M.D. Atrey

Experimental investigation on Mixed Refrigerant Joule Thomson (MR J-T) cryocooler

Mixed Refrigerant Joule Thomson (MR J-T) cryocoolers have obvious advantages, such as low cost, high reliability, higher cooling effect at 80 K, low vibrations and simplicity in design layout. As a result of this, their use for different applications has become a major threat to conventional cryocoolers such as Stirling coolers. The performance of the MR J-T cryocooler, in terms of cooling power at low temperatures, depends significantly on the components of the gas mixture and their concentration. An experimental set up has been developed in our laboratory to analyse various gas mixtures. An efficient Hampson type counter flow heat exchanger has been fabricated and tested in the set up. The present paper gives experimental results for various gas mixtures so as to get maximum cooling effect for a given temperature. The paper also presents the effect of working pressure of the optimized gas mixture on the performance of the cooler.

FIBER BRAGG GRATING SENSORS FOR MEASURING TEMPERATURE AND STRAIN SIMULTANEOUSLY AT CRYOGENIC TEMPERATURE

Feasibility of employing Fiber Bragg Gratings (FBG) for measuring thermodynamic parameters of superconducting (SC) coils has been studied. The distribution of mechanical stress and temperature inside the coil are important for an optimized magnet design. Standard sensors with electrical connections like resistance thermometer and strain gauges cannot be placed inside the coil. So it is impossible to access local stress and temperature data. The superimposed dual wavelength bi—metallic recoated Bragg gratings, fabricated in one fiber at same location are better suited for this purpose. Coil temperature and stress will vary the gratings periods which can be read out with a tuneable laser. The spectral position of the reflections may be correlated with the spatial position of the gratings. The shift of the gratings maximum reflection indicates the change of the gratings period. This, in turn, measures temperature and stress. The simultaneous temperature and strain measurement response of an aluminium—indiu...

In-Field Performance Evaluation of a Large Size Multistream Plate Fin Heat Exchanger Installed in a Helium Liquefier

Abstract A large size multistream plate fin heat exchanger (MSPFHE) is developed for an in-house-developed modified Claude cycle-based helium liquefier. The development of this heat exchanger, including thermal sizing and mechanical design, is described in this article. A pressure drop analysis is also carried out to compute the pressure drops across various components of the MSPFHE. The MSPFHE, after fabrication, pressure and leak testing, is integrated with the rest of the process equipment and piping of the developed helium liquefier cold box. Experiments are conducted to evaluate the performance of the developed MSPFHE under different modes of operations of the helium liquefier. Experimental results are compared with the computed predictions based on the two-dimensional numerical model as well as the commercial software Aspen MUSETM and reported in this article. Good agreement between the computed and measured performances is observed during the field testing of the MSPFHE.

Modification in charging composition in order to arrive at desired circulation composition in the context of sorption compressor based J-T cooler

A sorption compressor based cooler works on thermal swing adsorption process followed by a Linde-Hampson cycle for producing the cooling effect. A mixed gas refrigerant provides low temperatures in the cryogenic range with pressure around 20 bar as compared to a high pressure of about 100 bar with a single pure working fluid such as Nitrogen. With gas mixture, the through put of individual gas in the sorption compressor varies from gas to gas. This is attributed to the different quantities of gases retained by the adsorbent at the end of desorption process. Therefore, in adsorption compressor, the mixture in circulation varies in composition when compared to the gas mixture composition while charging in the system. Necessary corrections in the mixture composition to be charged need to be incorporated so that the mixture in circulation has the desired composition. A mathematical model, based on the adsorption data for individual component gas is used to arrive at the correct composition of the gas mixture in circulation.

Numerical simulation of tubes-in-tube heat exchanger in a mixed refrigerant Joule–Thomson cryocooler

Mixed refrigerant Joule-Thomson (MRJT) cryocoolers can produce cryogenic temperatures with high efficiency and low operating pressures. As compared to the high system pressures of around 150–200 bar with nitrogen, the operational pressures with non-azeotropic mixtures (e.g., nitrogen-hydrocarbons) come down to 10–25 bar. With mixtures, the heat transfer in the recuperative heat exchanger takes place in the two-phase region. The simultaneous boiling and condensation of the cold and hot gas streams lead to higher heat transfer coefficients as compared to single phase heat exchange. The two-phase heat transfer in the recuperative heat exchanger drastically affects the performance of a MRJT cryocooler. In this work, a previously reported numerical model for a simple tube-in-tube heat exchanger is extended to a multi tubes-in-tube heat exchanger with a transient formulation. Additionally, the J-T expansion process is also considered to simulate the cooling process of the heat exchanger from ambient temperature conditions. A tubes-in-tube heat exchanger offers more heat transfer area per unit volume resulting in a compact design. Also, the division of flow in multiple tubes reduces the pressure drop in the heat exchanger. Simulations with different mixtures of nitrogen-hydrocarbons are carried out and the numerical results are compared with the experimental data.

