- Email: [email protected]
refrigerant enhanced surface tubing
13
Space heating and cooling
Effectiveness of a thermal 97lO5003 using phase-change materials
energy
storage
system
El-Dessouky, H. and Al-Juwayhel, F. Energy Coni,er.r. Mgmt. 1997, 3X, (6) 601-617. For a phase-change thermal energy storage system, a second law analysis was developed. The methodology starts with determining the number of entropy generation numbers (N) for a cyclic operation consisting of heat storage and retrieval processes. During the cyclic operation, the phasechange material is maintained at its melting temperature. The entropy generation number is determined as a function of the Reynolds number of the operating fluid, specific heat transfer area, operating fluid inlet temperatures during the cyclic processes and the thermo-physical properties of both the storage materials and the heat transfer fluids. A case study is presented to demonstrate the applicability of the model developed. The storage material considered was either paraffin wax or calcium chloride hexahydrate, whereas the heat transfer fluid used was air or water.
97/05004
Exergetic
optimisation
of a heat exchanger
Cornelissen, R. L. and Hirs, G. G. Energy Convers. Mgmt, 1997, 38, (1517). 1567-1576. The paper aims to show that for the optimal design of an energy system, where there is a trade-off between exergy saving during operation and exergy use during construction of the energy system, exergy analysis and life cycle analysis should be combined. The two methods are often used separately, but some studies have been conducted in which they are combined. On the basis of the life cycle analysis method, an exergetic optimisation of a heat exchanger has been carried out. The optimization takes into account irreversibilities due to frictional pressure drops and the temperature difference between the hot and cold stream and irreversibilities due to the production of the materials and the construction of the heat exchanger. A water to water heat exchanger in a city heating system has been selected to exemplify this type of heat exchanger. The influence of the configuration of the heating system, including the energy conversion, on the optimisation of the heat exchangers has been shown. The analysis of the heat exchanger combining exergy and life cycle analysis are combined gives the design conditions of the heat exchangers with lowest life cycle irreversibility.
Experimental analysis and numerical modeling of 97105005 inward solidification on a finned vertical tube for a latent heat storage unit Velraj, R. et al. Sol. Energy, 1997, 60, (5) 281-290. The paper discusses the enhancement of heat transfer in a thermal storage system consisting of a cylindrical vertical tube with internal longitudinal fin arrangement. This configuration forms a V-shaped enclosure for the phase change material (PCM) giving maximum benefit to the fin arrangement. A theoretical model that also accounts for the circumferential heat flow through the tube wall is developed using enthalpy formulation and is employed in conjunction with the fully implicit finite difference method to solve the solidification in the convectively cooled vertical tube. A generalized enthalpy-temperature (H-T) relationship, developed by A.W. Date (1992) is suitable for constant phase change temperature. In the present work, this is modified to accommodate materials having either constant or a range of phase change temperatures. The numerical model is validated with experimental data. The instantaneous surface heat flux and the solidified fraction for different number of fins and tube radii, applicable to the design of latent heat thermal storage system consisting of an array of vertical cylindrical tubes containing PCM, are also discussed.
97105006 collectors
Experimental in a refrigerant
investigation of solar concentrating ejector refrigeration machine ItIt. J. EnergyRes., 1Y97, 21, (12) 1123-1131.
Al-Khalidy, N. Experiments were carried out to study the energy output and efficiency of solar collectors in a refrigerant ejector refrigeration machine using RI 13 as the refrigerant. Based on a theoretical study, the oil transformer is used as a working fluid in the solar system. The concentrating collectors show encouraging results for operating the system. The maximum collector efficiency was 20%. The system can be used for both cooling and heating purposes.
Flow boiling and condensation of tetrary refrigerant 97105007 mixtures inside water/refrigerant enhanced surface tubing Sami, S. M. and Song, B. Int. J. Emrgy Res., 1997, 21, (14) 1305-1320. Experimental results concerning the heat transfer characteristics of twophase flow condensation and boiling of tetrary (R-32/R-l25/R143alR134a) refrigerant mixtures inside water/refrigerant horizontal enhanced surface tubing are presented. Heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops OF the tetrary refrigerant mixtures, have been predicted and compared with other mixtures during flow condensation and boiling inside enhanced surface tubing. It was found that the tetrary refrigerant blend has higher transfer coefficients than R502, and the lowest pressure drop among the refrigerants studied.
