Dittus-Boelter

The mathematical formulation of forced convection heat transfer from fuel rods is well described in the Hterature. Notable are the Dittus-Boelter correlation (26,31) for pressurized water reactors (PWRs) and gases, and the Jens-Lottes correlation (32) for boiling water reactors (BWRs) in nucleate boiling. 

In the macroscopic heat-transfer term of equation 9, the first group in brackets represents the usual Dittus-Boelter equation for heat-transfer coefficients. The second bracket is the ratio of frictional pressure drop per unit length for two-phase flow to that for Hquid phase alone. The Prandd-number function is an empirical correction term. The final bracket is the ratio of the binary macroscopic heat-transfer coefficient to the heat-transfer coefficient that would be calculated for a pure fluid with properties identical to those of the fluid mixture. This term is built on the postulate that mass transfer does not affect the boiling mechanism itself but does affect the driving force. 

Najjar, Bell, and Maddox studied the influence of physical property data on calculated heat transfer film coefficients and concluded that accurate fluid property data is extremely important when calculating heat transfer coefficients using the relationships offered by Dittus-Boelter, Sieder-Tate, and Petukhov. Therefore, the designer must strive to arrive at good consistent physical/thermal property data for these calculations. 

E. For heating and cooling turbulent gases and other low viscosity fluids at DG/p > 8,000 the Dittus-Boelter relation is used. See Figures 10-46, 10-51, and 10-52. 

The convection heat transfer rate inside the tubes is expressed by the Dittus-Boelter equation ... 

Data on thermal performance are not readily available on all heat exchangers because of the proprietary nature of the machines. To exemplify typical thermal data, heat transfer can best be described by a Dittus-Boelter type equation ... 

The values of /"fh and ffc (the film resistances for the hot and cold fluids, respectively) can be calculated from the Dittus-Boelter equations previously described and the wall metal resistance / from the average metal thickness and thermal conductivity. The fouling resistances of the hot and cold fluids /"dh and are often based on experience, but a more detailed discussion of this will be presented later in this chapter. 

Wtnterton"5 has looked into the origins of the Dittus and Boelter equation and has found that there is considerable confusion in the literature concerning the origin of equation 9.61 which is generally referred to as the Dittus-Boelter equation in the literature on heat transfer. 

For the case of turbulent flow the Dittus-Boelter(9) correlation given in Volume 1, Chapters 9 and 10, is used ... 

The thermodynamic approach does not make explicit the effects of concentration at the membrane. A good deal of the analysis of concentration polarisation given for ultrafiltration also applies to reverse osmosis. The control of the boundary layer is just as important. The main effects of concentration polarisation in this case are, however, a reduced value of solvent permeation rate as a result of an increased osmotic pressure at the membrane surface given in equation 8.37, and a decrease in solute rejection given in equation 8.38. In many applications it is usual to pretreat feeds in order to remove colloidal material before reverse osmosis. The components which must then be retained by reverse osmosis have higher diffusion coefficients than those encountered in ultrafiltration. Hence, the polarisation modulus given in equation 8.14 is lower, and the concentration of solutes at the membrane seldom results in the formation of a gel. For the case of turbulent flow the Dittus-Boelter correlation may be used, as was the case for ultrafiltration giving a polarisation modulus of ... 

The procedure of taking a ratio of two coefficients, as done in equations (91) and (92), using a Dittus-Boelter equation for the hypothetical liquid-only case, leads to the relation ... 

Application of dimensional analysis to Eq. (42) leads to the dimensionless groups of the Dittus-Boelter equation for Eq. (43), one obtains... 

The tube-side heat-transfer coefficient may be calculated in two ways. The first uses the Dittus-Boelter equation for heat-transfer in a turbulent environment. This equation is given below. 

The outside him coefficient ha in the jacket can be estimated from the Dittus-Boelter equation for how through a pipe if a suitable equivalent diameter is used ... 

What is the Dittus-Boelter equation When does it apply ... 

Assuming the same temperatures and emissivities of the surfaces as in Prob. 8-56, estimate the heating or cooling required for the inner and outer surfaces to maintain them at these temperatures. Assume that the convection heat-transfer coefficient may be estimated with the Dittus-Boelter equation (6-4). 

When a liquid is forced through a channel or over a surface maintained at a temperature greater than the saturation temperature of the liquid, forced-convection boiling may result. For forced-convection boiling in smooth tubes Rohsenow and Griffith [6] recommended that the forced-convection effect be computed with the Dittus-Boelter relation of Chap. 6 [Eq. (6-4)] and that this effect be added to the boiling heat flux computed from Eq. (9-33). Thus... 

For computing the forced-convection effect, it is recommended that the coefficient 0.023 be replaced by 0.019 in the Dittus-Boelter equation. The temperature difference between wall and liquid bulk temperature is used to compute the forced-convection effect. 

The heat transfer coefficient for the fluid in the pipe is estimated using the Dittus-Boelter correlation ... 

For Npe > 10,000, 0.7 < Nf, < 170, for properties based on the bulk temperature and for beating, the Dittus-Boelter equation [Boel-ter. Cherry, Johnson and Martinelli, Heat Transfer Notes, McGraw-Hill, New York (1965)] may be used ... 

For fully-developed laminar forced convection in microchannels, Nu is proportional to Re° , while for the fully-developed turbulent heat transfer Nu is predicted by the Dittus-Boelter correlation by modif3ung only the empirical constant coefficient from 0.023 to 0.00805. 

If the Nusselt number is plotted as a function of the Reynolds number, the curve in figure 6 is obtained. One can see the constant value for laminar flow. In the turbulent regime the Gnielinski correlation is compared to a Dittus-Boelter type correlation. 

Recently, Phattaranawik et al. [48] have used several equations to estimate the heat transfer coefficient in laminar and turbulent flow regimes. They found that Equation 19.22 is the most suitable for laminar flow, while the Dittus-Boelter equation was most suitable for turbulent conditions. 

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