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

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

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

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

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

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

Numerical Model and Experimental Analysis of the Thermal Behavior of Electric Radiant Heating Panels

Electric radiant heating panels are frequently selected during the design phase of residential and industrial heating systems, especially for retrofit of existing buildings, as an alternative to other common heating systems, such as radiators or air conditioners. The possibility of saving living and working space and the ease of installation are the main advantages of electric radiant solutions. This paper investigates the thermal performance of a typical electric radiant panel. A climatic room was equipped with temperature sensors and heat flow meters to perform a steady state experimental analysis. For the dynamic behavior, a mathematical model was created and compared to a thermographic measurement procedure. The results showed for the steady state an efficiency of energy transformation close to one, while in a transient thermal regime the time constant to reach the steady state condition was slightly faster than the typical ones of hydronic systems

Investigation of surface inclination effect during dropwise condensation of flowing saturated steam

When a pure vapor condenses over a surface, it can form a continuous liquid film or a multitude of discrete droplets, thus realizing the so-called dropwise condensation (DWC). In the literature, most of the experimental data refer to DWC on vertical condensing surfaces with quiescent vapor. However, in many applications, the condensing vapor usually has a non-zero flow velocity with a consequent effect on the sliding motion of droplets. Moreover, the drag force due to vapor velocity may be the only mechanism for liquid removal on a horizontal surface or in space applications. A systematic investigation of the effects of vapor drag and surface inclination on the heat transfer and droplet population during DWC is needed and is addressed in the present paper. Here, DWC of flowing steam is experimentally studied on sol-gel silica-based coated aluminium substrates at three different inclinations: vertical, inclined at 45°, and horizontal. Heat transfer coefficient (HTC) and droplet population measurements are performed in a wide range of heat flux (260–610 kW m−2) and average vapor velocity (3.3–13.8 m s−1). When decreasing the tilt angle, from vertical to horizontal, due to the lower contribution of the gravity force, the average droplet size increases, and a strong HTC reduction is observed above all at low vapor velocities. Because of the vapor drag force, the HTC increases with steam velocity and, at the highest mass velocity, the HTC is independent from the surface inclination. A model for the droplet departing radius in the presence of vapor velocity, initially proposed by the present authors for the sole case of vertical surfaces, is here modified to account also for the effect of surface inclination and then assessed against the present experimental data. Hence, we propose to predict the heat flux during DWC by coupling the new equation for the departing radius with the available models of heat transfer through a single droplet and drop-size distribution. The developed calculation method is found to provide satisfactory predictions of the HTC for the whole range of vapor velocity, heat flux and surface inclination

Sequential coupled simulation of a dual source heat pump and shallow geothermal reservoir

The numerical simulation is an important tool for the assessment of exploiting geothermal energy. It can be used in shallow geothermal applications to evaluate the different production scenarios and the sustainability of the system (geothermal reservoir and heat pump) on long term. Moreover, in shallow geothermal projects, to simulate the real behaviour of the system, the load profiles of the end user and variations of the working mode of the heat pump should be taken into account. The present work introduces and describes a coupled numerical model, in which a dedicated Matlab\uae script has been realized to allow a sequential coupled simulation of a shallow geothermal reservoir exploited with a dual source heat pump. A mathematical model of a dual source heat pump that can work with the ground or the air as source/sink has been developed in Matlab\uae environment. Each component of the heat pump has been modelled considering the equations that govern the physical phenomena. The dynamic numerical simulator FEFLOW\uae 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 on the long term, and it is important for understanding the advantages and limits of the dual source heat pump in assuring the sustainability over time when heat is exchanged with the ground, avoiding the depletion of geothermal resources. The mathematical models have been validated with experimental data from a geothermal plant located in Tribano (Padova, IT). This is one of the four pilot sites realized within the framework of the H2020 GEOTeCH Project. It consists of eight coaxial borehole heat exchangers 30 m deep, connected to the 16 kW dual source heat pump prototype realized by HIREF S.p.A. The geothermal heat pump system has been working, and monitored, since October 2017 and it provides heating and air conditioning to an office area. Experimental results have been used to verify the new coupled model, and although the preliminary results are encouraging, further study and work are necessary to make it robust and stable for future routine work