Performance optimization of a miniature Joule–Thomson cryocooler using numerical model

The performance of a miniature Joule–Thomson cryocooler depends on the effectiveness of the heat exchanger. The heat exchanger used in such cryocooler is Hampson-type recuperative heat exchanger. The design of the efficient heat exchanger is crucial for the optimum performance of the cryocooler.In the present work, the heat exchanger is numerically simulated for the steady state conditions and the results are validated against the experimental data available from the literature. The area correction factor is identified for the calculation of effective heat transfer area which takes into account the effect of helical geometry. In order to get an optimum performance of the cryocoolers, operating parameters like mass flow rate, pressure and design parameters like heat exchanger length, helical diameter of coil, fin dimensions, fin density have to be identified. The present work systematically addresses this aspect of design for miniature J–T cryocooler.     

Design and optimization of a two-stage 28 K Joule–Thomson microcooler

Micro Joule–Thomson (JT) coolers made from glass wafers have been investigated for many years at the University of Twente. After successful realization of a single-stage JT microcooler with a cooling capacity of about 10 mW at 100 K, a two-stage microcooler is being researched to attain a lower temperature of about 30 K. By maximizing the coefficient of performance (COP) of the two-stage microcooler, nitrogen is selected as the optimum working fluid for the first stage and hydrogen as that for the second stage. A dynamic finite-element model is developed for analyzing the cooler performance and to calculate the smallest cooler geometry. The optimized overall cooler dimensions are 20.4 × 85.8 × 0.72 mm for a net cooling power of 50 mW at 97 K at the first stage and 20 mW at 28 K at the second stage. The cool-down time to 28 K is calculated to be about 1.7 h with mass-flow rates of 14.0 mg/s for nitrogen and 0.94 mg/s for hydrogen at steady state.     

Heat Capacities and Joule–Thomson Coefficients of HFC Refrigerants

In this report, we have examined the behavior of heat capacities and Joule–Thomson coefficients in low- and moderate-density regions based on recent theoretical studies of the ideal-gas heat capacity and virial coefficients of R-32, R-125, R-134a, R-143a, and R-152a. The results have been compared with those derived from empirical equations of state which have been recently developed, based on a large quantity of experimental data for these refrigerants. Both results are in good agreement. Proper behaviors for these second-derivative properties justify the use of the empirical equations of state in low-temperature and low-density regions where no experimental data are available.     

Closed-Cycle Joule–Thomson Cryocooler for Resistance Thermometer Calibration down to 0.65K

A closed-cycle Joule–Thomson cryocooler for resistance thermometer calibration has been developed. It consists of a Gifford–McMahon mechanical refrigerator and a closed-cycle ³He Joule–Thomson expansion circuit that utilizes the isenthalpic expansion of ³He for cooling. The developed cryocooler can reach temperatures as low as 0.6K and can operate for months with a simple procedure. The typical cooling power of the cryocooler is 1mW at 0.65K with a molar flow rate of 160μmol ·s⁻¹ through the ³He Joule–Thomson circuit. The possible mechanical vibration level experienced by the resistance thermometers was measured with a laser vibrometer. It was confirmed that the maximum acceleration level is 0.1m· s⁻² and will not cause a problem for thermometer calibration.     

Performance of the one-stage Joule–Thomson cryocooler fed with a nitrogen–hydrocarbon mixture and built from mass-produced components made for the refrigeration industry

This paper describes the theoretical performance and working parameters of a Joule–Thomson (J-T) cryocooler that is supplied with a nitrogen–hydrocarbon mixture and works in a closed cycle. Nowadays, they are the subject of intensive research in different laboratories around the world, especially in Asia and the USA. The industrial application of this type of cooler is significantly limited by the high values of working pressure for pure nitrogen. Supplying the system with a mixture of nitrogen and hydrocarbons makes it possible to reduce the level of the working pressure down to that which is achieved by commercially available compressors produced for the refrigeration industry. A theoretical analysis of the performance of the cooler is presented, along with the experimental results for different mixtures. The described cooler is characterized by high reliability, simple construction in the low-temperature section, and relatively low manufacturing costs. The system produces about 10 W of cooling power at an approximate temperature of 90 K. The cooling power can be used to cool down high-temperature superconductor magnets, in nanotechnology, for cryomedical applications, and to liquefy small amounts of nitrogen, argon, oxygen, or methane.     

