Pub Date : 2021-01-01DOI: 10.1299/JTST.2021JTST0021
S. Kadowaki, T. Aung, Taisei Furuyama, K. Kawata, T. Katsumi, Hideaki Kobayashi
Effects of pressure and heat loss on the unstable motion of cellular-flame fronts in hydrogen-air lean premixed flames were numerically investigated. We adopted the reaction mechanism for hydrogen-oxygen combustion, modeled with seventeen reversible reactions of eight reactive species and a diluent. Two-dimensional unsteady reactive flow was treated, and the compressibility, viscosity, heat conduction, molecular diffusion and heat loss were taken into account. A sufficiently small disturbance was superimposed on a planar flame to obtain the relation between the growth rate and wave number, i.e. the dispersion relation, and the linearly most unstable wavelength, i.e. the critical wavelength. As the pressure became higher, the maximum growth rate increased and the unstable range widened. These were due mainly to the decrease of flame thickness. As the heat loss became larger, the former decreased and the latter narrowed, which were due mainly to the decrease of burning velocity. To investigate the characteristics of cellular-flame fronts, a disturbance with the critical wavelength was superimposed. The superimposed disturbance developed owing to intrinsic instability, and then the cellular shape of flame fronts appeared. The burning velocity of a cellular flame normalized by that of a planar flame increased as the pressure became higher and the heat loss became larger. This indicated that the pressure and heat loss affected strongly the unstable motion of cellular-flame fronts. The burning velocity of a cellular flame increased monotonically with an increase in the space size. This was attributed to long-wavelength components of disturbances. Moreover, we estimated the fractal dimension of flame fronts through the box counting method. As the pressure and heat loss increased, the fractal dimension became larger, which denoted that the flame shape became more complicated.
{"title":"Effects of pressure and heat loss on the unstable motion of cellular-flame fronts caused by intrinsic instability in hydrogen-air lean premixed flames","authors":"S. Kadowaki, T. Aung, Taisei Furuyama, K. Kawata, T. Katsumi, Hideaki Kobayashi","doi":"10.1299/JTST.2021JTST0021","DOIUrl":"https://doi.org/10.1299/JTST.2021JTST0021","url":null,"abstract":"Effects of pressure and heat loss on the unstable motion of cellular-flame fronts in hydrogen-air lean premixed flames were numerically investigated. We adopted the reaction mechanism for hydrogen-oxygen combustion, modeled with seventeen reversible reactions of eight reactive species and a diluent. Two-dimensional unsteady reactive flow was treated, and the compressibility, viscosity, heat conduction, molecular diffusion and heat loss were taken into account. A sufficiently small disturbance was superimposed on a planar flame to obtain the relation between the growth rate and wave number, i.e. the dispersion relation, and the linearly most unstable wavelength, i.e. the critical wavelength. As the pressure became higher, the maximum growth rate increased and the unstable range widened. These were due mainly to the decrease of flame thickness. As the heat loss became larger, the former decreased and the latter narrowed, which were due mainly to the decrease of burning velocity. To investigate the characteristics of cellular-flame fronts, a disturbance with the critical wavelength was superimposed. The superimposed disturbance developed owing to intrinsic instability, and then the cellular shape of flame fronts appeared. The burning velocity of a cellular flame normalized by that of a planar flame increased as the pressure became higher and the heat loss became larger. This indicated that the pressure and heat loss affected strongly the unstable motion of cellular-flame fronts. The burning velocity of a cellular flame increased monotonically with an increase in the space size. This was attributed to long-wavelength components of disturbances. Moreover, we estimated the fractal dimension of flame fronts through the box counting method. As the pressure and heat loss increased, the fractal dimension became larger, which denoted that the flame shape became more complicated.","PeriodicalId":17405,"journal":{"name":"Journal of Thermal Science and Technology","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66343313","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-01-01DOI: 10.1299/JTST.2021JTST0029
Y. Okumura
Although rapid pyrolysis affords higher volatile yield than slow pyrolysis, the change in the yield of components at different heating rates have not been reported in detail. Moreover, few studies have assessed the changes in the tar component yield with respect to biomass type. Therefore, from a practical point of view, this study quantitatively determined the effects of the heating rate and biomass type on the tar component yield through gas chromatography–mass spectrometry. In addition, the mechanism of tar formation was investigated. The main results of the study are as follows: (1) Changes in the yield and component of biomass tar due to increase in the heating rate by a factor of 100 were quantitatively determined. An increased heating rate resulted in a higher yield of aromatic compounds and induced the formation of benzene, toluene, and other compounds. At slow heating rates, the yield of odorous components such as vanillin, furfural, acids, and aldehydes increased. At the middle heating rate (1.0 K/s), a significant increase in the amount of phenols containing OH and O groups was observed. (2) For woody biomass, acetic acid, cellulose-derived glucose, catechol, phenols, and furfural were identified as the major tar components. (3) The tar components volatilized from the wood trunk, bark, and grass are affected by the primary content of the biomass constituents.
