Hybrid PCM\u2014aluminium foams\u2019 thermal storages: an experimental study

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 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 capabilities. This work experimentally studies the use of aluminum foams as heat transfer medium to improve the overall heat transfer of paraffin waxes that can be possible phase change materials to be implemented in hybrid sensible-latent water thermal energy storages. The experimental tests were run in a dedicated setup designed, developed, and built at the Department of Management and Engineering of the University of Padova. The effects of the use of aluminum foams as enhancing heat transfer medium were studied by comparing the loading and unloading processes of a paraffin wax with melting temperature around 40 \ub0C, with and without metal foams, in a water thermal storage unit. The effects of three different foams with 5, 20, and 40 Pores Per Inch (PPI) were investigated

Numerical analyses of concrete thermal energy storage systems: effect of the modules' arrangement

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3D CFD Simulation of a New Ventilated Roof

In the last decades, energy management and saving have become challenging issues. Considering the building sector (residential or industrial), different technologies have been developed in order to realize tangible energy savings, such as: ventilated roof, double facades, glazed surfaces, etc. Nonetheless, it is important for these new technologies to contemporary assure the human thermal comfort. Passive cooling (or heating) technologies are of actual interest. Low or near-zero energy buildings can only be realized as a result of the good design of all their components; specifically, the roofs call for particular attention as they take large parts of a building’s total surface area. This paper presents a comparison between an innovative ventilated roof, based on an original design of the support and a traditional one. A 3D numerical model is developed to analyze the air flow and to compute the achievable benefits in terms of reduction of the summer heating gains. The simulations were performed by varying the solar irradiance from 600 to 1000 W m2. The investigation is conducted comparing a ventilated roof assembly to the same traditional structure, assuming buoyancy-driven airflow. Two roof types are studied: an insulated roof and a non-insulated one. The results reveal that the ventilated roof leads to a great reduction of the total amount of solar heat gains for all the simulated scenarios

R134a Flow Boiling Heat transfer on an Electrically Heated Carbon/Carbon Surface

With the increase of heat flux densities following the Moore’s law, electronic cooling challenge is focused on the high heat flux to be dissipated by the operating fluid and more and more efficient heat spreaders, dissipators, and compact heat exchangers are in great demand for various applications. Considering the device efficiency, the boiling heat transfer ensures very high heat transfer coefficients, which can even be improved via specific surface treatments that have been shown to be very effective. In particular, several authors, experimentally demonstrated the interesting enhancement capabilities of microparticles coatings on the Critical Heat Flux. Furthermore, the recent work on nanoscale domain has led to new concepts for surface modification. In the last decade, nano-structured materials (i.e. nanowires coatings, nanoporous layers, Carbon Nano Tube arrays, etc.) have been proved to enhance the boiling heat transfer. Unfortunately, almost all of this kind of surface treatments fail when scaled up to industrial implementation because of the relatively high costs and complex operations involved. Furthermore, compactness and lightness of cooling systems are becoming even more challenging design constraints leading the research efforts towards new light and efficient materials. In this scenario, the Carbon/Carbon material appears to be a viable option for future thermal management devices because it exploits interesting properties having a low density and a high thermal conductivity; moreover, it is already used in many industrial applications where it is shaped in various forms even complex. This paper presents the experimental measurements carried out during flow boiling heat transfer of R134a on a Carbon/Carbon surface. The test section with the Carbon/Carbon sample, is electrically heated from the bottom and it is instrumented with 18 wall thermocouples to monitor the temperature distribution at an imposed heat fluxes of 50 kW m-2, and refrigerant mass flow rates from 50 to 200 kg m-2 s-1, at constant saturation temperature of 30 °C. The sample is tested in a new experimental facility built at the Nano Heat Transfer Lab of the Department of Management and Engineering of the University of Padova especially designed to study the flow boiling heat transfer process on innovative materials and enhanced micro- and nano-structured surfaces.

