A DISTRIBUTED MODEL FOR PREDICTING THE TRANSIENT BEHAVIOUR OF A MICROFIN TUBE EVAPORATOR
The microfin technology is gaining popularity because of it enhances heat transfer substantially without causing a large increase in pressure drop. It is known that, per unit tube length, heat transfer coefficients of helical microfin tubes are about twice as large as those for plain tubes. So in order to improve the thermal performance of the air-conditioning (AC) and the heat pump (HP) systems, the tube-fin heat exchangers of these systems are made of microfin tube with different diameter and geometries. The present paper investigates the transient behaviour of the microfin tube evaporator for a window air-conditioner charged with R22 and R410A as refrigerants. As R22 is one of the chemicals, being phase-out for the environmental protection pursuant to an international agreement the Montreal Protocol on substance that Deplete Ozone Layer (UNEP, 1991), the R410A (R32/R125, 50％/50％) an azeotropic mixture is introduced as an acceptable substitute for it (EPA, 1996). The knowledge of the transient behaviour is important for an evaporator for controlling the position of the interface (liquid-vapour) point close to the exit. This will allow the maximum efficiency and prevent any liquid refrigerant to enter in the compressor. A distributed parameter model is developed by discritizing the continuity, momentum and energy equations over the space and time span, using implicit finite difference approach. An efficient iterative method is used to obtain the numerical solution of the model. Simulation programme is developed using MATLAB software package linking with REFPROP for refrigerant properties. The liquid-vapour slip in the two-phase region is accounted by the void fraction model (VFM) proposed by Yashar et al., 2001 and the heat transfer coefficient is calculated by the correlation proposed by Kandlikar et al., 1997 for a microfin tube evaporator. The simulation results include the comparison of transient behaviour of a smooth and microfin tube evaporator using R22 and R410A as refrigerants under the step increase and decrease in refrigerant mass flow rate. It is observed that due to the higher heat transfer coefficient for the microfin tube evaporator the liquid dry-out occurs 1.296 m earlier than the smooth tube. It is also observed that the step change in the refrigerant mass flow can control interface position to get uniform air temperature distribution of the coil.
R.G.KAPADIA SANJEEV JAIN R.S.AGARWAL
Department of Mechanical Engineering, S.V.M.Institute of Technology, College Campus,Old NH No-8, Bha Department of Mechanical Engineering, Indian Institute of Technology, Delhi, Hauz Khas,New Delhi, 11
国际会议
The 22nd International Congress of Refrigeration(第22届国际制冷大会)
北京
英文
2007-08-21（万方平台首次上网日期，不代表论文的发表时间）