Enhanced heat transfer surface

FIGURE 1L5 Pool boiling from smooth and structured surfaces on the same apparatus [40]. (a) sketch of cross sections of three enhanced heat transfer surfaces tested (b) boiling curves for three enhanced tubes and smooth tube. 

Greater efficiency resulting from enhanced heat transfer surfaces and better energy economy. 

Enhanced heat transfer surfaces must be provided with adequate venting in order to fully achieve the benefits of the enhancement. Higher rates of heat transfer suffer greater loss of efficiency for the same inert load. 

Another major area in which improvements are certain is in the application of enhanced heat transfer surfaces. Enhancement will enable more efficient operation and also permit the use of lower grade heat. Consequently, streams at temperatures not normally considered usable will be used to provide energy for evaporation. 

For ice on coil type ice storage tank, water inside the tank is sometimes agitated to enhance heat transfer on coil surface. Small water or air pumps are used for this purpose. This agitation is effective until ice is formed on the coil, because temperature during ice forming is uniformly freezing point. Degree... 

The second approach assigns thermal resistance to a gaseous boundary layer at the heat transfer surface. The enhancement of heat transfer found in fluidized beds is then attributed to the scouring action of solid particles on the gas film, decreasing the effective film thickness. The early works of Leva et al. (1949), Dow and Jacob (1951), and Levenspiel and Walton (1954) utilized this approach. Models following this approach generally attempt to correlate a heat transfer Nusselt number in terms of the fluid Prandtl number and a modified Reynolds number with either the particle diameter or the tube diameter as the characteristic length scale. Examples are ... 

In a nutshell, the performance of weapons and munitions increases with the use of nanosized particles because of the increased surface area and enhanced heat transfer resulting in reduced ignition delay, burn time, improved mechanical properties and high density-specific impulse. Further, formulations based on micron-sized materials with a wide distribution suffer from defects such as slow energy release, incomplete combustion and inability to support rapid combustion which can be overcome with the use of nanoparticles or nanomaterials [102]. 

In general, on increasing the reactor volume, the required heat transfer surface increases faster than S3 this effect is enhanced by the decrease of the heat transfer coefficient. This prescription cannot be obeyed by the lateral surface of the reactor (which increases as S2) so that an internal or external additional heat exchange surface, whose dimensions can be fixed independently from the reactor dimensions, must be provided. 

In this chapter, emphasis will be given to heat transfer in fast fluidized beds between suspension and immersed surfaces to demonstrate how heat transfer depends on gas velocity, solids circulation rate, gas/solid properties, and temperature, as well as on the geometry and size of the heat transfer surfaces. Both radial and axial profiles of heat transfer coefficients are presented to reveal the relations between hydrodynamic features and heat transfer behavior. For the design of commercial equipment, the influence of the length of heat transfer surface and the variation of heat transfer coefficient along the surface will be discussed. These will be followed by a description of current mechanistic models and methods for enhancing heat transfer on large heat transfer surfaces in fast fluidized beds. Heat and mass transfer between gas and solids in fast fluidized beds will then be briefly discussed. 

Fins are used to enhance heat transfer, and the use of fins on a surface cannot be recommended unless the enhancement in heat ivansfer justifies the added cost and complexity associated with the fins. In fact, there is no assurance that adding fins on a surface will enhance heat transfer. The performance of the fins is judged on the basis of the enhancement in heat transfer relative to the no-fin case. The performance of fins is expressed in terms of Ihe fin effectiveness Cf. defined as Fig. 3-44. 

Finned surfaces are commonly used in practice to enhance heat transfer, and they often increase the rate of heat transfer from a surface severalfold. 

Fins enhance heat transfer from a surface by enhancing surface area. 

Finned surfaces are commonly used in practice to enhance heat transfer. Fins enhance heal transfer from a surface by exposing a larger surface area to convection. The temperature distribution along the fin for very long fins and for fins with negligible heat transfer at the fin tip arc given by... 

SC Explain how the fins enhance heat transfer from a surface. Also, explain how the addition of fins may actually decrease heat transfer from a surface,... 

Enclosures arc frequently encountered in practice, and heal transfer through them is of practical interest. Heat transfer in enclosed spaces is complicated by the fact that the fluid in the enclosure, in general, does not remain stationary. In a vertical enclosure, the fluid adjacent to the hotter surface rises and the fluid adjacent to the cooler one falls, setting off a rolationary motion within the enclosure that enhances heat transfer through the enclo.surc. Typical flow patterns in vertical and horizontal rectangular enclosures are shown in Figs. 9-21 and 9-22. 

In assisting flow, the buoyant motion is in the same direction as the forced motion. Therefore, natural convection assists forced convection and enhances heat transfer, An example is upward forced flow over a hot surface. 

The pool boiling heal transfer relations given above apply to smooth surfaces Helow we discuss some methods to enhance heat transfer in pool boiling. 

When (he tube is firmed on one side to enhance heat transfer, the total heat transfer surface area on the finned side becomes... 

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