NH3/C02复叠制冷系统实验研究

对NH3/CO2复叠式制冷系统进行了性能实验,并对NH3/CO2复叠系统、两级NH3系统以及单级NH3系统的性能进行了比较.结果表明,当CO2冷凝温度升高时,复叠系统的COP先增人后降低:随着冷凝蒸发器中换热温差的降低、CO2蒸发温度的升高,系统COP逐渐升高.在较低的蒸发温度下,NH3/CO2复叠系统的COP高丁两级NH3、单级NH3系统.结果表明自然工质的NH2/CO2复叠式制冷系统在低温工况卜具有良好的心用前景.     

NH3／CO2复叠制冷系统实验研究

对NH3／CO2复叠式制冷系统进行了性能实验,并对NH3／CO2复叠系统、两级NH3系统以及单级NH3系统的性能进行了比较。结果表明,当CO2冷凝温度升高时,复叠系统的COP先增大后降低;随着冷凝蒸发器中换热温差的降低、CO2蒸发温度的升高,系统COP逐渐升高。在较低的蒸发温度下,NH3／CO2复叠系统的COP高于两级NH3、单级NH3系统。结果表明自然工质的NH3／CO2复叠式制冷系统在低温工况下具有良好的应用前景。     

NH3／CO2复叠制冷系统实验研究

对NH3／CO2复叠式制冷系统进行了性能实验,并对NH3／CO2复叠系统、两级NH3系统以及单级NH3系统的性能进行了比较。结果表明,当co2冷凝温度升高时,复叠系统的COP先增大后降低;随着冷凝蒸发器中换热温差的降低、CO2蒸发温度的升高,系统COP逐渐升高。在较低的蒸发温度下,NH3／CO2复叠系统的COP高于两级NH3、单级NH3系统。结果表明自然工质的NH3／CO2复叠式制冷系统在低温工况下具有良好的应用前景。     

NH3／CO2复叠制冷系统实验研究

对NH3／CO2复叠式制冷系统进行了性能实验,并对NH3／CO2复叠系统、两级NH3系统以及单级NH3系统的性能进行了比较。结果表明,当CO2冷凝温度升高时,复叠系统的COP先增大后降低;随着冷凝蒸发器中换热温差的降低、CO2蒸发温度的升高,系统cOP逐渐升高。在较低的蒸发温度下,NH3／CO2复叠系统的COP高于两级NH3、单级NH3系统。结果表明自然工质的NH3／CO2复叠式制冷系统在低温工况下具有良好的应用前景。     

CO2低温制冷循环热力学分析

通过对CO2单级压缩和双级压缩制冷循环的热力学分析得出,在一定的蒸发温度和冷凝温度下,CO2单级压缩制冷循环的COP比CO2双级压缩制冷循环的COP低、压差大、压比高.因此,CO2低温制冷循环系统应采用双级压缩制冷循环,为提高CO2双级压缩制冷循环的循环效率,应尽可能升高蒸发温度、降低冷凝温度,可以看出自然工质CO2双级压缩制冷循环有很好的发展前景.     

自然工质在小型超市制冷系统中应用的理论分析

本文对低温级以CO2为工质的超市复叠式制冷系统进行了热力学理论分析,计算了不同蒸发温度、冷凝温度和不同传热温差下的COP,并与传统的超市复叠制冷循环进行对比分析。结果发现自然工质CO2／NH,复叠制冷系统的性能效率为最好,而R290／CO2复叠制冷系统的COP与使用传统工质的相当。因此,将自然环保工质复叠式制冷系统应用于小型超市具有很好的应用前景。     

余热驱动的吸收-压缩复叠制冷循环性能及经济性分析

本文提出一种新型吸收-压缩复叠制冷循环,该循环由内燃机余热驱动的高温级复合吸收-压缩制冷循环与动力驱动的低温级CO2亚临界压缩制冷循环复叠构成。对不同制冷工质对在此循环中的性能进行了对比分析;并进一步研究了关键参数对复叠制冷循环性能的影响规律;最后进行了经济性分析。结果表明：该循环的理论性能优于两级复叠制冷循环;在冷凝温度为40 ℃、蒸发温度为-35 ℃条件下,R124-DMAC/R744性能优异,COP可达2.864,是较为理想的工质对,且年总成本较低为15 150.14 美元。     

