Akio Miyara | Saga University (original) (raw)
Papers by Akio Miyara
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A numerical investigation is carried out for single phase flow behavior and heat transfer charact... more A numerical investigation is carried out for single phase flow behavior and heat transfer characteristics inside herringbone microfin tube by using a commercial CFD code FLUENT. It solves the three-dimensional Navier-Stokes equations in strong conservation ...
2021 Purdue Conferences. 18th International Refrigeration and Air-Conditioning Conference at Purdue., 2021
In this study, boiling and condensation heat transfer characteristics of refrigerant R1234yf flow... more In this study, boiling and condensation heat transfer characteristics of refrigerant R1234yf flowing inside a channel of plate heat exchanger were experimentally investigated. The test section consists of 8 plates where the refrigerant flow channel is formed by two plates grooved chevron pattern. Other plates are used to measure local temperature distribution and to form cooling /heating water channel. In this experiment three vertical flow channel exist in the test section and where the refrigerant flows in the middle channel (upward or downward directions) depending on evaporation or condensation process. In case of evaporation process the refrigerant flows upward in the middle channel and heat source water flows downward in the outer two channels. In condensation, refrigerant flows downward in the middle channel and the cooling water flows upwards in the two outer channels. In the test section, there are 160 measurement points in total with 4 sets of 8 symmetrical points in the horizontal direction and 5 symmetrical points in the vertical direction. In order to measure local heat transfer characteristics of the R1234yf, 20 thermocouples were set in the test section for measurement of wall temperature of the refrigerant and heat source water side. Local heat transfer coefficient was obtained under the experimental conditions of the mass flux of 10kg/(2 s) and 50 kg/(2). The local heat transfer coefficient is varied in horizontally and vertically. Local dryout in evaporation process was identified. The data of the R1234yf was also compared with the R32.
Plate heat exchangers (PHEs) are a type of compact heat exchanger widely used for industrial appl... more Plate heat exchangers (PHEs) are a type of compact heat exchanger widely used for industrial applications, such as refrigeration, heating, cooling, chemical processing, etc. They provide a large heat transfer surface area per volume, which makes them particularly suited for installation in confined spaces. Consequently, they have a reduced refrigerant charge and require lighter structural supports. Generally, PHEs consist of thin, rectangular, pressed steel plates (most often stainless steel) that are stacked together, such that hot and cold fluid streams alternate through the inter-plate passages. The plates are stamped with corrugated patterns that not only provide a larger effective heat transfer surface area but also modify the flow field in order to promote enhanced thermal-hydraulic performance. The use of compact heat exchangers with reduced channel hydraulic diameter and flow channel length has increased over the last years. To keep the pressure loss at acceptable levels, an implication of the diameter down-scaling is an increase in the number of parallel flow channels through the heat exchanger. The growing number of parallel flow channels increases the challenge of distributing twophase flow equally among the channels. Due to the complexity of the two-phase flow process, a rigorous theoretical analysis is not feasible. The fundamental understanding of the flow distribution and other mechanisms in this type of channel is rather limited. In this thesis, two-phase hydraulic performances of a prototype chevron plate heat exchanger have been investigated, modelled and simulated in detail. The hydrodynamic characteristics and flow distribution in two cross-corrugated channels have been investigated numerically. The main advantages of the CFD model of the PHE are the detailed void fraction and velocity distributions obtained and the fact that it is not necessary to collect extensive experimental data to adjust the model parameters. The 3D velocity fields have been obtained through numerical simulation. It is found that the flow around the contact points is separated in two streams and a considerable mixing occurs. Resorting to the results of velocities field it was possible to conclude that a laminar flow occurs in the present operation conditions; the existence of corrugations in the plates confers to velocity a sinusoidal behaviour in the main flow direction. The local condensation heat transfer characteristics of R1234ze(E) in the plate heat exchanger were investigated experimentally at different mass flux conditions. The experiments were conducted by varying the mass fluxes. The local condensation heat transfer coefficient decreased with increasing of wetness with different values in horizontal direction. At low refrigerant mass flux (G = 10, 20 kg/m 2 s), the heat transfer coefficients are not dependent on iii mass flux and probably condensation is controlled by gravity. For higher refrigerant mass flux (Gr = 50 kg/m 2 s) the heat transfer coefficients depend on mass flux and forced convection condensation occurs. The condensation heat flux decreases with increase of distance along the direction from inlet to outlet of plate heat exchanger which is the direction of condensation progress. The local wall temperature distribution decreases with increase of distance along the downstream. More advanced development in computer technology will be helpful for further investigations by CFD. It is very good method to obtain some detail information about two-phase flow in flow field which cannot be achieved by experiment only. It is possible to obtain details of two phase flow inside plate heat exchanger by numerical simulations. Some evidence has not been revealed yet and become a good challenge in future research.
