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Experimental investigation on exponential heating of laminar flow
136 Abstracts for The 3rd World Conference heavy water moderator enters the calandria through 12 inlet diffusers and leaves the calandria through four outlet nozzles. Flow and temperature fields in the calandria are greatly influenced by the configuration of the inlet diffusers and their location, among other parameters. In view of the importance, experimental studies on different types of diffusers were carried out to determine velocity distribution at diffuser outlet, pressure loss coefficient, etc. and to visualize the flow patterns. Experiments were conducted in a high flow test facility on full-scale diffuser models submerged in a pool of water. Dye and tracer paint were used for flow visualization. Large separated flow zones were observed at diffuser outlet. Introduction of partition plates reduced the peak velocity. This paper deals with the details of the tests conducted and the results obtained.
Effect of Fin Length on Solidification Rate in a Latent Heat Energy Storage System Z. Ilken M. Toksoy
Dokuz Eylul University Mechanical Engineering Department Izmir, Turkey The main problem encountered in latent heat energy storage (LHES) systems is to find and apply the methods increasing phase change rate. The use of finned-heat exchange surfaces seems to be an attractive way, among several methods, of handling that problem. The aim of this work was to ascertain the usefulness of finned surfaces in a LHES unit including a definite amount of phase change material and to determine the effect of fin length on phase change rate. For that purpose, solidification of liquid water whose initial temperature was at the fusion value was analyzed for three different types of heat exchange surfaces, namely, without fin, with a short fin, and with a long fin. Experiments were carried out for three different average base temperatures of the finned surface. During the experiments, slides were taken from the frontal face of the solidification unit at predetermined time intervals. The images were projected on a flat surface in full scale, and the areas defined by solidification contours were evaluated with a digital planimeter. Solidification quantity versus time curves obtained for three different types of heat exchange surfaces are compared. It is concluded that long fins are more effective in enhancing solidification rate.
Measurement Technique for Determination of the Thermophysical Properties of Thermochemicai Storage Materials Valko Valkov
Institut National de la Recherche Scientifique Universit~ du Quebec Varennes, QuJbec, Canada In this paper, a simple and reasonably accurate technique for simultaneously measuring the effective thermal diffusivity and conductivity of thermochemical storage materials is presented. The sample of the material (a salt-binder mixture) was surrounded with a solid whose thermophysical properties are known. An electrical heater was placed
symmetrically in the sample. By recording the temperature at the boundary between the sample and the solid and the temperature inside the solid as a function of time from the start of the instantly applied heat flux, it is possible to get information on the thermophysical properties of the material surrounding the heat source. The short duration of the heat pulse (max. 2 s) is necessary to avoid the thermal influence on the studied sample. Experimental results of the effective thermal conductivity and diffusivity of different mixtures (ammoniated salt and expanded graphite 12% and 25%) are presented.
Forced Convection A Study on Heat/Mass Transfer from Rectangular Cylinders Using Naphthalene Sublimation Technique C. H. Chung M. K. Chung
Thermal Hydraufics Department Korea Atomic Energy Research Institute Taejeon, Korea S. Y. Yoo
Department of Mechanical Design Engineering Chungnam National University Taejeon, Korea A naphthalene sublimation technique is used to investigate heat and mass transfer from rectangular cylinders with different ratios of the width to height. Local and average mass transfer rates were obtained by using an automated depth measurement system. The experimental results are compared with the previous heat-transfer data by employing the heat/mass transfer analogy. Mass-transfer coefficients for the faces of the cylinder are quite different from each other and changed drastically as the width-to-height ratio was varied. Such variation is closely connected with the complex flow phenomena around rectangular cylinders such as separation, reattachment, and vortex shedding. The comparison of mass- and heat-transfer data shows good agreement in average transfer rates, and the trends of the data are similar in local transfer rates. Thus, the naphthalene sublimation technique is appropriate to explore the heat-transfer process in complex flow situations which are not applicable to the conventional heat transfer measurement techniques.
Experimental Investigation on Exponential Heating of Laminar Flow S. Piva G. Pagliarini
Istituto di Fisica Tecnica Facolta' di Ingegneria Universita' di Bologna Bologna, Italy The forced convection of a fluid flowing in a circular duct, in the region upstream of a heating section, is analyzed by means of an experimental apparatus designed, built, and used specifically for this. A wide range of laminar Reynolds numbers (150 _< Re < 1000) and heating rates (1 < P(W) _< 20) were covered in the experimental study. The experiments, carried out with a fully developed velocity profile,
Experimental Heat Transfer, Fluid Mechanics, and Thermodynamics 137 show the wall temperature growing exponentially in the direction of flow. The experimental data were reduced in order to evaluate the exponent of the wall temperature and the Nusselt number distribution along the duct. Furthermore, the strong dependence shown by the characteristic thermal parameters suggests that this experimental arrangement may be utilized for the estimation of unknown parameters like the thermal conductivity of the wall and the mass flow rate.
