Ángel Á. Pardiñas

Heat Transfer Performance of Functionalized Graphene Nanoplatelet Aqueous Nanofluids

The low thermal conductivity of fluids used in many industrial applications is one of the primary limitations in the development of more efficient heat transfer systems. A promising solution to this problem is the suspension of nanoparticles with high thermal conductivities in a base fluid. These suspensions, known as nanofluids, have great potential for enhancing heat transfer. The heat transfer enhancement of sulfonic acid-functionalized graphene nanoplatelet water-based nanofluids is addressed in this work. A new experimental setup was designed for this purpose. Convection coefficients, pressure drops, and thermophysical properties of various nanofluids at different concentrations were measured for several operational conditions and the results are compared with those of pure water. Enhancements in thermal conductivity and in convection heat transfer coefficient reach 12% (1 wt %) and 32% (0.5 wt %), respectively. New correlations capable of predicting the Nusselt number and the friction factor of this kind of nanofluid as a function of other dimensionless quantities are developed. In addition, thermal performance factors are obtained from the experimental convection coefficient and pressure drop data in order to assess the convenience of replacing the base fluid with designed nanofluids.

EXPERIMENTAL DETERMINATION OF THE BOILING HEAT TRANSFER COEFFICIENTS OF R-134a AND R-417A ON A SMOOTH COPPER TUBE

Flooded evaporators are widely used as compact cooling units to cool liquids. They consist of a shell-and-tube heat exchanger, with the fluid to cool flowing inside the tubes of the bundle and a refrigerant that evaporates over those tubes. Pool boiling on the external surface of the tubes is a very complex process, and therefore the boiling heat transfer coefficients (HTCs) should be determined experimentally. Copper and copper alloys tubes are commonly employed in such heat exchangers, due to their high thermal conductivity and relative low cost. On the other hand, refrigeration and air conditioning sectors are undergoing significant changes caused mainly by the necessity of replacing existing refrigerants with more environmentally friendly ones. This paper reports the experimental determination of the pool boiling HTCs of R-134a and R-417A blend on a smooth copper tube of 18.87 mm diameter, at two saturation temperatures of 10°C and 7°C. Although smooth tubes are not commonly used in shell-and-tube evaporators nowadays, it is a first approach to pool boiling of drop-in refrigerants. The experimental setup and data acquisition are described, the experimental procedure is explained, the data reduction methodology is detailed, and the results are presented and discussed.

Modelling vapour compression cycles with R744 for residential heat pumps adapted to the spanish climate.

R744 heat pumps appear as an ideal solution to produce heating, cooling and domestic hot water (DHW). The transcritical cycle has the potential to produce high temperature hot water simultaneously with other demands as heating or air conditioning, however the low trans-critical temperature of CO2 is a natural limitation for applications in warmer climates. R744 heat pumps are now used in Nordic countries, but they are still a challenge for hotter climates, for which there are R&D efforts to enhance their COP. Among the technologies to implement, ejectors appear to improve the cycle performance in percentages that go from 10 to 20%, according to previous works from the literature. This paper details the study of residential heat pumps with R744 to produce combined heating/cooling and DHW under different climatic conditions that occur in Spain. The COP for R744 heat pumps supplying combined heating/cooling and DHW demand is improved by implementing ejectors, however the improvement is not as high as stated in other research studies.