Pengcheng Shu

Simulation of discharge pressure pulsation within twin screw compressors

Abstract Even in the absence of self-acting valves, flow through the discharge port of a twin screw compressor is oscillatory in nature. This unsteady but periodic flow variation at the discharge port excites the pressure pulsation, which is the main source of noise and vibration. In this paper, a mathematic model based on the one-dimensional unsteady gas flow equations is established to describe the discharge pressure pulsation, which considers the effects of friction and heat transfer between the gas and the pipe. The boundary conditions of the discharge pressure pulsation model are given in detail. The two-step Lax—Wendroff scheme is applied to solve the one-dimensional unsteady gas flow equations. In order to verify the theoretical analysis, the discharge pressure pulsations under various working conditions are measured and compared. It is shown that the model established in this paper is a useful tool to obtain a better understanding of the behaviour of the pressure pulsation in discharge pipe. It is found that the most important factor that affects the discharge pressure pulsation is the pressure difference between the pressure upstream of the discharge port in the compression chamber and the pressure in the discharge line some way downstream. The minimum fluctuation occurs when the discharge pressure is equal to the pressure corresponding to the built-in volume ratio. For a constant discharge pressure, the pressure pulsation amplitude increases with the rotational speed.

An Experimental Study on Energy Recovery by a Pelton-Type Expander in a Domestic Refrigeration System

A Pelton-type two-phase expander was developed to replace the throttling valve for energy recovery in a domestic vapor-compression refrigeration system. The Pelton-type expander was designed and fabricated based on thermodynamic and mathematical analysis. It consists of two Laval nozzles and one simplified twin-arc blade impeller. A combined expander-generator prototype was validated in a R-410A domestic refrigeration system and demonstrated a stable operation within a wide speed range of up to 26,500 rpm. Experimental results showed the feasibility of integrating the Pelton-type expander into a domestic refrigeration system. A maximum isentropic efficiency of 32.8% was achieved under the experimental conditions. The relationships between the recovery work, the expander efficiency, and the impeller rotational speed were investigated. The results indicated that the velocity ratio was around 0.17 when the recovery work achieved the maximum value. The results also showed that the increase in the cooling capacity by 6.5% and the coefficient of performance (COP) by 5.4% could be obtained under the testing conditions.

Development of the free piston expander for work recovery in transcritical CO2 refrigeration cycle

Abstract Replacing the throttle valve with an expander is beneficial to the performance improvement on the transcritical CO2 refrigeration cycle. The expander is hereby developed for this purpose in this paper. With the required working conditions, performance potential, reliability, etc. considered, the free piston machine is selected as the most suitable concept after a comparison among some candidates. The single acting structure is adopted, in which an auxiliary compressor is arranged on a common shaft to utilize the work recovered by the expander. A slider-based scheme is applied to control the expander inlet/outlet, and an adjustable clearance volume is designed to modulate the operating ranges of the low/high pressures. A design model is established to determine the geometric parameters of the expander-compressor unit. A prototype expander is manufactured and preliminary test is carried out on a test rig with both air and CO2 as the working fluid. The experimental results show that the expander-compressor unit can work in some ranges of the low/high pressures, which proves that this type of expander concept as well as the inlet/outlet control method is feasible. A serious pressure pulsation is observed at the expander inlet, resulting in considerably large pressure drop during the filling process of the expander. The work to improve the expander is therefore being done, with the attenuation of the pressure pulsation focused on.

Research on oil-free hermetic refrigeration scroll compressor:

Abstract A prototype of the oil-free hermetic scroll compressor for the application of the refrigeration system of the aviation devices was constructed. The performance of this prototype oil-free scroll compressor was simulated using a thermodynamic model, and was measured. And it was concluded that the prototype oil-free scroll compressor is useful for the refrigeration system of the aviation devices. The simulated results were consistent with the measured ones within less than 10 per cent of the mean deviation. When the evaporating and condensing temperatures were 5 and 55°C, respectively, the volumetric efficiency, discharge temperature, and coefficient of performance were 0.61, 135°C, and 1.45, respectively. The prototype oil-free scroll compressor had been operating reliably for more than 200 h under 85.7 per cent duty cycle and 20 h for 100 per cent duty cycle.

