Phase distribution of a refrigerant two-phase flow over an impacting T-junction

Climate change is a major global concern. Heating and cooling of buildings contributes significantly to the climate change. Currently, 40 % of the total energy use and 36 % of the total CO2 emissions in the European Union (EU) arise from buildings [1]. To decrease the emissions of buildings, the insulation grade of the current building stock should be improved and the current heating and cooling installations should be replaced with ones not depending on fossil fuels. A promising technology is a heat pump, which can be powered by renewable energy and has a higher efficiency than conventional systems. A heat pump uses a thermodynamic cycle to convert heat from a low temperature to a higher temperature. The heat pump cycle is a closed cycle containing a refrigerant and consisting of 4 components: a compressor, a condenser, an expansion valve and an evaporator. When the refrigerant enters the evaporator, it is typically in the two-phase region. A two-phase flow is a flow consisting of two phases which are in this case liquid and vapour. To distribute the two-phase flow over the parallel tubes of the evaporator a distributor is used. However, this distribution is often not homogeneous. Maldistribution can occur due to improper placement of the heat pump, production tolerances, varying heat loads, fouling and indirect causes which affect the pressure gradient in the parallel sections like frosting and dirt accumulation at the air side. This maldistribution results in a significant drop in coefficient of performance (COP) and capacity of the heat pump [2]. This work limits its scope to a tubular distributor head with only two outlets. This geometry can be reduced to an impacting T-junction. An impacting T-junction is a T-junction of which the two outlets are perpendicular to the inlet tube. The purpose of this work is to fill the gaps in literature concerning the phase distribution over an impacting T-junction and to develop a new phase distribution model. To start, this work gives the overview of the current state of art and tries to indicate the gaps. Several authors found an inconsistency of the influence of the inlet superficial velocities when there is a flow regime transition. Hence, a first goal of this work is to study the influence of the inlet superficial velocities on the phase distribution in the vicinity of flow regime transitions. Further, most experiments found in literature are executed with water-air mixtures. Hence, little information is available on the influence of fluid properties on the phase distribution. This work will add extra data to literature for different refrigerants and discusses the influence of different fluid properties. To fill these gaps in literature, an experimental setup was developed which allows to test the phase distribution of two-phase refrigerant flows over an impacting T-junction. The setup is capable of testing refrigerant flows with a mass flux up to 700 kg/(m²s) at a saturation temperature between 10°C and 20°C and with a vapour quality between 0 and 1. The diameter of the impacting T-junction is 8 mm. In total 696 experiments were performed with four different refrigerants: R32, R125, R1234ze an R134a. In other words, the phase distribution over the whole mass fraction range of 60 different inlet flows was tested. The conservation of energy has an average error of 2 % and is always smaller than 5 %. The consistency of the experimental setup was verified by repeating random experiments. To compare the experimental results, a new quantitative method was proposed. Based on the experimental results, a strong influence of the flow regime on the phase distribution was observed. While sweeping through a range of inlet superficial vapour velocities, discontinuities in the phase distribution were observed at the flow regime transitions. Further, the liquid has a decreasing preference of flowing to the branch with the lowest mass flow rate with increasing inlet superficial vapour velocity for an inlet superficial liquid velocity equal or higher than 0.2 m/s. In contrast, for an inlet superficial liquid velocity equal or lower than 0.1 m/s, the liquid has a increasing preference of flowing to the branch with the lowest mass flow rate with increasing inlet superficial vapour velocity. The influence of three fluid properties (density, viscosity and surface tension) was also investigated. The viscosity does not have any influence on the phase distribution. The phases are distributed more homogeneous when the density ratio (ρ_g/ρ_l) increases. The maldistribution of the phases increases with increasing surface tension. During the experiments, the pressure gradient over the T-junction was measured. These pressure gradient measurements were used to create a model which predicts the pressure drop over the T-junctions. This new pressure drop model is more accurate for this work’s data and expands the prediction capabilities to other flow regimes compared to models found in literature. Before a new phase distribution model was proposed, the seven existing models were evaluated using the data from literature and this work’s data. The models available were designed for either water-air or water-steam flows. In general, the models have the highest predictive capability for their design two-phase mixture. Hence, none of the models captures the influence of the fluid properties. Also, none of the models was able to predict this work’s data properly. A new model was proposed based on the insights gained from the experimental results. The model is based on three fundamental laws: conservation of mass, momentum and energy. The new model is then evaluated using this work’s data and the data from literature. The new model works well for this work’s data and is acceptable for the water-steam and water-air data. Finally, the model is extended to inclined impacting T-junctions. The inclined model is able to predict the liquid mass fraction F_l correctly for this work’s data. However, the prediction of the vapour mass fraction F_g is less accurate compared to the horizontal model

Experimental investigation of a forced convection heat transfer of the organic fluid R-125 at supercritical pressures and under organic rankine cycle conditions

