Experimental study of two-phase flow and heat transfer during vaporization, filmwise and dropwise condensation of refrigerants and steam inside small diameter channels

In this Thesis, film condensation, flow boiling and adiabatic two-phase flow in minichannels are investigated. Also an experimental study on dropwise condensation on plane surfaces in a small diameter channel is presented. Two are the main goals of this work: the investigation of environmentally friendly refrigerants and the enhancement of two phase heat transfer. Over the past decades, increased attention to environmental problems coupled with the request for compact and efficient heat transfer devices has brought substantial changes and improvements in the technique of refrigeration. The use of new refrigerants with very low values of global warming potentials (GWP) in minichannels, microchannels and nanochannels have gained quite an attention in engineered systems. In particular the fluid R1234yf, which has a GWP equal to 4, has emerged as the global replacement refrigerant of R134a in future mobile air conditioning systems. In the present Thesis R1234yf single phase and two-phase heat transfer and pressure drop are investigated with experiments inside two different minichannels. Tests have been performed inside a circular and a square single minichannel having 0.96 and 1.23 hydraulic diameter respectively. Heat transfer coefficients measured during filmwise condensation, at 40°C saturation temperature, and during vaporization, at 30°C saturation temperature, are compared to the ones previously obtained with R134a. A comparison of two-phase pressure drop during adiabatic flow of these two fluids is also performed. With the aim of investigating the validity of conventional theory in the micro-scale, two new test sections made of circular minichannels, with 0.96 and 2 mm inner diameter, are designed and realized specifically for single phase and two phase pressure drop measurements. The good agreement found with classical macroscale models is used to validate a technique for determining the hydraulic diameter of ducts from pressure drop measurement in laminar flow. With the motivation for improved energy efficiency and miniaturization of the heat exchangers a study in the area of enhanced condensation is also performed. Nanoengineered copper surfaces are investigated with the aim of enhancing the heat transfer coefficient during condensation by promoting the dropwise condensation mode (DWC) instead of the film mode. A thermosyphon twophase test loop is designed to measure the condensation heat transfer coefficients attained during steam condensation at a saturation temperature of 110 °C. A small size rectangular channel (3.6 mm hydraulic diameter) is realized in order to reproduce the effect of the vapour shear, reproducing the real conditions that occur inside a plate heat exchanger. The experimental apparatus allows the measurements of heat transfer coefficient during condensation and the visualization of the droplets movements. A simple wet-chemical fabrication process is used to obtain a superhydrophobic copper surface. The heat transfer performances of the superhydrophobic surface and of some polished copper surfaces are investigated during dropwise condensation.In questa Tesi sono studiati la condensazione a film, l’ebollizione convettiva e il deflusso adiabatico bifase all’interno di minicanali. Inoltre è presentato uno studio sperimentale riguardante la condensazione a gocce su superfici piane all’interno di un canale di piccole dimensioni. Due sono gli obiettivi principali di questo lavoro: lo studio di fluidi refrigeranti eco-compatibili e il miglioramento dello scambio termico bifase. Negli ultimi decenni, la sempre maggiore attenzione ai problemi ambientali, abbinata alla richiesta di dispositivi di scambio termico sempre più compatti ed efficienti ha portato notevoli cambiamenti e miglioramenti nella tecnica di refrigerazione. L'uso di nuovi refrigeranti con ridotto effetto serra (GWP) in minicali, microcanali e nanocanali ha assunto particolare attenzione nei sistemi ingegneristici. In particolare il fluido R1234yf, che ha un GWP pari a 4, è emerso come il sostituto del refrigerante R134a nei futuri impianti di climatizzazione nell’industria automobilistica. In questa Tesi sono presentati i coefficienti di scambio termico e le perdite di carico misurati durante deflusso monofase e bifase di R1234yf all’interno di due differenti minicanali. Le prove sperimentali sono state realizzate all’interno di un minicanale a sezione circolare e di un minicanale a sezione quadrata aventi rispettivamente un diametro idraulico pari a 0.96 e 1.23 mm. I coefficienti di scambio termico, misurati durante condensazione a film alla temperatura di saturazione di 40°C e durante vaporizzazione convettiva alla temperatura di saturazione di 30°C, sono stati poi confrontati con quelli ottenuti in precedenza con il fluido R134a. E’ stato fatto inoltre un confronto delle perdite di carico bifase misurate con questi due fluidi. Con l’obiettivo di verificare la validità della teoria convenzionale nella micro-scala, due nuovi minicanali a sezione circolare, aventi 0,96 e 2 mm di diametro interno, sono stati dimensionati e realizzati appositamente per misurare le perdite di carico durante deflusso bifase e monofase. Il buon accordo trovato con i modelli classici validi per i canali convenzionali ha permesso di convalidare una tecnica per determinare il diametro idraulico dei condotti dalle misure di perdita di carico durante deflusso in regime laminare. Nella presente Tesi è presentato inoltre uno studio riguardante il miglioramento del processo di condensazione con l’obiettivo di aumentare l’efficienza energetica degli scambiatori di calore e di diminuirne le dimensioni. Sono state studiate superfici in rame nano strutturate con lo scopo di promuovere la condensazione a gocce piuttosto che quella a film. Un nuovo impianto sperimentale costituito da un termosifone bifase è stato realizzato per misurare i coefficienti di scambio termico durante condensazione di vapor d’acqua alla temperatura di saturazione di 110 °C. Per investigare l’effetto della velocità del vapore, riproducendo le reali condizioni degli scambiatori a piastre, la sezione sperimentale è stata realizzata mediante un canale rettangolare di piccole dimensioni (3.6 mm di diametro idraulico). L’impianto sperimentale permette di misurare i coefficienti si scambi termico durante la condensazione, ma anche la visualizzazione dei movimenti delle goccioline. Un semplice processo a immersione chimica è stato utilizzato per ottenere una superficie in rame superidrofoba. Le prestazioni di scambio termico di tale superficie e di alcune superfici in rame lucidato sono state studiate durante la condensazione a gocce

