Distributed and Non-Steadv State Modelling of an Air Cooler

Th•s paper presents the equation derivation as well as the stmulation results of a non-steady state distributed model for a commonly used type of evaporators -air coolers. In contraSt to others, the model puts the SU"Css on the rcfr~gcram mass transponation occuring in the two-phase flow region mside an air cooler. The so~led slip-effect between the vapour and liquid IS taCkled by using an advanced computer package PHOENICS (sec [27)). Because the model is purely distributed, it is apphcable to various kinds of tube circuit ammgements of air coolers. The purpose of the model is studying and opumiution of non-steady state behaviour of refrigerating systems with capacity control. Nomenclature Symbols Physical Meanings Unirs A Cross-secuon area [m'] B Constant c Constant Spectfic heat [kJ/(kg · K)] D Outer diameter of pipes [mj d Inner diameter of pipes [m] F Friction force [N/m' l f Interfacial momenwm-transpon coefficient [kgl(m' s)] Specific enthalpy [kJ/kgJ L Length [m] M Mass [kg] M Mass flow rate [kgls] m Interfacial mass-lranspon rate [kg/(m' sll p Pressure [N/m' I Q Heat transfer [kwl q Heat flux [kw/m' J s Surface area [m' I T Absolute temperature [KI I Time lsi u Velocity [m/sl Velocity fm/s] v Volume [m' l w Water-vapour concentration Jn air [kg/kg] w Water-vapour flux [kgl(m' s)l Quality Coordinate [mj Greek Symbols <X Heat transfer coefficient [kw/(K · m' ) ) <CD Void fracuon Latent heat of water fkJ/kgJ '1 Fin efficiency Temperature r c1 Friction factor Density [kglm' J