S.L. Bapat

Experimental investigations of multilayer insulation

Abstract Multilayer insulation (MLI), using alternate layers of shield and spacer in high vacuum, is the most effective cryogenic insulation developed to date. Due to unpredictable changes in parameters such as winding pressure, uniform contact pressure and interstitial pressure, accurate theoretical prediction of MLI performance is very difficult. Thus, an experimental investigation has been carried out on a few indigenous MLI materials. The investigations are centred on the influence of the number of layers and layer density, with the cold boundary at the same temperature as liquid nitrogen. The experiments have been carried out using a cylindrical vessel with guard vessels on the top and bottom flat surfaces. The interstitial pressure, which depends on conditions pertaining to specific parameters, such as outgassing rate of materials, cryopumping speed and time for evacuation, has also been measured. The results are compared with those obtained from a theoretical analysis carried out for the same combination of shield and spacer materials.

Performance prediction of multilayer insulation

Abstract Multilayer insulation (MLI), using alternate layers of shield and spacer material, is the most effective insulation developed so far for cryogenic applications. The effective thermal conductivity of MLI is a function of various parameters, such as shield and spacer materials, number of layers and layer density, contact pressure, vacuum maintained and interstitial pressure. This paper presents an analysis involving fabric spacer material. The model considers the heat transfer by various modes such as solid conduction, conduction through contact surfaces, radiation through the exposed area between adjacent shields and radiation through the fibrous spacer. Gas conduction has also been considered, taking into account the estimated interstitial pressure. Results are presented for the radiation mode alone, as well as for radiation, conduction and gas conduction considered collectively.

Solubility characteristics of R22-DMF refrigerant-absorbent combination

This Paper presents the solubility data of Refrigerant 22 (R22) - dimethyl formamide (DMF) measured over a wide range of temperature (−25°C to 120°C) and over a complete range of composition. The mixtures show large negative deviation from Raoults law and there is large difference in normal boiling point its components. Thus this combination is suited for vapour absorption refrigerant systems. The In P-1/T diagram and temperature-composition diagram for the mixtures of R22-DMF have also been plotted for use in the analysis of vapour absorption systems.

Cyclic simulation of Stirling cryocoolers

Abstract Analysis of Stirling cryocoolers has been an important research topic over the last 30 years. The actual and the ideal Stirling cycles are distinctly different and performance prediction of the actual cycle has been a subject of interest to scientists. In this paper cyclic simulation of the actual Stirling cycle cryocooler has been performed on the microcomputer. The results are compared with the actual measured results and they show better agreement than for exisiting analyses. The analysis has been further extended to fit different machines.

Dynamic and thermodynamic analysis of doubly motorized miniature Stirling cryocooler using double coil linear motors

Abstract Small capacity Stirling cryocoolers (∼ 1 Wat 80 K) for space-borne applications generally use linear electromagnetic drives (also called linear motors) for long, maintenance-free life and high reliability. Usually, such motors employ a single moving coil. Recent trends, however, favour a double coil configuration for higher motor efficiency. Earlier, in free piston, free displacer (FPFD) Stirling cryocoolers the displacer was driven by the pressure pulses from the compressor. The latest units have a split configuration, with the displacer also being driven by a small capacity linear motor, giving better control over its mean position, stroke and phase lead over the compressor piston. After establishing a mathematical model for the working of a double coil linear motor, this paper analyses the link between thermodynamic performance and the dynamics of the two reciprocating components in a doubly motorized FPFD Stirling cryocooler, thus presenting a design tool for such cryocoolers.

