 Author
 Marijn Billiet (UGent)
 Promoter
 Michel De Paepe (UGent)
 Organization
 Abstract
 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 twophase region. A twophase flow is a flow consisting of two phases which are in this case liquid and vapour. To distribute the twophase 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 Tjunction. An impacting Tjunction is a Tjunction 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 Tjunction 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 waterair 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 twophase refrigerant flows over an impacting Tjunction. 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 Tjunction 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 Tjunction was measured. These pressure gradient measurements were used to create a model which predicts the pressure drop over the Tjunctions. 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 waterair or watersteam flows. In general, the models have the highest predictive capability for their design twophase 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 watersteam and waterair data. Finally, the model is extended to inclined impacting Tjunctions. 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.
Downloads

phdfinalmarijnbilliet.pdf
 full text
 
 open access
 
 
 6.61 MB
Citation
Please use this url to cite or link to this publication: http://hdl.handle.net/1854/LU8586599
 MLA
 Billiet, Marijn. “Phase Distribution of a Refrigerant Twophase Flow over an Impacting Tjunction.” 2018 : n. pag. Print.
 APA
 Billiet, M. (2018). Phase distribution of a refrigerant twophase flow over an impacting Tjunction. Ghent.
 Chicago authordate
 Billiet, Marijn. 2018. “Phase Distribution of a Refrigerant Twophase Flow over an Impacting Tjunction”. Ghent.
 Chicago authordate (all authors)
 Billiet, Marijn. 2018. “Phase Distribution of a Refrigerant Twophase Flow over an Impacting Tjunction”. Ghent.
 Vancouver
 1.Billiet M. Phase distribution of a refrigerant twophase flow over an impacting Tjunction. [Ghent]; 2018.
 IEEE
 [1]M. Billiet, “Phase distribution of a refrigerant twophase flow over an impacting Tjunction,” Ghent, 2018.
@phdthesis{8586599, abstract = {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 twophase region. A twophase flow is a flow consisting of two phases which are in this case liquid and vapour. To distribute the twophase 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 Tjunction. An impacting Tjunction is a Tjunction 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 Tjunction 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 waterair 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 twophase refrigerant flows over an impacting Tjunction. 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 Tjunction 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 Tjunction was measured. These pressure gradient measurements were used to create a model which predicts the pressure drop over the Tjunctions. 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 waterair or watersteam flows. In general, the models have the highest predictive capability for their design twophase 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 watersteam and waterair data. Finally, the model is extended to inclined impacting Tjunctions. 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.}, author = {Billiet, Marijn}, isbn = {9789463551915}, language = {eng}, pages = {166}, school = {Ghent University}, title = {Phase distribution of a refrigerant twophase flow over an impacting Tjunction}, year = {2018}, }