Flow across cylinders

Forced convection outside tubes Flow across single cylinders... 

Air at I atm and 35°C flows across a 5.0-cm-diameter cylinder at a velocity of 50 m/s. The cylinder surface is maintained at a temperature of I50°C. Calculate the heat loss per unit length of the cylinder. 

A 5-cm-diameter cylinder maintained at 100°C is placed in a nitrogen flow stream at 2 atm pressure and 10°C. The nitrogen flows across the cylinder with a velocity of 5 m/s. Calculate the heat lost by the cylinder per meter of length. 

Air at 70 kPa and 20°C flows across a 5-cm-diameter cylinder at a velocity of 20 m/s. Compute the drag force exerted on the cylinder. 

Helium at l atm and 325 K flows across a 3-mm-diameter cylinder which is heated to 42S K. The flow velocity is 9 m/s. Calculate the heat transfer per unit length of wire. How does this compare with the heat transfer for air under the same conditions ... 

Air flows across a 4-cm-square cylinder at a velocity of 10 m/s. The surface temperature is maintained at 85°C. Free-stream air conditions are 20°C and 0.6 atm. Calculate the heat loss from the cylinder per meter of length. 

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 across cylinders and splieres is frequently encountered in practice. For example, the lubes in a shell-and-tube heat exchanger involve both internal flow through the lubes and external flow over the tubes, and both flows must be considered in the analysis of the heal exchanger. Also, many sports such as soccer, tennis, and golf involve flow over spherical balls. 

The characteristic length for a circular cylinder or sphere is taken to be the external diameter D. Thus, the Reynolds number is defined as Re = VD/v where V is Ihe uniform velocity of Ihe fluid as it approaches the cylinder or splicre. The critical Reynolds number for flow across a circular cylinder or sphere is about Re s 2 X 10. That is, the boundar) layer remains laminar for about Re < 2 X K) and becomes turbulent forRc 2 X l(y. ... 

Flows across cylinders and spheres, in general, involve flow separation, which is difficult to handle analytically. Therefore, such flows must be studied experimentally or numerically. Indeed, flow across cylinders and spheres has been studied experimentally by numerous investigators, and several empirical correlations have been developed for (he heat transfer coefficient. 

Figure 48-7 Designers treat radial wind forces as a static load on the stack. Such forces act this way As a steady wind flows across a cylinder, there is an uneven pressure distribution and a reversal of force, creating a suction on the leeward side.

Fig. 3.24 Local heat transfer coefficients in flow across a circular cylinder, according to Giedt [3.14]...

A third common technique for separating mixtures of proteins, as well as other molecules, is based on the principle that molecules dissolved in a solution will interact (bind and dissociate) with a solid surface. If the solution is allowed to flow across the surface, then molecules that interact frequently with the surface will spend more time bound to the surface and thus move more slowly than molecules that interact infrequently with the surface. In this technique, called liquid chromatography, the sample Is placed on top of a tightly packed column of spherical beads held within a glass cylinder. The nature of these beads determines whether the separation of proteins depends on differences in mass, charge, or binding affinity. 

---