Mixed convection regimes

The domain of the mixed convection regime depends on the fluid the flow configuration and the flow pattern (17). It is usually defined by a region a < Gr/Re < b where a and b are the lower and upper bounds of the domain respectively and Gr/Re is the buoyanct force parameter n being a constant that varies with the flow configuration. Conversely when the buoyant force is the dominant mode of transport Re /Gr or some power of it becomes the important parameter for mixed convection. 

The mean heat transfer in the mixed convection regime for assisting flows has been correlated by Churchill [55] using the equation... 

Horizontal Flow. For laminar flow over the upper surface of a horizontal heated plate (or over the bottom surface of a cooled plate), the center of the mixed convection regime can again be estimated by equating the forced convection Nusselt number from Eq. 4.154 to that for natural convection from Eq. 4.39c (for detached turbulent convection). This results in... 

The details of the flow in the mixed convection regime have been clarified by Gilpin et al. [113]. After an initial development of the laminar forced convection boundary layer, rolls with axes aligned with the flow appear at the location marked Onset in Fig. 46. These persist until the end of the transition regime, marked Breakup, after which the motion appears as fully detached turbulent natural convection flow. 

Horizontal Cylinders. For a heated horizontal cylinder in perpendicular cross flow, the angle of the approaching stream, ( > in Fig. 4.47, greatly affects the heat flow in the mixed convection regime. For ( > = 0 the forced flow assists the natural convection and the dependence of the average Nus-selt number on Re resembles path A in Fig. 4.44. For = 90° there is a sharper transition from natural to forced convection than when 4> = 0, while for opposed flow (( > = 180°) there is a minimum as shown by path B in Fig. 4.44. For a cooled cylinder the same description applies except that ( > is measured from the vertical axis extending upward from the cylinder. 

A procedure for calculating the heat transfer in the mixed convection regime for the problem has also been proposed by Morgan [198] on the basis of work of Borner [22] and Hatton et al. [131]. For a given Ra and Re, the value of Nuw is computed from Eq. 4.45. For the given Re, the constants a and n are chosen from Table 4.14. The value of Re,- is then found from Eq. 4.163 with Nuf = Nuw that is,... 

On the basis of data available up to 1964, Metais and Eckert [190] established the forced convection boundary of the mixed convection regime, and their results are presented in Fig. 4.50. The line was drawn where natural convection was thought to alter the heat transfer from that for pure forced convection by 10 percent. Figure 4.49 defines the nomenclature for this problem. 

Heat transfer relations in the mixed convection regime have been proposed by several investigators, but the equation proposed by Depew and August [74] appears to be most successful. This relation agrees with their measurements for water (Pr = 6.5), ethyl alcohol (Pr 15), and a glycerol-water mixture (Pr = 375), as well as with the data from other authors, to about 40 percent ... 

Regimes for purely forced, purely free, and mixed convection for flow through vertical pipes... 

Hsieh, J.C., Chen. T.S.. and Armaly. B.F.. Mixed Convection Along a Nonisothermal Vertical Rat Plate Embedded in a Porous Medium the Entire Regime. Int. J. Heat Mass Transfer, Vol. 36, No. 7, pp. 1819-1825, 1993. 

According to Fig. 7-14, the mixed-convection-flow regime is encountered. Thus we must use Eq. (7-67). The Graetz number is calculated as... 

In a cold wall reactor, the convection regime is mixed. On the one hand, the temperature gradient between the substrate and the walls of the reactor tends to establish a system of natural convection (laminar flow). On the other hand, the flow of gas induces a forced convection (turbulent flow). A laminar flow is necessary to ensure a good uniformity of the epitaxial film thickness. 

FIGURE 4J0 Regimes of forced and mixed convection for flow through horizontal tubes with uniform wall temperature, for 10 2 < (Pr DIL) < 1. From Metais and Eckert [190]. 

Vertical Tubes. The flow regime chart for vertical tube flow shown in Fig. 4.51 was prepared by Metais and Eckert [190] for either a uniform-heat-flux or uniform-wall-temperature boundary condition. The two boundaries of the mixed convection are defined in such a way... 

For prediction of subassembly coolant flow rate and temperature distributions a wide range of coolant flow and thermal convection regimes must be considered including laminar and turbulent flow natural, forced and mixed (forced + natural) convection and steady state and transient reactor conditions. 

Separation layer mixers use either a miscible or non-miscible layer between the reacting solutions, in the first case most often identical with the solvent used [48]. By this measure, mixing is postponed to a further stage of process equipment. Accordingly, reactants are only fed to the reaction device, but in a defined, e.g. multi-lamination-pattem like, fluid-compartment architecture. A separation layer technique inevitably demands micro mixers, as it is only feasible in a laminar flow regime, otherwise turbulent convective flow will result in plugging close to the entrance of the mixer chamber. 

The separation-layer technique benefits from the unique feature of micro mixers, such as to operate in a laminar flow regime [135], By the absence of convective recirculation patterns, at least close to the inlet, the separation layer remains as a barrier between the solution to be mixed, as long as it is not passed by molecules owing to diffusive transport. 

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