The organic Rankine cycle (ORC) is a suitable technology for utilizing low-grade temperature heat sources of ~100 °C from various industry processes. In the ORC cycle an organic fluid with a lower boiling point is used as a working medium. The performance of the ORCs has advanced significantly in the last decades. However, there is still a possibility of improving the efficiency of this cycle. The supercritical heat transfer in the heat exchanger ensures better thermal match between the heating and working fluids temperatures glides in the heat exchanger. Hence, better understanding of the heat transfer phenomena to a fluid at supercritical state in a horizontal flow and in a large diameter tube is of great importance. Therefore, the tests are performed in a counter-current tube-in-tube test section positioned horizontally with a total length of 4 m and a tube diameter of 0.0286 m. R-125 is used as a working fluid in the experiments. During the measurements the temperature of the heating fluid was 90 °C, the mass flow rate and the pressure of the working fluid R-125 was in the range of 0.2–0.3 kg/s and 38–55 bar respectively. Furthermore, results from the pressure and temperature measurements obtained at the inlet and at the outlet of the test section are reported. The results show that the overall heat transfer coefficient is influenced by the mass flow rate of the organic fluid. At pressures close to the critical pressure of R-125 higher values of the overall heat transfer coefficients are determined. Deteriorated heat transfer is not likely to occur at these operating conditions because the critical heat flux is higher than the one obtained from the measurements. A comparison between the experimental Nusselt number with heat transfer (Nusselt) correlations from the literature is done and the measurement points fall within the uncertainty ranges of both heat transfer correlations.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers

Experimental assessment of a helical coil heat exchanger operating at subcritical and supercritical conditions in a small-scale solar organic rankine cycle

In this study, the performance of a helical coil heat exchanger operating at subcritical and supercritical conditions is analysed. The counter-current heat exchanger was specially designed to operate at a maximal pressure and temperature of 42 bar and 200 °C, respectively. The small-scale solar organic Rankine cycle (ORC) installation has a net power output of 3 kWe. The first tests were done in a laboratory where an electrical heater was used instead of the concentrated photovoltaic/thermal (CPV/T) collectors. The inlet heating fluid temperature of the water was 95 °C. The effects of different parameters on the heat transfer rate in the heat exchanger were investigated. Particularly, the performance analysis was elaborated considering the changes of the mass flow rate of the working fluid (R-404A) in the range of 0.20–0.33 kg/s and the inlet pressure varying from 18 bar up to 41 bar. Hence, the variation of the heat flux was in the range of 5–9 kW/m2. The results show that the working fluid’s mass flow rate has significant influence on the heat transfer rate rather than the operational pressure. Furthermore, from the comparison between the experimental results with the heat transfer correlations from the literature, the experimental results fall within the uncertainty range for the supercritical analysis but there is a deviation of the investigated subcritical correlations

Experimental two-phase heat transfer study of R245fa in horizontal mini-channels at high saturation temperatures

Heat transfer measurements for R254fa were conducted. The heat transfer coefficient was determined for a smooth stainless steel tube with an inner tube diameter of 3 mm. The experiments were conducted for three heat fluxes (10, 30 and 50 W/m^2), five mass fluxes (100, 300, 500, 700 and 1000 kg/(m^2.s)) and at three saturation temperatures (40°C, 70°C and 125°C). The experimental data was used to determine the influence of the saturation temperature, mass flux, heat flux and vapour quality on the heat transfer coefficient. At a low saturation temperature, the heat transfer coefficient increases with an increasing mass flux. However, at a high saturation temperature the heat transfer coefficient decreases with an increasing mass flux. Furthermore, the heat transfer coefficient increases with increasing vapour quality at a low saturation temperature. On the contrary, the heat transfer coefficient decreases at higher saturation temperatures

Flow regime based heat transfer correlation for R245fa in a 3 mm tube

241 heat transfer measurements for R254fa were conducted. The heat transfer coefficient was determined for a smooth stainless steel tube with an inner tube diameter of 3 mm. The experiments were conducted for five mass fluxes (100, 300, 500, 700 and 1000 kg/(m2 s)), three heat fluxes (10, 30 and 50 kW/m2) and at three saturation temperatures (40 °C, 70 °C and 125 °C). The experiments were used to determine the influence of the saturation temperature, mass flux, heat flux, vapour quality and flow regime on the heat transfer coefficient. At a low saturation temperature, the heat transfer coefficient increases with an increasing mass flux. However, at a high saturation temperature the heat transfer coefficient decreases with an increasing mass flux. Furthermore, the heat transfer coefficient increases with increasing vapour quality at a low saturation temperature. On the contrary, the heat transfer coefficient decreases at higher saturation temperatures. Due to the fact that most heat transfer models found in literature are developed for low saturation temperatures and one flow regime, the heat transfer coefficients predicted by the existing models do not comply very well with the experimental data. Thus, a new heat transfer correlation for R254fa was proposed. The new correlation has a Mean Absolute Error of 11.7% for the experimental data of a tube with an inner tube diameter of 3 mm. Finally, this new correlation was also verified with R245fa datasets of other authors

