Convection film coefficients

Yazdanian, M., and Klems, J. (1994). Measurement of the Exterior Convective Film Coefficient for Windows m Low-Rise Buildings. ASIIRAE Transactions 100. 

For organic liquids, evaluate the natural convection film coefficient from Figure 10-103. Equation 10-29 may be used for the inside horizontal tube by multiplying the right side of the equation by 2.25 (1 + 0.010 Gr,i/")/logRe. 

Rohsenow and Hartnetd recommend the Briggs and Young convection film coefficient relation for externally finned tubes. 

HORIZONTAL TUBES NATURAL convection FILM coefficient FDR PETROLEUM FRACTIONS CHi h Visco9i-tj Ra ge)... 

Igure 2-43. Natural convection film coefficient for petroleum Figure 2-44. Natural convection film coefficient for petroleum actions in the low viscosity range—horizontal tubes. fractions in the high viscosity range—horizontal tubes. 

Using a procedure similar to that used to develop the simplified equation for horizontal tubes, the natural convection film coefficient for vertical tubes is ... 

Natural convection film coefficient, Btu/ hr. sq. ft.°F Specific gravity... 

NATURAL CONVECTION FILM COEFFICIENT FDR PETROLEUM FRACTIONS... 

The forced convection film coefficient h and the frost surface temperature Ts must be known to evaluate (5) for comparison with the data reported in [ ]. In this study it was decided to modify an existing correlation for Nu to obtain an expression for h. Two thorough studies were considered and the Hegge-Zijnen expression arbitrarily chosen,... 

Figure 2.9 shows some convection (film) coefficients, he. Table 4.5 of reference 51 lists many specific values for he. 

Fig. 2.9 Convection (film) coefficients, ha for hot air or poc. F = flow parallel to a flat surface of length F D = flow across a cylinder of diameter D. Courtesy of North American Mfg. Co. (See also table 3.2.)...

Simultaneous Loss by Radiation The heat transferred by radiation is often of significant magnitude in the loss of heat from surfaces to the surroundings because of the diathermanous nature of atmospheric gases (air). It is convenient to represent radiant-heat transfer, for this case, as a radiation film coefficient which is added to the film coefficient for convection, giving the combined coefficient for convection and radiation (h + hf In Fig. 5-7 values of the film coefficient for radiation are plotted against the two surface temperatures for emissivity = 1.0. 

The overall heat transfer coefficient, U, is a measure of the conductivity of all the materials between the hot and cold streams. For steady state heat transfer through the convective film on the outside of the exchanger pipe, across the pipe wall and through the convective film on the inside of the convective pipe, the overall heat transfer coefficient may be stated as ... 

Film Coefficients with Fluid Inside Tubes, Forced Convection... 

Figure 10-51. Convection inside film coefficient for gases and low viscosity fluids inside tubes—heating and cooling. (Used by permission McAdams, W. H. Heat Transmission, 2"= Ed., 1942. McGraw-Hill, Inc. All rights reserved.)...

Determine the inside film coefficient by methods previously oudined for convection. 

Determine the tube-side film coefficient for convection or condensation as required, by methods previously described. 

In most cases where convective heat transfer is taking place from a surface to a fluid, the circulating currents die out in the immediate vicinity of the surface and a film of fluid, free of turbulence, covers the surface. In this film, heat transfer is by thermal conduction and, as the thermal conductivity of most fluids is low, the main resistance to transfer lies there, Thus an increase in the velocity of the fluid over the surface gives rise to improved heat transfer mainly because the thickness of the film is reduced. As a guide, the film coefficient increases as (fluid velocity)", where 0.6 < n < 0.8, depending upon the geometry. 

Figure 9.21. Film coefficients of convection for flow of water through a tube at 289 K...

Equation 9.1-15 equates the rate of heat transfer by conduction at the surface to the rate of heat transfer by conduction/convection across a thermal boundary layer exterior to the particle (corresponding to the gas film for mass transfer), expressed in terms of a film coefficient, h, and the difference in temperature between bulk gas at Tg and particle surface at Ts ... 

When a fluid is heated, the hot less-dense fluid rises and is replaced by cold material, thus setting up a natural convection current. When the fluid is agitated by some external means, then forced convection takes place. It is normally considered that there is a stationary film of fluid adjacent to the wall and that heat transfer takes place through this film by conduction. Because the thermal conductivity of most liquids is low, the main resistance to the flow of heat is in the film. Conduction through this film is given by the usual relation (74), but the value of h is not simply a property of the fluid but depends on many factors such as the geometry of the system and the flow dynamics for example, with tubes there are significant differences between the inside and outside film coefficients. 

For a straight tube immersed in a well-stirred fluid, the inside film coefficient, h, may be calculated from the Nusselt number given by the general relationship applying to forced convection in tubes when Re > 10 and0.7[Pg.30]

The gas film coefficient due io free convection (Figure 8.15a) is described by the relation of Shulyakovskyi (1969) ... 

The relationships developed from field measurements have been made dimensionless with the assumptions that v = 1.33 x 10 m /s and AijO = 2.6 x 10 m /s to facilitate comparisons between relations and avoid dimensional problems. They are given in Table 9.2. The early measurements were to investigate the loss of water from the reservoirs of the Colorado River in the United States, and the later measurements were designed to investigate heat loss from heated water bodies. A revelation occurred in 1969, when Shulyakovskyi brought in buoyancy forces as related to natural convection to explain the heat loss from heated water at low wind velocities. This was picked up by Ryan and Harleman (1973), who realized that natural convection could explain the need for a constant term in front of the relationship for gas film coefficient, as had been found by Brady et al. (1969), Kohler (1954), Rymsha and Dochenko (1958), and Shulyakovskyi (1969). Finally, Adams et al. (1990) rectified... 

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... 

In figuring heat transfer between equipment and surroundings, it is adequate to take account of the resistances of only the insulation and the outside film. Coefficients of natural convection are in Table 8.9 and properties of insulating materials at several... 

The convective heat transfer coefficients hi and h0 must be calculated from equations that involve the geometry of the system, the physical properties of the fluid, and the velocity with which it is flowing. These equations are obtained variously by more or less fundamental analysis of the heat transfer and fluid flow mechanisms, or by correlation of experimental data, or by combinations of these methods. A few typical values of the film coefficients are... 

The heat transfer rate, q, is taken as positive in the direction wall-to-fluid so that it will have the same sign as(Tw -T/) and h will always, therefore, be positive. A number of names have been applied to h including convective heat transfer coefficient , heat transfer coefficient , film coefficient , film conductance , and unit thermal convective conductance . The heat transfer coefficient, h, has the units W/m2-K or, since its definition only involves temperature differences, W/m2oC, in the SI system of units. In the imperial system of units, h has the units Btu/ft2-hr-°F. 

Table 4 presents equations for use in the estimation of heat-transfer film coefficients for condensation, boiling, and natural convection. Values showing the general range of film coefficients for various situations are indicated in... 

Gs = shell-side mass velocity across tubes based on the minimum free area between baffles across the shell axis, lb/(hXft2) h = film coefficient of heat transfer, Btu/(hXft2X°F) subscript c indicates convection subscript d represents dirt or fouling subscript co indicates conduction... 

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