{"title":"Quantitative analysis of primary tar yields volatilized from biomass (Effect of heating rate and biomass type on tar components)","authors":"Y. Okumura","doi":"10.1299/JTST.2021JTST0029","DOIUrl":"https://doi.org/10.1299/JTST.2021JTST0029","url":null,"abstract":"Although rapid pyrolysis affords higher volatile yield than slow pyrolysis, the change in the yield of components at different heating rates have not been reported in detail. Moreover, few studies have assessed the changes in the tar component yield with respect to biomass type. Therefore, from a practical point of view, this study quantitatively determined the effects of the heating rate and biomass type on the tar component yield through gas chromatography–mass spectrometry. In addition, the mechanism of tar formation was investigated. The main results of the study are as follows: (1) Changes in the yield and component of biomass tar due to increase in the heating rate by a factor of 100 were quantitatively determined. An increased heating rate resulted in a higher yield of aromatic compounds and induced the formation of benzene, toluene, and other compounds. At slow heating rates, the yield of odorous components such as vanillin, furfural, acids, and aldehydes increased. At the middle heating rate (1.0 K/s), a significant increase in the amount of phenols containing OH and O groups was observed. (2) For woody biomass, acetic acid, cellulose-derived glucose, catechol, phenols, and furfural were identified as the major tar components. (3) The tar components volatilized from the wood trunk, bark, and grass are affected by the primary content of the biomass constituents.","PeriodicalId":17405,"journal":{"name":"Journal of Thermal Science and Technology","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66343712","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-01-01DOI: 10.1299/jtst.2021jtst0030
Seiichi Takeuchi, S. Asao, M. Yamakawa
Theoretical examinations based on absorption line databases were carried out to investigate the influence of turbulence-radiation interaction on the radiative heat transfer arriving at the wall of large-scale industrial furnaces including hydrocarbon flame, where the re-absorption of radiative energy by combustion gas on its path toward objects to be heated cannot be neglected. In this study, we combined an improved version of our previous method for reducing the calculation load required for tracing turbulent fluctuation in temperature in great detail and an efficient method proposed in our previous papers to reduce the enormous calculation load contingent on detailed non-gray analysis. When we combined these methods with a governing equation solver for obtaining the spatial distribution of time-averaged values of temperature, concentration, velocity, and so on, we could evaluate the heat transfer including radiation in large-scale industrial furnaces enveloping turbulent hydrocarbon flame with sufficient accuracy equivalent to Line-by-Line analysis and with a feasible calculation load. Our application of this calculation method to large-scale furnaces enveloping hydrocarbon flame revealed that neglecting the turbulence-radiation interaction in numerical simulation gave rise to an obvious change in the heat flux distribution on the side wall and in the spatial distribution of the time-averaged temperature. In addition, change in the total amount of radiative energy arriving at the side wall caused by neglecting the turbulence-radiation interaction was fairly small compared with the change observed in our previous report on a model optical path imaging the typical course of radiative energy in large-scale industrial furnaces fueled by propane.
