Internal flow forced convection

TABLE 17.13 Important Dimensionless Groups for Internal Flow Forced Convection Heat Transfer... 

Gondim, R.R., Cotta, R.M., Santos, C.A.C., and Mat, M. (2003) Internal Transient Forced Convection with Axial Diffusion Comparison of Solutions Via Integral Transforms, ICHMT International Symposium on Transient Convective Heat And Mass Transfer in Single and Two-Phase Flows, Cesme, Tmkey, August 17 - 22. 

J. P. Hartnett, Single Phase Channel Flow Forced Convection Heat Transfer, in 10th International Heat Transfer Conference, vol. 1, pp. 247-258, Brighton, England, 1994. 

Source Incropera, F. P. et aL Fundamentals cfHeat and Mass Transfer, 6th Edition. 2007. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission Shah, R. K. and London, A. L. Laminar Flow Forced Convection in Ducts. Academic Press, New York, 1978 Asako, Y. et al. International Journal cf Heat and Mass Transfer, 31,2590-2593,1988. 

In this section the correlations used to determine the heat and mass transfer rates are presented. The convection process may be either free or forced convection. In free convection fluid motion is created by buoyancy forces within the fluid. In most industrial processes, forced convection is necessary in order to achieve the most economic heat exchange. The heat transfer correlations for forced convection in external and internal flows are given in Tables 4.8 and 4.9, respectively, for different conditions and geometries. 

Convection is the heat transfer in the fluid from or to a surface (Fig. 11.28) or within the fluid itself. Convective heat transport from a solid is combined with a conductive heat transfer in the solid itself. We distinguish between free and forced convection. If the fluid flow is generated internally by density differences (buoyancy forces), the heat transfer is termed free convection. Typical examples are the cold down-draft along a cold wall or the thermal plume upward along a warm vertical surface. Forced convection takes place when fluid movement is produced by applied pressure differences due to external means such as a pump. A typical example is the flow in a duct or a pipe. 

There are many types of internal forced convection. This chapter examines selected examples. Flows with heat transfer between parallel plates and flows in pipes, tubes and ducts are considered. 

There have bee.i many studies of internal mixed convection, particularly in circular pipes. The conditions under which flow in a circular pipe can be assumed to be purely forced convective, purely free, and mixed convective have been presented in graphical form by Metais and Eckert [62], the form of these graphs being given in Figs. 9.26 and 9.27. Figure 9.26 applies to flow in a vertical pipe while Fig. 9.27 is for flow in a horizontal pipe. j... 

This chapter has been concerned with flows in wb ch the buoyancy forces that arise due to the temperature difference have an influence on the flow and heat transfer values despite the presence of a forced velocity. In extemai flows it was shown that the deviation of the heat transfer rate from that which would exist in purely forced convection was dependent on the ratio of the Grashof number to the square of the Reynolds number. It was also shown that in such flows the Nusselt number can often be expressed in terms of the Nusselt numbers that would exist under the same conditions in purely forced and purely free convective flows. It was also shown that in turbulent flows, the buoyancy forces can affect the turbulence structure as well as the momentum balance and that in turbulent flows the heat transfer rate can be decreased by the buoyancy forces in assisting flows whereas in laminar flows the buoyancy forces essentially always increase the heat transfer rate in assisting flow. Some consideration was also given to the effect of buoyancy forces on internal flows. 

Forced convection heat transfer is probably the most common mode in the process industries. Forced flows may be internal or external. This subsection briefly introduces correlations for estimating heat-transfer coefficients for flows in tubes and ducts flows across plates, cylinders, and spheres flows through tube banks and packed beds heat transfer to nonevaporating falling films and rotating surfaces. Section 11 introduces several types of heat exchangers, design procedures, overall heat-transfer coefficients, and mean temperature differences. 

Flow In Round Tubes In addition to the Nusselt (NuD = hD/k) and Prandtl (Pr = v/a) numbers introduced above, the key dimensionless parameter for forced convection in round tubes of diameter D is the Reynolds number Re = (.7 ) u where G is the mass velocity G = m/Ac and Ac is the cross-sectional area Ac = kD2I4. For internal flow in a tube or duct, the heat-transfer coefficient is defined as... 

Convection is classified as natural (or free) and forced convection, depend ing on how the fluid motion is initiated. In forced convection, the fluid is forced to flow over a surface or in a pipe by external means such as a pump or a fan. In natural convection, any fluid motion is caused by natural means such as the buoyancy effect, which manifests itself as the rise of warmer fluid and the fall of the cooler fluid. Convection is also classified a.s external and internal, depending on whether the fluid is forced to flow over a surface or in a pipe. 

On the other hand, the heat fransfer literatiue of the last decade has demonstrated a vivid and growing interest in thermal analysis of flows in micro-channels, botii tiirough experimental and analytical approaches, in connection with cooling techniques of micro-electronics and witii tiie development of micro-electromechanical sensors and actuators (MEMS), as also pointed out in recent reviews [12-16]. Since tiie available analytical information on heat fransfer in ducts could not be directly extended to flows witiiin microch mels with wall slip, a number of contributions have been recentiy directed towards the analysis of internal forced convection in the micro-scale. In the paper by Barron et al. 

Peng, X. and Wang, B., (1993), Forced convection and flow boiling heat transfer in flat plates with rectangular microchannels. International Journal of Heat and Mass Transfer 36,14, pp. 3421-3427. 

In this work, heat and fluid flow in some common micro geometries is analyzed analytically. At first, forced convection is examined for three different geometries microtube, microchannel between two parallel plates and microannulus between two concentric cylinders. Constant wall heat flux boundary condition is assumed. Then mixed convection in a vertical parallel-plate microchannel with symmetric wall heat fluxes is investigated. Steady and laminar internal flow of a Newtonian is analyzed. Steady, laminar flow having constant properties (i.e. the thermal conductivity and the thermal diffusivity of the fluid are considered to be independent of temperature) is considered. The axial heat conduction in the fluid and in the wall is assumed to be negligible. In this study, the usual continuum approach is coupled with the two main characteristics of the microscale phenomena, the velocity slip and the temperature jump. 

Dong, Z. F. Department of Mechanical Engineering, Florida International University (chap. 5, Forced Convection, Internal Flows), e-mail zdong agilis.com... 

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