R32吸气状态对变频压缩机效率的影响

制冷剂R32作为R22热门过渡替代制冷剂,运行排气温度高严重影响压缩机长时间运行的可靠性,湿压缩方法可以降低压缩机的排气温度,但也降低了压缩机的效率。通过实验对比研究在定压比变频和定频变压比工况下,不同吸气状态对滚动转子式压缩机效率的影响,得出等熵压缩效率和容积效率与吸气状态的变化关系。实验结果表明:吸气过热时,均能保持较高的等熵压缩效率和容积效率,过热度对其影响不大,吸气带液时压缩机效率的变化趋势与吸气过热时不同,应区分开;相对x=1,x=0.9时等熵压缩效率和容积效率分别降低约7%和5%,总体降幅较小。定频下,压比越大,压缩机效率降低幅度越大,定压下,频率越高,降低幅度越小;等熵压缩效率和综合效率系数的变化趋势相同。     

空气源热泵系统湿压缩与电子膨胀阀调控方式的研究

针对空气源热泵吸气带液问题,通过调节电子膨胀阀开度改变压缩机吸气口制冷剂状态,研究了四种不同调控工况对系统性能和参数的影响,分析不同调控工况下系统性能的变化。实验结果表明:较大的电子膨胀阀开度可提高蒸发压力和制冷剂质量流量,从而改善系统性能,但这不是湿压缩所产生的影响,相反后期大量的吸气带液会导致系统性能降低;吸气带液可以有效降低压缩机排气温度,同时也会使单位制热量和比功降低;在实际运行过程中,应采取定过热度控制方法,且过热度应尽量小,但要避免压缩机吸气带液。     

不完全湿压缩应用于转子式制冷系统的可行性研究

针对部分制冷工质排气温度较高的现象,将不完全湿压缩方法应用于转子式制冷系统,通过在不同频率下改变压缩机的吸气状态(过热至不完全湿压缩),研究了系统循环性能及核心部件的热力特性。实验结果表明:不完全湿压缩下存在一个最佳干度控制范围0x0.96,在该区间内制冷量提升5.1%～5.5%,制冷性能系数(Coefficient of Performance,COP)提升5.4%～5.8%,排气温度较过热段改善20%以上,系统性能得到了明显提升;当吸气干度范围在0x0.96时,压缩机的热力性能基本维持良好,相较于过热段的极大值处,容积效率降低仅1%～2%,电效率降低亦仅1%～2%;不完全湿压缩下,压缩机频率的变化对系统性能及压缩机热力性能影响均较大;将压缩机运行于最佳干度范围区间0x0.96的较低频或是额定频率附近将获得最优综合效率。     

空气源热泵系统压缩机吸气带液问题的研究

以空气源热泵热水机组为实验研究对象,通过改变电子膨胀阀开度,研究压缩机吸气状态对系统性能的影响,研究结果表明:(1)在加热前期(水箱平均温度20~30℃),膨胀阀开度越大,系统COP(能效比)越大;在加热后期(水箱平均温度40~55℃),膨胀阀开度越大,系统COP越小;(2)压缩机吸气带液可以有效改善冷凝器的换热性能,提高总换热系数,但在加热后期,压缩机大量吸气带液时,制冷剂质量流量降低造成的影响更大,因此系统制热量先上升后下降;(3)以压缩机吸气过热度为控制对象调节电子膨胀阀,使压缩机处于少量吸气带液状态,可以有效提高系统COP。     

变频滚动转子式压缩机变工况性能的实验研究

以R32变频滚动转子式制冷系统为研究对象,通过改变电子膨胀阀开度、压缩机运转频率、蒸发温度和吸排气压比,研究压缩机在不同工况尤其是不同吸气状态下的运行性能,结果表明:在过热段,压缩机容积效率基本不随过热度的变化而变化;在两相段,容积效率随吸气干度的减小呈线性下降趋势,且各工况斜率基本相同。蒸发温度基本不影响压缩机容积效率,而压比越高,容积效率越低;压缩机电效率在过热段和两相段分别随过热度和吸气干度的减小而线性减小,且两相段斜率大于过热段斜率。在相同蒸发温度下,压比越高,电效率越小;在相同压比下,蒸发温度越高,电效率越小;在各工况下,系统COP在过热段基本不变,在两相段随干度减小而减小。吸气干度0.90处的COP比吸气饱和点的COP平均降低了5.5%;容积效率随压缩机频率的提高而增大,电效率和系统COP随压缩机频率的提高而减小。     