R404A与CO2复叠式制冷系统的理论分析

本文针对所建立的新型R404A／CO2复叠式制冷设备进行了理论研究,该系统可提供零下40℃以下的低温环境。根据R404A和CO2的物性特征及复叠式循环流程,通过数值模拟寻找一定工况下CO2低温级的最佳冷凝温度及二者的最佳质量流量比,分析冷凝蒸发器的工作温度、CO2侧蒸发温度、R404侧的冷凝温度等对R404A／CO2复叠式系统COP的影响。结果表明,为了提高循环效率并保证循环的安全运行,应尽可能地升高低温段蒸发温度、降低高温段冷凝温度,缩小冷凝蒸发器的传热温差,环保工质R404A和CO2的复叠式制冷系统在低温制冷条件下有良好的发展前景。     

二氧化碳复叠式制冷循环的热力性能分析

分析了CO2的热力特性和CO2复叠式制冷循环的优势及应用现状,概述了复叠式制冷循环的原理及组成,从理论和实验两个方面对NH3/CO2和R290/CO2复叠式制冷循环进行了分析,得出了COP以及高低压级质量流量比与蒸发温度、冷凝温度、冷凝蒸发温差之间的关系.     

自动复叠蒸发冷凝器参数变化影响的研究

介绍自动复叠制冷循环中蒸发冷凝器的几种结构形式及工作特点,分析自制的循环及实验数据,得到自动复叠实验系统自身运行规律,并通过改变运行工况,研究蒸发冷凝器的参数变化对自动复叠制冷循环的影响。     

NH3／CO2复叠式制冷系统的仿真与实验研究

运用集总参数法建立了NH3／CO2复叠式制冷系统的稳态仿真数学模型,计算和分析了低温循环冷凝温度对系统COP值和质量流量比的影响;最优冷凝温度随蒸发温度的变化：蒸发温度和冷凝温度对系统COP值的影响,以及不同冷凝蒸发传热温差下系统COP值的变化关系,比较了某工况下仿真结果与实验测试数据,验证了仿真模型正确性,对进一步深入研究NH3／CO2复叠式制冷系统具有一定的指导作用。     

四种跨临界CO2压缩式热泵系统的热力性能研究

CO2是具有很大潜力的天然替代工质之一,CO2跨临界循环放热过程中具有较大温度滑移,与水侧温升过程相匹配,因此适合用于热泵热水器系统。国内外学者提出了许多提高跨临界CO2循环效率的方法,其中包括引入回热器、喷射器等设备,从不同角度对比分析在常规跨临界CO2热泵系统中引入回热器、喷射器后系统的性能变化。本文在前人工作的基础上,建立相关热力学计算模型,并进一步对四种不同形式的跨临界CO2热泵系统（常规跨临界CO2热泵系统（TCHS）、带回热器的跨临界CO2热泵系统（TCHSI）、带喷射器的跨临界CO2热泵系统（TCHSE）及带喷射器与回热器的跨临界CO2热泵系统（TCHSEI））的性能进行研究,对比分析排气压力一定的情况下四种循环的热力性能;从最优排气压力的角度出发,分析对比不同系统中气冷器出口温度变化对系统最优排气压力和制热系数的影响,以及喷射器等熵效率对系统性能的影响。以上研究为CO2压缩式热泵系统的实用化进展奠定良好的理论基础。     

Effects of vapor injection techniques on the heating performance of a CO2 heat pump at low ambient temperatures

The objective of this study is to investigate the effects of vapor injection techniques on the heating performance of a CO₂ heat pump. The performances of the flash tank vapor injection (FTVI), sub-cooler vapor injection (SCVI) and FTVI with a suction line heat exchanger (FTSX) cycles were measured and analyzed with variations of the outdoor temperature, compressor frequency, and injection mass flow rate. At the outdoor temperature of −15 °C and compressor frequency of 55 Hz, the heating capacity and COP of the optimized SCVI cycle were 12.1% and 12.7% higher than those of the optimized FTVI cycle, respectively, because the total mass flow rate in the SCVI cycle was higher than that in the FTVI cycle by the large temperature and pressure differences in the sub-cooler of the SCVI cycle. In addition, the optimum injection flow rate ratios in the vapor injection CO₂ cycles yielding the maximum COP were determined at various compressor frequencies.     

Thermodynamic analysis of an R744–R717 cascade refrigeration system

A thermodynamic analysis of carbon dioxide–ammonia (R744–R717) cascade refrigeration system is presented in this paper to optimize the design and operating parameters of the system. The design and operating parameters considered in this study include (1) condensing, subcooling, evaporating and superheating temperatures in the ammonia (R717) high-temperature circuit, (2) temperature difference in the cascade heat exchanger, and (3) evaporating, superheating, condensing and subcooling in the carbon dioxide (R744) low-temperature circuit. A multilinear regression analysis was employed in terms of subcooling, superheating, evaporating, condensing, and cascade heat exchanger temperature difference in order to develop mathematical expressions for maximum COP, an optimum evaporating temperature of R717 and an optimum mass flow ratio of R717 to that of R744 in the cascade system.     

