Showing papers in "Heat Transfer Engineering in 2003"
TL;DR: In this article, the authors provide a roadmap of development in the thermal and fabrication aspects of microchannels as applied in microelectronics and other high heat-flux cooling applications.
Abstract: This paper provides a roadmap of development in the thermal and fabrication aspects of microchannels as applied in microelectronics and other high heat-flux cooling applications. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 200 micrometers. The impetus for microchannel research was provided by the pioneering work of Tuckerman and Pease [1] at Stanford University in the early eighties. Since that time, this technology has received considerable attention in microelectronics and other major application areas, such as fuel cell systems and advanced heat sink designs. After reviewing the advancement in heat transfer technology from a historical perspective, the advantages of using microchannels in high heat flux cooling applications is discussed, and research done on various aspects of microchannel heat exchanger performance is reviewed. Single-phase performance for liquids is still expected to be describable by conventional equations; however, the gas flow may...
672 citations
TL;DR: The effect of surface roughness on pressure drop and heat transfer in circular tubes has been extensively studied in literature as discussed by the authors, and a detailed experimental study is undertaken to investigate the roughness effects in small diameter tubes.
Abstract: The effect of surface roughness on pressure drop and heat transfer in circular tubes has been extensively studied in literature. The pioneering work of Nikuradse [1] established the sand grain roughness as a major parameter in defining the friction factor during laminar and turbulent flows. Recent studies have indicated a transition to turbulent flows at Reynolds number values much below 2300 during single-phase flow in channels with small hydraulic diameters. In the present work, a detailed experimental study is undertaken to investigate the roughness effects in small diameter tubes. The roughness of the inside tube surface is changed by etching it with an acid solution. Two tubes of 1.032 mm and 0.62 mm inner diameter are treated with acid solutions to provide three different roughness values for each tube. The Reynolds number range for the tests is 500-2600 for 1.067 mm tube and 900-3000 for 0.62 mm tube.
253 citations
TL;DR: In this paper, a literature search is presented on plate heat exchangers and new correlations for evaporation heat transfer coefficient and friction factor are introduced, which are applicable to various system pressure conditions and plate chevron angles.
Abstract: Plate heat exchangers are used regularly in the heating, ventilating, air conditioning, and refrigeration industry. There is an urgent need for detailed and systematic research regarding heat transfer and the fluid flow characteristics of these types of exchangers. As an initiative in this respect, a literature search is presented on plate heat exchangers. New correlations for evaporation heat transfer coefficient and friction factor are introduced, which are applicable to various system pressure conditions and plate chevron angles. The correlations are based on actual field data collected during several years of installation and operation of chillers, and they are intended to serve as design tools and perhaps as a starting point for future research.
243 citations
TL;DR: In this paper, the authors examine important processes that must be considered when channel diameters decrease, including flow distribution issues in single, parallel, and split flows; flow instability in parallel passa...
Abstract: Since the 1950s, the research and industrial communities have developed a body of experimental data and set of analytical tools and correlations for two-phase flow and heat transfer in passages having a hydraulic diameter greater than about 6 mm. These tools include flow regime maps, pressure drop and heat transfer correlations, and critical heat flux limits, as well as strategies for robust thermal management of HVAC systems, electronics, and nuclear power plants. Designers of small systems with thermal management by phase change will need analogous tools to predict and optimize thermal behavior in the mesoscale and smaller sizes. Such systems include a wide range of devices for computation, measurement, and actuation in environments that range from office space to outer space as well as living systems. This paper examines important processes that must be considered when channel diameters decrease, including flow distribution issues in single, parallel, and split flows; flow instability in parallel passa...
188 citations
TL;DR: A two-phase flow pattern map was originally proposed by Kattan, Thome, and Favrat as part of a new flow pattern-based flow boiling model for predicting local heat transfer coefficients during evaporation in horizontal tubes.
