Three-dimensional numerical simulation of the basic pulse tube refrigerator

A three-dimensional physical and numerical model of the basic pulse tube refrigerator (PTR) was developed. The compressible and oscillating fluid flow and heat transfer phenomenon in the pulse tube were numerically investigated using a self-developed code. Some cross-section average parameter variations such as velocity, temperature and pressure wave during one cycle were revealed. The variations of velocity and temperature distributions in the pulse tube were also analyzed in detail for further understanding of the working process and refrigeration mechanism of PTRs.     

Three-dimensional numerical simulation of the basic pulse tube refrigerator

A three-dimensional physical and numerical model of the basic pulse tube refrigerator (PTR) was developed. The compressible and oscillating fluid flow and heat transfer phenomenon in the pulse tube were numerically investigated using a self-developed code. Some cross-section average parameter variations such as velocity, temperature and pressure wave during one cycle were revealed. The variations of velocity and temperature distributions in the pulse tube were also analyzed in detail for further understanding of the working process and refrigeration mechanism of PTRs. __________ Translated from Journal of Engineering Thermophysics, 2006, 27(5): 737–740 [译自: 工程热物理学报]     

Two-dimensional numerical simulation and performance analysis of tapered pulse tube refrigerator

Numerical simulation of two-dimensional viscous compressible oscillating flow was carried out for the uniform cross-section and tapered pulse tubes. Based on the numerical results, it was found that for the taper pulse tube refrigerator, there was an optimum taper angle, with which the performance of a cryogenic refrigerator can be greatly improved. However, on the other hand, the results also demonstrated that when the taper angle becomes much larger than this optimum value, the cooling performance becomes weaker than that of the circular tube. We discussed the effect of the tapered pulse tube on the performance of pulse tube refrigerator based on an evaluation of the secondary flow in the pulse tube. It was found that in comparison with the uniform cross-section pulse tube, the magnitude of the secondary flow in the tapered pulse tube decreases while its distribution becomes less uniform, which explains why the performance of the tapered pulse tube can be improved compared with the uniform cross-section pulse tube.     

CFD simulation of a Gifford–McMahon type pulse tube refrigerator

Pulse tube refrigerator has the advantages of long life and low vibration over the conventional cryocoolers, such as Gifford–McMahon (GM) and Stirling coolers because of the absence of moving parts in low temperature. This paper performs a two-dimensional computational fluid dynamic (CFD) simulation of a Gifford–McMahon type double inlet pulse tube refrigerator (DIPTR), operating under a variety of thermal boundary conditions. A commercial Computational Fluid Dynamics (CFD) software package Fluent 6.1 is used to model the oscillating flow inside a pulse tube refrigerator. Helium is used as working fluid for the entire simulation. The simulated DIPTR consists of a transfer line, an after cooler, a regenerator, a pulse tube, a pair of heat exchangers for cold and hot end, an orifice valve with connecting pipe, a double inlet valve with connecting pipe and a reservoir. The simulation represents fully coupled systems operating in steady-periodic mode. The externally imposed boundary condition is sinusoidal pressure inlet by user defined function at one end of the tube and constant temperature or heat flux boundaries at the external walls of the hot end and cold-end heat exchangers. The general results, such as the cool down behaviors of the system, phase relation between mass flow rate and pressure at pulse tube section and the temperature profile along the wall of the cooler are presented.The simulation shows the minimum decrease in temperature at cold-end heat exchanger for a particular combination of cryocooler assembly. The CFD simulation results are compared with available experimental data. Comparisons show that there is a reasonable agreement between CFD simulation and experimental results.     

First and second law analysis of pulse tube refrigerator

The first and second law of thermodynamics was used to analyze the orifice type and the double-inlet type of pulse tube refrigerator (PTR). Detailed dynamic characteristics of the thermodynamics, flow and heat transfer processes in the PTR were revealed, including the dynamic pressure variations, transient gas temperature, mass flow rate in the PTR. The exergy loss method was used to analyze each component in the PTR for the first time, and the performance coefficients of all components of PTR have been obtained. It was found that the performance coefficient of the double-inlet PTR was 0.108, 9% higher than that of the orifice PTR. The analysis also showed that the exergy efficiency of the double-inlet PTR was 29.95%, significantly higher than that of the orifice PTR (25.04%). In addition, it was found that the exergy losses in the regenerator and orifice were substantially larger than in other components of the PTR system. The optimal design of these two key components is, therefore, essential for the further improvement of the PTR performance.     

