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Da Riva, Enrico (2009) Two-phase Heat Transfer in Minichannel Heat Exchangers: Heat Pump Applications, Design, Modelling. [Tesi di dottorato]

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Abstract (inglese)

Refrigerant charge minimization is one of the most important targets for heating and air conditioning applications when using natural refrigerants like hydrocarbons or ammonia to cope with the new environmental challenges.
Some applications of minichannels for charge minimization in heat pumps are presented and discussed in this thesis.
The design of an innovative condenser, an evaporator and an internal heat exchanger is presented. These devices are shell-and-tube heat exchangers using 2 mm i.d. minichannels and realized for the use with propane. Computational procedures based on empirical correlations available in the literature and a simplified model of the heat transfer and pressure drop processes have been developed and used for the design. Experimental performance data of the heat exchangers when using R22 and propane is reported and compared against the predictions given by the computational procedures.
The shell-and-tube minichannel heat exchangers have been installed in a 100 kW heat pump using propane as the refrigerant. The unit has been designed for laboratory tests and the minichannel shell-and-tube heat exchangers have been installed in the facility together with a conventional plate condenser and an evaporator. Different configurations have been tested in order to quantify the advantages of operating the heat pump using the low charge heat exchangers, with regard to both energy performance and propane charge. In particular, the experimental performance when using the minichannel condenser is compared to the one obtained when using the plate condenser, and the influence of the internal heat exchanger on the performance of the equipment is measured and discussed. Experimental data about the efficiency with propane of the semihermetic compressor installed in the heat pump is also reported.
Besides empirical correlations to predict the global thermal performance, a more complete understanding of the two-phase flow and heat transfer in minichannels is needed for the design and optimization of heat exchangers.
Some CFD simulations are presented in this thesis using the innovative Volume Of Fluid (VOF) method, which is able to directly compute multiphase flows without using any empirical closure law to model the interaction between the phases.
In order to assess the capability of the method to compute the motion of the gas-liquid interface, which is crucial for two-phase flow and heat transfer, simulations of the adiabatic churn flow regime of air-water mixture at different pipe diameters and liquid and gas superficial velocities have been initially performed. A comparison of the numerical results with experimental visualizations is reported and a simplified theoretical model of the wave levitation process has been developed and used to explain the numerical results. The VOF simulations were then extended to the study of condensation of R134a inside a minichannel with 1 mm internal diameter. Computational results about the evolution of the vapour-liquid interface and the heat transfer coefficient along the channel are reported.

Abstract (italiano)

La riduzione della carica di refrigerante nelle applicazioni di condizionamento e riscaldamento è uno dei vincoli di progetto principali quando vengono utilizzati, per motivi di carattere ambientale, refrigeranti naturali come idrocarburi ed ammoniaca.
Alcune applicazioni dei minicanali per la minimizzazione della carica nelle pompe di calore vengono presentate e discusse nella presente tesi.
Viene presentato il progetto di un condensatore, un evaporatore ed uno scambiatore di calore rigenerativo innovativi. Questi componenti sono degli scambiatori di calore a fascio tubiero utilizzanti minicali del diametro di 2 mm e progettati per l’uso con propano. Delle procedure di calcolo basate su di correlazioni disponibili in letteratura ed un modello semplificato del processo di scambio termico sono state utilizzate per il progetto. Le prestazioni sperimentali degli scambiatori con R22 e propano vengono riportate e confrontate con le stime fornite dalle procedure di calcolo.
Gli scambiatori di calore sono stati installati in una pompa di calore della capacità termica di 100 kW utilizzante propano come fluido frigorigeno. Nell’impianto della pompa di calore, destinata a test di laboratorio, sono stati installati anche un condensatore ed un evaporatore a piastre convenzionali. In questo modo è stato possibile confrontare diverse configurazioni al fine di quantificare in via sperimentale i vantaggi apportati dall’utilizzo degli scambiatori a minicanali, in termini sia di prestazioni energetiche, sia di carica di propano richiesta. In particolare, le prestazioni delle configurazioni utilizzanti il condensatore a minicanali vengono confrontate con quelle delle configurazioni utilizzanti lo scambiatore a piastre, e l’influenza sulle prestazioni energetiche dello scambiatore rigenerativo viene misurata e discussa. Vengono inoltre riportati dati sperimentali relativi all’efficienza con propano del compressore semiermetico installato nella pompa di calore.
Oltre a correlazioni empiriche in grado di stimare le prestazioni termiche globali, il progetto e l’ottimizzazione di scambiatori di calore richiede una più approfondita conoscenza del deflusso e dello scambio termico all’interno di minicanali.
Vengono presentate in questa tesi delle simulazioni di termofluidodinamica computazionale tramite l’innovativo metodo VOF (Volume Of Fluid) in grado di simulare direttamente deflussi multifase senza la necessità di utilizzare correlazioni empiriche per la modellazione dell’interazione tra le fasi.
Al fine di validare l’efficacia di questo metodo nel calcolare il moto dell’interfaccia gas-liquido, il quale è un aspetto cruciale nello scambio termico bifase, sono state in un primo momento eseguite delle simulazioni del regime di deflusso ”churn flow” per una miscela aria-acqua nel caso di un tubo liscio verticale adiabatico, a differenti valori di diametro del tubo e di velocità superficiale delle due fasi. I risultati sono stati confrontati con visualizzazioni sperimentali ed un modello teorico semplificato del processo di levitazione delle onde è stato sviluppato ed utilizzato per commentare i risultati numerici.
Le simulazioni con il metodo VOF sono state in un secondo momento estese allo studio della condensazione di R134a all’interno di un minicanale del diametro di 1 mm. Vengono riportati risultati computazionali relativi all’evoluzione dell’interfaccia vapore-liquido e dei coefficienti di scambio termico lungo il minicanale.

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Tipo di EPrint:Tesi di dottorato
Relatore:Cavallini, Alberto
Correlatore:Del Col, Davide
Dottorato (corsi e scuole):Ciclo 21 > Scuole per il 21simo ciclo > INGEGNERIA INDUSTRIALE > FISICA TECNICA
Data di deposito della tesi:30 Gennaio 2009
Anno di Pubblicazione:Gennaio 2009
Parole chiave (italiano / inglese):Minichannel, natural refrigerants, VOF, condensation, churn flow, heat pump, propane
Settori scientifico-disciplinari MIUR:Area 09 - Ingegneria industriale e dell'informazione > ING-IND/10 Fisica tecnica industriale
Struttura di riferimento:Dipartimenti > Dipartimento di Fisica Tecnica
Codice ID:1784
Depositato il:30 Gen 2009
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