Abstract: A two-phase flow pattern map was originally proposed by Kattan, Thome, and Favrat as part of a new flow pattern-based flow boiling model for predicting local heat transfer coefficients during evaporation in horizontal tubes in the fully stratified flow regime, the stratified-wavy regime, the intermittent flow regime, the annular flow regime, and for annular flow with partial dryout. Zurcher, Thome, and Favrat improved on this flow pattern map by empirically correcting two of the transition boundaries based on extensive new flow pattern observations for ammonia, but it still requires an iterative solution of numerous equations. Zurcher, Favrat, and Thome have since proposed an even more detailed method for predicting flow pattern transitions by taking into account the intertwined effects of void fraction on flow transition and flow pattern on void fraction, resulting in a map that is very effective but very complex to implement. In the newest version of this map, the goal of which was to simplify the metho...
112 citations
TL;DR: In this article, the transitions between these intertube flow modes were observed and measured for rows of plain tubes, Thermoexcel-C enhanced condensing tubes, Turbo-Bii enhanced boiling tubes, and 26 fpi (1024 fpm) low-finned tubes for water, glycol, and a 50/50 water-glycol mixture.
Abstract: Intertube flow modes for falling film condensers and evaporators with plain and enhanced horizontal tube bundles are thought to have a key influence on their tube row effects Three principal types of flow modes are typically observed—droplet mode, column mode, and sheet mode—together with intermediate modes—coexistence of droplet-column modes and of column-sheet modes All act as falling jets from the upper tube onto the tube directly below In a previous study by Roques, Dupont, and Thome [1], the transitions between these intertube flow modes were observed and measured for rows of plain tubes, Thermoexcel–C enhanced condensing tubes, Turbo-Bii enhanced boiling tubes, and 26 fpi (1024 fpm) low-finned tubes for water, glycol, and a 50/50 water-glycol mixture Significant differences were observed in the transition thresholds for these tube surface geometries In the present study, this investigation has been extended to 19 fpi and 40 fpi (748 and 1575 fpm) low-finned Wieland tubes A comparison between p
54 citations
TL;DR: In this paper, a re-examination of extended surface heat transfer basic treatment that appears in heat transfer textbooks is presented, and the emphasis on efficiency should be shifted to effectiveness instead.
Abstract: This article is devoted to the re-examination of extended surface heat transfer basic treatment that appears in heat transfer textbooks. It demonstrates that—unlike what is suggested in textbooks—the emphasis on efficiency should be shifted to effectiveness instead. Graphs display how the heat dissipation of a given fin is directly obtained without any reference to the efficiency, which is now the standard approach, and total effectiveness is proposed to replace the total efficiency. It is further shown that the effectiveness approach aids in verifying some of the simplifying assumptions and shows that extended surfaces must be thermally and geometrically thin in order to fulfill their function. The results of a brief excursion to the optimum fin problem will help students become involved in a preliminary fin design, a topic conspicuously absent from many textbooks. Three postulates extend certain results of the unidirectional analysis of the constant thickness fins to other shapes and two-dimensional pro...
52 citations
TL;DR: In this paper, the combined effects of the inlet angle and mass flow rate on flow maldistribution have been studied and the study is useful in the optimum design of plate-fin heat exchangers.
Abstract: The plate-fin heat exchanger is normally designed with the assumption that the fluid is uniformly divided among all the parallel passages. In practice, however, the design of the exchanger, the heat transfer process, the operation of the external system, etc., may create high flow maldistribution. The performance deterioration of plate-fin heat exchangers due to flow maldistribution may be serious. In this review, the flow distribution performance in a plate-fin heat exchanger has been experimentally studied and the distribution performance of different distributors' inlet angles has been measured. The combined effects of the inlet angle and mass flow rate on flow maldistribution have been studied. The study is useful in the optimum design of plate-fin heat exchangers.
40 citations
TL;DR: General thermal design methods for two-phase heat exchangers are emerging that are based on local two- phase flow patterns and the flow structure of the two-phases, which promise to be much more accurate and reliable than the older, statistically-derived empirical design methods.
