Temperature Fluctuation of a Closed-Cycle Helium Joule-Thomson Cryocooler with Two-Stage Precooling

For quantum communications,single-photon detection,millimeter wave detection and other space projects,all of them need to work at liquid helium temperatures to achieve excellent performance.The closed-cycle helium Joule-Thomson cryocooler(JTC)is currently one of the mainstream solutions to realize the liquid helium temperature.While realizing the liquid helium temperature,the detector has strict requirements on the temperature fluctuation of the JTC,because the thermal noise caused by the JTC temperature fluctuation will have a critical impact on the detection performance.The typical closed-cycle helium JTC is precooling by a two-stage precooler.When the operating parameters of the JTC compressor remain unchanged,the change of the precooler is the main factor that affects the temperature fluctuation of the JTC.To explore the influence mechanism of JTC temperature fluctuations,experimental and theoretical studies are carried out.Based on the real gas equation of state,the influence of various parameters on the evaporator temperature fluctuations is explained.Research results show that the increase in temperature of each stage will cause the temperature of the JTC to increase.Especially,the change of the secondary precooling temperature(T_(pre2))has the most obvious influence on JTC temperature.Furthermore,the influence of the JT compressor’s buffer tank volume Vb on temperature fluctuation is studied.By increasing the Vb,the JTC temperature fluctuation caused by the temperature change of the precooler can be effectively reduced.

Impact of ventilation design on the precooling effectiveness of horticultural produce-a review

Optimizing the ventilation design of packaging system is of crucial importance for improving the efficiency of the forced-air precooling process to maintain the quality of horticultural produce and extend the shelf life in food cold chain.Many efforts had been devoted to the study about the impact of ventilation design on airflow and temperature distribution inside ventilated packages.This paper reviews relevant research methods,commonly used quantities for the measurement of precooling effectiveness,attractive design parameters,and their impact on precooling effectiveness.These allow us to know exactly the characteristic and deficiency of each research method,identify dominant design parameters,and seek a promising way for the future improvement of the ventilated packaging system.

Effect of precooling temperature on physiological quality of cold stored Agaricus bisporus

Effect of precooling temperatures(2℃,6℃,10℃and 14℃)on the physiological quality of postharvest Agaricus bisporus during cold storage was investigated.After six hours’precooling,Agaricus bisporus was stored at 3℃and sampled on day 3,6 and 9,respectively,for physiological quality analysis.Results showed that physiological quality of the Agaricus bisporus increased with the decrease of precooling temperature in the range of 2-14℃.Precooling at 2℃before cold storage had a positive impact on the storage quality of Agaricus bisporus.The decrease of hardness,whiteness and pH value was delayed,while the increase of cell membrane permeability and PPO and POD activities was restrained.Whiteness value of the Agaricus bisporus precooled at 2℃was above 80 on day 9,which means it was still acceptable,but the Agaricus bisporus precooled at 6℃and 10℃lost their commercial values.

Investigation on Precooling Effects of 4 K Stirling-Type Pulse Tube Cryocoolers

Stirling-type pulse tube cryocoolers(SPTCs)working at liquid-helium temperatures are appealing in space applications because of their promising advantages such as high reliability,compactness,etc.Worldwide efforts have been put in to develop SPTCs operating at liquid-helium temperatures especially with helium-4 as the working fluid.Staged structure is essential to reach such low temperatures.Generally,both the regenerator of the last section and the pulse tube together with the phase shifter are precooled by its upper stage or by external cold source to a low temperature of around 20 K.However,the precooling effects on the regenerator and the pulse tube are synthetic in previous studies,and their independent effects have not been studied clearly.In this manuscript,the precooling effects on the regenerator and on the pulse tube together with the phase shifter are tested independently on a unique-designed precooled SPTC.The tested precooling temperature is between 13.3 K and 22 K,and the no-load refrigeration temperature gets down to 3.6 K.Further analyses and numerical calculations have been carried out.It is found that the influence on the regenerator is remarkable,which is different from previous conclusions.It is also found that the precooling effects on the pulse tube are relatively weak because of the large pressure-induced enthalpy flow of a real gas working at the temperatures near to the critical point.Furthermore,the phase shifting capacity is analyzed with two cases and with both helium-4 and helium-3 as working fluids,and it keeps quite constant after optimizing the frequency and the precooling temperature for each case.The investigation on these independent effects will provide valid reference on the precooling mechanism study of SPTCs working down to liquid-helium temperatures.