434
Fuel and Energy Abstracts
November 1997
97105006 Heat transfer and friction characteristics of turbulent flow in circular tubes with twisted-tape inserts and axial interrupted ribs Zhang, Y. M. et al. Enhmced Heat Trcrmfcr, 1997, 4, (4). 297-308. This paper investigates local heat transfer distributions and pressure drop in smooth circular tubes with twisted-tape inserts and axial interrupted ribs. Experimental data for isothermal friction factors and regionally averaged Nusselt numbers are presented for turbulent air flows for Reynolds numbers ranging from 17,000 to 82.000. The circular tube is composed of ten isolated copper sections with a tube length-to-diameter ratio (L/D) of 15. Three different 360 twisted-tape ratios are studied: H/D = 6 (2.5 turns), 7.5 (2 turns), and 10 (1.5 turns). Three axial interrupted rib configurations attached to the inner wall of three test tubes with twisted-tape inserts are investigated: e/D = 0.0625 in-line rib, e/D = 0.0625 staggered rib. and e/D = 0.125 staggered rib. The results show that the heat transfer and pressure drop in the tuhe with twisted-tape inserts increase by increasing the number of twisted-tape turns. Individual results for the varying conditions.
Heat transfer and pressure drop in a heat exchan97lO5009 ger with a helical pipe containing inside springs Yildiz, C. et al. Emrfiv Cmwer.s. Mgmt.. 1997, 38, (h), 619-624. Spring-shaped wires with varying pitch within a helical pipe comprise the heat exchanger under investigation in this work. The pressure drop and the overall heat transfer rates were measured for the case of air flow at various Reynolds numbers inside and constant water flow outside. The Nuaselt number was found to increase with decreasing pitch/wire diameter ratto. as much as five times with respect to an empty pipe for the same Dean number. For this relationship. a tentative empirical formula is suggested. Although a rise up to IO times in the inlet/outlet pressure drop values with respect to the conventional empty helical case is observed, the increase in Nusselt number, naturally, reflects an increase of ahout 30% in the effectiveness of the helical heat exchanger.
97lO5010 Improved load-frequency control with capacitive energy storage Tripathy, S. C. Energy Conw~. Mgmt, 1997, 38. (h), 551-562. Fast acting energy storage devices can effectively dampen electromechanical oscillations in a power system. Such an effect arises because additional energy storage capacity is provided as a supplement to the kinetic energy storage in the moving mass of the generator rotor. The energy storage devices share the sudden changes in power requirement in the load. This work proposes a new scheme using a small rating capacitive energy storage unit for improvement of the dynamic performance of a power system having load-frequency control. A comprehensive digital computer model of a twoarea interconnected power system, which includes the governor dead-band non-linearity, steam reheat turbine, boiler dynamics and the generator rate constraint, is developed for the analysis.
97/05011 properties tics Sarevski,
Influence of the new refrigerant thermodynamic on some refrigerating turbocompressor characteris-
M. N.
froc.
Int. Cmgr.Refrig.,19th,
19Y.5, 3A. 483-400.
Investigation of heat transfer in a polymeric change material for low level heat storage Rovon, L. et al. Enemy Cowem. Mgmt, 1997, 38, (6) 517-524.
97105012
phase
The paper introduces a new material for low level heat storage. It is a stable mixture of water with a water soluble polymerised and cross-linked monomer such as polyacrylamide. Quantitative results regarding the thermophysical properties of the material are presented. These parameters are used in a theoretical approach describing the progression of a phase change front in a slab of finite thickness, and the computed results are found to be in good agreement with the experimental data for the considered duration of freezing and thawing in samples.
97lO5013 PV powered
Microprocessor-based overcurrent air-conditioning system Anis, W. R. et al. Energy Comers. Mgmt., 1997, 38, (I),
protection
for
29-37. In order to convert the d.c. power to ac. power. d.c./a.c. inverters are necessary in photovoltaic (PV) systems used to drive a.c. loads. Dynamic a.c. loads, driven by single phase induction motors, require high starting current. A theoretical and experimental investigation of the instantaneous starting current showed that it reaches values much higher than seven times the rated r.m.s. current. If the A/C is driven by an infinite a.c. source, such a high instantaneous current does not affect the source. However, if the A/C is driven by a PV array through an electronic inverter, such a high starting current can damage the electronic components of the inverter. In this work, a microprocessor-based control system to limit the high starting current is discussed. The proposed technique measures the rate of change of current and, hence, predicts the current at the forthcoming moments. According to this measured value, the microprocessor determines whether the A/C should be disconnected from the inverter or not. The microprocessor is programmed in such a way as to give the disconnect command hefore the current reaches the maximum allowable limit. The proposed microprocessor control system offers protection against the inrush starting current and keeps monitoring the motor current even after the starting period. The system provides continuous protection for the motor and the source.