R134a And Its Low GWP Substitutes R1234yf And R1234ze(E) Flow Boiling Inside A 4mm Horizontal Smooth Tube

The substitution of HFC134a with low GWP refrigerants is one of the most important challenge for refrigeration and air conditioning. The possible substitutes include natural refrigerants, such as HC600 (Butane) and HC600a (Isobutane), and also synthetic refrigerants, such as HFO1234yf and HFO1234ze(E). The HC refrigerants exhibit very low GWP, 3 and 4 HC600a and HC600 respectively, good thermodynamic and transport properties, and pressure and volumetric performance very similar to HFC134a. The major drawback of HC refrigerants is their high flammability, being classified in class A3 according to ASHRAE classification. Also the HFO refrigerants present a mild flammability, being classified in class A2L. In fact, it is very difficult to found low GWP substitutes for traditional HFC refrigerants with no flammability, as a weak chemical stability and / or a big chemical reactivity are presuppositions for low GWP. Both HFO1234yf and HFO1234ze(E) seem to be very promising as substitute for HFC134a, showing a GWP lower than 1 together with pressure and volumetric properties closely near to those of HFC134a. This paper presents the comparative analysis of HFC134a HFO1234yf and HFO1234ze(E) during saturated flow boiling inside a 4 mm horizontal smooth tube: the effects of heat flux, refrigerant mass flux, mean vapour quality and saturation temperature (pressure) are investigated separately to rank the superposed effects of different heat transfer regimes (nucleate boiling or/and forced convection boiling). 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, mean vapour quality, and saturation temperature (pressure). The refrigerant mass flux ranges from 200 to 600 kg m-2s-1 and the heat flux from 15 to 30 kW m-2. The experimental measurements were reported in term of boiling heat transfer coefficients and frictional pressure drops. Heat transfer coefficients have a positive slope versus vapour quality and the slope increases with refrigerant mass flux and decreases with heat flux. Saturation temperature (pressure), refrigerant mass flux and mean vapour quality have a remarkable impact on the frictional pressure drop, whereas the effect of heat flux appears marginal or negligible. Convective boiling seems to be the prevailing heat transfer regime in present experimental tests. HFO1234ze(E) and HFO1234yf exhibit heat transfer coefficients and pressure drops similar to HFC134a. Present heat transfer coefficients and pressure drops were also compared against different correlations for refrigerant boiling inside tube. The universal correlation proposed by Kim and Mudawar (2014) and the Friedel (1979) correlation show the best performance in predicting heat transfer coefficients and pressure drops, respectively

HFO1234ze(E) And HFC134a Flow Boiling Inside a 4mm Horizontal Smooth Tube

Nowadays, the substitution of HFC134a with low GWP refrigerants is one of the most important challenge for refrigeration and air conditioning. The possible substitutes include natural refrigerants, such as HC600 (Butane) and HC600a (Isobutane), and also synthetic refrigerants, such as HFO1234yf and HFO1234ze(E). The HC refrigerants exhibit very low GWP, 3 and 4 HC600a and HC600 respectively, good thermodynamic and transport properties, and pressure and volumetric performance very similar to HFC134a. The major drawback of HC refrigerants is their high flammability, being classified in class A3 according to ASHRAE classification. Also the HFO refrigerants present a mild flammability, being classified in class A2L. In fact it is very difficult to found low GWP substitutes for traditional HFC refrigerants with no flammability, as a weak chemical stability and / or a big chemical reactivity are presuppositions for low GWP. In particular HFO1234ze(E) seems to be very promising as substitute for HFC134a, showing a GWP lower than 1 together with pressure and volumetric properties closely near to those of HFC134a. This paper presents the comparative analysis of HFC134a and HFO1234ze(E) during saturated flow boiling inside a 4 mm horizontal smooth tube: the effects of heat flux, refrigerant mass flux, mean vapour quality and saturation temperature (pressure) are investigated separately to rank the superposed effects of different heat transfer regimes (nucleate boiling or/and forced convection boiling). 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, mean vapour quality, and saturation temperature (pressure). The refrigerant mass flux ranges from 200 to 600 kg m-2s-1 and the heat flux from 15 to 30 kW m-2. The experimental measurements were reported in term of boiling heat transfer coefficients and frictional pressure drops. The heat transfer coefficients have a positive slope versus vapour quality and the slope increases with refrigerant mass flux and decreases with heat flux. Saturation temperature (pressure), refrigerant mass flux and mean vapour quality have a remarkable impact on the frictional pressure drop of both HFO1234ze(E) and HFC134a whereas the effect of heat flux appear marginal or negligible. Convective boiling seems to be the prevailing heat transfer regime in present experimental tests. HFO1234ze(E) exhibits heat transfer coefficients similar to HFC134a and slightly higher frictional pressure drops. Present heat transfer coefficients were compared against different heat transfer correlations for refrigerant boiling inside tube. The universal correlation proposed by Kim and Mudawar (2014) shows the best performance with a mean absolute percentage deviation of 6.1% both for HFO1234ze(E) and HFC134a data, respectively. Present frictional pressure drops were compared against different correlation for two-phase pressure drop inside tube: Friedel (1979) correlation shows the best performance with a mean absolute percentage deviation of 11.7% and 12.6% for HFO1234ze(E) and HFC134a respectively

HFO1234ze(E) Boiling Inside a Brazed Plate Heat Exchanger

HFC134a has been probably the most important refrigerant of the two past decades as it dominated the application in domestic refrigeration, mobile air conditioning and large chillers and it took part as component in several refrigerant mixtures such as HFC404A, and HFC407C. Unfortunately HFC134a exhibits a relatively large Global Warming Potential (GWP), and it will be subjected to a gradual reduce in the use up to a complete phase out in the next future according to the different national and international regulations. The HydroFluoroOlefins (HFO) refrigerants HFO1234yf and HFO1234ze(E) seem to be the most promising substitutes for HFC134a as they exhibit very low GWP values (1 or less) together with pressure and volumetric properties closely near to those of HFC134a. The unique drawback of HFO refrigerants seems to be their mild flammability. The Brazed Plate Heat Exchangers (BPHE), which involve a reduction of the refrigerant charge of one order of magnitude as compared to the traditional tubular heat exchangers, are particularly interesting for limiting the risk of flammable or mildly flammable refrigerants such as HFO1234ze(E). In fact the first attempt to reduce the risk of flammable refrigerants is to decrease the refrigerant charge. This paper presents the experimental heat transfer coefficients and pressure drop measured during HFO1234ze(E) boiling inside a small BPHE: the effects of heat flux, refrigerant mass flux, saturation temperature (pressure) and outlet conditions are investigated. The evaporator tested is a BPHE consisting of 10 plates, 72 mm in width and 310 mm in length, which present a macro-scale herringbone corrugation with an inclination angle of 65° and a corrugation amplitude of 2 mm. The experimental tests have been carried out at three different saturation temperatures (10, 15 and 20°C) and four different evaporator outlet conditions (vapour quality around 0.80 and 1.00, vapour super-heating around 5 and 10°C), whereas the inlet vapour quality ranges between 0.2 and 0.3. The refrigerant mass flux ranges from 11 to 31 kg/m2s and the heat flux from 4 to 17 kW/m2. The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show great sensitivity to heat flux, outlet conditions and fluid properties and weak sensitivity to saturation temperature (pressure). The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on refrigerant mass flux. The heat transfer and pressure drop measurements are complemented with an IR thermography analysis carried out during the vaporisation tests. The saturated boiling heat transfer coefficients were compared with a new model for refrigerant boiling inside BPHE (Longo et al., 2015): the mean absolute percentage deviation between calculated and experimental data is 7.2%. The present data points were also compared with those of HFC134a and HFO1234yf previously measured inside the same BPHE under the same operating conditions: HFO1234ze(E) exhibits heat transfer coefficients very similar to HFC134a and HFO1234yf and frictional pressure drops slightly higher than HFC134a and HFO1234yf

R1234yf FLOW BOILING HEAT TRANSFER INSIDE A 3.4 mm ID MICROFIN TUBE

The use of synthetic refrigerants with a non-negligible Global Warming Potential or, on the contrary, of natural but flammable or toxic natural fluids calls for the charge minimization of the refrigerating and air conditioning equipment. The refrigerant charge minimization as well as the use of eco-friendly fluids can therefore be considered two of the most important targets for these applications to cope with the new environmental challenges. Traditional microfin tubes are also widely used in air and water heat exchangers for heat pump and refrigerating applications during condensation or evaporation. The possible downsizing of microfin tubes can lead to more efficient and compact heat exchangers and thus to a reduction of the refrigerant charge of the systems. Furthermore, over the last several years, much research and development effort has been focused on potential refrigerants possessing low GWPs. Among the fluorinated propene isomers which have normal boiling point temperature data published in the public domain, several have low GWP and normal boiling temperatures relatively close to R134a; among them, R1234yf has as a normal boiling temperature approximately 3.4°C lower than that of R134a, with a GWP=4. This paper presents the R1234yf flow boiling heat transfer and pressure drop measurements inside a small microfin tube with internal diameter at the fin tip of 3.4 mm. This study is carried out in an 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. The microfin tube is brazed inside a copper plate and electrically heated from the bottom. Several T-type thermocouples are 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, the vapour 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, onset dryout vapour quality, and frictional pressure drop as a function of the operative test conditions