Table of contents

The 2022 UIT (Italian Union of Thermo-Fluid Dynamics) International Conference, hereafter referred as 39^th UIT 2022, was organized by the Department of Civil and Mechanical Engineering of the University of Cassino (Italy), in collaboration with the UIT, on June 20-22 2022, at the Castello di Gaeta, Italy.

The 39^th UIT 2022 was organized by the Organizing Committee of the 38th UIT Heat Transfer Conference, initially scheduled for June 21^st-23^rd 2021 at the Castello di Gaeta, Italy, and then organized completely online considering the criticalities induced by the spread of COVID-19.

The 39^th UIT Heat Transfer Conference 2022 program scheduled two keynote lectures given by Prof. Perumal Nithiarasu from the Swansea University, Swansea, UK, titled "Physics-Informed Neural Networks (PINNs) for solving thermal problems", and by Prof. Wilson K. S. Chiu, from University of Connecticut Storrs, Connecticut, USA, titled "Thermal Transport in Architected Open Cell Foams". A total of 68 abstracts were submitted to the 39^th UIT Heat Transfer Conference 2022, 55 of which presented by the Authors in oral sessions, while the remaining 13 abstracts were presented in one poster session.

About 100 researchers participated to the 39th UIT 2022. The Conference was a useful occasion to stimulate discussion, further the understanding of heat transfer and related phenomena, present the state-of-the-art of some topics, discuss emerging trends and promote collaborations. The Organizing Committee hope that the event results constituted significant contribution to the knowledge in the following topics: Computational fluid dynamic and heat transfer; Conduction, radiation, thermophysical properties and porous media; Forced, natural and mixed convection; Heat and mass transfer in nuclear plants and energy systems; Measurement techniques for heat and mass transfer; Multiphase fluid dynamics and heat transfer.

A special tank to UIT, international advisory committee, local organizing committee and all the participants.

With Kind Regards

Paolo Vigo, Università di Napoli "Parthenope", Italy

Marco Dell'Isola, Università degli Studi di Cassino e del Lazio Meridionale, Italy

Fausto Arpino, Università degli Studi di Cassino e del Lazio Meridionale, Italy

Alfonso Niro, Politecnico di Milano, Italy

List of Committees, Contact details for the declaration are available in this pdf.

All papers published in this volume have been reviewed through processes administered by the Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

• Type of peer review: Single Anonymous

• Conference submission management system: Morressier

• Number of submissions received: 29

• Number of submissions sent for review: 29

• Number of submissions accepted: 29

• Acceptance Rate (Submissions Accepted / Submissions Received × 100): 100

• Average number of reviews per paper: 2

• Total number of reviewers involved: 32

• Contact person for queries:

Name: Fausto Arpino

Email: f.arpino@unicas.it

Affiliation: University of Cassino and Southern Lazio

Ever-increasing heat fluxes of electronic components ask for higher performance of devices responsible for their cooling. Against that background, PCM-based heat sinks – because of their compactness, and effectiveness – are well-established solutions for thermal management issues. Even though solutions for the thermal enhancement of PCM devices have been widely presented, new ways to optimize such systems are emerging. Among them, this work investigates the application of density-based topology optimization to define innovative heat sinks design able to minimize thermal resistance under constant wall temperature. A 2D numerical model is developed by means of a finite element tool. The heat equation is solved for the topology optimization problem with the objective of minimizing the average temperature, considering further manufacturing restrictions. Solid-isotropic-material-with-penalization (SIMP) method is applied to link the design variable to material properties. Parameterization of Helmholtz filter's minimum feature size and projection based on the hyperbolic tangent function is performed, showing improved performance as well as feature size decrease. The optimized prototype – with PCM – is then simulated with the enthalpy-porosity model to assess the benefits, i.e., reduction in the melting time, with respect to the baseline. Results show the potential of optimizing heat sinks via a topology-based approach and confirm it as a promising tool for finding new heat sink geometry, whatever the application.

