A general geometrical model of scroll compressors based on discretional initial angles of involute

Based on discretional initial angles of involute of scroll compressors, a general mathematical representation of scroll wraps, working chamber volume and leakage areas is presented. The scroll wraps' geometrical expressions, including interaction arc, of the involute angle with discretional initial angles of involute are developed. By using it, a new calculation formula of working chamber volume without restriction to special involute initial angles is set up and the expression of the volume during all the suction, compression and discharge processes in a general subsection function style is given. A geometrical model of leakage areas, including flank in, flank out, radial in and radial out, is also developed based on the discretional initial angles condition. Finally, this geometrical model is applied in a thermodynamic model and the simulation results are compared with some former experimental results. It is found that this model has a satisfactory accuracy and is easy to be used in thermodynamic simulation.     

Parametric studies on hermetic reciprocating compressors

An advanced numerical simulation model for the thermal and fluid dynamic optimization of hermetic reciprocating compressors has been developed. The quality of the numerical solution has been verified by means of a critical analysis of the different sources of errors, and validated through an extensive experimental comparison. This work is focused on presenting different parametric studies of hermetic reciprocating compressors, based on the numerical simulation model developed. Results presented show the influence of different aspects (geometry, motor, valves, working conditions, etc.) on the basis of the meaningful non-dimensional parameters, which describe the compressor behaviour (volumetric and isentropic efficiency, coefficient of performance, etc.). The idea of this paper is to show the possibilities offered by the simulation model and its final objective, a better understanding of the thermal and fluid dynamic compressor behaviour to improve the design of these equipments.     

Experimental investigation on convective heat transfer mechanism in a scroll compressor

This paper describes an experimental study on the convective heat transfer inside the scroll compressor. An experimental refrigeration system is composed with extensive instrumentations in the compressor that is operated at variable speeds. The 13 thermocouples installed inside the compressor monitor the temperatures of the scroll wrap during compression process of refrigerant. The temperature and the pressure of refrigerant at suction, and the pressure at discharge ports are measured, and applied to the numerical simulation as the operating condition parameters. The temperature measured at the discharge port is used to verify the simulation result with relevant heat transfer coefficient. This paper describes the effect of motion of the orbiting scroll on the convective heat transfer in the scroll wraps. Separate experiments are performed to investigate the heat transfer in such a peculiar physical condition. With this experimental result, the effect of the oscillation of the wall on the heat transfer is quantitatively analyzed and applied to the simulation of compression process in scroll compressor. The whole consecutive compression processes in the scroll compressor is simulated in detail by solving equations of mass and energy balance for the refrigerant. The modified heat transfer coefficient correlation considering the effect of motion of the orbiting scroll predicts the discharge temperature better than other typical heat transfer coefficients.     

Instability of automotive air conditioning system with a variable displacement compressor. Part 2. Numerical simulation

A test system is built first in order to investigate the instability of the automotive air conditioning (AAC) system with a variable displacement compressor (VDC), and hunting phenomena caused by the large external disturbance in the AAC system with a VDC and a thermal expansion valve, and in the AAC system with a VDC and a fixed-area throttling device are investigated experimentally in part 1 of this paper. The experimental results indicate that there also exist the hunting phenomena in the AAC system with a fixed-area throttling device. The system stability is found to be dependent on the direction of the external disturbance, and the system is apt to cause hunting when the condensing pressure decreases excessively since it may cause two-phase state at the throttling device inlet and make a large disturbance to the system. The piston stroke length will oscillate only when the oscillation amplitudes of forces acting on the wobble plate are great enough, otherwise the piston stroke length will be kept invariable, and then the system instability rule is also suitable for the AAC system with a fixed displacement compressor. From the experimental results, it is concluded that the two-phase flow at the throttling device inlet or at the evaporator outlet is the necessary condition but not sufficient condition for system hunting. Finally, a new concept, conservative stable region, is proposed based on the experimental results and theoretical analysis.     

Comparative study of the impact of the dummy port in a scroll compressor

A dummy port plays an important role in the porting process and the improvement of the performance of a scroll compressor. This paper documents an investigation on the working mechanism of the dummy port in a scroll compressor. To characterize the dummy port effects on the different parts of the scroll compressor, two scroll compressors, one with and the other without a dummy port, are studied comparatively. The flow through the dummy port is examined in the background of an integrated compressor working process. The assembly of the compressor under investigation includes the upper bearing housing, scrolls, check valve, and discharge plenum. The Navier–Stokes equations with a k– turbulence model are solved at the standard operating conditions of a scroll compressor. Refrigerant-22 is used as the working fluid. The thermodynamic and transport properties of the refrigerant gas are modeled by the Martin–Hou equation of state and power laws, respectively. Global flow physics is investigated first to lay a foundation to understand the working mechanisms that control the porting process before averaging techniques are applied. The behavior of the gas pockets in the porting process is characterized in both geometric and dynamic nature. The time-dependent variation of volume, mass, energy, and volume-averaged field quantities inside the gas pockets are studied throughout the porting process. The impact of the dummy port on the compressor performance is defined.     

Detailed thermodynamic characterization of hermetic reciprocating compressors

The aim of this paper is the detailed analysis of different well-known thermodynamic efficiencies usually used to characterize hermetic compressors. Attention is focussed on the volumetric efficiency, the isentropic efficiency, and the combined mechanical–electrical efficiency. A procedure is presented to detach these efficiencies into their main components (physical sub-processes) to get deeper insight on the overall behavior.The volumetric efficiency is split into partial efficiencies related to pressure drop and heat transfer effects, supercharging effects, superdischarging effects, leakages, etc. The isentropic efficiency is detached using two different points of view: the work associated to the individual sub-processes (compression, discharge, expansion, suction), and the work associated to the underpressures, overpressures, and between the inlet and outlet mean compressor pressures. Finally, the combined mechanical–electrical efficiency is related to the heat transfer losses/gains, and to the exergy transfers and exergy destroyed.Even though some of the concepts introduced in the paper can be applied to different kinds of compressors, the discussion is specially focussed on hermetic reciprocating compressors. An advanced simulation model developed by the authors has been used to generate data to illustrate the possibilities of the detailed thermodynamic characterization proposed. The criteria developed are useful tools for comparison purposes, to characterize compressors, and to assist designers during the optimization process.     

Effect of throat diameters of the ejector on the performance of the refrigeration cycle using a two-phase ejector as an expansion device

This paper is a part in a series that reports on the experimental study of the performance of the two-phase ejector expansion refrigeration cycle. In the present study, three two-phase ejectors are used as an expansion device in the refrigeration cycle. The effects of throat diameter of the motive nozzle, on the coefficient of performance, primary mass flow rate of the refrigerant, secondary mass flow rate of the refrigerant, recirculation ratio, average evaporator pressure, compressor pressure ratio, discharge temperature and cooling capacity, which have never before appeared in open literature, are presented. The effects of the heat sink and heat source temperatures on the system performance are also discussed.     

Design of experimental bench and internal pressure measurement of scroll compressor with refrigerant injection

Experiments on the inner compression process of scroll compressor with refrigerant injection can reveal the essence of refrigerant injection. The difficulty of the experiment is the design of location of measuring ports, measuring system of dynamic pressure and design of the injection system. Focusing on the dynamic pressure measurement of inner compression process during refrigerant injection, an integrated bench design method for refrigerant injection research in scroll compressor is presented in this paper. The location design of injection ports and measuring ports, frequency spectrum analysis of pressure signal, selection of the sensor type and configuration, and design of the pressure-leading system are expressed, respectively. Finally, a test bench is set up. Based on it, several elementary experiments were carried out. The results show that: this design method solves most problems in the experimental research of scroll compressor with refrigerant injection and works reliably; the refrigerant injection effects the majority of the inner compression process and should not be considered as a transient process; gas injection can increase the system performance greatly and there is an optimal injection pressure for a certain scroll compressor.     

