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

Marijn Billiet (UGent)
(2018)
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Promoter
(UGent)
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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 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.

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Citation

Please use this url to cite or link to this publication:

MLA
Billiet, Marijn. “Phase Distribution of a Refrigerant Two-phase Flow over an Impacting T-junction.” 2018 : n. pag. Print.
APA
Billiet, M. (2018). Phase distribution of a refrigerant two-phase flow over an impacting T-junction. Ghent.
Chicago author-date
Billiet, Marijn. 2018. “Phase Distribution of a Refrigerant Two-phase Flow over an Impacting T-junction”. Ghent.
Chicago author-date (all authors)
Billiet, Marijn. 2018. “Phase Distribution of a Refrigerant Two-phase Flow over an Impacting T-junction”. Ghent.
Vancouver
1.
Billiet M. Phase distribution of a refrigerant two-phase flow over an impacting T-junction. [Ghent]; 2018.
IEEE
[1]
M. Billiet, “Phase distribution of a refrigerant two-phase flow over an impacting T-junction,” 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 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.},
  author       = {Billiet, Marijn},
  isbn         = {978-94-6355-191-5},
  language     = {eng},
  pages        = {166},
  school       = {Ghent University},
  title        = {Phase distribution of a refrigerant two-phase flow over an impacting T-junction},
  year         = {2018},
}