In contrast to traditional mesh-based methods for the numerical solution of boundary value problems, e.g., Finite Element (FEM) and Finite Volume (FVM), in the recent period many meshfree approaches have been proposed in order to avoid those typical issues due to the mesh. For example, the quality of the mesh greatly affects the reliability of the final solution in the case of CFD problems and the human intervention of a professional is often still needed when dealing with complex-shaped domains. This in turn increases both cost and time required for the reliable simulation of problems of engineering relevance. Meshless methods, on the other side, usually rely on a simpler distribution of nodes and do not require the storage of connectivity information. Among others, one of the most promising meshless methods in terms of accuracy and flexibility is the one based on the Radial Basis Function – Finite Difference (RBF-FD) scheme. RBF-FD methods, however, are usually affected by severe ill conditioning issues when Neumann boundary conditions are employed. This fact is the main responsible for the appearance of large discretization errors near the boundary and for the lack of stability of traditional time integration schemes. In order to address this issue, some new algorithms for the robust treatment of boundary conditions have been developed and successfully employed to solve fluid flow problems with heat transfer. Furthermore, it is well acknowledged that the efficient resolution of boundary layers arising in this class of problems requires an adequate spatial discretization in the neighbourhood of the boundary, i.e., increased node/mesh density along the direction of large gradients only. This result is achieved by employing anisotropic node distributions, which is a novelty in the context of the RBF-FD method to the best of the authors' knowledge. The method described above is successfully employed for the accurate solution of a representative 3D heat transfer problem with incompressible fluid flow.

In secondary steel making process, steel scrap is melted in an electric arc furnace, then poured into a ladle where ferroalloy are added to obtain the desired steel composition. Energy is also supplied in LF (Ladle Furnace) to guarantee steel temperature required in the teeming phase. In this paper a simplified, but sound numerical model of the process the ladle undergoes is presented: it is meant to provide a way to compare energy demand for different choices of materials used in the ladle or for changes in the ladle work cycle. As an example of application of the model, energy demand in LF are evaluated and compared for two different choices of the refractory material used as working lining: magnesia and alumina. To take into account the reduction of the working lining due to wear processes, energy demand has been calculated for three different thicknesses. In addition, the beneficial effect of a lid during the waiting phase after cleaning is investigated.

In the present work, a numerical investigation of the Latent Heat Thermal Energy Storage system (LHTESs) is performed on a vertical shell and tube geometry, made of a porous medium filled with a phase change material (PCM). The LHTESs is a cylinder with a corrugated inner surface at a constant temperature above the PCM melting temperature; on the external surface there are heat losses to the outside and the top and bottom surfaces are adiabatic. The PCM used is a pure paraffin wax, the metal foam, instead, is made of aluminum. The phase change process is modelled with the enthalpy-porosity theory, while the Local Thermal Non-Equilibrium (LTNE) and the Darcy-Forchheimer models are adopted to analyse the heat transfer between the paraffin and the metal foam. The solutions of the governing equations are computed with Ansys-Fluent commercial code. The survey considers different conductivity variations on the external wall and distinct corrugated wall geometric parameters, with different wavelengths and wave amplitudes. The results of the numerical simulations, concerning the LHTESs charging phase, are reported as a function of time and compared in terms of melting time, enthalpy stored, and energy loss.

In this article, an application of the macroscopic porous media approach, suitable for CFD simulations in oscillating flows, is proposed for a simplified Standing Wave Thermo-Acoustic Engine (SWTAE), composed of a hot buffer tube, a stack - where the energy conversion takes place - and the rest of the resonator. While for a Travelling Wave Thermo-Acoustic Engine (TWTAE) a macroscopic model for porous media has been successfully applied to both regenerators (similar to the stack in a SWTA) and heat exchangers, for SWTAE this is not true. The results illustrate that a Local-Thermal Non-Equilibrium model is required to start up the SWTAE, otherwise (with the Local Thermal Equilibrium model) the thermoacoustic instability cannot arise. Furthermore, the comparison between the simulation conducted at the microscopic scale and that one at the macroscopic level, depicts that a purely Darcy-linear model employed for the macroscopic model, characterized for oscillating flows, overpredicts the pressure amplitude at periodic steady-state. For this reason, a Forchheimer-like coefficient needs to be implemented to fit the macroscopic solution with the microscopic one.

Ejectors are classified as fluid-dynamics controlled devices where the "component-scale" performances are imposed by the local-scale fluid dynamic phenomena. For this reason, ejector performances (measured by the pressure-entrainment ratio coordinate of the critical point) are determined by the connection of operation conditions, working fluid and geometrical parameters. Given such a connection, variable geometry ejector represents a promising solution to increase the flexibility of ejector-based systems. The present study aims to extend knowledge on variable geometry systems, evaluating the local and global performances of the R290 ejector equipped with a spindle. The prototype ejector was installed at the R290 vapour compression test rig adapted and modified for the required experimental campaign. The test campaign considered global parameter measurements, such as the pressure and the temperature at inlets and outlet ports together with the mass flow rates at both inlet nozzles, and the local pressure drop measurements inside the ejector. In addition, the experimental data were gathered for different spindle positions starting from fully open position the spindle position limited by the mass flow rate inside the test rig with the step of 1.0 mm.

Natural circulation loops are systems used to transport heat by the free convection of a working fluid in a closed thermo-hydraulic circuit. Many examples of the Lattice Boltzmann method applied to free convection can be found in the literature, but most of them are limited to model simple enclosures. This work is one of the first approaches to an engineering system as the Natural Circulation Loops using this numerical method. This study describes the implementation and validation of a numerical model of a rectangular loop characterized by a small inner diameter. This research is focused on the thermohydraulic response of the NCL varying the working fluid. We have obtained satisfactory results demonstrating that accepted analytical correlations for the steady-state behavior are reproduced by the thermal Lattice Boltzmann Method (double distribution function). Overall, these results suggest that the temperature and velocity oscillations during the transient increase directly with the fluid Prandtl number; instead, the temperature difference between the vertical legs is inversely correlated and decreases.

Plume fires are characterized by a turbulent nature with a large number of different scales. LES is often used to solve the largest structures and to model the smallest ones. Grid size and time steps become decisive to place the limit between resolved and modelled turbulence. Significant information on this limit and its placement can be obtained with spectral analyses of the specific turbulent kinetic energy. While frequency analysis is relatively easy, an analysis in the wavenumber domain is more challenging. The IWC method, typically used in structures and acoustics, is used here for this purpose. IWC method allows to obtain wavenumber spectra with a better resolution than those obtained with a direct approach. Furthermore, in this paper the IWC method is also used in its reverse form to obtain frequency spectra. Although rather dense grids have been chosen, the number of nodes along the plume and their spacing is not such as to guarantee detailed wavenumber spectra with the direct approach and consequently with the reverse IWC. On the contrary, the IWC method provides wavenumber spectra in agreement with those obtained directly, but of much higher quality.

Computational Fluid-Dynamics (CFD) simulations are widely used for designing components and devices for heat transfer enhancement, and to this end the Reynolds Averaged Navier-Stokes (RANS) equations are often chosen, as they are computationally efficient. In this paper, several numerical simulations have been carried out on convective heat transfer of an air flow through a rectangular channel of 1:10 aspect ratio, 120 mm wide, 840 mm long. Numerical results have been compared to analytical values and experimental data. The configuration of the described numerical model will be used as starting point for detailed investigations of fluid-dynamic and thermal performances of ribbed channels in further analysis.

In this paper, a Computational Fluid-Dynamics (CFD) analysis on heat transfer characteristic using Reynolds Averaged Navier-Stokes (RANS) equations on a rectangular channel with ribbed surfaces is presented. Several numerical simulations have been carried out on convective heat transfer of an air flow through a rectangular channel of 1:10 aspect ratio, 120 mm wide, 840 mm long, with 90° ribs. Ribs have a square cross section with 4 mm side and three different values of dimensionless pitch, namely, 10, 20 and 40. Results on convective global heat transfer coefficient, i.e., averaged over the whole channel, have been compared to experimental data obtained in a channel with the same geometry and boundary conditions. Agreement between numerical and experimental data is discussed for the three different pitches considered. As expected, comparisons show a decreasing reliability for increasing complexity. The aim of this work is to find a suitable configuration of a CFD model with RANS that will permit authors to apply it to the range of dimensionless pitches.

Mini channel solution is used in devices that require a high density of transmitted thermal power as a very large-scale integration design in computer systems and compact exchangers. Furthermore, the mini channels are extensively investigated in the literature for turbulent and laminar regimes. In this project, different configurations of mini channels have been studied to enhance heat transfer, using simulations with a commercial multi-physics code. Thanks to the results of the models, more promising configurations with 3D printing technique may be built. The project challenge is improving convective thermal power extracted by the exhaust gases of a mini-catalytic combustor. The combustor feeds six modules for thermoelectric power production (TEMs). As the first step, three different mini channel geometries have been chosen; the first one with 19 channels with rectangular cross-section, the second one with 6 channels with a convergent profile, and the latter with 2 channels with a fractal branching geometry. Simulations started from studying fluid dynamic to investigate the velocity field at the exit of the mini channels. The analysis has been extended by adding the conjugate heat exchange between fluid and combustor wall. The results show an increase in heat exchange compared to the base case for all configurations, with a maximum value for the 19 mini channels configuration.

The aim of this work is to gain an insight on the effect of the wall heating on the aeroacoustic sound radiated by bluff bodies in laminar flows. In particular, the local thermal treatment of the wall boundary was investigated as a possible method for active controlling the emitted noise. This technique was studied performing direct numerical simulations of the aeroacoustic noise produced by an isolated square cylinder operating at a Reynolds and Mach numbers equal to 150 and 0.2, respectively. In the considered case, the Karman vortex street deriving by the flow/cylinder interaction, produces a lift and drag pulsation on the body surface, leading to a dipolar-like noise emission. In this context, different local thermal fluxes were applied to the cylinder wall in order to reduce its aerodynamic forces fluctuation and, consequently, the associated pressure disturbance that produces the radiated sound. The computations are performed using an OpenFOAM solver that adopts an explicit Runge-Kutta scheme for time integration and a second-order, energy conserving scheme for the convective part of the Eulerian flux. Moreover, the spurious numerical waves reflections at the far-field boundary are damped adopting a sponge-layer approach.

Electro-osmotic flows are a means of circulating polar fluids through microchannels without resorting to mechanical pumping. The lack of moving parts, of noise and the ease of integration in silicon chips make them an interesting option for microchip cooling and miniaturized total analysis systems. This paper describes an optimization in terms of first- and second-law analysis of the cross-section of a microchannel subject to electro-osmotic flow. Starting from rectangular cross-sections of different aspect ratios, its corners are progressively smoothed and the resulting Poiseuille and Nusselt numbers computed. Performance evaluation criteria are then used to assess the change in, among others, heat transfer rate, temperature difference between wall and bulk fluid, and equivalent pumping power. The entropy number is also computed and the results commented. It is found that the latter criterion highlights a configuration of minimum entropy generation, whereas the trends of the heat transfer and temperature difference are opposite to that of the equivalent pumping power.

Thanks to their high effective thermal conductivity, specific surface area, and tortuosity, open-cell foams are well-known for their capability to enhance heat transfer in applications such as heat exchangers and volumetric solar air receivers. In the very recent years, innovative manufacturing techniques, including 3D designing and printing, have been looked very helpful to find foam morphologies that allow to maximize heat transfer and minimize pressure drop. Optimal foam structures can be obtained by means of pore-scale simulations, employing an exhaustive search with a bearable computational effort. A multi-objective optimization of convective heat transfer and pressure drop in Kelvin's foams with air is presented in this paper. A pore-scale numerical model, with a uniform heat flux at the solid/fluid interface, is used to predict the interfacial convective heat transfer coefficient, h_c, and pressure drop, Δp, in the foam. The cell size, porosity, cell anisotropy stretching factor, as well as the inlet velocity and the direction of the air, are assumed as the design variables for the optimization model, while the interfacial convective heat transfer coefficient and pressure drop are chosen as the objective functions to be maximized and minimized, respectively. Pareto fronts ranging from h = 110 W/m² K and Δp = 0.77 Pa to h_c = 460 W/m² K and Δp = 51 Pa are predicted, within which the optimum point for the chosen foam morphology, air velocity and direction can be selected, according to the chosen criterion.

