Forced convection heat transfer inside tubes

In the forced convection heat transfer, the heat-transfer coefficient, mainly depends on the fluid velocity because the contribution from natural convection is negligibly small. The dependence of the heat-transfer coefficient, on fluid velocity, which has been observed empirically (1—3), for laminar flow inside tubes, is h for turbulent flow inside tubes, h and for flow outside tubes, h. Flow may be classified as laminar or... 

Hauf and Grigull [133-135] precisely measured the natural convection heat transfer inside a tube following a step change in the temperature of a fluid in forced convection over the outside of the tube. In this case the heat transfer coefficient on the outer surface is constant throughout the transient, and the heat capacity of the wall plays an important role. Cheng et al. [50] have studied conditions leading to the formation of ice inside horizontal tubes (without throughflow), also with uniform heat transfer coefficient between the outside boundary and a cold environment. 

Where hi and are the convection heat transfer coefficients inside and outside the tube, respectively, v/hich are to be determined using the forced convection relations. 

This type of exchanger is used to reject heat from a fluid inside the tubes (and associated headers) directly to ambient air. To be effective, the air must flow in forced convection to develop acceptable transfer coefficients. Figures 10-174, 10-175, and 10-176 illustrate the two types, designated by the type of air movement, induced draft or forced draft. 

A fluid whose properties are essentially those of o-dichlorobenzene is vaporised in the tubes of a forced convection reboiler. Estimate the local heat-transfer coefficient at a point where 5 per cent of the liquid has been vaporised. The liquid velocity at the tube inlet is 2 m/s and the operating pressure is 0.3 bar. The tube inside diameter is 16 mm and the local wall temperature is estimated to be 120°C. 

When a fluid is heated, the hot less-dense fluid rises and is replaced by cold material, thus setting up a natural convection current. When the fluid is agitated by some external means, then forced convection takes place. It is normally considered that there is a stationary film of fluid adjacent to the wall and that heat transfer takes place through this film by conduction. Because the thermal conductivity of most liquids is low, the main resistance to the flow of heat is in the film. Conduction through this film is given by the usual relation (74), but the value of h is not simply a property of the fluid but depends on many factors such as the geometry of the system and the flow dynamics for example, with tubes there are significant differences between the inside and outside film coefficients. 

Specific correlations of individual film coefficients necessarily are restricted in scope. Among the distinctions that are made are those of geometry, whether inside or outside of tubes for instance, or the shapes of the heat transfer surfaces free or forced convection laminar or turbulent flow liquids, gases, liquid metals, non-Newtonian fluids pure substances or mixtures completely or partially condensable air, water, refrigerants, or other specific substances fluidized or fixed particles combined convection and radiation and others. In spite of such qualifications, it should be... 

Our discussion of film condensation so far has been limited to exterior surfaces, where the vapor and liquid condensate flows are not restricted by some overall flow-channel dimensions. Condensation inside tubes is of considerable practical interest because of applications to condensers in refrigeration and air-conditioning systems, but unfortunately these phenomena are quite complicated and not amenable to a simple analytical treatment. The overall flow rate of vapor strongly influences the heat-transfer rate in the forced convection-condensation system, and this in turn is influenced by the rate of liquid accumulation on the walls. Because of the complicated flow phenomena involved we shall present only two empirical relations for heat transfer and refer the reader t.o Rohsenow [37] for more complete information. 

HEAT-TRANSFER COEFFICIENTS FOR FLUIDS FLOWING INSIDE TUBES FORCED CONVECTION, SENSIBLE HEAT 7.26... 

Heat transfer by forced convection inside micro tube, generally referred as the Graetz problem, has been extended by Barron et al. [11] and Larrode and al. [12] to include the velocity slip described by Maxwell in 1890 [13] and the temperature jump [14] on tube surface, which are important in micro scale at ordinary pressure and in rarefied gases at low-pressure. 

Micro heat pipe effect -l- two-phase forced convection in the coaxial gap (porous tube inside the glass tube) increase the heat transfer 2-2.5 times as higher as in liquid pool at low and moderate heat fluxes. 

The local heat transfer coefficient for forced convection inside tubes is determined from experiments conducted with electrically heated tubes. This coefficient requires that the inner temperature of the tube walls Tt be known. However, T turns out to be more difficult to measure than the outer temperature of the tube walls, To. The usual practice is to measure To and relate it to 7i by an analytical expression. We wish to obtain this expression in terms of a tube with an inner radius J i, an outer radius f 2> and internal energy generation u ". 

The mechanism of heat flow in forced convection outside tubes differs from that of flow inside tubes, because of differences in the fluid-flow mechanism. As has been shown on pages 59 and 106 no form drag exists inside tubes except perhaps for a short distance at the entrance end, and all friction is wall friction. Because of the lack of form friction, there is no variation in the local heat transfer at different points in a given circumference, and a close analogy exists between friction and heat transfer. An increase in heat transfer is obtainable at the expense of added friction simply by increasing the fluid velocity. Also, a sharp distinction exists between laminar and turbulent flow, which calls for different treatment of heat-transfer relations for the two flow regimes. 

Cox et al. [101] used several kinds of enhanced tubes to improve the performance of horizontal-tube multiple-effect plants for saline water conversion. Overall heat transfer coefficients (forced convection condensation inside and spray-film evaporation outside) were reported for tubes internally enhanced with circumferential V grooves (35 percent maximum increase in U) and protuberances produced by spiral indenting from the outside (4 percent increase). No increases were obtained with a knurled surface. Prince [102] obtained a 200 percent increase in U with internal circumferential ribs however, the outside (spray-film evaporation) was also enhanced. Luu and Bergles [15] reported data for enhanced condensation of R-113 in tubes with helical repeated-rib internal roughness. Average coefficients were increased 80 percent above smooth-tube values. Coefficients with deep spirally fluted tubes (envelope diameter basis) were increased by 50 percent. 

D. P. Traviss, W. M. Rohsenow, and A. B. Baron, Forced Convection Condensation inside Tubes A Heat Transfer Equation for Condenser Design, ASHRAE Trans., 79, pp. 157-165,1972. 

S. Koyama, J. Yu, S. Momoki, T. Fujii, and H. Honda, Forced Convective Flow Boiling Heat Transfer of Pure Refrigerants Inside a Horizontal Microfin Tube, in Convective Flow Boiling, J. C. Chen ed., pp. 137-142, Taylor Francis, Washington, DC, 1996. 

Introduction. The use of fins or extended surfaces on the outside of a heat exchanger pipe wall to give relatively high heat-transfer coefficients in the exchanger is quite common. An automobile radiator is such a device, where hot water passes inside through a bank of tubes and loses heat to the air. On the outside of the tubes, extended surfaces receive heat from the tube walls and transmit it to the air by forced convection. 

Sensible heat transfer in most applications involves forced convection inside tubes or ducts or forced convection over exterior surfaces. 

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