Yanzhong Li

Three-Dimensional Numerical Simulation on the Laminar Flow and Heat Transfer in Four Basic Fins of Plate-Fin Heat Exchangers

In this paper, four basic fins of the plate-fin heat exchangers, rectangular plain fin, strip offset fin, perforated fin, and wavy fin, are modeled and simulated by taking account of fin thickness, thermal entry effect, and end effect. Three-dimensional numerical simulations on the flow and heat transfer in the four fins are investigated and carried out at laminar flow regime. Validity of the modeling technique is verified by comparing computational results with both corresponding experimental data and three empirical correlations from literatures. Global average Colburn factor (j factor) and friction factor (f factor) and their local 1D streamwise-average distributions along the fins are presented by introducing data reduction method. The heat transfer behaviors in both the developing and developed regions are analyzed by examining variations of the local Nusselt number along the flow direction. It is found that the thermal entry length of the four fins might be expressed in the format of L e =c l Re c2 Pr D h, which has the same form as the one in a circular tube.

Fluid Flow Distribution and Heat Transfer in Plate-Fin Heat Exchangers

Single-phase and two-phase flow distribution in plate-fin heat exchangers and the influence of nonuniform fluid flow distribution on the thermal performance of such heat exchangers were experimentally investigated. The experimental results show that flow maldistribution can be a serious problem in plate-fin heat exchangers because of nonoptimized header configurations. The uneven distribution of two-phase flow in plate-fin heat exchangers is more pronounced than that of single-phase flow. It is shown that the uneven distributions result in a significant deterioration of the heat transfer performance. The relationship between the flow maldistribution characteristics and the resulting loss in heat exchanger effectiveness has been studied in this work. Certain improved header configurations with perforated plates were proposed in order to solve the maldistribution problem. It was found that the new header configurations could effectively improve the thermal performance of plate-fin heat exchangers. By changing the header configuration, the degree of flow and temperature nonuniformity in the plate-fin heat exchanger was reduced to 16.8% and 74.8%, respectively, under the main test condition.

Annual performance evaluation and experimental study on variable speed compressors for room air conditioners

Energy efficiency for room air conditioners has received increasing attention in recent years with the intense emphasis on environmental concern and energy consumption. As the main energy consumption component of air conditioner, compressor awaits further work to accomplish the transition of performance evaluation from one operation condition to its actual usage. In the present study, an evaluation index called annual efficiency is proposed to rate the annual performance of air conditioner compressor. Experimental study is conducted on four variable-speed compressors, and the results show that the best fit determined from annual efficiency is different from that of ASHRAE standard. The compressor efficiency and dynamic pressure are quite different at rated and half-capacity test conditions. Compressor running under half-capacity conditions is more energy efficient and peak phenomenon of dynamic pressure is less obvious when compared with rated conditions. Moreover, more attention should be paid to half-capacity conditions to improve the annual efficiency of compressor and energy-saving capabilities of air conditioners.

Numerical modeling and analysis of ejector in the proton exchange membrane fuel cell system

Computational Fluid Dynamics (CFD) technique is employed to study the performance of ejector in the fuel delivery line of polymer electrolyte membrane (PEM) fuel cell system. Four different ejector geometries are established and tested under 34 different working conditions. It is found that the humidity at the ejector exit increases with the pressure ratio αP at first then decreases with it. The humidity always decreases with increasing the diameter ratio αD. Results show that there is an inverse relationship between the humidity and the entrainment ratio of the ejector.

Study on thermal stratification in rocket liquid oxygen tank with natural circulation precooling loop

A numerical investigation is performed on the physical field of rocket tank with natural circulation precooling loop based on computational fluid dynamics (CFD) technique. The results show that convection heat transfer is dominant in the bulk above the return inlet position and the heat conduction takes place chiefly below the position. The axial thermal stratification is produced in the main region of tank and the radial stratification appears only in the bottom, and the lowest temperature region moves towards the opposite bottom header region depart from the sidewall near the return position.

Transient Analysis of a Rotor-Journal Bearing System under Large and Dynamic Loads

A special rotor-journal bearing system under large and dynamic loads is studied in this paper, which originates from the rotary compressor. Its a flexible system because of the slender shaft and two thin wall bearing housings.It is considered as a fluid-structure interaction problem. The finite element method is applied to approach the rotor system, bearing housings and nonlinear fluid films. The coupling equations are resolved with Newton-Raphson method. Based on the method, the dynamic behaviors of the system with different balancing configurations at different speeds are analyzed.