低频率下电子膨胀阀调节对水源热泵系统参数的影响

压缩机低频运行可以减少电能消耗,但对系统性能也会产生一定的影响。以滚动转子式压缩机水源热泵系统为研究对象,通过改变电子膨胀阀开度,研究压缩机在低频运行时系统性能参数的变化特性。实验结果表明:压缩机在低频(25～35 Hz)运行时,电子膨胀阀对过热度或干度的调节区间为1%～9%,提高冷冻水进水温度,低频调控区间变为2%～18%,控制精度提高;制热量与COP在过热度1 K左右达到最大值,较于常规控制过热段(5～10 K)制热量提升10.3%～34.2%;COP提升11%～34.5%;在低频范围内,电子膨胀阀对质量流量的调节区间小于16%,对排气温度的调控区间小于18%;压缩机低频运行时,应避免其吸气口少量带液,此时系统性能骤降,恶化压缩机。     

使用电子膨胀阀的空气源热泵热水器的实验研究与优化

为充分利用电子膨胀阀调节灵活的特性,对空气源热泵热水器在定阀开度和定过热度下的系统性能进行对比实验,并提出了电子膨胀阀的排气温度控制法。实验结果表明:定阀开度下运行易发生吸气带液现象,吸气带液量的大小对系统性能的影响较大,但可由排气温度加以判断,适当增大阀开度可提升平均制热效率(COPa)并降低排气温度;与定阀开度相比,定过热度下运行时COPa普遍较高,排气温度高,但改变定过热度对COPa的影响较小,这为过热度的简单控制提供了依据;提出的排气温度控制法以排气温度判断吸气带液量大小,对过热度进行简单控制,满足了大制冷剂流量和小过热度的制热要求,试验测试中COPa提升8.24%,排气温度降低7.16%,系统性能达到了预期的优化效果。     

滚动转子式压缩制冷系统润滑油的流动

润滑油的流动对制冷系统的性能和可靠性有重要影响.建立滚动转子式压缩系统实验台,观测和研究膨胀阀出口和蒸发器出口制冷剂/油混合物的两相流流型.结果发现:在蒸发器出口处混合物的流动表现为"油渍"蠕动、"油膜"线状流、"油膜"环状流和雾状湿蒸汽流等流型;在膨胀阀出口有液气分相流和泡气分相流等流型.在一定的运行工况下,压缩机正过热度越小,"油膜"流动速度越快,越利于压缩机回油;当压缩机排气温度等于冷凝温度时,高含油量的液体节流后形成泡状流,使得系统性能恶化甚至造成压缩机损坏.     

采用喷水降温螺杆压缩机的水蒸气热泵性能研究

为拓宽水蒸气热泵在余热回收中的工作温区,降低水蒸气压缩机的排气温度,对采用喷水降温螺杆压缩机的水蒸气热泵系统及其主要部件建立热力学模型,研究了螺杆压缩机喷水温度,及最佳喷水温度下蒸发温度、冷凝温度对系统性能的影响。结果表明:喷水可以有效降低压缩机排气温度,喷水温度在73～87℃之间可保证压缩机运行在报警温度之下;压缩机耗功和冷凝器放热量随喷水温度的升高先增加后降低,在喷水温度为80℃时系统性能系数(EER)最佳;最佳喷水温度下,蒸发温度提高,蒸发器吸热量、压缩机耗功、冷凝器放热量及EER增大,压缩机压比降低;冷凝温度提高,压缩机耗功、压比增大,冷凝器放热量和EER降低。     

寒冷地区用空气源热泵热水器准双级压缩循环特性研究

空气源热泵热水器在寒冷地区运行时会出现排气温度过高、制热性能弱等弊端,准双级压缩循环技术可有效改善其在寒冷地区的运行特性。采用带中压补气的转子压缩机,研制了以R410A为循环工质的空气源热泵热水器,试验研究了系统在寒冷地区低温环境下的制热性能。结果表明:带中压补气的空气源热泵热水器系统排气温度较无补气有所降低,当室外温度从7.0下降到-25.0℃时,与无补气系统相比,补气系统制热量提升了6.2%～15.5%,压缩机功率提升了2.8%～9.5%,COP_h提升了3.3%～9.6%。     