Nucleation of supercooled water on solid surfaces

Heterogeneous nucleation of water was investigated using molecular dynamics simulation. Solid with fcc (111) surface was placed at the bottom of a cell consisting of 864 water molecules. ST2 model with NPT ensemble was used. The pressure and temperature were set to be 0.1 MPa and 275 K, respectively. The interaction between water and the solid was based on the equations proposed by Spohr. Exception was made on the lattice constant which was slightly modified to fit with that for ice structure. The shape of the solid surface was considered. It was found that the only one layer of water molecules was adsorbed in a case of a flat surface, whereas ice nucleation occurred by removing some of the atoms from the surface. Spohr's interaction was also modified so that the dipole moment of water became anti-ferroelectric. It was found that the modification increased the ice growth, further. The effect of lattice constant of solid on nucleation was also investigated. It was found that the variation on lattice constant with a few percent from that of ice was acceptable for nucleation, especially on shrinking side. On expanding side, however, it gave some gaps for water molecules to fit in other than that for ice structure, and it prevented the growth of ice. Hence, a guideline for the selection of ice nucleus material was obtained.     

Twin screw oil-free wet compressor for compression–absorption cycle

The performance of a twin screw compressor operating under wet (two-phase) compression conditions in an ammonia–water compression absorption heat pump cycle is investigated both theoretically and experimentally. The paper reports on the influence of the location of liquid intake or, depending what applies, injection angle and mass flow rate of the injected liquid on compressor performance. Labyrinth seals separate the oil-free process side from oil lubricated bearing housing. Labyrinth seals leakage is modelled and its impact on performance is theoretically and experimentally investigated. The need for liquid injection from the discharge side to obtain acceptable performance is discussed based on experimental results.     

Loss-efficiency model of single and variable-speed compressors using neural networks

Compressor is the critical component to the performance of a vapor-compression refrigeration system. The loss-efficiency model including the volumetric efficiency and the isentropic efficiency is widely used for representing the compressor performance. A neural network loss-efficiency model is developed to simulate the performance of positive displacement compressors like the reciprocating, screw and scroll compressors. With one more input, frequency, it can be easily extended to the variable speed compressors. The three-layer polynomial perceptron network is developed because the polynomial transfer function is found very effective in training and free of over-learning. The selection of input parameters of neural networks is also found critical to the network prediction accuracy. The proposed neural networks give less than 0.4% standard deviations and ±1.3% maximum deviations against the manufacturer data.     

Influence of liquid refrigerant injection on the performance of an inverter-driven scroll compressor

The operation of a scroll compressor at high compression ratios can cause excessively high discharge temperatures, which can be detrimental to the reliability and efficiency of the compressor. In the present study, the performance of an inverter-driven scroll compressor with liquid refrigerant injection was measured with a variation of compressor frequency, injection pressure, and injection location. The influence of the liquid injection on the performance is presented as a function of operating parameters and injection location by comparing the results with those for the non-injection case. It was found that liquid injection under high frequency was very effective at attaining higher performance and reliability of the compressor, but injection under low frequency showed some disadvantages. For high frequency at a given injection ratio, the injection at α=180°, for an injection angle at an injection port, yielded slightly better performance of the compressor as compared to that at α=90°.     

Heat and mass transfer characteristics in a spray chamber

This paper presents a one-dimensional mathematical model for heat and mass transfer of water droplets in a spray chamber. The model includes drop size distribution and velocity of the droplets generated by a nozzle of inlet diameter 3.2 mm. By using the conservation of mass and energy, the changes in water temperature, air temperature and humidity along the spray cone in the spray chamber can be calculated. This model is tested with two different water mass flows. The results look reasonable from practical point of view and they also show that higher water mass flow results in a higher air temperature drop and higher humidity.     