Wrap of cylinder and its effect on main features of rotary vane compressor for automobile air conditioning system

Abstract The rotary vane compressor has been used in the automobile air conditioning system. One of the key factors of its design is the wrap of its cylinder. From the working principle of this type of compressor, it is explained in mathematics why the wrap is composed of four parts: arc of circle, main curve, and two segments of transit curves, and the conditions that the wrap needs to satisfy are derived simultaneously. At the same conditions, we compared to the gas force F p , contact force F nc of the blade head, the force acting on rotor by blades, and the suction volume V s for three kinds of main curves. The wrap composed of trigonometric function can form the biggest value of V s , while the least suction volume V s will be reached as the cubic polynomial is taken. The wrap composed of trigonometric function forms the biggest absolute value of the gas force F p and the middle peak value of the force acting on rotor by blades, while the least absolute value of the gas force F p and the biggest contact force F nc of blade head will be reached as the cubic polynomial is taken. The force acting on the rotor by blades takes the biggest peak value if the main curve is the ellipse curve. The frequency of the force acting on the rotor by blades is the same as the number of blades at the range of the crank angle from 0 to 180°.

Experimental Analysis of Mass Composition of R417A in Presence of Leak/Recharge in a Heat Pump Water Heater

As a substitute for R22, an environmentally friendly zeotropic mixture R417A is often employed. The large glide temperature of the zeotropic mixture R417A could cause capacity and efficiency reductions due to changes in its composition after an isothermal vapor leak/recharge process. It is, therefore, necessary to predict the composition change in R417A under all leak conditions. This paper presents the experimental analysis of the mass composition of R417A in the presence of a 0–50% isothermal vapor leak/recharge in a heat pump water heater. Based on the experimental data, a simple model is proposed to describe the composition change in R417A for various leaks and recharge scenarios. The effect of composition change in the performance of a heat pump water heater is also dealt with. Experimental results show that the composition changes in R417A increase as ambient temperature decreases. Isothermal vapor leaks have a great effect on the composition of R417A and the performance of a heat pump water heater at lower temperatures. It is also found that when the ambient temperature is higher than 7°C with a 10% leak and R417A recharge, there is almost no difference in the performance of the plant with respect to the original state except that the compressor discharge temperature is lower.

Numerical modelling of heat transfer between gas and solid in a vibrated fluidised bed

Abstract Due to its high gas–solids heat transfer efficiency, a vibrated fluidised bed is suggested to be suitable in cryogenic comminution system as a pre-cooler. It is of great significance to use high efficient facility to minimise the effect of low thermal conductivity of rubber on the heat transfer between gas and rubber particles. The purpose of this paper is to establish a mathematical model to describe the characteristics of heat transfer between gas and solid in vibrated fluidised bed with the aid of computer simulations. In order to simplify the modelling, this paper presents the hypothetic particle motion style and gas velocity distribution. After the control equations have been presented, discretised and solved, the temperature profile in rubber particle was obtained according to certain boundary and initial conditions. Computational results were verified by means of comparing with experimental data. There was a good consistency between them.

Research on the reliability of a scroll compressor in a heat pump system

Abstract The scroll compressor is used in heat pump and refrigeration system equipment because of important features such as its compact size, high efficiency, low vibration and noise level, etc. In the refrigeration cycle the scroll compressor runs smoothly and perfectly, but in the heat pump cycle some problems appear because of deterioration in the working condition. The effect of different factors such as atmospheric condition, electric voltage fluctuation, the configuration of the scroll compressor, etc., on the running reliability of the heat pump system are discussed in this paper, thermodynamic behaviour is examined and some basic design concepts for running reliability of the scroll compressor in the heat pump system are pointed out. Experimental results show that the temperature difference is about 10 K between the gas discharged from the outlet of the compressor and the wound wire of the motor in a high-pressure shell compressor. Calculated and test results show good agreement. The wound wire temperature of the motor in a low-pressure shell is much lower than that in a high-pressure shell scroll compressor. Hence, the low-pressure shell scroll compressor has higher running reliability than the high-pressure one.

A Study on Running Reliability of Scroll Compressor in Heat Pump System

Scroll type of compressor has been used in heat pump & refrigeration system because of its compact size, high efficiency, low vibration and noise level, and excellent running reliability. In the refrigeration cycle the scroll compressor runs smoothly and perfectly, but in the heat pump cycle some troubles appear because its working condition becomes worse. The effect of different factors such as atmospheric condition, electric voltage fluctuation, structure of scroll compressor etc. on running reliability of heat pump system are discussed in this paper. The thermodynamic and aerodynamic behaviors will be emphasized. Some basic design concepts for the running reliability of scroll compressor in the heat pump system are pointed out.Copyright