Distributed and non-steady-state modelling of an air cooler

Abstract The refrigerant flow inside the coils of a dry expansion plate-finned air cooler can be distinguished into two completely different types: two-phase flow and single-phase flow. The most difficult part of non-steady-state modelling of an air cooler is to describe the liquid and vapour mass transport phenomena occurring in the two-phase flow region, as this determines the boundary position between the two regions and then the superheat temperature, which is in turn the feedback signal of the thermostatic expansion valve. In fact, the mass transport is mainly governed by the momentum exchange between refrigerant liquid and vapour, which is usually called slip-effect. Because the momentum or force equilibrium is so fast compared to the thermal equilibrium, the slip-effect can be considered as a steady-state phenomenon. With this assumption, the mass transport in an air cooler can be described by using a simple propagation equation. The steady-state slip-effect, however, is found by solving the momentum equations for one-dimensional two-phase flow using advanced computer packages such as phoenics . This paper presents the derivation of the equations in non-steady-state modelling of an air cooler as well as the results obtained from the model. Because the model is purely distributed, it is applicable to various kinds of tube circuit arrangements of air coolers. The purpose of the model is studying and optimization of non-steady-state behaviour of refrigerating systems with capacity control.

[1]  W. F. Stoecker.  Stability of an Evaporator-Expansion Valve Control Loop , 1966 .

[2]  R W James,et al.  A MODEL OF A MARINE CHILLED WATER PLANT FOR MICROPROCESSOR CONTROL DEVELOPMENT , 1980 .

[3]  J. S. Van der Meer.  Simulation of a refrigerant evaporator , 1987 .

[4]  F. C. McQuiston.  Finned tube heat exchangers: State of the art for the air side , 1980 .

[5]  B. L. Bhatt,et al.  A system mean void fraction model for predicting various transient phenomena associated with two-phase evaporating and condensing flows , 1978 .

[6]  Rolf Isermann,et al.  Modeling and Control of a Refrigerant Evaporator , 1985 .

[7]  S. Levy,et al.  Steam Slip—Theoretical Prediction From Momentum Model , 1960 .

[8]  G. Wallis.  One Dimensional Two-Phase Flow , 1969 .

[9]  S. Zivi.  Estimation of Steady-State Steam Void-Fraction by Means of the Principle of Minimum Entropy Production , 1964 .

[10]  J. Thome,et al.  Convective Boiling and Condensation , 1972 .

[11]  W. T. Hancox,et al.  A general technique for the prediction of void distributions in non-steady two-phase forced convection , 1971 .

[12]  J. MacArthur.  Transient heat pump behaviour: a theoretical investigation , 1984 .

[13]  R. W. James,et al.  Dynamic Analysis of an Industrial Refrigeration System to Investigate Capacity Control , 1975 .

[14]  D. Didion,et al.  A simulation model of the transient performance of a heat pump , 1982 .

[15]  W. F. Stoecker,et al.  Theoretical Model for Predicting the Transient Response of the Mixture-Vapor Transition Point in Horizontal Evaporating Flow , 1968 .

[16]  Piet M.T. Broersen,et al.  Hunting of Evaporators Controlled by a Thermostatic Expansion Valve , 1980 .

[17]  J. MacArthur,et al.  Unsteady compressible two-phase flow model for predicting cyclic heat pump performance and a comparison with experimental data , 1989 .