Transactions of the Japan Society of Refrigerating and Air Conditioning Engineers, 2001
TOP > J-EAST > List of Journal Titles (T) > Transactions of the Japan Society of Refrige... more TOP > J-EAST > List of Journal Titles (T) > Transactions of the Japan Society of Refrigerating and Air Conditioning Engineers(2001) > Effects of Fin Shape on Condensation Heat Transfer and Pressure Drop inside Herringbone Micro Fin Tubes. ... Effects of Fin Shape on ...
Proceedings of the... …, 2000
抄録 Heat transfer coefficient and pressure drop of in-tube condensation of R410A were measured wit... more 抄録 Heat transfer coefficient and pressure drop of in-tube condensation of R410A were measured with a herringbone type micro fin tube, a helical micro fin tube, and a smooth tube. Flow state of the herringbone micro fin tube was observed by using R123 as a working ...
E3S Web of Conferences, 2019
Ground source cooling system (GSCS) uses a ground heat exchanger (GHE) for exchanging heat with t... more Ground source cooling system (GSCS) uses a ground heat exchanger (GHE) for exchanging heat with the ground. A spiral-tube GHE is gaining interest in recent year. This study presents an experimental analysis of thermal performance of shallow spiral-tube ground heat exchanger (GHE) installed in the ground at 3 m depth in series and parallel configurations. These GHE configurations offer a compromise between the conventional vertical and horizontal GHEs. The spiral-tube GHE which is consist of spiral pipe installed in the borehole provides a better performance in application of GSCS. The thermal performances ofspiraltube GHE in series and parallel configurations were investigated under actual condition. Inlet and outlet temperatures of the both configurations were measured and periodically recorded. The average heat exchange rates of the GHEs are 122.4 W m –1 in parallel configuration and 86.2 W m –1in series configuration. Heat exchange rate of the spiral-tube GHEs in parallel configu...
Journal of Engineering …, 2012
The flow configuration and distribution behavior of two-phase flow in a distributor made of acryl... more The flow configuration and distribution behavior of two-phase flow in a distributor made of acrylic resin have been investigated experimentally. In this study, air and water were used as two-phase flow working fluids. The distributor consists of one inlet and two ...
International Journal of Engineering, Science and Technology, 2011
Vapor-liquid equilibrium (VLE) data were predicted for the binary mixture of carbon dioxide (CO 2... more Vapor-liquid equilibrium (VLE) data were predicted for the binary mixture of carbon dioxide (CO 2) and dimethyl ether (DME) at ten temperatures ranging from 273.15 to 386.56 K and pressure upto 7.9 MPa to observe this mixture's potential of COP enhancement and capacity modulation as a working fluid in a refrigeration system. Since the mixtures are zeotropic in nature and the components of the mixtures have good thermophysical properties, zero ozone depleting potential (ODP) and low global warming potential (GWP), they are considered as promising alternative refrigerants. The Benedict-Web-Rubin (BWR) and the modified Benedict-Web-Rubin (MBWR) equations of state (EoS) have been used for the prediction of VLE data. For the BWR and MBWR equations of state, respective constant binary interaction parameters have been determined by using the available experimental VLE data of CO 2 /DME mixtures. The predicted VLE data have been compared with the experimental data and the data obtained from REFPROP version 8.0. Among the comparison results, BWR EoS shows good agreement with the experimental data.
Proceedings of the 15th International Heat Transfer Conference, 2014
This study investigates thermal performance and characteristics of spiral-tube ground heat exchan... more This study investigates thermal performance and characteristics of spiral-tube ground heat exchanger (GHE) for ground source heat pump in laminar and turbulent flows. The thermal performances are evaluated by numerical method using a CFD code. Various models of spiral-tube GHEs installed in a borehole and backfilled with silica-sand were simulated. In comparison with the conventional U-tube GHE, the performance of spiral-tube GHE increases by 62.7 % in the laminar flow and by 33.5 % in the turbulent flow. Due to the interest in applying the GHE in concrete foundation piles, the spiral-tube GHE installed in a concrete pile is also simulated. Its performance shows a slight better performance of 5 % in the laminar flow and 6 % in the turbulent flow compared with that of a borehole backfilled with silica sand. Furthermore, the performance of spiral-tube GHEs with outlet pipe installed inside and outside the spiral tube in the borehole, and varying spiral pitches of p = 0.05 m; 0.1 m and 0.2 m are investigated. The effects of outlet pipe positions on the performances are shown. The spiral-tube with outlet pipe installed outside gives a better performance, of 2 % in the laminar flow and 10 % in the turbulent flow, than that of the outlet pipe installed inside. Water temperature distributions of the both spiral-tube GHEs are presented. Varying the spiral pitch significantly affects the performance of the spiral-tube GHEs. Increasing the spiral pitch reduces the number of spirals per meter borehole depth.
