ISSN:0145-7632    

DOI:10.1080/01457639708939886

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Because of its importance in industrial processes, overall natural convective heat transfer from horizontal cylinders has been the subject of many experimental and analytical studies. However, because of the dispersions in these results, there is still uncertainty about the value of the Nusselt number Nu at a particular Rayleigh number Ra, especially for Ra > 103. The values of Nu obtained from the most commonly used correlations do not reduce this uncertainty because of their own dispersion. To avoid the effect of higher Prandtl numbers Pr on the heat transfer, a review is made of the results from 34 experimental studies in air (Pr = 0.7) in the range 10 ≤ Ra ≤ 107. A review is also made of 23 analytical and numerical studies. It is concluded that the mean values of Nu within each of the two groups agree to within 4% in the range 10 ≤ Ra ≤ 104. At higher Ra, the mean of the experimental values of Nu is higher than the mean of the analytical and numerical values, the difference increasing from 7.86% at Ra = 105to 11.3% at Ra = 107. Possible reasons for this discrepancy are discussed

ISSN:0145-7632    

DOI:10.1080/01457639708939887

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Heat transfer enhancement and pressure drop penalty in annuli with spirally fluted inner tubes were evaluated for the laminar, transition, and turbulent flow regimes for the Prandtl number range 4.37 < Pr < 10.31. Recent experimental studies that reported Nusselt number and friction factor correlations for these annuli were used as the basis for this parametric evaluation. On the basis of flow-visualization tests and tube-surface temperature-profile measurements, it is known that the fluted inner lubes induce a significant degree of swirl in the flow. In the laminar regime, friction factor enhancements relative to smooth-tube annuli are typically between 1.1 and 2.0, while enhancements of up to 10 occur in the turbulent regime. Nusselt numbers in the laminar regime are between 4 and 20 times the smooth annulus values, while turbulent enhancements are between 1.1 and 4.0. These respective enhancements are functions of the flute depth, pitch and angle, and annulus radius ratio. The enhancement efficiency of these tubes was defined as the ratio of the Nu enhancement to the friction factor enhancement. This figure of merit for spirally enhanced annuli was characterized for a wide range of annulus geometries and explained in terms of the fluid flow mechanisms. One of the more significant findings of this study is that in the low Reynolds number range (Re < 3000), fluted inner tubes in annuli cause enhancements in heat transfer that are up to 7 times the corresponding increase in friction factor. The dependence of heal transfer and friction factor enhancement on geometric and flow variables was used to develop guidelines for the optimum range of applicability of spirally fluted annuli.

ISSN:0145-7632    

DOI:10.1080/01457639708939888

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This paper is basically the text of the Kern Lecture for 1991 (the 1990 Kern Award). The paper begins with some remarks about Dr. Kern. By way of introduction to heat transfer enhancement, historical notes and the evolution of literature in this area are presented. Comments are made about the increasing practical applications of enhancement technology. Developments in single-phase convection are presented, with particular emphasis on offset strip fins and twisted-tape inserts. Pool boiling and flow boiling (particularly microfin tubes) are then considered in some detail. It is concluded that enhancement represents a powerful technology to improve heat exchanger performance.

ISSN:0145-7632    

DOI:10.1080/01457639708939889

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Leakages are inherently associated with the heat transfer process in a rotary heat exchanger, owing to clearances brought about by construction difficulties and rotor-housing thermal distortions. The method proposed permits one to evaluate leakage distributions within steam boiler rotary regenerators. In principle, the method is based on a gas flow diagram of the regenerator, where gas streams are arranged into a network in which the corresponding mass flow rates are governed by pressure drops in the seals and matrix rotation. Moreover, energy and mass balances are taken into account where appropriate. On this basis, a nonlinear equation system modeling gas flows within the rotary regenerator is derived. Supplementing this system with data determined experimentally, one obtains a closed form of the gas flow problem, the solution of which gives particular gas mass flow rates within the regenerator and the leakage distribution resulting from it. On the basis of experiments carried out on real devices, the approach presented is exemplified with two particular cases accompanied by evaluation of uncertainties in the final results. A comparison of the results presented with those available by another method concludes the article.

ISSN:0145-7632    

DOI:10.1080/01457639708939890

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

In single-phase shell-and-tube heat exchangers, thermal performance prediction is customarily accomplished with an idealization that the number of baffles used is very large and can be assumed to approach infinity. Under this idealization, the temperature change within each baffle compartment is very small in comparison with the total temperature change of the shell fluid through the heat exchanger. Thus the shell fluid can be considered as uniform (perfectly mixed) at every cross section (in a direction normal to the shell axis). It is with this model that the mean temperature difference correction factor is normally derived for single-phase exchangers. In reality, a finite number of baffles are used, and the condition stated above can be achieved only partially. In this article, a comprehensive review is made and new results are derived where needed to assess the influence of a finite number of baffles on heat transfer performance for 1-1, 1-2, and 1-N TEMA E, 1-2 TEMA J, and 1-2 TEMA G and H single-phase shell-and-tube exchangers. It is shown that the number of baffles required to achieve the performance within about 2% of an exchanger with an infinite number of baffles varies with the type of exchanger and the performance parameters. The new results are presented in tabular form.

ISSN:0145-7632    

DOI:10.1080/01457639708939891

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor