Two-Phase Flow and Heat Transfer of a Non-Azeotropic Mixture inside a Single Microchannel

In the recent years much attention has been paid to the use of fluorinated propene isomers for the substitution of high-GWP refrigerants. However, the HFOs (hydrofluoroolefins) cannot cover all the air-conditioning, heat pump, and refrigeration applications. In a recent study, it was found that the coefficient of performance (COP) and the capacity of heat pump cycles using HFO-1234ze(E) are significantly lower than those of R410A (Koyama et al., 2010). The main causes are the small latent heat and vapor density of R1234ze(E). To improve the COP and capacity, in the latest literature it was attempted to blend R1234ze(E) into another refrigerant, R32 Although R32 is one of the HFCs, it has relatively low GWP and excellent thermodynamic characteristics. Therefore, a zeotropic mixture of R1234ze(E) and R32 can be used in the field of air-conditioning due to its mild impact on environment. In this paper, a mixture of R1234ze(E) and R32 (0.5/0.5 by mass) is under study. In particular the frictional pressure gradient and the local heat transfer coefficients during flow boiling and condensation of this mixture in a single minichannel with 0.96 mm diameter are measured. Tests are carried out in the experimental apparatus available at the Two Phase Heat Transfer Lab of the University of Padova. As a peculiar characteristic of the present technique, the flow boiling 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. For the determination of the local heat transfer coefficient, three parameters are measured: the local heat flux, the saturation temperature and the wall temperature. The heat flux is determined from the temperature profile of the secondary fluid in the test section. The wall temperature is directly measured along the test section and the saturation temperature is obtained from the pressure measurements at the inlet and outlet of the test tube. During condensation tests, the heat is subtracted from the fluid by using cold water. As in flow boiling, the heat transfer coefficient is obtained through the measurement of the local heat flux and the saturation-to-wall temperature difference. The heat transfer coefficients are compared against predicting models available in the literature. The new experimental data are also compared to heat transfer data of pure R1234ze(E) and R32. This allows to analyze the heat transfer penalization due to the mass transfer resistance of this zeotropic mixture and to assess available predicting models for condensation and evaporation of zeotropic mixtures in minichannels. Pressure drop data are also used to assess predicting pressure gradient correlations

Effect Of The Refrigerant Charge On The System Performance And Mass Distribution In Air-To-Water Heat Pumps

Recent regulations in the matter of climate change and environmental protection are pushing to reduce the release of greenhouse gases into the atmosphere. The overall environmental impact of a refrigeration system can be reduced by optimizing and possibly minimizing the amount of refrigerant charge in the system. Even in the case of natural and low-GWP synthetic refrigerants, due to the well-known problems with toxicity and flammability, it is required to minimize the amount of refrigerant charged into the system to reduce the associated risks. The charge minimization process requires to know the refrigerant distribution to identify and redesign the critical components in terms of charge retention. This paper analyses numerically the influence of the refrigerant charge on the system performance and on the mass distribution in an air-to-water reversible heat pump working with R32. A mathematical model has been developed to simulate the unit during the cooling mode operation. The model uses the finite volume method to predict the refrigerant charge within the heat exchangers; the amount of refrigerant dissolved in the compressor oil is also accounted for. The results show that most of the charge is stored into the condenser and highlight the existence of an optimum charge that maximizes the system COP. The same model allows to compare various refrigerants in terms of direct and indirect impact on the greenhouse effect

Investigation of Evaporator Performance with and without Liquid Overfeeding

In the present work, the performance of a segmentally baffled shell-and-tube evaporator working with liquid overfeeding is investigated. The refrigerant is R134a that flows inside the tubes, while water flows on the shell side. A single shell pass has been adopted for the water with one tube pass for the evaporating fluid. The test rig used for the experimental measurements consists of a primary refrigerant loop plus the condenser and the evaporator water auxiliary loops. The evaporator can be fed with two-phase mixture from the expansion valve or with saturated liquid coming from the liquid-vapor separator (in this case a variable speed recirculation pump is used). Inlet and outlet temperatures have been measured for both fluids together with the flow rate allowing the determination of the overall heat transfer coefficient. In addition, pressure drop have been measured on the refrigerant side. Tests have been performed both without overfeeding and with overfeeding at different values of recirculation ratio. The recirculation ratio is defined as the ratio between refrigerant flow rate at the evaporator and the vaporized refrigerant flow rate. Furthermore, measurements have been taken at fixed water outlet temperature and varying the heat duty. In order to study the evaporator behavior, a computational procedure has been developed. Finally, the numerical model of the heat exchanger has been validated against experimental data