Study on performance of a heat pump water heater using suction stream liquid injection

Liquid refrigerant injection into a suction line is an effective and practical method to reduce the discharge temperature when a scroll compressor operates at high compression ratios. In the present study, correlations among the compressor suction temperature, discharge temperature, heat pump heating capacity, power consumption, coefficient of performance (COP) and the quantity of suction liquid injection are established. The paper presents experimental analysis and a comparison with calculated results of the heat pump water heater (HPWH) performance with suction liquid injection in different conditions. It is found that the suction liquid injection explicitly lowers the discharge temperature of the compressor and the heating capacity of the unit, but the power consumption increases with COP decreasing. In addition, the highest injection ratio must be controlled fewer than 5%. The suction liquid injection has a better effect on the HPWH at the temperature ranging from ?15 °C to 20 °C. Within this temperature range, the 5% ratio suction liquid injection decreases the discharge temperature of the compressor by 10 °C, while the heating capacity of the HPWH decreases by less than 5%, power consumption increases by less than 1.5%, and COP decreases by less than 7%.     

Dynamic and energy analysis of a liquid piston hydrogen compressor

Liquid piston compressor is the most promising compressor to be used for hydrogen-refueling stations. However, their energy transfer and the energy dissipation processes of are poorly studied and not well understood. In this paper, a new energy analysis method for an ionic-liquid type liquid piston compressor is proposed. In the compressor section, porous media is used to promote heat transfer from the hydraulic oil during the compression process. A mathematical model has been formulated considering the heat transfer and damping effects of the porous media on the compressor performance. Moreover, the compressibility of the hydraulic oil and its overflow loss on the compressor performance were also established. In the model, the seven stages of the entire working cycle of the compressor were look into in detail, alongside with its energy efficiency. The results show that the key parameters governing the energy efficiency of the compressor are the heat transfer efficiency of the compressor and the overflow losses of the hydraulic oil.     

Analysis of non-adiabatic excess enthalpy spray flames

A steady, one dimensional, low speed flame propagating in a dilute, monodisperse, sufficiently off stoichiometric and weakly heterogeneous spray with external heat recirculation is analyzed using activation energy asymptotics. A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified. Heat recirculation is achieved by transferring heat through a tube wall within a given distance L. The external heat transfer results in either globally external heat loss or excess enthalpy burning (which is globally adiabatic) to the spray system with increasing wall temperature. The influences of external heat recirculation and liquid fuel spray on the combustion characteristics of the spray flames are examined with five parameters, namely the heat transfer length for excess enthalpy burning, the heat transfer coefficient, the amount of external heat transfer, the liquid fuel loading and the droplet size. It is found that the extent of flammability is enlarged with increasing heat transfer length and heat transfer coefficient or decreasing external heat loss. The range of flammability is also enlarged with increasing liquid loading or decreasing droplet size for lean sprays, while the opposite holds for rich sprays.     

Liquid piston gas compression

A liquid piston concept is proposed to improve the efficiency of gas compression and expansion. Because a liquid can conform to an irregular chamber volume, the surface area to volume ratio in the gas chamber can be maximized using a liquid piston. This creates near-isothermal operation, which minimizes energy lost to heat generation. A liquid piston eliminates gas leakage and replaces sliding seal friction with viscous friction. The liquid can also be used as a medium to carry heat into and out of the compression chamber. A simulation is presented of the heat transfer and frictional forces for a reciprocating piston and a liquid piston. In the application of an air compressor, with a pressure ratio of 9.5:1 and a cycle frequency of 20 Hz, the liquid piston decreased the energy consumption by 19% over the reciprocating piston. The liquid piston and the reciprocating piston exhibited a total efficiency of 83% and 70% respectively. The liquid piston demonstrated significant improvements in the total compression efficiency in comparison to a conventional reciprocating piston. This gain in efficiency was accomplished through increasing the heat transfer during the gas compression by increasing the surface area to volume ratio in the compression chamber.     