Heat Transfer Augmentation from a Bundle of Heated Cylinders Situated in Two Opposite Ribbed Plates of a Duct K. Torikoshi M. Kawazoe
Mechanical Engineering Laboratory Daikin Industries, Ltd. Sakai, Osaka, Japan Convective/radiative heat-transfer and pressure-drop characteristics from a bundle of five heated cylinders situated in a streamwise center portion of a duet were investigated experimentally. The system considered in the present study was an array of rectangular ribs welded on both plates of a duct at regular intervals. The rib was made up of stainless-steel block, and its height was chosen to be 1 or 2 mm. In each case, the rib pitch was decided so as to set the ratio of the pitch to a height 5 or 10. Both the heated cylinders and ribbed plates were painted with heat resisting black paint having an emissivity of 0.94. Experiments were performed using air ranging in Reynolds number from 1000 to 10000. The present results indicate that the installation of the ribbed plates into a duct brought about an increase in overall heat transfer in the cylinder bank.
Heat-Transfer Augmentation by Longitudinal Vortices Rows Jurandir Itizo Yanagihara
Department of Mechanical Engineering Escola Polit£cnica da Universidade de SKo Paulo SKo Paulo, Brazil Kahoru Torii
Department of Mechanical Engineering Yokohama National University Yokohama, Japan Longitudinal vortices can potentially enhance heat transfer with small pressure loss penalty. The objective of this experimental work was to investigate the influence of arrays of longitudinal vortices generated by half-delta wings on the local and average heat transfer and fluid flow of an otherwise laminar boundary layer. Heat-transfer experiments and hot-wire velocity measurements were conducted to clarify the mechanisms of heat-transfer augmentation. Experiments with arrays of co-rotating and counter-rotating longitudinal vortices indicated that both laminar and turbulent effects play a key role in enhancing heat transfer. Counter-rotating longitudinal vortices tend to merge in the common flow up region between generators; in this case, smaller distances between generators with the common flow down and large angles of attack were found to enhance the heat transfer. Arrays of co-
rotating longitudinal vortices show lower vortex merging; arrays of vortices produced by half-delta wings with higher angles of attack significantly distort the flow field and produce a noticeable growth of near-wall turbulence intensity. An overall performance analysis indicated that arrays of counter-rotating longitudinal vortices are more suitable for heat-transfer enhancement than arrays of co-rotating vortices.
Experimental Study of Vortex Heat-Transfer Enhancement Mechanisms V. N. Afanasiev Yh. P. Chudnovsky A. I. Leontiev
Department of Power Machinery Building Moscow Technical University Moscow, Russia In this paper, comparative analyses of various heat-transfer enhancement method effects and the experimental study results of vortex heat-transfer enhancement physical mechanisms are presented. Experimental investigations were made of friction and heat transfer on surfaces shaped by systems of spherical cavities streamlined by a turbulent flow. Flow structure in single, two-, and three-dimensional cavities is also investigated. The experiments were run on a special aerodynamic test bed using a traditional procedure for diagnosing a turbulent boundary layer. The results obtained allow us to conclude that such a shaping of a heating surface has no appreciable effect on the hydrodynamics of a flow with considerable (up to 30-40%) heat transfer enhancement. Experimental data on heat transfer are generalized, and a relationship suitable for engineering estimation of heat transfer on surfaces shaped by a regular relief of spherical cavities and valid for the investigated geometric parameters of shaping and flow regimes were obtained. Results were also obtained that allow us to formulate the physical flow model in the single cavity dependent on the geometric parameters and flow regime. The vortex technology problems, practical application of heat-transfer enhancement by vortex method, and progress prospectives are elucidated.