带约束条件的CO_2热泵热水系统高压控制方程

系统高压的优化与控制在提升跨临界CO_2系统效率方面至关重要。在CO_2热泵热水系统中,传统的高压控制方程中仅考虑了气体冷却器侧的出口温度而忽略实际应用中对出水温度的要求和温度夹点,导致系统性能大大降低。本文针对温度夹点和进出水温等约束的影响,建立了有约束的气冷器模型,并对系统高压进行了优化。采用实验设计以及统计学方法确定影响最优高压的主要因素,通过最小二乘法回归出最优高压控制方程。使用该高压控制方程的系统平均COP损失1.8%,最大COP损失8.7%,可以更好地满足实际应用。     

Ammonia evaporation in a mixed configuration chevron plate heat exchanger with and without miscible oil

An experimental study was conducted to determine the effects of miscible lubricant oil on evaporation of ammonia in a vertical chevron plate heat exchanger. The heat exchanger was configured in a U-type counter flow arrangement with mixed (30°/60°) chevron plate configuration. Experiments were carried out for four saturation temperatures ranging from −25 °C to −2 °C for a fixed ammonia mass flux rate of 6.5 kg m⁻² s⁻¹ and over a range of heat flux levels resulting in a vapor quality at the heat exchanger exit ranging between 0.5 and 0.9. For a given saturation temperature, experiments were performed for 0%, 3%, 6% and 9% oil concentrations, by volume in ammonia. The oil concentration, exit vapor quality, heat flux and saturation temperature were found to have significant effects on the heat transfer coefficient and pressure drop of ammonia. Based on the experimental data, correlations to estimate two phase Nusselt number and friction factor, generalized for the whole range of oil concentration have been presented.     

R417A热泵空调器运行特性分析

在焓差实验台和热泵性能测试系统中,对1台R417A热泵空调器在高/低温工况下的运行特性进行研究。通过改变蒸发器侧空气的温度、湿度等参数,测得压缩机吸/排气温度、蒸发温度、冷凝温度、制热性能系数,并用数码相机对室外侧翅片管蒸发器的结霜情况进行记录。实验数据和结果表明,在高/低温工况下,R417A热泵空调器的排气温度相对较低,制冷系统的冷凝温度、蒸发温度、制热系数和功耗在高温工况时变化不大,低温工况时,由于室外侧翅片管蒸发器结霜系统各性能变化较大,在结霜后期系统的性能有较大的衰减。     

Experimental investigation of a CO2 automotive air conditioner

In this study, a CO₂ automotive air conditioner prototype was designed and constructed. The compressor was of swash plate design; the gas cooler and evaporator were made of fin-tubes; a manual expansion valve and an internal heat exchanger accumulator were used. The lubricant, the CO₂ charge, the evaporator outlet pressure, the compressor speed, the air inlet temperature and flow rate of the gas cooler and the air flow rate of the evaporator were varied and the performance of the prototype was experimentally investigated in detail. The cooling capacity, compressor power consumption, CO₂ mass flow rate, and COP value were analyzed. The experimental results showed that the CO₂ system performance was greatly affected by different lubricants; the CO₂ system performance was sensitive to the mass charge; the high side pressure affected the system performance greatly and a high side pressure controller was needed.     

Boiling heat transfer of HFO-1234yf flowing in a smooth small-diameter horizontal tube

The flow boiling heat transfer coefficient of the low-GWP (global warming potential) refrigerant HFO-1234yf inside a smooth small-diameter horizontal tube (inner diameter: 2 mm) was experimentally investigated. The local heat transfer coefficient was measured at heat fluxes of 6-24 kW m⁻², mass fluxes of 100-400 kg m⁻² s⁻¹, an evaporating temperature of 288.15 K, and an inlet vapor quality of 0-0.25. The results show that the effect of heat flux on the heat transfer was large at low vapor quality, while the effect of mass flux was large at high vapor quality. The heat transfer coefficient of HFO-1234yf was almost the same as that of R-134a. The heat transfer coefficients calculated based on correlations with Saitoh et al. agreed well with the measured values compared to other correlations. The measured pressure drop agreed well with that predicted by the Lockhart-Martinelli correlation.