Theoretical and experimental investigation of pressure drop and refrigeration effect in pulse tube cryocooler

The design of a highly efficient pulse tube cryocooler (PTC) is a subject of recent research activities. The PTC performance depends on various operating and design parameters. Regenerator is one of the very important components of the PTC which decides the low temperature that the PTC can attain. Efficiency of regenerator should be high enough, 96% or above, in order to reach very low temperature while the pressure drop in the regenerator is one of the parameters which needs to be analysed in detail. In the present work, theoretical and experimental investigations are carried out on two different single stage U type PTC. The volumes of regenerators and pulse tubes, in both the cases are kept same while the length to diameter (L/D) ratios of regenerators are changed. Investigations are carried out on these PTC with respect to pressure drop in the regenerator and net refrigeration effect obtained from the PTC at 80K. The pressure drop increases from 0.29bar to 2.07bar with an increase in L/D ratio from 1.93 to 9, resulting in decrease in refrigeration effect from 6.1W to 1.7W at 80K with 300W input power. The study is further extended to understand the effect of coarse size stainless steel mesh size in the regenerator. Coarse size meshes filled up to 60% of regenerator length improved the refrigeration effect from 1.7W to 2.8W; however, further filling degrades the performance of the PTC. The experimental results are compared with theoretical results obtained by Sage software and Isothermal model

Theoretical and experimental investigations on Stirling-type pulse tube cryocoolers with U-type configuration to achieve temperature below 20K

To achieve lower temperatures is a subject of recent research and development activities in the field of pulse tube cryocoolers. To reach a temperature of 20K, multi-staging is necessary in Stirling-type pulse tube cryocoolers. In the present work, Sage software is used to design a three-stage gas-coupled as well as thermal-coupled pulse tube cryocooler. A single-stage and a two-stage pulse tube cryocoolers are developed, tested and are coupled by a thermal link to build up a three-stage thermal-coupled pulse tube cryocooler. The lowest temperature of 19.61K is obtained with a cooling capacity of 220mW at 30K at the third stage operating at 17bar charge pressure and 68Hz frequency. The phase-shifting mechanism used is a double inlet valve at the third stage while the inertance tube is used for the other stages

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. (C) 2014 Elsevier Ltd. All rights reserved

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. (C) 2015 Elsevier Ltd. All rights reserved

SecureCSearch: Secure Searching in PDF over Untrusted Cloud Servers

© 2019 IEEE. The usage of cloud for data storage has become ubiquitous. To prevent data leakage and hacks, it is common to encrypt the data (e.g. PDF files) before sending it to a cloud. However, this limits the search for specific files containing certain keywords over an encrypted cloud data. The traditional method is to take down all files from a cloud, store them locally, decrypt and then search over them, defeating the purpose of using a cloud. In this paper, we propose a method, called SecureCSearch, to perform keyword search operations on the encrypted PDF files over cloud in an efficient manner. The proposed method makes use of Shamir's Secret Sharing scheme in a novel way to create encrypted shares of the PDF file and the keyword to search. We show that the proposed method maintains the security of the data and incurs minimal computation cost

Experimental investigation on temperature profile and pressure drop in two-phase heat exchanger for mixed refrigerant Joule-Thomson cryocooler

The heat exchanger of a mixed refrigerant Joule-Thomson (MR J-T) cryocooler forms a very important component of the cycle. The working fluid in such a heat exchanger consists of a mixture of gases which undergo condensation and boiling heat transfer simultaneously. The design of these heat exchangers, therefore, is crucial; however, heat transfer data related to such heat exchangers is not available. In the present work, temperature distributions of hot and cold fluid along the length of the helical coil heat exchanger are measured experimentally. The effect of mixture compositions on the temperature distributions in the heat exchanger is studied. The performance of the heat exchanger is analyzed in terms of overall heat transfer coefficient and the heat transfer rate. It was found that the higher values of overall heat transfer coefficient leads to decreasing cool-down time for MR J-T cryocooler. However, to achieve lower refrigeration temperatures, the greater part of the heat exchanger should experience two-phase flow and the temperature profiles should be linear. Pressure drop studies reveal that the total pressure drop for the evaporating cold stream is crucial, which strongly depends on the mixture composition and operating conditions. (C) 2014 Elsevier Ltd. All rights reserved

Experimental investigation on mixed refrigerant Joule-Thomson cryocooler with flammable and non-flammable refrigerant mixtures

The mixed refrigerant Joule-Thomson (MR J-T) cryocoolers have a wide application area covering the temperature range from 80 K to 200 K. The significant advantages of the system are simplicity of its design and working reliability with high level performance. The present paper discusses the experimental results of MR J-T cooler with different flammable and non-flammable mixture compositions. The work highlights the use of pressure-enthalpy and temperature-enthalpy diagrams for these mixtures to support the experimental results. A record lowest temperature of 65 K and a cooling capacity of 6 W at 80 K are obtained for a single stage MR J-T system starting at 300 K. Further, using a mixture of minimum flammable refrigerants, temperatures below 100 K is achieved

Optimization of design parameters of 2-stage, split type, free displacer stirling cryocooler

A computer model for multistage Stirling cryocoolers has been developed. Following the validation by experimental results the model has been applied to optimise the design parameters of a two stage, split type, free displacer plastic Stirling cryocooler using gap regeneration. This paper presents the determination of optimum design parameters considering two important design options. The paper also shows the effect of variations of design parameters on the optimisation function. The design parameters are determined for compression volume of 50 cc and for stable temperatures at 1(st) and 2(nd) stage of expansion of 160 K and 77 K respectively

Numerical investigation of transient behaviour of the recuperative heat exchanger in a MR J-T cryocooler using different heat transfer correlations

In J-T cryocoolers operating with mixed refrigerants (nitrogen-hydrocarbons), the recuperative heat exchange takes place under two-phase conditions. Simultaneous boiling of the low pressure stream and condensation of the high pressure stream results in higher heat transfer coefficients. The mixture composition, operating conditions and the heat exchanger design are crucial for obtaining the required cryogenic temperature. In this work, a one-dimensional transient algorithm is developed for the simulation of the two-phase heat transfer in the recuperative heat exchanger of a mixed refrigerant J-T cryocooler. Modified correlation is used for flow boiling of the high pressure fluid while different condensation correlations are employed with and without the correction for the low pressure fluid. Simulations are carried out for different mixture compositions and numerical predictions are compared with the experimental data. The overall heat transfer is predicted reasonably well and the qualitative trends of the temperature profiles are also captured by the developed numerical model. (C) 2016 Elsevier Ltd. All rights reserved