Refrigerant dissolves in refrigeration oil which is used to lubricate the compressor. Because the... more Refrigerant dissolves in refrigeration oil which is used to lubricate the compressor. Because the viscosity of the oil in which the refrigerant is dissolved is significantly reduced, the rate of the reduction is important information to select a proper oil. However, viscosities have not been sufficiently measured for mixtures of refrigeration oil and new refrigerants which are HFOs and mixtures of HFO and HFC. In order to utilize HFO refrigerants in alternative refrigeration systems, viscosity data of mixtures of refrigeration oil and HFO are essential. In this study, mixture of a refrigeration oil and refrigerant (R410A and R1234yf) viscosity are measured with temperature from 40 to 80 ℃ (313 – 353 K) and the oil mass concentration from 80 to 100 % using the tandem capillary tubes method. In this method, the test fluid flows inside two different length and same diameter capillary tubes connected in series in order to eliminate the pressure drop at the inlet and outlet therefore the...
Distributor plays a major role on uniformly of the fluid flow distribution through multiple chann... more Distributor plays a major role on uniformly of the fluid flow distribution through multiple channels, particularly in two phase flow. Uniformity and low pressure drop are the major issue which shall be solved. This study emphasized on the shape of branching points for two phase flow distributor to minimize mal-uniformity and pressure drop. The present distributor, the merge pipe distributor was developed to curved round distributor models with three types of reducer namely; curve reducer, T-reducer and straight reducer. The work was done thru investigating the performance of merge pipe distributor and comparing it to the performance of developed distributors. The analyses were performed through CFD modeling by using Fluent@ software, the two dimensional approach and high superficial velocity. It was founded that, the merged pipe distributor has tended to lack uniformity of phase distribution and slightly lower pressure drop than curved reducer distributor. While the curve round distributor models with T-reducer has exhibited better performance due to smaller pressure drop and optimum phase uniformity.
Journal of Thermal Science, 2007
Numerical simulations have been carried out for two-dimensional wavy falling liquid film in order... more Numerical simulations have been carried out for two-dimensional wavy falling liquid film in order to investigate kinetic energy and surface energy of that liquid film. Governing equations, which are continuity equation, Navier-Stokes equation, and equations of interfacial boundary conditions including surface movement and effect of surface tension, have been solved directly by means of a numerical scheme based on the finite difference method. In most cases, periodic disturbances superimposed at inflow boundary grow to fully developed waves which retain the given periodic behavior. In some cases, however, random waves appear after the fully developed waves. Variations of kinetic energy and surface energy of the periodically developed waves and the random waves have been discussed.
In loop heat pipes (LHPs), wick materials and their structures are important in achieving continu... more In loop heat pipes (LHPs), wick materials and their structures are important in achieving continuous heat transfer with a favorable distribution of the working fluid. This article introduces the characteristics of loop heat pipes with different wicks: (i) sintered stainless steel and (ii) ceramic. The evaporator has a flat-rectangular assembly under gravity-assisted conditions. Water was used as a working fluid, and the performance of the LHP was analyzed in terms of temperatures at different locations of the LHP and thermal resistance. As to the results, a stable operation can be maintained in the range of 50 to 520 W for the LHP with the stainless-steel wick, matching the desired limited temperature for electronics of 85 °C at the heater surface at 350 W (129.6 kW·m−2). Results using the ceramic wick showed that a heater surface temperature of below 85 °C could be obtained when operating at 54 W (20 kW·m−2).
Applied Sciences, 2018
This study was made to investigate the condensation local heat transfer and adiabatic pressure dr... more This study was made to investigate the condensation local heat transfer and adiabatic pressure drop of R134a inside a 2.5 mm outside diameter smooth and microfin tube at low mass flux condition. Data were measured for mass fluxes from 50 to 300 kg m−2s−1, vapor qualities from 0 to 1 and saturation temperatures from 20 to 30 °C. The effects of mass flux, vapor quality, saturation temperature, microfin and diameter of test tube were analyzed. The pressure drop of the microfin tube was about 1.5 times higher than that of the smooth tube at mass flux 50 kg m−2s−1. The heat transfer coefficient of the microfin tube was about 2–5 times higher than that of the smooth tube at mass flux 100 kg m−2s−1. Experimental results were also compared with typical correlations. For the microfin tube, however, all the existing correlations do not adequately predict the present data. Poor predictions may be attributed to the lack of the small diameter microfin tube with low mass flux data in their databa...
