Surface orientation, heat transfer

Indicate why each of these quantities would be expected to influence the heat transfer coefficient and explain how the orientation of the surface affects the process. 

Steam-liquid flow. Two-phase flow maps and heat transfer prediction methods which exist for vaporization in macro-channels and are inapplicable in micro-channels. Due to the predominance of surface tension over the gravity forces, the orientation of micro-channel has a negligible influence on the flow pattern. The models of convection boiling should correlate the frequencies, length and velocities of the bubbles and the coalescence processes, which control the flow pattern transitions, with the heat flux and the mass flux. The vapor bubble size distribution must be taken into account. 

Nishikawa, K., Y. Fujita, S. Uchida, and H. Ohta, 1983, Effect of Heating Surface Orientation on Nucleate Boiling Heat Transfer, Proc. ASME-JSME Thermal Engineering Joint Conf, Honolulu, HI, vol. 1, pp. 129-136, ASME, New York. (2)... 

The mathematical formulation of the fiber-spinning process is meant to simulate and predict the hydrodynamics of the process and the relationship between spinning conditions and fiber structure. It involves rapid extensional deformation, heat transfer to the surrounding quenching environment, air drag on the filament surface, crystallization under rapid axial-orientation, and nonisothermal conditions. 

Without the external mass and heat transfer resistances, the initial and boundary conditions with the y-coordinate oriented from the centerhne (y = 0) to the surface (y L) arc... 

Experimental studies have been conducted by Bi et al. (1990) in a fast fluidized bed 186 mm in inside diameter at ambient temperature. FCC particles with a mean size of 48 fan were employed, and three cylindrical heat probes, 10 mm in diameter and 40,80 and 160 mm in length, respectively, were used. The probes, made of copper-sheathed inner heating elements, were instrumented with thermocouples 40 mm apart on the surface for the measurement of surface temperature. Located 2.9 m above the distributor, the probe was installed either upward or downward (see Fig. 5), and was moved along the radial direction by two connecting sticks for the measurement of heat transfer coefficients for different orientations and at different radial positions. 

Fig. 6. Effect of surface orientation on heat transfer coefficients (Bi et al., 1990).

The difference between the rates of radiation emitted by the surface and the radiation absorbed is the net radiation heat transfer, If the rate of radiation absorption is greater than the rate of radialion emission, the surface is said to be gaining energy by radiation. Otherwise, the surface is said to be losing energy by radialion. In general, the determination of the net rate of heat transfer by radiation between two surfaces is a complicated matter since it depends on the properties of the surface.s, their orientation relative to each other, and the interaction of the medium between the surfaces with radiation. 

The inner surface heat transfer coefficient A, remains fairly constant throughout the year, but the value of varies considerably because of its dependence on the orientation and wind speed, which can vary from less than 1 km/h in calm weather to over 40 km/h during storms. The-commonly used values of h, and /i for peak load calculation.s are... 

Building compunents often involve trapped airspaces between various layers. Thermal resistances of such air spaces depend on the thickness of the layer, the temperature difference across the layer, the mean air temperature, tb eniissivity of each surface, the orientation of the air layer, and the directioy Of heat transfer. The emissivities of surfaces commonly encountered in buildings are given in Tabic 3—10. The effective emissivity of a plane-pafaiict air space is given by... 

The relations for cylinders above are for single cylinders or cylinders oriented such that the flow over them is not affected by the presence of others. Also, they are applicable to smooth surfaces. Surface roughness and ti t free-stream turbulence may affect the drag and heat transfer coefficients significantly. Eq. 7 -37 provides a simpler alternative to Eq. 7-35 for flow over cylinders. However, Eq. 7-35 is more accurate, and thus should be preferred in calculations whenever possible. 

Natural convection heat transfer on a surface depends on the geometry of the surface as well as its orientation. It also depends on the variation of temperature on the surface and (he thermophysical properties of the fluid involved. 

The experiments confirm what we suspect for the lower surface of a hot plate, but the opposite is observed on the upper surface. The reason for this curious behavior for the upper surface is that the force component initiates upward motion in addition to the parallel motion along the plate, and thus the boundary layer breaks up and forms plumes, as shown in the figure. As a result, the thickness of the boundary layer and thus the resistance to heat transfer decreases, and the rate of heal transfer increases relative to the vertical orientation,... 

Radiation heat transfer between surfaces depends on the orientation of the surfaces relative to each other as well as their radiation propeities and temperatures, as illustrated in Fig. 13-1. For example, a camper can make the most use of a campfire on a cold night by standing as close to the fire as possible and by blocldng as much of the radiation coming from the fire by turning his or her front to the fire instead of the side. Likewise, a person can maximize the amount of solar radiation incident on him or her and take a sunbath by lying down on his or iier back instead of standing. 

To account for the effects of orientation on radiation heat transfer between two surfaces, we define a new parameter called the vieu factor, which is a purely geometric quantity and is independent of the surface properties and temperature. It is also called the shape factor, configuration factor, and angle factor. The view factor based on the assumption that the surfaces are diffuse emitters and diffuse reflectors is called the diffitse view factor, and the view factor based on the assumption that the surfaces are diffuse emitters but specular reflectors is called the specular view factor. In lliis book, we consider radiation exchange between diffuse surfaces only, and ihu.s the term view factor simply means diffuse view factor. 

---