Giulia Righetti

University of Padua | UNIPD · Department of Management and Engineering

Metal Additive Manufacturing (MAM) is a non-traditional technology recently introduced to manufacture multifunctional mechanical components. In fact, recent developments in Laser Powder Bed Fusion (LPBF) process have enabled the production of materials characterized by high density and high thermal conductivity properties, such as copper alloys and...

This work presents some new experimental measurements collected on a very innovative system proposed for electronic cooling. This setup combines a classical heat sink with a latent thermal storage. The storage material is a phase change nanoemulsion made of water and 5 wt% of a commercial RT40HC paraffin wax with 1-octadecanol as nucleating agent (...

This work presents some new experimental measurements collected on a very innovative system proposed for electronic cooling. This setup combines a classical heat sink with a latent thermal storage. The storage material used is a phase change nanoemulsion made of water and 5 wt. % RT40HC paraffin wax with 1-octadecanol as nucleating agent (weight fr...

Phase change material nanoemulsions were synthesised and characterised to be used as possible storage and heat transfers fluids in low temperature thermal management. Thermal energy storage densities of different paraffin / water nanoemulsions were studied, and RT44HC was selected as the optimized PCM. Seven different nucleating agents, including p...

This paper presents the local heat transfer coefficients of R290 (propane) and R1270 (propylene) condensation inside a Brazed Plate Heat Exchanger (BPHE). A new test section was designed for measuring the local heat transfer coefficient in refrigerant two-phase heat transfer in a BPHE. The test section includes two corrugated plates instrumented wi...

The transportation of perishable products is a crucial activity that must be carefully managed along the cold chain. The refrigerated transport aims at ensuring the suitable transportation conditions by maintaining the desired temperature level in the refrigerated compartment. Moreover, the carriage of perishable fresh foodstuffs calls for addition...

This paper presents the local heat transfer coefficients of R290 (propane) and R1270 (propylene) boiling inside a Brazed Plate Heat Exchanger (BPHE). A new test section was designed for measuring the local heat transfer coefficient in refrigerant two-phase heat transfer in a BPHE. The test section includes two thick AISI316L stainless steel corruga...

This paper presents the local heat transfer coefficients of R32 and R410A complete condensation inside a Brazed Plate Heat Exchanger (BPHE). A new test section, with one channel on the refrigerant-side and two channels on the water-side, was specifically designed and manufactured for measuring the local heat transfer coefficient in refrigerant two-...

It is well known that latent thermal energy storages can help in store large quantities of energy in small volumes, but that their charging and discharging periods are prohibitively long. In this work it was experimentally demonstrated that the addition of a foam can drastically shorten the phase change materials charging and discharging times up t...

This paper presents the measurement of liquid phase density, thermal diffusivity, thermal conductivity, and dynamic viscosity of five commercial PCM (RT2HC, RT5HC, OM05-P, OM08, CrodaTherm9.5) in the temperature range 2 − 30 °C. The density of RT2HC, RT5HC, OM05-P is 23-24% lower than water, that of OM08 is 6% higher than water and that of CrodaThe...

The low thermal conductivity of Phase Change Materials (PCM) can be improved with extended surfaces such as additively manufactured 3D periodic lattice structures. Three different aluminum alloy-based lattices (base sizes 10, 20, and 40 mm) with average porosity of 0.95 filled with paraffin wax, with a nominal phase change temperature of 55°C, were...

This paper presents the local heat transfer coefficients of R32 and R410A boiling inside a Brazed Plate Heat Exchanger. A new and innovative test section, with one channel on the refrigerant-side and two channels on the water-side, was specifically designed and manufactured for measuring the local heat transfer coefficient in refrigerant two-phase...

This manuscript presents an experimental study on heat transfer and pressure drop of two aluminum lattice-frame materials obtained via additive manufacturing, varying the air flow rate and the electrical power. A round shape and an airfoil shape samples were compared. The second one was proposed to reduce pressure drops. It was measured that pressu...

Latent Thermal Energy Storage (LTES) systems adopting Phase Change Materials (PCMs) have been proposed to be implemented along the cold chain over the last years. Hence, in this work, a novel insulated wall concept for refrigerated truck is proposed. A 2D transient numerical model of the truck cell is developed. Results are obtained considering the...

Open-cell foams offer several interesting possibilities in numerous technological fields. In fact, they present high surface area to volume ratio as well as enhanced flow mixing and attractive stiffness and strength. However, their complete and reliable characterization has not been completed yet. In fact, there is still no a comprehensive work tha...

This paper shows the promising capabilities that the aluminum metal foams offer in enhancing the water pool boiling on flat surfaces. Different metal foam samples, with pore density ranging between 5 and 40 Pore Per Inch (PPI), with similar porosity values, around 0.92, and two different thickness values, 5 and 10 mm, were tested to investigate the...

This study performs the comparative analysis of R32 and R410A boiling inside a 4.2 mm ID microfin tube. The experimental tests were carried out at different saturation temperatures, different vapour qualities, heat fluxes and mass velocities to evaluate the specific contribution of each operating parameter. The boiling heat transfer coefficients sh...

This work aims at explaining the effect of the operating conditions on the performance of passive electronic thermal management systems based on Phase Change Materials. The low thermal conductivity of the Phase Change Materials is usually felt as one of their major limitations that hinders the effective heat transfer capability of the whole passive...

In this work, an innovative insulated wall concept for refrigerated truck is proposed. A 2D transient numerical model of the truck cell is developed and simulated considering the solar radiation from 6 AM to 4 PM of a typical summer day in Vicenza (Italy). The innovative composite wall consists of a traditional polyurethane (PU) insulation layer wr...

This study performs the comparative analysis of R32 and R410A condensation inside a 4.2 mm ID microfin tube: the effects saturation temperature, vapour quality and mass velocities are evaluated. The heat transfer coefficients show a great sensitivity to vapour quality, a non-negligible sensitivity to saturation temperature/pressure, and a non-monot...

This paper presents a Gradient Boosting Machines (GBM) model for predicting refrigerant two-phase frictional pressure gradient inside Brazed Plate Heat Exchangers (BPHE) based on an extensive database that includes 1624 boiling data-points, 925 condensation data-points, 16 different plate geometries, and 16 different refrigerants (including 4 natur...

R134a was recognized as probably one of the most important refrigerants of the two past decades. Among the proposed alternative fluids, there are certainly isobutane (R600a) and R152a. This article presents about 200 new heat transfer coefficient and pressure drop data obtained during flow boiling of R152a and R600a inside a smooth copper tube havi...

This paper presents an Artificial Neural Network (ANN) model for predicting refrigerant boiling heat transfer coefficients inside Brazed Plate Heat Exchangers (BPHE). The model accounts for the effect of plate geometry, operating conditions and refrigerant properties. The model shows a fair agreement with a database of 1760 data points comprising 1...

This paper presents a detailed discussion on the hysteresis phenomenon caught during the flow boiling heat transfer of R245fa on a Carbon/Carbon surface. Due to the partial hydrophilicity of the Carbon/Carbon surface, a hysteresis in flow boiling heat transfer is observed and studied to understand the underlining heat transfer mechanisms on the bas...

This paper presents an Artificial Neural Network (ANN) model for predicting refrigerant condensation heat transfer coefficients inside herringbone-type Brazed Plate Heat Exchangers (BPHE). The model accounts for the effect of plate geometry, operating conditions and refrigerant properties both in the saturated and in the superheated vapour condensa...

With current growth in electronic systems, their physical sizes decrease, and the spacing between components decreases, both the total amount of heat generated and the power density increase significantly. There is a general agreement in the scientific community that current air-cooling technologies are asymptotically approaching their intrinsic li...

R1233zd(E) and R1224yd(Z) are two HCFO (Hydro Chloro Fluoro Olefin) low-GWP refrigerants recently proposed as substitutes of the HFC (Hydro Fluoro Carbon) fluids commonly used in air conditioning, high temperature heat pumps and ORC applications. A few experimental works regarding the two phase heat transfer in small diameter smooth and microfin tu...

The interest in Phase Change Materials (PCMs) has been continuously growing, since they were identified as a suitable way to store large quantities of thermal energy. Despite many PCMs being available on the market, almost all present a relatively low thermal conductivity, which limits the efficiency and the convenience of their use inside Latent T...

This paper presents an assessment of new low-GWP refrigerants through coupling the experimental measurement of thermal and hydraulic performances to a theoretical evaluation based on the Total Temperature Penalisation (TTP) criterion for the specific case study of two-phase heat transfer inside a small-diameter smooth tube. The TTP analysis was als...

In the scientific and industrial community, the interest on Latent Thermal Energy Storages (LTESs) is continuously increasing. These components can be easily coupled with intermittent renewable energy sources and with heat sources that undergo to cycling operations. Phase Change Materials (PCMs) are considered convenient and reliable media to be us...

This paper performs the thermodynamic and the heat-transfer assessment of the low-GWP refrigerants R600a, R1234ze(Z) and R1233zd(E) as alternative to the traditional low-pressure HFC refrigerants, such as R245fa, for Heat Pump (HP) and Organic Rankine Cycle (ORC) applications. The thermodynamic assessment shows that R1234ze(Z) exhibits efficiency v...

R134a is ongoing a phase-down process in several countries and R1234yf can be a viable low GWP alternative together with R1234ze(E). Since new experimental data are always requested by researchers and industry to increase the global database, to assess the existing correlations, and to expand the knowledge about the refrigerant applications, in thi...

In this paper, experimental data pertinent to deionized water pool boiling across 10 mm thick aluminum foams are presented. Two foam samples with different pore densities, 5 and 10 pores per inch, yet an identical mean porosity of 0.92 are tested. Compared to a heated flat plate, of the same base size, the foams offer higher heat transfer area albe...

This paper presents the heat-transfer assessment of different low GWP substitutes for traditional HydroFluoroCarbon (HFC) refrigerants obtained by applying both an experimental analysis based on the direct measurement of thermal (heat transfer coefficients) and hydraulic (pressure drops) performances and a theoretical analysis based on a specific P...

Free convection in a partially foam-filled enclosure is theoretically investigated. Two independent approaches are undertaken. Scale analysis has been conducted along with a thermal resistance network model to predict the performance of metal foams as heat sinks to remove heat from a horizontal heated plate. Results from scale analysis were used to...

It is well-known that the use of the latent heat absorption phenomenon associated with the melting process of a suitable Phase Change Material (PCM) can be considered an effective way to improve energy storage capabilities in many applications. This work experimentally investigates the effects of six open-cell aluminum foams and of a 3-D periodic a...

This paper presents the heat transfer coefficients and the frictional pressure drops of R1234ze(Z) and R1233zd(E) boiling inside a commercial Brazed Plate Heat Exchanger (BPHE): the effects of heat flux / mass flux, saturation temperature / pressure, outlet conditions and fluid properties are investigated. The boiling heat transfer coefficients are...

R1233zd(E) has been proposed as low GWP alternative to R245fa. This paper compares the R1233zd(E) and R245fa performance during flow boiling inside a small diameter microfin tube having internal diameter at the fin tip equal to 4.2 mm, 40 fins, 0.15 mm high, and a helix angle of 18°. The experimental measurements were carried out at a constant mean...

This study performs the comparative analysis of R134a, R152a, R1234yf and R1234ze(E) condensation inside a 4 mm ID smooth tube. The experimental tests were carried out at three different saturation temperatures, 30, 35, and 40 °C, at different vapour qualities and mass velocities to evaluate the specific contribution of refrigerant mass flux, mean...

The use of phase change materials (PCM) can be considered an effective way to improve the energy storage capabilities of hybrid water thermal energy storage (TESs) in solar heating and cooling plants. However, due to a few shortcomings, their use is still limited. This paper aims to give a direct estimation of the considerable advantages achievable...

This study performs the comparative analysis of R134a, R152a, R1234yf and R1234ze(E) condensation inside a 4 mm ID smooth tube. The experimental tests were carried out at three different saturation temperatures, 30, 35, and 40 °C, at different vapour qualities and mass velocities to evaluate the specific contribution of refrigerant mass flux, mean...

The latent heat absorption phenomenon associated with melting of a suitable phase change material can be an effective way to improve the thermal energy storage (TES) behaviour in many applications. However, the most suitable materials to be used in heating and refrigeration systems find intrinsic limitations due to their poor heat transfer capabili...

Carbon fiber reinforced carbon matrix (C/C) composites have been widely used in aeronautics and aerospace industries for several decades. Some new experimental measurements carried out during flow boiling heat transfer of R245fa on a C/C surface are presented. The test section with the C/C sample is electrically heated from the bottom and it is in...

Chicken meat is among the most largely consumed meat worldwide. The shelf life of chicken meat is rather short so freezing is largely used to reduce water activity and stop the microorganism proliferation. In this paper experimental data available in the literature for food freezing, including poultry products, are briefly reviewed. Then, some new...

In this paper some new experimental data collected during R1233zd(E), a new hydrochlorofluoroolefin molecule, flow boiling inside a microfin tube having an inner diameter equal to 4.3 mm are reported. The tube was made of copper, set up in horizontal position and electrically heated from the bottom. The data, presented in terms of heat transfer coe...

This paper presents the development of a new electronic cooling concept especially conceived for helicopter avionics thermal management in the framework of 7 th FP EU project TOICA (Thermal Overall Integrated Conception of Aircraft, www.toica-fp7.eu). A prototype was developed and tested. It consists of a hybrid active-passive cooling system that c...

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 poo...

This paper presents the R245fa flow boiling heat transfer and pressure drop measurements inside a mini microfin tube with internal diameter at the fin tip of 4.2 mm, having 40 fins, 0.15 mm high with a helix angle of 18°. The tube was brazed inside a copper plate and electrically heated from the bottom. Sixteen T-type thermocouples are located in t...

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 inle...

Carbon/Carbon (C/C) is one of the most high-tech materials developed by the aerospace industry and then applied to critical components in several different applications. This composite material appears to be a viable option for future thermal management devices because it exploits interesting properties having a low density and a relatively high th...

This paper presents the comparative analysis of HC290 (Propane), HC1270 (Propylene), and HFC404A during vaporisation inside a 4 mm smooth tube. The experimental tests were carried out at three different saturation temperatures (5, 10, and 20 °C) at increasing vapour quality up to incipient dryout to evaluate the specific contribution of heat flux,...

This paper presents the comparative analysis of HFC404A and its low GWP substitutes HC290, and HC1270 in saturated vapour condensation inside a 4 mm ID horizontal smooth tube. The experimental tests were carried out at 30, 35, and 40 °C of saturation temperatures, with refrigerant mass flux in the range 75–800 kg m⁻² s⁻¹ at decreasing vapour qualit...

This paper compares HFC404A and two possible hydrocarbon substitutes HC290 (propane) and HC1270 (propylene) in condensation inside a commercial brazed plate heat exchanger (BPHE) at saturation temperatures between 25 and 40 °C. The effects of saturation temperature (pressure), refrigerant mass flux, and vapour super-heating on heat transfer and hyd...

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 h...

This paper identifies different commercially available peristaltic and diaphragm micro-pumps suitable for the application in a portable / wearable artificial kidney and carries out their flow dynamic and energetic assessment in a specifically designed experimental rig. This analysis provides information on pressure and flow rate behaviours for deve...

Since 2010, HydroFluoroOlefines (HFOs) 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 tu...

This paper presents the heat transfer coefficients and the pressure drops measured during HFC404A vaporisation inside a commercial BPHE and the comparison of this data with previous measurements carried out during HC290 (Propane) and HC1270 (Propylene) vaporisation inside the same BPHE and similar operating condition in order to assess the capabili...