Horizontal ground heat exchanger in ground source heat pump systems is susceptible to ground surf... more Horizontal ground heat exchanger in ground source heat pump systems is susceptible to ground surface variations thus affecting its thermal performance. However, this configuration is desirable due to low installation costs as it mainly involved burying pipes in shallow trenches. In this study, experimental investigation of thermal performance for slinky horizontal ground heat exchangers (GHEs) in several operation conditions were conducted. The slinky type horizontal ground heat exchangers (HGHEs) in two orientations were considered such as reclined (parallel to ground surface) and standing (perpendicular to ground surface). Copper tube with outer surface coated using lowdensity polyethylene was considered for GHEs pipes. The GHEs were buried in 1.5 m depth of the ground. Ground temperature distribution was monitored up-to 10 m depth by placing T-type thermocouples at different depth position. During the operation, performance of GHEs decreases with operation time because of ground ...
The thermal performances of three types of ground heat exchangers (GHEs) installed in a steel pil... more The thermal performances of three types of ground heat exchangers (GHEs) installed in a steel pile foundation including double-tube, U-tube and multi-tube have been investigated in short-time period of operation. The performance of the GHEs descends gradually due to the heat buildup in the surrounding ground with operating time in continuous operation. Alternative operation modes (cooling, heating, and hot water heating) over a short-time period of operation for ground source heat pump (GSHP) system can be alternative solution to increase its performance. Experimental study was carried out in discontinuous operation in 2 hours operation with constant interval of off-time. As a result, the heat exchange rate in the short-time period of operation in discontinuous operation increases of 53.1 % for double-tube, 33.3 % for U-tube, and 29.1 % for multi-tube compare with in continuous operation. Geothermal energy system is receiving interest as environmentally friendly energy source with w...
In this study, the performance analysis of heat pump cycle using carbon dioxide (CO2) and dimethy... more In this study, the performance analysis of heat pump cycle using carbon dioxide (CO2) and dimethyl ether (DME) zeotropic mixture has been carried out by cycle calculation in order to clarify the characteristics and coefficient of performance (COP) of CO2/DME heat pump cycle. The calculation conditions were established as a hot-water supply system and the calculations were conducted by considering the heat transfer between the refrigerant and heat source/sink water. The heat pump cycle is formed around the critical point for high CO2 concentration mixtures and it is formed under the critical point for low CO2 concentration mixtures. The COP has the maximum at a certain pressure for each mixture and the COPs of the mixture and pure DME are higher than that of pure CO2. Operating pressure and refrigerant mass flow rate decreases with increase of DME concentration. The effects of concentration on characteristics of the cycle have also been discussed.
The viscosity of working fluids is an essential thermophysical property to design heat exchanger,... more The viscosity of working fluids is an essential thermophysical property to design heat exchanger, efficient processes, equipment, and simulation. Different types of conventional methods are used to measure the viscosity of fluids. The capillary tube method is based on the theoretical principle, and it is a reliable method. Nevertheless, it is tough to measure viscosity precisely because of problems involved in an equipment. In the present study, the tandem capillary tube method is proposed, and reliability of this method is explained. In this method, the pressure changes at the ends of the tube can be canceled, and the viscosity can be easily obtained from the Hagen-Poiseuille equation. A way to eliminate an effect of diameter difference between tubes is also proposed. The reliability and accuracy have been verified by comparing the measured viscosities of R134a to the reference values calculated by REFPROP ver.9.1. Most of the data agreed well within ±1.5 % deviation. These results...
This study investigates thermal performance of vertical ground heat exchangers (GHEs) with differ... more This study investigates thermal performance of vertical ground heat exchangers (GHEs) with different inlet water temperatures and borehole depths. The performances of three types of GHEs namely U-tube, double-tube and multi-tube GHEs are evaluated by numerical method using a CFD code. The simulation results show that heat exchange rates per unit borehole depth increase in the cooling mode and decrease in the heating mode of 3.4 W/m for U-tube, 5.7 W/m for double-tube, and 3.3 W/m for multi-tube with increasing of 1 C of the temperature difference between inlet water and ground. In addition, increasing the depth of vertical GHE lowers the heat exchange with the ground. By comparing with 20 m depth, the heat exchange rates per unit borehole depth lower of 32.5 % in 60 m depth, 47.9 % in 100 m depth for U-tube GHE and 29 % in 60 m depth, 42.7 % in 100 m depth for multi-tube GHE, respectively.
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A numerical investigation is carried out for single phase flow behavior and heat transfer charact... more A numerical investigation is carried out for single phase flow behavior and heat transfer characteristics inside herringbone microfin tube by using a commercial CFD code FLUENT. It solves the three-dimensional Navier-Stokes equations in strong conservation ...
2021 Purdue Conferences. 18th International Refrigeration and Air-Conditioning Conference at Purdue., 2021
In this study, boiling and condensation heat transfer characteristics of refrigerant R1234yf flow... more In this study, boiling and condensation heat transfer characteristics of refrigerant R1234yf flowing inside a channel of plate heat exchanger were experimentally investigated. The test section consists of 8 plates where the refrigerant flow channel is formed by two plates grooved chevron pattern. Other plates are used to measure local temperature distribution and to form cooling /heating water channel. In this experiment three vertical flow channel exist in the test section and where the refrigerant flows in the middle channel (upward or downward directions) depending on evaporation or condensation process. In case of evaporation process the refrigerant flows upward in the middle channel and heat source water flows downward in the outer two channels. In condensation, refrigerant flows downward in the middle channel and the cooling water flows upwards in the two outer channels. In the test section, there are 160 measurement points in total with 4 sets of 8 symmetrical points in the horizontal direction and 5 symmetrical points in the vertical direction. In order to measure local heat transfer characteristics of the R1234yf, 20 thermocouples were set in the test section for measurement of wall temperature of the refrigerant and heat source water side. Local heat transfer coefficient was obtained under the experimental conditions of the mass flux of 10kg/(2 s) and 50 kg/(2). The local heat transfer coefficient is varied in horizontally and vertically. Local dryout in evaporation process was identified. The data of the R1234yf was also compared with the R32.
Plate heat exchangers (PHEs) are a type of compact heat exchanger widely used for industrial appl... more Plate heat exchangers (PHEs) are a type of compact heat exchanger widely used for industrial applications, such as refrigeration, heating, cooling, chemical processing, etc. They provide a large heat transfer surface area per volume, which makes them particularly suited for installation in confined spaces. Consequently, they have a reduced refrigerant charge and require lighter structural supports. Generally, PHEs consist of thin, rectangular, pressed steel plates (most often stainless steel) that are stacked together, such that hot and cold fluid streams alternate through the inter-plate passages. The plates are stamped with corrugated patterns that not only provide a larger effective heat transfer surface area but also modify the flow field in order to promote enhanced thermal-hydraulic performance. The use of compact heat exchangers with reduced channel hydraulic diameter and flow channel length has increased over the last years. To keep the pressure loss at acceptable levels, an implication of the diameter down-scaling is an increase in the number of parallel flow channels through the heat exchanger. The growing number of parallel flow channels increases the challenge of distributing twophase flow equally among the channels. Due to the complexity of the two-phase flow process, a rigorous theoretical analysis is not feasible. The fundamental understanding of the flow distribution and other mechanisms in this type of channel is rather limited. In this thesis, two-phase hydraulic performances of a prototype chevron plate heat exchanger have been investigated, modelled and simulated in detail. The hydrodynamic characteristics and flow distribution in two cross-corrugated channels have been investigated numerically. The main advantages of the CFD model of the PHE are the detailed void fraction and velocity distributions obtained and the fact that it is not necessary to collect extensive experimental data to adjust the model parameters. The 3D velocity fields have been obtained through numerical simulation. It is found that the flow around the contact points is separated in two streams and a considerable mixing occurs. Resorting to the results of velocities field it was possible to conclude that a laminar flow occurs in the present operation conditions; the existence of corrugations in the plates confers to velocity a sinusoidal behaviour in the main flow direction. The local condensation heat transfer characteristics of R1234ze(E) in the plate heat exchanger were investigated experimentally at different mass flux conditions. The experiments were conducted by varying the mass fluxes. The local condensation heat transfer coefficient decreased with increasing of wetness with different values in horizontal direction. At low refrigerant mass flux (G = 10, 20 kg/m 2 s), the heat transfer coefficients are not dependent on iii mass flux and probably condensation is controlled by gravity. For higher refrigerant mass flux (Gr = 50 kg/m 2 s) the heat transfer coefficients depend on mass flux and forced convection condensation occurs. The condensation heat flux decreases with increase of distance along the direction from inlet to outlet of plate heat exchanger which is the direction of condensation progress. The local wall temperature distribution decreases with increase of distance along the downstream. More advanced development in computer technology will be helpful for further investigations by CFD. It is very good method to obtain some detail information about two-phase flow in flow field which cannot be achieved by experiment only. It is possible to obtain details of two phase flow inside plate heat exchanger by numerical simulations. Some evidence has not been revealed yet and become a good challenge in future research.