Abstract: General thermal design methods for two-phase heat exchangers are emerging that are based on local two-phase flow patterns and the flow structure of the two-phases. These methods promise to be much more accurate and reliable for predicting two-phase heat transfer coefficients and pressure drops than the older, statistically-derived empirical design methods that completely ignore flow regime effects or simply treat flows as stratified (gravity-controlled) or nonstratified (shear-controlled) flows, which greatly limits their accuracy, validity, and reliability and often results in prediction errors surpassing 100% within their supposed range of application. These new flow pattern and flow structure types of design methods are particularly suited for use in modern heat exchanger design software, which are typically incremental and hence require local methods that capture the real trends in experimental data. The status of these new developments is reviewed here for intube two-phase flow and heat transfer proc...
36 citations
TL;DR: In this paper, a flow visualization of a 6-port microchannel tube with a hydraulic diameter of 1.5 mm under adiabatic conditions was performed using R134a, a medium pressure refrigerant.
Abstract: Results are presented from a flow visualization investigation of a multiport microchannel tube using R134a, a medium pressure refrigerant. The study covers mass fluxes from 50-300 kg/s.m 2 and qualities ranging from 10-90%, with a 6-port microchannel tube with a hydraulic diameter of 1.5 mm under adiabatic conditions. The results from the flow visualization studies indicate that several flow configurations may exist in multiport microchannel tubes at the same time while constant mass flux and quality flow conditions are maintained. Based on these observations, development of a typical flow regime map does not appear to be an appropriate manner for describing the flow field if the flow conditions for each port are not known. Flow mapping of the fluid regimes in this multiport microchannel is accomplished by developing functions that describe the fraction of time or the probability that the fluid exists in an observed flow configuration. An application of the alternative method of flow description for press...
34 citations
TL;DR: In this article, the problem of determining the bulk temperature of semitransparent oxide particles from the experimental data on the color temperature is analyzed by using a recently proposed approximate model for the radiation transfer inside a nonisothermal refracting spherical particle.
Abstract: A considerable temperature difference in semitransparent oxide particles due to intensive heating in plasma spraying makes it difficult to interpret the optical measurements of their temperature. The problem of determining the bulk temperature of such particles from the experimental data on the color temperature is analyzed by using a recently proposed approximate model for the radiation transfer inside a nonisothermal refracting spherical particle. The same approximation is also employed for developing an improved model of particle heating, taking into account the radiation-conduction interaction inside the particle. Calculations for Al 2 O 3 and ZrO 2 particles in a typical plasma jet show that the color temperature of oxide particles may be less than or greater than their bulk temperature, depending on the spectral absorption coefficient of particle substance. This temperature difference during the melting of particles may reach the value of 200-300 K. A procedure for in situ evaluation of the absorpti...
TL;DR: In this article, a comprehensive review of the published literature on inside-tube condensation heat transfer correlations for smooth and microfin tubes with an emphasis on correlations used in air conditioning and refrigeration is provided.
Abstract: This article provides a comprehensive review of the published literature on inside-tube condensation heat transfer correlations for smooth and microfin tubes with an emphasis on correlations used in air conditioning and refrigeration. The correlations presented are discussed and evaluated with experimental data from different authors for different fluids and flow conditions. This review is divided in two main parts: condensation inside smooth tubes and condensation inside microfin tubes. According to the comparison between empirical correlation and experimental data for smooth tubes, correlations proposed by Dobson et al., Dobson and Chato, and Cavallini et al. appear to be the most accurate ones to be used for different fluids and boundary conditions. In the case of microfin tubes, much additional work is needed to develop more general and accurate correlations.
TL;DR: In this article, a finite element method with the frontal solver was used to examine the flow parameters and heat transfer characteristics of the top cover of a dome and the effect of various "offset" of the dome and inclined roof on convective heat transfer.
