Average heat transfer coefficient

Frequently experimental data report average heat-transfer coefficients based upon an arbitrarily denned temperature difference, the two most common being... 

In general, the average heat-transfer coefficient on immersed bodies is predicted by... 

Panel Coils Immersed in Liquid Overall Average Heat-Transfer Coefficients ... 

For low values of the Reynolds number, such as 10, where sn eamline flow should certainly apply, the Nusselt number has a value of about 2, and a typical value of the average heat transfer coefficient is 10 ". For a Reynolds number of 104, where the gas is certainly in turbulent flow, the value of the Nusselt number is typically 20. Hence there is only a difference of a factor of ten in the heat transfer coefficient between tlrese two extreme cases. 

It may be used, the relation of the time-averaged heat transfer coefficients on the top and bottom, as a criterion for determination of dryout. It was assumed that the relation he/hi < 1 indicates dryout, i.e., the surface superheat Tw -7f is greater than that, when the surface contacts single-phase water only (hg is the heat transfer at the bottom of the channel). This method can be applied to connect dryout with hydraulic conditions, if the value of he may be associated with intermittent flow parameters. 

Figure 1. Average heat transfer coefficients at surface of horizontal tube in bubbling fluidized bed. (From Chandran, Chen and Staub, 1980.)...

Figure 4. Effect ofmixed particle sizes, (a) Particle size distribution, (b) Circumferential-average heat transfer coefficients. (Data ofBiyikli and Chen, 1982).

The first type of model considers the heat transfer surface to be contacted alternately by gas bubbles and packets of closely packed particles. This leads to a surface renewal process whereby heat transfer occurs primarily by transient conduction between the heat transfer surface and the particle packets during their time of residence at the surface. Mickley and Fairbanks (1955) provided the first analysis of this renewal mechanism. Treating the particle packet as a pseudo-homogeneous medium with solid volume fraction, e, and thermal conductivity (kpa), they solved the transient conduction equation to obtain the following expression for the average heat transfer coefficient due to particle packets,... 

Noe, A. R., and Knudsen, J. G., Local and Average Heat Transfer Coefficients in a Fluidized Bed Heat Exchanger, Chem. Eng. Prog. Symp. Series, 64(82) 202-211 (1968)... 

Currently, there are no correlations available between the surface wave function, the film thickness and the average heat transfer coefficient that successfully describe the experimental results. The increase in the average heat transfer coefficient for an increasing rotational speed may be due to better shear mixing, resulting from thinner films and smaller and more concentrated surface waves. Similar phenomena have been observed by both... 

The average heat-transfer coefficient is thereafter calculated from... 

Therefore, we obtain the following expressions for the average heat transfer coefficient for two extreme cases For rapid renewal when fc < C we have... 

As h x0 25, the average heat transfer coefficient from 0 to L is given by h = 4hL/3. The average Nusselt number is then... 

By choosing the appropriate average property values in these correlations, demonstrate that the average heat transfer coefficient may be expressed as... 

Eq. (1.2) defined the local heat transfer coefficient, ht in terms of the local rate of heat transfer rate per unit area, the local wall temperature and, in those cases where it is changing, the local fluid temperature. In general, all of these quantities, i.e., h, q, Tw, and Tf, vary with position on the surface. For the majority of applications it is convenient to define, therefore, a mean or average heat transfer coefficient, h, such that if Q is the total heat transfer rate from the entire surface of area A then ... 

In this case, the average heat transfer coefficient for the pipe will depend on both and D. If dimensionless analysis is applied to this problem, assuming for simplicity that the buoyancy force effects are negligible, then one possible result would be ... 

It will be seen from the above that the average heat transfer rate for the entire plate surface is twice the local heat transfer rate existing at the end of the plate, i.e., at x L. The relation between the local and average heat transfer coefficients is shown diagrammatically in Fig. 3.6. 

A vertical plate 10 cm high is immersed in a stagnant fluid. The plate is maintained at a temperature of 50°C and the fluid temperature is 10°C. Determine the average heat transfer coefficient for this situation if the fluid is ... 

This improved estimate of the Reynolds number can now be used in Eq. (11.42) to recalculate the average heat transfer coefficient,... 

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