Simone Mancin

Air Flow in Aluminum Foam: Heat Transfer and Pressure Drops Measurements

Abstract Metal foams are cellular structure materials that present open cells, randomly oriented and mostly homogeneous in size and shape. Cellular structure materials, and particularly open-cell metal foams, have been proposed as possible substitutes for traditional finned surfaces in electronics cooling applications. This article presents the heat transfer and pressure drops measurements obtained during air flow through an aluminum foam, which has 40 pores per inch with 0.63 mm pore diameter. The specimen has been inserted in a new open-circuit type wind channel with a rectangular cross-section that has recently been built at the Department of Fisica Tecnica of the University of Padova. The experimental heat transfer coefficients and pressure drops have been collected by varying the air flow rate supplied by the screw compressor that provides a variable volumetric air flow ranging between 0–90 m3h−1 at a constant gauge pressure of 7 bar. The specific heat flux has been simulated by powering with a 25-kWm−2 copper heater attached at the bottom of the aluminum foam base plate. The experimental results are reported in terms of heat transfer coefficients, mean normalized wall temperatures, and pressure drops.

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.

Saturated R134a flow boiling inside a 4.3 mm inner diameter microfin tube

The refrigerant charge minimization in refrigerating and air-conditioning systems represents a challenging issue due to the new environmental national and international regulations. The use of smaller smooth tubes, such as with the outer diameter around 5 mm, is becoming more and more common in many applications. More recently, the microfin tubes have also started to be reduced in size to cope with the continuously increasing demand of new, efficient, and compact heat exchangers for air-conditioning and refrigeration equipment. This work investigates the performance of R134a during saturated flow boiling inside a microfin tube with internal diameter at the fin tip of 4.3 mm. Boiling heat transfer coefficients, frictional pressure drops, and critical vapor qualities were measured at 30°C of saturation temperature, by varying the refrigerant mass velocity between 100 and 800 kg m−2 s−1 and the vapor quality from 0.1 to 0.95 at four different heat fluxes: 15, 30, 60, and 90 kW m−2. Moreover, the reliability of several models for flow boiling heat transfer and pressure drop estimations was assessed by comparing the experimental results with the calculations.

A review on in-tube two-phase heat transfer of hydro-fluoro-olefines refrigerants

Since 2010, hydro-fluoro-olefines have been experimentally studied during both boiling and condensation with the research productions continuously growing. At a glance, most of the data collected regards the two most available fluids, R1234yf and R1234ze(E), which were tested during two-phase flow both inside and outside smooth and microfin tubes or in brazed plate heat exchangers, as well on other different enhanced surfaces. A few data were obtained with noncommercially available hydro-fluoro-olefines, such as R1234ze(Z). In addition, relevant work has been dedicated to study the heat transfer capabilities of hydro-fluoro-carbon/hydro-fluoro-olefine mixtures. This article presents a critical review on the published research articles on in-tube two-phase heat transfer during boiling and condensation of hydro-fluoro-olefine refrigerants. In particular, a large database of experimental measurements was collected, reviewed, discussed, and then compared against common models to identify the most suitable for all the investigated two-phase heat transfer processes.

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.

Visualization of the Heat Transfer Enhancement During Condensation in a Microfin Tube

Microfins tubes are largely used in refrigeration industry for in-tube refrigerant condensation, because of the heat transfer enhancement when compared to equivalent smooth tubes under the same operating conditions. But not much evidence about the effect of microfins on the condensation flow patterns is available in the open literature. There is agreement in the open literature that the mechanisms of heat transfer are intimately linked with the prevailing two-phase flow regime. The present authors have recently measured the heat transfer coefficient during condensation of R410A in a microfin tube. The heat transfer enhancement in this tube can be experimentally evaluated by comparing those coefficients to the ones measured by Cavallini et al. (2001) in a plain tube, at the same operating conditions. The same operative conditions (saturation temperature, vapor quality and mass flux), occurring during the heat transfer measurements, were reproduced in a different section for visualization of flow patterns during condensation of R410A. The flow visualization has been carried out both in the plain tube and in the microfin tube. The objective of the present paper is to present the heat transfer enhancement during condensation of R410A and to show the flow visualized at the same operating condition for both the smooth and the microfin tube, aiming to link the heat transfer enhancement to the flow pattern variation.Copyright

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 Pipe Finned Heat Exchanger for Heat Recovery: Experimental Results and Modeling

ABSTRACT This paper presents an experimental and theoretical analysis of a 6-row horizontal microfin Heat Pipe Finned Heat Exchanger (HPFHE) used for energy recovery purposes inside an air conditioning unit. The experimental campaign investigated both the summer and the winter conditions for European countries by varying the operating conditions at the inlet of the HPFHE. New experimental tests are presented for the identification of low – global warming potential refrigerants, environmental friendly substitutes of the more traditional HFC134a. The results showed the interesting heat transfer capabilities of HFC152a as an alternative HPFHE working fluid. A simulation model previously developed by present authors was validated against the new experimental data collected and then used to simulate the thermal performance of the HPFHE under different operating test conditions, in order to assess the potentiality of seasonal energy savings with HFC152a.

Nanoparticle Deposition During Cu-Water Nanofluid Pool Boiling

The present research activity aims to rigorously investigate nanofluid pool boiling in order to definitively assess this as a technique for controlled nanoparticle coating of surfaces, which can enhance the nucleate boiling performance. This paper presents preliminary nanoparticle deposition results obtained during Cu-water (0.13 wt%) nanofluid pool boiling on a smooth copper surface. The tests were run in an experimental setup designed expressly to study water and nanofluid pool boiling. The square test sample block (27.2 mm × 27.2 mm) is equipped with a rake of four calibrated T-type thermocouples each located in a 13.6-mm deep holes drilled every 5 mm from 1 mm below the top surface. The imposed heat flux and wall superheat can be estimated from measurement of the temperature gradient along the four thermocouples. The samples are characterized by scanning electron microscopy (SEM) to analyse the morphological characteristics of the obtained thin, Cu nanoparticle coating.