Abstract: In this article, the analyses of heat transfer and free convective motion have been carried out numerically for various structures. The solution is based on a finite element method with the frontal solver to examine the flow parameters and heat transfer characteristics. Several dome configurations--such as flat, inclined, and dome shapes--are considered for the top of the enclosure. A general conic equation is considered to represent the dome as circular, elliptical, parabolic, or hyperbolic shape. The findings from this study indicate that the convective phenomenon is greatly influenced by the shape of the top cover dome and tends to form a secondary core even at a moderate Rayleigh number when compared with an equivalent rectangular enclosure. In addition, the circular and elliptical shapes of the dome give higher heat transfer rate. The effect of various "offset" of the dome and inclined roof on convective heat transfer is also found to be quite significant. However, beyond 0.3 of offset of the top cov...
TL;DR: A greater awareness in environmental issues such as benign refrigerants and global warming has been an impetus to the development of heat-driven adsorption chillers in industrialized countries, as evidenced by the plethora of published literature and patents.
Abstract: A greater awareness in environmental issues, such as benign refrigerants and global warming, has been an impetus to the development of heat-driven adsorption chillers in industrialized countries, as evidenced by the plethora of published literature and patents [1–29]. The engineering rubic “chillers” here refers to both heating and cooling systems that are encountered in the refrigeration and air conditioning industries. The development of adsorption chillers fits appealingly the above-mentioned environmental goals because they utilize low-temperature waste heat directly that is available in abundance from process, oil, and chemical industries or renewable sources. Adsorption chillers recover the waste heat sources into useful heating or cooling and, consequently, mitigate the effects of global warming as fewer fossil fuels are being burned. Secondly, the working adsorbent-adsorbate pair employed in adsorption chillers is made from harmless substances (having no ozone depletion potential), such as the commercially available silica gel-water,
TL;DR: In this article, heat transfer and friction characteristics for a new type of enhanced rectangular duct with winglets have been investigated experimentally, and the results indicate that, in the range of Reynolds number from 5 2 10 3 to 4.7 2 10 4, the heat transfer performance of the enhanced duct with the winglets is superior to the enhanced rectilinear duct with transverse disturbances.
Abstract: Heat transfer and friction characteristics for a new type of enhanced rectangular duct with winglets have been investigated experimentally. The results indicate that, in the range of Reynolds number from 5 2 10 3 to 4.7 2 10 4 , heat transfer performance of the enhanced duct with winglets is superior to the enhanced duct with transverse disturbances. Comparisons under the identical pumping power condition reveal that the Nusselt number ratio Nu/Nu o of the winglet duct to the smooth duct varies from 1.7 to 3.5, while this ratio is usually less than 1.5 for the enhanced ducts with transverse disturbances. For condition of same mass flow, the ratio Nu/Nu o of the winglet duct to the smooth duct varies between 2.7 and 6.0.
TL;DR: In this paper, a mathematical model for the design of rotary regenerators is presented, which is valid for all kinds of channel geometries that form the matrix of a rotary generator and can be used for all rotational speeds.
Abstract: Rotary regenerators are used widely for energy recovery, both in industrial and air-conditioning applications, because of the advantages they have over other types of heat exchangers. In this study, a mathematical model for the design of rotary regenerators is presented. The method is valid for all kinds of channel geometries that form the matrix of a rotary regenerator and can be used for all rotational speeds. The validity of the method is tested with the experimental and numerical results available for rotational speeds between 0.05 and 7 rev/min. A good agreement between the model and the experimental numerical results is obtained. The method presented can therefore be recommended for the safe design of rotary regenerators.
TL;DR: In this article, the authors used a remote control mobile robot unit to measure the velocity and temperature fields of the air and water flows in a selected reference vertical segment of cooling tower in the form of phenomenological dependence.