experimental and numerical study of a parabolic trough linear cpvt system

Abstract The electric and thermal performance of a parabolic trough linear concentrating photovoltaic-thermal (CPVT) system operating in Padova (northern Italy) is experimentally investigated. The system moves about two axes and exhibits a geometrical concentration ratio around 130. The receiving module placed on the focus line displays a secondary optics made of two flat mirrors to gather some reflected radiation and to contribute to the concentrated flux on two lines of triple junction photovoltaic cells soldered on a ceramic substrate. The substrate is in thermal contact with a aluminium heat exchanger with water flow channels to cool the PV cells. During the test runs, the inlet water temperature ranges from 20 °C to 80 °C and the heat yield is obtained from mass flow rate and temperature measurements while a rheostat and a power analyzer are connected to the electric terminals of the module to assess the electrical production. The direct normal irradiation (DNI) is measured by a pyrherliometer mounted on a solar tracker. Experimental results are used to assess a numerical model of the solar receiver and the whole concentrator

DNI Estimation Procedures for the Assessment of Solar Radiation Availability in Concentrating Systems

Abstract DNI (Direct Normal Irradiance) is the resource utilized by solar concentrators. Besides, the determination of DNI is needed in the models for the estimation of global irradiance on tilted planes, which is the input to flat-plate systems. This paper describes a study of different estimation procedures for the assessment of the DNI , using experimental data with a time scale of 1 min, taken at two different latitudes. The analyzed approaches include measuring techniques and models. The results show that the different estimation methods can lead to quite different conclusions when comparing the solar radiation availability in concentrating and flat-plate systems and this can affect the energy and economic evaluations. Based on the experimental analysis, indications for reducing the uncertainty in the estimation of DNI are discussed

Two-Phase Heat Transfer of Low GWP Ternary Mixtures

Refrigerant blends obtained mixing hydrofluorocarbons (HFC) and hydrofluoroolefins (HFO) have recently been proposed as substitutes for high GWP (Global Warming Potential) fluids employed in refrigeration and air-conditioning systems. As a general trend, the dimension of pipes used in heat exchangers is decreasing: diameters around 5 mm are often employed in finned-tube coil heat exchangers and minichannels heat exchangers (with internal diameter around 1-2 mm) are also a common solution for the automotive sector and for air-cooled chillers. Condensation and flow boiling heat transfer coefficients of zeotropic ternary mixtures R455A (R32, R1234yf and R744 at 21.5/75.5/3.0% by mass composition) and R452B (R32, R1234yf and R125 at 67.0/26.0/7.0% by mass composition) have been measured inside a minichannel (0.96 mm diameter) and inside a conventional tube (8.0 mm diameter). R455A exhibits a temperature glide around 10 K at 35 °C bubble temperature whereas R452B presents a temperature glide around 1 K at 40 °C bubble temperature. The experimental results are compared with selected correlations for condensation and flow boiling heat transfer which account for the additional mass transfer resistance occurring during two-phase heat transfer of zeotropic mixtures. It emerges the importance of including the mass transfer resistance for the prediction of heat transfer coefficient when considering high temperature-glide mixtures

Experimental Analysis of Optimal Operation Mode of a Ground Source Heat Pump System

Abstract This paper presents experimental data and modeling of a ground source heat pump (GSHP) with variable speed compressor, variable speed water pumps and variable speed fans in the coils, installed at Hiref Spa (Italy) in the framework of the European Project Ground-Med. The present model has been developed to evaluate the operating conditions that lead to the maximum seasonal coefficient of performance and to analyze the behavior of the system at partial loads since variable capacity heat pumps do not work at nominal power for most of the time. The control parameters of the model that can be varied are the followings: frequency of compressor, frequency of water pump to the borehole heat exchangers, frequency of water pump to the user, velocity of the fans and water temperature to the user. The model has been compared with experimental data taken during a heating season and it can be the baseline to develop a control strategy with the final objective of maximizing the seasonal coefficient of performance of the system

