Heat transfer, by forced convection

Critoph, R.E. and Thorpe, R.N., Momentum and heat transfer by forced convection in fixed beds of granular active carbon. Applied Thermal Engineering, 1996, 16,419 427. 

Thorpe, R.N., Heat transfer by forced convection in beds of granular adsorbent material for solid adsorption heat pumps. Ph.D. Thesis, University of Warwick, UK, 1996. 

Assuming spherical prills and heat transfer by forced convection, the heat transfer coefficient between the prill and air can be determined from a relation of the form ... 

Fand, R. M. Heat Transfer by Forced Convection from a Cylinder to Water in Crossflow, Int. J. Heat Mass Transfer, vol. 8, p. 995, 1965. 

In Chapters 7 and 8, we considered heat transfer by forced convection, where a fluid was forced to move over a surface or in a tube by external means such as a pump or a fan. In this chapter, we consider natural convection, where any fluid motion occurs by natural means such as buoyancy. The fluid motion in forced convection is quite noiicenhle, since a fan or a pump can transfer enough momentum to the fluid to move it in a certain direction. Tlie fluid motion in natural convection, however, is often not noticeable because of the low velocities involved. 

Heat transfer by forced convection inside micro tube, generally referred as the Graetz problem, has been extended by Barron et al. [11] and Larrode and al. [12] to include the velocity slip described by Maxwell in 1890 [13] and the temperature jump [14] on tube surface, which are important in micro scale at ordinary pressure and in rarefied gases at low-pressure. 

This relation is analogous to the expression for the heat transfer by forced convection given earlier. The dimensionless group kd/D corresponds to the Nusselt group in heat transfer. The parameter rj/pD is known as the Schmidt number and is the mass-transfer counterpart of the Prandtl number. For example, the evaporation of a thin liquid film at the wall of a pipe into a turbulent gas is described by the equation... 

HEAT TRANSFER BY FORCED CONVECTION IN LAMINAR FLOW... 

Heat Transfer by Forced Convection in Laminar Flow 333... 

The rate of heat transfer by forced convection, steam condensation, and boiling is significantly improved by vibrating the heat transfer surface. 

Heat Transfer between a Fluid and the External Surfece of a Tube (Cylinder) Heat transfer by forced convection from a fluid to the surface of a cylinder for cross flow is given by the following empirical correlation (Cengel, 2002) ... 

Heat transfer by forced convection is characterized by the following parameters ... 

The term multiplying 39/3 is also dimensionless and is called the Peclet number, Pe, and represents the ratio of the heat transfer by forced convection to that by conduction. The boundary and initial conditions given in Eq. 5.20 become... 

Bulk polymerization offers real hazards. The thermal conductivities of monomers and polymers are low, and the viscosity buildup limits heat transfer by forced convection. Removal of unreacted monomer from the final product is difficult because of the low surface-to-volume ratio. However, the level of impurities can be held down by use of low initiator levels and diligent monomer removal. 

Figure 10-171. How air velocity over heated pipe increases heat transfer through forced convection. (Used by permission Chapman, F. S., and Holland, F. A. Chemical Engineering, Dec. 20, 1965, p. 79. McGraw-Hill, Inc. All rights reserved.)...

Single-phase flow region at the inlet the liquid is below its boiling point (sub-cooled) and heat is transferred by forced convection. The equations for forced convection can be used to estimate the heat-transfer coefficient in this region. 

The normal practice in the design of forced-convection reboilers is to calculate the heat-transfer coefficient assuming that the heat is transferred by forced convection only. This will give conservative (safe) values, as any boiling that occurs will invariably increase the rate of heat transfer. In many designs the pressure is controlled to prevent any appreciable vaporisation in the exchanger. A throttle value is installed in the exchanger outlet line, and the liquid flashes as the pressure is let down into the vapour-liquid separation vessel. 

We stait this chapter with a discussion of the physical mechanism of natural convection and the Grashof number. We then present the correlations to evaluate heat transfer by natural convection for various geometries, including fmned surfaces and enclosures, (finally, we discuss simultaneous forced and natural convection. 

The presence of a temperature gradient in a fluid in a gravity field always gives rise to natural convection currents, and thus heat transfer by natural convection. Therefore, forced convection is always accompanied by natural convection. 

Cox et al. [101] used several kinds of enhanced tubes to improve the performance of horizontal-tube multiple-effect plants for saline water conversion. Overall heat transfer coefficients (forced convection condensation inside and spray-film evaporation outside) were reported for tubes internally enhanced with circumferential V grooves (35 percent maximum increase in U) and protuberances produced by spiral indenting from the outside (4 percent increase). No increases were obtained with a knurled surface. Prince [102] obtained a 200 percent increase in U with internal circumferential ribs however, the outside (spray-film evaporation) was also enhanced. Luu and Bergles [15] reported data for enhanced condensation of R-113 in tubes with helical repeated-rib internal roughness. Average coefficients were increased 80 percent above smooth-tube values. Coefficients with deep spirally fluted tubes (envelope diameter basis) were increased by 50 percent. 

Natural convection heat transfer occurs when a solid surface is in contact with a gas or liquid which is at a different temperature from the surface. Density differences in the ffuid arising from the heating process provide the buoyancy force required to move the ffuid. Free or natural convection is observed as a result of the motion of the fluid. An example of heat transfer by natural convection is a hot radiator used for heating a room. Cold air encountering the radiator is heated and rises in natural convection because of buoyancy forces. The theoretical derivation of equations for natural convection heat-transfer coefficients requires the solution of motion and energy equations. 

Temperature can be measured from heat transfer by conduction, convection, or radiation. Household thermometers use either the expansion of metals or other substances or the increase in resistance with temperature. Thermocouples measure the electromotive force generated by temperature difference. Pyrometers measure infrared radiation from a heat source. Spectroscopic thermometry compares the spectrum of radiation against a blackbody spectrum. Temperature-sensitive paints and liquid crystals change intensity of radiation in certain wavelengths with temperature. 

Convection involves the transfer of heat by the motion and mixing of macroscopic portions of a fluid (i.e., the flow of a fluid past a solid boundary). The term natural convection is used if this motion and mixing is cansed by density variations resnlting from temperature differences within the fluid. The term forced convection is used if this motion and mixing is cansed by an outside force, such as a pump or fan (as shown in Figure 6.4). The transfer of heat from a hot-water radiator to a room is an example of heat transfer by natural convection. The transfer of heat from the surface... 

---