Evaluation and optimization of air-based precooling for higher postharvest quality: literature review and interdisciplinary perspective

Precooling is of significant importance for postharvest fruits and vegetables to control the quality degradation and prolong the shelf-life.Current precooling methods include room cooling,forced-air cooling,hydrocooling,vacuum cooling,contact or package icing,and cryogenic cooling,all of which have their advantages and disadvantages.The first two methods with the cooling medium of air are extensively used because of the wide applicable range of fruits and vegetables.Numerous studies have been devoted to cope with the drawbacks of these two air-based precooling methods with various evaluation criteria and optimization methods.A systematic literature review on these studies is firstly conducted with respect to experimental and numerical investigations respectively for the two methods.The main contributions from the previous studies are also summarized respectively with the research objectives and performance metrics.The literature review indicates that the current performance evaluation is limited to apparent parameters and the optimal design is only proposed based on the performance evaluation and comparison.Furthermore,with inspiration from the research in other domains,a scheme of advanced evaluation and optimization for air-based precooling methods is proposed with thermodynamic evaluation metrics and constructal optimization methods from the interdisciplinary perspective.

Thermal Efficiency Optimization of a Ground Source Heat Pump System by Assistant Preheating/Precooling: A Case Study on a Heating-Load Dominated Building

Operation strategies of Ground Source Heat Pump System(GSHPS) such as continuous or intermittent approaches have been extensively studied. In this work, a novel strategy was proposed to maximize the energy efficiency of GSHPS. In this approach, the buildings was first air-conditioned by the preheating/precooling mode and the rest thermal load was covered by the heat pump(HP) mode. The system performance can then be optimized by considering the combination of these two operating modes. A case study was made to examine the thermal performance of a GSHPS installed in Nuremberg, Germany. Thermal performance of the HP and the preheating/precooling operating modes was examined. The system was optimized by deploying the assistant preheating/precooling approach and over one-year period monitoring showed that the seasonal Coefficient of Performance(COP) of the GSHPS was 4.12 in winter and 5.01 in summer. These COP values are higher than that of the conventional GSHPS. Thus, this proposed strategy could be an efficient way to improve the thermal performance of GSHPS.

Thermodynamic Analysis and Optimization of the C3/MRC Liquefaction Process

In the natural gas liquefaction process,the mixed refrigerant natural gas liquefaction process is widely used in LNG liquefaction plants because of its advantages of low energy consumption.This paper focuses on the influences of important parameters in the C3/MRC liquefaction process,that is,the comparison between propane precooling temperature and the number of moles of methane in mixed refrigerant,power consumption and loss.In addition,the total process was optimized with the optimizer and manual adjustment in HYSYS software to minimize the total power consumption.The results show that with increasing propane precooling temperature,the propane flow rate is almost unchanged,while the mixed refrigerant flow rate decreases significantly,and the loss of the heat exchanger increases significantly.The power consumption of the propane precooling cycle and hybrid refrigeration cycle increases with increasing methane content in the refrigerant,so the power consumption of the whole process increases accordingly.The effect of the methane content in the mixed refrigerant on the process evaluation index is more significant than that of the propane precooling temperature.

The Effect of Pre-Exercise Cooling on Performance Characteristics: A Systematic Review and Meta-Analysis

Exercising in high environmental temperatures may cause precocious hyperthermia induced fatigue resulting in a decreased athletes’ performance output. This systematic review with meta-analysis investigated the possible effects of pre-exercise cooling on performance output. This study was performed according to the PRISMA guidelines and the PICO-model was used to establish the research question. The Cochrane Risk of Bias Tool was applied to assess the validity of the included studies. Study eligibility was given when the studies compared the effects between any kind of pre-cooling and non-cooling strategies prior to exercise on performance output. Twenty-nine studies met the inclusion criteria for quantitative analysis. Risk of bias was high or unclear but the performance bias was low. The estimated standardized mean difference revealed that external pre-cooling (21 studies) enhanced performance (Hedges’ g = 0.49 [95% CI: 0.33 to 0.64]), with the main effect observed in endurance cycling or running. Internal (7 studies) and mixed-method (5 studies) pre-cooling failed to significantly affect performance parameters. However, the main output parameter, evaluated in these studies, was peak power output. Subgroup analysis for different outcome measures was not possible because meaningful grouping was not plausible. Limitations of this meta-analysis were the high or unclear risk of bias and the comparability of the included studies. Future studies should also determine the effects of different pre-cooling applications on female and untrained participants. Based on the results of this meta-analysis, it can be concluded that there is some evidence in favour of external pre-cooling to avoid precocious hyperthermia induced fatigue in endurance athletes exercising in hot environments.

