ANALISI TERMICA ED OTTICA DELLO SCAMBIO TERMICO BI-FASE DENTRO CANALI DI PICCOLO DIAMETRO CON FLUIDI PURI E MISCELE

In the present work, the condensation and flow boiling heat transfer and two-phase pressure drop of ternary azeotropic mixture R516A (R1234yf/R152a/R134a 77.5/14/8.5% by mass), binary azeotropic mixture R513A (R1234yf/R134a, 56/44% by mass), binary azeotropic mixture R515B (R1234ze(E)/R227ea, 91/9% by mass) and R450A (R1234ze(E)/R134a, 56/44% by mass) are investigated inside two smooth tubes (3.38 mm and 0.96 mm inner diameter). Also, the condensation heat transfer of pure hydrochlorofluoroolefin R1233zd(E) was investigated. The four mixtures have been recognized as compatible replacement fluids for R134a, due to their comparable volumetric cooling capacity and cycle efficiency, together with their similar thermophysical properties. For the same reasons, R1233zd(E) has been suggested as substitution for R245fa. The effect of tube diameter, mass velocity, vapour quality and saturation-to-wall temperature difference on the condensation heat transfer is investigated. Several condensation heat transfer and frictional pressure drop models are assessed using the present experimental database. In the present work, non-invasive optical techniques are employed to evaluate the flow regime inside a 3.38 mm channel. The employed test section is composed of two tube-in-tube heat exchangers for the evaluation of the quasi-local heat transfer coefficient and a specifically designed glass tube used to visualize the flow. The same 3.38 mm test section was used to study the condensation heat transfer with R1233zd(E) under micro-gravity conditions. The investigation of the liquid film thickness and heat transfer coefficients inside small diameter channels is important also for space applications and, in particular, for the design of the two-phase thermal management systems, where the changing gravity level strongly affects the observed flow patterns and, consequently, the condensation heat transfer. While several experimental and numerical investigations have been conducted to examine the gravity effect on nucleate and flow boiling, research studies on condensation under reduced gravity are still limited in the literature, especially at low mass velocity. Hence, in the present research the effect of gravity is experimentally assessed at mass velocity equal to 30 kg m-2 s-1 and 40 kg m-2 s-1 during condensation of pure fluid R1233zd(E) inside a 3.38 mm ID horizontal channel. Condensation tests were carried out during the 84th ESA Parabolic Flight Campaign on board the Novespace Airbus A-310 AirZeroG by simultaneously measuring the heat transfer coefficient and the liquid film thickness inside a horizontal channel in both normal gravity and microgravity conditions. The reduced gravity condition is responsible for a heat transfer penalization with respect to normal gravity. The effect of a change in the gravity level on the characteristics of the interfacial waves is discussed in the present work. The condensate thickness is evaluated by combining the local measurements provided by optical sensors, such as the chromatic confocal sensor or the interferometer. The liquid film thickness is determined simultaneously with quasi-local measurements of heat transfer coefficients and flow pattern visualizations; the results are compared and analysed together to improve the knowledge of the occurring local mechanisms. A statistical analysis is performed to characterize the wave structures in terms of amplitude, frequency and velocity and better understand the influence of the interfacial waviness on the heat transfer. The accuracy of several models for the prediction of the heat transfer coefficient and the liquid film thickness during condensation in vertical down-flow is discussed. As the considered models provide inaccurate predictions at low mass velocity, a new heat transfer correlation is proposed and validated against the experimental data.