Investigation of transient chill down phenomena in tubes using liquid nitrogen

Chill down of cryogenic transfer lines is a crucial part of cryogenic propulsion as chill down ensures transfer of single phase fluid to the storage tanks of cryogenic engines. It also ensures single phase liquid flow at the start of the engine. Chill down time depends on several parameters such as length of the pipe, pipe diameter, orientation, mass flux etc. To understand the effect of these parameters, experiments are carried out in a set up designed and fabricated at Indian Institute of Technology Bombay using tubes of two different diameters. Experiments are conducted at different inlet pressures and mass flow rate values to understand their effect. Two different pipe sizes are taken to study the effect of variation in diameter on chill down time and quantity of cryogen required. Different orientations are taken to understand their effect on the chill down time, heat transfer coefficient and critical heat flux for the same inlet pressure and mass flux. Pipe inner wall temperature, heat transfer coefficient for different boiling regimes and critical heat flux are calculated based on measured outer surface temperature history for each case. A one dimensional energy conservation equation is solved for transient chill down process considering constant mass flux and inlet pressure to predict the chill down time. Temperature variation during chill down obtained from the numerical simulations are compared with the measured temperature history.

Optimum design of liquid helium dewar with vapour cooled shields

The work presented aims to develop a numerical model which can be used to design a vapour shielded liquid helium (LHe) Dewar without LN2 Shield. This model can compute the position of shields and the Net Evaporation Rate (NER) based on the capacity and number of vapour cooled shields (VCS) given by the user. The effect of neck dimensions on the heat in-leak is also given by this model. Heat in-leak design calculations are done using Finite Difference Method (FDM). A computer program is developed using MATLAB®. The results can be obtained either in graphical form or in terms of minimum NER and optimal positions of the VCS. The modelis validated against available results in literature and additional results are discussed in detail. This program can be extended to estimate the neck dimensions and position of shields for any given capacity, number of shields and allowable boil-off rate.

Development of experimental test facilities for validation of multistream plate finheat exchanger design codes

Compact plate-fin heat exchangers (PFHE) having very high effectiveness (>0.95) are key equipment of modern helium liquefaction/refrigeration systems. Effectiveness (e) of these heat exchangers strongly influences the overall system performance. Apart from basic fluid film resistances, various secondary parameters such as axial heat conduction (AHC) through the heat exchanger metal matrix, parasitic heat in-leak from surroundings, variation in fluid/metal properties and flow mal-distribution etc need to be considered while sizing/rating such high effectiveness PFHE. Need of multiple streams in a single heat exchanger further complicates thermal designs. In-house codes are developed at BARC/IIT Bombay for rating of such high effectiveness PFHEs. For experimental performance evaluation of PFHEs, their characterization at various operating conditions and validation of in-house developed numerical codes, a dedicated closed loop experimental test facility is developed. Development of the dedicated closed loop experimental test facility is presented in this paper along with initial test results.

Prediction of temperature distribution in heat exchanger for mixed refrigerant Joule–Thomson cryocooler

The overall performance of a mixed refrigerant Joule-Thomson (MR J–T) cryocooler depends on the efficiency of the heat exchanger used to cool the refrigerant prior to J-T expansion. The multi-component refrigerant mixture gets condensed and evaporated simultaneously at different pressures in the recuperative heat exchanger which is responsible to increase its performance. However, at present, the design of such heat exchanger is difficult due to lack of experimental heat transfer data.

Effects of axial heat conduction, property variation and parasitic heat in-leakon performance of compact plate-fin heat exchangers

Modern helium liquefaction/refrigeration systems employ compact plate-fin heat exchangers having very high effectiveness (>0.95). Performance of such systems is a strong function of effectiveness of heat exchangers used. The calculation of heat exchanger effectiveness in such cases needs considerations of several secondary parameters apart from basic fluid film resistance. In the present paper, the combined effects of secondary parameters like axial heat conduction through heat exchanger matrix and parasitic heat in-leak from the surroundings has been studied numerically. Large temperature changes in cryogenic heat exchangers may result in correspondingly larger changes in fluid properties and metal matrix conductivity, which shall be taken care of during numerical calculations. Numerical model developed in the present work is based on the one given in the literature. Numerical technique to solve the system of equations is implemented in MATLAB®. Real properties of helium at each node are evaluated using HEPAK®, which is linked to the developed code. Using this model, performance of heat exchangers is studied at four different temperature levels of (300-77)K, (77–20)K, (20-8)K and (10-5.1)K. The results highlight the effects of each of the above secondary parameters.