scholarly journals How Big Is an Error in the Analytical Calculation of Annular Fin Efficiency?

Energies ◽  
2019 ◽  
Vol 12 (9) ◽  
pp. 1787 ◽  
Author(s):  
Mladen Bošnjaković ◽  
Simon Muhič ◽  
Ante Čikić ◽  
Marija Živić
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Heat Exchange ◽  
Transfer Coefficient ◽  
Surface Area ◽  
Analytical Calculation ◽  
Base Surface ◽  
Fin Efficiency ◽  
Annular Fin ◽  
Original Definition

An important role in the dimensioning of heat exchange surfaces with an annular fin is the fin efficiency. The fin efficiency is usually calculated using analytical expressions developed in the last century. However, these expressions are derived with certain assumptions and simplifications that involve a certain error in the calculation. The purpose of this paper is to determine the size of the error due to the assumptions and simplifications made when performing the analytical expression and to present what has the greatest impact on the amount of error, and give a recommendation on how to reduce that error. In order to determine the error, but also to gain a more detailed insight into the physics of heat exchange processes on the fin surface, computational fluid dynamics was applied to the original definition of fin efficiency. This means that a numerical simulation was performed for the actual fin material and for the ideal fin material with infinite thermal conductivity for the selected fin geometry and Re numbers from 2000 to 18,000. The results show that fin efficiency determined by numerical simulations is greater by up to 12.3% than the efficiency calculated analytically. The greatest impact on the amount of error is the assumption of the same temperature of the fin base surface and the outer tube surface and the assumption of equal heat transfer coefficient on the entire fin surface area. Using a newly recommended expression for the equivalent length of the fin tip, it would be possible to calculate the fin efficiency more precisely and thus the average heat transfer coefficient on the fin surface area, which leads to a more accurate dimensioning of the heat exchanger.

scholarly journals Heat-Up Performance of Catalyst Carriers—A Parameter Study and Thermodynamic Analysis

Energies ◽  
2021 ◽  
Vol 14 (4) ◽  
pp. 964
Author(s):  
Thomas Steiner ◽  
Daniel Neurauter ◽  
Peer Moewius ◽  
Christoph Pfeifer ◽  
Verena Schallhart ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Surface Area ◽  
Internal Surface ◽  
Installation Space ◽  
Parameter Study ◽  
Thin Wall ◽  
Primary Role ◽  
Internal Surface Area

This study investigates geometric parameters of commercially available or recently published models of catalyst substrates for passenger vehicles and provides a numerical evaluation of their influence on heat-up behavior. Parameters considered to have a significant impact on the thermal economy of a monolith are: internal surface area, heat transfer coefficient, and mass of the converter, as well as its heat capacity. During simulation experiments, it could be determined that the primary role is played by the mass of the monolith and its internal surface area, while the heat transfer coefficient only has a secondary role. Furthermore, an optimization loop was implemented, whereby the internal surface area of a commonly used substrate was chosen as a reference. The lengths of the thin wall and high cell density monoliths investigated were adapted consecutively to obtain the reference internal surface area. The results obtained by this optimization process contribute to improving the heat-up performance while simultaneously reducing the valuable installation space required.

scholarly journals THE INFLUENCE OF THE CHANGES IN WIND VELOCITY ON THE OUTER HEAT EXCHANGE OF THE BUILDINGS

2021 ◽  
pp. 148-155
Author(s):  
D.V. Tarasevych ◽  
◽  
O.V. Bogdan ◽  
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Heat Exchange ◽  
Transfer Coefficient ◽  
Heat Loss ◽  
Wind Velocity ◽  
Air Temperature ◽  
Outer Surface ◽  
Climatic Parameters ◽  
The Heat Transfer Coefficient