A strategy to optimize the charge amount of the mixed refrigerant for the Joule–Thomson cooler

The performance of the mixed refrigerant (MR) Joule–Thomson cooler is mainly dependent on the MR circulation composition for a given hardware. However, it is difficult to charge the MR to the desired circulation composition, due to the composition shift. In the present study, a novel strategy was proposed to solve this problem. In this strategy, the MR Joule–Thomson cooler is first charged with the initial charge amount, which is obtained by estimating the MR inventory in the cooler using the homogeneous model. Afterwards, the cooler is started and the MR circulation composition is adjusted to the corresponding optimal composition by adding the MR charge amount stepwise. Additionally, this strategy was verified by an experiment with a ternary mixture of methane, ethane and i-butane. The experimental results indicated that the MR circulation composition was able to be adjusted to the corresponding optimal circulation composition approximately within the relative deviation of ±5%.     

Thermophysical Properties of Ag and Ag–Cu Liquid Alloys at 1098K to 1573K

The surface tension and density of liquid Ag and Ag–Cu alloys were measured with the sessile drop method. The sessile drop tests were carried out at temperatures from 1098K to 1573 K, on cooling (temperature decreasing stepwise) under a protective atmosphere of high purity Ar (6N). The density of liquid Ag and Ag–Cu alloys decreases linearly with increasing temperature, and an increase in concentration of copper results in a lower density. The surface tension dependence on temperature can be described by linear equations, and the surface tension increases with increasing Cu content. The results of the measurements show good agreement with existing literature data and with thermodynamic calculations made using the Butler equation.     

Transient simulation of a miniature Joule–Thomson (J–T) cryocooler with and without the distributed J–T effect

The aim of this work is to develop a transient program for the simulation of a miniature Joule–Thomson (J–T) cryocooler to predict its cool-down characteristics. A one dimensional transient model is formulated for the fluid streams and the solid elements of the recuperative heat exchanger. Variation of physical properties due to pressure and temperature is considered. In addition to the J–T expansion at the end of the finned tube, the distributed J–T effect along its length is also considered. It is observed that the distributed J–T effect leads to additional cooling of the gas in the finned tube and that it cannot be neglected when the pressure drop along the length of the finned tube is large. The mathematical model, method of resolution and the global transient algorithm, within a modular object-oriented framework, are detailed in this paper. As a part of verification and validation of the developed model, cases available in the literature are simulated and the results are compared with the corresponding numerical and experimental data.     

Isochoric Heat Capacities of Propane + Isobutane Mixtures at Temperatures from 280 to 420 K and at Pressures to 30 MPa

The isochoric heat capacity (c_v) and pressure–volume–temperature-composition (pvTx) properties were measured for propane + isobutane mixtures in the liquid phase and in the supercritical region. The expanded uncertainty (k = 2) of temperature measurements is estimated to be ±13 mK, and that of pressure measurements is ±8 kPa. The expanded relative uncertainty for c_v is ±3.2% for the liquid phase, increasing to ±4.8% for near-critical densities. The expanded uncertainty for density is estimated to be ±0.16%. The present measurements for {xC₃H₈ +(1−x)i-C₄H₁₀} with x = 0.0, 0.498, 0.756, and 1.0, were obtained at 659 state points at temperatures from 270 to 420 K and at pressures up to 30 MPa. The experimental data were compared with a published equation of state. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

The cool-down behaviour of a miniature Joule–Thomson (J–T) cryocooler with distributed J–T effect and finite reservoir capacity