{"title":"Influence of turbulence-radiation interaction on radiative heat transfer to furnace wall and temperature distribution in large-scale industrial furnaces enveloping hydrocarbon flame","authors":"Seiichi Takeuchi, S. Asao, M. Yamakawa","doi":"10.1299/jtst.2021jtst0030","DOIUrl":"https://doi.org/10.1299/jtst.2021jtst0030","url":null,"abstract":"Theoretical examinations based on absorption line databases were carried out to investigate the influence of turbulence-radiation interaction on the radiative heat transfer arriving at the wall of large-scale industrial furnaces including hydrocarbon flame, where the re-absorption of radiative energy by combustion gas on its path toward objects to be heated cannot be neglected. In this study, we combined an improved version of our previous method for reducing the calculation load required for tracing turbulent fluctuation in temperature in great detail and an efficient method proposed in our previous papers to reduce the enormous calculation load contingent on detailed non-gray analysis. When we combined these methods with a governing equation solver for obtaining the spatial distribution of time-averaged values of temperature, concentration, velocity, and so on, we could evaluate the heat transfer including radiation in large-scale industrial furnaces enveloping turbulent hydrocarbon flame with sufficient accuracy equivalent to Line-by-Line analysis and with a feasible calculation load. Our application of this calculation method to large-scale furnaces enveloping hydrocarbon flame revealed that neglecting the turbulence-radiation interaction in numerical simulation gave rise to an obvious change in the heat flux distribution on the side wall and in the spatial distribution of the time-averaged temperature. In addition, change in the total amount of radiative energy arriving at the side wall caused by neglecting the turbulence-radiation interaction was fairly small compared with the change observed in our previous report on a model optical path imaging the typical course of radiative energy in large-scale industrial furnaces fueled by propane.","PeriodicalId":17405,"journal":{"name":"Journal of Thermal Science and Technology","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66343816","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-01-01DOI: 10.1299/jtst.2021jtst0034
Ryo Yoshiie, A. Yamada, Yoko Nunome, Yasuaki Ueki, I. Naruse
When the biomass gasifier is connected with a gas engine system directly, tar should be removed from the syngas to prevent the engine from breaking down. A downdraft packed bed gasifier has the advantage for low tar emission because the syngas passes through the char gasification zone downstream of the reactor, where tar compounds can be trapped and decomposed. Then, objective of this study is to confirm the tar decomposition behaviors inside the downdraft packed bed reactor. Woody biomass gasification experiments were carried out, using an auto-thermal downdraft packed bed gasifier. The reactor’s height and inner diameter were 1000mm and 100mm, respectively. Black pine pallets were continuously fed into the reactor from the top. The gasifying agent was air, which was introduced into the reactor at the air-fuel equivalent ratio of 0.49. The packed bed height was kept to be constant at 600mm. The reactor has eleven thermo-couples and eleven sampling ports at the wall along the flow direction. They were used for measurements of temperature profiles and gas compositions in the reactor. Micro-GC was used for the measurement of N 2 , O 2 , CO, CO 2 and H 2 , and FIDGC was used for other hydrocarbons. In some ports among them, tar in syngas was also sampled via dichloromethane scrubbing in ice-bath, and analyzed for molecular weight distributions of tar compounds by TOF-MS. As a result, tar and larger hydrocarbons were confirmed to be generated in the upstream, and then decomposed downstream inside the downdraft reactor.
