Dittus Boelter equation

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. 

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

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). 

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. 

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 ... 

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. 

Note that Equation (9.85) is basically the same general form as the familiar Dittus-Boelter equation for heat transfer in tubes. The basic heat-transfer mechanism is identical. It is dependent on the flow of fluid next to the heat-transfer surfaces, whether these are the vessel walls or some internals. Differences in the correlations are therefore mainly due to the differences in flow characteristics generated by the different impellers relative to the surface under consideration. This is reflected in the value of K. 

EMPIRICA QUATIONS. To use Eq. (12.27) or (12.28), the function 0> or i must be khovihu. One empirical correlation for long tubes with sharp-edged entrances is the Dittus-Boelter equation... 

From equation (2-74), Sh = 51.7 from Table 2.1, kc = 0.0192 mol/m2-s-kPa. To estimate the heat-transfer coefficient, use the Dittus-Boelter equation for cooling (Incropera et al., 2007) ... 

The coil inside coefficient is given by the Dittus-Boelter equation with a correction for curvature of the helical coil [3] ... 

The inside film coefficient, hi, may be estimated using the correlations available in the literature such as the well known Dittus-Boelter equation for low viscosity coolants, as recommended in the literature [Coulson and Richardson, 1999] ... 

In double-pipe and shell-and-tube heat exchangers, fluids flow through straight, smooth pipes and tubes of circular cross section. Many correlations have been published for the prediction of the inside-wall, convective heat transfer coefficient, /i when no phase change occurs. For turbulent flow, with Reynolds numbers, = D,G/ ji, greater than 10,000, three empirical correlations have been widely quoted and applied. The first is the Dittus-Boelter equation (Dittus and Boelter, 1930) for liquids and gases in fully developed flow (Z>,/L < 60), and with Prandtl numbers, = Cp[iJk, between 0.7 and 100 ... 

The Colburn equation (Colburn, 1931) also applies to liquids and gases and is almost idra-tical to the Dittus-Boelter equation, but is usually displayed in a j-factor form in terms of a Stanton number, = hJGCp. It is considered valid to a Prandtl number of 160 ... 

Several methods of computing an analytical Reynolds number for the two-phase flow were tried. The fluid properties were evaluated at various temperatures including wall, film and average bulk conditions. Both gaseous and two-phase densities were related to the above temperatures. These Reynolds numbers along with corresponding Prandtl numbers were used in calculating an estimated Nusselt number by the Dittus-Boelter equation. 

NUf = Nusselt number computed from Dittus-Boelter equation and (1)... 

The principal uncertainty in non-Newtonian heat transfer in the transition and turbulent region is the criterion for the onset of turbulence and range of the transition region. Data taken in fully developed turbulent flow with a variety of solid-liquid suspensions may be correlated satisfactorily with the Dittus-Boelter equation (or variations of it), with some uncertainty about the best viscosity to use in the correlation [138-142]. 

In obtaining the heat-transfer coefficients for comparison with bismuth, the sodium coefficients were calculated from the Martinelli-Lyon relationship. The coefficients for molten salt and water were calculated from the conventional Dittus-Boelter equation. 

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