The linear stability analysis of a mixed convection viscous flow in a vertical porous pipe is here investigated. The contribution of viscous heating is assumed to be non negligible. A fully developed flow regime is assumed for the basic state. The local balance equations for this state display dual stationary solutions. The dual branches of stationary solutions are determined numerically. Since the pipe is characterised by an isothermal lateral surface, the viscous heating is the sole cause of the buoyancy force. In order to investigate the stability of the basic dual solutions, small amplitude disturbances with the form of normal modes are superposed to the basic state. The solution of the eigenvalue problem obtained allows one to determine the growth rate associated to both the basic solution branches. The sign of the growth rate determines whether the particular basic solution is stable or unstable.

In the present study it is proposed a solution approach of the Inverse Heat Conduction Problem (IHCP) intended at assessing the local heat transfer coefficient on the inner wall of a tube, under a forced convection problem. The estimation method is established on the Boundary Element Method (BEM) combined with the Truncated Singular Value Decomposition (TSVD) methodology, employed to manage the ill-conditioned nature of the problem. The numerical results of the direct problem, built by the BEM, are firstly validated, and consequently adopted as synthetic data inputs to resolve the IHCP and corroborate the whole assessment procedure. This approach is also tested with experimental data about forced convection problem in coiled pipes. In these geometries, the convective heat transfer coefficient changes considerably along the wall periphery and for this reason, it constitutes a perfect example to test the ability of the presented method to infer a spatially varying internal heat transfer coefficient distribution.

In this work the over-time behaviour of a shell-and-tube type heat exchanger applied to a commercial and small-scale wood biomass gasification system was investigated. The heat exchanger, equipped with twisted tape turbulators, was used to cool down the syngas produced by the power plant. An experimental campaign was conducted to evaluate the performance trend in the first 15 hours of operation. The results showed an increase in heat transfer which led to a progressive reduction of the gas outlet temperature from 105.4 °C to 90.0 °C. The data collected confirm the literature studies on the positive effect that the deposits of particulate matter and tars have in reducing the clearance between the heat exchanger pipes and the turbulators, providing information for the temporal optimization of the cleaning strategy of the heat exchanger itself.

Modelling of evaporative condensers relies heavily on the correct choice of the correlations for transport coefficients to give an accurate estimate of the dissipated thermal load and evaporated water mass flowrate, yet the literature does not cover all flow arrangements, nor operating conditions. In order to avoid expensive, and often impractical, experimental campaigns devoted to the determination of these quantities for a specific piece of equipment, a statistical approach based on the analysis of variance can be adopted: by analysing the sensitivity of the dependent variables on the choice of the transport coefficients, the best set of correlations can be determined, depending on whether a more accurate estimate is desired for the heat flux or evaporated mass flowrate, or if both are equally important.

The main outcome of the present paper is the feasibility analysis of SIRIO (Sistema di rimozione della Potenza di decadimento per Reattori InnOvativi) facility with conditions based on those of its reference facility. The aim of SIRIO project is to study an innovative Decay Heat Removal System (DHRS) for liquid metal reactor and advanced Light Water Reactor (LWR). Such system must ensure passive control of the power removed from the primary system in abnormal condition, and must ensure reactor cooling in both short and long term. This study present numerical simulations developed with RELAP5/MOD3.3, of two operational procedures: the first one is a steady-state and the second one is a transient phase with decay heat generation. The thermal-hydraulic model, developed with RELAP5/MOD3.3, simulates the whole facility including lines, valves, water and gas tanks, and the Molten Salts (MS) gap. Since there is not experimental data, the present paper is a pre-test study based on SIRO facility design.

Natural gas hydrates are the largest reservoir of natural gas on earth and represent an important solution for the transition from the actual energy scenario to a renewable one. Methane, contained in hydrate reservoirs, can be replaced with carbon dioxide, so that the obtained fuel is neutral in terms of climate-changing emissions and therefore equivalent to renewable energy sources. The experimental tests presented in this paper are devoted to study the CO₂-CH₄ replacement process in natural gas hydrates in presence of water salt. Test are carried out in a lab scale reactor of 1 l volume. The investigation aims at obtaining data on methane recovery rate, CO₂ sequestration, composition of the obtained outlet CO₂/CH₄ mixture, as well as spatial distribution of hydrates thanks to the analysis of temperature profiles. Temperature analysis is used to determine hydrates' spatial distribution within the sediment. The number of methane hydrate moles formed with salt water is lower than that with pure water. During CO₂ injection, not only CH₄-CO₂ replacement took place, but also new CO₂ hydrate formation. The exchange factor, in fact, ranges from 0.11 to 0.4 for tests with salt.