{"title":"Tar generation and decomposition in downdraft packed bed reactor for woody biomass gasification","authors":"Ryo Yoshiie, A. Yamada, Yoko Nunome, Yasuaki Ueki, I. Naruse","doi":"10.1299/jtst.2021jtst0034","DOIUrl":"https://doi.org/10.1299/jtst.2021jtst0034","url":null,"abstract":"When the biomass gasifier is connected with a gas engine system directly, tar should be removed from the syngas to prevent the engine from breaking down. A downdraft packed bed gasifier has the advantage for low tar emission because the syngas passes through the char gasification zone downstream of the reactor, where tar compounds can be trapped and decomposed. Then, objective of this study is to confirm the tar decomposition behaviors inside the downdraft packed bed reactor. Woody biomass gasification experiments were carried out, using an auto-thermal downdraft packed bed gasifier. The reactor’s height and inner diameter were 1000mm and 100mm, respectively. Black pine pallets were continuously fed into the reactor from the top. The gasifying agent was air, which was introduced into the reactor at the air-fuel equivalent ratio of 0.49. The packed bed height was kept to be constant at 600mm. The reactor has eleven thermo-couples and eleven sampling ports at the wall along the flow direction. They were used for measurements of temperature profiles and gas compositions in the reactor. Micro-GC was used for the measurement of N 2 , O 2 , CO, CO 2 and H 2 , and FIDGC was used for other hydrocarbons. In some ports among them, tar in syngas was also sampled via dichloromethane scrubbing in ice-bath, and analyzed for molecular weight distributions of tar compounds by TOF-MS. As a result, tar and larger hydrocarbons were confirmed to be generated in the upstream, and then decomposed downstream inside the downdraft reactor.","PeriodicalId":17405,"journal":{"name":"Journal of Thermal Science and Technology","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66344458","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jtst.2020jtst0018
Kota Kawasaki, Mitsuho Nakakura, K. Matsubara
Conjugate radiation-convection-conduction simulation was conducted for a solar volumetric receiver of silicon carbide honeycomb for high temperature heat absorption at 1,000°C and higher. Simulation was made for three cases of channel cell size: 0.6mm; 1.5mm; 2.9mm. At two levels of incident heat flux 1,400 kW m and 4,200 kW m, air mass flux was changed variously for optimization of working conditions. When the cell size is reduced from d = 2.9 mm to 0.6 mm, the receiver efficiency together with the air temperature at the receiver exit increase at each level of incident heat flux. At 1,400 kW m, the receiver efficiency exceeds 0.8 when the air temperature is as high as 1000°C in the case of the smallest cell size: d = 0.6 mm. At 4,200 kW m , the efficiency surpasses 0.80 when the air temperature is almost 1500°C in the case of d = 0.6 mm. The heat losses from the receiver was analyzed through budget of energy balance equation. It was found that the thermal radiation was attenuated by reduction of channel cell size which resulted in enhancement of the receiver efficiency. The mean temperature at the top edge of the receiver decreased with the reduction of channel size in consistency with the attenuation of thermal radiation. The numerical result demonstrated that the reducing cell size is essential to absorb concentrated solar light at very high temperatures beyond 1000°C and higher.
{"title":"Conjugate simulation of solar honeycomb receiver for high temperature heat absorption at constant incident heat flux","authors":"Kota Kawasaki, Mitsuho Nakakura, K. Matsubara","doi":"10.1299/jtst.2020jtst0018","DOIUrl":"https://doi.org/10.1299/jtst.2020jtst0018","url":null,"abstract":"Conjugate radiation-convection-conduction simulation was conducted for a solar volumetric receiver of silicon carbide honeycomb for high temperature heat absorption at 1,000°C and higher. Simulation was made for three cases of channel cell size: 0.6mm; 1.5mm; 2.9mm. At two levels of incident heat flux 1,400 kW m and 4,200 kW m, air mass flux was changed variously for optimization of working conditions. When the cell size is reduced from d = 2.9 mm to 0.6 mm, the receiver efficiency together with the air temperature at the receiver exit increase at each level of incident heat flux. At 1,400 kW m, the receiver efficiency exceeds 0.8 when the air temperature is as high as 1000°C in the case of the smallest cell size: d = 0.6 mm. At 4,200 kW m , the efficiency surpasses 0.80 when the air temperature is almost 1500°C in the case of d = 0.6 mm. The heat losses from the receiver was analyzed through budget of energy balance equation. It was found that the thermal radiation was attenuated by reduction of channel cell size which resulted in enhancement of the receiver efficiency. The mean temperature at the top edge of the receiver decreased with the reduction of channel size in consistency with the attenuation of thermal radiation. The numerical result demonstrated that the reducing cell size is essential to absorb concentrated solar light at very high temperatures beyond 1000°C and higher.","PeriodicalId":17405,"journal":{"name":"Journal of Thermal Science and Technology","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66340947","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jtst.2020jtst0002
Liezheng Tang, J. Ruan, Guannan Li, Xuefeng Yin
The surface temperature measurement is susceptible to the surrounding air for the cable or the insulated busbar laid in free air. Therefore, an approach for improving their surface temperature measurements by covering the temperature sensor with a heat insulated layer is put forward. Firstly, the surface temperatures of the cable and the insulated busbar attached by a platinum resistance thermometer with and without a heat insulated layer under rated current are obtained using the thermal analyses in Comsol. Subsequently, the temperature rise test of the insulated busbar was carried out for the indirect verification of the previous analyses. The measured surface temperatures were used to calculate the conductor temperature based on the transient thermal network. By comparison with the measured conductor temperature, it is found that the deviation of the surface temperature measurement without the heat insulated layer is about 4~7 K while that with the heat insulated layer is only ±1 K. Further, the generalization of the presented method to the distributed temperature sensing system is analyzed. This study demonstrates that the accuracy of the surface temperature measurement of the cable and the insulated busbar can be effectively improved by wrapping a suitable heat insulated layer around the sensor.
