Andrea Diani

Experimental Measurements of R134a Flow Boiling Inside a 3.4-mm ID Microfin Tube

The minimization of the refrigerant charge in refrigerating and air conditioning equipment is now an important issue for the new environmental challenges. This paper presents R134a flow boiling heat transfer and pressure drop measurements inside a mini microfin tube with internal diameter of 3.4 mm. This study was carried out in a new experimental facility built at the Dipartimento di Ingegneria Industriale of the University of Padova, especially designed to study both single- and two-phase heat transfer processes in microstructured surfaces. The microfin tube was brazed inside a copper plate and electrically heated from the bottom by means of a wire resistance. Several T-type thermocouples were inserted in the wall to measure the temperature distribution during the phase-change process. In particular, the experimental measurements were carried out at constant saturation temperature of 30 °C, by varying the refrigerant mass velocity between 190 kg m−2 s−1 and 940 kg m−2 s−1, and the vapor quality from 0.2 to 0.99 at three different heat fluxes: 10, 25, and 50 kW m−2. The experimental results are presented in terms of two-phase heat transfer coefficient, vapor quality at the onset of dryout, and frictional pressure drop.

R1234yf Flow Boiling Heat Transfer Inside a 2.4-mm Microfin Tube

ABSTRACT This paper presents an experimental study on R1234yf flow boiling inside a mini microfin tube with an inner diameter at the fin tip of 2.4 mm. R1234yf is a new refrigerant with an extremely low global warming potential (GWP <1), proposed as a possible substitute for the common R134a, whose GWP is about 1300. The mass flux was varied between 375 and 940 kg m−2 s−1, heat flux from 10 to 50 kW m−2, and vapor quality from 0.1 to 1. The saturation temperature at the inlet of the test section was kept constant and equal to 30°C. The wide range of operative test conditions permitted highlighting the effects of mass flux, heat flux, and vapor quality on the thermal and hydraulic behavior during the flow boiling mechanism inside such a mini microfin tube. The results show that at low heat flux the phase-change process is mainly controlled by two-phase forced convection, and at high heat flux by nucleate boiling. The two-phase frictional pressure drop increases with increasing both mass velocity and vapor quality. Dry-out was observed only at the highest heat flux, at vapor qualities of around 0.94–0.95.

Experimental and numerical analyses of different extended surfaces

Air is a cheap and safe fluid, widely used in electronic, aerospace and air conditioning applications. Because of its poor heat transfer properties, it always flows through extended surfaces, such as finned surfaces, to enhance the convective heat transfer. In this paper, experimental results are reviewed and numerical studies during air forced convection through extended surfaces are presented. The thermal and hydraulic behaviours of a reference trapezoidal finned surface, experimentally evaluated by present authors in an open-circuit wind tunnel, has been compared with numerical simulations carried out by using the commercial CFD software COMSOL Multiphysics. Once the model has been validated, numerical simulations have been extended to other rectangular finned configurations, in order to study the effects of the fin thickness, fin pitch and fin height on the thermo-hydraulic behaviour of the extended surfaces. Moreover, several pin fin surfaces have been simulated in the same range of operating conditions previously analyzed. Numerical results about heat transfer and pressure drop, for both plain finned and pin fin surfaces, have been compared with empirical correlations from the open literature, and more accurate equations have been developed, proposed, and validated.

R1234yf Flow Boiling Heat Transfer in a Rectangular Channel Heated from the Bottom

ABSTRACT This paper presents some preliminary experimental measurements collected during flow boiling heat transfer of low global warming potential refrigerant R1234yf in an asymmetrically heated rectangular plain channel. The asymmetrical heating is the common boundary condition that occurs in many different applications, for instance, in almost all the electronic devices, which are now pushing the cooling demands to more and more greater requirements. From this standpoint, the analysis of the flow boiling heat transfer of efficient and eco-friendly refrigerants can open new frontiers to the electronic thermal management. The experimental measurements were carried out at the Department of Industrial Engineering of the University of Padova by imposing two different heat fluxes, 50 and 100 kW m−2, at a constant saturation temperature of 30°C; the refrigerant mass velocity was varied between 50 and 200 kg m−2 s−1, while the vapor quality varied from 0.2 to 0.95. The developed measuring technique permits to estimate the flow boiling heat transfer coefficient and the critical value of vapor quality at the onset of the dryout.

Flow boiling heat transfer of R1234yf on a microparticle coated copper surface

This article investigates the flow boiling heat transfer of the low global warming potential refrigerant R1234yf on a microparticle coated surface obtained via high-pressure cold spray, a simple and nonexpensive technique. The sample was obtained by depositing pure copper particles with average size of 20 μm obtaining a 0.1 mm thick coating on a smooth copper plate 10 mm wide and 200 mm long. The experimental measurements were carried out at constant saturation temperature of 30°C, by varying the heat flux from 50 to 100 kW m−2, the refrigerant mass flux from 30 to 200 kg m−2 s−1, and the vapor quality from 0.2 to 0.99. The coating was found to be hydrophilic, leading to hysteresis on the heat transfer behavior, which is discussed in detail. Furthermore, the experimental results are compared against similar measurements obtained during R1234yf flow boiling over a plain copper surface.

Heat and Mass Transfer to Air in a Cross Flow Heat Exchanger with Surface Deluge Cooling

Cross flow heat exchangers, when applied to cool data center rooms, use external air (process air) to cool the air stream coming from the data center room (primary air). However, an air–air heat exchanger is not enough to cope with extreme high heat loads in critical conditions (high external temperature). Therefore, water can be sprayed in the process air to increase the heat dissipation capability (wet mode). Water evaporates, and the heat flow rate is transferred to the process air as sensible and latent heat. This paper proposes an analytical approach to predict the behavior of a cross flow heat exchanger in wet mode. The theoretical results are then compared to experimental tests carried out on a real machine in wet mode conditions. Comparisons are given in terms of calculated versus experimental heat flow rate and evaporated water mass flow rate, showing a good match between theoretical and experimental values.

Material and height effects on the heat transfer performance of metal foams cooled by air in forced convection

In this paper, copper and aluminum foams with different porosity, number of pores per inch (PPI) and foam core height, are experimentally studied during air forced convection. The experimental measurements permit to understand how each parameter (i.e. porosity, PPI, material, and foam thickness) affects the heat transfer and fluid flow behavior of the metal foams. The paper presents the experimental heat transfer coefficients, permeability and inertia coefficients; moreover, it reports the normalized mean wall temperature as a function of the pumping power per unit of heat transfer area: two meaningful parameters that allow quantitative comparisons of different enhanced surfaces, which can be considered suitable for electronic thermal management.