Abstract: In this paper, the development of natural draft cooling towers diagnostics is presented. Diagnostic method is based on measurements of velocity and temperature fields of the airflow in the entire surface area of cooling tower and the raised phenomenological model of heat and mass transfer in a selected reference vertical segment of cooling tower. Velocity and temperature fields of the airflow were measured with the aid of a remote control mobile robot unit that was developed to enable measurements in an arbitrary measurement point above the spray zones over the entire cooling tower area. Topological structures of the humid air velocity profiles and temperature profiles above the spray zones were obtained at constant integral parameters of a power plant. Measurement results of temperature and mass flow characteristics of the air and water flows in a selected reference vertical segment of cooling tower are presented in the form of phenomenological dependence. Phenomenological dependence links local cooling ...
TL;DR: In this article, the authors analyzed the three-dimensional laminar natural convection flow with three chips at various positions by employing the computational fluid dynamics (CFD) code PHOENICS.
Abstract: The three-dimensional laminar natural convection flow with three chips at various positions was analyzed by employing the computational fluid dynamics (CFD) code PHOENICS. The SIMPLEST algorithm with the Hybrid Scheme was used to simulate these flows. Three chips, arranged in five different positions with isothermal and insulated walls, were solved. The temperature distribution of our computational results was similar to the experimental data trend and very close to the numerical results achieved by Beak et al. The calculating results show that different chip position arrangements strongly influence the chip average temperature. The highest temperature occurred with the vertical chip arrangements. The findings herein establish a fundamental numerical study of three-dimensional heat transfer using three chips and a basis for further analysis of the associated heat transfer for more complicated chip position arrangements.
TL;DR: A theoretical analysis of the cooling effect of a heat sink is presented in this paper, where the optimum heat transfer equation can be utilized to obtain the optimum length of fins in a sink, which affects the overall thermal effectiveness of the heat sink.
Abstract: A theoretical analysis of the cooling effect of a heat sink is presented in this study. With the input data of Biot number, Bi, and heat transfer coefficient ratios, H and H *, the optimum heat transfer equation can be utilized to obtain the optimum length of fins in a heat sink, which affects the overall thermal effectiveness of the heat sink. This optimum equation is in transcendental form, which involves three dimensionless parameters, $\sqrt{\hbox{Bi}}$ S opt. , $\sqrt{\hbox{Bi}}$ H , and $\sqrt{\hbox{Bi}}$ H *. Finally, the thermal resistance of a heat sink is derived and examples are provided to illustrate the effect on the cooling performance of a heat sink under various design conditions.
TL;DR: In this article, a finned plate heat exchanger and a connected, twin active bed system are used to enable heat recuperation in an adsorption refrigeration system.
Abstract: A new refrigeration system that uses a specially designed finned plate heat exchanger and works with zeolite and methanol is proposed. The integration of heat transfer and adsorption via a finned surface coated with zeolite CBV 901 and the use of a connected, twin active bed system to enable heat recuperation are novel features. The thermophysical properties of zeolite and methanol were first studied with the intention of designing a high performance heat exchanger (generator) for the adsorption refrigeration system. Here, the major problem is related to poor conductivity at the interface between the heat exchanger and the zeolite. The adsorbent must be heated (desorption phase) and then cooled (adsorption phase) back to ambient temperature in order to complete a thermodynamic cycle. To manufacture a sufficiently small system, there must be high rates of heat transfer in and out of the adsorbent. Therefore, the surface of the heat exchanger is finned in order to increase the heat transfer area (the fins a...
TL;DR: A history of the MIT Heat Transfer Laboratory can be found in this paper, where the authors present a detailed account of the history and history of MIT's Heat Transfer Program (HTP).
Abstract: (2003). A History of the MIT Heat Transfer Laboratory. Heat Transfer Engineering: Vol. 24, No. 2, pp. 3-17.
TL;DR: In this article, a new and unique piece of equipment for the thermal treatment of gas wastes (i.e., the incineration of volatile organic compounds contained in polluted air) has been developed.