{"title":"Improving surface temperature measurement of the power cable and insulated busbar using the heat insulated layer","authors":"Liezheng Tang, J. Ruan, Guannan Li, Xuefeng Yin","doi":"10.1299/jtst.2020jtst0002","DOIUrl":"https://doi.org/10.1299/jtst.2020jtst0002","url":null,"abstract":"The surface temperature measurement is susceptible to the surrounding air for the cable or the insulated busbar laid in free air. Therefore, an approach for improving their surface temperature measurements by covering the temperature sensor with a heat insulated layer is put forward. Firstly, the surface temperatures of the cable and the insulated busbar attached by a platinum resistance thermometer with and without a heat insulated layer under rated current are obtained using the thermal analyses in Comsol. Subsequently, the temperature rise test of the insulated busbar was carried out for the indirect verification of the previous analyses. The measured surface temperatures were used to calculate the conductor temperature based on the transient thermal network. By comparison with the measured conductor temperature, it is found that the deviation of the surface temperature measurement without the heat insulated layer is about 4~7 K while that with the heat insulated layer is only ±1 K. Further, the generalization of the presented method to the distributed temperature sensing system is analyzed. This study demonstrates that the accuracy of the surface temperature measurement of the cable and the insulated busbar can be effectively improved by wrapping a suitable heat insulated layer around the sensor.","PeriodicalId":17405,"journal":{"name":"Journal of Thermal Science and Technology","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66340435","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jtst.2020jtst0008
P. Promthaisong, V. Chuwattanakul, S. Eiamsa-ard
This paper presents 3D numerical investigation of the turbulent flow and heat transfer characteristics of a twisted square duct installed with multi-twisted tapes. Air was used as the working fluid with flow rates in terms of Reynolds numbers ranging from 3000 to 20,000. The effects of (1) multi-twisted tape width ratios (w/H) of 0.2 to 1.0 and (2) the number of channels (N = 2 and 4) on heat transfer and flow mechanisms were studied at constant twist ratio of y/D = 3.5. The numerical results showed that twisted square duct combined with twisted tape caused swirl flows which effectively promoted fluid mixing and provided heat transfer over those of both a straight smooth square duct and twisted square duct. Increasing w/H led to increases in both heat transfer and the friction factor. At a given multi-twisted tape width ratio (w/H), the heat transfer and friction factor with N = 4 were higher than those with N = 2, while thermal enhancement factor showed the opposite trend. A maximum thermal enhancement factor of 1.93 was obtained at tape width ratio of w/H = 1.0, channel number of N = 2 and Re = 3000.