An energy and economic analysis on small-scale LAES (liquefied air energy storage) system is presented. The LAES operative parameters were analyzed via MATLAB simulations. The optimal case is given with a flowrate of 1000 kg/h and 4 turbine expansions, resulting in a net electric power output in the discharging process of 260 kW and the optimal values are 450 K for inlet temperature and 150 bar for the discharge pressure. For the charging process the specific consumption is significantly affected by the storage pressure. Also the round trip efficiency is influenced by the storage pressure: for a storage pressure of 8 bar, it is about 25% compared to the 12% in case of a storage pressure equal to 1.10 bar. The LCOS obtained is between 1.2 €/kWh and 1.8 €/kWh. This values are higher than other results in literature due to the scale of the system.

The Vacuum Vessel Pressure Suppression System (VVPSS) is designed to protect the ITER reactor (Vacuum Vessel, VV) from an overpressurization accident. The VVPSS is mainly composed of four Vapour Suppression Tanks (VSTs) partially filled with water inside which venting pipes (spargers) are immersed to direct the steam in the subcooled pool to condense it. Since the maximum pressure allowed inside the VV is only 0.15 MPa, a sub-atmospheric pressure must be maintained inside the VSTs. During a series of experimental campaigns funded by ITER Organization to study and validate the correct operability of the VVPSS and conducted in a large-scale plant built at the B. Guerrini laboratory of the University of Pisa, pressure instability and reverse flow through the sparger holes have been observed. The analysis of acceleration signals recorded during this pressure inversion, showed large amplitude oscillations at low frequency due to the Condensation Induced Water Hammer (CIWH).

In this paper, we present a new BOS (Background Oriented Schlieren), based on a hidden grid, for studying heat flows. In the setup, we record a grid-based intensity pattern whose phase map carries information about the temperature gradient. The background (undistorted) pattern is hidden in the light source. Quantitative analysis is obtained by a windowed Fourier transform approach. Some experimental results are given to demonstrate the feasibility of the technique.

The ventilation flow in a car cabin has been experimentally investigated. The study has been carried out in a car commercially available, by testing one ventilation mode (panel-vent mode) at one fan strength (level 3 of the 4 available) with fresh air intake (without any re-circulation). The flow velocity at the exit of the vents has been measured using a 5-hole pressure probe. The flow velocity fields inside the car cabin have been measured by particle image velocimetry (PIV) in three longitudinal sections: (i) the car centre plane, including both the front and rear area; (ii) the driver's seat centre plane, only in the front area; (iii) the passenger's seat centre plane, only in the front area. At these longitudinal planes, the time-average flow velocity is presented and discussed. The experimental results provide new insights in the ventilation flow in a car cabin.

Refrigerant R513A represents an interesting solution for the retrofit of conventional high-GWP fluorinated gases, such as R134a, R401A, R401B and R409A for low and medium temperature applications. R513A is an azeotropic mixture (almost zero-temperature glide at any operating pressure) made up of R134a and R1234yf (44% and 56% in mass, respectively), allowing at the same time a very low GWP of 580 and favourable safety characteristics such as no flammability and no toxicity (A1 ASHRAE class). The boiling performance of this blend is scarcely explored and studied in scientific literature, especially in case of commercial tubes typically adopted for refrigeration purposes. For this reason, this paper presents two-phase flow boiling experiments of refrigerant R513A in a 6.00 mm horizontal stainless-steel tube. Heat is provided by means of Joule effect directly on the tube surface, and the peripheral average heat transfer coefficients are obtained by measuring the temperatures at four sides (top, bottom, left and right) of the channel. The effect of the operating conditions is experimented and discussed, by varying the mass flux between 150 and 300 kg/m² s, saturation temperature between 20 and 50°C and imposed heat flux between 5 and 20 kW/m². Also, a comparison with the boiling performance of refrigerant R134a is proposed within the same operating conditions. Finally, the assessment of well-known flow boiling prediction methods is presented and discussed.