Abstract: A quite new and unique piece of equipment for the thermal treatment of gas wastes (i.e., the incineration of volatile organic compounds contained in polluted air) has been developed. This compact equipment is characterized by a cylindrical combustion chamber placed inside a heat exchanger—a polluted air preheater. This cylindrical preheater consists of several concentric stainless sheets. Both flue gas from the combustion chamber and polluted air heated by flue gas flow in the spaces between the cylindrical sheets. Narrow distance strips placed between the sheets form helical rectangular ducts, through which we can achieve a counter-current flow of process fluids. A mathematical model for the calculation and/or simulation of the equipment consists of submodels of a heat exchanger and combustion chamber and the annular space between them. Based on the results of measurements on an industrial scale experimental facility, new correlations for the thermal and hydraulic calculations of the heat exchanger were ...
TL;DR: In this article, the authors present a computational method that implements an analytical approach toward determining the thermal processes that take place in an electrically heated oven, where integral equations of radiative heat transfer are specialized to the three-dimensional temperature field in the oven and numerically evaluated.
Abstract: This article presents a computational method that implements an analytical approach toward determining the thermal processes that take place in an electrically heated oven. The integral equations of radiative heat transfer were specialized to the three-dimensional temperature field in the oven and numerically evaluated. The radiative fluxes at the surfaces of a thermal load situated in the oven were thus obtained and combined with natural convection fluxes extracted from a well-established correlation. The net result was a set of predictions for the temperature history of a thermal load of which size, material properties, radiative surface characteristics, and location in the oven were varied. In addition, two modes of oven operation were considered: a preheated and a non-preheated oven. An alternative model for the experimental data was built on an algebraic representation utilizing a one-term, time-based exponential. That model enabled the identification of the effect of the thermal inertia factor; load...
TL;DR: In this paper, the single-phase heat transfer and pressure drop characteristics were determined for an angled spiraling tape that was inserted into the annulus of a tube-in-tube heat exchanger.
Abstract: Heat transfer augmentation in heat exchangers has received much attention in recent years, mainly due to energy efficiency and environmental considerations. Many active and passive techniques are currently being employed in heat exchangers, with twisted tape inserts providing a cost-effective and efficient means of augmenting heat transfer. This article describes how the single-phase heat transfer and pressure drop characteristics were determined for an angled spiraling tape that was inserted into the annulus of a tube-in-tube heat exchanger. Four sets of experimental measurements were taken: a normal tube-in-tube heat exchanger that was used as a reference and three heat exchangers with different angled spiraling tape inserts. Semiempirical heat transfer and pressure drop correlations were derived that predict the experimental data to within 4.3%. The angled spiraling tape inserts resulted in an increase in the heat transfer coefficients and pressure drop penalties, which varied with tape pitch and the d...
TL;DR: In this article, the authors proposed a network heat spreader with overall dimensions of 78 2 62 2 3.2 mm and external cooling fins are attached to enhance air-cooling heat transfer rate.
Abstract: To achieve a high heat-flux level and reduce manufacturing costs associated with conventional heat pipes, the concept of network heat spreaders employing a boiling heat-transfer mechanism in a narrow space had been proposed, and several flat-plate wickless heat spreaders had been designed and fabricated. The heat spreaders had been tested under different working conditions and orientations relative to gravity with very good results. The previously tested network heat spreaders, however, were based on plates with a relatively large size for general heat spreading purposes. In the present study, network heat spreaders with overall dimensions of 78 2 62 2 3.2 mm are designed and fabricated. Spreaders of this size are intended for use as heat sinks of high-power electronic components. External cooling fins are attached to enhance air-cooling heat transfer rate. The network heat spreaders are tested under various working conditions with water as the working fluid. The maximum heat input rate achieved is about ...
TL;DR: In this paper, the authors investigated the effects of viscous dissipation of working fluid on the optimum heat sink parameters for the case of air cooling with a micro-narrow-channeled compact heat sink.