An experimental investigation on performance of bubble pump with lunate channel for absorption refrigeration system

An experimental research on the performance of the bubble pump for absorption refrigeration units was made. The bubble pump provides the drive for the absorption cycle and is a decisive component of the absorption refrigeration unit. The bubble pump's property determines the efficiency of the absorption refrigeration system. A continuous experimental system with different size of bubbles pumps were designed, constructed and successfully worked. The experiments were performed by changing some of the parameters affecting the bubble pump performance. The experimental results shows that the performance of the bubble pump depends mainly on the driving temperature, the solution head and the combining tube diameters. With the suitable size of section area of the pump tubes the net elevating height of solution is 2.5 times as high as the solution submergence. The lunate channel has several outstanding characteristics, such as low starting temperature (minimum 68 °C), wide operating temperature range and lower requirement for vacuum condition (under 10 kPa). Then the elevating capability of the bubble pump with lunate channel is much better than others currently. It would provide well foundation for practical applications.     

Introduction to the field of cryobiology and overview of selected papers

The article is a synopsis of the 9 papers in the field of cryobiology, particularly regarding the use of modern cryopreservation and related techniques for conservation of genetic resources of endangered species and economically important breeds of wild and domesticated animals and plants published in this issue of the International Journal of Refrigeration. The papers are related to vitrification, primarily of reproductive and progenitor cells as a swiftly expanding and effective approach of biostabilization, as well as to cryopreservation of spermatozoa, leucocytes, and other cells of the humans, sturgeon species, European mink, marine invertebrates, and plants. A brief introduction to cryobiology and related fields is also provided.     

Lubrication oil pumping by utilizing vane motion in a horizontal rotary compressor

For a horizontal rotary compressor which utilizes reciprocating motion of the vane for oil supply into lubrication elements, an analytical study has been carried out on the oil pumping mechanism. Energy equation has been applied to the oil flow inside the oil-conveying pipe with oil feeding hole in the middle. Oil distributions into individual lubrication elements such as various bearing elements have also been analyzed by applying electric circuit network theory to the oil flow network. Fairly good agreement between calculations and experiments for the oil pumping rate has been obtained in a wide range of compressor speed.     

Fundamental study of adhesion of ice to cooling solid surface (Discussion on influence of copper oxide layer on ice adhesion force)

In many situations, ice often adheres to a cooling solid surface, frequently causing serious accidents. It is critical to clarify the mechanism of ice adhesion to the cooling surface in order to prevent ice adhesion. In a past study, the shearing stresses of two kinds of test plates with a copper surface having the higher thermal conductivity were measured. The shearing stress corresponds to ice adhesion force. Both shearing stresses were significantly different; however, the cause remains unclear.Therefore, the present study focuses on an oxide layer as the main factor causing the difference of both shearing stresses; the influence of the oxide layer formed on shearing stress was discussed. And in the removal and reformation processes of the oxide layer, the time variation of the shearing stress was clarified. Moreover, the relationship between the state of the copper surface and the shearing stress was also clarified by surface analysis.     

Two-stage heat pump system with vapor-injected scroll compressor using R410A as a refrigerant

Refrigerant vapor-injection technique has been well justified to improve the performance of systems in refrigeration applications. However, it has not received much attention for air conditioning applications, particularly for air conditioning in hot climates and for heat pumping in cold climates. In this study, the performance of an 11 kW R410A heat pump system with a two-stage vapor-injected scroll compressor was experimentally investigated. The vapor-injected scroll compressor was tested with the cycle options of both flash tank and internal heat exchanger configurations. A cooling capacity gain of around 14% with 4% COP improvement at the ambient temperature of 46.1 °C and about 30% heating capacity improvement with 20% COP gain at the ambient temperature of −17.8 °C were found for the vapor-injected R410A heat pump system as compared to the conventional system which has the same compressor displacement volume.