Transactions of the Japan Society of Refrigerating and Air Conditioning Engineers, 2001
TOP > J-EAST > List of Journal Titles (T) > Transactions of the Japan Society of Refrige... more TOP > J-EAST > List of Journal Titles (T) > Transactions of the Japan Society of Refrigerating and Air Conditioning Engineers(2001) > Effects of Fin Shape on Condensation Heat Transfer and Pressure Drop inside Herringbone Micro Fin Tubes. ... Effects of Fin Shape on ...
Proceedings of the... …, 2000
抄録 Heat transfer coefficient and pressure drop of in-tube condensation of R410A were measured wit... more 抄録 Heat transfer coefficient and pressure drop of in-tube condensation of R410A were measured with a herringbone type micro fin tube, a helical micro fin tube, and a smooth tube. Flow state of the herringbone micro fin tube was observed by using R123 as a working ...
E3S Web of Conferences, 2019
Ground source cooling system (GSCS) uses a ground heat exchanger (GHE) for exchanging heat with t... more Ground source cooling system (GSCS) uses a ground heat exchanger (GHE) for exchanging heat with the ground. A spiral-tube GHE is gaining interest in recent year. This study presents an experimental analysis of thermal performance of shallow spiral-tube ground heat exchanger (GHE) installed in the ground at 3 m depth in series and parallel configurations. These GHE configurations offer a compromise between the conventional vertical and horizontal GHEs. The spiral-tube GHE which is consist of spiral pipe installed in the borehole provides a better performance in application of GSCS. The thermal performances ofspiraltube GHE in series and parallel configurations were investigated under actual condition. Inlet and outlet temperatures of the both configurations were measured and periodically recorded. The average heat exchange rates of the GHEs are 122.4 W m –1 in parallel configuration and 86.2 W m –1in series configuration. Heat exchange rate of the spiral-tube GHEs in parallel configu...
Journal of Engineering …, 2012
The flow configuration and distribution behavior of two-phase flow in a distributor made of acryl... more The flow configuration and distribution behavior of two-phase flow in a distributor made of acrylic resin have been investigated experimentally. In this study, air and water were used as two-phase flow working fluids. The distributor consists of one inlet and two ...
International Journal of Engineering, Science and Technology, 2011
Vapor-liquid equilibrium (VLE) data were predicted for the binary mixture of carbon dioxide (CO 2... more Vapor-liquid equilibrium (VLE) data were predicted for the binary mixture of carbon dioxide (CO 2) and dimethyl ether (DME) at ten temperatures ranging from 273.15 to 386.56 K and pressure upto 7.9 MPa to observe this mixture's potential of COP enhancement and capacity modulation as a working fluid in a refrigeration system. Since the mixtures are zeotropic in nature and the components of the mixtures have good thermophysical properties, zero ozone depleting potential (ODP) and low global warming potential (GWP), they are considered as promising alternative refrigerants. The Benedict-Web-Rubin (BWR) and the modified Benedict-Web-Rubin (MBWR) equations of state (EoS) have been used for the prediction of VLE data. For the BWR and MBWR equations of state, respective constant binary interaction parameters have been determined by using the available experimental VLE data of CO 2 /DME mixtures. The predicted VLE data have been compared with the experimental data and the data obtained from REFPROP version 8.0. Among the comparison results, BWR EoS shows good agreement with the experimental data.
Proceedings of the 15th International Heat Transfer Conference, 2014
This study investigates thermal performance and characteristics of spiral-tube ground heat exchan... more This study investigates thermal performance and characteristics of spiral-tube ground heat exchanger (GHE) for ground source heat pump in laminar and turbulent flows. The thermal performances are evaluated by numerical method using a CFD code. Various models of spiral-tube GHEs installed in a borehole and backfilled with silica-sand were simulated. In comparison with the conventional U-tube GHE, the performance of spiral-tube GHE increases by 62.7 % in the laminar flow and by 33.5 % in the turbulent flow. Due to the interest in applying the GHE in concrete foundation piles, the spiral-tube GHE installed in a concrete pile is also simulated. Its performance shows a slight better performance of 5 % in the laminar flow and 6 % in the turbulent flow compared with that of a borehole backfilled with silica sand. Furthermore, the performance of spiral-tube GHEs with outlet pipe installed inside and outside the spiral tube in the borehole, and varying spiral pitches of p = 0.05 m; 0.1 m and 0.2 m are investigated. The effects of outlet pipe positions on the performances are shown. The spiral-tube with outlet pipe installed outside gives a better performance, of 2 % in the laminar flow and 10 % in the turbulent flow, than that of the outlet pipe installed inside. Water temperature distributions of the both spiral-tube GHEs are presented. Varying the spiral pitch significantly affects the performance of the spiral-tube GHEs. Increasing the spiral pitch reduces the number of spirals per meter borehole depth.