When choosing architectural and planning solutions, such climatic factors as air temperature and humidity, having scalar quantities, as well as solar radiation, wind and precipitation having vector characteristics, must be taken into account. The calculated climatic parameters for the design of building enclosing structures, heat loss calculations and heat supply regulation are provided in the current documentation on norms and standards. The practical exploitation of various buildings demonstrates that in terms of initial climatic data, the choice of design parameters is not always efficiently justified; hence, the influence of the environment on the heating regime of the structures is insufficient in the estimations and sometimes erroneous. The wind is one of such climatic parameters. Its velocity and repeatability impact the heat exchange of the building structure with the environment as well as the alteration in temperature regime. The wind current towards the building creates additional pressure on the facade of the construction from the wind side direction. This leads, firstly, to air infiltration via the enclosing structures, and secondly, to the rise of heat exchange from the outer surface of the wall on the windward side. Based on estimated and analytical research, the values of the change in wind velocity depending on the altitude were analyzed, and its influence on the heat loss during heating of multi-storey buildings was assessed. The alterations in the wind velocity depending on the altitude were analyzed in the conditions of dense (urban) and broad construction. Besides, the authors presented the dependence of the convective component of the heat transfer coefficient of the outer surface of the structure on the values of the wind velocity. Based on the performed and presented calculations, it can be noticed that the heat transfer of the external structure will be much higher for multi-storey buildings than for mid-rise constructions. Thus, the convective component of the heat transfer coefficient of the outer surface rises by 36 % when the wind velocity increases from 5 m/s to 7 m/s. If not taking into consideration this dependence in the design, it can significantly influence the estimation of heat loss and energy efficiency of buildings, especially when it is about the increased percentage of facades glazing. The authors of the article assessed the heat loss for heating the windward and leeward facades at average values of the outside air temperature during the heating season in Ukraine. Hence, for constructions higher than 70 m with a calculated wind velocity of 5 m/s, heat losses increase from 10 % to 19 %. Such great difference in heat loss between the windward and leeward walls of the building requires increased thermal protection from the prevailing winter winds. Therefore, when designing multi-storey buildings, it is necessary to take into account changes in wind velocity according to the altitude. The obtained results can be useful both for choosing architectural and planning solutions, like the materials for external enclosing structures and for the objective assessment of the wind protection degree of individual buildings and territories.

Increasing Condenser Capacity Without Adding Tubes to Support a Station Uprate

2010 ◽  
Author(s):  
Warren C. Welch ◽  
Timothy J. Harpster ◽  
Joseph W. Harpster
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Surface Area ◽  
Low Cost ◽  
Cooling Water ◽  
Steam Generation ◽  
Before And After ◽  
Generation Capacity ◽  
Almost All

A station uprate provides an economical opportunity to improve the generation capacity of a power plant if all the major system components are able to handle the effects of increased generation. The magnitude of uprate from increased steam generation will be limited by the maximum capacity of the weakest link in the cycle, which for many plants is the condenser. The condensers on many units are already pushed to their limit. This is especially true if a cooling tower is employed, where the condenser inlet cooling water temperatures are high on high wet-bulb temperature days. This condition forces many units to throttle down load to prevent excursions above the backpressure limits on their turbines. For condensers limited by the present duty, however, the options have been historically limited to rebundling the whole condenser with a larger surface area design and perhaps changing the tube material to a material with a higher heat transfer coefficient. Recently, a very low cost option has been demonstrated that should be considered by any plant looking to increase condenser duty or prevent station power reductions. Advances in the proper management of steam, condensate and noncondensable flows have permitted an upgrade for almost all vintage condensers, unlocking inactive surface area without a bundle replacement or complete redesign. This paper reports the results of a condenser retrofit effort, with emphasis on an upgrade applied to a load limited condenser concurrent with a major reduction in its operating backpressure. The performance of the condenser is presented before and after the upgrade showing significant backpressure reduction and heat transfer improvement accompanied by exceptional condensate chemistry results. It will be shown that 30% of the effective condenser surface area (or similarly, an additional 30% average heat transfer coefficient) was unlocked by activating the previously idle surface area.

scholarly journals Determination of long-term permissible currents in wires of power supply systems of railways

2019 ◽  
Vol 78 (2) ◽  
pp. 90-95 ◽  
Author(s):  
E. P. FIGURNOV ◽  
Yu. I. ZHARKOV ◽  
V. I. KHARCHEVNIKOV
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Solar Radiation ◽  
Transfer Coefficient ◽  
Surface Area ◽  
Meteorological Conditions ◽  
The Body ◽  
Additional Heating ◽  
The Heat Transfer Coefficient