Simplified steady-state modeling for variable speed compressor

This paper presents a detailed analysis of semi-empirical methods to calculate mass flow rate, shaft power and discharge temperature for three types of variable speed compressors: reciprocating, scroll and piston rotary. The proposed methods are an integration of physical-based models for constant speed compressor and the physical characteristics of volumetric efficiency and isentropic efficiency between different speeds. The physical-based models were first validated with good agreement with experimental data from publication for the three types of constant speed compressors. The comparison between modeling results and experimental data from publication for the three types of variable speed compressors shows the RMS errors are less than 3%, 3% and 3 °C for refrigerant mass flow rate, compressor power input and discharge temperature, respectively. The model of variable speed compressor will allow the reduction of the number of experimental data required to characterize variable speed compressor behavior in the modeling of refrigeration systems because of its physical mechanisms.     

Optimization of ejector-expansion transcritical CO2 heat pump cycle

Optimization studies along with optimum parameter correlations, using constant area mixing model are presented in this article for ejector-expansion transcritical CO₂ heat pump cycle with both conventional and modified layouts. Both the energetic and exergetic comparisons between valve, turbine and ejector-expansions-based transcritical CO₂ heat pump cycles are also studied for simultaneous cooling and heating applications. Performances for conventional layouts are presented by maximum COP, optimum discharge pressure and corresponding entrainment ratio and pressure lift ratio of ejector, whereas for modified layout by maximum COP, optimum discharge pressure and corresponding pressure lift ratio. The optimization for modified layout can be realized for certain entrainment ratio, evaporator and gas cooler exit temperature combinations. Considering the trade-off between the system energetic and exergetic performances, and cost associated with expansion devices, the ejector may be the promising alternative expansion device for transcritical CO₂ heat pump cycle.     

Miniature heat pipes as compressor cooling devices

The purpose of this paper is an analytical and experimental study on heat transfer and temperature distribution for hermetic refrigeration compressor using miniature heat pipes as two-phase thermal control system. Heat pipe based coolers, such as miniature and micro heat pipe spreaders, loop heat pipes and loop thermosyphons ensure the temperature decrease of the most important parts of the compressor – cylinder head, cylinder, oil and compressor shell down to 10–15 °C. The experimental validation of the analytical study was performed for four different designs of miniature heat pipes.     

Theoretical and experimental studies of water injection scroll compressor in automotive fuel cell systems

A water injection scroll compressor to supply clean compressed air to an automotive fuel cell system is researched. The water is used as both the lubricant and coolant in the compressor. A thermodynamic model of the water injection scroll compressor considering leakage and heat exchange for use with an automotive fuel cell system was developed using the conservation of energy and mass equations and the equation of state. The results show that the scroll compressor has nearly isothermal compression when injecting water in it. Increasing the compressor rotation speed increases the discharge loss and the volumetric efficiency of the scroll compressor. The difference between the calculated power and the isothermal power increases as the compressor rotation speed rises, which means the efficiency of the compressor decreases. Increasing the flow rate of water injected increases the indicated isothermal efficiency and decreases the discharge temperature. Under the condition studied, the mass flow rate of water has the greatest effect on the discharge temperature.     

Theoretical study of the dynamic characteristics of a self-commutating liquid piston hydrogen compressor

Hydrogen is being more and more widely deployed in various fields for its ‘clean’ character. For applications in automobiles where hydrogen has already been adopted for years, higher pressure means better mileage. To improve the pressure of the hydrogen compressor, a novel self-commutating liquid piston hydrogen compressor is proposed in the present study. A two-stage hydrogen booster is designed on both sides of the hydraulic cylinder piston, which is driven by a spool installed in the cylinder piston. The benefits of the novel hydrogen compressor are reducing the throttling loss and enhancing the response of the piston. Furthermore, the principle of the hydrogen compressor is illustrated, based on which a dynamic model is established while taking oil compressibility, leakage and flow force in the compression process into consideration. Moreover, system simulation model is established by applying the simulation software, verifying the feasibility and validity of the novel structure. Accordingly, the energy efficiency on the mechanical-hydraulic structure is improved.