Optimization of design parameters of Stirling cycle machine

Abstract The performance of a Stirling cryocooler is governed by six principal design parameters, namely speed, pressure limits, the temperature of the working fluid in the compression and expansion spaces, the ratio of the compression and expansion space swept volumes, the ratio of total dead volume and expansion space swept volume and the phase angle between the volume variations of the expansion space and compression space. For optimizing the performance of a cryocooler, its cyclic analysis should be considered along with a loss analysis. The present paper attempts to find out the optimum combination of these design parameters for machines PLN-106 and PLN-108s, for which design data are known. The objective functions of optimization can be based on COP maximization or any design constraints such as the total volume or space taken up by the machine, maximum pressure generation, etc. This paper first applies the optimization routine to the Philips machine and identifies the optimization function for which the machines are designed. The routine is then further extended to develop a generalized procedure for designing a cryocooler of any capacity. The procedure is applied to determine the different dimensions of machines with a liquid nitrogen producing capacity of 8 dm 3 h −1 . The design developed using this procedure is then compared with the actual existing design.

Theoretical analysis and performance investigation of Stirling cycle regenerators

Abstract This paper attempts to simulate the actual conditions existing in the regenerator of Stirling cycle cryocoolers. The mathematical model presented is solved by finite difference techniques and the results are then compared with those obtained by two other models reported in the literature. The analysis is further extended to study the performance behaviour of the regenerators under different operating conditions.

Investigations on Performance of Stirling Engine Regenerator Matrix

Stirling engine technology has attracted attention due to recent environmental and energy problems. The regenerator is the main component in high efficiency Stirling engines. A suitable regenerator must be designed for each Stirling machine to provide high performance. The aim of the present work is to find a feasible number of screens in regenerator by taking into account the pressure drop, dead volume, the thermal penetration depth and geometry of regenerator. The second order cyclic analysis with realistic assumptions is carried out for a single cylinder, beta Stirling engine with rhombic drive for predecided operating conditions, such as pressure of 30 bar, hot side temperature of 750 K, speed of 1440 rpm and hydrogen as the working fluid. It is intended to design and develop the Stirling engine with capacity ≥ 1.5 kWe and the efficiency of drive mechanism and alternator is assumed as 85% each. Miyabe’s and Martini’s approaches are used to simulate regenerator performance considering non-sinusoidal motion of displacer and piston. The results reveal that the flow loss increases remarkably to attain higher value of regenerator effectiveness. However, increase in the speed results into an increase in the mass flow rate of the working fluid. It is observed that regenerator effectiveness decreases only marginally over the range of speeds considered. It is also ensured for selected regenerator screen that the thermal penetration depth (239 μm) should be greater than wire radius of mesh (20.5 μm). For present set of operating and geometrical parameters, length of regenerator is fixed as 22 mm which gives regenerator effectiveness as 0.965. Further, the practice to fill more screens than the designed number of screens in the regenerator, while assembling is not advantageous. It increases pressure drop which results in reduced power output. These are some of the important conclusions.Copyright

Numerical investigations on the Dish–Stirling engine system

ABSTRACT The Stirling engine is an environmentally friendly external combustion heat engine and reduces the complexities of the combustion process, and indirectly helps in reduction of CO2 emission. Modelling based on cyclic analysis is performed for a Beta configuration Stirling engine of 1.5 kWe capacity using a rhombic drive for the solar-dish-supported Stirling engine. The analysis helps in estimating the overall efficiency of the system using the experimental correlation of the solar concentrator ARUN160 at the engine operating temperature. The analysis shows that the system will have overall efficiency around 25% in the range of 750–1050 K at the expansion space. The degradation of performance compared to that at an operating temperature of 1025 K is only marginal and makes 750 K a more preferred temperature. The present study evaluates a range of possible design goals and provides suitable alternatives and thus provides a clear understanding of the system design considerations.

Development of a computer model for three-stage, split type, free displacer Stirling cryocooler

Abstract Development of a model for a multiple stage Stirling cycle cryocooler has been a topic of research for the last four to five years. The model can help to optimize the design parameters of the machine and to predict the performance of the machine under different operating conditions. In this paper, the authors have given a computer model for a three-stage, free displacer, split type Stirling cryocooler. The model so developed has been validated by experimental results.