Surface heat transfer coefficient

The surface heat transfer coefficient is the quantity of heat flow per unit time normal on the surface across unit area at the interface between two materials with unit temperature difference across the interface. Obviously, it depends on the nature of this interface, and especially on the nature of the material that is in contact with the rubber. 

When a fluid surrounds the rubber piece, the effect of the coefficient h defined in Equation 2.29 is inversely proportional to the thickness of the rubber. 

When the rubber is surrounded with a mold with a good contact, Equation 2.26 holds, as it expresses that the values of the heat flux on both sides of the interface are equal. 

As said already in Part 2.3, considerable work has been done on the subject of heat transfer through a solid and a fluid, especially for evaluation of heat transfer systems [2], and various dimensionless numbers have been established, whatever the nature of the fluid motion. In any case, either with a motionless or a stirred fluid, the Niisselt number expressed in Equation 2.29, appears as the most important parameter. 

Perfect insulation, expressed by Equation 2.24, does not exist in practice. The only solution is to maintain the same temperature on both sides of the interface this solution was applied in measuring the thermal conductivity in order to avoid the heat loss from the apparatus. 

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Bed-to-Surface Heat Transfer. Bed-to-surface heat-transfer coefficients in fluidized beds are high. In a fast-fluidized bed combustor containing mostly Group B limestone particles, the dense bed-to-boiling water heat-transfer coefficient is on the order of 250 W/(m -K). For an FCC catalyst cooler (Group A particles), this heat-transfer coefficient is around 600 W/(600 -K). 

Circulation of air at velocities of I to 10 m/s is desirable to improve the surface heat-transfer coefficient and to eliminate stagnant air pockets. Proper air flow in tray dryers depends on sufficient fan capacity, on the design of ductwork to modify sudden changes in direction, and on properly placed baffles. Nonuniform airflow Is one of the most serious problems in the operation of tray di yers. 

The surface heat transfer coefficient can be dramatically increased since the gas space between fin or tube and the adsorbent can be greatly reduced or eliminated. 

Thermal characteristics of material layers for each type of wall must be specified, including thickness, conductivity, density, and specific heat. Moreover, the features of internal and external surfaces of each wall must be specified, including solar absorptance and roughness, which affect surface heat transfer coefficients. 

Nusselt Nu Thermal conductivity of fluid Dimension of surface Heat transfer coefficient... 

In general, gas-to-particle or particle-to-gas heat transfer is not limiting in fluidized beds (Botterill, 1986). Therefore, bed-to-surface heat transfer coefficients are generally limiting, and are of most interest. The overall heat transfer coefficient (h) can be viewed as the sum of the particle convective heat transfer coefficient (h ), the gas convective heat transfer coefficient (h ), and the radiant heat transfer coefficient (hr). 

Overall bed-to-surface heat transfer coefficient = Gas convective heat transfer coefficient = Particle convective heat transfer coefficient = Radiant heat transfer coefficient = Jet penetration length = Width of cyclone inlet = Number of spirals in cyclone = Elasticity modulus for a fluidized bed = Elasticity modulus at minimum bubbling = Richardson-Zaki exponent... 

Ksa = surface heat transfer coefficient between cooling medium and the freezing zone. 

The thermal conductivity of ice and of dried products is relatively well known, but the surface heat transfer coefficient, Ksu during freezing and the total heat transfer coefficient K(ot during freeze-drying vary largely as described in the various chapters. Table 1.3 gives a survey of some data of interest in freeze-drying. 

Table 1.3 Surface heat transfer coefficient, total heat transfer coefficient and thermal conductivity.

The experimental setup, described in Example 8.1, for calculating the bias in a dynamic environment will be used here to discuss the parameter estimation methodology. In this case both the surface heat transfer coefficient (h) and the thermal conductivity (A) of the body in the condition of natural convection in air are considered (Bortolotto et al., 1985). 

Normally, the heat-transfer rate is between 5 and 25 times that for the gas alone. Bed-to-surface-heat transfer coefficients vary according to the type of solids in the bed. Group A solids have bed-to-surface heat-transfer coefficients of approximately 300 J/(m2s-K) [150 Btu/(h-ft2-°F)]. Group B solids h ave bed-to-surface heat-transfer coefficients of approximately 100 J/(m2- s-K) [50 Btu/(h-ft2-°F)], while group D solids have bed-to-surface heat-transfer coefficients of 60 J/(m2-s-K) [30 Btu/(hft2 oF)]. 

Here o is the density of the reactant mixture (perhaps measured in units of kg m-3), c the specific heat capacity (J K-1 kg-1), S the surface area, and X the surface heat transfer coefficient (Wm 2K 1). We have assumed a Newtonian cooling term for the transfer of heat to the surroundings. 

Discuss how the surface heat-transfer coefficient h depends on the strain rate and the Prandtl number. 

Consider the behavior of the surface heat-transfer coefficient h for air (Pr = 0.7) in semi-infinite stagnation flow and semi-infinite rotating-disk flow. Assuming that Nu =... 

This is a functional equation for the boundary position X and the unknown constant parameter n. Upon substituting Eq. (256) into Eq. (251) an ordinary differential equation is obtained for X(t, n), and a family of curves in the phase plane (X, X) can be obtained. For n sufficiently close to unity two functions in the phase plane can be determined which serve as upper and lower bounds for the trajectories. The choice is guided by reference to the exact solution for the limiting case of constant surface temperature. It is shown that the upper and lower bounds are quite close to the one-parameter phase plane solution, although no comparison is made with a direct numerical solution. The one-parameter solution also agrees well with experiments on the solidification of aluminum under conditions of low surface heat transfer coefficient (hi = 0.02 cm.-1). 

The bed-to-surface heat transfer coefficient may vary with the radial position in the bed. Wender and Cooper (1958) correlated the data of coefficients for heat transfer to immersed vertical tubes and proposed the following empirical correlation for 0.01 < Rep < 100... 

Step 13. In this step, Ux, the step 1 assumed value, and the overall extended surface heat-transfer coefficient are calculated ... 

To determine whether the thin body approximation may be used, one should compare the surface heat transfer coefficient, and the thermal conductance of the solid, ksom/8. Their ratio is the Biot number,... 

The overall mass and heat interphasc exchange coefficients can be obtained by summing the resistances from bubble to cloud and from cloud to emulsion (dense phase) in series. The overall bed to surface heat transfer coefficient is assumed to be mostly atuibuted to... 

The inner surface heat transfer coefficient A, remains fairly constant throughout the year, but the value of varies considerably because of its dependence on the orientation and wind speed, which can vary from less than 1 km/h in calm weather to over 40 km/h during storms. The-commonly used values of h, and /i for peak load calculation.s are... 

Interior and Exterior Surface Heat Transfer Coefficients... 

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