Space heating and cooling
Effectiveness of a thermal 97lO5003 using phase-change materials
energy
storage
system
El-Dessouky, H. and Al-Juwayhel, F. Energy Coni,er.r. Mgmt. 1997, 3X, (6) 601-617. For a phase-change thermal energy storage system, a second law analysis was developed. The methodology starts with determining the number of entropy generation numbers (N) for a cyclic operation consisting of heat storage and retrieval processes. During the cyclic operation, the phasechange material is maintained at its melting temperature. The entropy generation number is determined as a function of the Reynolds number of the operating fluid, specific heat transfer area, operating fluid inlet temperatures during the cyclic processes and the thermo-physical properties of both the storage materials and the heat transfer fluids. A case study is presented to demonstrate the applicability of the model developed. The storage material considered was either paraffin wax or calcium chloride hexahydrate, whereas the heat transfer fluid used was air or water.
97/05004
Exergetic
optimisation
of a heat exchanger
Cornelissen, R. L. and Hirs, G. G. Energy Convers. Mgmt, 1997, 38, (1517). 1567-1576. The paper aims to show that for the optimal design of an energy system, where there is a trade-off between exergy saving during operation and exergy use during construction of the energy system, exergy analysis and life cycle analysis should be combined. The two methods are often used separately, but some studies have been conducted in which they are combined. On the basis of the life cycle analysis method, an exergetic optimisation of a heat exchanger has been carried out. The optimization takes into account irreversibilities due to frictional pressure drops and the temperature difference between the hot and cold stream and irreversibilities due to the production of the materials and the construction of the heat exchanger. A water to water heat exchanger in a city heating system has been selected to exemplify this type of heat exchanger. The influence of the configuration of the heating system, including the energy conversion, on the optimisation of the heat exchangers has been shown. The analysis of the heat exchanger combining exergy and life cycle analysis are combined gives the design conditions of the heat exchangers with lowest life cycle irreversibility.
Experimental analysis and numerical modeling of 97105005 inward solidification on a finned vertical tube for a latent heat storage unit Velraj, R. et al. Sol. Energy, 1997, 60, (5) 281-290. The paper discusses the enhancement of heat transfer in a thermal storage system consisting of a cylindrical vertical tube with internal longitudinal fin arrangement. This configuration forms a V-shaped enclosure for the phase change material (PCM) giving maximum benefit to the fin arrangement. A theoretical model that also accounts for the circumferential heat flow through the tube wall is developed using enthalpy formulation and is employed in conjunction with the fully implicit finite difference method to solve the solidification in the convectively cooled vertical tube. A generalized enthalpy-temperature (H-T) relationship, developed by A.W. Date (1992) is suitable for constant phase change temperature. In the present work, this is modified to accommodate materials having either constant or a range of phase change temperatures. The numerical model is validated with experimental data. The instantaneous surface heat flux and the solidified fraction for different number of fins and tube radii, applicable to the design of latent heat thermal storage system consisting of an array of vertical cylindrical tubes containing PCM, are also discussed.
97105006 collectors
Experimental in a refrigerant
investigation of solar concentrating ejector refrigeration machine ItIt. J. EnergyRes., 1Y97, 21, (12) 1123-1131.
Al-Khalidy, N. Experiments were carried out to study the energy output and efficiency of solar collectors in a refrigerant ejector refrigeration machine using RI 13 as the refrigerant. Based on a theoretical study, the oil transformer is used as a working fluid in the solar system. The concentrating collectors show encouraging results for operating the system. The maximum collector efficiency was 20%. The system can be used for both cooling and heating purposes.
Flow boiling and condensation of tetrary refrigerant 97105007 mixtures inside water/refrigerant enhanced surface tubing Sami, S. M. and Song, B. Int. J. Emrgy Res., 1997, 21, (14) 1305-1320. Experimental results concerning the heat transfer characteristics of twophase flow condensation and boiling of tetrary (R-32/R-l25/R143alR134a) refrigerant mixtures inside water/refrigerant horizontal enhanced surface tubing are presented. Heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops OF the tetrary refrigerant mixtures, have been predicted and compared with other mixtures during flow condensation and boiling inside enhanced surface tubing. It was found that the tetrary refrigerant blend has higher transfer coefficients than R502, and the lowest pressure drop among the refrigerants studied.
