Experimental study of phase change heat transfer in minichannels for ground and space applications

Two-phase heat transfer is widely encountered in minichannels heat exchangers such as air-cooled condensers and evaporators for automotive, compact devices for electronic cooling, aluminum condenser for air-conditioning applications, loop heat pipes and capillary pumped loops for space applications. Nowadays there is also an increasing interest on new refrigerants, pure and blended, with a low-GWP. This thesis aims at studying the physical phenomena of condensation by investigating the effect of gravity inside small channels, presenting an experimental analysis during normal gravity and microgravity conditions. Another objective of this thesis is to analyze the two-phase heat transfer performance of non-azeotropic mixtures, with low-GWP, in minichannels. Regarding mixtures the work focused not only on condensation heat transfer but also on flow boiling and two phase pressure drop. In the first part of this thesis the design and the realization of a new test section to study the effect of gravity during convective condensation is presented. The new test section has an internal diameter of 3.38 mm and it allows to measure quasi-local heat transfer coefficients and to visualize the flow pattern. Quasi-local heat transfer coefficients measured on ground during condensation of R134a at 40°C and mass velocities between 50 and 200 kg m-2 s-1 are presented. The flow patterns visualized are compared with flow pattern maps available in the literature and the experimental heat transfer coefficients with different models for condensation. The new test section has been also used to study convective condensation in microgravity conditions during the 62nd ESA Parabolic Flight Campaign. This has been one of the first attempts to perform heat transfer coefficients measurements and flow pattern visualizations on normal gravity and microgravity conditions during convective condensation inside tubes. The deep connection between gravity effect and flow pattern are shown in the images. Regarding on-ground applications, in the recent years much attention has been paid to the possible use of fluorinated propene isomers for the substitution of HFC fluids being in most cases high-GWP refrigerants. However, the available HFOs (hydrofluoroolefins) cannot cover all the air-conditioning, heat pump, and refrigeration systems when used as pure fluids because their thermodynamic properties are not suitable for all operating conditions; therefore some solutions may be found using blends of refrigerants, to satisfy the demand for a wide range of working conditions. The second part of this thesis presents an experimental study of the two-phase heat transfer for a R32/R1234ze(E) non-azeotropic mixture, at different mass compositions, inside a 0.96 mm inner diameter minichannel. As first step, frictional pressure gradient during adiabatic two-phase flow has been investigated. The experimental data are also compared with several models available in the literature. Local heat transfer coefficients have been then measured during condensation of the R32/R1234ze(E) mixture at three different mass compositions (23/77%, 46/54 and 75/25% by mass). The new experimental data are compared to those of pure R1234ze(E) and R32. This allows to analyze the heat transfer penalization due to the mass transfer resistance occurring during condensation of this zeotropic mixture and to assess about suitable predicting models. The present work reports also the local heat transfer coefficient measured during flow boiling of the R32/R1234ze(E) mixture (50/50% by mass) at a pressure of 14 bar, which corresponds to a bubble temperature of 26.2 °C. The flow boiling data, taken in the 0.96 mm minichannel, are discussed with particular regard to the mass transfer mechanism. The new experimental data are also compared to flow boiling data of pure R1234ze(E) and pure R32. This flow boiling database, encompassing more than 300 experimental points at different values of mass velocity, heat flux and vapor quality, is compared with available correlations in the literature. The introduction of a correction to account for the additional mass transfer resistance is discussed and such correction is found to be necessary and proper to provide a correct sizing of the evaporator

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

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

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

Condensation Heat Transfer Coefficient Measurements and Flow Pattern Visualizations of R515B and R450A Inside a 3.4 mm Diameter Channel