{"title":"Thermal and swirl flow topologies in a twisted square duct with a multi-twisted tape installed","authors":"P. Promthaisong, V. Chuwattanakul, S. Eiamsa-ard","doi":"10.1299/jtst.2020jtst0008","DOIUrl":"https://doi.org/10.1299/jtst.2020jtst0008","url":null,"abstract":"This paper presents 3D numerical investigation of the turbulent flow and heat transfer characteristics of a twisted square duct installed with multi-twisted tapes. Air was used as the working fluid with flow rates in terms of Reynolds numbers ranging from 3000 to 20,000. The effects of (1) multi-twisted tape width ratios (w/H) of 0.2 to 1.0 and (2) the number of channels (N = 2 and 4) on heat transfer and flow mechanisms were studied at constant twist ratio of y/D = 3.5. The numerical results showed that twisted square duct combined with twisted tape caused swirl flows which effectively promoted fluid mixing and provided heat transfer over those of both a straight smooth square duct and twisted square duct. Increasing w/H led to increases in both heat transfer and the friction factor. At a given multi-twisted tape width ratio (w/H), the heat transfer and friction factor with N = 4 were higher than those with N = 2, while thermal enhancement factor showed the opposite trend. A maximum thermal enhancement factor of 1.93 was obtained at tape width ratio of w/H = 1.0, channel number of N = 2 and Re = 3000.","PeriodicalId":17405,"journal":{"name":"Journal of Thermal Science and Technology","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1299/jtst.2020jtst0008","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66340474","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jtst.2020jtst0025
N. Giannetti, Kiyoshi Saito, Hiroaki Yoshimura
The complexity of the actual operation of thermal engineering systems comprises multiphase interfacial phenomena evolving out of equilibrium. Therefore, their generalised formulation can contribute towards better understanding and control of these phenomena, eventually pushing the existing related technologies beyond the state-of-the-art. In this respect, variational principles are significant for a more comprehensive physical representation and for closing the problem, while obtaining relatively simpler mathematical formulations. In this study, a general variational formulation of dissipative two-phase flows based on the minimum entropy production is developed. In particular, this study provides a general expression of the entropy generation rate, which introduces interfacial contributions due to surface tension between different phases, and is used to estimate two-phase flow fraction based on Prigogine's theorem of minimum entropy generation. Subsequently, this formulation is investigated in terms of different assumptions and pressure drop models, and employed to clarify the implementation of Prigogine's theorem to obtain the widely-accepted Zivi's expression of void fraction and the effect of different assumptions on the deviation from his expression. A new expression is finally obtained to cover laminar flow conditions, which are implicitly excluded from the applicability of Zivi’s expression.
{"title":"Formulation of steady-state void fraction through the principle of minimum entropy production","authors":"N. Giannetti, Kiyoshi Saito, Hiroaki Yoshimura","doi":"10.1299/jtst.2020jtst0025","DOIUrl":"https://doi.org/10.1299/jtst.2020jtst0025","url":null,"abstract":"The complexity of the actual operation of thermal engineering systems comprises multiphase interfacial phenomena evolving out of equilibrium. Therefore, their generalised formulation can contribute towards better understanding and control of these phenomena, eventually pushing the existing related technologies beyond the state-of-the-art. In this respect, variational principles are significant for a more comprehensive physical representation and for closing the problem, while obtaining relatively simpler mathematical formulations. In this study, a general variational formulation of dissipative two-phase flows based on the minimum entropy production is developed. In particular, this study provides a general expression of the entropy generation rate, which introduces interfacial contributions due to surface tension between different phases, and is used to estimate two-phase flow fraction based on Prigogine's theorem of minimum entropy generation. Subsequently, this formulation is investigated in terms of different assumptions and pressure drop models, and employed to clarify the implementation of Prigogine's theorem to obtain the widely-accepted Zivi's expression of void fraction and the effect of different assumptions on the deviation from his expression. A new expression is finally obtained to cover laminar flow conditions, which are implicitly excluded from the applicability of Zivi’s expression.","PeriodicalId":17405,"journal":{"name":"Journal of Thermal Science and Technology","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66341092","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jtst.2020jtst0004
Nae-Hyun Kim
Corrugated louver fin, which has louvers on wavy surface, may be a promising mean to augment the air-side heat transfer of fin-and-tube heat exchangers. However, only limited prior studies are available. In this study, two kinds of corrugated louver fin-and-tube heat exchangers – one having one corrugation per row and the other having two corrugations per row – were tested, and the results were compared with those of the standard louver fin and the plain fin samples. The highest j and f factor were obtained for the standard louver fin sample, followed by the single corrugated louver fin, the double corrugated inclined louver fin and then the plain fin sample. The high j and f factor of the standard louver fin sample may be due to the large louver fraction on the fin surface. Furthermore, larger fin surface area of the double corrugated louver fin compared with that of the single corrugated fin may be the reason for the smaller j and f factor. All the enhanced fin samples yielded larger heat transfer capacity than the plain fin sample at the same pumping power. Furthermore, the largest heat transfer capacity per pumping power was obtained for the standard louver fin sample. The single corrugated louver fin sample yielded higher heat transfer capacity per pumping power than the double corrugated sample.