Determining heat losses in a reheat furnace : a case study

In a previous study of an actual reheat furnace with a capacity of 45 ton/h, it was found that a staggering 20% of the input energy was unaccounted for in the heat balance. It was hypothesized that this missing energy was lost to the environment as heat losses. In this work, the main losses are identified and quantified. First, the heat transfer through the wall of the furnace was determined. For this, an extensive measurement campaign was performed. Based on the measured wall temperatures and emissivity values, the heat transfer from the walls for the operating conditions at the time of the measurements was estimated. The heat rejected through the walls amounts to approximately one fifth of the total heat loss. Secondly, when the furnace door is opened, a relatively large flow rate of hot gas leaves the furnace, and a net heat loss occurs due to the radiative heat exchange between the furnace interior and the environment. As the aforementioned heat losses are very difficult to measure, a simplified theoretical model was made based on physical principles. The corresponding results indicate that the opening of the furnace accounts for a large part of the remaining heat loss

The distribution of two-phase R32 over an impacting T-junction

This experimental work studies the distribution of a two-phase refrigerant flow over a horizontal impacting T-junction. A setup was built which consists of two parts: a flow conditioner and a test section. The flow conditioner creates a two-phase mixtures (R32) at a saturation temperature between 10 °C and 20 °C with a mass flux of 150 to 700 kg/(m².s) and a vapour quality between 0 and 1. In the test section, the two-phase flow is distributed over two identical parallel sections using an impacting T-junction. The backpressure and heat input of each parallel section can be regulated. The mass flow rates and vapour qualities are measured before and after the T-junction. Further, the pressure gradient over the T-junction is measured. Also the void fraction before the T-junction is determined using a capacitive void fraction sensor. Using design of experiments, the main effects of superficial vapour velocity, superficial liquid velocity and saturation pressure on the distribution of R32 were studied. For R32, the two phases only distribute uniformly over the T-junction when the mass flow rate through the two outlet branches is equal. Furthermore, the experiments show a decreased tendency of the liquid to exit through the outlet with the lowest mass flow rate with increasing superficial vapour velocity. The opposite is noticed with an increased superficial liquid velocity at a high superficial vapour velocity. Finally, no effect of the saturation pressure was found. The obtained results were then compared with the results of water-air mixtures found in literature

The distribution over an impacting T-junction of two-phase R32 under heat pump conditions

This experimental work studies the distribution of a two-phase refrigerant flow over a horizontal impacting T-junction. A setup was built which consists of two parts: a flow conditioner and a test section. The flow conditioner creates a two-phase mixtures (R32) at a saturation temperature between 10 °C and 20 °C with a mass flux of 150 to 700 kg/(m².s) and a vapour quality between 0 and 1. In the test section, the two-phase flow is distributed over two identical parallel sections using an impacting T-junction. The backpressure and heat input of each parallel section can be regulated. The mass flow rates and vapour qualities are measured before and after the T-junction. Further, the pressure gradient over the T-junction is measured and the void fraction is determined before the T-junction using a capacitive void fraction sensor. Using design of experiments, the main effects of superficial vapour velocity, superficial liquid velocity and saturation pressure on the distribution of R32 were studied. For R32, the two phases only distribute uniformly over the T-junction when the mass flow rate through the two outlet branches is equal. Further, the experiments show a decreased tendency of the liquid to exit through the outlet with the lowest mass flow rate with increasing vapour superficial velocity. The influence of the superficial liquid velocity was less pronounced and dependent on the superficial vapour velocity. Finally, no effect of the saturation pressure was found. The obtained results were then compared with the results of water-air mixtures found in literature

Design of an experimental set-up to determine the influence of corrosion on heat transfer

In the exploitation of geothermal energy, heat exchangers are essential to distribute heat to energy conversion systems (e.g. organic Rankine cycles) or district heating networks. The geothermal brine found in Belgium however has a high temperature and a high salinity which makes it extremely corrosive. In such environments, the classic solution is to construct a heat exchanger with a highly corrosion resistant metal such as titanium or nickel. However, since these metals are very expensive, alternatives are investigated. One such alternative is using heat exchangers made of less corrosion resistant materials, but where detailed information about the corrosion process is available. This information is then used during design and for predictive maintenance. An experimental set-up to determine the corrosion rate and the influence of corrosion on the heat transfer is designed.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers

Experimental study of the effects of foam height, orientation and radiative heat transfer on buoyancy-driven convection

Air-saturated buoyancy-driven convection in open-cell aluminium foam is studied. The effects of foam height, radiative heat transfer and orientation are experimentally investigated. Two aluminium foam heat sinks with the same baseplate dimensions (6″ by 4″) are tested. Their respective foam height is 22.2 mm and 40 mm. The aluminium foam has a porosity of 0.946 and a pore density of 10 pores per linear inch. The heat sinks are tested in a vertical and a horizontal orientation. The effect of radiation is studied by comparing untreated heat sinks with painted versions. During the experiments the power dissipated by the heat sinks is measured as function of the temperature difference between the baseplate of the heat sink and the ambient. This temperature difference is varied from 10 to 70°C