Refrigerant dissolves in refrigeration oil which is used to lubricate the compressor. Because the... more Refrigerant dissolves in refrigeration oil which is used to lubricate the compressor. Because the viscosity of the oil in which the refrigerant is dissolved is significantly reduced, the rate of the reduction is important information to select a proper oil. However, viscosities have not been sufficiently measured for mixtures of refrigeration oil and new refrigerants which are HFOs and mixtures of HFO and HFC. In order to utilize HFO refrigerants in alternative refrigeration systems, viscosity data of mixtures of refrigeration oil and HFO are essential. In this study, mixture of a refrigeration oil and refrigerant (R410A and R1234yf) viscosity are measured with temperature from 40 to 80 ℃ (313 – 353 K) and the oil mass concentration from 80 to 100 % using the tandem capillary tubes method. In this method, the test fluid flows inside two different length and same diameter capillary tubes connected in series in order to eliminate the pressure drop at the inlet and outlet therefore the...
Distributor plays a major role on uniformly of the fluid flow distribution through multiple chann... more Distributor plays a major role on uniformly of the fluid flow distribution through multiple channels, particularly in two phase flow. Uniformity and low pressure drop are the major issue which shall be solved. This study emphasized on the shape of branching points for two phase flow distributor to minimize mal-uniformity and pressure drop. The present distributor, the merge pipe distributor was developed to curved round distributor models with three types of reducer namely; curve reducer, T-reducer and straight reducer. The work was done thru investigating the performance of merge pipe distributor and comparing it to the performance of developed distributors. The analyses were performed through CFD modeling by using Fluent@ software, the two dimensional approach and high superficial velocity. It was founded that, the merged pipe distributor has tended to lack uniformity of phase distribution and slightly lower pressure drop than curved reducer distributor. While the curve round distributor models with T-reducer has exhibited better performance due to smaller pressure drop and optimum phase uniformity.
Journal of Thermal Science, 2007
Numerical simulations have been carried out for two-dimensional wavy falling liquid film in order... more Numerical simulations have been carried out for two-dimensional wavy falling liquid film in order to investigate kinetic energy and surface energy of that liquid film. Governing equations, which are continuity equation, Navier-Stokes equation, and equations of interfacial boundary conditions including surface movement and effect of surface tension, have been solved directly by means of a numerical scheme based on the finite difference method. In most cases, periodic disturbances superimposed at inflow boundary grow to fully developed waves which retain the given periodic behavior. In some cases, however, random waves appear after the fully developed waves. Variations of kinetic energy and surface energy of the periodically developed waves and the random waves have been discussed.
In loop heat pipes (LHPs), wick materials and their structures are important in achieving continu... more In loop heat pipes (LHPs), wick materials and their structures are important in achieving continuous heat transfer with a favorable distribution of the working fluid. This article introduces the characteristics of loop heat pipes with different wicks: (i) sintered stainless steel and (ii) ceramic. The evaporator has a flat-rectangular assembly under gravity-assisted conditions. Water was used as a working fluid, and the performance of the LHP was analyzed in terms of temperatures at different locations of the LHP and thermal resistance. As to the results, a stable operation can be maintained in the range of 50 to 520 W for the LHP with the stainless-steel wick, matching the desired limited temperature for electronics of 85 °C at the heater surface at 350 W (129.6 kW·m−2). Results using the ceramic wick showed that a heater surface temperature of below 85 °C could be obtained when operating at 54 W (20 kW·m−2).
Applied Sciences, 2018
This study was made to investigate the condensation local heat transfer and adiabatic pressure dr... more This study was made to investigate the condensation local heat transfer and adiabatic pressure drop of R134a inside a 2.5 mm outside diameter smooth and microfin tube at low mass flux condition. Data were measured for mass fluxes from 50 to 300 kg m−2s−1, vapor qualities from 0 to 1 and saturation temperatures from 20 to 30 °C. The effects of mass flux, vapor quality, saturation temperature, microfin and diameter of test tube were analyzed. The pressure drop of the microfin tube was about 1.5 times higher than that of the smooth tube at mass flux 50 kg m−2s−1. The heat transfer coefficient of the microfin tube was about 2–5 times higher than that of the smooth tube at mass flux 100 kg m−2s−1. Experimental results were also compared with typical correlations. For the microfin tube, however, all the existing correlations do not adequately predict the present data. Poor predictions may be attributed to the lack of the small diameter microfin tube with low mass flux data in their databa...