{"title":"An experimental investigation on the airside performance of fin-and-tube heat exchangers having corrugated louver fins - Part I; dry surface","authors":"Nae-Hyun Kim","doi":"10.1299/jtst.2020jtst0004","DOIUrl":"https://doi.org/10.1299/jtst.2020jtst0004","url":null,"abstract":"Corrugated louver fin, which has louvers on wavy surface, may be a promising mean to augment the air-side heat transfer of fin-and-tube heat exchangers. However, only limited prior studies are available. In this study, two kinds of corrugated louver fin-and-tube heat exchangers – one having one corrugation per row and the other having two corrugations per row – were tested, and the results were compared with those of the standard louver fin and the plain fin samples. The highest j and f factor were obtained for the standard louver fin sample, followed by the single corrugated louver fin, the double corrugated inclined louver fin and then the plain fin sample. The high j and f factor of the standard louver fin sample may be due to the large louver fraction on the fin surface. Furthermore, larger fin surface area of the double corrugated louver fin compared with that of the single corrugated fin may be the reason for the smaller j and f factor. All the enhanced fin samples yielded larger heat transfer capacity than the plain fin sample at the same pumping power. Furthermore, the largest heat transfer capacity per pumping power was obtained for the standard louver fin sample. The single corrugated louver fin sample yielded higher heat transfer capacity per pumping power than the double corrugated sample.","PeriodicalId":17405,"journal":{"name":"Journal of Thermal Science and Technology","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1299/jtst.2020jtst0004","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66340123","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jtst.2020jtst0020
Xiaofeng Yang, Yanxia Du, Shenshen Liu, G. Xiao, Yewei Gui, Wei Liu
Planetary spacecrafts, such as return capsules and Mars entry vehicles, enter the atmosphere at extremely high speeds, and therefore high-enthalpy dissociated aerodynamic environment occurs around the heat shield (Gnoffo, 1999). Such severe environment brings complex interface heat transfer processes between the gas and solid domains. Moreover, deep space exploration, such as hypersonic Mars entry mission, causes an additional carbon-oxygen (C-O) reacting environment, which further brings some new challenges to the evaluation of interface heat transfer characteristics (Reynier, 2014). Accurate and reliable prediction of interface heat transfer is the premise and basis for effectively ensuring the safety of the spacecraft thermal protection system (TPS), reducing design redundancy and increasing the effective payload (Duffa, 2013). With the continuous and in-depth development of high-performance computing, using the numerical technology to characterize fluid mechanics, structural heat transfer, interface chemistry and their interaction has become an effective way to solve this problem (Milos and Rasky, 1994). Early researches on interface heat transfer characteristics were done by solving the compressible gas dynamics equations with simple mathematical closure on the solid surface (Wright, et al., 2010). No interface chemistry was the * State Key Laboratory of Aerodynamics, China Aerodynamics Research and Development Center 6 Second Ringroad South Section, Mianyang, Sichuan, 621000, P. R. China E-mail: xiaofeng.yang@cardc.cn ** College of Aeronautics and Astronautics, National University of Defense Technology 109 Deya Road, Changsha, Hunan, 410073, P. R. China
行星航天器，如返回舱和火星进入飞行器，以极高的速度进入大气层，因此在隔热罩周围会发生高焓解离空气动力环境(Gnoffo, 1999)。这种恶劣的环境带来了复杂的气固界面换热过程。此外，深空探测，如高超声速火星进入任务，造成了额外的碳氧(C-O)反应环境，这进一步给界面传热特性的评估带来了一些新的挑战(Reynier, 2014)。准确可靠的界面传热预测是有效保证航天器热防护系统(TPS)安全、减少设计冗余、增加有效载荷的前提和基础(Duffa, 2013)。随着高性能计算的不断深入发展，利用数值技术表征流体力学、结构传热、界面化学及其相互作用已成为解决这一问题的有效途径(Milos and Rasky, 1994)。早期对界面传热特性的研究是通过在固体表面上求解具有简单数学闭包的可压缩气体动力学方程来完成的(Wright, et al.， 2010)。无界面化学是*空气动力学国家重点实验室，中国空气动力研究与发展中心，四川绵阳二环路南段6号，621000，中国。E-mail: xiaofeng.yang@cardc.cn **国防科技大学航空航天学院，湖南省长沙市德雅路109号，410073
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