Heat and Momentum Transfer From Circumferential Finned Rods in Longitudinal Flow R.-A. Henne E. Obermeier
Fachbereich Maschinentechnik Uniuersitiit-Gesamthochschule-Siegen Institut fiir Fluid- u. Thermodynamik Siegen, Germany Compactness and efficiency of heat exchangers can be improved by enhancement of heat transfer. Extended surfaces and surfaces with so-called turbulence promoters constitute the most effective design features for promoting heat transfer between a surface and a stream of fluid. In this study, heat-transfer and pressure-drop experiments were performed for tubes with circumferential fins. In most cases, these finned tubes are used for transverse flow. This investigation is concerned with finned tubes in longitudinal annular flow. The most important parameter
Effect of Fin Length on Solidification Rate in a Latent Heat Energy Storage System Z. Ilken M. Toksoy
Dokuz Eylul University Mechanical Engineering Department Izmir, Turkey The main problem encountered in latent heat energy storage (LHES) systems is to find and apply the methods increasing phase change rate. The use of finned-heat exchange surfaces seems to be an attractive way, among several methods, of handling that problem. The aim of this work was to ascertain the usefulness of finned surfaces in a LHES unit including a definite amount of phase change material and to determine the effect of fin length on phase change rate. For that purpose, solidification of liquid water whose initial temperature was at the fusion value was analyzed for three different types of heat exchange surfaces, namely, without fin, with a short fin, and with a long fin. Experiments were carried out for three different average base temperatures of the finned surface. During the experiments, slides were taken from the frontal face of the solidification unit at predetermined time intervals. The images were projected on a flat surface in full scale, and the areas defined by solidification contours were evaluated with a digital planimeter. Solidification quantity versus time curves obtained for three different types of heat exchange surfaces are compared. It is concluded that long fins are more effective in enhancing solidification rate.
Measurement Technique for Determination of the Thermophysical Properties of Thermochemicai Storage Materials Valko Valkov
Institut National de la Recherche Scientifique Universit~ du Quebec Varennes, QuJbec, Canada In this paper, a simple and reasonably accurate technique for simultaneously measuring the effective thermal diffusivity and conductivity of thermochemical storage materials is presented. The sample of the material (a salt-binder mixture) was surrounded with a solid whose thermophysical properties are known. An electrical heater was placed
symmetrically in the sample. By recording the temperature at the boundary between the sample and the solid and the temperature inside the solid as a function of time from the start of the instantly applied heat flux, it is possible to get information on the thermophysical properties of the material surrounding the heat source. The short duration of the heat pulse (max. 2 s) is necessary to avoid the thermal influence on the studied sample. Experimental results of the effective thermal conductivity and diffusivity of different mixtures (ammoniated salt and expanded graphite 12% and 25%) are presented.
Forced Convection A Study on Heat/Mass Transfer from Rectangular Cylinders Using Naphthalene Sublimation Technique C. H. Chung M. K. Chung
Thermal Hydraufics Department Korea Atomic Energy Research Institute Taejeon, Korea S. Y. Yoo
Department of Mechanical Design Engineering Chungnam National University Taejeon, Korea A naphthalene sublimation technique is used to investigate heat and mass transfer from rectangular cylinders with different ratios of the width to height. Local and average mass transfer rates were obtained by using an automated depth measurement system. The experimental results are compared with the previous heat-transfer data by employing the heat/mass transfer analogy. Mass-transfer coefficients for the faces of the cylinder are quite different from each other and changed drastically as the width-to-height ratio was varied. Such variation is closely connected with the complex flow phenomena around rectangular cylinders such as separation, reattachment, and vortex shedding. The comparison of mass- and heat-transfer data shows good agreement in average transfer rates, and the trends of the data are similar in local transfer rates. Thus, the naphthalene sublimation technique is appropriate to explore the heat-transfer process in complex flow situations which are not applicable to the conventional heat transfer measurement techniques.
Experimental Investigation on Exponential Heating of Laminar Flow S. Piva G. Pagliarini
Istituto di Fisica Tecnica Facolta' di Ingegneria Universita' di Bologna Bologna, Italy The forced convection of a fluid flowing in a circular duct, in the region upstream of a heating section, is analyzed by means of an experimental apparatus designed, built, and used specifically for this. A wide range of laminar Reynolds numbers (150 _< Re < 1000) and heating rates (1 < P(W) _< 20) were covered in the experimental study. The experiments, carried out with a fully developed velocity profile,
Experimental Heat Transfer, Fluid Mechanics, and Thermodynamics 137 show the wall temperature growing exponentially in the direction of flow. The experimental data were reduced in order to evaluate the exponent of the wall temperature and the Nusselt number distribution along the duct. Furthermore, the strong dependence shown by the characteristic thermal parameters suggests that this experimental arrangement may be utilized for the estimation of unknown parameters like the thermal conductivity of the wall and the mass flow rate.