This paper investigates the effects of heat flux, saturation temperature, and outlet conditions on HFO1234ze(E) boiling inside a Brazed Plate Heat Exchanger (BPHE). The effect of the heat flux on the heat transfer coefficients was remarkable. Similar consideration applies for outlet condition effects whereas the impact of saturation temperature was...

This paper presents the comparative analysis of HFO1234ze(E) and HFC134a during vaporisation inside a 4 mm smooth tube. The experimental tests were carried out at three different saturation temperatures (10, 15, and 20 °C) at increasing vapour quality up to incipient dryout to evaluate the specific contribution of heat flux, refrigerant mass flux,...

Abstract This paper presents a new model for refrigerant boiling inside Brazed Plate Heat Exchangers (BPHEs) based on a set of 251 experimental data previously obtained by the authors which includes data points relative to HFC refrigerants (HFC236a, HFC134a, HFC410A), HC refrigerants (HC600a-Isobutane, HC290-Propane, HC1270-Propylene), and also a n...

This paper presents the comparative analysis of HFC32 and HFC410A during vaporisation inside a 4 mm smooth tube. The experimental tests were carried out at three different saturation temperatures (5, 10, and 20 °C) at increasing vapour quality up to incipient dryout to evaluate the specific contribution of heat flux, refrigerant mass flux, mean vap...

Cooling and freezing food is one of the most widely discussed topics in the refrigeration literature. Some semi-empirical models have been proposed to estimate the total freezing time but the achievable results can be significantly different depending on the model used. This paper focuses on the freezing time of poultry products. Thanks to a calibr...

This paper presents HFC32 average boiling heat transfer coefficients and pressure drops measured inside a small Brazed Plate Heat Exchanger (BPHE): the effects of heat flux, saturation temperature (pressure), and outlet conditions are investigated. The experimental tests were carried out at four different saturation temperatures (5, 10, 15, and 20°...

This paper presents a new computational procedure for refrigerant condensation inside herringbone-type Brazed Plate Heat Exchanger (BPHE). A transition point between gravity controlled and forced convection condensation was found for an equivalent Reynolds number around 1600. At low equivalent Reynolds number (<1600) the heat transfer coefficients...

This paper presents the comparative performance analysis of the low GWP refrigerants HFO1234yf, HFO1234ze(E) and HC600a inside a commercial roll-bond evaporator for household refrigerators. The vaporisation performances were evaluated at two evaporation temperatures, -15 and -20 °C, and different refrigerant mass flow rates and compared with those...

This paper presents the experimental heat transfer coefficients and pressure drops measured during refrigerant HFC32 condensation inside a commercial Brazed Plate Heat Exchanger (BPHE) and compares this data with similar measurements previously obtained for refrigerant HFC410A to assess its capability as low GWP substitute for HFC410A in medium siz...

This paper presents the comparative analysis of a traditional raw milk dispenser and an innovative prototype based on nanofluid technology. The traditional raw milk dispenser consists of an off-the-shelf upright air-cooled refrigerator sold on the market, whereas the innovative prototype presents a tank equipped with a serpentine tube jacket operat...

This paper presents the experimental heat transfer coefficients and pressure drop measured during refrigerant HFO-1234ze(Z) saturated vapour condensation inside a commercial Brazed Plate Heat Exchanger (BPHE) and compares this data with similar measurements previously obtained for refrigerant HFC-236fa, HFC-134a, HC-600a, HFO-1234ze(E) in order to...

In the present paper the experimental and theoretical analysis of a Heat Pipe Heat Exchanger (HPEH) is reported. The HPHE consists of 54 horizontal 12.7 mm micro-fin copper tubes 700 mm in length and aluminium continuous fins 0.115 mm in thickness with 3 mm of fin spacing. The HPHE presents an evaporator section of 270 mm, a condenser section of 27...

This paper presents heat transfer coefficients and pressure drops measured during condensation of the low GWP refrigerant HFO1234ze(E) inside a BPHE. The condensation temperature ranges between 24.8 and 40.2 degrees C, the refrigerant mass flux from 10.7 to 39.9 kg M-2 s(-1) and the heat flux from 5.3 to 26 kW m(-2). The heat transfer coefficients...