In the standard for contact wires made from copper and its alloys, the values of long-term permissible temperatures have significantly decreased. This requires recalculation of previously valid values of long-term permissible currents. Authors considered revised method for calculating the long-term permissible currents, based on a more rigorous consideration of the laws of heat transfer and experimental studies of the conditions of heating and cooling of shaped (contact) and stranded wires. Technique is based on heat balance conditions, using which the sources of greatest inaccuracies become such quantities as cooled surface area, influence of wind direction, meteorological conditions, laws of change in heat transfer coefficient, effect on additional heating of solar radiation. Deviations when these indicators are taken into account by existing methods can cause errors of 40 % or more. Formulas for calculating the actual outer surface of stranded and shaped wires are given. The inadmissibility of calculating the surface area of the wires by their reference diameter is noted. Updated law of the change in heat transfer coefficient for stranded and shaped wires, as well as the degree of its dependence on wind speed and cooled surface, is given based on a summary of extensive domestic and foreign research. It is shown that with the longitudinal direction of the wind, the reduction of this coefficient occurs to a lesser extent than has been assumed so far. Authors propose method for taking into account an increase in the heat transfer coefficient under meteorological conditions characteristic of ice formation. The heat transfer coefficient of shaped and stranded wires in no case can not be taken as for round pipes with smooth surface. Existing method of accounting for solar radiation, which influences the additional heating of wires, leads to an unjustified and repeated exaggeration of this effect, since previously only the radiation incident on the wire was taken into account in the calculations. According to the laws of heat transfer, the temperature of the irradiated body does not depend on the incident, but on the resulting radiation, defined as the difference between the radiations incident on the body and emitted by it in accordance with its temperature. A formula for accounting for such heat transfer is proposed. The above methodology and calculation formulas allow performing reasonable calculations to determine the long-term permissible currents of individual stranded and shaped wires, as well as the contact network as a whole.

scholarly journals Heat Exchange Modeling in Cooling Fins

2020 ◽  
pp. 669-676
Author(s):  
Evgeniy N. Vasil'ev
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Heat Exchange ◽  
Transfer Coefficient ◽  
Heat Conduction Problem ◽  
Rectangular Cross Section ◽  
One Dimensional ◽  
Wide Range ◽  
Simulation Results ◽  
The Heat Transfer Coefficient

The article discusses the process of heat exchange of a finned wall with a coolant. The temperature field in the wall volume was determined on the basis of a numerical solution of the two-dimensional heat conduction problem, and the analysis of the characteristics of temperature distributions was carried out according to the simulation results. The values of the heat transfer coefficient of cooling fins with rectangular cross section were calculated for two variants of heat transfer conditions at the end of the fins in a wide range of dimensionless parameters. The error in calculating the heat transfer coefficient in the approximation of a thin fin was determined by means of a one-dimensional computational model

scholarly journals TAKING INTO ACCOUNT DEPENDENCE OF VISCOSITY COEFFICIENT ON TEMPERATURE DURING VAPOR CONDENSATION ON A VERTICAL WALL

2020 ◽  
Vol 63 (5) ◽  
pp. 94-101
Author(s):  
Aleksandr I. Moshinskiy ◽  
Pavel G. Ganin ◽  
Alla V. Markova ◽  
Larisa N. Rubtsova ◽  
Vladislav V. Sorokin
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Heat Exchange ◽  
Transfer Coefficient ◽  
Dynamic Viscosity ◽  
Viscosity Coefficient ◽  
Vapor Condensation ◽  
Condensation Temperature ◽  
Heat Exchange Equipment ◽  
The Heat Transfer Coefficient

In the present study, the problem of vapor condensation on a flat vertical surface is investigated in the case of an arbitrary dependence of the dynamic viscosity coefficient on temperature according to a fairly general law. At a constant value of this coefficient and other characteristics of a condensing liquid (heat conductivity coefficient, density) this task was considered by Nusselt in a constant gravitational field. The results obtained by Nusselt formed the basis (with certain modifications) for the computational practice of heat exchange equipment of chemical technology in the presence of steam condensation of any heat carrier. Formation of a condensate film occurs due to heat transfer through the liquid film, vapor condensation at the outer edge of the film and the flow of liquid along the surface. The article generalizes the Nusselt theory for the heat transfer coefficient under the indicated conditions, and as a result, convenient calculation formulas for the heat transfer coefficient, which are necessary to describe the operation of heat and mass exchange equipment. Approximate relations are proposed for calculating the dynamic viscosity coefficient, which are useful for calculating film flow on a flat surface. A comparison is made with the previously used ratios in an approximate manner taking into account the dependence of viscosity coefficient on temperature. When in technical applications one wants to determine the average value of two parameters, which are then used to calculate certain characteristics of a process, then, traditionally, the average of these parameters is considered. This article shows that by simplifying the dependence of the effective dynamic viscosity coefficient, more accurate results are obtained by dividing the interval of the width of the current film in the ratio of three to one, where three fourths refer to the wall temperature, and one fourth to the condensation temperature. The analytical dependencies presented in this paper can be used for practical calculations of the heat exchange equipment.

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