Abstract: The effects of viscous dissipation of working fluid on the optimum heat sink parameters are investigated for the case of air cooling with a micro-/narrow-channeled compact heat sink. For this purpose, an optimization method is introduced first on the basis of dimensionless groups while employing several assumptions. This method yields minimum pumping work or pressure drop with a set of optimized geometric/hydrodynamic parameters when outer dimension of a heat sink and imposed thermal load are specified. Especially for the case of laminar flow, the procedure presents an explicit existence of cooling limit by the viscous heat generation, giving an analytical expression of the maximum removable heat Q max . The relationships between thermal load and each parameter are calculated for both laminar and turbulent regimes under the conditions of compact heat sink dimension (20 mm 2 20 mm 2 2 mm) and circular cross-sectional shape of channels. The results show that the cooling capability under such conditions is l...
TL;DR: In this article, the authors investigated the thermal behavior of a thin slab under the effect of a fluctuating surface thermal disturbance, as described by the dual phase-lag heat conduction model.
Abstract: In the present work, the thermal behavior of a thin slab under the effect of a fluctuating surface thermal disturbance, as described by the dual-phase-lag heat conduction model, is investigated. It is found that using the dual-phase-lag heat conduction model is essential at large frequencies of the surface disturbance. Mathematical criteria that specify the limits beyond which both the hyperbolic wave and the dual-phase-lag heat conduction models deviate from the diffusion model are derived.
TL;DR: In this paper, the authors apply sustainability criteria to the design of "heat sinks" used to cool advanced microelectronic components, including the use of natural resources, the environment, social welfare, and economic impact.
Abstract: The present effort addresses the application of sustainability criteria to the design of "heat sinks" used to cool advanced microelectronic components. The sustainability assessment is based on several criteria, including the use of natural resources, the environment, social welfare, and economic impact. The development of forced convection heat sinks, which are compatible with sustainable development, involves a subtle balance between the achieved thermal performance and the investment of material and energy in the fabrication and operation of the heat sink. It is shown that sustainability criteria can be used to select the environmentally optimal configuration among the most promising heat sink designs, including the lowest pumping power, the least mass of material, and the lowest total (fabrication and operation) energy for a specified application. Of the options considered for cooling a 100 W microprocessor with an aluminum heat sink operating at an excess temperature of 25 K, the heat sink design wit...
TL;DR: In this article, the effects of different parameters such as phase change material size and melting point temperature on the irreversibility of a thermal energy storage system are examined, and the results show that the irrevability of thermal storage module is strongly affected by the size of PCM (diameter and length of the external cylinder) and melting temperature.
Abstract: In this paper, irreversibility of a thermal energy storage system is numerically investigated. The system consists of two concentric cylinders. The outer cylinder is filled with phase change material (PCM), while working fluid flows inside the inner pipe. The system works periodically. The related governing equations are solved by a control volume-based finite difference method. The effects of different parameters such as PCM size and melting point temperature are examined on the irreversibility of the system. The results show that the irreversibility of thermal storage module is strongly affected by the size of PCM (diameter and length of the external cylinder) and melting temperature. Based on the obtained results, the irreversibility of the system can be reduced by proper selection of PCM size and melting temperature.
TL;DR: In this article, the optimum dimensions of trapezoidal profile radiating and convective-radiating circular fins have been determined in a step-by-step derivation of the governing equations, which are set forth in a non-dimensional form and solved numerically.
Abstract: The optimum dimensions of trapezoidal profile radiating and convective-radiating circular fins have been determined. A step-by-step derivation of the governing equations is given, which are set forth in a non-dimensional form and solved numerically. Consideration is given to both zero and non-zero environmental temperature and environmental irradiation. The analysis is based on the following simplifying assumptions: one-dimensional conduction, insulated tip, length of arc idealization (LAI), and negligible fin-to-base radiant interaction. The influence of the thermal conductivity, emissivity and other parameters upon the optimum dimensions has been analyzed. The results are generalized by expressing the optimum bore thickness, tip radius, heat dissipation, and volume in dimensionless form. Simple correlations have been derived that can be employed to readily obtain the optimum fin dimensions for a specified heat transfer rate or fin's weight without the use of any graphs and/or interpolations. Two example...