434
Fuel and Energy Abstracts
November 1997
97105006 Heat transfer and friction characteristics of turbulent flow in circular tubes with twisted-tape inserts and axial interrupted ribs Zhang, Y. M. et al. Enhmced Heat Trcrmfcr, 1997, 4, (4). 297-308. This paper investigates local heat transfer distributions and pressure drop in smooth circular tubes with twisted-tape inserts and axial interrupted ribs. Experimental data for isothermal friction factors and regionally averaged Nusselt numbers are presented for turbulent air flows for Reynolds numbers ranging from 17,000 to 82.000. The circular tube is composed of ten isolated copper sections with a tube length-to-diameter ratio (L/D) of 15. Three different 360 twisted-tape ratios are studied: H/D = 6 (2.5 turns), 7.5 (2 turns), and 10 (1.5 turns). Three axial interrupted rib configurations attached to the inner wall of three test tubes with twisted-tape inserts are investigated: e/D = 0.0625 in-line rib, e/D = 0.0625 staggered rib. and e/D = 0.125 staggered rib. The results show that the heat transfer and pressure drop in the tuhe with twisted-tape inserts increase by increasing the number of twisted-tape turns. Individual results for the varying conditions.
Heat transfer and pressure drop in a heat exchan97lO5009 ger with a helical pipe containing inside springs Yildiz, C. et al. Emrfiv Cmwer.s. Mgmt.. 1997, 38, (h), 619-624. Spring-shaped wires with varying pitch within a helical pipe comprise the heat exchanger under investigation in this work. The pressure drop and the overall heat transfer rates were measured for the case of air flow at various Reynolds numbers inside and constant water flow outside. The Nuaselt number was found to increase with decreasing pitch/wire diameter ratto. as much as five times with respect to an empty pipe for the same Dean number. For this relationship. a tentative empirical formula is suggested. Although a rise up to IO times in the inlet/outlet pressure drop values with respect to the conventional empty helical case is observed, the increase in Nusselt number, naturally, reflects an increase of ahout 30% in the effectiveness of the helical heat exchanger.
97lO5010 Improved load-frequency control with capacitive energy storage Tripathy, S. C. Energy Conw~. Mgmt, 1997, 38. (h), 551-562. Fast acting energy storage devices can effectively dampen electromechanical oscillations in a power system. Such an effect arises because additional energy storage capacity is provided as a supplement to the kinetic energy storage in the moving mass of the generator rotor. The energy storage devices share the sudden changes in power requirement in the load. This work proposes a new scheme using a small rating capacitive energy storage unit for improvement of the dynamic performance of a power system having load-frequency control. A comprehensive digital computer model of a twoarea interconnected power system, which includes the governor dead-band non-linearity, steam reheat turbine, boiler dynamics and the generator rate constraint, is developed for the analysis.
97/05011 properties tics Sarevski,
Influence of the new refrigerant thermodynamic on some refrigerating turbocompressor characteris-
M. N.
froc.
Int. Cmgr.Refrig.,19th,
19Y.5, 3A. 483-400.
Investigation of heat transfer in a polymeric change material for low level heat storage Rovon, L. et al. Enemy Cowem. Mgmt, 1997, 38, (6) 517-524.
97105012
phase
The paper introduces a new material for low level heat storage. It is a stable mixture of water with a water soluble polymerised and cross-linked monomer such as polyacrylamide. Quantitative results regarding the thermophysical properties of the material are presented. These parameters are used in a theoretical approach describing the progression of a phase change front in a slab of finite thickness, and the computed results are found to be in good agreement with the experimental data for the considered duration of freezing and thawing in samples.
97lO5013 PV powered
Microprocessor-based overcurrent air-conditioning system Anis, W. R. et al. Energy Comers. Mgmt., 1997, 38, (I),
protection
for
29-37. In order to convert the d.c. power to ac. power. d.c./a.c. inverters are necessary in photovoltaic (PV) systems used to drive a.c. loads. Dynamic a.c. loads, driven by single phase induction motors, require high starting current. A theoretical and experimental investigation of the instantaneous starting current showed that it reaches values much higher than seven times the rated r.m.s. current. If the A/C is driven by an infinite a.c. source, such a high instantaneous current does not affect the source. However, if the A/C is driven by a PV array through an electronic inverter, such a high starting current can damage the electronic components of the inverter. In this work, a microprocessor-based control system to limit the high starting current is discussed. The proposed technique measures the rate of change of current and, hence, predicts the current at the forthcoming moments. According to this measured value, the microprocessor determines whether the A/C should be disconnected from the inverter or not. The microprocessor is programmed in such a way as to give the disconnect command hefore the current reaches the maximum allowable limit. The proposed microprocessor control system offers protection against the inrush starting current and keeps monitoring the motor current even after the starting period. The system provides continuous protection for the motor and the source.