Heat transfer and pressure drop during condensation of the low GWP refrigerant R1234yf

The present paper reports local heat transfer coefficients measured during condensation of R1234yf within a single circular 0.96 mm diameter minichannel and compares them to the ones of R134a. This experimental work is carried out in a unique test apparatus which allows to determine the local heat flux extracted from the condensing fluid from the temperature profile of the coolant. For this purpose, the temperatures of the coolant and of the wall are measured along the test section. The saturation temperature is determined from the saturation pressure which is measured at inlet and outlet of the test channel. Condensation tests are carried out at mass fluxes ranging between 200 and 1000 kg m2 s1 and the heat transfer coefficients result to be lower as compared to the ones of R134a. Since the saturation temperature drop directly affects the heat transfer rate, the pressure drop during adiabatic two-phase flow of R1234yf is also measured and compared to R134a

Flow boiling of R1234yf in a 1 mm diameter channel

Since many HFC refrigerants have relatively large values of GWP, recent development efforts have been initiated to investigate halogenated olefins as possible refrigerants, with fluorinated propene isomers, in particular, having emerged as possible solutions. Specifically, R1234yf has emerged as an R134a replacement for automotive applications. In this paper, the local heat transfer coefficient measured during flow boiling of R1234yf in a 1mm diameter circular microchannel is reported and compared to R134a. During tests, the heat is provided to the boiling fluid by using a secondary fluid. Therefore the heat flux is not imposed but instead it is the result of the inlet temperatures of the two fluids and the thermal resistances on the two sides, as it occurs in actual heat exchangers for automotive and air-conditioning applications. Flow boiling tests are carried out at 31\ub0C saturation temperature and mass fluxes ranging between 200 kg m-2 s-1 and 600 kg m-2 s-1

Flow Condensation on Copper-Based Nanotextured Superhydrophobic Surfaces

Superhydrophobic surfaces have shown excellent ability to promote dropwise condensation with high droplet mobility, leading to enhanced surface thermal transport. To date, however, it is unclear how superhydrophobic surfaces would perform under the stringent flow condensation conditions of saturated vapor at high temperature, which can affect superhydrophobicity. Here, we investigate this issue employing \u201call-copper\u201d superhydrophobic surfaces with controlled nanostructuring for minimal thermal resistance. Flow condensation tests performed with saturated vapor at a high temperature (110 \ub0C) showed the condensing drops penetrate the surface texture (i.e., attain the Wenzel state with lower droplet mobility). At the same time, the vapor shear helped ameliorate the mobility and enhanced the thermal transport. At the high end of the examined vapor velocity range, a heat flux of 600 kW m\u20132 was measured at 10 K subcooling and 18 m s\u20131 vapor velocity. This clearly highlights the excellent potential of a nanostructured superhydrophobic surface in flow condensation applications. The surfaces sustained dropwise condensation and vapor shear for five days, following which mechanical degradation caused a transition to filmwise condensation. Overall, our results underscore the need to investigate superhydrophobic surfaces under stringent and realistic flow condensation conditions before drawing conclusions regarding their performance in practically relevant condensation applications