Convection individual coefficient

Now that the overall coefficient U has been broken down into its component parts, each of the individual coefficients /q, hi, and hi must be evaluated. This can be done from a knowledge of the nature of the heat transfer process in each of the media. A study will therefore be made of how these individual coefficients can be calculated for conduction, convection, and radiation. 

For example, for heat transfer through a flat wall, where heat is transferred by convection and radiation from the wall, the overall heat transfer coefficient is related to the individual coefficients by... 

The magnitude of the individual coefficients will depend on the nature of the heat transfer process (conduction, convection, condensation, boiling, or radiation), on the physical properties of the fluids, on the fluid flow rates, and on the physical arrangement of the heat transfer surface. As the physical layout of the exchanger cannot be determined until the area is known, the design of an exchanger is of necessity a trial-and-error procedure. The steps in a typical design procedure are as follows ... 

Water is heated from 15 to 65°C in a steam-heated horizontal 50-mm-ID tube. The steam temperature is 120 C. The average Reynolds number of the water is 450, The individual coefficient of the water is controlling. By what percentage would natural convection increase the total rate of heat transfer over that predicted for purely laminar flow Compare your answer with the increase indicated in Example 12.4. 

The description of mass transfer requires a separation of the contributions of convection and mutual diffusion. While convection means macroscopic motion of complete volume elements, mutual diffusion denotes the macroscopically perceptible relative motion of the individual particles due to concentration gradients. Hence, when measuring mutual diffusion coefficients, one has to avoid convection in the system or, at least has to take it into consideration. 

Specific correlations of individual film coefficients necessarily are restricted in scope. Among the distinctions that are made are those of geometry, whether inside or outside of tubes for instance, or the shapes of the heat transfer surfaces free or forced convection laminar or turbulent flow liquids, gases, liquid metals, non-Newtonian fluids pure substances or mixtures completely or partially condensable air, water, refrigerants, or other specific substances fluidized or fixed particles combined convection and radiation and others. In spite of such qualifications, it should be... 

Direct evaluation of the convective heat transfer coefficient (h ) of subjects clothed in undergarments and socks (normal ventilated environment) was achieved by observing the sublimation rate of naphthalene balls uniformly positioned three centimeters from the body surface. Equations were developed for prediction of h as a function of metabolic activity and posture, calculation o average skin temperature, and estimation of maximum evaporative heat losses from the body (U2 ). In another approach, the coefficients of dry heat transfer at varying wind speeds for nude and clothed sectional mannequins were determined (U3). At air flow rates above 2 m/sec, percentage contributions of individual body sections to total heat transfer remain constant for the nude and clothed mannequin, yet increased for normally uncovered units such as the face and hands. Generally, the ratio of total heat flow for the nude to clothed mannequin increased with air flow. 

Individual Heat-Transfer Coefficient The local rate of convective heat transfer between a surface and a fluid is given by Newton s law of cooling... 

Individual heat transfer coefficients for convection are defined by the equation ... 

It is commonly observed that increases in average heat transfer occur at a sound pressure level of about 134 to 140 dB (well above the normal human tolerance of 120 dB), and that these increases are associated with the formation of an acoustically induced flow (acoustic or thermoacoustic streaming) near the heated surface. Large circumferential variations in heat transfer coefficient are present [298], and it has been observed that local improvements in heat transfer occur at intensities well below those that affect the average heat transfer [299]. Correlations have been proposed for individual experiments however, an accurate correlation covering the limits of free convection and fully developed vortex motion has not been developed. 

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