Nusselt number combined convection

At low Reynolds numbers, the Nusselt number will tend to the constant value that would exist in purely free convection, this being designated as Afa.v, whereas at high Reynolds numbers, when the effects of the buoyancy forces are small, the Nusselt numbers will tend to the values that would exist in purely forced convection at the same Reynolds number as that being considered. These forced convection Nusselt numbers are here designated as Nur- In the combined convection regions between these two limits, the Nusselt number variation can be approximately... 

Typical variation of Nusselt number with Reynolds number in assisting combined convective flow. 

In convection studies,-it is common practice to nondimensionalize the governing equations and combine the variables, which group together into dimensionless numbers in order to reduce the number of total variables. It is also common practice to nondimensionalize the heat transfer coefficient h with the Nusselt number, defined as... 

Coefficients of heat transfer by natural convection from bodies of various shapes, chiefly plates and cylinders, are correlated in terms of Grashof, Prandtl, and Nusselt numbers. Table 8.9 covers the most usual situations, of which heat losses to ambient air are the most common process. Simplified equations are shown for air. Transfer of heat by radiation is appreciable even at modest temperatures such data are presented in combination with convective coefficients in item 16 of this table. 

Heat transfer by free convection can thus be presented as a relation between three dimensionless groups Cpt]/k is known as the Prandtl number, the combination l ATagp /tf as the Grashof number, and ql/AT, the Nusselt number may also be written hl/k. 

Heat transfer by convection occurs in liquids and gases where there is a velocity field caused by extorted fluid motion or by natural fluid motion caused by a difference in density. The former case involves forced convection, and the latter case free convection. Combined convection occurs when both forced and free convection are present. The convection coefficient of surface heat transfer, a, defining the heat exchange in the contact boundary layer between fluid and soUd, is determined. Coefficient or is often expressed by equations containing criteria numbers, such as those of Nusselt (Nu), Prandtl (Pr), Reynolds (Re) and Grashof(Gr) ... 

Jaturonglumlert and Kiatsiriroat (2010) considered heat and mass transfer for the combined convective and FIR drying of fruit leather, and found that the ratio between the heat and mass transfer coefficients for the combination technique could not be obtained from the heat-mass transfer analogy. Hence, modified correlations for predicting the ratio of heat and mass transfer coefficients in term of the heat transfer Nusselt number were developed. 

In 2004, Bettis, with the help of the Pennsylvania State University (PSU), performed a literature search in the area of forced an mixed convection heat transfer. Bettis and PSU were able to develop a laminar heat transfer correlation relating calculated Nusselt numbers to their geometiy dependent counterparts by combining the experimental results for three separate geometries. The Reynolds number dependent laminar Nusselt number takes the form ... 

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