Innovative Minichannel Condensers and Evaporators for Air Conditioning Equipment

The use of aluminum heat exchangers for refrigeration and air-conditioning equipment is very interesting since it allows to reduce weight and manufacturing costs while maintaining high performance. In this paper a two-phase heat transfer characterization of an innovative aluminum minichannel heat exchanger is presented. The heat exchanger (HX) is composed by rectangular channels with internal perforated turbolators. A special test section has been projected and realized in the Two Phase Heat Transfer Lab of the University of Padova in order to measure the heat transfer coefficient (HTC) on the refrigerant side during flow boiling and condensation. The test section has a single refrigerant channel with a perforated fin to make the minichannels. The test section is provided with 14 water flow modules installed at top and bottom of the refrigerant channel to promote boiling or condensation of the refrigerant. Therefore, the test section is made of seven different zones: each of them is equipped with 8 thermocouples to measure the wall temperature during the refrigerant phase change. The heat flow rate in each zone is calculated by an energy balance on the water side. Pressure transducers and thermocouples on the refrigerant side allow to determine the saturation temperature and thus the heat transfer coefficient of the refrigerant. The operating refrigerant used during tests is R410A. The particular scheme adopted for the test section enables to measure HTC at varying vapor quality and heat flow rate. Vaporization and condensation tests were carried out with different saturation temperatures, specific heat flow rate (from 40 to 150 kW/m2) and refrigerant mass flux (50÷150 kg/(m2 s) ). Data acquired have been compared with vaporization and condensation predictions from various correlations available in literature. This part of the work is very interesting since no data is available in the literature for such a geometry in vaporization nor in condensation. Therefore, the present paper will investigate the potential performance of these innovative minichannel heat exchangers as condensers and evaporators in air-conditioning equipment

A New Model for the Analysis of Performance in Evacuated Tube Solar Collectors

Solar collectors can provide a useful response to the heat demand in buildings, such as heating of domestic water and spaces. Among the different types of solar collectors, the evacuated tube ones can better display their features when the temperature difference between operating fluid and ambient air is high, which are the typical operating conditions during space heating of buildings. Beside the heating application, there is also need for addressing the increasing energy consumption due to the summer air conditioning. Solar cooling can be an interesting opportunity, because the cooling demand matches the period with the higher solar radiation availability. Also in this application the evacuated collectors are an adequate type, due to the temperature level of the heat required as an input to the absorption machines. Therefore, accurate modeling and simulation of this type of collectors is highly recommended to fully assess the performance of the collectors already available in the market and propose improved designs, for example with higher concentration ratio, to get more benefits particularly at higher levels of temperature. This paper reports a new model for the performance analysis of evacuated tube solar collectors. The analyzed collectors are truncated CPC (compound parabolic collectors). An original software is developed under MATLAB environment for the simulation purposes. A novel numerical procedure is implemented to obtain the solution for the nonlinear set of equations representing the mathematical model. The length of the tube is divided into a specified number of segments. The analysis is performed for each segment along the tube length in order to obtain the variation of the different parameters. The model analyzes separately the optics and the heat transfer in the evacuated tubes and this approach allows to extend the analysis to new configurations. The model can simulate the efficiency curve under steady state conditions, according to the standard EN 12975-2 (EN 12975-2. Thermal solar systems and components - solar collectors - part 2: test methods. Brussels: CEN; 2006), but it is also able to describe the performance during the day with a quasi-dynamic approach. A comparison with experimental data shows the accuracy of the model. The results will provide a comprehensive characterization of the performance of evacuated tube collectors, discussing modeling and experimental data of efficiency and heat production at different levels of temperature. Particular attention will be paid to the applications of heating and cooling of buildings, industrial process heat generation and district heating

Monitoring Of a Commercial Refrigeration CO2 System And Comparison With Simulations

The demand for natural refrigerants is growing in commercial refrigeration systems. In the recent years, carbon dioxide has increased its market share in the field of commercial refrigeration and has proven to be a viable solution for the replacement of hydrofluorocarbon (HFC) systems. This success is mainly due to the ongoing technological evolution of carbon dioxide refrigeration systems. In the present paper, a CO2 commercial refrigeration unit serving a supermarket located in Northern Italy is presented. The unit consists of a booster compressor rack with parallel compression, with ten compressors arranged to provide around 20 kW cooling capacity at low temperature and 90 kW cooling capacity at medium temperature. Four out of ten compressors, provided by Frascold®, are arranged in parallel. The installation, in addition to the cooling load, provides all the thermal functions in one unit: it integrates a heat exchanger for the air conditioning and the possibility of two stage heat recovery, for sanitary hot water production and for space heating. The refrigeration unit is equipped with pressure and temperature sensors, power consumption and load analysers for the compressors. A computer model has been developed to evaluate the acquired data of the system and to analyse the key parameters. The preliminary results from the monitoring of this unit are presented in this paper and used to calibrate the model of the system. Afterwards, simulations have been performed at variable operating conditions in a cold month to evaluate the performance of the unit. The results of the model have been compared to an independent set of monitoring data