Stefano Bortolin

Condensation in a Square Minichannel: Application of the VOF Method

A number of steady-state numerical simulations of condensation of R134a at mass fluxes of 400 kg m−2 s−1 and 800 kg m−2 s−1 inside a 1-mm square cross section minichannel are proposed here and compared against simulations in a circular cross section channel with the same hydraulic diameter. The volume of fluid (VOF) method is used to track the vapor–liquid interface, and the effects of interfacial shear stress, surface tension, and gravity are taken into account. A uniform wall temperature is fixed as a boundary condition. At both mass velocities the liquid film and the vapor core are treated as turbulent; a low-Re form of the SST k-ω model has been used for the modeling of turbulence through both the liquid and vapor phases. Numerical simulations are validated against experimental data. The influence of the surface tension on the shape of the vapor–liquid interface may provide some heat transfer enhancement in a square cross section minichannel, but this depends on the mass flux and it may be not significant at high mass velocity, as confirmed by experimental data and by the present numerical work. The gravity force is shown to be responsible for the liquid film thickness increase at the bottom of the channel in the circular cross section, but the gravity force has a minor effect in the square minichannel; at these mass velocities, the heat transfer mechanism is dominated by shear stress and surface tension.

Condensation Heat Transfer and Pressure Losses of High- and Low-Pressure Refrigerants Flowing in a Single Circular Minichannel

A 0.96 mm circular minichannel is used to measure both heat transfer coefficients during condensation and two-phase pressure losses of the refrigerants R32 and R245fa. Test runs have been performed at around 40°C saturation temperature, corresponding to 24.8 bar saturation pressure for R32 and 2.5 bar saturation pressure for R245fa. The pressure drop tests have been performed in adiabatic flow conditions, to measure only the pressure losses due to friction. The heat transfer experimental data are compared against predicting models to provide a guideline for the design of minichannel condensers.

Flow Boiling of R245fa in a Single Circular Microchannel

This paper describes an experimental setup for the investigation of two-phase heat transfer inside microchannels and reports local heat transfer coefficients measured during flow boiling of HFC-245fa in a 0.96-mm-diameter single circular channel. The test runs have been performed during vaporization at around 1.85 bar, corresponding to 31°C saturation temperature. As a peculiar characteristic of the present technique, the heat transfer coefficient is not measured by imposing the heat flux; instead, the boiling process is governed by controlling the inlet temperature of the heating secondary fluid. In the data, mass velocity ranges between 200 and 400 kg m−2 s−1, with heat flux varying from 5 to 85 kW m−2 and vapor quality from 0.05 up to 0.8. Since these data are not measured at uniform heat flux conditions, a proper analysis is performed to enlighten the influence of the different parameters and to compare the present data to those obtained when the heat flux is imposed. Besides, the test runs have been carried out in a double mode: by increasing the water-to-refrigerant temperature difference and by decreasing it. Finally, the experimental data are compared to models available in the literature for predicting the heat transfer coefficients inside microchannels.

Two-phase pressure drop and condensation heat transfer of R32/R1234ze(E) non-azeotropic mixtures inside a single microchannel

Much attention has been paid in the recent years to the possible use of fluorinated propene isomers for the substitution of high global warming potential refrigerants. Among the fluorinated propene isomers, R1234ze(E) may be a substitute of R134a for refrigeration applications. R1234ze(E) has a global warming potential lower than 1 (considering a period time of 100 years), and it is receiving some attention also as a component of low global warming potential mixtures. In this article, a mixture of R1234ze(E) and R32 has been investigated at two different mass compositions (23/77% and 46/54% by mass) with regard to the performance at the condenser. The local heat transfer coefficients during condensation in a single microchannel with 0.96 mm diameter are measured and analyzed. The frictional two-phase pressure drop in the same channel is also investigated. The present tests are carried out on the experimental apparatus available at the Two-Phase Heat Transfer Lab of the University of Padova. The new experimental data are compared to those of pure R1234ze(E) and R32. This allows the heat transfer penalization due to the mass transfer resistance occurring during condensation of these zeotropic mixtures to be analyzed and suitable predicting models to be assessed. The knowledge of heat transfer coefficient and pressure drop allows evaluation of the overall performance of these mixtures when used in condensers.

Predicting Methods for Flow Boiling Heat Transfer of a Non-Azeotropic Mixture Inside a Single Microchannel

At this time, a widely accepted model that can predict flow boiling heat transfer in microchannels with different fluids, geometries, and operative conditions is still missing. Depending on the working fluid, a predicting correlation can lead to accurate estimation or give rise to errors up to 50% and higher. The situation is further complicated when the working fluid is a zeotropic mixture of two components, due to the additional mass transfer resistance that must be estimated. In the recent years much attention has been paid to the possible use of fluorinated propene isomers in substitution for high-global-warming-potential refrigerants. The available hydrofluoroolefins cannot cover all the air-conditioning, heat pump, and refrigeration applications when used as pure fluids because their thermodynamic properties are not suitable for all the operating conditions, and therefore some solutions may be found using blends of refrigerants, to satisfy the demand for a wide range of working conditions. The adoption of new mixtures poses the problem of how to extend the correlations developed for pure fluids to the case of flow boiling of mixtures in microchannels. In this work, a mixture of R1234ze(E) and R32 (0.5/0.5 by mass) has been considered: The local heat transfer coefficient during flow boiling of this mixture in a single microchannel with 0.96 mm diameter has been measured at a pressure of 14 bar, which corresponds to a bubble temperature of around 26°C. This flow boiling database, encompassing more than 300 experimental points at different values of mass velocity, heat flux, and vapor quality, is compared with available correlations in the literature. The introduction of a correction to account for the additional mass transfer resistance is discussed, and such correction is found to be necessary and proper to provide a correct sizing of the evaporator.

Experiments on dry-out during flow boiling in a round minichannel

Temperature measurements during flow boiling of R134a in a 0.96 mm single circular channel are reported in order to provide a criterion for the determination of the critical conditions in the channel. The flow boiling heat transfer is obtained by using a secondary fluid; the wall temperature displays larger fluctuations in the zone where dryout occurs. These temperature fluctuations in the wall denote the presence of a liquid film drying up at the wall with some kind of an oscillating process. These temperature fluctuations never appear during condensation tests, neither are present during flow boiling at low vapour qualities. The fluctuations also disappear in the post-critical condition zone.Experimental values of dryout quality measured with the above method are reported in this paper at mass velocity ranging between 300 and 600 kg m−2s−1.In the practical applications of flow boiling, the dryout quality is a key parameter in the two-phase systems for cooling of devices, both for ground and microgravity applications. The test conditions reported here refer to relatively high mass velocities, and are obtained at earth gravity. Nevertheless, since the critical heat flux differences between the two gravity environments decrease with increasing velocity, the present data may also be used for inertia dominated systems at low g.

Impact of two-phase distribution systems on the performance of a microchannel evaporator

The design and the performance of an innovative shell-and-tube evaporator using round copper microchannels are presented in this article. This prototype has been designed aiming at the minimization of the refrigerant charge, which can be required by safety or environmental restrictions. Experimental data of heat transfer and pressure drop are reported in the present article. The measurements have been obtained with two different evaporator inlet headers and two different working fluids (i.e., R22 and R410A) to investigate the mutual influence of the design of the distribution system and the refrigerant properties on possible maldistribution issues. A computational procedure implementing different correlations has also been developed and validated against experimental data; this procedure allows the prediction of the performance of the same evaporator with a hydrocarbon, such as propane, and comparison of the prototype to a brazed-plate heat exchanger.

Frictional Pressure Drop during Two-Phase Flow of Pure Fluids and Mixtures in Small Diameter Channels

Abstract Two-phase flow is widely encountered in minichannels heat exchangers such as air-cooled condensers and evaporators for automotive, compact devices for electronic cooling and aluminum condenser for air-conditioning applications. In the present work, frictional pressure drop during adiabatic liquid-vapor flow is experimentally investigated inside a single 0.96 mm diameter minichannel. Tests have been run with three mixtures of R32/R1234ze(E) (23/77%, 50/50% and 75/25% by mass composition) at mass flux ranging between 200 and 600 kg m−2 s−1. Since pressure drop has a strong influence on the two-phase heat transfer, it is crucial to have reliable pressure drop prediction methods for two-phase heat transfer modeling and optimization. Therefore, with the aim of extending its validity range, a model to calculate the frictional pressure gradient during two-phase flow in small diameter channels is tested against the present two-phase pressure drop database. An assessment is also done with two low-GWP refrigerants: the halogenated olefin R1234ze(E) and the hydrocarbon R290. The present model accounts for the effect of internal surface roughness as a function of the liquid-only Reynolds number.

CFD study on electrolyte distribution in redox flow batteries

The most important component in a redox flow battery (RFB) cell is the MEA (membrane electrode assembly), a sandwich consisting of two catalyzed electrodes with an interposed polymeric membrane. In order to allow electrolyte flow toward the electroactive sites, the electrodes have a porous structure that can be obtained with carbon base materials such as carbon felts. The RFB cell is closed by two plates containing the distribution flow channels. Considering that a uniform electrolyte distribution in the reaction region is a prerequisite for high-efficiency operation, the flow pattern is an important parameter to be investigated for the optimization of the cell.In the present work, the effect of different channels patterns on the electrolyte distribution and on the pressure drop is numerically investigated. Three-dimensional simulations have been carried out with ANSYS Fluent code and four different layouts have been considered. Calculations have been performed both in the distribution channels and in the felt porous region.

Experimental Study of Condensation Inside a Horizontal Single Square Minichannel

This work is aimed at presenting experimental heat transfer coefficients measured during condensation inside a single square cross section minichannel, having a 1.18 mm side length. The experimental heat transfer coefficients are compared to the ones previously obtained in a circular minitube. This subject is particularly interesting since most of the mini and microchannels used in practical applications have non circular cross sections. The test section used in the present work is obtained from a thick wall copper tube which is machined to draw a complex passage for the water; its geometry has been studied with the aim of increasing the external heat transfer area and thus decreasing the external heat transfer resistance. This experimental technique allows to measure directly the temperature in the tube wall and in the water channel. The heat flux is determined from the temperature profile of the coolant in the measuring sector. The wall temperature is measured by means of thermocouples embedded in the copper tube, while the saturation temperature is obtained from the saturation pressure measured at the inlet and outlet of the measuring sector. On the whole, more than seventy thermocouples have been placed in the 23 cm long measuring section. Tests have been performed with R134a at 40°C saturation temperature, at mass velocities ranging between 200 and 800 kg m−2 s−1 . As compared to the heat transfer coefficients measured in a circular minichannel, in the square minichannel the authors find a heat transfer enhancement at the lowest values of mass velocity; this must be due to the effect of the surface tension. No heat transfer coefficient increase has been found at the highest values of the mass velocity where condensation is shear stress dominated.Copyright