The objective of this work is to study the effect of the reservoir pressure and volume on the cool-down behaviour of a miniature Joule–Thomson (J–T) cryocooler considering the distributed J–T effect. As the supply pressure to the J–T cooler reduces in case of a reservoir with finite capacity, the volume and the initial pressure of the reservoir are crucial for the operation of the cryocooler. These parameters affect the cool down time, cooling effect and the time for which the cooling effect is obtained at the required cryogenic temperature. A one dimensional transient model is formulated for the fluid streams and the solid elements of the recuperative heat exchanger of the cryocooler. Argon gas is used as the working fluid and its physical properties are evaluated at the local conditions of temperature and pressure. Cases with different reservoir capacities and pressures are worked out to study their effect on the transient behaviour of the cryocooler.     

Thermal Conductivity of Carbon Dioxide–Methane Mixtures at Temperatures Between 300 and 425 K and at Pressures up to 12 MPa

The thermal conductivities of carbon dioxide and three mixtures of carbon dioxide and methane at six nominal temperatures between 300 and 425 K have been measured as a function of pressure up to 12 MPa. The measurements were made with a transient hot-wire apparatus. The relative uncertainty of the reported thermal conductivities at a 95% confidence level is estimated to be ±1.2%. Results of the low-density analysis of the obtained data were used to test expressions for predicting the thermal conductivity of nonpolar mixtures in a dilute-gas limit developed by Schreiber, Vesovic, and Wakeham. The scheme was found to underestimate the experimental thermal conductivity with deviations not exceeding 5%. The dependence of the thermal conductivity on density was used to test the predictive scheme for the thermal conductivity of gas mixtures under pressure suggested by Mason et al. and improved by Vesovic and Wakeham. Comparisons reveal a pronounced critical enhancement on isotherms at 300 and 325 K for mixtures with methane mole fractions of 0.25 and 0.50. For other states, comparisons of the experimental and predicted excess thermal conductivity contributions showed a smaller increase of the experimental data with deviations approaching 3% within the examined range of densities.     

Phase behavior and thermal conductivity of urea at pressures up to 1 GPa and at temperatures in the range 50–370 K

The thermal conductivity of the solid phases I and III of urea was measured at temperatures in the range 50–370 K for pressures up to 1 GPa. Phase III, previously detected only at pressures above 0.5 GPa, was observed here at low pressures ( <0.07 GPa) below about 230 K. Extrapolation of the I–III phase line indicates that phase III might be obtained at 218 K at atmospheric pressure and, consequently, that urea might exhibit two solid phases at atmospheric pressure. The temperature dependence of the thermal conductivity of both phase I and phase III could be described by the Debye model for thermal conductivity assuming phonon scattering by three phonon umklapp processes only. Despite a volume decrease at the I III transition, the thermal conductivity decreased by about 20%. Normally, thermal conductivity increases at a phase transition at which volume decreases. This rather unusual behavior of urea might be due to an increase in the nearest-neighbor distance at the I III transition.     

Density and Viscosity Measurements of Dimethoxymethane and 1,2-Dimethoxyethane from 243 K to 373 K up to 20 MPa

Measurements of the density and viscosity of dimethoxymethane and 1,2-dimethoxyethane are reported over the temperature range from 243 K to 373 K and at pressures up to 20 MPa. The measurements were performed simultaneously using a vibrating-wire instrument operated in the forced mode of oscillation. The overall uncertainties of these results are 2.0% in viscosity and 0.2% in density. The measurements were correlated with a Tait-type equation for density and a hard-sphere model for viscosity. The maximum absolute deviation and the average absolute deviation (AAD) of the density measurements from the correlation for dimethoxymethane are 0.065% and 0.012%, respectively, and for 1,2-dimethoxyethane, are 0.16% and 0.044%. With regard to viscosity, the maximum absolute deviation and the AAD of the present results from the correlation for dimethoxymethane are 1.55% and 0.40%, respectively, and for 1,2-dimethoxyethane, are 1.05% and 0.26%. Comparisons of the experimental data and measurements from the literature with values calculated by the correlations at different temperatures and pressures are presented.     

Experimental Study of the Velocity of Sound in Liquid n‐Tetradecane at Temperatures from 303.15 to 433.15 K and Pressures to 100 MPa

The velocity of sound in liquid ntetradecane has been studied experimentally in the interval of temperatures from 303 to 433 K and pressures to 100 MPa. The maximum measurement error amounts to 0.1%. Experimental data on the velocity of sound in liquid ntetradecane in the region of the state variables p = 50–100 MPa and T > 373 K have been obtained for the first time. It has been shown that the new data demonstrate the reliability of the structure–property quantitative correlation proposed earlier for the acoustic quantity in the series of nalkanes.     

Measurements of Vapor Pressures from 280 to 369 K and (p, ρ, T) Properties from 340 to 400 K at Pressures to 200 MPa for Propane

Measurements of (p, ρ, T) properties for compressed liquid propane have been obtained by means of a metal-bellows variable volumometer at temperatures from 340 to 400 K at pressures up to 200 MPa. The volume- fraction purity of the propane sample was 0.9999. The expanded uncertainties (k = 2) of temperature, pressure, and density measurements have been estimated to be less than 3 mK; 1.5 kPa ( MPa), 0.06% (7 MPa MPa), 0.1% (50 MPa MPa) , and 0.2% (p>150 MPa); and 0.11%, respectively. Four (p, ρ, T) measurements at the same temperatures and pressures as literature values have been conducted for comparisons. In addition, vapor pressures were measured at temperatures from 280 to 369 K. Furthermore, comparisons of available equations of state with the present measurements are reported.Paper presented at the 17th European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Density and Viscosity of the 1-Methylnaphthalene+2,2,4,4,6,8,8-Heptamethylnonane System from 293.15 to 353.15 K at Pressures up to 100 MPa

The dynamic viscosity of the binary mixture 1-methylnaphthalene+2,2,4,4,6,8,8-heptamethylnonane was measured in the temperature range 293.15 to 353.15K (in progressive 10K steps) at pressures of 0.1, 20, 40, 60, 80, and 100MPa. The composition of the system is described by nine molar fractions (0 to 1 in 0.125 progressive steps). The density was measured at pressures from 0.1 to 60MPa in progressive 5 MPa steps. The measurements of are used to determine the excess viscosity ^E and the excess activation energy of flow G ^E as a function of pressure, temperature, and composition. Some models have been used to represent the viscosity of this binary mixture.     

Thermophysical Properties of Binary Mixtures of Methanol with Chlorobenzene and Bromobenzene from 293 K to 313 K

Densities and viscosities have been measured for the binary mixtures of methanol with chlorobenzene and with bromobenzene from 293 K to 313 K over the complete composition range. Densities were used to compute the excess molar volume ( , for these binary systems. The results have been discussed in terms of molecular interactions. Furthermore, viscosity results were compared with a corresponding-states model. The average absolute deviation was found to be 1.9 %.     

The thermal conductivity and emissivity of DE-24 graphite at temperatures of 2300–3000 K

The results of an investigation of the heat-transfer and radiation properties of DE-24 isostatic graphite under steady conditions in the 2300–3000 K temperature range are presented. The thermal conductivity and emission characteristics of the material were determined by the two-cylinder method. The sample was heated by passing a constant electric current through it.     

Transport properties of nitrogen,oxygen, carbon dioxide,and air at low densities and temperatures from 50 to 3000°K

We calculate the viscosity and thermal conductivity, the Prandtl number, and the Eucken factor for a (12-7, ) pair model potential. The calculated values agree with correlated experimental data within the limits of error of the measurements.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 1, pp. 77–81, July, 1982.