Heat Transfer Augmentation from a Bundle of Heated Cylinders Situated in Two Opposite Ribbed Plates of a Duct K. Torikoshi M. Kawazoe
Mechanical Engineering Laboratory Daikin Industries, Ltd. Sakai, Osaka, Japan Convective/radiative heat-transfer and pressure-drop characteristics from a bundle of five heated cylinders situated in a streamwise center portion of a duet were investigated experimentally. The system considered in the present study was an array of rectangular ribs welded on both plates of a duct at regular intervals. The rib was made up of stainless-steel block, and its height was chosen to be 1 or 2 mm. In each case, the rib pitch was decided so as to set the ratio of the pitch to a height 5 or 10. Both the heated cylinders and ribbed plates were painted with heat resisting black paint having an emissivity of 0.94. Experiments were performed using air ranging in Reynolds number from 1000 to 10000. The present results indicate that the installation of the ribbed plates into a duct brought about an increase in overall heat transfer in the cylinder bank.
Heat-Transfer Augmentation by Longitudinal Vortices Rows Jurandir Itizo Yanagihara
Department of Mechanical Engineering Escola Polit£cnica da Universidade de SKo Paulo SKo Paulo, Brazil Kahoru Torii
Department of Mechanical Engineering Yokohama National University Yokohama, Japan Longitudinal vortices can potentially enhance heat transfer with small pressure loss penalty. The objective of this experimental work was to investigate the influence of arrays of longitudinal vortices generated by half-delta wings on the local and average heat transfer and fluid flow of an otherwise laminar boundary layer. Heat-transfer experiments and hot-wire velocity measurements were conducted to clarify the mechanisms of heat-transfer augmentation. Experiments with arrays of co-rotating and counter-rotating longitudinal vortices indicated that both laminar and turbulent effects play a key role in enhancing heat transfer. Counter-rotating longitudinal vortices tend to merge in the common flow up region between generators; in this case, smaller distances between generators with the common flow down and large angles of attack were found to enhance the heat transfer. Arrays of co-
rotating longitudinal vortices show lower vortex merging; arrays of vortices produced by half-delta wings with higher angles of attack significantly distort the flow field and produce a noticeable growth of near-wall turbulence intensity. An overall performance analysis indicated that arrays of counter-rotating longitudinal vortices are more suitable for heat-transfer enhancement than arrays of co-rotating vortices.
Experimental Study of Vortex Heat-Transfer Enhancement Mechanisms V. N. Afanasiev Yh. P. Chudnovsky A. I. Leontiev
Department of Power Machinery Building Moscow Technical University Moscow, Russia In this paper, comparative analyses of various heat-transfer enhancement method effects and the experimental study results of vortex heat-transfer enhancement physical mechanisms are presented. Experimental investigations were made of friction and heat transfer on surfaces shaped by systems of spherical cavities streamlined by a turbulent flow. Flow structure in single, two-, and three-dimensional cavities is also investigated. The experiments were run on a special aerodynamic test bed using a traditional procedure for diagnosing a turbulent boundary layer. The results obtained allow us to conclude that such a shaping of a heating surface has no appreciable effect on the hydrodynamics of a flow with considerable (up to 30-40%) heat transfer enhancement. Experimental data on heat transfer are generalized, and a relationship suitable for engineering estimation of heat transfer on surfaces shaped by a regular relief of spherical cavities and valid for the investigated geometric parameters of shaping and flow regimes were obtained. Results were also obtained that allow us to formulate the physical flow model in the single cavity dependent on the geometric parameters and flow regime. The vortex technology problems, practical application of heat-transfer enhancement by vortex method, and progress prospectives are elucidated.
Heat and Momentum Transfer From Circumferential Finned Rods in Longitudinal Flow R.-A. Henne E. Obermeier
Fachbereich Maschinentechnik Uniuersitiit-Gesamthochschule-Siegen Institut fiir Fluid- u. Thermodynamik Siegen, Germany Compactness and efficiency of heat exchangers can be improved by enhancement of heat transfer. Extended surfaces and surfaces with so-called turbulence promoters constitute the most effective design features for promoting heat transfer between a surface and a stream of fluid. In this study, heat-transfer and pressure-drop experiments were performed for tubes with circumferential fins. In most cases, these finned tubes are used for transverse flow. This investigation is concerned with finned tubes in longitudinal annular flow. The most important parameter