Heat transfer entrance region

Entrance andExit SpanXireas. The thermal design methods presented assume that the temperature of the sheUside fluid at the entrance end of aU tubes is uniform and the same as the inlet temperature, except for cross-flow heat exchangers. This phenomenon results from the one-dimensional analysis method used in the development of the design equations. In reaUty, the temperature of the sheUside fluid away from the bundle entrance is different from the inlet temperature because heat transfer takes place between the sheUside and tubeside fluids, as the sheUside fluid flows over the tubes to reach the region away from the bundle entrance in the entrance span of the tube bundle. A similar effect takes place in the exit span of the tube bundle (12). 

Limiting Nusselt numbers for laminar flow in annuli have been calculated by Dwyer [Nucl. Set. Eng., 17, 336 (1963)]. In addition, theoretical analyses of laminar-flow heat transfer in concentric and eccentric annuh have been published by Reynolds, Lundberg, and McCuen [Jnt. J. Heat Ma.s.s Tran.sfer, 6, 483, 495 (1963)]. Lee fnt. J. Heat Ma.s.s Tran.sfer, 11,509 (1968)] presented an analysis of turbulent heat transfer in entrance regions of concentric annuh. Fully developed local Nusselt numbers were generally attained within a region of 30 equivalent diameters for 0.1 < Np < 30, lO < < 2 X 10, 1.01 <... 

The temperature maps shown in Fig. 20 illustrate the development of the temperature field as the flow enters a tube heated at the wall. The first (left-hand) map shows the initial heating of the gas at the tube entrance. The development of the boundary layer near the walls is clear and represents one contribution to the heat transfer resistance in the wall region. The more rapid... 

In Ulrichson and Schmit s work on laminar flow heat transfer in the entrance region of circular tubes the following results were obtained. 

Sparrow, E.M., Analysis of Laminar - orced Convection Heat Transfer in the Entrance Region of Rat Rectangular Ducts , NACA TN 3331, 1955. 

Sparrow, E.M., Hallman, T.M., and Siegel, R., Turbulent Heat Transfer in the Thermal Entrance Region of a Pipe with Uniform Heat Rux , Appl. Sci. Rest. Section A, Vol. 7, p. 37. 1957. 

Deissler, R.G., Turbulent Heat Transfer and Friction in the Entrance Regions of Smooth Passages , Trans. ASME, Vol. 77, pp. 1211-1234, 1955. 

Wang, M., Tsuji, T., and Nagano, Y., Mixed Convection with Row Reversal in the Thermal Entrance Region of Horizontal and Vertical Pipes , Int. J. Heat Mass Transfer, Vol. 37, pp. 2305-2319,1994. 

Narusawa. U., Numerical Analysis of Mixed Convection at the Entrance Region of a Rectangular Duct Heated from Below", Int. J. Heat and Mass Transfer, Vol. 36, pp. 2375-2380, 1993. 

An annulus consists of the region between two concentric tubes having diameters of 4 cm and 5 cm. Ethylene glycol flows in this space at a velocity of 6.9 m/s. Entrance temperature is 20°C and the exit temperature is 40°C. Only the inner tube is a heating surface and it is maintained constant at 80°C. Calculate the length of annulus necessary to effect the heat transfer. 

The Nusselt numbers and thus the convection heat transfer coefficients are much higher in the entrance region. 

Precise correlations for the friction and heat transfer coefficients for the entrance regions are available in Ihe literature. However, the tubes used in practice in forced convection are usually -several times the length of either entrance region, and thus the flow through the tubes is often assumed to be fully developed for the entire length of the tube. This simplistic approach gives reasonable results for the rate of heat transfer for long tubes and conservative results for short ones. 

R. G. Deissler. Analysis ofThrbulent Heat Transfer and Flow in the Entrance Regions of Smooth Passages. 1953. Referred lo in Handbook of Single-Phase Convective Heal Transfer, ed. S. Kaka9, R. K. Shah, and W. Aung. New York Wiley Intcrscience, 1987. 

Gui, F. and Scaringe, R.P., Enhanced Heat Transfer in the Entrance Region of Microchannels, lECEC Paper No. ES-40, ASME, 1995, 289-294. 

Prediction of the heat-transfer coefficient in the transition flow regime is uncertain due to the strong effects of entrance conditions and instability of the flow pattern. Gnielinski [18] modified the Petukhov-Popov equation to accommodate the transition region and extend it into the turbulent flow range ... 

For equilateral triangular ducts having rounded corners with a ratio of the corner radius of curvature to the hydraulic diameter of 0.15, Campbell and Perkins [180] have measured the local friction factor and heat transfer coefficients with the boundary condition on all three walls over the range 6000 < Re < 4 x 104. The results are reported in terms of the hydrodynamically developed flow friction factor in the thermal entrance region with the local wall (Tw) to fluid bulk mean (Tm) temperature ratio in the range 1.1 < TJTm < 2.11, 6000 < Re < 4 x 10 and 7.45 [Pg.382]

The research conducted by Hogg [213] has indicated that turbulent flow entrance length in coils with circular cross sections is much shorter than that for laminar flow. Turbulent flow can become fully developed within the first half-turn of the coil. Therefore, most of the turbulent flow and heat transfer analyses concentrate on the fully developed region. 

Prakash and Liu [266] have numerically analyzed laminar flow and heat transfer in the entrance region of an internally finned circular duct. In this study, the fully developed / Re is compared with those reported by Hu and Chang [265] and Masliyah and Nandakumar [267]. The incremental pressure drop K(°°) and hydrodynamic entrance length L+hy together with /Re are given in Table 5.48, in which the term n refers to the number of fins, while / denotes the relative length of the fins. 

A trapezoidal duct is displayed in the inset of Fig. 5.50. Fully developed laminar flow and the heat transfer characteristics of trapezoidal ducts have been analyzed by Shah [172]. The fully developed /Re, NuHn and NuH2 are given in Figs. 5.50 and 5.51. Farhanieh and Sunden [276] numerically investigated the laminar flow and heat transfer in the entrance region of trapezoidal ducts. The fully developed values off Re, K(°°), and Nu were in accordance with the results from Shah [172]. 

J. W. Ou, and K. C. Cheng, Viscous Dissipation Effects on Thermal Entrance Region Heat Transfer in Pipes with Uniform Wall Heat Flux, Appl. Sci. Res., (28) 289-301,1973. 

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