Optimization of an inertance pulse tube refrigerator using the particle swarm optimization algorithm

In this study, a particle swarm optimization method is employed to find the optimal operating parameters and geometrical parameters, which maximize the coefficient of performance (COP) of an inertance pulse tube refrigerator (IPTR). The considered decision variables of the IPTR are the charging pressure, which varies from 15 to 25 bar, operating frequency varying from 20 to 60 Hz, geometrical parameters, such as diameter varying from 15.0 to 25.00 mm, and length varying from 40.0 to 70 mm of the regenerator; diameter varying from 12.0 to 20.00 mm and length varying from 40.0 to 80 mm of the pulse tube; and diameter varying from 2.0 to 6.00 mm and length varying from 1.0 to 3.0 m of the inertance tube. A 1D numerical model, based on the finite volume discretization of governing equations has been selected to build the initial design matrix and solve the governing continuity, momentum, and energy equations. Analysis of variance is performed using the result obtained from the numerical simulation to visualize the variations of COP as a combination of various input parameters. It is observed that after optimizing the input parameters, the COP of the IPTR increases by 15.14%.     

Experimental flow field investigations of a film cooling hole featuring an orifice

This paper presents the flow field downstream of a film cooling hole geometry featuring orifice, referred to as nozzle hole, on a flat plate using PIV. The experiments were performed with blowing ratios from 0.5 to 2.0, density ratio of 1.0 and mainstream Reynolds number of 115,000. Velocity fields and vorticity fields of nozzle hole jet are compared with that of cylindrical hole jet. The results indicate that nozzle hole jet features double-decker vortices structure, resulting in vortices canceling out and significant reduction in CRVP strength. The streamwise vorticity of nozzle hole jet averages a drop of 55% at low blowing ratio 0.5 in comparison to cylindrical case. At high blowing ratio from 1.0, 1.5 and 2.0, the average drop is 30%–40%. A round jet bulk is observed to merge from the two legs of a typical kidney-shaped jet and the merged jet brings better coverage over the surface. In addition, it is found that CRVP strength might not have strong impact on jet lift-off but influences jet-mainstream mix characteristics.     

Ecological optimization of an irreversible Ericsson cryogenic refrigerator cycle

The ecological optimization and parametric study of an irreversible Ericsson cryogenic refrigerator cycle with finite heat capacities of external reservoirs is studied. The ecological function is defined as the cooling load minus the loss of the cooling load (the irreversibility) due to the entropy generation rate. The ecological function is optimized with respect to working fluid temperatures and the values of the cooling load, power input, the loss rate of the cooling load and COP are calculated for a typical set of operating parameters. The effects of different operating parameters on the ecological function, cooling load, the loss rate of the cooling load and COP are studied. The loss rate of the cooling load and the power input are found to be increasing functions of the cycle temperature ratio and decreasing functions of COP while the COP is found to be a decreasing function of the cycle temperature ratio. On the other hand, there exist the optimal values of the cycle temperature ratio and COP at which the ecological function and cooling load attain their maximum values. Also the ecological function and the cooling load are found to be increasing functions of the sink‐side heat capacitance rate and the effectiveness on the source‐, sink‐, and regenerative‐side heat exchangers while the decreasing functions of the source‐side heat capacitance rate. Copyright © 2005 John Wiley & Sons, Ltd.     