燃氢燃气轮机热力学特性分析

本文提出用金属氢化物解决氢能燃气轮机中式的贮存、携带和加压问题。用预冷进气的方案,提高燃气轮机的效率和比功率。文章推导了循环基本热力学方程。对影响循环效率的几个主要因素进行了分析,并用一般回热式燃气轮机循环与本循环进行比较,指出了预冷进气燃氢燃气轮机的优越性能。

Micromanufacturing technologies of compact heat exchangers for hypersonic precooled airbreathing propulsion: A review

The Hypersonic Precooled Combined Cycle Engine(HPCCE), which introduces precooler into traditional hypersonic engine, is regarded as the most promising propulsion system for realizing a single-stage-to-orbit vehicle. The unique demands lead to the application of the compact heat exchangers, which can realize high thrust-to-weight ratio, sufficient specific impulse and high compression ratio. However, it is challenging to accurately manufacture the compact heat exchanger due to its extremely high heat dissipation capacity, remarkable compactness, superior adaptability and harsh operating condition. This review summarizes the precooling schemes of combined cycle propulsions and describes the demands and key issues in the fabrication of a compact heat exchanger for HPCCE. The investigation focuses on the application of various micromanufacturing methods of heat exchangers constructed from tubes of less than 1 mm in diameter and microchannels of less than 200 micrometers. Various micromanufacturing processes, which include microforming, micromachining, stereolithography, chemical etching, 3 D printing, joining and other advanced microfabricating processes, were reviewed. In addition, the technologies are compared in terms of dimensional tolerance, material compatibility, and process applicability. Furthermore, the boundaries of the micromanufacturing constraints are specified as references for the design of compact heat exchangers. Ultimately, the technological difficulties and development trends are discussed for the fabrication of compact heat exchangers for HPCCE.

A refined design method for precoolers with consideration of multi-parameter variations based on low-dimensional analysis

The precooler is a distinctive component of precooled air-breathing engines but constitutes a challenge to conventional thermal design methods.The latter are based upon assumptions that often reveal to be limited for precooler design.In this paper,a refined design method considering the variations of fluid thermophysical properties,flow area and thermal parameters distortion,was proposed to remediate their limitations.Firstly,the precooler was discretized into a fixed number of sub-microtubes based on a new discretization criterion.Next,in-house one-dimensional(1D)and two-dimensional(2D)segmented models were established for rapid thermal design and precooler rating with non-uniform airflow,respectively.The heat transfer experimental studies of supercritical hydrocarbon fuel were performed to verify the Jackson correlation for precooler design and the in-house models were validated against the reported data from open literature.On this basis,the proposed method was employed for the design analysis of hydrocarbon fuel precoolers for precooled-Turbine Based Combined Cycle(TBCC)engines.The results show that the local performance of precoolers is intrinsically impacted by the aforementioned three variations.In the case study,the local heat transfer performance is drastically affected by coolant flow transition.While the circumferential temperature distortion of airflow is weakened by heat transfer.With consideration of additional parameter variations,this novel method improves design accuracy and shortens the design time.

Microstructural evolution and mechanical properties of brazed IN718 ultrathin-walled capillary structure using different particulate reinforced filler alloy

The development of hypersonic precooled combined cycle engine provides a reliable power system for the future intercontinental navigation and reusable space orbital transportation.The compact heat exchanger precooler is the key component of the cooling cycle system,which provides efficient heat management for the hypersonic engine.This investigation mainly focused on the manufacturing process of precooler,and the typical structure of the precooler,ultrathin-walled capillary structure,was facilitated by the vacuum brazing using the particle reinforced composite filler alloy.The effects of different types and contents of the reinforced particle on the spreading behaviour,microstructural evolution,mechanical performance changes were investigated in detail.During the cooling process,the nucleation and growth of the c-Ni solid solution on the surface of the reinforced particles was the main reason for the microstructure homogenization.Compared to the Ni particles,the more obvious mechanical properties improvement could be achieved by using IN718 particles reinforced composite filler alloy.The tensile strength and elongation of the brazed ultrathin-walled structure present 9.36%and 47.27%higher than that using initial filler alloy.Moreover,the micro-hardness distribution result revealed that the reinforced particles reduced the overall micro-hardness of the brazed region,dispersed the eutectic structure and increased the hardness dispersion,which would reduce the stress concentration tendency and improve the structural properties.