Horizontal ground heat exchanger in ground source heat pump systems is susceptible to ground surf... more Horizontal ground heat exchanger in ground source heat pump systems is susceptible to ground surface variations thus affecting its thermal performance. However, this configuration is desirable due to low installation costs as it mainly involved burying pipes in shallow trenches. In this study, experimental investigation of thermal performance for slinky horizontal ground heat exchangers (GHEs) in several operation conditions were conducted. The slinky type horizontal ground heat exchangers (HGHEs) in two orientations were considered such as reclined (parallel to ground surface) and standing (perpendicular to ground surface). Copper tube with outer surface coated using lowdensity polyethylene was considered for GHEs pipes. The GHEs were buried in 1.5 m depth of the ground. Ground temperature distribution was monitored up-to 10 m depth by placing T-type thermocouples at different depth position. During the operation, performance of GHEs decreases with operation time because of ground ...
The thermal performances of three types of ground heat exchangers (GHEs) installed in a steel pil... more The thermal performances of three types of ground heat exchangers (GHEs) installed in a steel pile foundation including double-tube, U-tube and multi-tube have been investigated in short-time period of operation. The performance of the GHEs descends gradually due to the heat buildup in the surrounding ground with operating time in continuous operation. Alternative operation modes (cooling, heating, and hot water heating) over a short-time period of operation for ground source heat pump (GSHP) system can be alternative solution to increase its performance. Experimental study was carried out in discontinuous operation in 2 hours operation with constant interval of off-time. As a result, the heat exchange rate in the short-time period of operation in discontinuous operation increases of 53.1 % for double-tube, 33.3 % for U-tube, and 29.1 % for multi-tube compare with in continuous operation. Geothermal energy system is receiving interest as environmentally friendly energy source with w...
In this study, the performance analysis of heat pump cycle using carbon dioxide (CO2) and dimethy... more In this study, the performance analysis of heat pump cycle using carbon dioxide (CO2) and dimethyl ether (DME) zeotropic mixture has been carried out by cycle calculation in order to clarify the characteristics and coefficient of performance (COP) of CO2/DME heat pump cycle. The calculation conditions were established as a hot-water supply system and the calculations were conducted by considering the heat transfer between the refrigerant and heat source/sink water. The heat pump cycle is formed around the critical point for high CO2 concentration mixtures and it is formed under the critical point for low CO2 concentration mixtures. The COP has the maximum at a certain pressure for each mixture and the COPs of the mixture and pure DME are higher than that of pure CO2. Operating pressure and refrigerant mass flow rate decreases with increase of DME concentration. The effects of concentration on characteristics of the cycle have also been discussed.
The viscosity of working fluids is an essential thermophysical property to design heat exchanger,... more The viscosity of working fluids is an essential thermophysical property to design heat exchanger, efficient processes, equipment, and simulation. Different types of conventional methods are used to measure the viscosity of fluids. The capillary tube method is based on the theoretical principle, and it is a reliable method. Nevertheless, it is tough to measure viscosity precisely because of problems involved in an equipment. In the present study, the tandem capillary tube method is proposed, and reliability of this method is explained. In this method, the pressure changes at the ends of the tube can be canceled, and the viscosity can be easily obtained from the Hagen-Poiseuille equation. A way to eliminate an effect of diameter difference between tubes is also proposed. The reliability and accuracy have been verified by comparing the measured viscosities of R134a to the reference values calculated by REFPROP ver.9.1. Most of the data agreed well within ±1.5 % deviation. These results...
This study investigates thermal performance of vertical ground heat exchangers (GHEs) with differ... more This study investigates thermal performance of vertical ground heat exchangers (GHEs) with different inlet water temperatures and borehole depths. The performances of three types of GHEs namely U-tube, double-tube and multi-tube GHEs are evaluated by numerical method using a CFD code. The simulation results show that heat exchange rates per unit borehole depth increase in the cooling mode and decrease in the heating mode of 3.4 W/m for U-tube, 5.7 W/m for double-tube, and 3.3 W/m for multi-tube with increasing of 1 C of the temperature difference between inlet water and ground. In addition, increasing the depth of vertical GHE lowers the heat exchange with the ground. By comparing with 20 m depth, the heat exchange rates per unit borehole depth lower of 32.5 % in 60 m depth, 47.9 % in 100 m depth for U-tube GHE and 29 % in 60 m depth, 42.7 % in 100 m depth for multi-tube GHE, respectively.