An alternative low Global Warming Potential (GWP) refrigerant that could be used to replace R134a in heat pumps, refrigeration and air-conditioning systems is the hydrofluoroolefin R1234ze(E). As a drawback, R1234ze(E) is classified as a mildly flammable fluid (A2L ANSI/ASHRAE classification) and it presents a lower volumetric cooling capacity compared to R134a. In the search for non-flammable R134a substitutes, hydrofluorocarbon/hydrofluoroolefin binary mixtures can be considered. R515B (R1234ze(E)/R227ea at 91.1/8.9% by mass) and R450A (R1234ze(E)/R134a at 58.0/42.0% by mass) are two alternatives classified as A1 (not flammable). R515B is an azeotropic mixture with GWP100-years = 299, whereas R450A is a near-azeotropic blend (temperature glide 0.6 K at 40 °C) with GWP100-years = 547. In this work, condensation tests are performed with R515B and R450A inside a circular cross-section channel with an inner diameter equal to 3.4 mm. The test section is composed of two copper heat exchangers designed for the measurement of the quasi-local heat transfer coefficient. A glass tube, located between the two diabatic parts of the test section allows the visualization of the two-phase flow patterns by a high-speed camera. Heat transfer coefficients are measured at 40 °C mean saturation temperature and mass flux between 50 and 300 kg m-2 s-1. The prediction accuracy of condensation heat transfer models is then assessed against the experimental results. Measured heat transfer coefficients are also compared with those of R1234ze(E) at the same operative conditions. Regarding the diameter of the present test tube, it is worthy to point out that small diameter tubes are often employed in finned coil heat exchangers and minichannel heat exchangers are a common solution for air-cooled condensers

Sequential coupled numerical simulations of an air/ground-source heat pump: Validation of the model and results of yearly simulations

Numerical simulations are important tools for the assessment of exploiting geothermal energy in heat pump applications. They can be used to evaluate the performance of the system, the long-term production scenarios and the sustainability of the geothermal reservoir. The present work introduces and describes a numerical model, in which a dedicated Matlab script has been realized to allow sequentially coupled simulations of a shallow geothermal reservoir and of a heat pump system. A mathematical model of a dual-source heat pump, working alternatively with the ground or the air as heat source/sink, has been developed in Matlab environment. The heat exchangers of the heat pump have been modelled considering the equations that govern the physical phenomena. The dynamic numerical simulator FEFLOW, based on the finite element method, has been used to simulate the behaviour of the geothermal reservoir, subjected to heat extraction/reinjection by a closed loop vertical heat exchangers field. This methodological approach is useful to evaluate the performance of the coupled system in the long term, and it is important for understanding the advantages and limits of the dual-source heat pump in assuring sustainability over time avoiding the depletion of geothermal resources. The models and their coupling have been calibrated and validated with experimental data from a shallow geothermal plant located in Tribano (Padova, IT). It consists of eight coaxial borehole heat exchangers 30 m deep, connected to a 16 kW dual-source heat pump prototype. The heat pump system provides heating and cooling to an office area. The coupled model has been used to compare the performance of the heat pump when working in air-mode only or in ground-mode only. This allowed the development of a switching control strategy between the two thermal sources. Yearly simulations with the switching strategy have shown that the seasonal performance factor of the dual-source heat pump during the heating mode can be 13.8 % higher compared to the one obtained with a conventional air source heat pump and 3.8 % higher with respect to a ground source heat pump

Design And Testing Of a Microchannel Heat Exchanger Working As Condenser And Evaporator

In the recent years, international agreements and regulations urge for a reduction of production and utilization of Hydrofluorocarbons (HFCs), while achieving high efficiency remains a crucial aspect for refrigeration and air conditioning systems. One of the possible candidates to replace the high global warming potential (GWP) fluid currently employed in heat pump systems (R410A) is the refrigerant R32, which belongs to A2L class. In addition to adopting low-GWP refrigerants, charge minimization is a major design objective for such systems, mainly in the case of flammable refrigerants. In the case of reversible heat pumps, a reduced volume of the heat exchangers limits the refrigerant charge migration between condenser and evaporator when switching between the operation modes. The refrigerant charge minimization coupled with the use of new refrigerants can therefore be considered one of the most important objectives for new heat pump developments. The microchannel technology helps for this purpose. The present paper presents an air-to-refrigerant microchannel heat exchanger working with R32, realized in the framework of the European Project GEOTeCH. The prototype heat exchanger, working both as condenser and as evaporator, has been tested on an innovative dual source (air and ground) heat pump, which can operate in heating and cooling modes. A model of the microchannel heat exchanger has also been developed and the predicted performance have been compared with the experimental measurements. In the end, the model has been used to estimate the refrigerant charge trapped in the minichannel when it works as condenser and the results have been compared with those obtained using a traditional finned coil heat exchanger