Dynamic Experimental Study of a Multi—bypass Pulse Tube Refrigerator with Two—bypass Tubes

INTRoDUCTIONAfterthebasicpulsetuberefrigeratorwasfirstin-ventedbyGmerdandLongsworthl1],theorilicepulsetuberefrigeratorwasintroducedbyMikulinetal.[2]Double-inletpulsetuberefrigeratorwasreportedbyZhuetal.l3jStillothermodificationswerepresentedbyIshizakietall4landMatsubaraetal.l5]Illterestsinpulsetuberefrigeratorshavegrownrapidlyduetothefactthatithasnomovingpartsatthecoldend,soithasconsiderablesystemadvalltagesovermostoftheothertypesofrefrigeratorsinreliabilitylifetime,vibrationandcostI6-7…     

Experimental and numerical investigations of hydrogen–air premixed combustion in a converging–diverging micro tube

Experimental and numerical studies of hydrogen–air premixed combustion in a converging–diverging micro tube with inner diameters of the inlet, throat, and outlet of 2, 1, and 2 mm, respectively, have been performed to study the combustion and flame characteristics. The influences of the equivalence ratio (Φ) and inlet velocity (v_in) are investigated. The experiments reveal that the v_in range for stable combustion—between 3.4 and 41.4 m/s—was significantly expanded, particularly when Φ = 1.4. This effect can primarily be attributed to the converging–diverging structure. As Φ increased, both the wall and the flame temperatures exhibited an increasing–decreasing trend; the largest heat loss ratio occurred at Φ = 1.0. The ignition position initially moved upstream and then moved downstream. The flame thickness increased and then decreased, reaching its peak value at Φ = 1.2. The flame length decreased monotonously. As v_in increased, the wall temperature increased, the flame temperature decreased, and the flame moved downstream to grow thicker and longer.     

Effect of expander shape on cooling characteristics by using a model pulse tube refrigerator

Pulse tube refrigerators do not have moving parts in the cold section, and they have low vibration, high reliability, and long life. The expander in refrigerators typically has an inverted U or coaxial shape because this attains a wider absorber area, lower height, and compactness. However, the performance of a Stirling-type pulse tube refrigerator is inferior to that of a Stirling refrigerator. Cooling characteristics of the pulse tube refrigerator greatly depend on the shape of the expander. In this study, an inertance-type refrigerator, which uses ambient air for the working gas, was developed to examine the effect of expander shape. This refrigerator model with changeable expander operated with a Stirling cycle, and it was composed of a reciprocating compressor, after-cooler, regenerator, absorber, pulse tube, hot-end, and inertance tube with reservoir. The following expander shapes were tested: in-line, L shape, L-L shape, and coaxial shape. The effect of expander shape on cooling capacity was examined experimentally and numerically using the model pulse tube refrigerator. The results of experiments showed that the L shape expander had the highest performance and the coaxial expander had the lowest performance. In addition, the characteristics of the gas flow in each expander were confirmed by fluid dynamics analysis.     

Numerical simulation of a two‐stage pulse tube refrigerator based on adiabatic model

Cryocoolers are devices that are capable of achieving and maintaining cryogenic temperatures for a number of applications such as high‐energy physics, cooling of superconducting magnets, sensors, high‐vacuum production, cryotronics, cryonics, and so on. All the above applications need coolers with high reliability, efficiency, low maintenance, and low cost. The absence of moving parts at the cryogenic temperatures makes the pulse tube (PT) coolers quite suitable for the above applications. In spite of considerable developments in the area of PT cryocoolers, many of the fundamental processes responsible for the cold production are not fully understood. In this work, we present the results of numerical simulations of two‐stage pulse tube refrigerators (PTR) using adiabatic flow of gas through the pulse tube system. A two‐stage PTR is the improved version of single‐stage system to achieve temperature close to 4 K. Assuming adiabatic gas flow through PTs, the algebraic equations for pressure, mass flow, and volumes at different locations have been derived and solved by a MATLAB based program. Using the above, the performance of PTR has been optimized for different operational parameters. The cooling powers predicted by the model have been compared with the experimental data, and they are in good agreement with each other.     

