressurized combustion process provides the purification and compression unit with a concentrated carbon dioxide stream. For the purpose of our analysis, a flue gas purification and compression process including de-SO[subscript x], de-NO[subscript x], and low temperature flash unit is examined. We compare a case in which the combustor operates at 1.1 bars with a base case in which the combustor operates at 10 bars. Results show nearly 3% point increase in the net efficiency for the latter case.Aspen Technology, Inc.Thermoflow Inc

Analysis of a Mixing Chamber Developed for Use in a Gravity-Independent Dilution Refrigerator

International audienceWe present thermodynamic models for a mixing chamber used in dilution refrigerators designed for gravity-independent operation. We develop models for calculating the cooling power of such a dilution refrigerator as a function of the load temperature. We compare our models with experimental results from the open-cycle Planck dilution refrigerator and the currently-under-development closed-cycle variant of the Planck refrigerator. In doing so, we resolve what was believed to be a discrepancy in the cooling power of the Planck system

A closed-cycle dilution refrigerator for space applications

International audienceWe discuss the development of a gravity-insensitive dilution refrigerator adapted from the open-cycle refrigerator used for the Planck mission. Since the 3He and 4He components are circulated (the 4He by a fountain-effect pump operating at about 2 K, the 3He by a compressor at room temperature) rather than ejected into space, the lifetime of a closed-cycle refrigerator is not limited by the quantity of 3He and 4He available. In this work, we concentrate on the design and performance of the cold end (counterflow heat exchanger and mixing chamber) of the refrigerator. We discuss the sizing of the heat exchanger and present cooling power measurements. We detail the working of the fountain pump. We also briefly touch upon some practical issues including the choice of a 3He compressor and the pre-cooling requirements for the dilution refrigerator

Development of the cold end of a gravity-insensitive closed cycle dilution refrigerator

International audienceThis work presents the experimental results and analytical modelling of the cold end of a closed-cycle gravity-insensitive dilution refrigerator adapted from the open-cycle dilution refrigerator used for the Planck mission. The refrigerator is designed to provide 1 μW of cooling at a temperature of 50 mK. The cold end of the refrigerator comprises a counterflow heat exchanger (which pre-cools the 3He and 4He components down from a temperature of about 1 K to below 100 mK), a mixing chamber and a load heat exchanger at about 50 mK. We discuss the counterflow heat exchanger designs that were considered and present experimental results. The best configuration shows a cooling power of 1 μW at 45 mK. We develop analytical models for the heat exchangers and the mixing chamber and compare them with experimental data

ScienceDirect IUTAM Symposium on Multiphase flows with phase change: challenges and opportunities, Modelling of Flashing in Capillary Tubes using Homogeneous Equilibrium Approach

Abstract A model for the two phase flow of refrigerant through a capillary tube has been developed. This model assumes mechanical & thermal equilibrium between the phases. The flashing phenomenon occurs when a liquid is out of thermodynamic equilibrium such as sudden changes in the pressure or temperature of a liquid system. The rate of flashing is modeled as proportional to the deviation from the equilibrium vapor pressure. Results of this model compared with experimental data for flow through capillary tube. Also this model is used to calculate the critical flow rate for various inlet conditions. The predictions are compared with experimental data. The concordance between the predictions and the experiments has been discussed in this paper