Transient simulation of a sub- to transcritical CO2 refrigeration cycle

Garces de Gois, Michael (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The purpose of this project is to develop a numerical transient simulation model of the CO2 refrigeration cycle containing a capillary tube as expansion device. A design procedure for CO2 refrigerators is also developed; which makes use of a steady-state capillary tube model for acquiring the initial design of the capillary tube, and uses the transient simulation program to determine the low pressure side internal volume and validate the complete design of the refrigerator. Using this design procedure a CO2 refrigerator was designed, built and then tested to validate the simulation model developed. The project was conducted at the Department of Mechanical and Mechatronic Engineering at the University of Stellenbosch. A literature review on the history of refrigeration, the vapour-compression refrigeration cycle, the transcritical CO2 refrigeration cycle, refrigerant properties, CO2 properties, and a comparison of refrigerant performance was done as background to the use of CO2 as refrigerant. Furthermore, literature review of an alternative expansion device, the vortex tube, was included, but found not practical for implementing with CO2 which behaves more like a real gas and not an ideal gas. Lastly, theory on the development of the one-dimensional CFD finite volume method in terms of a general variable is shown. This provides background to the development of discretised conservation equations for the simulation model. The transient model developed is capable of predicting transient operation from stand-still starting conditions through to steady-state conditions. The model makes use of well-known pipe friction and heat transfer correlations. Furthermore, it makes use of the real gas equation of state for CO2 from Span and Wagner (1996). Moreover, the model determines whether the flow is single - or two-phase and then calculates the appropriate properties. Lastly, it has the whole refrigerant circuit, from compressor outlet back to compressor inlet, as the simulation domain. The simulation results for the pressure and temperature distributions show the transient behaviour at start-up. The transient and steady-state results also agree fairly well with the experimental results. The steady-state pressure graphs are as expected and the constant evaporation temperature is a confirmation that the model approximates real life operation quite well. In conclusion, the results show that the simulation model is a useful tool for designing and understanding capillary tube CO2 refrigeration cycles. Future work involves developing a nonadiabatic compressor model and models for alternative expansion devices to be included in the current refrigerator simulation model.

AFRIKAANSE OPSOMMING: Die doel van hierdie projek is om ’n numeriese oorgangsfase-simulasie-model van ʼn CO2 verkoelingstelsel met ’n haarbuis as uitbreidingstoestel te ontwikkel. ’n Ontwerp prosedure vir CO2 yskaste word ook ontwikkel; wat gebruik maak van ʼn bestendige toestand haarbuis model vir die verkryging van die aanvanklike ontwerp van die haarbuis, en maak gebruik van die oorgangsfase-simulasie program om die lae druk kant se interne volume te bepaal en die geldigheid van die volledige ontwerp van die yskas te bevestig. Deur die toepassing van hierdie ontwerp prosedure is ʼn CO2 yskas ontwerp, gebou en dan getoets om die simulasie program wat ontwikkel is te valideer. Die projek is voltooi by die Departement van Meganiese en Megatroniese Ingenieurswese aan die Universiteit van Stellenbosch. ’n Literatuuroorsig oor die geskiedenis van verkoeling, die damp-samedrukkings verkoelingsiklus, die transkritiese CO2 verkoelingsiklus, koelmiddel eienskappe, CO2 eienskappe, en ’n vergelyking van koelmiddel prestasie is gedoen as agtergrond vir die gebruik van CO2 as koelmiddel. Verder is ’n literatuuroorsig van ’n alternatiewe uitbreidingstoestel, die vortex buis, ingesluit, maar daar is gevind dat hierdie toestel nie praktiese is vir die implementering met CO2 nie; omdat CO2 eerder as ʼn werklike en nie ʼn ideale gas optree nie. Laastens word die teorie oor die ontwikkeling van die een-dimensionele numeriese vloeidinamika eindige volume metode in terme van ’n algemene veranderlike getoon. Dit bied agtergrond tot die ontwikkeling van die gediskritiseerde behoudsvergelykings vir die simulasie model. Die ontwikkelde oorgangsfase-model is in staat om die oorgangsfase te voorspel van die rustende begintoestand tot by die bestendigetoestand. Die model maak gebruik van bekende pyp wrywing en hitte-oordrag korrelasies. Verder, maak dit gebruik van die werklike gas toestandsvergelyking vir CO2 van Span en Wagner (1996). Nog verder, bepaal die model of die vloei enkel- of twee-fase is, en bereken dan die geskikte eienskappe. Laastens, dit het as die simulasie domein die hele koelmiddel kringloop vanaf die kompressor uitlaat terug na die kompressor inlaat. Die simulasie resultate vir die druk- en temperatuurverspreidings wys die gedrag van die oorgangsfase vanaf begin toestand. Die oorgangsfase en bestendigetoestand resultate stem redelik goed ooreen met die eksperimentele resultate. Die bestendigetoestand drukgrafieke is soos verwag, en die konstante verdampingstemperatuur is ’n bevestiging dat die model werklike werking baie goed benader. Ten slotte, die resultate toon dat die simulasie model ʼn nuttige instrument is vir die ontwerp en begrip van haarbuis CO2 verkoelingsiklusse. Toekomstige werk behels die ontwikkeling van ’n nie-adiabatiese kompressor model en simulasie modelle vir alternatiewe uitbreidings toestelle wat in die huidige yskas-simulasie model geïmplementeer kan word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98630
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