Condensation of the water vapor present in the air is a heat and mass transfer process encountered in many applications as humid air dehumidification and water harvesting. Depending on the wettability characteristics of the surface, condensation can take place in filmwise mode or in dropwise mode with the formation of discrete liquid droplets over the condensing surface. While dropwise condensation (DWC) of pure steam was found to promote a considerable enhancement of the heat transfer compared to filmwise condensation, when dealing with humid air DWC more investigation is needed. Modeling of DWC from humid air requires the calculation of the heat flow rate through a single droplet and the determination of the drop-size distribution. The heat exchanged through a single droplet depends on the heat and mass transfer resistances, while the drop-size distribution is also affected by nucleation site density and droplets mobility. Therefore, to better understand the DWC phenomenon with humid air and for the validation of the models, it is necessary to measure the heat flux (total and latent), droplet population and nucleation site density. In the present work, condensation tests from humid air are performed over two square (40 mm x 40 mm) aluminum samples that display different wettability. The experimental apparatus consists of a closed air loop with two main components: the environmental chamber and the test chamber. The air is conditioned in the environmental chamber and then it flows inside the test section where the vapor present in the humid air is condensed over the vertical metallic sample. Two variable speed fans are used to circulate the air. The test section is designed for heat and mass transfer measurements and for simultaneous visualization of the condensation process. As a peculiar characteristic of the present experimental technique, all the test section assembly is suspended on a high precision balance allowing a precise measurement of the mass of condensate. The effect of surface wettability on the heat and mass transfer during DWC is investigated. Time-lapse videos of the condensation process are acquired at different magnifications. By using a homemade MATLAB^® program for droplet detection, recorded images are analysed allowing the determination of both the drop size density distribution (small and large droplet population) and the nucleation sites density.

Environmental concerns are forcing the replacement of the commonly used refrigerants and finding new fluids is a top priority. The hydro-fluoro-olefin (HFO) R1234ze(E), because of its smaller global warming potential (GWP) and shorter atmospheric lifetime, replaced R134a. Accordingly, for HVAC systems design, a detailed knowledge of the thermo-fluid-dynamic characteristics of the fluids and reliable predictive models are required. To improve the understanding, R134a and R1234ze(E) were employed in convective condensation experiments (saturation temperature T_sat = 35°C, mean quality x_m = 0.1~0.9, quality changes Δx = 0.05~0.6, mass flux G = 43~444 kg·m⁻²s⁻¹) inside a microfin tube (outer diameter D = 9.52 mm, fin number n = 60, fin height H = 0.2 mm). The results were used for two goals: the former is the comparison of the heat transfer features of the two fluids, while the latter aims at testing the performance of prediction models available in the open literature. At the saturation temperature T = 35°C, the two fluids show small differences in the thermal properties so that, as expected, the experiments highlighted a very similar behavior in the typical operating conditions of HVAC systems. In fact, for all the operating conditions marginal differences were observed in the pressure drop, the heat transfer coefficient and the flow pattern maps. The issue of prediction reliability, however, is still open. Actually, not all the models achieving good results for R134a show the same performance for R1234ze(E), especially for the pressure drop.

The recent restrictions on the use of refrigerants with high Global Warming Potential (GWP) have pushed the research to consider alternative solutions, such as the HydroFluoroOlefins, as possible replacements. Even though they are generally mildly flammable, their GWP is significantly lower compared to those of the HydroFluoroCarbons. This paper presents experimental measurements of R1234ze(E) and R134a heat transfer coefficients during condensation inside a brazed plate heat exchanger. Experimental tests during condensation heat transfer have been conducted considering different degrees of superheating, subcooling and outlet vapour quality. The condensation temperature varies between 34.6 °C and 42.3 °C and the refrigerant mass flux is between 9 kg m⁻² s⁻¹ and 49 kg m⁻² s⁻¹. The results showed that the heat transfer coefficients measured with R134a are between 4% and 8% higher than those of R1234ze(E). Complete condensation experiments showed that an increase in the liquid subcooling degree significantly reduces the thermal performance at low refrigerant mass velocities. In some cases, the plate area occupied by liquid refrigerant reached almost 30% of the overall heat transfer area, thus decreasing by 2.7 times the average heat transfer coefficient when passing from 3 K to 8 K subcooling degree. A comparison with the predictions of some empirical models is also presented to assess which one can better predict the experimental data.