Local heat transfer coefficient

Characterization and influence of electrohydro dynamic secondary flows on convective flows of polar gases is lacking for most simple as well as complex flow geometries. Such investigations should lead to an understanding of flow control, manipulation of separating, and accurate computation of local heat-transfer coefficients in confined, complex geometries. The typical Reynolds number of the bulk flow does not exceed 5000. 

X = distance film has fallen g = gravitational constant Pi = liquid density = latent heat of vaporization JL = liquid viscosity k = liquid thermal conductivity AT = temperature difference = (Tb bbi,p i -NrUj = local Nusselt number, h x/k, h = local heat transfer coefficient... 

Condensation of mixed vapours is considered further in Volume 6, Chapter 12, where it is suggested that the local heat transfer coefficient may be expressed in terms of the local gas-film and condensate-film coefficients. For partial condensation where ... 

The local heat transfer coefficients on the surface of the pipe may not be uniform, though the surface is heated by uniform heat flux. This irregularity is due to the distribution of the air and liquid phase in the pipe. The temperature distribution along the pipe perimeter shows a maximum at the top and a minimum at the bottom of the pipe. In Fig. 5.36a-c, the heat transfer coefficients are plotted versus angle 0. These results were compared to simultaneous visual observations of the flow pat-... 

In Fig. 5.39a-d the local heat transfer coefficients derived in the horizontal tube are compared to those obtained in the 8° upward inclined pipe and presented by Hetsroni et al. (2006). The results show a clear improvement of the heat transfer coefficient with the pipe inclination. Taitel and Dukler (1976) showed that the flow regimes are very sensitive to the pipe inclination angle. In the flow regime maps presented in their work, the transition from stratified to annular flow in the inclined tube occurs for a smaller air superficial velocity than for the case of the horizontal tube. 

On the other hand Bao et al. (2000) reported that the measured heat transfer coefficients for the air-water system are always higher than would be expected for the corresponding single-phase liquid flow, so that the addition of air can be considered to have an enhancing effect. This paper reports an experimental study of non-boiling air-water flows in a narrow horizontal tube (diameter 1.95 mm). Results are presented for pressure drop characteristics and for local heat transfer coefficients over a wide range of gas superficial velocity (0.1-50m/s), liquid superficial velocity (0.08-0.5 m/s) and wall heat flux (3-58 kW/m ). 

In this table the parameters are defined as follows Bo is the boiling number, d i is the hydraulic diameter, / is the friction factor, h is the local heat transfer coefficient, k is the thermal conductivity, Nu is the Nusselt number, Pr is the Prandtl number, q is the heat flux, v is the specific volume, X is the Martinelli parameter, Xvt is the Martinelli parameter for laminar liquid-turbulent vapor flow, Xw is the Martinelli parameter for laminar liquid-laminar vapor flow, Xq is thermodynamic equilibrium quality, z is the streamwise coordinate, fi is the viscosity, p is the density, <7 is the surface tension the subscripts are L for saturated fluid, LG for property difference between saturated vapor and saturated liquid, G for saturated vapor, sp for singlephase, and tp for two-phase. 

The mass and heat transfer performance of the SDR is indeed impressive. Aoune and Ramshaw [91] found local heat transfer coefficients ranging from about 10 000 to about 30 000 W m K and local mass transfer coefficients between about 4E-04... 

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. 

The values of Lc and 8f are calculated from standard theoretical relations for the velocity profile in the laminar film (parabolic relationship) and for the dependence of film thickness on vertical distance (one-fourth power relationship), respectively. Hsu and Westwater obtained the following expression for the local heat transfer coefficient in the turbulent region ... 

Figure 2. Local heat transfer coefficients around circumference of a horizontal tube in bubbling fluidized bed. (V - U) /(Ut- L,) 0.1. (From Chandran, Chen and Staub. 1980.)...

Biyikli, S., and Chen, J. C., Effect of Mixed Particle Sizes on Local Heat Transfer Coefficients Around a Horizontal Tube in Fluidized Beds, 7th Int. Heat Transfer Conf, Munich, Germany (1982)... 

Chandran, R., Chen, J. C., and Staub, F. W., Local Heat Transfer Coefficients Around Horizontal Tubes in Fluidized Beds, J. of Heat Transfer, 102(2) 152-157 (1980)... 

When we want to look at the connection between the flow behavior and the amount of heat that is transferred into the fixed bed, the 3D temperature field is not the ideal tool. We can look at a contour map of the heat flux through the wall of the reactor tube. Fig. 19 actually displays a contour map of the global wall heat transfer coefficient, h0, which is defined by qw — h0(Tw-T0) where T0 is a global reference temperature. So, for constant wall temperature, qw and h0 are proportional, and their contour maps are similar. The map in Fig. 19 shows the local heat transfer coefficient at the tube wall and displays a level of detail that would be hard to obtain from experiment. The features found in the map are the result of the flow features in the bed and the packing structure of the particles. 

Knowing the heat flux, the disc surface temperature and the liquid film temperature, it was possible to calculate the average and local heat transfer coefficients for a variety... 

Feind (F2), 1960 Experimental studies of film flow (water, aqueous glycol solutions) with countercurrent air flow in vertical tubes (2.0-5.0 cm. diameter). Data on mean film thicknesses, local heat transfer coefficients, pressure drop, wave heights, onset of flooding, gas/film interactions. 

General considerations of the heat transfer in a gas-solid flow require separate energy equations for the two phases that are coupled through a local heat transfer coefficient as well as local phase velocities. These approaches are complex and are introduced in Chapter 5. 

Consequently, the instantaneous local heat transfer coefficient is obtained as... 

Furthermore, the area-averaged local heat transfer coefficient can be expressed by h = J J (y hifix) dr dA = y/KemcpemS... 

Figure 12.14. Variation of local heat transfer coefficient around a horizontal tube immersed in a fluidized bed of alumina (from Botterill et al., 1984).

Tlie local heat transfer coefficient based on Tw - T n is expressed as... 

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