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Evaluation of dynamic models for refrigeration system components : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Process and Environmental Technology at Massey University
There is a paucity of proven models for predicting the energy transients of walls used in low temperature applications and pressure vessels commonly used in refrigeration plants. The aim of this work was to investigate how the accuracy of feasible models for these situations was affected by changing model complexity. Wall models were developed by assuming each wall layer could be represented by one of five possible thermal behaviours: null, resistance only, capacity only, alternating resistance and capacity (lumped) or fully distributed resistance and capacity. A range of feasible models for each of four common low temperature wall types was investigated by comparison of simulated behaviour to predictions by a finite element model (which was itself validated by comparison to experimental data for wall systems). Several model evaluation measures are presented to aid engineering judgement in selecting appropriate wall models for particular applications. Only resistance needs be considered for accurate prediction of mean heat flux entering a room. It is recommended that metal layers be represented by capacity only models, thin insulation layers by resistance only models, thicker insulation layers by lumped or fully distributed models, and concrete layers by lumped or fully distributed models. The recommended number of zones within a lumped or distributed model for a layer rises as the amplitude of the expected repeating temperature cycle for that layer increases. Four models of different complexity were derived to represent a typical industrial intercooler (pressure vessel). These models were tested by comparison of predictions to the measured time-temperature response of two pilot plant calorimeters containing R134a, when subjected to changing heat inputs. The measured response rate was most strongly influenced by sensible heat storage in the calorimeter shells and liquid refrigerant. Little difference in predictions by the four models was obtained in spite of the less complex models neglecting many known physical phenomena. A model considering only the thermal capacity in the shell and liquid refrigerant predicted rates of temperature change within 10% of predictions by all other models, and also close to the experimental data. An industrial case study suggested that the conclusions from the calorimeter study may be valid over much wider ranges. Suggestions are made on ways to improve the simplest model accuracy, and to gain greater benefit from the more complex models.
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Estrada-Flores, S., Cleland, A. C., & Cleland, D. J. (1995, August 20-25). Modelling of thermal behaviour of walls for low temperature applications: sandwich panel type. Paper presented at the 19th International Congress of Refrigeration, The Hague, the Netherlands.