Experimental and numerical investigation of thermal characteristics of a novel concentric type tube heat exchanger with turbulators

In this study, the heat transfer performance and friction characteristics of a novel concentric tube heat exchanger with different pitches of helical turbulators were investigated experimentally and numerically for a Reynolds number range from 3000 to 14 000. An experimental system was established to obtain experimental data. The numerical simulations were performed by using a three dimensional numerical computation technique, a commercial CFD computer code. Then, the heat transfer performance and friction characteristics of several helical turbulators were compared. The experimental, numerical and empirical correlation results were in a good agreement with each others. As a result, the heat transfer enhancements using turbulators were 2.91, 2.41, 2.18 and 1.99 times better than the smooth tube for pitch distances of p = 20, 40, 60 and 80 mm, respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Experimental and Numerical Investigation of Condensation Shock in Shock Tube

INTRODUCTIONThehomogeneousnucleationwithsubsequentspon-taneouscondensationofwater,pentanol,andethanolVaPorsinacarriergasareinvestigatedexperimen-tallyandtheoreticallyintheexpansionpartofashocktube.AswasshownbySislianandGlass(l976)forsupercriticalconditionsthecondensationshockwave(Cw)occurs.Thehomogeneousnucleationofwatervaporinacarriergaswasfirststudiedexperimentallyintheex-pansionpartofashocktubeusinglightscatteringbyBarschdorff(1975)andthenumericalsolutionwasaddedbySislianandGlaJss(1…     

Factors influencing the lowest refrigerating temperature of the miniature co‐axial pulse tube refrigerator

Experimental studies of the influencing factors on the lowest refrigerating temperature of a miniature co‐axial pulse tube refrigerator have been carried out in this paper. The results show that with the decrease of the mole fraction of hydrogen in the hydrogen‐helium mixture, the lowest refrigerating temperature decreases, and when the mole fraction of hydrogen is below 20%, the lowest refrigerating temperature is close to that obtained using pure helium. In addition, it is also found that the optimum frequency of the compressor is about 16.7 Hz for different hydrogen‐helium mixtures. When the charge pressure of the compressor increases, the lowest refrigerating temperature decreases; however, the decreasing trend gradually slows down with the increase of the charge pressure. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(4): 219–225, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20065     

Experimental study of detonation propagation in a square tube filled with orifice plates

DDT experiments were conducted in a 6000 mm long square cross-section (112 mm × 112 mm) tube with various obstacle configurations with hydrogen-air mixtures and ethylene-air mixtures at ambient pressure (101 kPa) and room temperature (298 K). Square orifice plates with inner side 86.8 mm and 70.8 mm (BR = 0.4 and 0.6) and round orifice plates with inner diameter 80.0 mm (BR = 0.6) were used to assemble the obstacle configurations. The plates were installed at 1, 2 and 3 times the tube inner side. Soot foils were placed between the two orifice plates at the end of the tube for $S=3D$, where $S$ is the obstacle spacing and $D$ is the tube inner side. The DDT limits were determined based on the flame velocity above the isobaric sound speed of the burnt products. The results show that at the DDT limits, the criterion $d_{eff}/\lambda \approx 1$ is not pervasive, i.e., $d_{eff}/\lambda$ decreases with the obstacle spacing increase, in which $d_{eff}$ and $\lambda$ are the effective diameter of the orifice and the detonation cell size. Within the limits, the measured velocity for BR = 0.6 square orifice plates is higher than that for round orifice plates. On the other hand, no obvious difference in the limits can be observed for the BR = 0.6 obstacle configurations. Soot foils provide insights into the detonation propagation mechanism in the orifice plate section. It is shown that hot spots formed via the interactions between the decoupled shock wave and the tube wall can be responsible for the re-initiation of detonation. In addition, overdriven detonations induced by shock focus at the corners, followed by a band of fine cells. For less sensitive mixture and smaller orifice, the re-initiation distance is longer. Near the limits, no cellular structure can be observed, indicating longer cycle period for detonation re-initiation. This also accounts for the significant velocity fluctuation for larger spacing ($S=2D$ and $S=3D$) when the limits are approached.     

Experimental and numerical investigations of mixing in raceway ponds for algae cultivation

The current high interest in the algae sector is leading to the development of several demo/commercial scale projects, either for the food market or bioenergy production. Raceway Ponds (RWPs) are a widely used technology for algae mass cultivation. RWPs were developed long time ago, and thus capital and operating costs are well assessed. Nevertheless, room still exists to further reduce operational costs. A possible route towards energy optimization and therefore operational cost reduction can be identified through a better understanding of the mixing phenomena.The focus of the present work is that vertical mixing, defined as the cyclical movement of the algal cells between surface and bottom layers of the culture, cannot be completely determined by considering only turbulence, and therefore it is not represented by the Re number.A 3D Computational Fluid Dynamic (CFD) analysis of a conventional RWP was carried out based on a multi-phase “Volume of Fluid” model, in order to investigate the flow field of the culture in the pond. The CFD results were compared with experimental measures on a 20 m² pilot RWP. Once agreement among CFD and experimental results was shown, a statistical evaluation of the trajectories calculated for algae particles in the flow was carried out. The aim of this statistical evaluation was to define the level of vertical mixing in different sections of the pond.The model proposed was then used to scale-up the results to a demo/pre-commercial size RWP (500 m²). The standard deviation of the actual trajectory was calculated with respect to the undisturbed trajectory for each section modeled.The results of the simulation showed that a limited mixing is to be expected in RWPs. In the long straight parts of the pond vertical mixing is poor and algae tend to settle to the bottom. Only in the bends the vortexes produced by flow separation move part of the culture from the bottom to the top and vice-versa. This result does not fit with the practice, typically observed in large scale ponds, of reducing vortexes around the bends by placing baffles. The method described can be applied to different pond designs operated at different culture velocities.     

Experimental and numerical investigations of hydrogen jet fire in a vented compartment

Hydrogen fires may pose serious safety issues in vented compartments of nuclear reactor containment and fuel cell systems under hypothetical accidents. Experimental studies on vented hydrogen fires have been performed with the HYKA test facility at Karlsruhe Institute of Technology (KIT) within Work Package 4 (WP4) - hydrogen jet fire in a confined space of the European HyIndoor project. It has been observed that heat losses of the combustion products can significantly affect the combustion regimes of hydrogen fire as well as the pressure and thermal loads on the confinement structures. Dynamics of turbulent hydrogen jet fire in a vented enclosure was investigated using the CFD code GASFLOW-MPI. Effects of heat losses, including convective heat transfer, steam condensation and thermal radiation, have been studied. The unsteady characteristics of hydrogen jet fires can be successfully captured when the heat transfer mechanisms are considered. Both initial pressure peak and pressure decay were very well predicted compared to the experimental data. A pulsating process of flame extinction due to the consumption of oxygen and then self-ignition due to the inflow of fresh air was captured as well. However, in the adiabatic case without considering the heat loss effects, the pressure and temperature were considerably over-predicted and the major physical phenomena occurring in the combustion enclosure were not able to be reproduced while showing large discrepancies from the experimental observations. The effect of sustained hydrogen release on the jet fire dynamics was also investigated. It indicates that heat losses can have important implications and should be considered in hydrogen combustion simulations.     

Experimental and numerical investigations on brittle failure probability and ductile resistance property

The scatter of measured fracture toughness data and the transferability of toughness between different crack configurations and loading conditions are major obstacles in applications of fracture mechanics. To address these issues, worldwide, significant efforts have been devoted to establishment of the local approach. The purpose of this work is to further investigate both brittle rupture and ductile rupture of typical nuclear materials by using miniature specimens. Systematic finite element analyses as well as corresponding fracture toughness tests are performed with respect to compact tension and pre-cracked V-notched specimens. Subsequently, assessment of brittle failure probabilities and estimation of fracture resistance curves are carried out. Promising results have been observed through comparisons between experimental and numerically predicted data, which enables one to determine practical safety margins of nuclear components containing a defect.     

Experimental study on the effect of frost parameters on domestic refrigerator finned tube evaporator coils

In this study, parameters affecting the frost formation on the evaporator of a refrigerator and the structure of frost were examined. Air velocity measurements both at the air inlet and outlet channels of the evaporator were performed, and the effect of air velocity on frost formation was examined. The rate of evaporation of water inside the refrigerator cabin was also recorded.